Composition and test of gray balance before printing

Gray balance is as important for the prepress process as the printing process. The inks used in the printing process are not as pure as the phosphors used in the screen display or the colors used in some digital proofing.

When the cyan, magenta, yellow, and black inks are overprinted with the same amount of ink and dots, the final effect is not neutral gray but brown. This is due to the impure color of the pigment used in the four-color printing ink. In order to make a good balance and reproduce the neutral gray with the existing ink, SNAP requires a larger cyan dot and a smaller dot for the magenta and yellow dots. In this way, the three-color superimposition effect appears neutral grey to the human eye. .

When the color is converted from the RGB color gamut to the CMYK color gamut, the software's conversion characteristics must be carefully considered and tested. Each time the proofing, digital proofing or mock proofing, should include the gray balance target (see the gray balance for details).

The gray balance is used to measure whether the cyan, magenta, and yellow inks are suitable for the actual printing process. The specific method is to reproduce the neutral gray by the light-color superimposition of cyan, magenta, and yellow. In this three-color overprinted neutral gray, the blue dot is slightly larger than the Huanghe article dot (SNAP standard). During the entire scanning, proofing, exporting film and printing process, we must effectively monitor and adjust the dot gain/TVI values ​​to ensure the final neutral gray effect. Printed Neutral Grey is an important indicator to monitor whether the dot gain/TVI value of each color component is balanced. Therefore, accurate reproduction of neutral gray is very important for printing.

Neutral ash is evaluated by subjective comparison of whether the cyanine tri-color superimposition effect and the monochromatic black intermediate density are equal. Neutral gray evaluation must be conducted in a standard observation environment. Neutral ash can also be measured with a densitometer. When measuring neutral gray, the absolute value of each major measurement channel of the density meter should be approximately equal.

Each print should have a neutral gray block or a series of absolute solid and light patches. These control bars should exist on each page or on as many pages as possible. Each bit listed in the table is measured by each measurement channel of the density. In practice, the error between the measured value and the calibration value should not be greater than the positive and negative O.03.

In addition to the gray balance control bar, it is also preferable to have a color control bar on each print. The color control bar should contain the following: All four-color field blocks; all four-color 50% dots; any two-color overprint blocks; if space permits, it would be best to include 25% of all four colors And 75% of outlets. All control strip color areas should be large enough for measurement. The length, width or diameter of each patch should be no less than 3/8 inch. If it is newsprint printing or other special printing, the size can be properly reduced. The color control bar does not need to be made continuous or across the entire page width.

Frequently, corrections should be made using the Transmittance Density Controller and the processor to ensure that the dot area/threshold values ​​of the film balance control strips they output are correct. The gray balance control strip width should be large enough to ensure that the transmission and reflection densitometers can make the correct measurements. Densitometers should be routinely corrected daily to ensure data stability.

If the ink used is of insufficient purity, the density of the gray balance color patches can be adjusted from dark to bright on the press if necessary. For example, cyan inks and color inks all have yellow impurities that affect the density of yellow; therefore, it is possible to reduce the density of yellow ink, and the corresponding density of blue ink and ink must also be properly adjusted. When reading data using a densitometer, make sure to measure the same color patch in the control strip of the same position at each time, so that the error value can be correctly read.

When measuring neutral gray, the error between the measured values ​​of the three measurement channels of the densitometer should be no more than +/-0.03. When you start printing, follow these steps:

1. Position the page correctly;

2. Keep the black density of the color blocks in the gray balance control bar of the entire page uniform;

3. Use reflection densitometer for measurement and detection;

4. Adjust density when necessary;

5. Retest at the same location.

Observations of transmissions, prints, mechanical objects, proofs, or other artwork require certain lighting, equipment, and surrounding physical environments (see ANSI/PH2.30-1989). This standard stipulates that the surroundings, including floors, walls and ceilings, should be painted with Mensil N8/gray pigments. At the same time, the standard also stipulates that the illumination source should use a standard illumination body with a color temperature of 5000K and a unique spectral curve.