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Difference between revisions of "Light Output Measurements"
Light Output Measurements (view source)
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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). For scale, a 60-watt incandescent lightbulb produces 890 lumens, a 40-watt bulb is 460 lumens, and a pretty decent flashlight produces about 200 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). For scale, a 60-watt incandescent lightbulb produces 890 lumens, a 40-watt bulb is 460 lumens, and a pretty decent flashlight produces about 200 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 | '''Emitter lumens''' LED manufacturers publish specifications listing the lumen output of their LEDs (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. [[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 CPF 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 CPF LED Flashlights] forum. This way different flashlights can be compared using consistent test procedures. | ||
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==Lux== | ==Lux== | ||
While lumens measure the total output of a light, lux measures the amount of light that strikes a point on a wall. 1 lux is equal to 1 lumen per square meter. This can be measured with a light meter. The problem is that the value will go down as the light is moved further from the wall. Also the value will vary depending on whether the reading is taken in the hotspot or spill of the flashlight beam. Still, it is an easy measurement to make and many flashlight reviewers will use it. It can also be used pretty accurately to get a profile of how the brightness of a light varies with time. Reviewers can take an initial reading, call that 100%, and then chart how the light intensity varies until the battery is drained. See [http://en.wikipedia.org/wiki/Lux Wikipedia article] for more information on lux. 0.25 lux is said to be roughly equal to the light of a full moon. On Mythbusters, they determined you needed about 4 lux to navigate a room if your eyes aren't adjusted to the darkness. They also said that 300 lux is needed to read by. | While lumens measure the total output of a light, lux measures the amount of light that strikes a point on a wall. 1 lux is equal to 1 lumen per square meter. This can be measured with a light meter. The problem is that the value will go down as the light is moved further from the wall. Also the value will vary depending on whether the reading is taken in the hotspot or spill of the flashlight beam. Still, it is an easy measurement to make and many flashlight reviewers will use it. It can also be used pretty accurately to get a profile of how the brightness of a light varies with time. Reviewers can take an initial reading, call that 100%, and then chart how the light intensity varies until the battery is drained. See [http://en.wikipedia.org/wiki/Lux Wikipedia article] for more information on lux. 0.25 lux is said to be roughly equal to the light of a full moon. On [http://en.wikipedia.org/wiki/MythBusters_%282011_season%29#Episode_169_.E2.80.93_Let_There_Be_Light Mythbusters], they determined you needed about 4 lux to navigate a room if your eyes aren't adjusted to the darkness. They also said that 300 lux is needed to read by. | ||
==Candela== | ==Candela== | ||
Candela is another SI unit, but is calculated in such a way so that it does not vary with distance. This is done by simply taking the value in lux and multiplying it by the square of the distance to the emitter (in meters). Taken at a distance of 1 meter, a reading of 1 lux indicates 1 candela. At 10 meters, a measurement of 1 lux would indicated 1 x 10² = 100 candela. Like a lux reading, it will vary depending on whether the reading is taken in the hotspot or spill of the light, so the measurement is usually taken at the brightest spot. Therefore the candela value is really a measurement of how far a flashlight will throw a beam. One candela represents the light intensity of a single candle and is roughly equal to the now obsolete non-SI unit "candlepower." See [http://en.wikipedia.org/wiki/Candela Wikipedia article] for more information on candela. | Candela (cd) is another SI unit, but is calculated in such a way so that it does not vary with distance. This is done by simply taking the value in lux and multiplying it by the square of the distance to the emitter (in meters). Taken at a distance of 1 meter, a reading of 1 lux indicates 1 candela. At 10 meters, a measurement of 1 lux would indicated 1 x 10² = 100 candela. Like a lux reading, it will vary depending on whether the reading is taken in the hotspot or spill of the light, so the measurement is usually taken at the brightest spot. Therefore the candela value is really a measurement of how far a flashlight will throw a beam. One candela represents the light intensity of a single candle and is roughly equal to the now obsolete non-SI unit "candlepower." See [http://en.wikipedia.org/wiki/Candela Wikipedia article] for more information on candela. | ||
==Throw== | ==Throw== | ||
An actual distance for throw can be directly calculated from the candela value and will be given in meters. The FL-1 standard does just this by providing a light intensity reading in candela and converting this to throw in meters by calculated the distance at which the light will generate 0.25 lux. Throw is just the square root of the light intensity in candela divided by 0.25 lux which gives meters. | An actual distance for throw can be directly calculated from the candela value and will be given in meters. The FL-1 standard does just this by providing a light intensity reading in candela and converting this to throw in meters by calculated the distance at which the light will generate 0.25 lux. Throw is just the square root of the light intensity in candela divided by 0.25 lux which gives meters. | ||
[[File:Eqn-throw.png|center|400px]] | |||