Nice project. You'll need some sort of light measuring device. There are many varieties. A photographic light meter will work but may cost you $100 or so. A cheap and straightforward way is to go down to Radio Shack and buy a CdS (Cadmium Sulfide) photoresistor pack. A 5-pack of them ,stock number 276-1657, costs about 3 bucks.
You'll also need a multimeter to measure the resistance of the device before and after the light is shone on it. If you don't have one you should buy one. It's a must for every young experimenter. Radio Shack has several models in the $20 region.
Take the largest one of the lot and use the ohm-meter to measure the resistance both pointed toward the light and then covered up. You should get about a factor of 10 or so difference. The reciprocal of the resistance (after you subtract the reciprocal of the dark resistance) is proportional to the light intensity. In other words I= k((1/R)-(1/RD
where I is the light intensity, R is the resistance, and RD
is the resistance in the dark. If you vary the distance from the light source to the photoresistor the intensity should fall off as the square of the distance.
(published on 05/16/2013)