measures the relative intensities of different Infra red light sources.
http://www.mitedu.freeserve.co.uk/Circuits/Testgear/irtest.htm
http://www.mitedu.freeserve.co.uk/Circuits/Interface/irlink.htm
Using this low-cost project one can reproduce audio from TV without disturbing others. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from TV to headphones.
http://electronicsworld.tripod.com/audioimages/audiockt3.html
http://circuitos.tripod.cl/schem/r53.gif
wiring IR modules to PC serial port.
http://www.lirc.org/receivers.html
http://www.restena.lu/convict/Jeunes/beacon.htm
http://academic1.bellevue.edu/robots/ircircuit.html
use infrared technology to remotely control a CD-ROM drive with an audio CD.
http://www.ee.washington.edu/conselec/A95/projects/mdw/fproj.html