Tooth Color and Reflectance as Related to Light Scattering and Enamel Hardness

Abstract

Tooth color is determined by the paths of light inside the tooth and absorption along these paths. This paper tests the hypothesis that, since the paths are determined by scattering, a relation between color and scattering coefficients exists. One hundred and two extracted incisors were fixed in formalin, mounted in a standardized position in brass holders, and pumiced. A facet was prepared near the incisal edge on the labial plane to allow for Knoop hardness measurements with a 500-gram load. Light scattering by the enamel was measured in a 45°/0° geometry; light scattering by both enamel and dentin was measured in a 0°/0° geometry. The reflection spectrum of the tooth was measured from the labial plane with a spectroradiometer in a 45°/0° geometry, with standard illuminant A and standard illuminant D65. To include all volume-reflected light, we used entire-tooth illumination and small-area measurement. CIELAB color coordinates were calculated from the spectra. Neither spectra nor coordinates showed evidence of a contribution of fluorescence to tooth color. Averaged values and standard deviations for L*,a*,b* were 69.9 (4.1), 1.22 (1.4), and 17.9 (2.9), respectively. Both scattering coefficients averaged to 0.6 (0.4) mm-1; Knoop hardness number was, on average, 271 (39) kg/mm 2. L* correlated with a* (r = -0.51), with the enamel scattering coefficient (r = 0.60), and slightly with hardness (r = 0.17, p = 0.03). The colors of 28 teeth from which the enamel was removed correlated strongly with the colors of the complete tooth. This study quantitatively confirms that tooth color is determined mainly by dentin, with enamel playing only a minor role through scattering at wavelengths in the blue range.