Without light, color does not exist. An object that we perceive as a certain color absorbs all light waves corresponding with other colors and reflects only those waves that we interpret as that object's color. For example, an object that absorbs blue and green light and reflects red appears red. The apparent color of an object is influenced by its physical properties, the nature of the incident light to which the object is exposed, the relationship to other colored objects; and the subjective assessment of the observer. These factors can cause a single tooth to look very different among different observers. Saleski' has pointed out that although lighting standards are available in industries as diverse as automobile and
lures give off relatively high concentrations of blue waves.
Artists have traditionally chosen studios illuminated by northern daylight, which can be close to full-spectrum white light and often is used as the "normal" standard for judging light from other sources (Fig. 23-3). It has a color rendering index (CRI) close to 100. The color rendering index, on a scale of 1 to 100, indicates how well a particular light source renders color as compared to a specific standard source.
Although daylight is often used as the standard against which other light sources are compared, tooth shades should never be selected in direct sunlight. The distribution of light waves from the sun depends on the time of day and on humidity and pollution. During morning and evening hours, the shorter light waves (blues and greens) are scattered, and only the longer ones (at the red end of the spectrum) penetrate the atmosphere. Consequently, incident daylight at dawn and dusk is rich in yellow and orange but lacking in blue and green. Northern daylight around the noon hour on a bright day is considered ideal because there is a harmonious balance of the full visible spectrum. Nevertheless, circumstances may dictate the use of artificial light for shade selection. In these cases, color-corrected fluorescent lighting is recommended because it approaches the necessary type of balance. Bergen and McCasland' have reported that two commercially available, color-corrected fluorescent tubes are acceptable full-spectrum sources with a CRI of greater than 90.* Another light source reference standard is color temperature, which is related to the color of a standard black body when heated. Color tempera ture is reported in degrees Kelvin (K), or absolute (0' K = -273' C). Accordingly, 1000' K is red; 2000' K is yellow; 5555' K is white; 8000' K is pale blue. Northern daylight has an average color temperature of around 6500' K, but this varies with the time of day, cloud cover, humidity, and pollution.
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