I think you’d usually want to sum at least a cv and and offset. One more feature is that U2C acts as an inverting precision rectifier, cutting of any negative part of the cv, preventing any current source down the line from driving into saturation trying to force a reverse current through the LEDs. You could replace it with a non-inverting rectifier, but (at least the design I know) that drives the rectifier into saturation when reverse biased. But maybe there’s different design.
Anyway, there’s 10s of ways of achieving all of this, this is just one example
Just stumbled across this — NOS incandescent vactrols! Only $20 each!
Also found this:
A writeup about photocells and vactrols from PerkinElmer. Differences between the long fall time and short fall time vactrols are attributed to differences in the LDR material; apparently the same LEDs are used for all (so no, it appears it’s not that some use infrared or far red LDRs and some use green). They talk about 7 materials used for photocells in vactrols, and their VTL5C2 and VTL5C4 used “type 0” and “type 4” respectively. Their commercial photocells apparently used only types 0 and 3, both peaking in the green. Unfortunately PerkinElmer is out of the business and it doesn’t appear their photocell line was picked up by anyone else.
One notable quote:
When light is suddenly applied, the photocells resistance drops very fast, typically reaching
63% (1-1/e conductance) of its final values in under 10 msec.
When the light is removed, the resistance increases initially at an exponential rate, approximately tripling in a few milliseconds. The resistance then increases linearly with time.
So what you call the fall time might be very different depending on how you define and measure it. For vactrols it’s described as the time it takes to reach either 100k or 1M (one or the other is used for different vactrols) after turn-off. But if you’re going by the time constant for the initial exponential change, that is far shorter. The GL55, NTE, and Advanced Photonix photocell datasheets quote fall times with no definition and no measurement description, so who knows. But the from the PerkinElmer catalog t_rise is the time to reach 1-1/e and t_fall is time to reach 1/e, and t_rise > t_fall. That makes sense, fast initial response to turn-off followed by very slow return to 100k or 1M. I think that explains the apparent disconnect between vactrol and photocell fall times.
