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n be written IR, 2R-9R, 3R-9R, 10R, followed by 1YR, 2YR, 3YR-9YR, 10YR, followed by 1Y, 2Y, 3Y, etc.; and these can be further subdivided. Thus a particular Hue might be 4.3Y or 8.1YR. Most natural teeth fan into a range between yellow and yellow-red. In a study of 6000 teeth Clark established a Hue range of 6YR to 9.3YR but found that the distribution of teeth within it was not at all uniform.
Chroma
This is defined as the intensity of a Hue. The term's saturation and Chroma are used interchangeably in the dental literature; both mean the strength of a given Hue or the concentration of pigment.
A simple way to visualize differences in Chroma is to imagine a bucket of water. When one cup of paint is added, a solution of low Chroma results. Adding a second cup of paint increases the Chroma, and m on, Until a solution is obtained that is almost all paint and consequently of high Chroma.
In the Munsell color system, maximum Chroma depends on the particular Hue but can ring from 10 to 14. Achromatic shades have a Chroma near 0. Natural teeth are found with Chroma ranges from 0 to 7.
Value
This is defined as the relative lightness or darkness of a color or the brightness of an object. The brightness of any object is a direct consequence of the amount of light energy that that objects reflects or transmits.
Lingt energy is measured in photons, and it is possible for objects of different Hues to reflect the same amount of photons and thus have the same brightness or Value. A common illustration would be the difficulty experienced in trying to tell a green from a blue object in a black and white photograph. The two objects will reflect the same amount of light energy and thus appear identical in the picture. This fact led to the once popular misconception that the Value of a color resided in its amount of grayness.
In the Mansell method of describing color, Value is divided into ten gradations, with 0 being black and 10 being white. Natural teeth range in Value from 4 to 8. A restoration that has too high. a Value (is too bright) may be readily detected by an observer and is a common esthetic fault in metal-ceramic prosthodontics.
Perception of color
Light from an object enters the eve and acts on receptors in the retina (rods and cones). Impulses from these are passed to the optical center 01 the brain, where an interpretation is made.
Different persons will make different interpretations of the same stimulus and thus shade selection is dependent on subjective assessment.
The eye
Under low lighting conditions only the rods are used (scotopic vision). These receptions allow and interpretation of just the brightness, not the color, of object is to be made. They are most sensitive to blue-green objects. Color vision is dependent on the cones, which are active under higher lighting conditions (photopic vision). The change from photopic to scotopic vision is termed dark adaptation and takes about 40 minutes to be complete.
Although the exact mechanism of color vision is not known, it has been demonstrated that there are three type of cone--sensitive to red, green, and blue light--which form an image in a manner similar to the additive effect of the pixels in a color television picture.
Color adaptation
Color vision decreases rapidly as an object is observed, the original color becoming appa