So I did some more research into photodiodes, and found some REALLY tiny ones, only 2mm long and 1mm wide. And they have a funny wavelength sensory curve that would be great for this project. TEMT6200X01
Considering a 17" monitor (what I'm working with), to get the number of diodes we would need we can use Pythagoreans theorem to help us out. If my math is wrong here help me out.
A² + B² = C²
C = 17" or roughly 432mm
C² = 432mm² = 186624mm
The monitor is not wide-screen so the aspect ratio is 4:3.
Thus, roughly:
186624 / 7 = 26660
A² = 26660 x 4 = 106640
B² = 26660 x 3 = 79980
Making:
A = [ch8730]106640 = 326
B = [ch8730]79980 = 283
If my math is right, the monitor is about 326mm wide by 282mm tall.
If we packed the photodiodes as close as possible but keeping a square foot print (2mm²) this would mean that we could make a 163 x 141 diode array. Unfortunately, this would give us a poor resolution, but the whole array would only be about 2-3mm thick. However, the 23,000 (163 x 141) photodiodes would cost about $8,300... which is about 55 times more than what the monitor cost when it was new!
So this is turning out to be a completely not cost effective porject :P. However, we get wonderfully thin hardware with a relatively decent resolution (considering you could fit about an 8 x 8 array of those diodes on your finger tip).
I guess I'm going back to the drawing board. Going to take a more detailed look at optical mouse sensors and possibly cellphone cameras.