How can a ldr resistor be stable over time? I think the ldr preamp are very subtle since it needs to convert electric to photo and then convert photo to resistance."
First of all it's not just ldr, it's 4 x quad matched led/ldr in a hermetically sealed packages with 4 legs on each.
2 legs for led, and 2 legs for ldr (resistor)
"Electric" is the LED (Light Emitting Diode) portion, and "photo" is the LDR (Light Dependent Resistor)
Here is the circuit diagram of it (2nd pic) and how to build one if you have the skills.
The MkII Lightspeed prototype has been powered on now 24/7 for over 6 years and is still in perfect calibration (stable), and we've never had one back out of hundreds for recalibration.
Just a couple of fried ones from owners doing strange things with the power supplies that have no basic electronic knowledge and trying the wrong thing.