The original discussion on this thread seemed to be with regard to CdS light dependent resistor photocells. These have a large change in resistance between light and dark, so they're pretty easy to work with. However, the "light resistance" varies with how long the cell has been exposed to light. The manufacturers give a "dark adapted" light resistance, and a light adapted light resistance. I think that getting a linear and repeatable output could be a challenge.
     In a dental application where I needed to measure mw/cm^2 of light, I used the TAOS TSL235R (http://www.taosinc.com/pdf/tsl235r.pdf) light to frequency converter. It is amazingly linear and easy to deal with on a PIC (just count the frequency and scale).

Harold


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