# Easy to Understand Lumens Vs Lux Explanation



## TEEJ

To understand lumens and lux, start with the concept that you can't SEE lumens. Lumens are a measure of the total light sent OUT by the flashlight.

What you SEE is the light that hits something, and bounces back to your eyes. THAT light that you SEE is referred to as LUX. The brighter the OBJECT looks to you, the higher the lux.

When a person says a light is "bright", they might mean it gives you high lux, or, high lumen values...or both....but, might not know why.




If you imagine that water is lumens, and its depth is brightness, so that the deeper the water, the brighter it looks to you...and you look down into a shot glass of water, say an inch deep...and see that brightness in that little glass...

...and then dump that shot glass of water into a kiddie pool, so it spreads out into a wide film of water....it will look "dimmer" but be the same number of lumens.



Humans suck at judging brightness in of itself...but you can see more with more lumens, but, most of the added vision is to the sides in a broad pool of light...and less is added to a central hotspot.

The hotspot generates glare, and the eye tells the brain that more glare = brighter....so people TYPICALLY will say a 100 L focused beam is "brighter" than a 1,000 L floody beam, as the floody one didn't glare and tell the brain how bright it was.



If you looked into the kiddie pool, and saw the dim light, you'd say, "that flashlight is dim".

If you looked into that shot glass, you'd say "that flashlight is bright!".


They both put out exactly the same amount of water/lumens though.


Lux is the lumens per square meter on the target....so, the more lumens you have, the easier to spread them out and still have enough lux to see targets.


If your beam spot is 10 square meters in size, you'd need ten times more lumens to have it look as bright as if it were focused down to only 1 square meter in size. 

IE: For the same "brightness", you'd need 1,000 lumens to get your 10 square meters to look as bright as the 1 square meter would look with only 100 lumens.


This is why a guy with a 100 lumen light with a tight beam might say "its impossible to read with it because there's too much glare", but a guy with a 500 lumen floody beam can read the same page with no glare.


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## Cataract

Excellent Way to put it!


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## NowIC

TEEJ said:


> To understand lumens and lux...



Nice job TEEJ!


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## TEEJ

sigh (There were some posts deleted for the sake of brevity/clarity)



For the record...10 square meters (The way I was using it) is not 10 meters by 10 meters, its representing Length X Width = 10.

As we are actually discussing a flashlight beam, its typically ROUND anyway....so a round area that covers 10 square meters is (In my world) might have a diameter of ~ 3.5682 meters.




∏ times the radius squared = 10 



For a SIMPLE explanation though, I try to avoid terms like ∏, etc.


For the inverse square law, I like to think of it as the light getting 4 times dimmer when you double the distance, or 4x brighter when you cut the distance in half, etc...



Can a mod trim off the superfluous posts after the first one to clean this up a bit?

- Thanks!

(Cataract, bro, you are cracking me up)


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## Poppy

deleted by Poppy.
Its all good


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## TEEJ

I've used that idea too, but I think people get lost in it when there's no visual.

Pressure = brightness seems harder to get than depth = brightness...as they can visualize depth and not pressure.

I wanted to explain lumens vs lux more than how to get throw vs flood per se...with an easy way to visualize what each is about.


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## Cataract

The pool analogy explains lumens VS lux best
The shower head analogy explains throw VS flood best. I believe that if you do a small revision with that in mind it would make an excellent way to explain throw and flood to newbs..


2 completely basic things that do have some common ground. Perhaps an organized cumulation of plainly explained concepts like these could be made into a full introduction course. For now, it might be best to keep these analogies separate and them point to them through a master thread.

p.s.: I take full responsibility for the deleted posts. I've been bad... Sorry TEEJ!


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## Fresh Light

I'd like an alcohol analogy.

Lux being the alcohol percentage
Lumens being the actual amount of alcohol in the container
Tint is the particular brand, flavor, or vintage
CRI is the score on ratebeer or winespectator


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## ShineOnYouCrazyDiamond

TEEJ - thanks for this explanation. I had a grasp on the concepts but this made it very easy to understand.

Poppy- BTW, the explanation of yours on the garden hose is also a perfect way to help explain to people the difference between Volts and Amps. Where volts is the pressure pushing the water out of the hose and Amps is the amount of water being pushed.


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## J-B

This cleared up a few things. Thanks!


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## TEEJ

ShineOnYouCrazyDiamond said:


> TEEJ - thanks for this explanation. I had a grasp on the concepts but this made it very easy to understand.
> 
> Poppy- BTW, the explanation of yours on the garden hose is also a perfect way to help explain to people the difference between Volts and Amps. Where volts is the pressure pushing the water out of the hose and Amps is the amount of water being pushed.



You are welcome.


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## TEEJ

J-B said:


> This cleared up a few things. Thanks!



De nada.


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## Cataract

ShineOnYouCrazyDiamond said:


> TEEJ [...]
> 
> Poppy- BTW, the explanation of yours on the garden hose is also a perfect way to help explain to people the difference between Volts and Amps. Where volts is the pressure pushing the water out of the hose and Amps is the amount of water being pushed.



Basic electronics are very easy to explain by using plumbing comparisons. That's exactly how my first electronics teachers taught us Volts, Amps and capacitors. In my particle/optical physics classes, we also had to experiment with water to understand the basic principles. Your statement is more than true. If only I had enough time I would try to write up similar threads on all the basic principles about flashlights and water analogies. I hope this thread helps start a new trend of teaching all the basics on flashlights.


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## TEEJ

Cataract said:


> Basic electronics are very easy to explain by using plumbing comparisons. That's exactly how my first electronics teachers taught us Volts, Amps and capacitors. In my particle/optical physics classes, we also had to experiment with water to understand the basic principles. Your statement is more than true. If only I had enough time I would try to write up similar threads on all the basic principles about flashlights and water analogies. I hope this thread helps start a new trend of teaching all the basics on flashlights.



I could see a new wave of analogies flowing from this source.


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## WWWW

Thanks for the explanation man, nice and simple.


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## TEEJ

WWWW said:


> Thanks for the explanation man, nice and simple.





Thanks!

I Try.


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## dwc032

Nice write up!!


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## TEEJ

Thanks!


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## njhart

Very useful! Actually this is my first trip to CPF (I've long been aware, but hesitant to jump full in to more gear collecting) and this was one of the things I wanted to know.


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## TEEJ

njhart said:


> Very useful! Actually this is my first trip to CPF (I've long been aware, but hesitant to jump full in to more gear collecting) and this was one of the things I wanted to know.



Excellent!

We try to help people learn what they came for.


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## njhart

And it is appreciated! I fully admit, I'm far more of a knife guy (at least at the moment) than a light guy. I have sort of specific wants that I'm having a hard time picking out. As such rather than just blindly asking around, I decided that I should at least have a moderate knowledge of what I'm looking at if I expect to get what I want.


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## japudjuha

So with lumens being the total light output, does Lux roughly equate to candle power/ intensity of the light?
I get lumens, but the Lux/Candella that I don't fully 'get' just yet. 
Thanks


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## TEEJ

japudjuha said:


> So with lumens being the total light output, does Lux roughly equate to candle power/ intensity of the light?
> I get lumens, but the Lux/Candella that I don't fully 'get' just yet.
> Thanks



Lux is a measure of how bright the target looks to you. Its the lumens per square meter....so the more lumens you send into a square meter of target, the brighter the target looks to you. 

The equivalent Lux at one meter is the candela (cd) rating for the light. This is a way of standardizing the specs.

So if a light puts 10k lux on a target one meter away, it's rated at 10k cd.


Examples:

If the beam angle is the same, for a given range, if I double the lumens it will double the lux (Because I am sending twice the lumens per square meter).

If I want to keep the lux the same at that range, I can make the beam angle wider, to spread the added lumens out over twice the area (So I end up with the same number of lumens per square meter).


The lux will decrease with range, so more distant targets are progressively dimmer.

The targets will look one quarter as bright at double the distance. (4X DIMMER at 2X distance)


That means that the 10k cd light in the above example, with 10,000 lux at one meter, would have 2,500 lux at 2 meters, and 625 lux at 4 meters, 6.25 lux on a target at 40 meters, and 1 lux at 100 meters, and, finally, 0.25 lux at 200 meters, and so forth.

The ANSI standard uses a 0.25 lux limit as defining a light's maximum range...so, if a light is advertised with a 200 meter range, that means it's rated at 10k cd, and so forth.


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## Poppy

Good job TEEJ :twothumbs

Something I learned from you is that if you take the square root of the published cd of the light, that will give you the distance in meters that it will produce 1 lux.
For example the square root of 10,000 cd = 100 meters
For me, 1 lux is much more usable light than 0.25 lux.


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## Asad

japudjuha said:


> So with lumens being the total light output, does Lux roughly equate to candle power/ intensity of the light?
> I get lumens, but the Lux/Candella that I don't fully 'get' just yet.
> Thanks


You're right. Lux is the unit of beam intensity, which is the number of lumens per unit area. As the beam spreads the beam intensity decreases. Moreover, the beam intensity is not uniform all across, usually more intense in the centre (the hotspot).


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## TEEJ

Poppy said:


> Good job TEEJ :twothumbs
> 
> Something I learned from you is that if you take the square root of the published cd of the light, that will give you the distance in meters that it will produce 1 lux.
> For example the square root of 10,000 cd = 100 meters
> For me, 1 lux is much more usable light than 0.25 lux.



Thanks Poppy!

A lot of people find, especially for longer ranges, that 1 lux is a lot more useful than 0.25. ANd that's its simply the square root of the cd is just _so_ darn convenient.


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## TEEJ

Asad said:


> You're right. Lux is the unit of beam intensity, which is the number of lumens per unit area. As the beam spreads the beam intensity decreases. Moreover, the beam intensity is not uniform all across, usually more intense in the centre (the hotspot).



Correct, for most lights...the hot spot is the brightest central part of the beam, and the corona surrounds that like a donut. The unfocused light that is not in the corona or hot spot, that just spills out is, strangely, simply called the spill. Some people confuse the spill with the corona, so its always a good idea to separate them in discussions.

Some lights, such as those with aspheric lenses simply project a picture of the LED itself, so there's little or no spill or corona, just a die shaped hot spot.


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## davyro

brilliant someone at work was asking me something similar but you've explained it 10x better than i could


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## TEEJ

davyro said:


> brilliant someone at work was asking me something similar but you've explained it 10x better than i could



Thanks! Its a VERY common question, and a tough concept for many to get their heads around, especially with all the advertising specs and so forth that muddy the waters.

Feel free to quote me as needed.


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## phosphor

Thank you TEEJ ! More than once I've come across a post of yours that has added meaning, depth, and understanding to these basic scientific principles.


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## TEEJ

phosphor said:


> Thank you TEEJ ! More than once I've come across a post of yours that has added meaning, depth, and understanding to these basic scientific principles.



That's great, as making these topics easier to get a handle on is my goal.


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## Razzle

Agreed, TEEJ.

This newb new nothing about handheld lights until I read some of your posts yesterday. I'm still in the dark, but it's becoming clearer!


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## Cataract

Razzle said:


> Agreed, TEEJ.
> 
> This newb new nothing about handheld lights until I read some of your posts yesterday. I'm still in the dark, but it's becoming clearer!



We're all here to learn and CPF is the best place to learn all about flashlights and lighting. Perhaps I personally learned nothing about the difference between lumen and lux (I do calibrate light meters as part of my job), but I did learn a simple and efficient way to explain it to others. This stuff is great for everyone!


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## TEEJ

LOL

That's why this is a simple to understand explanation...those who either need a way to explain it to others, or, just want to understand for the first time, can use it as a resource.

I'm glad so many have found it a valuable resource, that just makes me glow.

:thanks:


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## Beefcake the Mighty

Thanks. Makes sense.


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## TEEJ

Glad it helped.


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## Poppy

Hey TEEJ,
Congrats! 
:thumbsup: Someone made this thread a sticky! 

I was going to recommend it for the Unforgiven's "Threads of interest!" list.


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## TEEJ

Thanks guys!


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## peterharvey73

How is this?
Lumens vs Lux is the mechanical equivalent of Pressure = Force per Square Meter.

Lumens = Lux per Square Meter
Pressure = Force per Square Meter

So the Lux is like Force.
While he Lumen is like the Pressure.

I made it worse?


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## Cataract

Not completely bad, but a little confused or confusing I'd say. This one would be simpler:

Lumens = total amount of pressure inside a closed system
Lux = specific pressure on the square inch which has the most pressure is applied.

Lux is like the force of the flow of water going forward
Lumens is like the total amount of pressure inside the whole system, including side-wall pressure that barely gets the flow going anywhere.


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## jorn

Why not: Lumens is the amount of light emitted from a light. Lux is how bright it is.


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## TEEJ

jorn said:


> Why not: Lumens is the amount of light emitted from a light. Lux is how bright it is.



Mostly because "how bright it is" is typically interpreted to mean the lumens...and those needing the explanation would not be able to differentiate...and, they needed a way to differentiate lumens from lux in meaning.

Add to that - that lux is how bright it looks on a target, not how bright the light is anyway.


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## TEEJ

Cataract said:


> Not completely bad, but a little confused or confusing I'd say. This one would be simpler:
> 
> Lumens = total amount of pressure inside a closed system
> Lux = specific pressure on the square inch which has the most pressure is applied.
> 
> Lux is like the force of the flow of water going forward
> Lumens is like the total amount of pressure inside the whole system, including side-wall pressure that barely gets the flow going anywhere.



Pressure is harder to visualize though....especially for those with no plumbing experience, etc.

I want the explanation to be something that can be visualized...a mental picture. Using yet ANOTHER concept instead, is, IMHO, not as clear.


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## AbbyY

Thanks a lot TEEJ for this useful information!

If I correctly understand, a value of 250.000 cd will be traduced in a 500 m distance (using square root rule - 1 lux, instead the inverse square meter rule - 0.25 ANSI standard). That means i.e. SR95S UT beam (250.000 cd) could reach 500 m as *effective range* (what our eyes can clearly see) vs *ANSI range*?

I have also noticed that if you double the value of the square root you get the value based on ANSI standard (500x2 = 1.000 m for SR95S UT).


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## TEEJ

AbbyY said:


> Thanks a lot TEEJ for this useful information!
> 
> If I correctly understand, a value of 250.000 cd will be traduced in a 500 m distance (using square root rule - 1 lux, instead the inverse square meter rule - 0.25 ANSI standard). That means i.e. SR95S UT beam (250.000 cd) could reach 500 m as *effective range* (what our eyes can clearly see) vs *ANSI range*?
> 
> I have also noticed that if you double the value of the square root you get the value based on ANSI standard (500x2 = 1.000 m for SR95S UT).





250,000 cd would yield 1 lux at 500 M and 0.25 lux at 1,000 M, as predicted.

Because its 1/4 as bright (lux) at two times the distance, whatever range its at 1 lux, will be a 1/4 lux at double that distance.


I would caveat the "effective range" and "what our eyes can clearly see" phrases you mentioned though.

Essentially, the lux needed to resolve details INCREASES WITH DISTANCE.


Testing performed to calibrate rifle scopes for night use revealed for example that at 200 meters, 1 lux WOULD be enough to shoot a 3' x 2' white paper target, for some people, even a bit less, but others might need closer to 5 lux, but, for everyone, closer to 15 lux was needed to even FIND a rusty ~ 3' steel target.

The contrast of the target and its background CHANGES the needed lux dramatically.


The farther away the target, the more you depend on your fovea to resolve small details (Farther away, things LOOK smaller, etc...) and your fovea has TERRIBLE night vision, and, requires a LOT of light to work. 


So there is no one "Minimum Lux Level" that will always be "enough"....and that one lux figure is simply a common rule of thumb that is handy to work with....not a real hard figure.

:wave:


This is one of the reasons why someone might want a light that has a claimed 1,000 meter range, even though their friends chide that you can't see things 1,000 m away in daylight, so who needs a light with that kind of range, etc.



IE: That SR95 UT with its claimed 1,000 M (ANSI) range is probably NOT going to be USED to see things 1,000 M away....but, it CAN be used to see a low contrast target 100 M away...in practice.


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## AbbyY

Thank you TEEJ. A very good explanation!


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## experimentjon

I've been into flashlights for a few years now...good to finally know the difference between lumens and lux. Great analogy. I also like the alcohol analogy too.


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## Poppy

A couple of weeks ago I discovered something that I *think* may be pertinent to this discussion.

It appears that cd or lux is additive. In other words if you point a 5K cd light at an object you'll get 1 lux at 70 yards
If you point another 5K cd light at the same object, I believe that you'll get 2 lux at 70 yards.


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## TEEJ

Essentially, as the lux is the lumens per square meter, if you add lumens to an area, the lux goes up proportionally. The lux doesn't know if one LED, a triple LED, three flashlights each with an LED, a candle and a mirror and magnifying glass and a glow stick, etc, SENT the lumens, the target just bounces back the ones that hit it...which is the lux you see.

The thing to keep in mind that this DOESN'T mean that if I have a flashlight with one LED, and it has say 10k cd, that if I add two more LED to the same flashlight it will now have 30k cd....but if I shine two more of that flashlight at my target, I might get 3x the lux on my target.

This is akin to a chandelier. If you have one with three 60 watt bulbs, another one with six 60 watt bulbs would make the room look brighter (More lux), even though each individual bulb has the same cd.


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## Power Up

So would it mean that the amount of Lumens is directly proportional to the Lux in a mathematical equation?


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## TEEJ

Power Up said:


> So would it mean that the amount of Lumens is directly proportional to the Lux in a mathematical equation?



It might depend on the equation, but, its going to be directly proportional in relationship. Again, if the area's size the beam is ON is the same, but I have more lumens hitting it, I will have more lumens per square meter, which, = more lux. (Lux = lumens per square meter)

If I hit the same area with double the lumens, I will have double the lux, and so forth.

This is why two light bulbs in the same room's lamp will make the illuminated objects brighter even though the two bulbs have the same lumen output as each other...whatever lux I had with one bulb will essentially be doubled if that surface is hit by the lumens from the second bulb too.



(Its also why a multi-LED light can often have more throw than you'd think, as its dramatically increasing the lumen output, and in turn the resultant lux....despite the lumens form each individual LED being less focused due to the reflector focus efficiency compromises made to HAVE a head with multiple emitters, and why those reflectors are much larger, proportionally, to compensate, etc)


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## degarb

Yeah, I hate it when someone describes a light as, "bright". I think it is the job of the moderator to get the poster to better define their term. 

I think sellers, marketing folk, love the term "bright". Caveat Emptor.

I don't think it is a hard thing to understand, as long as it is not described in terms of fractions or inverse square law. The lux meter is a great tool to show people why a flashlight can be brighter to the Eye's Cones than a fixed light-even if the fixed light is 500 times the lumen output. I like also that photographer's lights (that I have purchased) are rated in lux at a distance, rather than lumens: meaning how bright the subject is illuminated is all that matters.


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## TEEJ

degarb said:


> Yeah, I hate it when someone describes a light as, "bright". I think it is the job of the moderator to get the poster to better define their term.
> 
> I think sellers, marketing folk, love the term "bright". Caveat Emptor.
> 
> I don't think it is a hard thing to understand, as long as it is not described in terms of fractions or inverse square law. The lux meter is a great tool to show people why a flashlight can be brighter to the Eye's Cones than a fixed light-even if the fixed light is 500 times the lumen output. I like also that photographer's lights (that I have purchased) are rated in lux at a distance, rather than lumens: meaning how bright the subject is illuminated is all that matters.




"Bright" is one of those words that can have different meanings, and should probably be replaced, albeit replacing words is easier said than done.


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## degarb

TEEJ said:


> "Bright" is one of those words that can have different meanings, and should probably be replaced, albeit replacing words is easier said than done.




I tried to coin the terms "Luxier and Lumier" ☺ I should relent, and we can go with "throwier" and "greater overall output". 

So, is "BrightGuy" really LuxierGuy , or just LumierGuy.com ? Its been a mystery for years.


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## TEEJ

degarb said:


> I tried to coin the terms "Luxier and Lumier" ☺ I should relent, and we can go with "throwier" and "greater overall output".
> 
> So, is "BrightGuy" really LuxierGuy , or just LumierGuy.com ? Its been a mystery for years.



LOL

We HAVE lumens and lux, but, few know how to USE them in a sentence, hence this very thread.



I have noticed over an over again that the average person considers the amount of glare/pupil stop down to be how they judge "brightness". If my 131 lumen LED maglight is shined next to my 900 L SC600, the giant area flooded by the 900 L is "not as bright" as the teeny dot made by the 131 L mag. IE: Glare = Bright, for the guy off the street...in practice.

As the average person is used to an average off the shelf light, and those lights HAVE TO concentrate the little lumen output they have as much as they can just to create enough lux to resolve anything/reach anything...the average person thinks of a flashlight beam a making a small spot of light....that's "Normal".

If the light is floody, to them, that's a "Lantern".


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## thedoc007

I really don't have a problem with "bright". I simply take it as "overall output", or lumens. "Focused" works for me to describe thrower lights, in a word. Virtually everyone understands what focused means, and how it makes a given output seem more intense, albeit over a smaller area.

It isn't perfect either, but it allows a quick summary to tell a non-flashaholic something useful. Personally I always look for a candela rating, which can give you a pretty good idea of what the beam will be useful for in a real world application. But you can't expect everyone to know what a candela rating means. Even some people on this forum have trouble with it, so the general public is hopeless...


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## TEEJ

thedoc007 said:


> I really don't have a problem with "bright". I simply take it as "overall output", or lumens. "Focused" works for me to describe thrower lights, in a word. Virtually everyone understands what focused means, and how it makes a given output seem more intense, albeit over a smaller area.
> 
> It isn't perfect either, but it allows a quick summary to tell a non-flashaholic something useful. Personally I always look for a candela rating, which can give you a pretty good idea of what the beam will be useful in a real world application. But you can't expect everyone to know what a candela rating means. Even some people on this forum have trouble with it, so the general public is hopeless...



Correct, except those people perceive glare (throw) as brightness, hence the lack of cross usage...as you think of bright as lumens, but, the great unwashed would be more inline with it meaning lux, even if they know neither term.

I see post after post from Newbs that say things like "I want the brightest light!" and "How many lumens do I need to see a fish 3' under water?" and "Is 1,000 lumens enough to let me shoot a coyote 200 m away?" etc.


Lux is not on the radar, and candle power is the gazillion thingies that the 55 watt halogen spot lights for $29.99 advertise as having millions of...so they are USED TO seeing lights on shelves advertised as having 15 MILLION Candlepower!!!!, next to the smaller ones with a measly 10 million cp, and so forth. That's their world as they know it...ours is new.


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## Power Up

TEEJ said:


> It might depend on the equation, but, its going to be directly proportional in relationship. Again, if the area's size the beam is ON is the same, but I have more lumens hitting it, I will have more lumens per square meter, which, = more lux. (Lux = lumens per square meter)
> 
> If I hit the same area with double the lumens, I will have double the lux, and so forth.
> 
> This is why two light bulbs in the same room's lamp will make the illuminated objects brighter even though the two bulbs have the same lumen output as each other...whatever lux I had with one bulb will essentially be doubled if that surface is hit by the lumens from the second bulb too.
> 
> 
> 
> (Its also why a multi-LED light can often have more throw than you'd think, as its dramatically increasing the lumen output, and in turn the resultant lux....despite the lumens form each individual LED being less focused due to the reflector focus efficiency compromises made to HAVE a head with multiple emitters, and why those reflectors are much larger, proportionally, to compensate, etc)



Got it loud and clear, thank you for enlightening a noob


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## Srargaz

Cool info about lumins.


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## Srargaz

A whole wealth of information.


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## Srargaz

What is the best way to measure lumens ?


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## dc38

Srargaz said:


> What is the best way to measure lumens ?



With a bonefide integrating sphere, of course!  It'll set you back quite a bit though.


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## TEEJ

Srargaz said:


> What is the best way to measure lumens ?



Lumens are very difficult to measure directly. Typically, an integrating sphere, as mentioned, is used. This is just a specially shaped container, made to take the lumens emitted, and spread them out very evenly, and then measure one spot with a device that measures lux.

If you know the surface area inside the container, and the entire surface is covered with the same intensity of light, because the light is spread evenly...if you know what the intensity is at one spot, you know what it is at all the other spots.

As lux = lumens per square meter, you can then calculate the lumens from the lux.

A LOT of people on this forum for example have made their own versions of this, using foam balls, cardboard boxes, and even just shining the lights at the ceiling and measuring what bounced down at a lux meter on the table, etc.

None of these will be as accurate as a real integrating sphere, and all of them have strengths and weaknesses, but, we like them nonetheless, as they do give a third party feel for a light's output, even if flawed. 

So lux is easier to measure, and, is also used to CALCULATE the lumens.


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## rdrfronty

Well said Teej. I agree homemade light boxes and spheres that I and others own do have their limitations. You do have to take these measured numbers with a grain of salt. How accurate they are is effected by the testing methods and the design itself. But mostly by the initial calibrating. That's the really difficult part.
Some have had a light actually tested in a sphere and try to use that as a baseline. Not a bad way to go. The possible flaw with that is your specific light box might not read consistent across the entire light size, beam pattern style, and mainly lumen level spectrum. And it counting on that said sphere and light being exact. 
Others, like selfbuilt & myself included, use as many ANSI rated lights as possible to try to create a good baseline and multiplier number. The flaw with this design is the lack of consistency between manufacturers published numbers. Over the years I have discovered certain brands always tend to read high, others low, and some right in the middle. For example, Eagletacs always appear to read higher than their published numbers. Zebralights tend to run lower than their published numbers. 4sevens always tend to read very close to published numbers. And etc. So the problem is - which manufacturer is correct? I'm sure all 3 of these companies use true integrated spheres. So that means that there might actually be variables between true spheres, or their testing methods vary and thus their results vary, who knows. Now I rarely find numbers drastically off, but inconsistencies are there. They are usually within about 10% of their ANSI ratings. 
So all we can do is find the best median from testing as many ANSI rated lights as possible. As long as your testing methods and equipment stays consistent, this will at least give consistent results from lights you test. Granted your numbers could actually be 10% high or 10% low - but at least it SHOULD give consistent numbers for comparison purposes.
I feel very confident in my lightboxes design that when I say one light tests at 900 lumens and the next light tests at 1000 lumens - rest assured that second light truly has about 10% more lumens. Now its possible beam types could affect these results, but I personally have never seen any noticeable variances in readings between beam types. In my lightbox anyway. My lightbox has absolutely no edges, corners, or flat spots. Nothing for light to "catch" on. So an intense spot should not be able reflect back on the sensor any easier than a powerful floody light. 
Anyway, almost any lightbox and testing method is going to be better than purely guessing. For a ANSI rated light purchase - testing said light might not make sense. After all, odds are the light will likely be very close to the manufactures published numbers. 
When these lightboxes truly come in handy is for testing modified lights, or of course non ANSI rated lights. Its nice to see what type of results you get when you swap in 3ea XML2 U2's on copper in an older model TK75 or whatever favorite light you own. Then you see the gains of your work. A lightbox and lux meter itself, make excellent tools for modders or light builders. They allow you to see what gains (or losses) you get when you try a certain LED, reflector, dedoming, lens, spring mod, or even different batteries. You see instant results. 
All this aside, the best thing about using lightboxes and meters is that they add another fun and interesting factor into this wonderful hobby of ours.


----------



## TEEJ

If I see one trend emerge in HOW the non-ANSI IS work, is that most of them seem to under represent flooders and over represent throwers....and as many of the baseline models ARE tighter beamed, the calibration tends to be closer to the thrower calibration and further from the flooders...and, sure, the spectrum response is going to be all over the place.

Most of the weakness of a DIYS IS is that the light MUST be evenly distributed to have a single point measurement be representative.

The simplest version of that is the ceiling bounce with the meter next to the light on the table, etc. MOST light from a tight hot spot will come straight down from the reflections off the ceiling over the table. If the (floody) light puts say, 60% of its spot OUTSIDE the table's diameter, most of the output is not measured with the same weighting given to the light coming down on the meter, on the table, etc.

So, given the high-tech coatings and engineering that goes into a real IS, a DIYS IS is going to be off, and, it will tend to be off the same way across the board, for example being "calibrated" more for throwers than for flooders, or to the emissions from one LED at a particular amperage/drive level vs another, etc.

It can get complicated, as variables include corona and spill proportions to hot spot proportions, and a lot of other variables that impact the difficulty of getting an even and representative distribution to fall on your lux meter's sensor.

The other issue you can't control is the consistency from light to light in actual output. A batch to batch variance in reflectors, LED, etc, can mean that if you measure 3 lights, you could have them 15% apart in output, and, the factory output spec, based upon a 3 light AVERAGE, means that YOUR light might be within 15% of that, high or low. 

Add that variable set to your DIYS IS's built in variable sets, your lux meter's built in variables, the IS variables, etc, and, when you add it all up, you COULD see dramatically different differences in perceived measurements, which, after crunching the stats, could be all the same.

Example: If the dogma is that SureFire is accurately measuring the lumens of their lights (Which can be correct, given gov contracts, etc...) - and, assuming that SF lumens are good for calibration...whatever the beam characteristics of the SF you use to calibrate are, become the reference you use as a baseline to measure the SF.

So SF lumens will, in that scenario, tend to be "right", because they were used to calibrate themselves....and lights that use different LEDs, beam angles, and so forth, will be "off", as they were not used to baseline themselves, but compared to another light's baseline.

If, for example, Zebralights's were used to establish the "baseline", then only ZL's would be found to be accurate, and all the throwy/different beam distribution and spectrum lights would be "off".


So, a real IS is the correct answer, and DIYS versions can ball park things, but, you would really need to do a lot of work to account for all the variables, and not just be satisfied to say, well, "its right for the known lights".

If your DIYS IS is actually as accurate as a real IS, and you could go to court and use it to prove that X light is over stating or understating its lumens, then, you could make a lot of them, and sell them to all the suckers paying many thousands of dollars for a real one, instead of buying, or just making your DIYS version. (You is the "royal you", not a poster or anyone in particular...)


That said, we ALL use our lux meters to bench line with...as we don't have an IS, and probably never will. Hence, we do take our DIYS measurements with the proverbial salt.


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## lightliker

Nicely explained!


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## coman

Nice Job TEEJ!


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## neutralwhite

+1 - this was the explanation I needed all this time. there was me buying more Lumens when I actually wanted more Lux. 
thanks Teej!. 



phosphor said:


> Thank you TEEJ ! More than once I've come across a post of yours that has added meaning, depth, and understanding to these basic scientific principles.


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## TEEJ

Thanks Guys!


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## JohnColter

Thanx, this is super helpful. Really love the analogy!

John


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## JeremyS

This has all been extremely helpful. Being relatively new to lights, I now have a better vocabulary to describe the characteristics of the lights I have, and the characteristics of the lights I want to have in the future.


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## Afroderp

If my retarded self could understand that explanation, anyone can!


Good job on summing that one up and putting it into laymans terms.


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## TEEJ

Glad to help...its one of those topics that really helps to understand what you want in a light.


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## JackP

_ONE OF THE BEST EXPLANATIONS I HAVE HEARD THAT MAKES SENSE. T.HANKS FOR THE TUTORIAL. kEEP UP THE GOOD WORK_


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## TEEJ

JackP said:


> _ONE OF THE BEST EXPLANATIONS I HAVE HEARD THAT MAKES SENSE. T.HANKS FOR THE TUTORIAL. kEEP UP THE GOOD WORK_



Thanks for the positive feedback!


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## Sea Urchin

Thanks for an analogy that helped me get my head together over Lux + Lumen. Brilliant!


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## TEEJ

Sea Urchin said:


> Thanks for an analogy that helped me get my head together over Lux + Lumen. Brilliant!



I guess now you're a See Urchin.


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## Gloh

Great analogy. Thanks for writing this!

Just a question though!
My torch has an 800 and 1020 Lumen setting and yet they look identical in terms of lux when I compare the two beams. 
Why is it that there is no discernible difference to the naked eye?


----------



## TEEJ

Gloh said:


> Great analogy. Thanks for writing this!
> 
> Just a question though!
> My torch has an 800 and 1020 Lumen setting and yet they look identical in terms of lux when I compare the two beams.
> Why is it that there is no discernible difference to the naked eye?



Probably because the beam is focused tightly enough that the 800 lumen output has already created enough glare that your "eye's light meter" has already pegged its needle, and you simply don't have the dynamic range in optical lux sensing to see further brightness.


The way you WOULD see the difference (Assuming your light actually goes from 800 to 1020 L...), is to pick a DISTANT target, one far away that you can barely see it with the high setting...then switch to the lower setting, and the object will disappear (Or at least get visibly dimmer). Typically, you need to get the lux on target down to a range that is within your eye's (terrible) dynamic range...to be able to resolve differences between lux output.


IE: You will typically be OK at seeing OBJECTS, but, NOT OK at judging brightness in of itself. 

If you shine at a wall, all you'll see is a bright spot on the wall, and, unless the spot gets larger or smaller, typically, most people can't tell the difference in output well at all between two hot bright spots.

If you shine the light out into a field, at a far away target, etc...you might see more, or less, target though, with different output.



The OTHER option is if your light's output is not changing, due to a circuit issue, the cells not having the juice to support a higher output, etc. 



For perspective, the lux is the lumens per square meter...so, the drop from 1,020 to 800 lumens will produce a proportional drop in lux. (A drop of ~ 22%)


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## Gloh

Right I see.
So you're saying that there should be a difference since 1050 is 'deeper' in terms of your analogy and is emitting more packets of light.
So whilst I don't see the difference close up, if I use it, as you said, in a field, technically the 1050 lumen setting would comparably be of higher lux than the 800 lumen setting and make more objects visible in the dark?


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## TEEJ

Gloh said:


> Right I see.
> So you're saying that there should be a difference since 1050 is 'deeper' in terms of your analogy and is emitting more packets of light.
> So whilst I don't see the difference close up, if I use it, as you said, in a field, technically the 1050 lumen setting would comparably be of higher lux than the 800 lumen setting and make more objects visible in the dark?



Yes...if you pick targets far enough away to be relevant, you will be able to see the added lux from the added lumens.

Otherwise, you're using a bathroom scale that goes up to 300 lb to see if 800 lb weighs more than 1,050 lb.





Its getting dark now (Around here at least...).

Go outside, and find some stuff a long way off that you can't see well due to the dark and distance.....and compare the outputs and what you see, and report back.

Most people can tell ~ 5-10% output difference under those circumstances...with 5% attainable if the target choices are better for differentiation.


I find most see it best if they have a good place to BACK UP from a target....so you do this:

1) Find a target you have a long line of sight to....turn the light to low, and start backing up until you can't see it any more....then hit high. 

2) You should see it again, and, you can drop something to mark the low's limit, and take more steps away to get the high's limit...and mark that too.


As the lux falls off according to the inverse square law, in practice, the lux will drop to a 1/4, if you double the distance. (Twice as far = quarter as bright, or, half the distance = 4x brighter, etc....)

By the same token, if your high was 4x brighter, you could DOUBLE the distance that the target looked as bright at.


As its only going to be ~ 22% brighter, the added distance will not be as dramatic as if it were 4x brighter....but, you WILL see the target from farther off than on low.


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## Gloh

Ahh what a perfect way of putting it.
I will test it out tonight to see if it is the battery or the distance that is limiting the difference.
Thank you for explaining!


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## TEEJ

Gloh said:


> Ahh what a perfect way of putting it.
> I will test it out tonight to see if it is the battery or the distance that is limiting the difference.
> Thank you for explaining!



If you are feeling ambitious, if you have a smart phone, many of them can download apps to measure lux.....and you can do some down and dirty lux measurements with that too.


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## Gloh

I didn't even know smartphones were capable of that. Well I downloaded one and will test it out later!
Don't think I'll be leaving my phone on the other side of a field though - especially not where I live!


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## TEEJ

Gloh said:


> I didn't even know smartphones were capable of that. Well I downloaded one and will test it out later!
> Don't think I'll be leaving my phone on the other side of a field though - especially not where I live!



The secret is to find a friend to stand at the other end of the field....with HIS phone.


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## Gloh

Hahaha and then leave him stranded in darkness whilst you head home with the aid of your beacon of light.


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## TEEJ

Gloh said:


> Hahaha and then leave him stranded in darkness whilst you head home with the aid of your beacon of light.




Hmmmm....the concept of "friend" might be different where you are?




Also....you might need the data, and/or repeat the process on another night.


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## degarb

Gloh said:


> Great analogy. Thanks for writing this!
> 
> Just a question though!
> My torch has an 800 and 1020 Lumen setting and yet they look identical in terms of lux when I compare the two beams.
> Why is it that there is no discernible difference to the naked eye?



1. Area = Length X width Therefore, you need to go 4x more lumens to perceive something doubly bright.

2. Your lower lumen torch may have even a slightly tighter hotspot and tighter corona. Therefore, it is possible to have higher lux in hotspot and, perhaps brighter corona. 


Your eyeball (source wikipedia/braingames/9 years of using lights for paint streak hunting) has a %2 fovia (also, according to slight of hand magicians, your area of attention), a %15 angle of cones (your sharp vision). Your peripheral vision is mostly for vague shapes and movement. It is also exponentially difficult, yes, exponentially harder to cover wider circles (in corona or hotspot), requiring exponentially larger battery and lumens).


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## TEEJ

degarb said:


> 1. Area = Length X width Therefore, you need to go 4x more lumens to perceive something doubly bright.
> 
> 2. Your lower lumen torch may have even a slightly tighter hotspot and tighter corona. Therefore, it is possible to have higher lux in hotspot and, perhaps brighter corona.
> 
> 
> Your eyeball (source wikipedia/braingames/9 years of using lights for paint streak hunting) has a %2 fovia (also, according to slight of hand magicians, your area of attention), a %15 angle of cones (your sharp vision). Your peripheral vision is mostly for vague shapes and movement. It is also exponentially difficult, yes, exponentially harder to cover wider circles (in corona or hotspot), requiring exponentially larger battery and lumens).



He implied it was one light with two outputs.



You are correct about the fovea. This is one reason that if you are in a very dark environment, and you look straight AT something, you might not see it....but, if you look to it's SIDE, you might see it.

And, yup, if the hot spots and corona/spill were different sizes/proportions, all bets are off as to visual comparison of brightness; Its why observers will report that the 131 lumen maglight (Tight beam) is "brighter" than the 900 lumen ZL SC600 (Floody beam). The 131 lumens in a teeny circle pegs our eye's light meter, and we perceive "brightness".


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## Samlittle

Yikes :thinking:. i've got to read this all over again but I'm closer to understanding the lux/lumen then ever before. Thanks TEEJ and others for the explanations.


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## Cankniver

sorry new guy here. Its a very helpful explanation. Do I assume that lumens are usually used in light spec s because lux is dependant on the surface being illuminated. Ie a black surface less lux than a white or is this way off or just dumb


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## thedoc007

Cankniver said:


> sorry new guy here. Its a very helpful explanation. Do I assume that lumens are usually used in light spec s because lux is dependant on the surface being illuminated. Ie a black surface less lux than a white or is this way off or just dumb



I'm sure TEEJ will answer you in more detail. But in fact, most lights do have both lumens and lux (of a sort) in the specs. The candela rating, and the beam distance (or range) all measure the intensity of a light. I.e., how many lux the brightest part of the beam will put on a target at any given range.


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## TEEJ

Cankniver said:


> sorry new guy here. Its a very helpful explanation. Do I assume that lumens are usually used in light spec s because lux is dependant on the surface being illuminated. Ie a black surface less lux than a white or is this way off or just dumb





Lumens were used historically, because the bulb or LED had a factory spec for the output.

The lights would be advertised as having the lumen rating being the LED's rating.....so if the light was powered at a particular drive level, CREE, etc, would have a chart that said at that power, the LED would produce X lumens. Sometimes they just used whatever the MAX for that LED was, w/o worrying about the drive level, just cherry picking what was on the chart as the top #, etc.


Later on, when ANSI standardized the testing, and that testing required that the lumens coming out from the flashlight, not from the LED, obviously, there were losses due to the reflectors, lenses, etc, and the lumen ratings dropped proportionally.


The lux you SEE is very dependent upon the color and reflectivity of the target, albeit if your target is a lux meter, this variable is eliminated from a measurement perspective.

So, the "lux at one meter" standard used to get the candelas (cd) of a light, doesn't care what color, etc, YOU will use the light to see. What the color etc changes, is HOW MUCH lux YOU might need TO SEE the target.

IE: You might be able to see something/someone (Lets call that "Bob") right next to you with night adapted eyes and 0.25 lux on Bob. If Bob is wearing a white disco dancing suit, you might be able to see Bob 200 meters away with 0.5 - 1 lux on him. If he were to change into a black ninja costume, at the same 200 meters, you may need well over 15 lux on him to see him.


So the target characteristics change how MUCH lux is needed _to resolve_ the target, but, the lux the light is providing, when measured, will be consistent for any target at a particular range.

The light's intensity (lux) drops with distance according to the inverse square law....so, if a light's specification gives a range in meters or feet, etc...you can back calculate the light's cd, as well as the lux at any given range.


So, you are correct in your interpretation that the light won't bounce back to your eyes if the target is dressed in flat black, as well as if it were wearing bike reflectors, etc...but, _for the ratings of lux_, the target is always the device that MEASURES the lux.

This eliminates a variable in COMPARING light's specs...in that they are all tested the same way, at least allegedly.


----------



## sjc

Many thanks to Teej for taking the time to illustrate. It makes sense. One question: you said that if you double the lumens at the same beam angle, you will double the lux. But if the initial beam angle is greater than 1 square meter, like a flood, then some of the doubled lumens will fall outside of the central square meter where lux is being measured, so will you get fewer than twice the lux in that case?


----------



## TEEJ

sjc said:


> Many thanks to Teej for taking the time to illustrate. It makes sense. One question: you said that if you double the lumens at the same beam angle, you will double the lux. But if the initial beam angle is greater than 1 square meter, like a flood, then some of the doubled lumens will fall outside of the central square meter where lux is being measured, so will you get fewer than twice the lux in that case?



The beam angle is an angle, not a "meter", etc...


The concept is that for a given DISTRIBUTION of light, (IE: The beam angle as a simplified way of expressing that), the lumens per square meter will be higher if the lumens are higher.

So if you double the lumens, for the same beam, you will double the lux...because you have doubled the lumens per square meter.

If the DISTRIBUTION is different (A different beam shape/proportion), then, yes, you are 100% correct, and all bets are off as to the effect.

So, sure, most beams have a corona and some spill, and, some parts of the hot spot that are less intense than others, etc...but if you ONLY increase the lumens (Say with more amperage, etc...), EACH of those will tend to be proportionally brighter, not JUST the hottest part of the hotspot.

So, the hottest part of the hotspot may get twice as bright if the lumens are doubled....doubling the lux. The corona and spill, etc, may be ALSO twice as bright too though.


Remember, the FIRST version of out hypothetical light's (lower lumen out put version of our hypothetical light) lux measurement did not represent 100% of the light's lumen output. It ONLY represented that light's hottest part of its hotspot.

The SECOND version's lux, with higher lumens, ALSO is not representative of 100% of ITS output. It ONLY represented that light's hottest part of ITS hotspot.

To simplify it, lets say, that for this flashlight's beam distribution pattern, 10% of the beam is the hotspot, and, 10% of the hotspot is the hottest part we measure. That might be 1% of the total output in this example.

If the light starts with 100 lumens, the hottest part of the hotspot might be 1 lumen's worth in this scenario.


If we make the same emitter double its lumen output, we now would get 2 lumens instead of 1 lumen at that same hot spot point.



Similarly, if the lumens stay the same, but you REDUCE the beam angle and/or distribution, so that the same lumens are hitting a smaller surface area, the lux will go up proportional to that change too.




So, if the beam angle is the same, when the target is further away, the square meters that the beam's spot will cover will get larger as the beam expands with distance. This lowers the lumens per square meter, as the lumens emitted remains constant, but, the square meters covered is increased, diluting those lumens.

The reduction is PROPORTIONAL though, so, with all else being the same, if you double the lumen output, you still double the lumens per square meter, at whatever range you are measuring at.


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## sjc

Thank you Teej. I was starting to follow you but your illustration got cut off in your second to last sentence in post #95. Thank you.


----------



## TEEJ

sjc said:


> Thank you Teej. I was starting to follow you but your illustration got cut off in your second to last sentence in post #95. Thank you.



I edited it to add back the parts that the computer gods had truncated.


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## GregP507

I prefer to think of a lumen as 3.8×10e15 photons/s (photons per second).


----------



## TEEJ

GregP507 said:


> I prefer to think of a lumen as 3.8×10e15 photons/s (photons per second).



That sort of thought doesn't work well in a lumens vs lux explanation though, as the units themselves have no everyday reference points for lay people. 

This is about the concepts more than about numbers.


----------



## CPF21

Thank you TEEJ, for explaining these confusing photonics terms in a layman's manner.


----------



## TEEJ

CPF21 said:


> Thank you TEEJ, for explaining these confusing photonics terms in a layman's manner.



My pleasure Sir.


----------



## TennesseeTony

Great explanation. Really clears that up. Thanks so much. This is a great forum for someone new. I've learned so much in the last few days.


----------



## TEEJ

TennesseeTony said:


> Great explanation. Really clears that up. Thanks so much. This is a great forum for someone new. I've learned so much in the last few days.



This place is like that...if you read the threads, every day you learn something new.


----------



## thedoc007

TEEJ said:


> I'm glad so many have found it a valuable resource, that just makes me glow.



You sure that isn't all the tritium you have around you?

Seriously, though, this is a great thread. Well explained, and a good reference for anyone who is just learning the concepts.


----------



## Full Power

TennesseeTony said:


> Great explanation. Really clears that up. Thanks so much. This is a great forum for someone new. I've learned so much in the last few days.



Sorry, I think I missed that explanation ... one needs to take it slow in order for all this info to make a little sense without risking overload! I had no idea a Flashlight could be so interesting, and most of all -- is more than just a Flashlight!


Sent from my iPhone using Candlepowerforums


----------



## kbuzbee

Full Power said:


> one needs to take it slow in order for all this info to make a little sense without risking overload!



Sure, that's one way. I prefer to overload, reset, overload, reset....  Keeping my brain fried on too much data is how I roll.  But beware of any posts I may make during a reset.  There have been more than a few....

Great thread, TEEJ,

Ken


----------



## Danielsan

I always think about a firefighter and a fire hose, the lumens is the total amount of water shooting out of that thing and the lux is jox concentrated the water jet is. Thats why even a low amount of water can be compressed to a powerful water jet, that would be a low lumen thrower flashlight. A high lumen light with low lux is when the fire hose sprays huge a fountain


----------



## TEEJ

Danielsan said:


> I always think about a firefighter and a fire hose, the lumens is the total amount of water shooting out of that thing and the lux is jox concentrated the water jet is. Thats why even a low amount of water can be compressed to a powerful water jet, that would be a low lumen thrower flashlight. A high lumen light with low lux is when the fire hose sprays huge a fountain



That's pretty much the analogy in this thread.

The qualification would be that the UNITS need the depth of the resultant puddles to be analogous to how bright the target would look, so that the actual level of illumination would follow, and be able to be applied to what you'd see over a larger vs smaller surface area, etc.


----------



## ER1C

Very well said and easy to understand! Thanks for clearing up the "technical" aspect of it :thumbsup:


----------



## Brianalaman

You just made a whole lot of sense. Thanks for the detail and explanation. :thumbsup:


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## TEEJ

You are quite welcome.


----------



## Skyraider59

*Is there a direct link between OTF lumens & lux readings out of an integration sphere*

Hi Guys

Can any one answer this question? Would an out of the front lumen figure relate to a lux figure obtained from a DIY integration sphere?
May be an odd question, but like many I always like to know if the lumen figure given by the manufacturer are close or a long way away from reality!
I give you a *random *example
Let say that you have a Gloworm x2 claimed at 1500lms which have been tested at approx 1400lms, 
if you would check it in an integration sphere got let say 1000 lux *(fictitious figure for easy calculation)*
Cloud you then assume that any light showing let say 500 lux in the same sphere would have an OTF output of roughly 750 lumen? and a light showing 2000lux would have a guesstimated out put of 3000lms?

If this assumption is correct would the light have to be of the same design ie single led versus twin or would this not matter?

I understand that this may be a very rough way of estimated OTF lumen, but this could be useful to many to just have a very rough idea of the actual lumen, specifically for the over claimed outputs found on some Chinese lights!!

Any idea on this?


----------



## TEEJ

*Re: Is there a direct link between OTF lumens & lux readings out of an integration sp*



Skyraider59 said:


> Hi Guys
> 
> Can any one answer this question? Would an out of the front lumen figure relate to a lux figure obtained from a DIY integration sphere?
> May be an odd question, but like many I always like to know if the lumen figure given by the manufacturer are close or a long way away from reality!
> I give you a *random *example
> Let say that you have a Gloworm x2 claimed at 1500lms which have been tested at approx 1400lms,
> if you would check it in an integration sphere got let say 1000 lux *(fictitious figure for easy calculation)*
> Cloud you then assume that any light showing let say 500 lux in the same sphere would have an OTF output of roughly 750 lumen? and a light showing 2000lux would have a guesstimated out put of 3000lms?
> 
> If this assumption is correct would the light have to be of the same design ie single led versus twin or would this not matter?
> 
> I understand that this may be a very rough way of estimated OTF lumen, but this could be useful to many to just have a very rough idea of the actual lumen, specifically for the over claimed outputs found on some Chinese lights!!
> 
> Any idea on this?



Well, the way an integrating sphere WORKS is that it homogenizes the total lumens so that they evenly cover the interior of the sphere, so a LUX reading on that surface would be representative of the lux present across the surface area.

As lumens are lux per square meter, if the interior is 1 square meter in surface area, and your representative lux reading = 1, it means you have 1 lux per square meter, or, 1 lumen.

IE: The way the IS works is exactly how your asking if it COULD work...so, yes, its actually its primary reason to exist.



The sphere is CALIBRATED to get the readings to be representative. So the actual sphere interior surface area and, baffles to prevent emitter light from shining directly ON the lux sensor, etc, are taken into account. KNOWN lumen sources are used to adjust the IS's response until the IS's equation is known, and you can then count on a calibration curve you can plot UNKNOWN lumen sources against...and find their value.

If its a DIYS IS, the distribution will NOT be homogeneous, and, the beam angle and other characteristics WILL cause the IS to be wrong by some amount. IE: The light will NOT be evenly distributed, and the lux will be higher or lower in some areas inside it...so a single lux measurement will be ONE of the lux levels, but may not be representative of an average of all of them, etc.

Its not realistic to expect a homemade $100 IS /Light box to be as good as a several thousand dollar laboratory made IS. There are REASONS the "Real One's" are so expensive and difficult to make. 

That said, with enough tweaking, many people have managed to get pretty close, at least for the lights THEY measure. The problem is that they have no good way of knowing for sure how a different light might be read, say due to multiple emiters or a really floody, or more typically, more throwy, beam pattern.

For example, the super expensive crystal impregnated IS coatings and perfectly round IS interiors with ports and surfaces designed to also evenly distribute impacting photons - CAN integrate the light from a powerful thrower, as the beam will hit the wall, and BE evenly distributed.

When a powerful beam hits your cardboard paper mache or Styrofoam box's white paint or interior, etc...some light might be absorbed and not reflected back, and be lost from the measurements...and some might reflect back in a very narrow band on on part of the box, but not others, so that the one (ONE) lux reading you have could be in a darker, or lighter, area...and your reading will be shy or inflated as a result.

Some home made IS tend to over estimate throwers and under estimate flooders, and some do the opposite. If people use a "Known Lumen Output" light, say a Surefire they trust the specs on...it might have a throw oriented beam, and, when they calibrate the IS, its therefore calibrated to represent the out put of the Surefire's beam pattern. They might then test another light of unknown output's lumen claims, and get a false low, or high,...making them assume the unknown brand's claims are inflated....or understated.

There are also "Wait Times". MANY lights start out brighter, and get dimmer with time. SOME are so dramatic, that their CLAIMED lumen outputs are based upon the max reading, say the first 30 seconds or so....and they then drop to half or less seconds later. So, if you put in the AA powered 1,000 L claim light, and it takes you more than 30 seconds or so to turn it on, and get a reading, the out put might have been 1,000 L, but, by the time you got a reading, it was 400 L....or, you GOT the 1,000 L, but didn't wait to see what it dropped to later, etc.

So, sure, what you ask is not only doable, its what its for....but, on a DIYS IS, YMWV...and you simply need to remember it if counting on the readings.


----------



## Skyraider59

*Re: Is there a direct link between OTF lumens & lux readings out of an integration sp*

Thanks very much TEEJ for your in depth answer to my question. 
So the bottom line is unless you have a lab IS, making assumptions of actual lumen outputs of several different lights(flooder/thrower/single/twin led etc) using a DIY IS which has been "roughly" calibrated with a know lumen output light is not a reliable way of quantifying an output! 

I did kind of expected this, so claimed lumen output given by manufacturer will have to stay unverified for many lights!

Thanks again


----------



## dc38

*Re: Is there a direct link between OTF lumens & lux readings out of an integration sp*

I developed a cheap way to ballpark it the lumen output using a cheap luxmeter...it works on the preface and assumption that an integrating sphere takes a cross section of total distributed dispersed light. I figured there qould be a coincidental point at which the luxmeter could be placed from a diffuser that would do the same thing. Ill pm you


----------



## Skyraider59

*Re: Is there a direct link between OTF lumens & lux readings out of an integration sp*

Sound good, looking forward to hear, thanks


----------



## TEEJ

*Re: Is there a direct link between OTF lumens & lux readings out of an integration sp*



dc38 said:


> I developed a cheap way to ballpark it the lumen output using a cheap luxmeter...it works on the preface and assumption that an integrating sphere takes a cross section of total distributed dispersed light. I figured there qould be a coincidental point at which the luxmeter could be placed from a diffuser that would do the same thing. Ill pm you



That could be a problem.

The diffuser would need to make the light coming out evenly distributed, and, to get the beam to hit it, and diffuse evenly, such that a reading anywhere would be representative, would be difficult to achieve. So, while a diffuser is supposed to DO that, in practice, it tends to be brighter in the middle....even if MORE evenly distributed.

Multiple diffusers might be a way to further homogenize the output, albeit each optic interface will cause losses, and your calibration curves would need to account for that percentage. This would include the parts of the spectrum that were being filtered out...as the lumen is a wavelength weighted value, etc.


----------



## silvercookie

I can honestly say that is the first time in a long time i have read a good thread.


----------



## OldNick

*Integrating sphereLux to lumens, final conversion confusion*

So you get an integrating sphere as described elsewhere on these forums. You make sure you surce cannot directkly strike the meter sensor. The whole idea is to make the light source as diffuse as possible. You measure the lux. Then what? One thread said you divide by 36. Why? Doesn't the ratio of the entire sphere's area to the area seen by the sensor (or the area of the sphere) come into play here? That was what I could not find, either here or looking elsewhere.

Thanks for any help


----------



## TEEJ

*Re: Integrating sphereLux to lumens, final conversion confusion*



OldNick said:


> So you get an integrating sphere as described elsewhere on these forums. You make sure you surce cannot directkly strike the meter sensor. The whole idea is to make the light source as diffuse as possible. You measure the lux. Then what? One thread said you divide by 36. Why? Doesn't the ratio of the entire sphere's area to the area seen by the sensor (or the area of the sphere) come into play here? That was what I could not find, either here or looking elsewhere.
> 
> Thanks for any help



This is explained earlier in this very thread...

..But, to summarize:

The reason its an INTEGRATING Sphere (IS), is because the light is spread out evenly across the SURFACE AREA of the sphere's interior.

This is SO that a reading of the lux, taken anywhere inside the sphere, will be representative of the average lux.

So, the SIZE of the sphere is what dictates the MATH you'd use to interpret that lux reading.


So if the lux is the lumens per square meter (m2), and the interior surface area of the sphere is 1 m2, then a reading of 1 lux means that there must have been 1 lumen to create that reading.

If the sphere is larger for example, then the surface area inside it might be 10 m2, so a reading of 1 lux would have required 10 lumens to achieve, or, the original 1 lumen source would have yielded 0.1 lumens as a result, and so forth. 


To further calibrate the IS, known sources are measured and compared to the actual measurements by the IS to see if they agree, and, if they are off, the INTERPRETATION of the reading itself is adjusted by correction factors that allow the reading to be INTERPRETED correctly. (Or at least more correctly, etc)



So, the sphere has a shape and coating that evenly integrates/distributes the light across its interior surfaces, and a baffle that prevents the sensor from being in the direct path of the source, so it reads an integrated/smoothly distributed average of the lux.

When you see a lux reading, you need to know the surface area it is representative of, and, the correction factors that were determined for that IS, to calculate the lumens that that lux READING would indicate.


Does that help?


----------



## reppans

*Re: Integrating sphereLux to lumens, final conversion confusion*

I happen to followed this post from the general section...



OldNick said:


> So you get an integrating sphere as described elsewhere on these forums. You make sure you surce cannot directkly strike the meter sensor. The whole idea is to make the light source as diffuse as possible. You measure the lux. Then what? One thread said you divide by 36. Why? Doesn't the ratio of the entire sphere's area to the area seen by the sensor (or the area of the sphere) come into play here? That was what I could not find, either here or looking elsewhere.
> 
> Thanks for any help



Not to take anything away from what TEEJ said on a proper sphere and calibration, but most of us use cheap homemade contraptions that are "close enough" and if nothing else, perfectly fine for at least relative measurements between similar lights. 

So, for DIY light boxes, your lux>lumen multiplier depends on the efficiency of your lightbox and is simply the "plug" multiplier between your lux reading and lumen output of your trusted calibration light. Mine happens to be a nice even 10x multiplier using a $5 PVC plumbing elbow rig . 

I calibrate to ti-force's lumen scale, the only reviewer on CPF to claim laboratory ANSI accuracy, and since his scale matches most of my lights/modes anyway . I also always use lower modes to calibrate since max output tends to have more sample variation, and is very battery sensitive.


----------



## OldNick

*Re: Integrating sphereLux to lumens, final conversion confusion*

5 characters


----------



## TEEJ

*Re: Integrating sphereLux to lumens, final conversion confusion*



reppans said:


> I happen to followed this post from the general section...
> 
> 
> 
> Not to take anything away from what TEEJ said on a proper sphere and calibration, but most of us use cheap homemade contraptions that are "close enough" and if nothing else, perfectly fine for at least relative measurements between similar lights.
> 
> So, for DIY light boxes, your lux>lumen multiplier depends on the efficiency of your lightbox and is simply the "plug" multiplier between your lux reading and lumen output of your trusted calibration light. Mine happens to be a nice even 10x multiplier using a $5 PVC plumbing elbow rig .
> 
> I calibrate to ti-force's lumen scale, the only reviewer on CPF to claim laboratory ANSI accuracy, and since his scale matches most of my lights/modes anyway . I also always use lower modes to calibrate since max output tends to have more sample variation, and is very battery sensitive.





It doesn't take away from what I said. 


Its just a question of degree. For example, the DIYS work best when comparing similar beam patterns, etc, but tend to fall short when the throw or flood is different, or the tints are different, etc.

A real IS of course uses a radiometer type meter, not a home style lux meter...etc.


On the other hand, if you feel your home made version is "close enough", you could submit it to ANSI for certification, and then sell a ton of them to fools who shelled out thousands of dollars for your competitor's ANSI certified IS.

:twothumbs


Seriously though, we ALL try to make out own versions, and, they CAN be pretty good...but, ALL will have the limitations imposed by the throw/flood issue, because none will be able to completely integrate the beam and homogenize the output.

Given the cost of a "real one", its typically the DIYS, or nothing.


----------



## domdbomb

Thanks for explaining! My friend kept telling me that it's not about the lumens when you're concerned about the brightness, and now I understand what he was talking about. Great way to put it.


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## Mark Yan

i think the lumen like pressure and the lux like intensity of pressure...:kiss:


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## scs

Mark Yan said:


> i think the lumen like pressure and the lux like intensity of pressure...:kiss:



I've always thought of lumen as the total force and lux as the pressure at a location (stress) at a given distance. Lumen is total output; lux is intensity at a given distance.


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## TEEJ

The lumens are the total supply, and, the lux is how its distributed...so, the higher the lux, the less distributed/more concentrated, those lumens are.

Again, as lux is the lumens per square meter, if you make the same lumens cover a larger surface area, the lux will be proportionally lower.

If you make the same lumens cover a smaller area, the lux will be proportionally higher.

If you keep the area covered the same size, but double the lumens, you are also doubling the lux...and so forth.


So, yes lux is definitely a way to express the intensity of the illumination produced on a target.

Lumens are just what you have to work with as a total supply....for your eventual lumens per square meter concentration.


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## reppans

I like the pressure/force analogy, I think I'll use "weight" going forward.

Imagine lying on the floor and a beautiful 115 lbs (lumen) woman lays directly on top of you > mule (no pun intended). Next she stands on a large book on your chest > flooder..... Coke can > thrower.... Long masonry nail > laser (and you die ).


----------



## TEEJ

reppans said:


> I like the pressure/force analogy, I think I'll use "weight" going forward.
> 
> Imagine lying on the floor and a beautiful 115 lbs (lumen) woman lays directly on top of you > mule (no pun intended). Next she stands on a large book on your chest > flooder..... Coke can > thrower.... Long masonry nail > laser (and you die ).



I didn't know how kinky you were.

Now I have laser mule woman in my head....


----------



## reppans

TEEJ said:


> Now I have laser mule woman in my head....


----------



## ACOLJNSD

It's nice to know that someone's differentiating between brightness, lumens and lux or footcandles. From an optical standpoint, there are equations that governs all of these parameters. And not only the parameters, but how they relate and what they mean. Brightness or Radiance is flux/(A*O) where Flux is lumens, A is the Area and O is the solid angle. Irradiance is flux/Area and Intensity is flux/Solid Angle. To convert Intensity to Irradiance, simply divide the Intensity by the distance squared. So if a flashlight has a centerbeam candlepower of 5000cd, the irradiance is 5000Lux at one meter or 5000 ft-candles at one foot. At 464ft, the irradiance is 0.023ft-candles or 0.25LUX. This corresponds to the ANSI FL1 standard for distance.


----------



## TEEJ

ACOLJNSD said:


> It's nice to know that someone's differentiating between brightness, lumens and lux or footcandles. From an optical standpoint, there are equations that governs all of these parameters. And not only the parameters, but how they relate and what they mean. Brightness or Radiance is flux/(A*O) where Flux is lumens, A is the Area and O is the solid angle. Irradiance is flux/Area and Intensity is flux/Solid Angle. To convert Intensity to Irradiance, simply divide the Intensity by the distance squared. So if a flashlight has a centerbeam candlepower of 5000cd, the irradiance is 5000Lux at one meter or 5000 ft-candles at one foot. At 464ft, the irradiance is 0.023ft-candles or 0.25LUX. This corresponds to the ANSI FL1 standard for distance.



LOL

This thread was for those who needed a lay version of how to understand the concept. If they got the math, it would not be needed.


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## renovipaz

Good information, Thanks!


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## kostas147

After this thread I understand the difference between Lumens and Lux!!! :thumbsup:


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## eric1565

I have always wondered about that. thanks


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## kill bill

Thank you for the info, makes it a little clearer!


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## Rivine

thanks for your clear explanation about lux vs lumen. it was very much helpful for me.


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## TEEJ

I'm glad it helped!


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## Rolz

Thanks for the easy explanation. To be honest I've never really looked for an explanation of the difference and assumed one could be derived from the other, but clealry not.


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## forbes617

very helpful


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## casnova

Thanks, in advance, for any info.
Very interesting thanks for sharing. :thumbsup:










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## bonefreak

Thank you TEEJ. Very well explained for a relatively new candle power head!


----------



## etc

Right. Good point. 

Point is, you can have 800 unfocused, very floody lumens that are almost useless unless for a very specific application. 

Or focused 120 lumens that are more useful in almost all applications.

Case in point, I have a Malkoff M60L, the original, 5+ year old device, "Low". It's just such a nice balance of thrown and high lux that I consistently found it more useful than even more recent, higher-powered units such as Malkoff M91. The latter had considerably more lumens (was it 800 or something to that effect) but it felt like holding a household lightbulb in your hand. All flood, no throw. Useless beyond room distance. Ended up selling M91. Still use M60L. 
M60L has higher lux than the higher powered M61.


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## TEEJ

etc said:


> Right. Good point.
> 
> Point is, you can have 800 unfocused, very floody lumens that are almost useless unless for a very specific application.
> 
> Or focused 120 lumens that are more useful in almost all applications.
> 
> Case in point, I have a Malkoff M60L, the original, 5+ year old device, "Low". It's just such a nice balance of thrown and high lux that I consistently found it more useful than even more recent, higher-powered units such as Malkoff M91. The latter had considerably more lumens (was it 800 or something to that effect) but it felt like holding a household lightbulb in your hand. All flood, no throw. Useless beyond room distance. Ended up selling M91. Still use M60L.
> M60L has higher lux than the higher powered M61.



Its the opposite for most people actually, in that most EDC lights at least are used most for closer illumination, such as task lighting...and too tightly focused beams make a glaring hot spot surrounded by darkness, and are far less useful.

Its like any tool of course, if its a Phillips headed screw....that's the best driver to use.


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## Draglid

this was easy for a newcomer of the flashlight world to understand , thanks !


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## summer1323

nice job, before read this, I so not now there difference


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## softsorter

Thanks for explaining that another thing I have learnt about leds lumens and lux, its falling into place a bit now.


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## TEEJ

softsorter said:


> Thanks for explaining that another thing I have learnt about leds lumens and lux, its falling into place a bit now.



Not just LED btw - This applies to any light.


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## cheey85

Never knew about lux until now lol

thanks


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## TEEJ

Glad it helped!


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## KBobAries

Oops. Hit the wrong button and it posted before I was finished.

I have the concept in general and I suppose by the 10th time I read the OP it will completely sink in. Is it ALWAYS the lux number ONLY that determines throw?

A couple examples pulled from the web. One light is 2350 lumens / 22K lux and the other is 3550 lumens / 23K lux. For the moment, disregarding the human eye & inverse square law about perceiving brightness. Both of the lights are listed as "great throwers." Is 1200 lumen difference for a 1K gain in lux a result of LED efficiency? Beam angle?


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## KeepingItLight

KBobAries said:


> A couple examples pulled from the web. One light is 2350 lumens / 22K lux and the other is 3550 lumens / 23K lux. For the moment, disregarding the human eye & inverse square law about perceiving brightness. Both of the lights are listed as "great throwers." Is 1200 lumen difference for a 1K gain in lux a result of LED efficiency? Beam angle?



The two flashlights you describe are both "general purpose" lights. For the most part, they are flooders. Due to the large number of lumens they output, they also gain some throw. 

Note, however, that even a single 18650 flashlight like my *ThorFire C8* can get more throw than they have. And it is only a 900-lumen flashlight.

Assuming the numbers are OTF (out-the-front), ANSI FL 1 lumen ratings, then the difference between the two flashlights you cited is most likely due to beam angle or a difference in spill/hot-spot brightness.

(BTW, I don't mean to nitpick, but just in case you do not already know, the proper term is "candela," rather than "lux." Without getting bogged down in too much detail, we can say that candela is the maximum lux that can be found, i.e., measured, at any position that is exactly 1 meter from the emitter. Usually, this means dead center in the hot spot, but not always.)


----------



## KBobAries

KeepingItLight said:


> ...For the most part, they are flooders. Due to the large number of lumens they output, they also gain some throw...
> 
> ...Assuming the numbers are OTF (out-the-front), ANSI FL 1 lumen ratings, then the difference between the two flashlights you cited is most likely due to beam angle or a difference in spill/hot-spot brightness.
> 
> (BTW, I don't mean to nitpick, but just in case you do not already know, the proper term is "candela," rather than "lux." Without getting bogged down in too much detail, we can say that candela is the maximum lux that can be found, i.e., measured, at any position that is exactly 1 meter from the emitter. Usually, this means dead center in the hot spot, but not always.)



Thanks for the reply. For the first part of the quote, if I'm following you correctly, the gain in throw is due to power. Even though it's a flood reflector enough light is being forced out the front and it has to go somewhere. Granted, if a thrower is wanted one would change the reflector. As is, the main gain is in having the area under flood be brighter and the increased throw is an incidental effect due to physics.

Regarding the second part of the quote. Not sure if the numbers are OTF. They came from 2 different lights from the same manufacturer. I used them just as an example to confirm my guess about beam angle. 

As to the third part of the quote, I didn't take it as nitpicking  Always glad to learn something. My inference is that this is similar to resistance & ohms. Lux is the topic & candela the numerical value. 

Dan


----------



## KeepingItLight

KBobAries said:


> If I'm following you correctly, the gain in throw is due to power. Even though it's a flood reflector enough light is being forced out the front and it has to go somewhere.



You got it!




KBobAries said:


> Regarding the second part of the quote. Not sure if the numbers are OTF. They came from 2 different lights from the same manufacturer. I used them just as an example to confirm my guess about beam angle.



Assuming that the manufacturer is consistent in its procedure for measuring lumens, we ought to be able to compare outputs of two different models it makes. 

Beam angle is one factor. If two flashlights output the same number of lumens, and one of them has a beam that is wider than the the other, then the beam intensity of the wide one would be dimmer than that of the narrow beam. 

Beam angle might not be the only reason two flashlights have different intensities. It is possible that they could use different optics that produced identical beam angles, but which divided the light between hot spot and spill in dissimilar ways. One could have a brighter spill and dimmer hot spot than the other. It would throw less. 

I am not an expert on reflector design. It is easy, however, to imagine that this could be accomplished by some combination of reflector and/or lens. One flashlight, for instance, might use a traditional reflector; the other, an aspheric lens.




KBobAries said:


> As to the third part of the quote, I didn't take it as nitpicking  Always glad to learn something. My inference is that this is similar to resistance & ohms. Lux is the topic & candela the numerical value.



This is not quite right. Lux and candela are both measurable quantities. 

Lux has units of lumens per square meter. It is a measure of the number of lumens that hit or pass through a 2-dimensional surface in space. In general, each point on the surface will have a different lux. 

On a smooth surface, there is a tangent plane at every point. Lux measures the number of lumens that hit or cross through the surface (at that point) perpendicular to the tangent plane. The calculation requires constructing a unit normal vector to the tangent plane, and taking the dot product with the vector representing the light beam. The result is to ignore everything except the lumens that are perpendicular. 

When the surface is a sphere of radius 1 meter, centered at the emitter of a flashlight, then the flashlight's beam intensity, measured in candela, is the highest lux measured anywhere on the sphere. Usually, but not always, the highest lux will be found in the center of the hot spot. If a beam has artifacts, however, the brightest part may not be in the center.

A more complete definition is more complicated, and gets involved with "solid angles," and such, but this hand-waving explanation is good enough for flashlights.

Sorry if this sound too technical! You can learn more at Wikipedia. But be forewarned, it gets pretty messy, very fast!


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## CivilGear

Thanks for sharing you explanation  I always like to collect metaphors to help explain lumens vs lux vs candles.


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## KeepingItLight

Thanks! I'm glad you liked it. 

It's one of the more technical explanations in this thread, but I think it gives a fairly accurate description of the difference between lux and candela, especially as the latter term applies to flashlights.

There is an unstated assumption that TEEJ has made throughout this thread. All of his targets are facing directly toward the flashlight, so that his flashlight beams strike them head on, i.e., perpendicularly. 

If, for instance, the target is the side of a building, the building must be facing the flashlight. If it is not, then you must use the dot product, as described above, in order to determine the lux on target. 

This makes sense. 

Suppose, for instance, that the flashlight beam is a circle 10 meters in diameter when it strikes the building. If the building is facing the flashlight, then a section 10 meters wide will be lit by the flashlight. If, however, the building is angled to the flashlight, a much longer section may be lit up. For purposes of discussion, let's say that a 20-meter section gets lit. 

The lumens have not changed, but the area being lit up has changed. Because the area is larger, the number of lumens per unit area will be less, i.e., the lux will be less.

From this example, you can see that the angle at which a light beam strikes a surface, i.e., a target, plays a role in determining lux!

The definition of a flashlight's beam intensity, as measured in candela, is designed to eliminate this dependence.

When you model a flashlight as a point source, and place a sphere of radius 1 meter around it, all the stuff about tangent planes, normal vectors, and dot products goes away. That's because every light beam is perpendicular to the surface of the sphere, no matter where it strikes it.


----------



## TEEJ

KeepingItLight said:


> Thanks! I'm glad you liked it.
> 
> It's one of the more technical explanations in this thread, but I think it gives a fairly accurate description of the difference between lux and candela, especially as the latter term applies to flashlights.
> 
> There is an unstated assumption that TEEJ has made throughout this thread. All of his targets are facing directly toward the flashlight, so that his flashlight beams strike them head on, i.e., perpendicularly.
> 
> If, for instance, the target is the side of a building, the building must be facing the flashlight. If it is not, then you must use the dot product, as described above, in order to determine the lux on target.
> 
> This makes sense.
> 
> Suppose, for instance, that the flashlight beam is a circle 10 meters in diameter when it strikes the building. If the building is facing the flashlight, then a section 10 meters wide will be lit by the flashlight. If, however, the building is angled to the flashlight, a much longer section may be lit up. For purposes of discussion, let's say that a 20-meter section gets lit.
> 
> The lumens have not changed, but the area being lit up has changed. Because the area is larger, the number of lumens per unit area will be less, i.e., the lux will be less.
> 
> From this example, you can see that the angle at which a light beam strikes a surface, i.e., a target, plays a role in determining lux!
> 
> The definition of a flashlight's beam intensity, as measured in candela, is designed to eliminate this dependence.
> 
> When you model a flashlight as a point source, and place a sphere of radius 1 meter around it, all the stuff about tangent planes, normal vectors, and dot products goes away. That's because every light beam is perpendicular to the surface of the sphere, no matter where it strikes it.




To clarify - This is incorrect regarding the beam spread on a target.

Remember that the light being emitted at the target has no idea of the target, etc.

So, if the target is a mouse, the beam is not more concentrated at its hot spot than if the target is an elephant, or a circus tent, etc.

The beam angle, for the most part, dictates the way the lumens are concentrated...and, if there's a target or targets out there for the lumens to hit and bounce back as lux...that in no way changes the output of the flashlight.


So, again - The candela (cd) is the lux at 1 meter........and, this is a CALCULATED value, for lights with more throw...as the beam is not typically fully formed AT 1 meter. IE: It might be MEASURED at 20 meters, or 3, etc, and back calculated to what it would be, mathematically, at 1 meter.

The entire reason for getting the cd is to allow calculation of the lux at OTHER ranges.


If a light provides the spec for lux at 1 meter, or, if not, the "range" of the light, say in meters, or feet, you can back calculate to find what cd would produce that spec..as the "range" is always to 0.25 lux on the target at that distance.


So, as far as raw throw goes, the higher the cd, the farther the throw.

If two lights have the same cd, but one has a higher lumen output, it will mean that the higher lumen output is part of a floodier beam (wider beam angle).



Keep in mind that the lux = the lumens per square meter.

So, if a flashlight puts a one square meter spot of light on a target 1 meter away, with an output of 1 lumen, it will also have an out put of 1 lux.

If the beam angle stays the same...but it puts out 2 lumens, the intensity will rise to 2 lux, because BOTH lumens are on the same square meter.

If I take the 1 lumen flashlight, and concentrate that 1 lumen onto a 1/2 square meter circle, we know have a beam with 2 lux, as we put 1 lumen on 1/2 a square meter.

If there is a mouse in that circle of light, we would see the entire mouse lit up by that 2 lux beam.

If there was an elephant in that circle of light, we would see 1/2 square meter of grey hide lit up by the 2 lux beam...and so forth.


In real life, the beams tend to have three main parts though: A hot spot, the brightest part in the center, the corona, a donut shaped ring of somewhat dimmer light around the hot spot, and spill, the light that was sent out past the reflector bowl without being focused.



If being used as a thrower though, the only part of the beam REACHING the target is the hot spot, and the corona and spill fell off in intensity enough to now be invisible/not usable at the light's maximum ranges.


So, 2 lights COULD have beam patterns that produced the same sized hot spot and cd, but one had more lumens, but the added lumens were "wasted" in spill and corona.......if you would rather see more at the max range.....or "spent" illuminating proximal stuff such as where you were walking, while looking way off in the distance, etc.



So, if shopping for "A Thrower"....the first question is how far you want to see what at.

That set's the cd you need......the range the light will hit what lux at can be calculated.



The second question is how large an area you want to light up at a time...at what distance.

The wider the beam angle, the wider the beam is, and, it spreads with distance. (Think of the light pattern sent out as an ice cream cone you are holding at the cone's tip...with the fat end, with the ice cream, as what you are lighting up your targets with).

Generally, for any given cd, if the cd is the same for two flashlights, but there are more lumens in the beam of one of them...a larger area will be lit up off in the distance....but you won't see FARTHER, just more of what's at the same max distance.


There are obviously other thrower issues to consider, albeit this thread's part is relevant here.


----------



## KeepingItLight

Thanks, TEEJ, for all the work you have done explaining these topics. I don't disagree with your analogies. I think they do a good job explaining the concepts. 

Thanks, as well, for explaining that a flashlight beam must achieve focus before you can measure its highest lux. As you say, that means candela must usually be found by measuring lux at a distance farther than 1 meter, and then calculating back to 1 meter. 

I think, however, you have misinterpreted my use of the word "angle." Your reply speaks about "beam angle." Beam angle is the "width" of a beam as measured at the flashlight head using a protractor. Beam angle is not what my post describes. 

The point is a subtle one. 

The angle in my post is the angle at which individual light rays strike the surface of a target. When they arrive exactly perpendicular to a flat target, then your description of the lux that falls on the target is correct. When they arrive at a different angle, one that is not perpendicular, then the lux is reduced! By this, I do not mean that flashlight candela is reduced. Obviously, the flashlight has not changed. What I mean is that the lux at each individual point of the target is reduced. 

Your discussions of lux on target, in this thread and others, implicitly assume that each individual light ray strikes the surface precisely perpendicular. 

You have said, for instance, that at the beam distance given by the ANSI FL 1 rating, the most brightly lit point on a target will have 0.25 lux falling on it. That is only true, however, when the target is facing the flashlight, so that its surface is perpendicular to the incoming light rays. When those rays are not perpendicular, the lux on target is reduced. 

This is hard to understand, and seems to violate common sense. 

That's why I cooked up my analogy of the building. It is easy to see that shining a flashlight along the side of a building causes the illuminated section to stretch out. A beam that makes a small circular target on the front of a building, will stretch into a larger oval when it is shined along the side. The number of lumens striking the target has not changed. The area being illuminated, however, has changed. It is larger. Same lumens; larger area. By definition, that means the lux on any point of the target has been reduced!

Now, using the ideas of calculus, shrink that building down to an infinitesimal size. The same result holds. When the surface is angled with respect to the incoming light rays, lux is reduced.

Here are a couple of relevant Wikipedia quotes. 

The cosine that is mentioned comes from dotting the unit normal vector of the target surface with a vector in the direction of an incoming light ray.



> *Illuminance*
> 
> Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance (lux) is inversely proportional to area when the luminous flux (lumens) is held constant.
> 
> One lux is equal to one lumen per square metre:
> 
> 1 lx = 1 lm/m2​ = 1 cd·sr/m2​





> The illuminance provided by a light source, on a surface *perpendicular *to the direction to the source, is a measure of the strength of that source as perceived from that location.
> 
> [Emphasis added.]





> The illumination provided on a surface by a point source equals the number of lux just described times the *cosine of the angle between a ray coming from the source and a normal to the surface.* The illumination provided by a light source that covers a large solid angle is proportional to the cosine of the angle between the surface normal and a sort of barycentre of the light source, so long as all of the source is above the plane of the surface. The number of lux falling on the surface equals this cosine times a number (in lux) that characterizes the source from the point of view in question.
> 
> [Emphasis added.]



This gets a little hairy! 

That's why I used the analogy of the face of a building. Hopefully, it helps put a little light on the subject!


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## KeepingItLight

Here is a concrete example of what I am talking about. You don't have to buy an *Acebeam K70* to carry out this experiment, but it will be more fun if you do!

So get your light set up for throw, and wander down field with your favorite book on the physics of light. Have an assistant sit with the flashlight, and turn it on and off as you move away. Use a cell phone to coordinate with your assistant. 

Find the shortest distance at which there is not enough light to read by. With the K70, that should be about 1300 meters! When you do this, hold the book exactly perpendicular to the flashlight beam. Now, take a couple of steps back towards to the light. Move just enough so that you can read, and no more. 

Okay, you have found the lowest lux at which you can read. Now tilt the book back 60 degrees. As the cosine of 60 degrees is 1/2, this will cut the lux in half. Voilà! You can no longer read. Even though you are still the same 1300 meters away from your K70 flashlight, the lux on target has changed. 

This is proof that the lux on target depends on the angle at which your flashlight beam strikes the target.


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## mikemalone

Great explanation!


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## TEEJ

KeepingItLight said:


> Here is a concrete example of what I am talking about. You don't have to buy an *Acebeam K70* to carry out this experiment, but it will be more fun if you do!
> 
> So get your light set up for throw, and wander down field with your favorite book on the physics of light. Have an assistant sit with the flashlight, and turn it on and off as you move away. Use a cell phone to coordinate with your assistant.
> 
> Find the shortest distance at which there is not enough light to read by. With the K70, that should be about 1300 meters! When you do this, hold the book exactly perpendicular to the flashlight beam. Now, take a couple of steps back towards to the light. Move just enough so that you can read, and no more.
> 
> Okay, you have found the lowest lux at which you can read. Now tilt the book back 60 degrees. As the cosine of 60 degrees is 1/2, this will cut the lux in half. Voilà! You can no longer read. Even though you are still the same 1300 meters away from your K70 flashlight, the lux on target has changed.
> 
> This is proof that the lux on target depends on the angle at which your flashlight beam strikes the target.




I understand what you are saying, but, lux is what bounces back to your eyes....the lumens per square meter.

If the surface is angled, less is reflected back to your eyes, so the flashlight sent out the same amount and quality of light, but, essentially, less was reflected back to you.

If comparing lights, the differences in how far you can see, at the maximum range, will therefore be proportional to the beam's cd.


This THREAD is to explain the difference between lux and lumens...and to allow readers to gain a feel for what they represent.

If we go off on tangents (pun intended), we dilute the light we are trying to cast on the subject.


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## bykfixer

Fun with pun.

Thanks for the thread.


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## dreze888

Thanks for the in depth info. Learning as I go.


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## vettex2

1 :devil:


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## vettex2

I appreciate the explanation !


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## Herb987

*Re: Integrating sphereLux to lumens, final conversion confusion*

Hi TEEj,

I would like to get your advice on calculating the lumens using an Integrating Sphere.

My issue is "not having enough area inside the sphere" in order to satisfy the quote in the next paragraph:

In your post you wrote: "So if the lux is the lumens per square meter (m2), and the interior surface area of the sphere is 1 m2, then a reading of 1 lux means that there must have been 1 lumen to create that reading."

===========================================

My equipment for measuring light is an Extech HD450.

===========================================

I was thinking of purchasing two 12" Smoothfoam Half Ball's from Hobby Lobby.
http://www.hobbylobby.com/Crafts-Hobbies/Basic-Crafts/Styrofoam/12"-Smoothfoam-Half-Ball/p/110834

and following the instructions here:
http://www.candlepowerforums.com/vb...Pseudo-Integrating-Sphere-photos-and-readings!

Let's ASSUME:
-- the INNER diameter of the above domes is 12" and 
-- that I purchased 2 of them and glued them together to form a sphere.

I would then calculate the inner surface area as:
Area = 4 pi r^2

I would convert the diameter from 12" to a radius of 6".

I would convert the radius of 6" to meters ... 

6 /39.37 = 0.1524 meters ... or ...

6*0.0254 = 0.1524 meters

and then plug the values into the formula for the area:

Area = 4 (3.14) (0.1524)^2
Area = 0.291716 square meters

===> At this point I have calculated that I do not have at least 1 square meter of surface area inside the sphere so I would not be able to have a 1 to 1 match when reading the lux on the meter and saying "the lux reading on the meter is the same as the lumens" because I do not have 1 square meter of surface area.

If I multiply the area I calculated above (0.291716 square meters) by 3.427 I would get an answer of 1 square meter.

Does that mean that since the area inside the sphere is less than 1 square meter that I would need to: 
-- divide my lux reading by 3.427 -or-
-- multiply my lux by 0.291716 
in order to get the lumens since I have less than 1 square meter?

Thanks.
H





TEEJ said:


> (I removed some of the quoted original post that I'm replying to - Herb)
> ...
> 
> So if the lux is the lumens per square meter (m2), and the interior surface area of the sphere is 1 m2, then a reading of 1 lux means that there must have been 1 lumen to create that reading.
> 
> If the sphere is larger for example, then the surface area inside it might be 10 m2, so a reading of 1 lux would have required 10 lumens to achieve, or, the original 1 lumen source would have yielded 0.1 lumens as a result, and so forth.


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## lamp flashlight

Fun with pun.

Thanks for the posting and have a good night


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## friskyplatypus

I think this was the easiest explanation I've ever seen. Made it so that I understood even after having looked at multiple different explanations when buying flashlights.


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## bulrush

This really helps. Thanks.


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## toasterz12

If I'm understanding this right, wouldn't the lux change depending on how far the flashlight was from the target and/or whether or not the flashlight was focused? If so, how far away is lux measured?


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## MC408

Great info. New to the forum and learning right away!


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## TEEJ

toasterz12 said:


> If I'm understanding this right, wouldn't the lux change depending on how far the flashlight was from the target and/or whether or not the flashlight was focused? If so, how far away is lux measured?



Exactly.

Lux can be MEASURED at any distance at all...as long as its far enough away for the beam to have fully formed....if the lux at that distance is to be used to calculate the lux at OTHER distances.

The cd of the flashlight is back calculated to represent the lux at 1 meter though.


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## ven

:twothumbs:twothumbs hope all is well TEEJ, good to see you


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## UrbanExplorer

I am by no means a specialist yet but I'm working towards it haha

Lumens is used to quantify the rate at which light flows from a source. For example it is like saying a hose puts out 5 gallons of water a minute. A flashlight might give off 1 lumen of light per second. Therefore although this is not a common practice you can technically convert lumens to a photon count if you take into account the wavelengths. One lumen of your average white light gives off 10^15 photons per second. 

As for lux it is also a rate of the flow of light. The only difference is that it has more strict parameters. Luminous flux (lumens) include ALL light regardless of direction or spread or area in which it covers. Illuminance (lux) just tells you how much light is supplied to a specific surface per second, per square meter. 
So let's say we have a beam of light that's circular and has 200lumens. The circle is 2 square meters. This means the luminous flux is 200 lumens but the spot it makes on the wall is 100 lumens per square meter. This can be changed by affecting the distance of the light from the wall or the focus of the light. 

Hope this helps!


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## Landonb

Good information here ,new to the addiction 
need to read couple more times to fully sink in 
thx


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## convolutionx

So laser points would have high lux due to small area of output, but low lumen due to just general less photons coming out of the flashlight?


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## Keitho

convolutionx said:


> So laser points would have high lux due to small area of output, but low lumen due to just general less photons coming out of the flashlight?


Yes, spot on. One way to think of it is to imagine the amount of power that the battery is producing. A laser pointer is bright enough to blind someone, even if it is inside a pen and is only powered by a tiny watch battery, because that tiny amount of power is focused into such a small area (and hopefully that area isn't on your retina). A huge 4x D cell maglight would have a hard time blinding someone, even at pretty close range, even though it is making a lot more lumens and consuming a lot more power--all those lumens are going all over the place. 

High lux = "I'm blind"
High lumens = "crap, my huge battery is already dead" and "crap, my flashlight is too hot to hold"


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## lumen aeternum

TEEJ said:


> Lux is a measure of how bright the target looks to you. Its the lumens per square meter....so the more lumens you send into a square meter of target, the brighter the target looks to you.
> 
> The equivalent Lux at one meter is the candela (cd) rating for the light. This is a way of standardizing the specs.
> 
> So if a light puts 10k lux on a target one meter away, it's rated at 10k cd.



How big is the target? Is it 1m^2 or an infinite wall, at 1m distance?

Wondering how to glean hotspot vs corona information.


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## Sabbs1

This was incredibly helpful for a new member, thank you. Excellent explanation!


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## Soverign

Excellent thread to read.
I find it difficult to compare lights when one manufacture uses Lumens and the other Lux.
Then again they will put their best numbers forth as that is what the marketing guys do!


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## fixit7

So it looks like lux is variable as it varies with the object it is striking. The lux reflected off a mirror would be higher than from reflection off a black cloth.

Whereas lumens is good indicator of how much light a light is giving out.


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## Bullzeyebill

Actually. the light reflected back from a medium would show the perceived brightness of that object. Go up to a black wall and point a light meter at the incoming light and you will get a lux reading no different than shining the light at a white wall, if the distance is the same.

Bill


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## brttina

Thanks for the sharing.:twothumbs


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## TEEJ

The "target" is a lux meter.

The meter is calibrated to the size of its sensor, so, the lux (Lumens per square meter) is calculated from whatever hits the sensor of known size.

The actual UNITS are _expressed_ in terms of lumens per square meter (Lux).

The throw unit (cd) is _expressed_ in terms of lumens per square meter at one meter distance.... even though the actual throw is basically never that number at that distance... as that number is merely useful for calculating the lux on target at long ranges, where a thrower shines, so to speak.


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## TEEJ

lumen aeternum said:


> How big is the target? Is it 1m^2 or an infinite wall, at 1m distance?
> 
> Wondering how to glean hotspot vs corona information.


The "target" is a lux meter.

The meter is calibrated to the size of its sensor, so, the lux (Lumens per square meter) is calculated from whatever hits the sensor of known size.

The actual UNITS are _expressed_ in terms of lumens per square meter (Lux).

The throw unit (cd) is _expressed_ in terms of lumens per square meter at one meter distance.... even though the actual throw is basically never that number at that distance... as that number is merely useful for calculating the lux on target at long ranges, where a thrower shines, so to speak.



As for corona and hot spot, spill, etc.... its information that is typically not available.

If looking for throw, assume there is no corona or spill, and concentrate upon the cd. The more cd, the more distance you can see to. 

If comparing two lights, for example, the one with the higher cd throws further.

After that, look at the lumens.

As a rule of thumb, if they have the same cd, but one has more lumens, the hot spot is USUALLY larger (same brightness, but a larger diameter beam spot).


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## lednight

Thank you for this in-depth conversation about Lux, Lumens and cd, wow! I started reading all the replies, but did not see my own question, for a smaller size light with good throw, should we look for a aspheric lense to minimize corona (wasted light?) Would this make a big difference in keeping the light concentrated?


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## Dave111

Thanks a ton! Always had been scratching my head between those two terms.


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## 37Blenman

Thanks for the info, quite interesting!!


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## dmichael

Very interesting. Thanks!!


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## msmith

Much appreciated - was trying to figure this exact conversion out while shopping for my new lights! Thanks! :twothumbs


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## magellan

I occasionally reread this thread and have always found it helpful.


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## desert.snake

Since this topic has already surfaced, I remembered one old explanation, in fact the same as the first few messages here, but with live pictures for better understanding



Flashlight Reviews and LED Modifications


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