9 December 2004 Specular highlights of plastic surfaces and the Fresnel coefficient
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Abstract
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 specular highlights for materials like plastics and ceramics can be approximated by the color of the incident light. We will 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 shows 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, we will show that 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 Nano-View SE MF 1000 Spectroscopic Ellipsometer. The computed spectral gradients of the tile specularities exhibited a less than 2.5% deviation from the predicted theoretical values.
© (2004) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Elli Angelopoulou, Sofya Poger, "Specular highlights of plastic surfaces and the Fresnel coefficient", Proc. SPIE 5578, Photonics North 2004: Photonic Applications in Astronomy, Biomedicine, Imaging, Materials Processing, and Education, (9 December 2004); doi: 10.1117/12.566776; https://doi.org/10.1117/12.566776
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KEYWORDS
Reflectivity

RGB color model

Refraction

Refractive index

Sensors

Cameras

Ceramics

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