Easy to Understand Lumens Vs Lux Explanation

[TEEJ]

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.

 

[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!

 

[TEEJ]

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.

 

[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!

 

[TEEJ]

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.

 

[Gloh]

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

 

[TEEJ]

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.

 

[degarb]

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).

 

[TEEJ]

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".

 

[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.

 

[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

 

[thedoc007]

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.

 

[TEEJ]

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]

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.

 

[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]

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.

 

[GregP507]

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

 

[TEEJ]

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.