[VAC] First LED measurements

[Dr. Gerald N. Johnson, electrical engineer]

The Brinkman LED flashlight produces 50 foot candles in a spot 1-3/4"
diameter at 1 foot. Power is 3 volts at 108 milliamps, 0.324 watt.

My Eveready 3 D-cell flashlight with alkaline cells and a PR-3 lamp (all
used) produces 200 foot candles in a spot 2" diameter at 1 foot. Power
was 4.5 volts at 415 milliamps, 1.87 watts. Four times the light for 6
times the power. Quartz lamps would produce more light.

The three LEDs from Hosfelt produced 7, 8.5 and 11.5 foot candles at 1
foot distance. Spread was soft edged about 2-1/2" diameter compared to
the first two illuminators measured above. Power was 3.28 volts at 30.5
milliamps, 0.1 watt.

The shop light fixture over my work bench did 65 foot candles over a
space 3' x 5' or larger at a 44.5" spacing consuming about 100 watts.

The Brinkman flashlight if defocused to cover a small sauce pan say 6"
diameter would produce about 5 foot candles. Workable but on the low
side. Generally about 30 foot candles is recommended for reading, while
5 is considered adequate for hallways, 1/2 foot candle for urban
streets. I once lit a main street to 1 foot candle and the power bill
was enormous...

The eye is very logarithmic, so that it take a 2:1 change in light
intensity to be noticeable, so by eye the Brinkman and Eveready
flashlights seem to produce nearly the same light level.

LED lifetime is somewhat inversely proportional to current. 20 or 30
milliamps per LED is considered good for 100,000 hours. In the lighting
situation, perhaps running 3 times the current could be considered a
safe trade off for loosing 90,000 of those lifetime hours. I'll abuse
the Hosfelt LEDs a bit more in nights to come.

I found the Lumiled company (www.lumileds.com) claims far greater light
productivity from white leds than most of the competition. By their
claims they get more light than anyone else. Diodes do vary
considerably. Colored LEDs for task illumination I'd not consider
useful. Definitely better than no light or a single wax candle, but not
useful for kitchen tasks and tiring for reading.

Gerald J.



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