Optical Properties of Teeth

Translucency

Translucency is the gradient between transparent and opaque and is considered the 4th dimension of color after hue, chroma, and value.^4,7,24 Translucent materials allow some light to pass through them. Increasing the translucency of a surface lowers its value as less light returns to the viewer’s eye.⁴ The different structures of the tooth are associated with varying degrees of translucency; Enamel is most translucent and transmits approximately 70% of light through 1mm of its thickness whereas dentin is less translucent transmitting approximately 30% of light through the same thickness.⁴² However, the translucency of enamel is influenced by several factors including the angle of incidence and the wavelength of light; the gloss, dehydration,⁴³ the refractive index of its surface; and its intercrystalline spatial configuration.^4,7 Translucency gives a tooth/restoration its color depth and vital appearance (Fig. 11).⁴⁴ Therefore, shade matching must be supplemented by recording the translucent areas.

Fig. 11 - Translucency gives the teeth their vital appearance.

Fig. 11 - Translucency gives the teeth their vital appearance.

Opacity

Opacity (of teeth) is another important optical feature that must be assessed during the fabrication of esthetic indirect restorations. It is the property of an object that blocks light from passing through it. Dental procedures like tooth bleaching, impression making, rubber dam isolation, and application of curing light can dehydrate teeth. Dehydrated teeth appear to have increased opacity making them appear brighter and whiter than their original color.⁴

Opaque restorations (Fig. 12) may reflect and/or absorb the incident light but will not transmit it. Opacity is directly associated with value; Increasing the opacity increases the value of a restoration.⁴

Fig. 12 - Nearly opaque dental restoration on teeth # 6, 7, 8, 9 and 10

Fig. 12 - Nearly opaque dental restoration on teeth # 6, 7, 8, 9 and 10

Opalescence

Opalescence is a phenomenon wherein a material appears to be one color when light is reflected from it and another color when light is transmitted through it.^4,7 Opals and tooth enamel exhibit opalescence;²⁸ They deflect different wavelengths in the light spectrum to various angulations.^4,28 This property of tooth enamel is associated with it appearing bluish-green color in reflected light and reddish-orange in transmitted light (Fig. 13).^28,45 The opalescent effect of enamel brightens the tooth and gives it, its optical depth and vitality.^4,28

Fig. 13 - Enamel appears reddish-orange in transmitted light.

Fig. 13 - Enamel appears reddish-orange in transmitted light.

Fluorescence

Fluorescence is the phenomenon of absorption of light by an object and the subsequent emission of light of a longer wavelength/lower energy.^4,7 The tooth structure associated with fluorescence is dentin as it is composed of a higher concentration of organic material.^4,7 Fluorescence is associated with teeth looking brighter, whiter, and vital.⁴ Natural teeth exhibit blue fluorescence under ultraviolet (UV) radiation (Fig. 14).²⁸ This optical property is relevant to patients who frequent nightclubs and entertainment shows.

Fig. 14 - Natural teeth exhibit fluorescence under ultraviolet light.

Fig. 14 - Natural teeth exhibit fluorescence under ultraviolet light.

The amount of dentin fluorescence is indirectly related to chroma;⁴ The greater the amount of dentin fluorescence, the lesser the chroma.^4,24 Rare earths are added to the dentinal layers of porcelain crowns to give them the property of fluorescence, thereby, making them appear similar to natural teeth under different lighting conditions.⁴ This is advantageous (when fabricating restorations with higher value) as it helps increase the value of a restoration without affecting the translucency.²⁸