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:
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.
The Smell of Molten Projects in the Morning 
Every good cheater inserts a data anomaly to deter suspicion…
I use only the finest hand-picked natural anomalies: nothing artificial here! [wince]
The 4.4X vs 4.0 data could also be reflection. Might be entertaining to try a different color of LED.
We had to do a detector for the HP bar code wands. The product didn’t/couldn’t use a thick layer of goop to settle out the reflections, so there were lots of complaints of sensitivity issues due to detector oxide thickness variations. We never got the issues resolved before the product went obsolete. (We were in a position of trying to use different types of screwdrivers to get the optimum way of hammering a nail. There were more frustrating projects, but can’t think of them…) I have a lot of respect for those who do anti-reflection coatings.
Aye, there’s undoubtedly light rattling around between the filters. The stuff is intended to knock back stage lighting, so the fact that it comes out anywhere close to the nominal attenuation may be pure, raw coincidence. I also have some 1-stop filter gel, but I’m just not going to venture into that circle of dweebdom…