LED brightness control circuit and a multimeter to compare light levels to LED level
http://avery.home.mindspring.com/LPmeter.htm
allows you to measure light levels using a regular digital voltmeter
http://www.edn.com/archives/1996/081596/17di3.htm
detects 295-nm UV radiation, which is dangerous to eyes and skin
http://www.edn.com/archives/1994/120894/25di1.htm
http://circuitos.tripod.cl/schem/r22.gif
http://www.psychtoolbox.org/measure.html
http://circuitos.tripod.cl/schem/r38.gif