# Thread: Why are Brightness and Lumens synonymous in the flashlight world?

1. ## Why are Brightness and Lumens synonymous in the flashlight world?

Brightness (Lumens) is the same as radiance (watts). The conservation of Radiance (Brightness) is essentially conservation of energy for light. So when someone states that a flashlight is bright or brighter than another flashlight, are they using the word "brightness" correctly? Meaning for typical flashlights, the brightness is set by the source radiance and is conserved throughout the system. Generally speaking, the brightness of the source is set by the luminous density or exitance of the source because all LEDs emit roughly the same distribution. So an XP-G2 is actually a littler "brighter" than an XM-L2 even though the XM-L can consume twice the wattage. Basic equations B = L/(n^2*A*O) where n is the index of refraction (assume 1 for air), A is the source area, and O is the solid angle (assume Pi() for most LEDs). That leaves L/A where L is the lumens or watts and A is the source area. The XP-G2 has a dome radius of 1.53mm and emits 515Lumens. So that's roughly a luminous density of 70lumens/mm^2. The XM-L2 has a dome radius of 2.26 and emits 1052Lumens., therefore the luminous density is 65.6lumens/mm^2. All lumen numbers are what's posted on Cree's website. So the XP-G2 is essentially brighter than the XM-L2.

So why do most of the posting state the XM-L2 flashlights are brighter. They may have more lumens, but they are definitely not "brighter" per definition.

There are brighter LED sources out there. And why does brightness matters for flashlights?

2. ## Re: Why are Brightness and Lumens synonymous in the flashlight world?

Check out Candela.

3. ## Re: Why are Brightness and Lumens synonymous in the flashlight world?

Maybe you are confusing lumens and lux? Lumens being total measured light output vs. lux being lumens per unit area? I guess it would depend on what you define "brighter" as: actual output or intensity on a spot. Light bulb = high lumens, low lux. Laser = low lumens, high lux. Which is brighter? How do you define brightness? I consider lumens and brightness to correlate directly.

4. ## Re: Why are Brightness and Lumens synonymous in the flashlight world?

I agree with nbp. The fact that your "simple" description involves equations with multiple variables means that it won't work for the average person. Put more simply, XP-G2 could be said to be more intense...but XM-L2 is still brighter (puts out more total light). Not everyone will agree on the definition, I'm sure, but over-complicating basic concepts won't help anyone.

Originally Posted by ACOLJNSD
There are brighter LED sources out there. And why does brightness matters for flashlights?
Obviously the total output of a light matters. You can either focus it, and get a more intense hotspot, or you can diffuse it, and get more lux over the same area, or some combination.

I think you might find this thread helpful:

http://www.candlepowerforums.com/vb/...ux-Explanation

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