Using a LED like a photodiode to measure light
You can use an LED as a photodiode. They are not the same however, a photodiode produces a leakage current in the opposite direction from LED's. One problem is the currents are also small (as with photodiodes in the mA to uA range). 

To gain the current a transimpedance amplifier is necessary and change it to a voltage that can be read by your micro controller