Light Output Measurements: Difference between revisions
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Lumens are a measure of the total light emitted by a LED or flashlight (see [http://en.wikipedia.org/wiki/Lumen_%28unit%29 Wikipedia article] for more information on lumens). Lumens are sometimes measured "at the emitter" or "out the front." Emitter lumens will always be higher than out the front lumens because there are some losses involved as the light bounces off the reflector and is filtered somewhat by the flashlight lens. | Lumens are a measure of the total light emitted by a LED or flashlight (see [http://en.wikipedia.org/wiki/Lumen_%28unit%29 Wikipedia article] for more information on lumens). Lumens are sometimes measured "at the emitter" or "out the front." Emitter lumens will always be higher than out the front lumens because there are some losses involved as the light bounces off the reflector and is filtered somewhat by the flashlight lens. | ||
'''Emitter lumens''' LED manufacturers publish specifications listing the lumen output of their LED's and sometimes flashlight manufacturers will just take the highest value for the LED in their flashlight and say that is the output. But actual output depends on the voltage and current delivered to the LED. Even then, these numbers are always going to be higher than the actual light output by the flashlight. | '''Emitter lumens''' LED manufacturers publish specifications listing the lumen output of their LED's (see [[Brightness Bins]]) and sometimes flashlight manufacturers will just take the highest value for the LED in their flashlight and say that is the output. But actual output depends on the voltage and current delivered to the LED. Even then, these numbers are always going to be higher than the actual light output by the flashlight. | ||
'''Out the front lumens''' To measure the actual light output of a flashlight, an integrating sphere (see [http://en.wikipedia.org/wiki/Integrating_sphere Wikipedia article]) is needed that captures all of the light and distributes the light equally so that it can be measured accurately regardless of whether the flashlight has a small hotspot or a wide flood. CPF users MrGman and bigchelis have calibrated integrating spheres, test flashlights that people send to them, and publish their results in posts that are stickied in the [http://www.candlepowerforums.com/vb/forumdisplay.php?f=45 LED Flashlights] forum. This way different flashlights can be compared using consistent test procedures. | '''Out the front lumens''' To measure the actual light output of a flashlight, an integrating sphere (see [http://en.wikipedia.org/wiki/Integrating_sphere Wikipedia article]) is needed that captures all of the light and distributes the light equally so that it can be measured accurately regardless of whether the flashlight has a small hotspot or a wide flood. [[Terminology#CPF|CPF]] users MrGman and bigchelis have calibrated integrating spheres, test flashlights that people send to them, and publish their results in posts that are stickied in the [http://www.candlepowerforums.com/vb/forumdisplay.php?f=45 LED Flashlights] forum. This way different flashlights can be compared using consistent test procedures. | ||
However, a flashlight rarely has consistent output. Instead the output usually drops off early as the LED heats up. So the initial output can be fairly high and then drop by 10-20% within a few minutes. Depending on how well regulated the light is, the output can continue to vary with battery voltage. | However, a flashlight rarely has consistent output. Instead the output usually drops off early as the LED heats up. So the initial output can be fairly high and then drop by 10-20% within a few minutes. Depending on how well regulated the light is, the output can continue to vary with battery voltage. | ||
'''FL-1 lumens''' The [[ANSI-NEMA FL-1|ANSI/NEMA FL-1]] standard requires an integrating sphere and uses a reading taken after the light has been on full power for 3 minutes. This number is even lower than "out the front" lumens. | '''FL-1 lumens''' or '''ANSI lumens''' The [[ANSI-NEMA FL-1|ANSI/NEMA FL-1]] standard requires an integrating sphere and uses a reading taken after the light has been on full power for 3 minutes. This number is even lower than "out the front" lumens. | ||
As an example of how lumens can vary, take the [[4Sevens]] Quark MiNi 123. This flashlight features a Cree XP-G LED with an R5 brightness bin that gives 139 lumens at 350mA. The MiNi probably drives the LED at closer to 1000mA, so the [http://www.cree.com/products/pdf/XLampXP-G.pdf XP-G specs] indicate 250% of the nominal luminous flux or 347.5 lumens at the emitter. However, 4Sevens advertises 189 out-the-front lumens. When [http://www.candlepowerforums.com/vb/showpost.php?p=3296637&postcount=121 bigchelis tested the MiNi 123] with a CR123A primary cell, he got a reading of 142 lumens when the light was turned on and 137 lumens at 3 minutes (with a higher voltage, faster draining IMR cell, MrGman measured 200 lumens at turn-on and 190 lumens at 2 minutes). Later on, 4Sevens introduced a version of this light with a S2 bin XP-G LED which should be 7% brighter. However for this light they used FL-1 testing and advertise only 135 lumens. | As an example of how lumens can vary, take the [[4Sevens]] Quark MiNi 123. This flashlight features a Cree XP-G LED with an R5 brightness bin that gives 139 lumens at 350mA. The MiNi probably drives the LED at closer to 1000mA, so the [http://www.cree.com/products/pdf/XLampXP-G.pdf XP-G specs] indicate 250% of the nominal luminous flux or 347.5 lumens at the emitter. However, 4Sevens advertises 189 out-the-front lumens. When [http://www.candlepowerforums.com/vb/showpost.php?p=3296637&postcount=121 bigchelis tested the MiNi 123] with a CR123A primary cell, he got a reading of 142 lumens when the light was turned on and 137 lumens at 3 minutes (with a higher voltage, faster draining IMR cell, MrGman measured 200 lumens at turn-on and 190 lumens at 2 minutes). Later on, 4Sevens introduced a version of this light with a S2 bin XP-G LED which should be 7% brighter. However for this light they used FL-1 testing and advertise only 135 lumens. | ||