Extech Light Meter Adapter

Here is my new Runtime plot and now Footcandle set-up:



From L>R...the light sensor one foot from the front of the Nano-Mate...the Light Adapter....DMM with RS-232 ....to the computer. With this setup, each mVDC equals one Footcandle. With the sensor to light distnce at one foot, then 1 mVDC = 1Fc = 1Fc at 1Ft equivalent to 1 Lux at 1M.

The sensor is mounted with Velcro so its position can be changed to center it in the beam.

The Extech Light Adapter comes in two models, caliibrated in Fc or calibrated in Lux. Cost was $60 at the time of purchase.

I just got an idea for a crude but zero-cost method of finding runtime to half brightness. I found an automatic nightlight I wasn't using and plugged it into the wall socket in the bathroom. I can see it from here. It's on. If I shine a flashlight on it, it goes off. My dimmer single-led flashlights can't turn it off at this distance; I have to move closer. So ... suppose I find the distance at which it turns it on and off for the flashlight I want to test, using the spill light rather than the hotspot for the brighter ones. Then I place the flashlight at the square root of that distance, turn it on, and start my stopwatch. When the nightlight comes on the flashlight will be at approximately half brightness. If I notice it ... should be better than my eyeball gauge, at the least. I'll look at your runtime charts, Roy, and see if you've tested a flashlight I have and see how closely I can duplicate your results....

-Cougar :{)

Correction to my late-night math: if distance with fresh batteries is D the formula is square root of (1/2 D squared).

-Cougar :{)

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Cougar_Allen said:
I just got an idea for a crude but zero-cost method of finding runtime to half brightness. I found an automatic nightlight I wasn't using and plugged it into the wall socket in the bathroom. I can see it from here. It's on. If I shine a flashlight on it, it goes off. My dimmer single-led flashlights can't turn it off at this distance; I have to move closer. So ... suppose I find the distance at which it turns it on and off for the flashlight I want to test, using the spill light rather than the hotspot for the brighter ones. Then I place the flashlight at the square root of that distance, turn it on, and start my stopwatch. When the nightlight comes on the flashlight will be at approximately half brightness. If I notice it ... should be better than my eyeball gauge, at the least. I'll look at your runtime charts, Roy, and see if you've tested a flashlight I have and see how closely I can duplicate your results....

-Cougar :{)


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Cougar,

After fiddling a bit with the problem, I found that this isn't a reliable way to do it as alignment on the new (shorter) position can't be controlled. Wide variations are possible with small changes in alignment.

The solution I came up to needs a ohm meter (which most of us seem to have) but provides 'run time to half light out' times to a percent or two:

Poor Man's Run Time Meter

Doug Owen

Whoops.

Does the "Inverse Square Law" (light decreases with the square of the distance) work for beamed Light? I thought that the Inverse Square Law only worked with a spherical geometry.

Roy,

You have a nice setup there.

The inverse square law works as long as the distance between meter and light is very large compared with the size of the illumination source. For example, it's valid at one meter for a light with a 3 inch bezel, but not valid for a 2-foot diameter searchlight at the same distance.

Paul

BTW, the intensity will be cut in half when the distance is increased by 41.4%. Square root of 2 equals 1.414

Good point about alignment -- now that I think of it I can see I'll never duplicate the alignment closely enough for meaningful results -- not without building an apparatus that will be more expensive than Roy's and defeat the whole purpose.

However narrow a flashlight beam may be it's not a line; it's a cone, and the cross-section is not a point; it's an area. Let's assume it's a perfect circle (doesn't matter what shape it is). The radius of that circle is directly proportional to the distance. The area is proportional to the square of the radius. It's the area of the beam that determines the brightness; we're spreading the same light energy over more or less area. Thus brightness is proportional to the square of the distance.

Call the distance with fresh batteries D. We want to find a distance X such that X squared equals 1/2 D squared, thus X = square root of the quantity (1/2 D squared).

-Cougar :{)

All this math is hurting my head! /ubbthreads/images/graemlins/grinser2.gif

[ QUOTE ]
Cougar_Allen said:
Call the distance with fresh batteries D. We want to find a distance X such that X squared equals 1/2 D squared, thus X = square root of the quantity (1/2 D squared).

-Cougar :{)


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Right. X=(0.707)(D)

I haven't been doing enough algebra lately -- when I don't do it often enough I forget how. For some reason I was thinking my equation couldn't be simplified, but of course it can -- all you need to remember is Doug's version, simply .707 * D, and if you want to measure to some other fraction of the original brightness just take the square root of the fraction and multiply it by D.

-Cougar :{)