Spectral measurements can be used to indicate tooth decay and to help developers of dental prostheses approximate tooth color. When researchers use the radiative transport equation to calculate light propagation within biological tissue, they usually consider it to be a medium that randomly scatters light, but it actually tends to have an ordered microscopic structure. Now scientists have solved the radiative transport equation based on measurements of dentin using computational algorithms called Monte Carlo simulations that consider how the tissue’s microstructure scatters light. To make the measurements, Alwin Kienle and Raimund Hibst at the Institut für Lasertechnologien in der Medizin und Mebtechnik in Ulm, Germany, focused a 633-nm unpolarized HeNe laser on cubes of dentin and measured the transmitted light with a cooled CCD camera. The cubes were in water to prevent them from drying. The researchers compared their measurements to their simulations. Scientists directed a laser at various points in a cube of dentin, and the scattered light was cone-shaped. Reprinted with permission of Physical Review Letters. Dentin contains numerous tubules that bend at one point, and the scientists considered these tubules as straight cylinders of infinite length. They noted that light directed at a cylinder forms a small cone around it. As the tubules bend, the cones of scattered photons follow the course of the tubules. Dentin collagen fibers, which decrease the light transmittance through the tissue, also were included in their simulations. In all cases, their simulations matched their measurements. The investigators would like to model light propagation in the whole tooth and are examining light propagation in tendons. They believe that the simulations of the light propagation should be modified for many biological tissues. “Up to now, it was [mostly] assumed that tissue light-scattering was random, but many tissue types have an aligned microstructure, which means that the model that has been used cannot be used,” Kienle said. Physical Review Letters, July 7, 2006, 018104.