The basic principle of color reproduction in the m

2022-07-25
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The basic principle of color reproduction in silk printing

the difference between color printing and monochrome printing lies in color reproduction. The color on the original is reproduced on the printing sheet by using the relationship between light and color

1. Color classification and characteristics

color comes from light, and light is accompanied by coloring. Color is closely related to light

(I) three primary colors of color light and color light additive method

let a beam of sunlight shine into the dark room, shine on the prism through the slit, and then project it onto the white screen, showing a light band composed of red, orange, yellow, green, blue, and purple. This light band is called spectrum. If the dispersion of white light by the prism is not sufficient, it can be found that red, green and blue light each account for 1/3 of the spectrum. If a series of color light synthesis experiments are conducted, it is found that the proper combination of red, green and blue light can simulate various colors in the natural world, so red, green and blue light are called the three primary colors of color light. In order to unify the chromaticity data, in 1931, the international lighting Commission stipulated that the wavelengths of red, green and blue primary colors were 700.2nm, 546.1nm and 534.8nm (1nm=1 10-7cm)

if the three primary color lights are added every 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) = green (c)

red (R) + green (g) + blue (b) = white (W)

the above formulas show that the brightness of the new color light obtained by the addition (mixing) of color light increases, so the mixing of color light is called additive method. By changing the mixing ratio of any two or three kinds of primary color lights, different color lights can be obtained. Light is the electromagnetic radiation that acts on human eyes and causes bright vision. It has energy. The more color light is mixed, the greater the light energy and value, and the brighter the color light is formed

if the three primary colors of red, green and blue are mixed with the three colors of green, magenta and yellow, their common feature is that they generally adopt new materials, new processes and new technology white light, that is,

red light + green light = white light

green light + magenta light = white light

blue light + yellow light = white light

when the two colors are added to obtain white light, the two colors complement each other. Therefore, red light and green light are complementary to each other, green light and magenta light are complementary to each other, and blue light and yellow light are complementary to each other

(II) pigment three primary colors and pigment subtraction method

if yellow, magenta and cyan are mixed in an appropriate proportion, the colors of the three primary colors of color and light can be obtained, that is,

Yellow + magenta = red

Yellow + cyan = green

magenta + cyan = blue

the mixing proportion of yellow, magenta and cyan can be changed to different colors due to selective absorption and reflection of color light. However, if any two or more colors are mixed, yellow, magenta and cyan cannot be obtained. Therefore, the three primary colors of the pigment are yellow, magenta and cyan

it can be seen from the relationship between color and light complement that the hue of the three primary colors of the pigment is the effect of subtracting a certain monochromatic light from white light to obtain another color light. Subtracting (absorbing) the three primary colors of red light, green light and blue light from white light respectively, we can get the complementary colors of blue, magenta and yellow. Therefore, yellow is called subtractive blue, magenta is called subtractive green, and green is called subtractive red, that is, yellow, magenta and green can also be called subtractive three colors

the lightness of the color obtained by the addition (mixing) of pigments decreases, so the mixing of pigments is called subtractive method

the color rendering principle of pigment subtraction 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)

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

it can be seen that black is obtained by mixing yellow and blue materials, black is obtained by mixing magenta and green materials, and black is also obtained by mixing cyan and red materials. When a certain color is mixed with another color to show black, the two colors are complementary to each other. Therefore, yellow and blue are complementary to each other, magenta and green are complementary to each other, and cyan and red are complementary to each other. The two colors are just the opposite. However, the three primary colors of light are the three primary colors of the complementary color material, and the complementary colors of the three primary colors of the color material are the three primary colors of light. Therefore, there is an interaction between light and color, and this connection has been cleverly applied to the color decomposition of color originals

(III) achromatic

colors are divided into achromatic and color. Non color refers to black, white and various grays from dark to brightest. They can be arranged into a series, called black and white series. The change from black to white in this series can be represented by a gray band, with pure black at one end and pure white at the other end. The material reflects all visible light, and the reflectance is 100% pure white; The substance absorbs all visible light and is pure black when the reflectivity is equal to 0%. In real life, there are no pure white and pure black substances. Magnesium oxide can only approximate pure white, and black velvet approaches pure black

the non color of black-and-white series can only reflect the change of light reflectivity of the material, and the visual feeling is a bright change

when the reflectivity of the surface of the printed matter to the radiation of all wavelengths of the visible spectrum has increased from 20 two years ago to 28 now, the visual feeling is white. If the reflectivity is less than 4%, it is black. White, black and gray objects have no selectivity for the reflection of various wavelengths of the spectrum. They are called neutral features

(IV) color

various 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 according to hue, such as red, yellow, green, etc. The hue is determined by the color light reflected from the surface of the object to the optic nerve of the human eye. Monochromatic light can be determined by its wavelength. If the color is composed of mixed light, the hue is determined by the proportion of the amount of light at various wavelengths of the mixed light. For example, under sunlight, the surface of a printed matter reflects color light with a wavelength of 500 ~ 550nm, while absorbs color light with other wavelengths, the visual impression of the printed matter is green

hue can be expressed by the shape of spectral reflectance curve. Curve a indicates that the hue of the object is green, and curve B indicates that the hue of the object is green and blue

2. brightness

in photometry, the brightness of color is described as the value of light (that is, the energy of light). It can be measured with a photometer. It is generally believed that the high reflectance of the surface of color objects will lead to the high brightness. The hues of a and B are the same, but the brightness of a is greater than that of B. therefore, there are differences between a and B visually

3. saturation

saturation (also known as chromaticity) refers to the purity of color, and various visible monochromatic lights are the most saturated colors. When the spectral color is added with more white light components, the more unsaturated it becomes

the saturation of an object's color depends on the selectivity of the reflected spectral radiation on the object's surface. If an object has high reflectivity to a narrow band of the spectrum, but has very low reflectivity to other wavelengths or does not reflect, the saturation of this color is high

1. juxtaposition of dots

the dot coverage of the highlighted part of the color print in the corresponding parts of the yellow, magenta, cyan and black printing plates and the original brightness is relatively small, and the dot distribution is sparse, so the dots of the highlighted part of the print are mostly in the juxtaposition state

when the yellow dot and magenta dot are juxtaposed, white light shines on the yellow dot, and the yellow dot absorbs blue light and reflects red and green light; White light irradiates the magenta dot, which absorbs green light and reflects red and blue light. The four colors are mixed in the space. According to the color light plus color method, red light, green light and blue light are mixed into white light, while the rest is red light. If the distance between two points is very small and they are very close to each other, the human eye will see red. In the same way, magenta and green dots are juxtaposed to see blue, and green dots and yellow dots are juxtaposed to see green

when two dots are juxtaposed, the color produced is biased to the side of the big dot. For example, when the big green dot and the small LV Hong dot are combined, the color produced is biased to cyan

when the yellow, magenta and cyan dots are juxtaposed, because the ink absorbs part of the color light, the paper also has different absorption of the color light, and cannot reflect 100% of the color light. When the distance between the dots is very small, the human eye sees gray

2. superposition of dots

in the dark tone part of color printing, the dot rate of the corresponding parts of the yellow, magenta, cyan and black plates and the original dark tone is increasing at a rate of 20% every year, and the dots are dense, so the dots of the dark tone part of the printing are mostly in the superposition state

when magenta dots are superimposed on yellow dots 3 After the equipment installation and commissioning, the white light first shines on the magenta dot, 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 dot, and the magenta dot is irradiated to white, and then only the red light is reflected from the paper, and the red light seen by the human eye. In the same way, the human eye sees blue when LV Hong and green dot overlap, and green when green dot and yellow dot overlap. When the yellow, magenta and cyan dots are superimposed together, the blue, green and red lights in the white light are absorbed, and the human eye sees black

the method of dot superimposed reproduction of color is affected by the transparency of the ink. The ink with low transparency has poor color rendering effect, and the completely opaque ink can only be used as the first color for printing

the middle tone part of the printed matter is rich in layers, and the way of color synthesis is both a little parallel and a little overlapping

according to the principle of color reproduction by point juxtaposition and point superposition, the amount of color light absorbed by the ink, the thickness of the ink layer, the color concentration of the ink, and the printing sequence will all affect the color reproduction effect

magenta + cyan + yellow = black (k)

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