An understanding of the nature of light and how the eye perceives and the brain interprets light as color is important for successful shade selection, particularly when metal ceramic of all-porcelain restorations are being made. Errors in shade matching continue to be a problem with these procedures and can be a source of frustration for the dentist and technician and a source of dissatisfaction for the patient. This chapter outlines some of the principles of light and color and applies them to a discussion of shade selection.
LIGHT AND COLOR
Without light, color does not exist. An object that we perceive to be of a certain color absorbs all light waves corresponding with other colors and reflects only those waves that we as the color of the object. Thus an object that absorbs blue and green light and reflects red appears red. Its physical properties, the nature or the incident light to which the object is exposed, the relationship to other colored objects, and the subjective assessment of the observer, influences the apparent color of an object. A single tooth therefore can look different to different people under differing light conditions. Saleski has pointed out that although lighting standards are available in industries as diverse as automobile and textile manufacturing there is no standard for lighting in dentistry.
Description of light
Scientifically, light is described as visible electromagnetic energy whose wavelength is measured in nanometers (nm) or billionths of a meter. The eye is sensitive only to the visible part of the electromagnetic spectrum, a narrow band with wavelengths of 380 to 750 nm.
Pure white light consists of relatively equal quantities of electromagnetic energy over the visible range. When it is passed through a prism, it is split into its component colors because the longer wavelengths are bent (refracted) less than the shorter ones.
Quality of light
The most common light sources in dental offices are incandescent and fluorescent, neither of which is pure white light. An ordinary incandescent light bulb emits relatively higher concentrations of yellow light waves than of blue and blue-green whereas fluorescent ceiling fixtures give relatively high concentrations of blue waves. Dental operations generally have incandescent light bulbs combined with blue reflectors, which give a rather bluish light.
Description of color
Munsell color order system. The most popular method for describing color is the Munsell system, which, despite having certain disadvantages, is widely used in the dental literature. In it the three attributes of color are called hue, value, and Chroma.
Hue
This is defined as the particular variety of a color, shade, or tint. The Hue of an object can be red, green, yellow, etc. and is determined by the wavelength of the reflected and/or transmitted light observed. The place of that wavelength (or wavelengths) in the visible range of the spectrum determines the Hue of the color. The shorter the wavelength, the closer the Hue will be to the violet portion of the spectrum; the longer the wavelength, the closer it win be to the red portion.
In the Munsell color system, Hues are divided into 10 gradations; yellow, yellow-red, red, red-purple, purple, purple-blue, blue, blue-green, green, and green-yellow. These are arranged in a wheel. Each gradation is subdivided; for example, red ca，英语毕业论文，英语毕业论文 |
