[elcritch]

For high accuracy, the brightness is really more the limiting factor, as most of the edge finding/binarization algorithms use a intensity factor for cutoff. I used camera measurement for small pieces of tissue samples in biomechanical measurement (~1mm x 5mm). We had a small window in our lab, and cloudy days would distort measurements. I solved it mostly by going to a SLDR camera from a handheld and a backdrop with a type of luminance chart. Much more accurate than calipers for soft squishy (and wet) samples. See [1] Figure 2E (pg 22), you can average out small variations pretty readily.

1: https://pdf.sciencedirectassets.com/274145/1-s2.0-S175161611...

Though your idea of a light source and adc could work well if you put it in a darkened enclosure. It’d probably require a bit more hardware design or tuning but the software / microcontroller side would b simpler. A RPi PiCam with a macro lens would work well too, but the reverse with less hardware tweaking and more software.