Leef
Enlightened
I've gotten my datalogging light meter working pretty well. I'm using a cardboard box with a white interior, the input hole and the sensor hole both located in the top so that there's no direct beam hitting the sensor. I've thought of other ways to collect all the light emerging from a flashlight, and when I decide which is best, I'll replace the cardboard box with something a bit nicer.
Meanwhile, as I start making measurements, I have lots of questions that many Forum members have figured out long ago. Can you help me?
Here are some questions:
1) As to maximum output of a LED light (my SuperFlashlight-III), I note that it drops quickly from initial turn on (with primary batteries, anyway). It looks like it drops for awhile, then rises slightly. Is there a typical (standard) way to assign the maximum output (e.g. at first turn on, at the short term rise point, or averaging some initial period)?
2) As I see it, the challenge to measuring total output is to capture all the photons without biasing the measurement in favor of location -- that is, not letting the hot spot disproportionately affect the measurement. That's why the box method is used -- all the photons are reflected and diffused. So:
a) Has anyone tried using a parabolic reflector or a Fresnel lens to capture all the output from a light for measurement? I don't know if Fresnel lenses shine equally at the focal point all the light striking them, or if they are biased in favor of the light that hits the center of the Fresnel. It seems like it may not matter since all the light -- hot spot & spill -- end up concentrated in one point. Anyone know? If they do collect without bias, they'd be a great measurement technique since the beam could be focused on the sensor across the sensor's entire area.
b) If I understand parabolic reflectors correctly, light hitting the reflector at any point and from any angle is focused at the focal point. Is this right? If so, one could use a relatively small sensor cell that wouldn't obscure much of the flashlight's beam, and get a good, unbiased measurement. Anyone know?
3) Another measurement that seems common is a maximum output in the center of the beam at 1 meter. I suppose this is a better indicator of throw than total output, right? Is this lumens? Can this measurement be made inside a tube, or does it need to be made in a large space (e.g. a room)? If the tube idea doesn't affect the measurement, one could use a cardboard mailing tube, maybe 12" diameter, and could make these measurements in a lighted room. But here's another thought -- maybe it's just as accurate to make the measurement in the open in a lighted room and simply subtract the background level from th emeasured level. Is this just as good?
4) When taking long term (e.g. a few hours) measurements of a light, is it necessary to cool the light? I gather that a LED's output for a given power input is inversely affected by temperature, right? A small ducted fan could be used, but this raises the question of "how cool is cool?" Is the temperature's effect on output sufficiently great to introduce a large error in measurement?
5) I'm curious about the relationship between measured output and what one sees. For example, the momentary output of my FS M6 is 8,700 lux, while the output of my quad-Lux (in a Mag C head running on 2x18650) is 9,000 lux. But when I go outside at night and play around with these two lights, they appear to be very different. Obvious differences are color and throw/spill. The M6 leaves the QL in the dirt as to throw, but the QL lights up a larger area up close, say 10 yds. And the difference in color makes it tricky to say which one is "brighter." Can anyone...uh...shed some light on this? BTW, my meter's spectral sensitivity peaks at 550 nm and is a fairly symmetric bell curve around the peak, dropping to near zero at 420 and 680 nm. I don't know how this corresponds to what normal eyes and what my eyes see, but I'd guess that the meter designers tried to emulate what the eye sees.
6) Is there a reasonable way to calibrate a light box to some standard?
Anyway, there's way too much fun to be had with this stuff. Thanx for any help.
Meanwhile, as I start making measurements, I have lots of questions that many Forum members have figured out long ago. Can you help me?
Here are some questions:
1) As to maximum output of a LED light (my SuperFlashlight-III), I note that it drops quickly from initial turn on (with primary batteries, anyway). It looks like it drops for awhile, then rises slightly. Is there a typical (standard) way to assign the maximum output (e.g. at first turn on, at the short term rise point, or averaging some initial period)?
2) As I see it, the challenge to measuring total output is to capture all the photons without biasing the measurement in favor of location -- that is, not letting the hot spot disproportionately affect the measurement. That's why the box method is used -- all the photons are reflected and diffused. So:
a) Has anyone tried using a parabolic reflector or a Fresnel lens to capture all the output from a light for measurement? I don't know if Fresnel lenses shine equally at the focal point all the light striking them, or if they are biased in favor of the light that hits the center of the Fresnel. It seems like it may not matter since all the light -- hot spot & spill -- end up concentrated in one point. Anyone know? If they do collect without bias, they'd be a great measurement technique since the beam could be focused on the sensor across the sensor's entire area.
b) If I understand parabolic reflectors correctly, light hitting the reflector at any point and from any angle is focused at the focal point. Is this right? If so, one could use a relatively small sensor cell that wouldn't obscure much of the flashlight's beam, and get a good, unbiased measurement. Anyone know?
3) Another measurement that seems common is a maximum output in the center of the beam at 1 meter. I suppose this is a better indicator of throw than total output, right? Is this lumens? Can this measurement be made inside a tube, or does it need to be made in a large space (e.g. a room)? If the tube idea doesn't affect the measurement, one could use a cardboard mailing tube, maybe 12" diameter, and could make these measurements in a lighted room. But here's another thought -- maybe it's just as accurate to make the measurement in the open in a lighted room and simply subtract the background level from th emeasured level. Is this just as good?
4) When taking long term (e.g. a few hours) measurements of a light, is it necessary to cool the light? I gather that a LED's output for a given power input is inversely affected by temperature, right? A small ducted fan could be used, but this raises the question of "how cool is cool?" Is the temperature's effect on output sufficiently great to introduce a large error in measurement?
5) I'm curious about the relationship between measured output and what one sees. For example, the momentary output of my FS M6 is 8,700 lux, while the output of my quad-Lux (in a Mag C head running on 2x18650) is 9,000 lux. But when I go outside at night and play around with these two lights, they appear to be very different. Obvious differences are color and throw/spill. The M6 leaves the QL in the dirt as to throw, but the QL lights up a larger area up close, say 10 yds. And the difference in color makes it tricky to say which one is "brighter." Can anyone...uh...shed some light on this? BTW, my meter's spectral sensitivity peaks at 550 nm and is a fairly symmetric bell curve around the peak, dropping to near zero at 420 and 680 nm. I don't know how this corresponds to what normal eyes and what my eyes see, but I'd guess that the meter designers tried to emulate what the eye sees.
6) Is there a reasonable way to calibrate a light box to some standard?
Anyway, there's way too much fun to be had with this stuff. Thanx for any help.
