Principles of Shade Selection
Christopher C.K. Ho
Shade selection is an important procedure providing patients with an aesthetic restoration that harmoniously blends to the remaining dentition. Knowledge of the scientific basis of colour, from understanding light to interpreting the artistic aspects of shade selection, ensures a successful result.
Shade selection involves the perception of colour, which depends on three entities:
- 1) Light source (illuminant)
- 2) Object
- 3) Detector (ocular or instrumental).
The colour of an object can be influenced by the illuminant, for instance tungsten light may cast a yellow colour compared to daylight. The property of influencing the colour of objects is called ‘colour rendition’. There are three main illuminants within any dental practice: natural, incandescent and fluorescent. Incandescent lighting is predominantly red/yellow and lacking in blue, while fluorescent lighting is high in blue tones and low in red. Initial shade selection should initially be made with daylight or colour-corrected lighting, and then the shade should be matched under different lights to avoid metamerism (the phenomenon that occurs when shades appear to match under one lighting condition and not another).
Colour possesses three dimensions: value, hue and chroma. A high-value object often reflects most of the light falling onto its surface and appears bright. The converse is true with a dark object, which absorbs most of the light and appears dull or of low value. Hue is the wavelength of light, and is dependent on the spectral reflectance from an object. Chroma is the concentration of colour or colour intensity.
The third part of the stimulus for colour is the spectral response of the detector, or the eye.
The difficulty of shade selection is that clinicians must be able to interpret a multilayered structure of varying thicknesses, opacities and optical surface characteristics. This can affect the way in which the eye perceives colour.
The basic hue of the tooth is determined by the colour of the underlying dentine, while value is a quality of the enamel overlay. Muia explains: ‘The dentine imparts all the colour. Enamel is like a fiberoptic structure conducting light through its rods.’1 Chroma is the saturation of colour in the dentine, but is influenced by the value and thickness of the enamel. Teeth are often termed ‘polychromatic’ and have variations in hue, value and chroma that give three-dimensional depth and characteristics. A young dentition is characterised by opaque, high-value enamel, which blocks the underlying dentine. As teeth age, the enamel becomes more translucent and dull (low value), revealing the underlying dentine. This layering can make reading of tooth colour difficult, since the value of the enamel and surface lustre often complicate colour evaluation of the underlying dentine.