Monitoring dental erosion by colour measurement: an in vitro study.

OBJECTIVES: The aim of this study was to develop a method to monitor dental erosion by evaluation of the colour change of teeth as a function of enamel loss, and to evaluate the reproducibility of the method used. METHODS: Light reflectance spectra of 12 extracted human incisors were measured using a spectroradiometer and diffuse illumination. From these spectra CIELab colour parameters L*, a* and b* were calculated. Erosive dental wear was simulated by incrementally removing enamel layers. We monitored the change of the colour parameters as a function of the enamel thickness removed. A clinical situation using a phantom head and ambient illumination was simulated with 8 incisors. In this set-up colour change due to polishing was evaluated. The teeth were immersed in coffee and tea to estimate the effects of nutritional dyes, and so, to determine reproducibility of the method used in clinical situations. RESULTS: A relationship between tooth colour measured and enamel loss was found, in particular for the b* value. The relation between the b*-value and the enamel thickness removed showed a slope of 15+/-3mm(-1), if the remaining enamel layer had a thickness of less than 0.5mm. Polishing of the teeth made them less yellow. Immersion in coffee darkened the teeth, but immersion in tea had no significant effect. CONCLUSIONS: Due to individual variation, it was impossible to use this relationship to estimate the remaining enamel thickness, but the method presented may be suitable for monitoring progression of erosive enamel loss.

Determination of Kubelka-Munk scattering and absorption coefficients by diffuse illumination.

The Kubelka-Munk theory, although it provides an equation that relates the reflection of a sample under diffuse illumination to certain of its properties, does not take boundary reflectance into account. Boundary reflection is always present because there is always a difference between the refractive indices of the sample and of the surrounding medium. We describe how a half-sphere is used to achieve diffuse illumination, and we present and exemplify equations that correct for boundary reflection with measurements of four composite restorative dental materials. The refractive index of the sample is measured with a matching technique that employs a glycerol-water mixture. Edge loss errors are estimated.

Optical properties of bovine muscle tissue in vitro; a comparison of methods.

We measured the optical properties of muscular tissue using several methods. Collimated transmission measurements of thin slabs showed spatial anisotropy of the scattering processes. Surface roughness of the sample disables the calculation of the extinction coefficient from these measurements. From angular intensity measurements we found a scattering asymmetry parameter g = 0.96. In fresh samples the optical diffusion constant D depends on the orientation with respect to the longitudinal direction of the muscular cells. From the D values we calculated s' perpendicular to the longitudinal direction as 0.19 mm(-1) (at 543 nm), 0.39 mm(-1) (at 594 nm) and 0.59 mm)(-1) (at 632 nm). The values for D which we measured from samples that were frozen and thawed did not show dependence on orientation. From spectral dependent reflectance measurements we found an oxygenation degree of 0.61 and a reduced scattering coefficient s' = 0.85 mm(-1) around 560 nm.

Anisotropy of volume-backscattered light.

In many measurements used for the determination of optical properties of biological tissues, a detector with a small cone of acceptance is used. Therefore a small part of the anisotropically distributed backscattered light is detected. We present the results of Monte Carlo calculations and study the anisotropy of the direction distribution of volume-backscattered photons. Close to the source it is impossible to give a general correction factor to relate the measurements to theoretical results. Far from the source, the measured radiation divided by the square of the sine of the accepting angle of the detector is a relatively good approximation of the radiant exitance at the surface of the sample under investigation. We recommend the use of a detector with a large angle of acceptance for these kinds of measurements.

HeNe-laser light scattering by human dental enamel.

Knowledge of the optical properties of tooth enamel and an understanding of the origin of these properties are necessary for the development of new optical methods for caries diagnosis and the measurement of tooth color. We measured the scattering intensity functions for HeNe-laser light of 80- to 100- micrometers-thick human dental enamel slabs. The asymmetry factors were calculated to be g = 0.68 at 633 nm. By measuring the collimated beam attenuation, we determined the scattering coefficient to be s = 6.6mm(-1). From Fraunhofer diffraction patterns, obtained from transmission of the laser beam, we calculated the periodicity of the prismatic structure as 5.4 micrometers. We present a model containing scattering by crystals and by prisms. It shows that the prisms are the most important scatterers but that the crystals are responsible for the back-scattering.

Theoretical model for the scattering of light by dentin and comparison with measurements.

A theoretical model of the scattering of light by dentin is presented. The model that results is a superposition of several scattering contributions, i.e., scattering by mineral crystals, collagen fibrils, and dentinal tubules. These tubules are oriented so that they cause an asymmetrical scattering process. The angular intensity functions are calculated for planes that are parallel or perpendicular to the plane of the tubules. The shape of the measured intensity function in the plane perpendicular to the tubules and the values of the scattering coefficient can be explained by the model that we present.

Reduced light-scattering properties for mixtures of spherical particles: a simple approximation derived from Mie calculations.

The reduced scattering cross section per unit of volume Sigma'(s) identical withSigma(s)(1 - g) is an important parameter to describe light propagation in media with scattering and absorption. Mie calculations of the asymmetry factor g for nonabsorbing spheres and Q(sca), the ratio of the scattering cross section Sigma(s) and the particle cross section, show that Q(sca)(1 - g) = 3.28x(0.37)(m - 1)(2.09) is true to within a few percent, when the Mie parameters for relative refractive index m and size x are in the ranges of 1 < m

Angular dependence of HeNe-laser light scattering by bovine and human dentine.

The scattering phase functions for HeNe-laser light of dentine sections 10-20 micron(s) thick were measured. The functions perpendicular to the tubules had first-order maxima at angles of 4 degrees for bovine dentine and 5 degrees for human dentine; those parallel to the tubules showed no first-order maxima. Several corrections were made before the asymmetry factors g for intertubular dentine were determined; the average values are for bovine dentine g = 0.37 and for human dentine g = 0.44.