A differential light meter is a nifty little tool that will tell you how much brighter the surrounding light is on one side of the meter than the other. With a little imagination and a little engineering, you could connect a whole bunch of these together in different directions, and make a little box that will tell you how bright the light in the room is from all directions. You might call that box a 3D camera.
This Instructable is more of a demonstration of concept, and an introduction to how operational amplifiers work, but it's cool to think about how this can be generalized!
What you will need:
1) A breadboard that can source +15 V and -15V OR a regular breadboard with a +/- 15 volt power supply
2) Jumper wires
3) An oscilloscope + oscilloscope probe (If you do not know how to use an oscilloscope, there are plenty of good resources online!)
4) A multimeter (not strictly required, but useful.)
1) An Operational Amplifier (We used a TL082 - http://tinyurl.com/d49o3yy)
2) Two photodiodes
3) A capacitor and resistor such that 1/RC is about 2*pi* 1 kHz and R is large (so you can see the tiny little current from the photodiode.)
We used a 1.5 nF capacitor and a 100 kOhm resistor.
(where R is the resistance of the resistor, and C is the capacitance of the capacitor)
Be VERY careful to connect the photodiode in the right orientation!! If you connect it the wrong way it will very quickly burn out and possibly shoot out of the board. And then, http://www.youtube.com/watch?v=YleZvTSDC6s