Specular highlights of plastic surfaces and the Fresnel coefficient

Details Specular highlights of plastic surfaces and the Fresnel coefficient Specular highlights of plastic surfaces and the Fresnel coefficient

Oct 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004spie.5578..465a&link_type=abstract

Photonics North 2004: Optical Components and Devices. Edited by Armitage, John C.; Fafard, Simon; Lessard, Roger A.; Lampropoul

Computer Science

Scientific paper

One of the biggest clues in specularity detection algorithms is the color of the specular highlights. There is a prevalent assumption that the color of specularities for materials like plastics and ceramics can be approximated by the color of the incident light. We show that such an assumption is not generally appropriate because of the effects of the Fresnel reflectance coefficient and its dependence on wavelength. Our theoretical analysis establishes that the sensitivity of the Fresnel term to the wavelength variations of the refractive index can be at least as large as 15%. Our experiments demonstrate that, even with traditional RGB color cameras, the recorded color of specular highlights is distinct from the color of the incident light. Furthermore, by computing the spectral gradients (i.e. the partial derivatives of the image with respect to wavelength) at specular regions we can isolate the Fresnel term up to an additive illumination constant. Our theory is supported by experiments performed on multispectral images of different colored plastic tiles. The refractive indices of the opaque plastics were measured using a specialized spectroscopic ellipsometer. The computed spectral gradients of the tile specularities exhibited a less than 2.5% deviation from the predicted theoretical values.

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