> Perhaps there's some accounting for this, and I'm curious to learn what it is

The slip itself is a calibration reference, so a clean photo of it could serve to compensate for the lamp and camera and calculate how accurate the readings is for different parts of the spectrum. (But good wide spectrum light would be ideal for high precision readout)

You're also still limited to visible light because of the camera uv and ir filter, for which the sun is a decent reference.

oh, yes of course! Thank you :)