Voltage comparator in a photo sensing circuit

[tachyon_man]

Hello, I attached a picture below but I am confused as to how the voltage comparator is comparing the two inputs (I know how a comparator works). The test points are running down to the two inputs but wouldn't the voltage always be identical across them since voltage doesn't drop in a parallel circuit but instead if it's 12V source, there would be 12V across each 'section' in a parallel circuit. Would someone please be able to explain to me how the voltages across those can be different from each other which would then explain how the comparator is working. Thank you so much.

 

[NascentOxygen]

I would presume that one branch gets ambient lighting, while the other side gets ambient plus some wanted signal? Photodiode's resistance varies with the intensity of incident light.

 

[tachyon_man]

At 2:30 am this morning I realized how silly I was sounding and figured out how the voltages vary. I know exactly how this works; I don't know where I went wrong before. :D