Lowest noise, low frequency, cheap photodetector configuration?
simulate this circuit &ndash; Schematic created using CircuitLab
Photodiodes back-to-back can be used in a transimpedance amplifier, where the op-amp maintains zero volts at diode junction. Output voltage will go positive for more illumination at D2, and negative for more illumination at D1. R1 can be increased for more gain. R1 may require a parallel capacitor of very small value to remain stable at high frequency.