I have a plastic disk 0.5" in diameter whose radius of refraction is supposedly non-uniform. It will vary by 0.015 at most from the center to the outside. I need to come up with some method of measuring the refractive index locally. I have access to: helium neon laser a photodetector and software optical table with moving mounts CNC machine shop lots of plastic/glass Every way I try seems to not work very well. I'll need some ideas from someone experienced please.