Colour-Shift / Rainbow Index
Quantify the angular colour shift ('rainbow') of a multilayer stack as seen by a human observer, using CIE colorimetry.
Validation: 2 / 2 reference cases passing in build ad9886a → /validation#colour-shift
Measure perceived colour shift of coated surfaces at different viewing angles. Compute CIE 1976 chromaticity trajectories, rainbow index, and CIEDE2000 colour differences under selectable illuminants. Essential for HUD and automotive display applications.