Humans live in a colorful world, and people can see colorful objects everywhere, so color is no stranger to people at all. But what is the nature of color? How to accurately express the color? Why is the color of porcelain crowns often not the same as the color of natural teeth? Not all of these questions can be answered. First, the basic knowledge of color people can perceive the existence of color must have three conditions, namely visible light, objects and the human visual system (eye, optic nerve, brain), one of the three is indispensable. Lu Yongjian, Department of Dentistry, Shanghai Xinhua Hospital The essence of color is electromagnetic waves. Electromagnetic waves can be divided into communication waves, infrared, visible light, ultraviolet, X-rays, γ-rays and cosmic rays due to the different wavelengths. The wavelength of visible light is 380～780nm, and visible light through the prism can present a spectrum consisting of red, orange, yellow, green, green, blue and violet, and there is a strict correspondence between these seven colors and wavelengths, the wavelength of red light is the longest, 680～780nm, and the wavelength of violet light is the shortest, 380～430nm. . Hue (hue) is the name of the color, is the appearance of the color, but also red, orange, yellow, green, green, blue, purple 7 basic colors in general. Different shades of color mixed with each other can form a variety of colorful and colorful. Such as yellow mixed with blue, can form emerald green, yellow green, grass green and other colors. Brightness (brightness) is a variety of colors from bright to dark degree of change, determined by the surface of the object to the reflectivity of light. In the colorless, the brightest is white, the darkest is black. The addition of different amounts of white in black, it can form a dark gray, medium gray, light gray and other different gray. Various colors also have different degrees of brightness, such as green plus white or black, can become light green, medium green, dark green and other colors. The closer to the high brightness area of the color the brighter, such as lemon yellow, light yellow, yellow; the closer to the low brightness area of the color the darker, such as dark green, dark blue, dark purple, etc.. Color degree (purity, saturation) refers to the degree of vividness of the color. Various colors in the pure color, its color degree is the highest. Hue, brightness of the same color can have a difference in chroma. There is no color difference in the colorless. Color hue, brightness, color must be illuminated in order to appear, the object that can emit lighting light is called the light source. There are two major types of light sources: sunlight and light. The stability of sunlight is poor, often affected by the region, season, climate, time, place, direction and other factors. Therefore, the use of sunlight as a light source, should be selected from 10:00 a.m. to 14:00 p.m., the sunlight reflected into the room by the north window, relatively stable. The color temperature of light at this time is about 5000-6000K, “K” is the unit of color temperature. There are incandescent lamps, fluorescent lamps and other varieties of light. The standard light is a special light source, color temperature of 6500 K, referred to as D65. standard light is prescribed by the International Commission on Illumination, good stability, not affected by season, time, climate and other factors, is a necessary device for color research. People in the observation, measurement, comparison of color sometimes error, the following are the possible causes of error: 1, the type of light source: the same color object in daylight and in the light presents a different color, in incandescent and in fluorescent lamps also have not a small difference. 2, the color of the light source: the same object in the red light exposure shows reddish color, in the blue light exposure shows bluish color. 3, the background of the object: in different color background, the same object will also show a different color. 4, the angle of the irradiation light: the angle of the irradiation light to the surface of the object will also affect the color effect. 5, the distance and angle of the observed object: when observing the color of the object, the distance and angle from the human eye to the surface of the object are also factors that affect the color. 6, the observer’s perception of color: people’s perception of color varies greatly, trained and untrained will have a big difference. To reduce color measurement, colorimetric error should be unified as follows three conditions. 1, a unified light source: the best conditions to use the standard light source (D65). No standard light source can be used when the sunlight, required in the daytime 10 to 14 hours, indoor north window next to. 2, the angle of the lighting light: the angle of the lighting light should be selected 45 ° angle. 3, the angle and distance of the observer The distance from the human eye to the object is 25 ~ 30mm, the angle of 90 ° II, the table color system People first conveyed information about color only used the hue, and only later learned that the same hue and the difference between lightness and color. In order to accurately express the color hue, lightness and color must be qualitative understanding of color up to the level of quantitative understanding, so the appearance of the table color system. Commonly used table color system are L*, a*, b* table color system, table color system, XYZ table color system, etc. L*a*b* color system This is the most commonly used color table method in the field of object color expression, which was recognized by the International Commission on Illumination (CIE) in 1976. l*a*b* color table system uses an imaginary spherical three-dimensional structure to describe the three basic parameters of color. ①Chromaticity variation is expressed in the spherical cross-section, with a indicating the red direction, -a the green direction, b the yellow direction, and -b the blue direction. The larger the value, the more to the periphery, the greater the chroma, the more vivid the color; the smaller the value, the closer to the vertical axis, the smaller the chroma, the less vivid the color. The smaller the value, the closer to the vertical axis, the smaller the chroma and the less vivid the color. ③ The brightness is shown on the vertical axis, the higher the brightness upwards, the lower the brightness downwards. According to the three basic parameters of the object color, can be precisely located in the color spherical structure, so that can be accurately described, expressed. The XYZ color system was developed in 1931 based on the principle of mixing the three primary colors (R=red, G=green, B=blue). y represents the reflectance of light, i.e. luminosity. xy represents chromaticity (hue and color). The shape of the diagram is similar to 1/6 of a circle, and the center of the diagram has no color and low color intensity, while the color intensity increases to the periphery. The above three color systems have their own characteristics, but their commonality is that they can quantify the hue, color and brightness of the object color, which lays the foundation for a comprehensive and accurate reflection and transmission of color information. In addition, there is the Lch color system. Third, the progress of dental color science In recent years, dental color science has made significant progress, especially in the colorimetric plate and computer colorimeter. When clinical colorimetry, the color of the natural periodontal environment has an obvious effect on the colorimetric color, especially the gums are closest to the teeth, the gum color contrasts with the tooth color, and the oral cavity between the upper and lower teeth is a darker background, all of which affect the accuracy of colorimetry. The gingival color plate introduced by Songfeng Company can reduce the influence of gingival and oral background contrast color in conjunction with the colorimetric plate. Gingival color plate has light, medium and dark three shades. Computer colorimeter The development of the color table color system and the improvement of the colorimetric plate have created important conditions for improving the colorimetric effect, but each physician and technician has different feelings about color, which also affects the correct determination of color and the accurate transmission of information, and still causes errors. Computer colorimeter better solve this problem. The computer colorimeter is composed of working head, microcomputer and printer. The working head is equipped with a small standard light source. When the colorimetric physician holds the body of the working head, let the probe touch the lip of the tooth being measured near the gum 2mm, press the switch 3 to 5 times, and the prescription can be printed out after 1 second. The contents include the shade, color, brightness of the tooth under test, as well as the type of shaded porcelain, body porcelain and enamel porcelain that should be used. The application of the computerized colorimeter excludes the influence of human factors on the color contrast effect.