I worked in the industry doing research. Photocells operate in either the photovoltaic mode or photoconductive mode.
When you hold the cell at 0V, which is known as Jsc or Isc, the current output is proportional to intensity. For a solar cell, the negative bias line should be flat for a good cell. Another datapoint is Voc or voltage open circuit. This is when I=0.
When people want to generate power, there is an operating point call Jmp or Imp. This is where the power is maximum.
J is current density e.g. mA/sqcm and I is current of say 10mA. The sun at AM1.5 Global spectrum is 100 mW/sqcm
Solar cells have a spectral response. The AM1.5 global spectrum. Each Solar cell material has it's own spectrum. It's modified slightly by adding an AR (anti-reflection coating).
Your sensor is two split solar cells. When the beam is aligned between the cells, each is producing a small, but identical current.