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Last Updated on: Monday, August 17, 2015 04:25 AM

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More  Light Sensors

Light Intensity Monitor
designed by David Johnson, P.E.

For an on-going consulting project, I had to determine the light output decay time of different fluorescent lamps. I wanted to know how fast the light from a lamp dropped to a low level, after current was interrupted in the lamp. I used a high voltage transistor circuit to drive the lamps under test and used the hobby circuit below to measure the decay time.
The hobby circuit was a simple PIN photo diode connected to a transimpedance Amp. The electronic circuit has an upper frequency cutoff of around 500KHz. The output signal was fed to an oscilloscope through a coax cable.

It turns out that typical cool white, warm white and kitchen lamps decay on the order of 30 milliseconds, while aquarium and full sunlight spectrum lamps are quite fast, down to about 40 microseconds or so. A black lamp, with no phosphor at all, decayed to 10% the peak value in about 20 microseconds. Perhaps at some future date I will experiment with using some of these powerful light sources to send data over a wide area, by encoding data in the modulating signal to the lamp.


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