# lux vs lumens vs candle power



## Avatar28 (Jun 18, 2007)

What, exactly is the difference? Okay, lux is generally what is used to measure the throw of a light and lumens is more a measure of the overall amount of light put out. How about candlepower? For example, those 1,000,000 candlepower spotlights you see all over the place, how would they compare to a light measured in lumens?


----------



## Nitro (Jun 18, 2007)

Lux and CP are both light intensity at a specific distance (throw). Lumens is overall light output.

I would forget CP though. Concentrate on Lumens and Lux for a good measure of light output.


----------



## OhMyGosh (Jun 18, 2007)

*Lumens* is the total light output in all directions

*Candlepower (Candela) *is the output over a certain angle. It is directly related to Lumens. 1 Lumen(Lm) = 1 Candela(Cd)/Steradian. (A complete sphere is 4*pi steradians). The difference is that the candlepower is usually measured at the brightest part of the beam to make the light look better. Lumens always measures the total light output.

*Lux *is a measure of how 'lit up' a surface is. 1 Lux = 1 Lumen per square meter. Unlike Lumens and candlepower, it depends on how far the light source is from the surface (unless you are talking about something like sunlight).


----------



## pmath (Jun 18, 2007)

I sort of knew all that but thank you for putting it so clearly. 

Peter


----------



## olight001 (Jun 19, 2007)

I just collect some infomation about the above question just now also, but here , all of you give us some hints,thanks all of you so much.


----------



## KingGlamis (Jun 19, 2007)

I guess this is a decent thread to ask this question. Are LED lumens any different than incan lumens? For example, if an LED is rated at 75 lumens and an incan is also rated at 75 lumens, would they be the same brightness? I'm not interested in colors, depth perception, etc. in this post. Just a question about lumens only.


----------



## WadeF (Jun 19, 2007)

While we are on this topic, who sells a good and affordable lumen/lux meter? If I buy one, does that make me an official flashaholic? 

I have a good light meter for photography use, anyway to convert it's readings to lux/lumens etc?

Like at ISO 100, f16, 1/250th = 5,000lux or whatever.

Edit: Found my answer: http://www.sekonic.com/support/support_2.asp

Not sure how accurate it is, but I get about 464,000.00 lux from my P1D-CE on Turbo, holding it righ up to the sensor.


----------



## OhMyGosh (Jun 19, 2007)

KingGlamis said:


> I guess this is a decent thread to ask this question. Are LED lumens any different than incan lumens? For example, if an LED is rated at 75 lumens and an incan is also rated at 75 lumens, would they be the same brightness? I'm not interested in colors, depth perception, etc. in this post. Just a question about lumens only.


 
The official lumens rating is weighted for the eyes sensitivity to different colors. A chart is shown Here at Wikipedia

Peak day vision is for green light ~550 nm, while peak night vision is at cyan ~510 nm. This would tend to make LED's somewhat more visible for night vision as their color is 'cooler'. Also note that night vision is very insensitive to red light ~650 nm. That is why red lights are recommended to preserve night vision.


----------



## Kilovolt (Jun 19, 2007)

Here a couple of rather complete and clear explanations:

http://www.flashlightreviews.com/features/lux.htm

http://files.intl-lighttech.com/handbook.pdf

it takes a while to read the second one but it is probably wise to keep it ready for future reference.


----------



## OhMyGosh (Jun 19, 2007)

Kilovolt said:


> Here a couple of rather complete and clear explanations:
> 
> http://www.flashlightreviews.com/features/lux.htm
> 
> ...


 
I'm sorry but the flashlightreviews article is just wrong. It keeps referring to lux when it is really talking about candlepower. It is easy to confuse since lux is what his meter displays. The key point is - when talking about the lux of a light, you must specify the distance. For example a light that reads 100 lux at 1m would read 1 lux at 10 m. Lux is not the output of a light - it is a measure of how much that light illuminates the object it is shining on. Lux is the important measure for profesional lighting designers and photographers. If you are designing a lighting system for a football stadium, you don't care about the light itself - you care about evenly lighting the field to a certain specified lux level.

Consider 2 perfect lights. Both are 100 lumens. Both have a perfect square beam that is even with sharp edges. One has a 5 degree beam and the other has a 10 degree beam. If you shine both lights at a wall, with the 5 degree beam 11.5 meters away and the 10 degree beam 5.75 meters away, both lights will make identical 1 meter square 'spots' that are 100 lux (100 lumens/square meter). However the candlepower of the 5 degree beam is 4x that of the 10 degree beam since the light is concentrated in a slimmer beam that has 1/4 the cross sectional area at a given distance.


----------



## Codeman (Jun 19, 2007)

OhMyGosh said:


> I'm sorry but the flashlightreviews article is just wrong. It keeps referring to lux when it is really talking about candlepower. It is easy to confuse since lux is what his meter displays. The key point is - when talking about the lux of a light, you must specify the distance. For example a light that reads 100 lux at 1m would read 10 lux at 10 m. Lux is not the output of a light - it is a measure of how much that light illuminates the object it is shining on. Lux is the important measure for profesional lighting designers and photographers. If you are designing a lighting system for a football stadium, you don't care about the light itself - you care about evenly lighting the field to a certain specified lux level.
> 
> Consider 2 perfect lights. Both are 100 lumens. Both have a perfect square beam that is even with sharp edges. One has a 5 degree beam and the other has a 10 degree beam. If you shine both lights at a wall, with the 5 degree beam 11.5 meters away and the 10 degree beam 5.75 meters away, both lights will make identical 1 meter square 'spots' that are 100 lux (100 lumens/square meter). However the candlepower of the 5 degree beam is 4x that of the 10 degree beam since the light is concentrated in a slimmer beam that has 1/4 the cross sectional area at a given distance.



If you'll take a closer look at the entire website, so that *you* understand the context in which that article was written, you'll find that he is correctly talking about lux readings. Heck, the very principles that you are using here *are* the basis for his measurements. :shakehead

In fact, Doug uses the inverse square law to derive his throw ratings, which is one of the most useful techniques I've seen for objectively evaluating lights. And it's all documented on his website.

For the most part on CPF, when you see lux measurements, they are taken at 1 meter. Otherwise, the measurements are meaningless, as you point out. After a while, though, most of CPF understands that 1 meter is the commonly-accepted distance to measure at, so it's often left off.

That article assumes all measurements are taken at the same distance. I think that if you re-read it, keeping that assumption in mind, that you'll find it's correct.


----------



## OhMyGosh (Jun 19, 2007)

Codeman said:


> If you'll take a closer look at the entire website, so that *you* understand the context in which that article was written, you'll find that he is correctly talking about lux readings. Heck, the very principles that you are using here *are* the basis for his measurements. :shakehead


 
I completely agree with the thrust of the artical. If it had said '200 lux at 1 meter' instead of just '200 lux' I would have no argument. I am bit touchy about manufacturers who rate their lights in candlepower without specifying the beam width. It encourages the creation of lights with needle thin beams that impresses your friends but is almost useless as a practical light (unless you are using in a boat or such on objects a mile away). A laser has a candlepower rating that is off the chart, but I think most of us would agree they make very poor flashlights.


----------



## Codeman (Jun 19, 2007)

OhMyGosh said:


> ...I am bit touchy about manufacturers who rate their lights in candlepower without specifying the beam width....



LOL - I think that's true about most CPF members, so welcome aboard!


----------



## SilverFox (Jun 19, 2007)

Hello OhMyGosh,

Welcome to CPF.

A point of clearification...

Foot candles and lux measure the same thing. One is English and the measurement is normalized to 1 foot, the other is Metric and the measurement is normalized to 1 meter.

You can transfer between the two measurements by using the following conversions:

1 lux = 0.0929 fc, or going the other way
1 fc = 10.764 lux.

This means that with 1 fc of illumination, you have 1 lumen per square foot, but when you move out to 1 meter for a lux measurement, you end up with only 0.0929 lumens per square foot due to the inverse square law and the greater distance.

Here is a thread where we compared various light meters, and got into some deeper discussions about light measurement.

Tom


----------



## robm (Jun 19, 2007)

OhMyGosh said:


> .... For example a light that reads 100 lux at 1m would read 10 lux at 10 m.



Actually (sorry the pedant in me got out again) a light that reads 100 lux at 1m would read 1 lux at 10 m, according to the inverse square law. 10x the distance so 1/10^2 (1/100) the light intensity.


----------



## OhMyGosh (Jun 19, 2007)

You know I noticed that after I fixed the same error in the last sentence. I perversely decided to leave the error in to see if anyone would notice - congrats:twothumbs


----------



## robm (Jun 19, 2007)




----------



## mdocod (Jun 19, 2007)

LUMENS: 
A *******ized measurement that at one time meant total light output of a light producing device. It is now a marketing buzzword.

LUX:
A *******ized measurement of light intensity that gets used incorrectly to compare lights.

CANDLEPOWER:
At one time, centuries ago, this actually existed, now it is nothing more than a *******ized marketing buzzword with absolutely no meaning or correlation to anything whatsoever.


----------



## winston (Jun 19, 2007)

mdocod said:


> LUMENS:
> A *******ized measurement that at one time meant total light output of a light producing device. It is now a marketing buzzword.
> 
> LUX:
> ...



Geez, Mr. Grumpy. :devil:

I'm just kidding. When I consider some topic about which I'm particularly well-informed, and the frustration I feel at all the limitations of various evaluative methods and comparisons, I get pretty irritated. Your credentials on this topic are beyond question, sir; so I hope you don't mind a little good-natured teasing from someone who doesn't know a tenth of what you do on the subject. :wave:
-Winston


----------



## mdocod (Jun 19, 2007)

yea, just jokin around mostly. but jokes are funny because of the truth that lies within.  muaha


another thought on the subject...

candlepower is the modern day "watts" in audio gear.

Nowadays, they are always trying to sell you more watts, as if more watts is really the answer to your audio problems, when in fact, the numbers on that garbage in the typical department store are total lies. It's the same thing with candlepower. If the customer were accidentally informed that if they picked out speakers that are 3db more efficient, they could get away with HALF the amplifier, then how would they sell big over-rated amps to unsuspecting victims? lol... Or if you accidentally informed the customer that the average listener will never listen to an audio program at more than ~2 watts per channel. Suddenly the huge buzzwords mean nothing.


----------



## BB (Jun 19, 2007)

Or the 5 hp air compressor or vacuum cleaner that runs on a standard 120 VAC circuit (about 1.5 hp maximum).

-Bill


----------



## SilverFox (Jun 19, 2007)

This is my take on the whole thing...

In order to identify something in the dark, I need to increase the level of illumination. 

I can predict how much illumination I need provided I know the distance to the object or person. Using the inverse square law, I can get a reasonable idea of how well a light is going to illuminate by checking the foot candles, if I am a certain number of feet away, or lux if I am a certain number of meters away.

If the object I am trying to illuminate is large, I need a large beam. If it is small, I can get buy with a smaller beam.

OK, that takes care of the hot spot measurement, now I have to decide if I want more than just the object illuminated. This is where spill and the beam profile comes in.

In order to project the amount of light that I need to illuminate my object, I need to start with a certain amount of lumens. If the beam is highly focused, I can get buy with less lumens than if the light has lots of spill and is a flood light.

So there you have it. I think foot candles and lux are the most important measurements because that is what lights up the object so you can see it. However, depending on beam shape, you also need an adequate amount of lumens to make it work.

Now, there are a couple of "issues" with my method...

First of all, light meters are not created equal, and a calibrated meter usually ends up being expensive. The light meter that I use may not totally agree with industry standards set up using a calibrated light meter. On top of that, my eyes may respond differently and I may find that I need more illumination, or can get buy with less illumination. This means that I have to get out in the dark and run some tests.

The next issue is that manufacturers lie!!! Well, it seems like it. They deal with theoretical maximums, and I am working in the real world. The differences are vast.

The final thing is that all objects do not require the same amount of illumination. It takes less illumination to simply notice an object. If you need to identify and classify the object, more illumination is needed. The nice thing about throw numbers is that it gives you an idea of what you can do. If your lux at 1 meter comes out to 40000 lux, you know that at 200 meters your light will be down to 1 lux. If you need more illumination, you can take the same numbers and change them to feet. You will have a little over 10 times the illumination at 200 feet.

While lumens are important, I feel that I can tell more from a beam profile and a lux measurement, along with some time in the field in the dark.

Tom


----------



## TorchBoy (Jun 19, 2007)

Thanks OhMyGosh, I don't think I've seen it put so clearly before.



OhMyGosh said:


> Also note that night vision is very insensitive to red light ~650 nm. That is why red lights are recommended to preserve night vision.


So for the red light to be any use it'll have to be bright enough to see with day vision... which means it'll be bright enough to wreck night vision. :thinking: But if it's sufficiently dim it won't be visible in one's peripheral vision.

I think I'll still go with a really dim white LED. Or maybe cyan would be the best idea for night vision lighting efficiency.


----------



## BB (Jun 19, 2007)

Here is a night vision thread from 4 years ago (now sticky in Night Vision Equipment)... I hate to summerize other than:

Rest is a post from QuickBeam:



> [ QUOTE ]
> Blue-green cockpit lighting is used in aircraft in which night-vision devices are used because, unlike the human eye, these devices are not sensitive to light at that end of the visual spectrum. In addition, blue-green light is the easiest for accommodative focus and is seen by the rods more readily than any other color. It is not seen as blue-green, however, but only as light.
> 
> [/ QUOTE ]
> ...


----------



## BB (Jun 19, 2007)

Also, should add, the "Pirate Solution" to the night vision dilemma...

Saw this on Myth Busters (Discovery Channel) within the last year or so--modern research apparently shows that Pirates were not missing more eyes than normal--but would wear a patch over one eye to develop and preserve night vision.

Cover one eye with a dark patch and only lift it when in the bowels of a dark ship (during the day), or when going from below at night (lit with lanterns) to up on deck (dark nights).

-Bill


----------



## abvidledUK (Jun 20, 2007)

WadeF said:


> Edit: Found my answer: http://www.sekonic.com/support/support_2.asp



Note: The "Print" version has an error in it, look around the 10 EV mark, missing line, highlighted info out of sequence.


----------



## OhMyGosh (Jun 23, 2007)

I just got this luxmeter.
As a reference I get 100,000 lux in direct sunlight. My work desk only measures 100 lux. I think I need a better desk light!


----------

