How to check photo diode? with Digital multimeter
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 
HOW TO TEST PHOTO DIODE WITH DIODE MODE?
Select DMM to Diode Mode How to use the Digital multimeter?
STEP-1. Connect DMM PROBE TO P-DIODE,
DMM Positive Red test lead to ANODE
Negative black lead to CATHODE
Negative black lead to CATHODE
DMM Display reading shows = 0.984v (984 mV)
STEP-2.
Connect DMM Negative Black test lead to ANODE
Positive Red lead to CATHODE
Positive Red lead to CATHODE
Verification:
If the DMM above reading shows the condition is GOOD.
PHOTO DIODE CIRCUIT
PHOTO DIODE WITH OP-AMP-AMPLIFIER
Verification:2.
If you get reading in forward bias as 0000 or OL or 1, and in reverse bias as 0000 (or) low values the PHOTO DIODE can be FAULTY and needs replacement.
