LED + Photodiode: Verifying Linearity

Given that test fixture, the obvious question is whether the PIN-10AP photodiode’s output current varies linearly with light intensity, just like the specs would lead you to believe. I excavated the sheet of 2-stop neutral density filter gel from the Parts Warehouse Wing and cut some 30 mm disks:

LED Photodiode test fixture - ND filter disks
LED Photodiode test fixture – ND filter disks

A single filter layer should reduce the light intensity by 2 f/stops = a factor of 4. Each successive layer reduces the intensity by another factor of 4. They’re all at least reasonably clean and free of defects, but they’re definitely not optical lens quality.

Running the LED with a 100 mA pulse at 20% duty cycle and stacking the disks in the fixture, one by one, between the LED and photodiode, produces this data:

Layers Attenuation Scale V I – uA Ratio
0 1 1.0000 8.7 87  
1 4 0.2500 1.9 19 4.58
2 16 0.0625 0.43 4.3 4.42
3 64 0.0156 0.097 0.97 4.43
4 256 0.0039 0.022 0.22 4.41

The Ratio column divides successive pairs of current values. The first step, from “no filter” to “one filter”, came out a bit larger than the rest, probably because the gel sheet isn’t anti-reflective and some light bounces off the top.

After that, though, it looks just like I’m cheating, doesn’t it?

The ratios should be 4.0, but the actual 4.4 means it’s a 2.1 stop filter. Close enough, methinks.