Hi Alex,
The diode is reverse-biased, and the reverse bias voltage (diode drop) is a function of the illumination that the diode is receiving. In other words the diode is in a photovoltaic topology. OpAmp amplifies the diode voltage to match the dynamic rage of the A/D converter. Here’s an app note about photodiode monitoring, if you want to read further **broken link removed**
By the way, I’d like to invite your question to a specialized group dedicated to sensors **broken link removed** . The group is fairly active and there is a good number of sensor experts.
- Nick