For giggles I placed 2 crystal clear body T13/4 LED's in a 3/4" long 13/64 diameter tube. So far the red works the best, yellow and green were not too responsive. Drive side is TTL level via a 510 ohm ballast and the reciever is driving a selfbiased darlington amp with 4.7K load resistor and a 1.1M bias resistor. The reciever LED is connected anode to ground and cathode to base. Amplitude is ~4.25Vpp at the darlington collector. Frequency response ~4KHz before adverse degradation of symmetry.
Boy you must have been bored.
But really cool information 
Not bored perse', but rather researching practical sensors one can scrounge readily at Radio Trash.(punn intended). Another user was looking for solutions for his windvane, I'm developing an optical encoder route.
Scientist/Electronics tinkerer, Forrest Mims, played with stuff in the early 70's and although only a few practical uses were ever really developed, it is sort of neat. Other people have used LEDS as low sensitivity SUN UP/SUN DOWN sensors.