Photodiode sensitivity and DMM diode test

Question

I have questions about the operation of a photodiode and the relationship between sensitivity and the diode voltage I mesaure with my DMM.
One of my products uses a photodiode to detect the magnitude of incoming light.  Recently there has been a dramatic step-change increase in the uncalibrated/default value of this measurement.  The technicians trying to salvage these units noticed that they could use their DMM's diode test to discriminate between "good" and "bad" photodiodes.  Every unit that failed calibration had a diode measuring 0.6V in this test; old production units contained photodiodes measuring 0.5V.  Replacing the 0.6V photodiode with one measuring 0.5V makes everything all right.
Is the DMM diode test result related to the photodiode's forward voltage?
Is the sensitivity of a photodiode related to the parameter measured by this test?
What do I even ask the manufacturer if I want what I used to get?
Thank you,
Jim
You're entirely bonkers. But I'll tell you a secret. All the best people are. ~ Alice

Answer 1

Jim,
I think you are on the right track in trying to ask the right questions. Unfortunately it may not be easy to determine what those questions are.
What is the wavelength or spectrum of your light source? What is the intensity at the photodiode? Do you have stray light or other noise issues? Is the illumination uniform over an area much larger than the photodiode or are you measuring a small spot? How fast are things changing? Has anything else changed in the product or the test setup? 
In response to one of your original questions: Every handheld DMM I have used applies a fixed current (often 100 uA or 1 mA) with a compliance voltage ranging from 1 to 4 V and then measures the voltage across the diode.
Any junction diode, including LEDs, is a photodiode if light can get to the junction.  The forward voltage of a diode depends on the materials including the doping levels, the size, and any internal resistance including lead and connection resistances. The sensitivity depends on those things as well as the optical configuration, exposed area, and probably other things. I doubt there is any simple correlation between forward voltage drop and sensitivity. You probably got lucky on this one.
How closely does the performance in your product correlate with any of the specifications on the photodiode datasheets?
Lynn