Most diodes are light sensitive, leds are sensitive near but not exactly at their emmitted color. Back in the dark ages of LEDs (late 60s) I worked on a voice over light beam system that used a yellow silicon carbide LED as both the transmitter and detector. Worked pretty well as a "push-to-talk" link for a hundred meters or so.
In the mid eighties I designed a frequency synthisizer for a satallite receiver. My boss, fun loving guy that he was, asked an engineer who didn't like me at all to certify it to FCC specs. After a couple of weeks screwing with it he thought he finally had me, there was 6dB excess 120 Hz noise on the signal that wasn't there when I passed it on to him. I went over check it out and took a brief look at his setup and told him he had a test setup problem and I'd be back after lunch to fix it. Went to lunch with my boss and some other Veeps and stopped in a drug store for some deep red nail polish. On the way back to our offices the three Veeps and I went by the test setup (and one fuming engineer). A drop of the "Magic Red Potion" on the frequency control varactor and the noise dropped 20dB. He had removed the cover from the box and the green emmission line from the flouresent lights was being picked up by the diode.
Geezer Engineer 1, Young Punk 0...
