Lux/Candela/Meters/Lumen Calculations

I'm sure this is already on here, but I couldn't find it. I'm looking for the equations relating lux, candela, lumen output, and meters (distance). I'm trying to do a quick and dirty brightness comparison to determine if I have the right product.

I have two lights from the same company, one a thrower (rated 1000lm) and one a weaponlight (rated 950lm). Now, I realize the beam profiles are very different, but the thrower *seems* to be putting out a heck of a lot more light than the weaponlight, which looks closer to (but still brighter than) a 300-some lm light with a similar beam profile. I've tried contacting the company once, but I got no response.

Using the rated candela, lumens, and whatnot, I would like to determine the distance from a wall at which the two flashlights (thrower, and weaponlight) would produce the same lux. If the weaponlight is obviously dimmer, I can bring my results to the company in question (since both lights are manufactured by them), and hopefully get a replacement. If there's no noticeable difference, I have no more problems. :twothumbs

Thanks!

The relationship between CD, throw distance in meters, and lux is: CD divided by the square of the throw distance equals lux at that throw distance.

For the same light, CD and the corresponding lux at a given distance scales roughly linearly with lumen. In other words, if at X lumens the CD is Y, then at 2X lumens, the CD is roughly 2Y.

Note lumens is total output. Lux is lumens per square meter. Lux is what you perceive as brightness. CD is a common metric by which the throw distance among lights can be compared. Max throw distance is calculated when the lux on target is 0.25. Also, CD equals lux at 1 meter.

Lights can have same lumen output but different CD depending on the focusing optic.

If you aren't in a huge hurry for them I have all of the formulas wrote out at home and I can upload a big of them tonight when I get back

The light with the higher cd will "look brighter"...because you can't SEE lumens.

You can see lux, which are photons that hit something and bounced back to your eye, where your eye then makes a picture with it.

The higher the lux, the brighter the illuminated target looks to you.

A light can have 10x the LUMENS, and look "dimmer", because those extra lumens are spread out over a wider area, so there's fewer per square meter, but they're covering more square meters.


So, if I have a 1 lumen light, and a 10 lumen light, and I have a beam angle on the 1 lumen light that make a spot on the wall that's 1 square meter in size, that light will be producing 1 lux (1 lumen per square meter) of illumination.

If the 10 lumen light had a wider beam angle, so the 10 lumens are making a 100 square meter spot on the wall, that light will only be producing 0.1 lux of illumination (A tenth of a lumen per square meter).

If I use the 10 lumen light with the same beam angle as the first 1 lumen light, so its now concentrated onto a 1 square meter spot, I now have 10 lumens per square meter, or, 10 lux of illumination.

And so forth.


So, its NORMAL for a thrower, even with less lumen output, to appear brighter than a less concentrated but higher lumen output light.

It would be very difficult to be able to perceive the difference between 950 lumens, and 1000 lumens.
Some say that you need twice the number of lumens to perceive the difference. I don't think that is quite accurate, but I do understand that there is a geometric relationship to lumen output, and perception.
It seems to me that you recognize that a light with fewer lumens may appear to be brighter, if it has a tighter more focused hot spot.

A U2 cool white tint, will put out more lumens than a T6 neutral white tint (perhaps a 10% difference).
Assuming the same LED, and same supposed bin/tint, they should pull the same ma at the tail cap, if you have a digital multi-meter, you can check that.
To put out 900-1000 lumens, a XM-L2 will pull about 3000 ma.

Our friend TEEJ has a much better understanding of all this than I, and of course, he makes terrific points!

I hadn't considered that you might be standing on your back porch shining the lights out into the woods.
If you are white wall hunting, you might see all of the lumens, ie. a combination of the hot spot, and spill.

Thanks for the help so far. I can get my hands on a DMM at some point being an engineering student.
CD/d^2=Lux is exactly what I need. Provided that the CD listed on the packaging are calculated based on the listed lumens.
I will do CD1/d1^2 = lux = CD2/d2^2, and determine a pair of distances to test at, where lux should be the same for both lights. (they should look equally bright on the wall)
Having removed the reflectors to try to get a more apples-to-apples visual test, it still looks like there's a brightness discrepancy.
Though I have also noticed that the tints are indeed different.

Thanks for the help so far guys. Will let you know what happens.

RyeBread said:

Thanks for the help so far. I can get my hands on a DMM at some point being an engineering student.
CD/d^2=Lux is exactly what I need. Provided that the CD listed on the packaging are calculated based on the listed lumens.
I will do CD1/d1^2 = lux = CD2/d2^2, and determine a pair of distances to test at, where lux should be the same for both lights. (they should look equally bright on the wall)
Having removed the reflectors to try to get a more apples-to-apples visual test, it still looks like there's a brightness discrepancy.
Though I have also noticed that the tints are indeed different.

Thanks for the help so far guys. Will let you know what happens.

Click to expand...

Your eye is a notoriously lousy light meter, so, if using your eyes to judge brightness, you will get rather unreliable results.



Just use this concept, that the inverse square law describes how the lux will drop with distance, in that it will be 1/4 the lux at double the distance, or, 4x as bright 2x as close, etc.

The DISTRIBUTION of the light will make determining the lumens from the lux impossible to do accurately, unless you have a way to integrate the total light output into a homogeneous field, so a lux measurement anywhere in that homogeneous field will yield the same result.


Essentially, most reflector based lights have a hotspot in the middle, and, within that hotspot, there are degree's of output, but generally, the closer to the enter, the higher the cd.

As you go farther outboard from the center of the hotspot, the cd drops...with a sharper drop typically observed at the hotspot to corona transition...producing a bright dot surreounded by a less bright donut of light around it.

Outside of the corona (the donut), there will be unfocused light that missed the reflector, and just spilled out to the edges (The "spill").

ALL of the light, including the spill, is part of the LED's total lumen output.


If you measure the lux at a given distance for a light, you can calculate the cd of the light, but NOT the lumens.

The cd will ONLY apply to the hottest part of the hotspot, and, not include any of the rest of the hotspot, or beam.


To get the lumens from a lux reading, again, you need a way to integrate the output into a homogeneous area, so a lux measurement can be representative.

If the AREA that you get the light to BE integrated on is, say a square meter, then, you COULD say that the lux you measure IS the lux per square meter (But that light better be evenly distributed across that square meter for this to work).

A device known as an integrating sphere (IS) is used to do all of the above...but they are hella expensive.

Some people make homemade versions, but, if you want to tell 950 from 1,000 lumens with it, you are wasting your time, its too small a difference to measure that way...you'd need a real IS to do that, etc.


For all practical purposes, I think you're probably just misled buy the thrower having a hotter hotspot, which your eye tells you "is brighter"...despite the lower lumens...as the weapon light's lumens are simply covering a wider area than the throwers, and, therefore, appear "dimmer" despite having MORE lumens, etc.

To verify the output you can do a simple ceiling bounce test, stand in the middle of a small to medium sized room, have both lights above your head aimed at the same spot on the ceiling.
Turn the first light on and look around the room[not the ceiling] and turn off, now turn on the second light [alternate between the two] and look around the room and repeat a few times.
At 1000 and 950 lumens the room should look very similar in brightness.

swan said:

To verify the output you can do a simple ceiling bounce test, stand in the middle of a small to medium sized room, have both lights above your head aimed at the same spot on the ceiling.
Turn the first light on and look around the room[not the ceiling] and turn off, now turn on the second light [alternate between the two] and look around the room and repeat a few times.
At 1000 and 950 lumens the room should look very similar in brightness.

Click to expand...

This test is notoriously thrown off if one beam is floodier though. Its best for two beams of the same characteristics.

IE: The throwier light bounces more light, proportionally, straight back down, than the floodier light does.

Essentially, the reason they use Integrating Spheres, and not Integrating Walls or Ceilings, is due to this problem.

Just to check relative brightness between the two, this is a good test the op can do with out shelling out for a lux meter. Sure it not 100% accurate test, but if you look around the room[not the ceiling] even though its a flooder or a thrower you will be able to tell if there is a difference between 2 lights with almost the same/similar output.
Personally i have my own lmd and lux meter set up and can check more accurately.

RyeBread said:

Thanks for the help so far. I can get my hands on a DMM at some point being an engineering student.
CD/d^2=Lux is exactly what I need. Provided that the CD listed on the packaging are calculated based on the listed lumens.
I will do CD1/d1^2 = lux = CD2/d2^2, and determine a pair of distances to test at, where lux should be the same for both lights. (they should look equally bright on the wall)
Having removed the reflectors to try to get a more apples-to-apples visual test, it still looks like there's a brightness discrepancy.
Though I have also noticed that the tints are indeed different.

Thanks for the help so far guys. Will let you know what happens.

Click to expand...

If he removes the reflectors, then it should remove the thrower vs flooder scenario.

Ok, would reflectors removed + identical tubes around LEDs be a better comparison? And then hold them an equal distance from the wall?

RyeBread said:

Ok, would reflectors removed + identical tubes around LEDs be a better comparison? And then hold them an equal distance from the wall?

Click to expand...

Yes. I think the other guys missed the detail that you planned to remove the reflectors to make things more equal.

As TEEJ (i believe) mentioned earlier, human eyes are a poor judge of brightness. Would a side-by-side comparison be enough to tell if one of these lights is not putting out as much light as it should be?

You thought that the difference was enough between one light and the other to complain to the manufacturer.

We explained that there is a difference in perception of brightness, and perhaps overall lumens because of beam shape, and different bin/tint.

We can't see your lights, and can't make that judgement call for you.

Can you see a 50 lumen difference if the light output is between 50 and 100 lumens? absoultely! BUT can you see a 50 lumen difference between 950 and 1000 lumen outputs? maybe, if they are side by side, then maybe.

Now... with that being said, often I will see that a manufacturer will list their lights with the CW tint, and then also offer it with a NW tint (and not make a correction for the 10% decrease in lumins or CD).
So.... if it is the 950 lumen light that seems low, and it has a NW tint, then it will appear less bright because, it may be putting out about 860 lumens due to the different tint. Now if you ask, "can I see the difference between a 860 lumen light with a neutral tint, and a 1000 lumen light with a U2-1A bin/tint" then I would say... yeah, most likely. If you put them side to side, or change between them quickly.

Does that make sense?


So, I'd say pull the reflectors, and make a comparison, then see if it is enough to complain about.

I once bought a high quality light, one that has three emitters in it, two were bright CW, and one was a dimmer, and neutral. Either they messed up at the factory and got their emitters mixed up, or one of them just wasn't putting out like it should. They swapped the light out for me, no problem, but I mention that because it is possible that you got a bad one in the batch!

Poppy said:

You thought that the difference was enough between one light and the other to complain to the manufacturer.

We explained that there is a difference in perception of brightness, and perhaps overall lumens because of beam shape, and different bin/tint.

We can't see your lights, and can't make that judgement call for you.

Can you see the difference between 50 and 100 lumens? absoultely! between 950 and 1000? maybe, if they are side by side, then maybe.
Now... with that being said, often I will see that a manufacturer will list their lights with the CW tint, and then also offer it with a NW tint (and not make a correction for the 10% decrease in lumins or CD).
So.... if it is the 950 lumen light that seems low, and it has a NW tint, then it will appear less bright because, it may be putting out about 860 lumens due to the different tint. Now if you ask, "can I see the difference between a 860 lumen light with a neutral tint, and a 1000 lumen light with a U2-1A bin/tint" then I would say... yeah, most likely. If you put them side to side, or change between them quickly.

Does that make sense?


So, I'd say pull the reflectors, and make a comparison, then see if it is enough to complain about.

I once bought a high quality light, one that has three emitters in it, two were bright CW, and one was a dimmer, and neutral. Either they messed up at the factory and got their emitters mixed up, or one of them just wasn't putting out like it should. They swapped the light out for me, no problem, but I mention that because it is possible that you got a bad one in the batch!

Click to expand...

Thanks!!!