Proper Method of Measuring Light Output

OK, I've been taking and reporting various lights readings for sometime now. The numbers I usually report are not important as it relates to FC, LUX, Candlepower, Etc, but are used for comparision puposes of two or more lights. I have a variety of ways I obtain those numbers, by sometimes putting the light right on the sensor, or sometimes using a white plastic cylinder to lessen the output for lower numbers, etc.

I'm curious to hear how you folks take your measurements and more importantly the "standards" for measuring light output in FC, LUX, ETC. There has to be some written method, so when Joe Blow says he light puts out 500 lux, it actually means something.

I would usually do my LUX measurements in the bathroom because there's no light pollution (no windows, ect)... Pull my digicam on a tripod to take pictures, or use it to position my Meterman, have a meter stick, and space it correctly from camera/meter to end to take the LUX measurements or pics... And I would always make sure new to near new batteries are placed in the light to test out. If I don't know the condition of the batteries, I won't test it with them.

I usually never do FC readings... /ubbthreads/images/graemlins/frown.gif

To measure absolute levels, I measure lux with my LM631 at a distance of 1 meter. It has to be 1 meter for the lux scale to be accurate. Similarly, it has to be a range of 1 foot for the foot-candle scale to be correct. In so doing, a light that registers 600 lux will also register 600 foot-candles. ( A one-candle source will measure 1 foot-candle at one foot and will measure 1 lux at one meter.)

To measure relative levels, I don't care about distances -- or even geometry -- just as long as it all is constant over the duration of the test (for a run-time test) or constant for each light (if I'm comparing two or more). As for geometry, I measure either direct center-of-beam or reflected-from-ceiling. Each geometry measures a different aspect of the lights (the old lux vs. lumen difference).

Following FalconFX, I use fresh batteries to show what the light is capable of. Before the tests and after I measure the "light pollution." I call this the ambient light level. If it's less that 1 per cent of the levels from the light, that's low enought for me.

Ok, that's a good start guys, but more details are needed. Are you shining the light directly into the meter, by moving the light to get the highest reading or does your light stay stationary and you move the meter to get the highest reading. These questions may sound silly, but I tyring to get the specifics of the "standard" way to do this.

Nascar,

Reread my post above. I added to it while you were typing. It may provide part of the answer to your new question.

Paul

P.S. I do move the meter to get the highest reading.

I move the light around to get the highest reading... The Meterman has a feature called "Peak", which allows you to get the highest reading and keep that reading on the dial long after you remove the light, and that's what I use for my stats...

Yes, that certainly helps. Now I know that if someone states a certain LUX reading, it was acheieved by having the light 1-meter from the light sensor of the meter, while adjusting side-to-side to get the highest reading within that 1-meter distance.

Or by having the light 1-foot from the light sensor of the meter on the fc scale, while adjusting side-to-side to get the highest reading within that 1-foot distance.

And of course, for really bright lights, one could use the lux scale, measure from 2 meters and divide the result by 4 (2 squared).

Edit: No, that's wrong. As FalconFX points out below, multiply by 4!

You mean divide by 4, or multiplied by 4...
/ubbthreads/images/graemlins/confused.gif

I need some Red Bull...
/ubbthreads/images/graemlins/banghead.gif

FalconFX,

You caught me in a moment of dimness. Yes, multiply by 4. /ubbthreads/images/graemlins/smile.gif

Paul