I've just tested 5 random Xvive 5C3s.

I discovered three things:

1) The LEDs aren't all identical. We're fussing about the LDR resistance, but the LED forward voltage varies a bit too. This means that for two different devices driven on the same voltage with the same series resistor, the LED current won't be identical. That has a knock-on effect on the LDR resistance.

2) LDR resistances were spread across a range, and much tighter at the low end than the high end. Once you're to very low currents like 0.1mA, the resistance varies a lot. The range from a few K to a few hundred K is fine.

3) At the low current end, the response is very slow. The LDR is clearly following an exponential response, and it gets most of the way there pretty quickly, but the result keeps altering a little bit for a long while. The datasheet specifies dark response times to 63%, as if it were a cap charging, so this is a reasonable way to think about it.

Here's my results. I used the same resistors for each reading, so the caveat about the LEDs applies. Everything was powered on 5V.

130R/24mA: 1.9K, 1.4K, 1.,9K, 1.5K, 1.5K (spread of x1.35)

270R/12mA: 2.7K, 2.2K, 3.1K, 2.3K, 2.5K. ( spread of x1.4)

470R/7mA: 3.8K, 3.2K, 4.8K, 3.4K, 4.1K. (spread of x1.5)

1K2/2.8mA: 8.3K, 7K, 10K, 8K 10.5K (spread of x 1.5)

2K5/1.3mA: 18K, 15K, 20K, 19.5K, 23.4K. (spread of x1.56)

5K1/0.65mA: 54K, 36K, 43K, 48K, 61K (spread of x1.7)

12K/0.27mA: 250K, 146K, 142K, 240K, 245K (spread of x1.76)

25K/0.14mA: 1.66M, 720K, 680K, 1.1M, 840K. (spread of x2.4)

The results are in line with the datasheet. Reckon on resistance values varying by roughly 2:1 between devices and you'll be fine.

Yes, they vary. We knew that already. Are they unusable junk? Not at all. MPH, if you got wildly different results to this, I'd start to wonder if you haven't got some dodgy parts.

One final thing to point out is that if you use a resistor in parallel with the LDR to reduce the maximum resistance, it also reduces the variability. I used 120K in the Filter FX. Assume I drop the LED current as low as 0.27mA, so the LDR could be 140K to 250K. That gives me a final output resistance of 65K to 81K (142K||120K and 250K||120K). That's not bad at all, only 25%.