INFLUENCE OF WINDOW SIZE IN SMALL-WINDOW COLOR MEASUREMENT, PARTICULARLY OF TEETH

Tooth colour is often measured with a small window for illumination an d measurement. This causes edge loss of the light, resulting in system atic errors in colour coordinates. This paper gives a quantification o f the edge losses, and explains their cause. We measured reflectance s pectra 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 th e colour coordinates La*b* from these spectra. Finally, 16 randomly c hosen teeth were illuminated with a pencil beam (lambda = 543 nm, and lambda = 633 nm) while the emerging light was measured as a function o f distance from the illuminated spot using a CCD detector. These data were used to calculate small-window edge losses, and thus to predict t he 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 teet h were always wet. Colour coordinates for the small-window colour meas urements deviate significantly from those determined using the spectro radiometer. These deviations can be explained from the wavelength-depe ndant edge loss that arises in small-window colour measurement.