The following video clip shows students using the overhead projector, 3 inch plexiglas squares, and a CBL with Light Intensity probe. While the best light source to use is a window with either indirect or direct sunlight, an overhead or flashlight can be used.
The initial light intensity may overpower the light probe. Thus a few squares may be needed before accurate readings can be obtained.
Tinted plexiglas can be found at most hardware / home improvements stores. Frequently this plexiglas absorbs as much as 70% of the light which limits the number of readings students can collect. A plexiglas with an absorption factor between 20-40% produces the best set of data. This kind of plexiglas is available through plexiglas distributors.
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