Photodiode Circuit for measuring light intensity
Photodiodes need to be treated in a special way !

1) use the diode in zero-bias or reverse mode (not in forward mode)

2) when dark, no current flows through the photodiode

3) a circuit is needed to "catch" the photocurrent and amplify it.

Here's an example of such a circuit:



This circuit is called a transimpedance amplifier.
Through feedback the opamp keeps the voltage across the diode zero so we're in zero-bias mode. Resistor RF converts the very small photocurrent 
to a voltage which you can measure at the opamp's output.