As berkeman has already said you can use an ammeter. Basically, you need to connect the positive lead from your ammeter to the cathode of the photodiode and the earth or negative lead to the anode of the photodiode. This will allow you to measure the rate of flow of electrons, which is proportional to the illuminiation of the photodiode (and dependant on the wavelength of light). If you plot current against distance from source, you should be able to determine a relationship between the two variables. Hoot, this was from another thread. I know this sounds stupid, but could you draw the diagram.