[Observing the tooth. The paths of light in visual and instrumental observation]

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.