Chapter II: Principles of Print Copy

Section 1: Colors and Colors A color's classification and characteristics The whole process by which a person produces a color sensation is: Light shines on a colored object. Reflected or transmitted light is accepted by the human eye and is recognized by the brain, giving rise to the four elements of color perception. It can be divided into two parts: objective stimuli (light source, color objects) and subjective sensations (eyes, brains). They are unified in the process of formation of color sensations. Both exist at the same time and are indispensable. Color can be divided into color and non-color. Non-color means white, black, and shades of gray. They form a series, from white to light gray, medium gray to dark gray, until black, known as black and white series. Pure white is an ideal total reflector with a reflectivity of 100%; pure black is an ideal reflector-less reflector with a reflectivity of zero. In nature, such pure white and pure black do not exist. Therefore, it is stipulated that when the reflectance of the surface of visible light is 80 to 90% or more, the object is white; when its reflectance is 40/ Below 0, the object is white; when the reflectance is below 40/0, ​​the object is black. All colors other than color are called color.
The three properties of the two colors, in order to analyze and distinguish the various shades of light and colors, have unifiedly defined the three attributes of the color in the world, namely, hue, lightness, and saturation. 1 Hue (hue).
Hue is a feature that distinguishes colors from each other. The different wavelengths of light in the visible spectrum are visually represented in various hues, including red, orange, yellow, green, cyan, blue, and violet. It is generally said that the three hue of red, orange, and yellow are warm colors, the cyan, blue, and purple are cool colors, and the green is the middle colors. The hue of the light source is determined by the human eye's perception of the spectral components it radiates; the hue of the object is determined by the human eye's perception of the spectral components of the light source and the wavelengths of radiation reflected (transmitted) at the surface of the object. For example, in sunlight, an object reflects radiation in the wavelength range of 480 to 560 nm and absorbs radiation at other wavelengths, the surface of the object is green. 2 Lightness.
Brightness refers to the degree of visual lightness produced by the stimulation of light. Since the color brightness is a psychophysical quantity, it is related to the illumination of the entire field of view, in addition to the influence of the reflection or transmission of light energy by the object itself. In an achromatic series, the closer the object color is to white, the higher the reflection of light and the higher the lightness, whereas the closer to black, the lower the reflectivity to light and the lower the lightness. For a colored object, the higher the light reflectivity of its surface, the higher its lightness and the brighter the color. For example, the surface of a tan colored object has a higher spectral reflectance than that of a red object and can produce a higher eye The brightness is bright, so the tan-brown object is brighter than the red object.
3 Saturation. Saturation refers to the purity of the color, which indicates the degree to which the object reflects or transmits light. The monochromatic light in the visible spectrum has the highest saturation.
In the color substitution law, it is pointed out that any color C can be matched by a certain amount of a mixture of monochromatic light Cλ with a wavelength λ and a certain amount of white light W. Formulated as:
C = Cλ + W
Therefore, the saturation of one color can be understood as the proportion of the monochromatic light that matches the color to the total color. S represents saturation, then S=Cλ/C=Cλ/(Cλ+W). For achromatic color, there is only difference in brightness, and there is no saturation of color saturation.
Section II: The principle of color rendering One-color light addition 1. The three primary colors of light. Different colors can be used to form a variety of colors in the vision, and colors that are not mixed by other two primary colors of light can be called primary colors of light. The three primary colors of light are green and blue. The specific wavelength of the three color lights specified by the International Standard Lighting Committee (CIE) in 1931 is: Red light (R) 700 nm
Green light (G) 546.1nm
Blue light (B) 435.8nm
Various primary colors in nature can be mixed by a certain proportion of these three primary colors of light.
2 color light additive method. According to the principle of additive color mixing of the three primary colors of red, green, and blue, a method of generating a new color light is a color light additive method.
In the area where blue light and green light overlap, the color is cyan, the overlap of blue light and red light is magenta light, and the overlap of green light and red light is yellow. The mixing of red and green light is used here to explain the principle of color light addition. We use two shades of light: one is the green light obtained by passing through the green filter, and the other is the red light obtained by passing through the red filter. The yellow light can be obtained by mixing these two colors of light. This is because the red and green systems in the eyes feel the same stimulus and feel yellow. It can be seen that not only the monochromatic light of a specific wavelength can stimulate the sense system to produce a yellow feeling, but also a sense of yellow can be produced when the sense values ​​of red sense and green sense are roughly equal. When different wavelengths of light are mixed, the spectral frequencies add up to produce a new, brighter shade of light. Therefore, the color light mixing is called additive color mixing. The regularity of color light additive color mixing can be expressed by the following figure.
Using the color equation can be expressed as:
Red + Green = Yellow Green + Blue = Green Blue + Blue = Magenta + Green + Blue = White Dichromatic Subtractive 1 Trichromatic primary colors. The three primary colors used for subtractive coloration are called the three primary colors of the colorants, and these three primary colors should have properties in which the two primary colors cannot be produced regardless of the mixing ratio. The three primary colors of pigment should be cyan, magenta, and yellow. Each of their colors should be able to subtract one-third of the white light and reflect or transmit two-thirds of the light. The magenta color magenta is often referred to as subtractive green, yellow is called minus blue, and cyan is called minus red. Because each color of subtractive primary colors is equivalent to the color of white light minus one spectral color, this name and the name of additive primary colors can all correspond.
2 Color subtractive method. Refers to the yellow, magenta, and blue primary pigments (such as pigments, inks) subtractive color mixing principle color method. We added red transparent paper to the incandescent lamp, and the light was red, as if the red transparent paper had a white color, which was actually the opposite. We know that only when the three kinds of color cells on the human retina receive the same amount of intense stimuli at the same time they have white sensation. White light is the compound of multiple shades, red transparent paper absorbs white light, and other shades only Red light gives a red feeling. If a yellow filter is used in place of the red transparent paper, not only the red light but also the green light are emitted, so that the yellow and green cells on the retina are stimulated at the same time, resulting in a yellowish feeling. It can be said that white light subtracts blue light and leaves red and green light to produce a yellow sensation. The effect of subtracting one or more color lights from white light (or composite light) to obtain another color light is called the subtractive effect, that is, the color reduction method.
If the red, green, and blue filters are stacked on top of a white light source, no light passes through. Because the wavelength range of the light filtered by the three color filters is irrelevant, that is, no one filter can transmit the light transmitted by the other two filters. The same test was performed using cyan, magenta, and yellow filters, and the situation was different. Because each color filter passes two-thirds of the color in the spectrum, other colors can be created if they overlap on the same light source. Any two color filters combined to subtract light through one of the three primary colors of light. For example, a cyan filter subtracts red light from white light, and a magenta filter subtracts green light. When the two filters are stacked together and the yellow filter subtracts blue light, all lights are Subtracted.
The cyan, magenta, and yellow pigments are mixed in equal amounts, and the color condition is similar to that of the color filter. For example, a cyan pigment and a yellow pigment are mixed to produce green, and an overlap of a love filter and a yellow filter also produces green. The essence of subtractive coloring is the fact that pigments or other colored substances selectively absorb certain color light in the white light spectrum, resulting in the effect of subtracting certain color light from white light. The color to be observed by the human eye is the white light minus. The shade of light left after some shades of light. The subtracted color light and the remaining color light complement each other.
The subtractive method shows that white light is the source of color, and all colors are contained in white light. Tri-complementary color law White light can be decomposed into three primary colors of red, green, and blue, and red, green, and blue colors of light combine to become white light. This shows that there is a certain link between colored light and white light.
Complementary color refers to two complementary color lights that can generate white light or gray light after additive color mixing; after subtractive color mixing, two complementary colorants of black or gray color are generated. That is, blue and red are complementary colors, magenta and green are complementary colors, and yellow and blue are complementary colors. Section III: Color copying principle Separation principle Separation is the process of decomposing a colorist manuscript into a monochromatic version. The principle of color separation is to make use of the selective absorption of red, green, and blue color filters to produce a complementary color version of the three primary colors of color light, namely, yellow, magenta, and blue. The color filter is a transparent medium that selectively absorbs and transmits visible light, and its role is to absorb only two colors of light (color light) by using one color light (primary light) of the three primary colors, so that only one color light is felt on the photosensitive sheet. The other two colors cannot be sensitive. The two kinds of non-photosensitive color light constitute a complementary color, that is, the medium color of the three primary colors of the color material. The photosensitive sheet is exposed and developed. After fixing, it forms a dichroic shade with shades of shades (the tone and gray tone are opposite to the object being copied).
The schematic diagram of the principle of color separation is a schematic diagram of the principle of color separation, which includes 8 colors that constitute the entire hue. When white light illuminates the original, the original may reflect or transmit a variety of colors. If a blue filter is inserted in front of the camera's lens so that the blue portion of the document is transmitted through, the latent image formed on the photosensitive sheet undergoes development processing to become a black image (formation density), while other portions are transparent. , as shown in Figure A1. After copying the positive image with this photographic film, the density of the green, yellow, and red regions on the positive image is large, as shown by A2 in the figure. Therefore, the yellow version (A3 in the figure) was obtained after printing with this positive picture.
When color separation is performed using a green filter, only the portion of the original that reflects or transmits green light passes through the filter and the lens, and the photosensitive material is exposed to light to form a high-density area, and red and blue portions are included in the photosensitive sheet. It is almost transparent, as shown in Figure B1. Use this color separation sheet to copy the positive image (as shown in Figure B2), and then use this positive image to print, and obtain a magenta version (Figure B3).
When the red filter is mounted in front of the lens, only red light reflected or transmitted through the original is transmitted. The green and blue light are absorbed by the color filter. After the exposed photosensitive sheet has been developed, the corresponding black portion of the original is changed. It is black, and the places containing green and blue are transparent. As shown in Figure C1, copy this photocopy into a positive image. The green and blue parts on the positive figure will have a high density (Figure C2), and then copy the positive picture. The printing plate was made; the cyan plate (Figure C3).
After separation by the color filter, the color original is decomposed into three separate shades. Each of these separations can only represent certain parts of the manuscript. These three Yin pictures have different densities in different parts of the screen. The size of the density is determined by the amount of light reflected and transmitted by the document and each portion. By overlaying three shades of images together, the color and tone of the original can be reproduced, as shown in Figure G.
Section IV: Color reproduction principle of color reproduction First, the necessity and possibility of printing dot images The dots are the pixels that make up the dot image. The density and color of the original can be reproduced by changing the area and/or amount of ink.
Consecutively tuned images are reproduced using images composed of dots, which is a commonly used copying method. When copying a continuous original, the continuous-density image is converted into a half-tone image consisting of halftone dots through a screen printing process. Prints such as lithographic printing and letterpress printing use dot-line images to reproduce continuous-tone images. Because only the ink and ink are not on the page, two levels of black and white are reproduced (as shown in Figure 2-24). When the amount is the same, the dots with different sizes can adjust the total ink amount per unit area; when the number of dots in the unit area is the same, the dot size is different, and the inked area rate is also different. The smaller the dot, the smaller the inked area rate and the lower the density.
For halftone images, their density when viewed beyond a certain distance