Hello, Can you please provide more information on how to calculate the reflectance of a non-grey standard?
“An ideal grey standard is perfectly grey (i.e. has equal reflectance across the entire spectral range the camera is sensitive to). However, sometimes a standard might be dirty, or you need to use a non-grey standard. This option allows you to specify different reflectance levels for the different image channels. You can work out what these levels are by photographing your imperfect/non-grey standard against a true grey standard, and measuring the reflectance of your non-grey standard relative to the true grey.“
You need to measure your standard relative to a “perfect” standard (not that such a thing really exists). E.g. a flattened pile of barium sulphate powder is pretty much 99% reflectance and diffuse. As are very expensive spectralon standards.
Photograph your imperfect standards next to this known “perfect” standard. Create an mspec image using only the known standard. Then measure the camera’s linear responses to your imperfect standards (as linear reflectance relative to the perfect standard in the mspec image). This will incorporate the imperfections in reflectance as viewed by that camera (so it might vary with other cameras). now just use those values for the imperfect standards. Run a test on that same calibration image backwards and check that it makes the “perfect” standard 99% in each channel.