Beyond the neutral interface reflection assumption in illuminant color estimation

The neutral interface reflection (NIR) assumption is a widely accepted theory in computer vision. According to the NIR the color of specularities of dielectric materials is the color of the incident illumination and the influence of the Fresnel reflectance is neglected. We show, that there is a material- and geometry-dependent shift between the color of the specularity and the color of the incident light due to the Fresnel effect which for human skin can be up to approximately 5.8%. As the NIR concept is often the core idea of specularity-based illuminant-color estimation techniques, the ignored Fresnel effect introduces a systematic error in the estimation result. We thus propose a material-dependent rectification method for correcting this color shift. Our experiments on human skin regions show an average improvement of the illuminant color estimation of about 30%.