Linear basis for metallic and iridescent colors

I have examined the correlation structure in goniochromism by principal-component analysis. Reflectance spectra were collected in synthetic samples that reproduce metallic, nacreous, and iridescent effects under different viewing angles. Although three principal components take into account 99% of the variance, between seven and eight are needed to reach 99.99%. The results were also confirmed by analyzing each viewing condition separately. It was found that although the viewing angle does not modify the first three basis functions, it affects the higher-order ones. These angle-dependent effects can be attributed to optical interference flakes. The implications for pigment identification are discussed.