Color separation

Photographs and transmissions often contain many color information that can be printed and reproduced. In the prepress process, designers often send photos to a prepress service center with image scanning and color correction services. The prepress service center then returns a low resolution file for use in the page design. When high-resolution files are placed on the final page, proofs are produced. However, designers may find subtle color changes in the image. These color changes cannot be a problem with the operation of the scanning operator. It may be that the original image contains more colors and printing density than the original color ink can print, and the density is higher. However, proofing should simulate the color gamut of printed proofs. If we suspect that the image to be processed may be problematic, we should ask the pre-press center to send a proof using a high resolution image to determine if there is a problem with the proof. In the early production process, adjusting the image would be cheaper and easier. If our studio still uses photographic technology to work, choose a photographer who understands how prepress processing centers work to minimize problems using test photography technology. Remember, the scanned image must be converted from RGB format to CMYK format before being used for proofing, and then inserted into the page layout software and sent to the service provider or printer. The best result is that when RGB is converted to CMYK, it is processed using a color management system. There should be a difference of 30 degrees between the screen angles of the primary colors of the color separation screen, and only the yellow version can have an angle difference of 15 degrees from the others, and this angle is between the cyan and magenta versions, or Between the cyan angle and the black angle. According to the main tone of the image, as long as the two versions of the screen number change, we can also accept the effect of setting the yellow version to the same angle as the cyan or black version. Although, the software may use slightly different algorithms when calculating the angle of the network, the angle between the angles can reach approximately 30 degrees. Normally the main color is best at 45 degrees (in most cases, the color is the main color). If the copy of the image, the use of full-color removal of the GCR method, then, the black version becomes the main color version, then its angle should replace the magenta, become 45 degrees. In two-color printing, the angle of the two screens should be different by 30 degrees. At this time, the angle of the black screen should be arranged at 45 degrees. On film, on proofs and during printing, screen angles and screens should use this method without a moiré effect. In general, electronic files should not include settings for screening parameters and dot shapes. When a print or film is produced directly by a printer using an electronic file, the printer must take full responsibility for the problem of moiré and, in any case, help solve the problem. If we need to use special angles to prevent the appearance of moire when we are making, we must inform the printing staff. Color Control Strips To facilitate the control of color on the press, we generally need to place the color control strip in the non-graphic area of ​​the entire printed sample sheet and perpendicular to the direction of the paper motion. The width of the color control strip varies from 1/8 inch to 1/2 inch. Larger signal bars are more useful, but require more range of non-image parts. The color control strip includes some image elements. In the recurrence pattern, the purpose of these image elements is to detect variables that affect the printed color effect. It typically includes a solid patch of each primary color and black, and several halftone patches. 50% tone blocks for each color, for dot gain value testing; 75% tone blocks and solid color blocks are required to test the print contrast; 25% tone blocks are used primarily to measure 1/4 tone areas; Solid color blocks are used to measure the degree of ink ink. The primary color ink can be used to measure the color difference and ashiness of the primary color ink; the two-color color overlay can be used to measure the ink overprint rate and the second tone condition. The tri-color overprint gray block with the correct tone value can be easily used to control the gray balance. The tri-color superimposed gray block with the correct dot is placed adjacent to the black tone block. When the gray balance of the tri-color superimposed color block changes compared with the black block, the printing operator can easily visually detect it. Densitometers can also measure changes in gray balance. Changes in gray balance indicate that the balance of colors is also changing. To ensure a stable product, color balance is the most important control variable. Include some special micro-line images in many control strips to test the ghosting during printing. Unwanted ghosting will increase the amount of dot gain, which will increase the density of midtones. The current color control bar also includes a blank "paper" block, which is a method of testing the same area on paper. Paper testing is often defined in terms of density (using blue filters) or chromaticity, and is used in some other density calculations such as dot area rate, print contrast, and ink overprint. Gray Balance Gray Balance refers to the ability to use the correct mix of yellow, magenta, and cyan to replicate pure neutral blacks and produce varying amounts of neutral grey. If the gray balance can be maintained throughout the processing of the job, the printed color gamut can be achieved. The factors that affect the gray balance are color density, color purity, dot gain, paper, and the observer's color perception. These factors are true for printing proofs and proofs. The gray balance can be measured using the 25%, 50%, 75% dot black color on the color control bar, and the 25%, 50%, 75% neutral tone pad of the tricolor overprint gray block. The following is an example of the tone of a typical color control bar: BK CMY 75% 75% 63% 63% 50% 50% 39% 39% 25% 25% 16% 16% If you use visual observation of the ash balance in the process, Standard observation conditions must be used during work: D50 lighting conditions and neutral viewing environment. Dot gain value The dot gain value is a significant increase in dot size or tone value, which can be obtained by comparing the measured value on the printed sample with the size defined in the digital file or the size measured on the color separation film. This expansion is caused by two types: optical expansion and mechanical expansion. * Optical Dot Augmentation Optical Dot gain is due to the absorption and scattering of light by the paper. The light spreads along the edges of the dots, making the tone appear darker. In this way, the apparent effect and test value of the dot will be larger than the actual physical size observed using a magnifying glass. The optical network point enlargement reaches the maximum in the midtone region where the perimeter of the network reaches the maximum. * The physical changes in the size of the expanded dots of mechanical dots are the film and printing exposure, the amount of ink, the length of the pigment, the viscosity of the ink, the adhesiveness of the ink, the printing color sequence, the balance of ink and ink, the temperature of the ink, the blanket, the plate and the impression cylinder The results of the combination of pressure, plate type, ghost blur, paper surface smoothness, absorbency, and whiteness. The physical change in the dot size is called mechanical dot enlargement. The amount of dot gain varies with the type of paper and the number of screen lines used. The amount of printed dot gain for non-coated paper or newsprint is relatively large. In the past, printers could use the number of coarse screen lines that produced a low number of dot gains to compensate for the problem of large dot gain in low-quality paper. Offset printing is generally easy to control when the dot gain value of all the primary colors is balanced. The print sequence of Cyan, Pin, and Yellow makes it more transactional to produce more uniform dot gain values ​​between colors. It is also helping print operators maintain gray balance and color balance when printing. When testing for dot gains, the standard practice is to first test the apparent dot area rate. This area rate includes dot gain for various reasons and is calculated using Murray-Davies (Mary Davis's formula). The dot gain value is then obtained by subtracting this value from the input value on the film or electronic file. For example, if the measured apparent dot area ratio is 78%, and the dot area ratio on the film is 50% by measurement, the dot gain for this color is 28%.