A photo diode is a p-n junction or PIN structure. When a photron of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron (and a positively charged electron hole). This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction’s depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photo current is produced. This photo current is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device.
PHOTO-DIODE 
Negative black lead to CATHODE
Positive Red lead to CATHODE
Verification: If the DMM above reading shows the condition is GOOD.
