The basic principle of color reproduction in screen printing

Where color prints differ from monochrome prints is the color reproduction. The color on the manuscript is reproduced on the printed sheet using the relationship between light and color.

First, color classification and characteristics

Color comes from light, light is accompanied by coloration, and color has a close relationship with light.

(A) Shade three primary colors and color light additive method

Let a beam of sunlight shine into the dark room, shine through the slits on the prisms, pass through the prisms, and project on a white screen. A light consisting of red, orange, yellow, green, cyan, blue, and violet will appear. Band, this band is called spectrum. If the dispersion of white light by the prism is not sufficient, it can be seen that the three colors of red, green and blue each occupy 1/3 of the spectrum. If a series of color light synthesis experiments were performed, it was found that selecting “appropriate” red, green, and blue light to combine can simulate various colors in the natural world, so that red, green, and blue light are said to be the three primary colors of colored light. In order to unify the data on chromaticity, the International Commission on Illumination in 1931 stipulated that the wavelengths of the three primary colors of red, green, and blue were: 700.2 nm, 546.1 nm, and 534.8 nm (1 nm=1×10-7 cm).

If the three primary colors of light are added each two or three, the following color lights can be obtained.

Red (R) + Green (G) = Yellow (Y)

Red (R) + Blue (B) = Magenta (M)

Blue (B) + Green (G) = Blue (C)

Red (R) + Green (G) + Blue (B) = White (W)

The above formulas indicate that the brightness of the new color light obtained by the addition (mixing) of the color lights increases, so the mixing of the color lights is an additive method. By changing the mixing ratio of any two or three shades of light in the three primary colors, various colors of shades can be obtained. Light is electromagnetic radiation that acts on the human eye and causes bright vision. It has energy. The more the color light is mixed, the larger the light energy and the value are, and the brighter the colored light is.

If the three primary colors of red, green, and blue are respectively mixed with the three colors of cyan, magenta, and yellow, white light can be obtained.

Red light + blue light = white light

Green light + magenta light = white light

Blu-ray + Yellow = White

When the two color lights are added together to obtain white light, the two color lights are complementary lights. Therefore, the red light and the cyan light are mutually complementing lights, and the green light and the magenta light are mutually complementary lights, and the blue light and the yellow light are complementary lights.

(b) Three primary colors of pigments and subtractive colorants

If the yellow, magenta, and cyan pigments are mixed in "proper" proportions for each of the two colors, the color of the three primary colors of the shaded light can be obtained.

Yellow + Magenta = Red

Yellow + blue = green

Magenta + blue = blue

By changing the mixing ratio of the three colorants yellow, magenta, and cyan, various colors can be obtained by selectively absorbing and reflecting color light. However, any two or more colors are mixed, and yellow, magenta, and blue cannot be obtained uniformly. Therefore, the three primary colors of the color material are yellow, magenta, and cyan.

According to the relationship of the color light complementary colors, the hue exhibited by the three primary colors of the coloring material is the effect obtained by subtracting one kind of monochromatic light from the white light and obtaining another kind of color light. By subtracting (absorbing) the three primary colors red, green, and blue light from the white light, the complementary color light, magenta, and yellow that are subtracted from the light are called minus blue, and the magenta is called minus green. Green is called reducing red, that is, yellow, magenta, and blue can also be called three subtraction.

Since the color obtained by the addition (mixing) of the color materials is reduced in brightness, the mixing of the color materials is called a subtractive method.

The color rendering principle of the color subtracting method can also be expressed by the following formula.

Y+M=W-B-G=R

Y+C=W-B-R=G

C+M=W-R-G=B

Y+M+C=W-B-G-R=k(black)

The yellow, magenta, and cyan pigments are mixed together. Blue, green, and magenta light are absorbed by yellow, magenta, and cyan materials, respectively, and they appear black.

It can be seen that the yellow material and the blue material are mixed to give a black color, the magenta material and the green material are mixed to obtain black, and the colored cyan material and the red material are mixed to obtain a black color. When a certain color material is mixed with another color material and appears black, the two color materials complement each other. Therefore, yellow and blue are color complementary colors, magenta and green are color complementary colors, and cyan and red color complementary colors. The complementary color of the colorant appears black, and the complementary color of the color light shows white light. The two are the opposite. However, the three primary colors of the complementary primary color of the three primary colors of light, and the complementary colors of the three primary colors of the colorant are the primary colors of light, and therefore, the light and the color are There is a mutual connection, which has been cleverly applied in the color decomposition of color manuscripts.

(III) Non-color

Colors are divided into two categories, achromatic and color. Achromatic colors are black, white, and various shades of gray from darkness to brightest. They can be arranged in a series called black and white series. The change from black to white in the series can be represented by a gray band. One end is pure. Black, the other end is pure white. The material reflects all visible light, and the reflectance equals 100% pure white; the material absorbs all visible light, and the reflectance equals 0% is pure black. There are no pure white and pure black substances in actual life. The magnesium oxide can only be approximated as pure white, and the black velvet is close to pure black.

The achromatic color of the black and white series can only reflect the change in the light reflectance of the substance, and the visual sensation is a bright change.

When the surface of the print has a reflectance of 80% to 90% of the radiation of all wavelengths in the visible spectrum, the visual sensation is white. If the reflectance is less than 4%, it is black. White, black, and gray objects are not selective for reflections at various wavelengths of the spectrum and are called neutral features.

(four) color

Colors other than black and white series are called color. Any color is represented by three quantities: hue, brightness, and saturation.

1. Hue

Hue is the most basic feature of color. People call colors such as red, yellow, and green according to hue. Hue is determined by the color of light reflected from the surface of the object to the optic nerve of the human eye. The monochromatic light can be determined by the wavelength of its light. If the color of the light composition is mixed, the hue is determined by the ratio of the light amounts of the various wavelengths of the mixed light. For example, in sunlight, the printed surface reflects colored light having a wavelength of 500 to 550 nm, while the light that absorbs other wavelengths is visually green.

The hue can be represented by the shape of the spectral reflectance curve. The curve A indicates that the hue of the object is green, and the curve B indicates that the hue of the object is greenish blue.

2. brightness

Photometrically describes the brightness of a color as the value of light (ie, the energy of light). Can be measured with a photometer. It is generally believed that the reflectance of the surface of a color object is high, and their brightness is large. The hue of the two colors A and B is the same, but the brightness of the color A is larger than that of B, and thus the colors A and B are different visually. .

3. saturation

Saturation (also called chroma) refers to the purity of the colors. The various colors of light visible are the most saturated colors. The more the white light component incorporated by the spectral color, the less saturated.

The saturation of the object's color depends on the selectivity of the reflected spectrum of the surface of the object. The object has a high reflectivity in a narrow band of the spectrum, and the reflectivity of other wavelengths is low or no reflection. The saturation of this color is high.

1. The juxtaposition of outlets

The bright spot of color prints has relatively small dot coverage in the corresponding parts of the yellow, magenta, cyan, and black plates and the bright highlights of the manuscript. Dots are sparsely distributed, and the dots of the bright spot of prints are mostly in parallel.

When the yellow dot and the magenta dot are juxtaposed, white light shines on the yellow dot. The yellow dot absorbs blue light and reflects red light and green light. White light shines on the magenta dot. The magenta dot absorbs green light and reflects red light. Blu-ray. The four shades of light are mixed in space. According to the additive color method, red light, green light, and blue light are mixed into white light, and the rest is red light. If the distance between two outlets is small, they are very close to each other and the human eye sees red. In the same way, magenta and blue dot are tied to the human eye to see blue, green dots and yellow dots are tied to the human eye to see green.

When the two outlets are juxtaposed, the resulting color is shifted to one side of a large outlet, such as a large blue dot and a small red dot, and the resulting color is partially cyan.

When the three dots of yellow, magenta, and cyan are juxtaposed, the ink absorbs part of the color light, and the paper absorbs different shades of light. The reflected light is not 100%. When the distance between dots is small, the human eye sees gray.

2. Overlapping of dots

In the dark tone portion of color prints, the dot ratios of the corresponding portions of the yellow, magenta, cyan, and black plates and the original dark tone are relatively large, and the dots are dense, so that the dots of the dark tone portion of the print are mostly in an overlapped state.

When the magenta dots are superimposed on the yellow dots, the white light first shines on the magenta dots, the green light in the white light is absorbed, the red light and blue light are transmitted to the yellow dot, the blue light is absorbed by the yellow dots, and the magenta is transmitted through the magenta dots. The dots were shining white, and only red light was reflected from the surface of the paper. The red eyes were seen by the human eye. By the same token, Lu Hong and the Qinghai dot overlap to see what the human eye sees is blue, and the blue dot and yellow dot overlap to see the green color. When the yellow, magenta, and blue three-color dots overlap, the blue, green, and red light in the white light are absorbed, and the human eye sees black color.

The manner in which the dots overlap and reproduce the colors is affected by the transparency of the ink. The ink with low transparency has no color rendering effect, and the completely opaque ink can only be printed as the first color.

The middle part of the printed matter is rich in layers, and the way of color synthesis is that the dots are juxtaposed and the dot overlaps.

According to the principle that the dots are juxtaposed and the dots overlap, the amount of light absorbed by the ink, the thickness of the ink layer, the color density of the ink, and the order of printing will all affect the effect of color reproduction.

Magenta + Blue + Yellow = Black (K)