Influence of window size in small-window colour measurement, particularly of teeth.

Tooth colour is often measured with a small window for illumination and measurement. This causes edge loss of the light, resulting in systematic errors in colour coordinates. This paper gives a quantification of the edge losses, and explains their cause. We measured reflectance spectra for 27 Formalin fixated extracted incisors using a small-window reflectance spectrophotometer equipped with external diaphragms of 3, 4, and 5 mm diameter, and using a spectroradiometer. We calculated the colour coordinates L*a*b* from these spectra. Finally, 16 randomly chosen teeth were illuminated with a pencil beam (lambda = 543 nm, and lambda = 633 nm) while the emerging light was measured as a function of distance from the illuminated spot using a CCD detector. These data were used to calculate small-window edge losses, and thus to predict the small-window reflectance factors relative to spectroradiometrically determined reflectance, at both 543 nm and 633 nm. In all instruments the same spot on the tooth was illuminated and measured, and the teeth were always wet. Colour coordinates for the small-window colour measurements deviate significantly from those determined using the spectroradiometer. These deviations can be explained from the wavelength-dependant edge loss that arises in small-window colour measurement.

[Observing the tooth. The paths of light in visual and instrumental observation]. / Kijken naar de tand. De gang van het licht bij visuele en instrumentele waarneming.

The light we observe when we look at a tooth has travelled a long way through the tooth. A large fraction has passed through the dentine. There is also a sideways displacement: the point of entrance into the tooth is about 2 mm away from the point of exit. This implies that tooth color cannot correctly be measured with an instrument employing a single circular window for both illumination and measurement. Visual caries diagnosis of smooth surfaces and with Fiber Optic Trans Illumination (FOTI) is discussed in terms of light paths through the tooth.

Method for measuring position-dependent volume reflection.

The position dependence of volume reflection caused by the illumination of a turbid material by a pencil beam of light yields information about the optical parameters of that material. Existing methods to measure the position dependence of the volume-reflection profile usually require scanning the sample surface and are hardly applicable to the area near the illuminating pencil beam. This paper describes an experimental set up based on a two-dimensional CCD camera with 14-bit dynamic range and a spatial resolution down to 20 µm. As a demonstration of its use, measurements are compared with theoretical results from the random-walk and diffusion theories. The conclusions are made that the equipment is suitable for measuring close to the illuminating beam without the need for scanning and that it gives an indication in terms of the product of radial distance toward the center of the beam and reduced scattering coefficient (rµ(s)') for the area where the theoretical results presented are valid.