since you are using RAW (uncompressed?), WB and color space should be irrelevant. It is important that you have vignette control, long exp NR, and hIgh ISO NR switched off. Which you do. Not shure about sharpening and clarity - I think it should be irrelevant on RAW, but would be better do set them to neutral in case of further measurements.
Looking at your data there is basically nothing new here - don't know what happens around 50sec, but anyway: There is linearity at higher signals, but not the expected linearity at low signal/short exposures. To me this looks like some geometric mean such as sqrt( noise*noise + signal*signal), where signal would be linear with exposure time, and noise would be a constant value. Or it may also (but slower) increase with exposure time (as Robin suggested).
Did you repeat the measurement under identical conditions but with the LED panel switched off (and completely shielded from any surrounding light source)? That would be a dark image. You need to do it at the same exposure times as your light images.
For scientific grade image analysis, you also need to do bias calibration using a very short exposure time (1/25sec or so) image. Did you acquire this as well, and did you use dark and bias calibration for full account of what happens in a digital sensor? If you have the images, Siril should be able to do the calibration for you.
One last point: what ISO did you use? 400 would be the optimal value for the D7500. And leave some time (1min or so) between individual exposures so the sensor can cool after it heated up during the exposure. The latter may not be the most important factor, but if you would repeat the measurement I would suggest doing it to be on the safe side. Otherwise you will have increasing levels of dark current because of increasing sensor temperature. This is a small drawback of using an (uncooled) DSLR, although the effect shouldn't be so dramatic.
You may consider opening the f-stop of your lens in the linearity measurement from f/32 by one or two steps, and reducing the exposure time to half of what you used previously, or a quarter. This would help reduce the contribution of dark current, in particular if you are doing this linearity measurement at room temperature.
Edited by LauraMS, 08 February 2023 - 02:38 AM.