CLINICAL RESEARCH
Fig 11 The longitudinal histological tooth section of a maxillary central incisor, 1 mm in thickness, was submerged in distilled water and photographed via transmissive illumination (upper) and reflective illumina- tion (lower) to epitomize the opalescent nature of enamel.
wide to 8 μ m high) meet this property be- cause the HAp subunit crystals exhibit thicknesses of 25 to 40 nm, and widths of 40 to 90 nm. For that reason the short blue wavelengths reflect preferentially from the enamel, while the longer am- ber wavelengths transmit accordingly through it (Fig 11). Fluorescence An example of photoluminescence is a phenomenon in which invisible UV light is absorbed and then re-emitted almost immediately 34 (10 -8 s) at a less energet- ic wavelength in the visible spectrum. Enamel and dentin both possess fluo- rescent properties, with dentin generally exhibiting three times the intensity than that of enamel 35 upon longitudinal sec- tion, emitting a white-blue luminescence after excitation, imparting additional vi- tality and brightness to the natural tooth appearance predominantly in UV rich environments only. The DEC also exhib- its elevated fluorescence due to the col-
Subjective optical elements
Translucency Translucency is defined as a gradi- ent between transparency (complete transmission of light) and opacification (complete reflection of light). The light transmission of enamel has been shown to be wavelength specific, age related and is influenced by its state of hydra- tion. A decrease in translucency during dehydration is explained as a result of an increased difference in refractive in- dices between the enamel prisms and the surrounding medium when water is replaced by air. 33 Opalescence Known as the Rayleigh scattering ef- fect; enamel demonstrates this dichroic effect, which is caused by scattering particles with typical dimensions much smaller than the wavelength of illumina- tion used. The mineral crystals present in the enamel prism (measuring 4 μ m
342 THE INTERNATIONAL JOURNAL OF ESTHETIC DENTISTRY VOLUME 9 • NUMBER 3 • AUTUMN 2014
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