Voltage drop across photodiode constant
I assume you're using the PD forward-biased. In that configuration, it behaves just like a plain old diode. You should reverse bias it, and look at the voltage across a resistor, like 







simulate this circuit &ndash; Schematic created using CircuitLab

Acceptable bias level depends on your PD. Resistor value depends on your PD size and the amount and wavelength of the light being observed. Bigger resistor gives better sensitivity, but slower response. If speed becomes an issue, step up to a transimpdance amplifier http://cds.linear.com/docs/en/design-note/dn399f.pdf