Offset production standardization (1)

First, the basic concept

If the separation products and prints are to be sold as goods, standardization is very important. In this way, the dichroic sheets from different workshops can be placed on a plate and printed with the same ink supply. This is done on the assumption that the pre-proofing done by each workshop is based on uniform standards. In practice, however, you can find that the color of each proof is so different that it is impossible to put it together on one plate.

1. Printing standardization should be pre-defined items.

Paper: roughness, gloss, color requirements, optical density, gray requirements, whiteness and its range, shades of light requirements.

Density range: Solid density Ds for each color and overprint.

Dot tones: Dot gain and gray balance.

2. Related concepts.

1Ds density in the field, this symbol is a universal symbol in the printing industry.

2 gray balance. When the relationship between the coverage of the three primary colors ink yellow, magenta, and blue is appropriate, they can be mixed into a neutral gray (from bright gray to black). The eye is most sensitive to the identification of neutral grey, and even small color distortions can be identified. However, the recognition of other complex colors by the eye, such as brown-green and blue-green, is completely different. Only when these colors are compared with the samples can the color difference be judged.

3 gray levels. This means that the drop in color brightness can be regarded as an increase in color in black. As the thickness of the ink layer increases, the color of the color will increase, such as the blue ink contained in the ink fountain, it looks almost black. In the density measurement, the gray scale becomes apparent as the density of the sub-density increases. Paper with low quality has a higher density of colors, because paper absorbs more strongly. Grayscale is compensated by electronic masks during color correction.

4 points density. When measuring the density of a certain basic color, the density value measured by two other filter packs except the complementary filter is used. Density values ​​are generally greater than zero. For example, the main density of a cyan ink is Dc=2.30, and the densities are Dm=0.83 and Dy=0.41, respectively. The densities measured on rough paper are larger than coated paper. Therefore, the density should be corrected by masking according to the paper type during the color separation stage. Only when the subject matter of the printed image is in a grey atmosphere can it be printed on poor quality paper. For example, a three-color ink combination with Y=100%, M=50%, C=0% overprinting, printing on coated paper can reproduce a bright and bright orange red, and can only reproduce brown on gray paper. Because the sub-density plays a role in this higher blue component.

If you don't have a colorimetric instrument to use and must work with a densitometer (such as Bruner and Macbeth AutoPilot, DuPont's Print Expert), then you can cleverly use the concept of neutral gray, neutral in the color space It is a fixed point.

It is, in principle, impossible to define a neutral gray with a densitometer based on the results of the three filter measurements. When the paper type and the ink are different, using a densitometer to measure a neutral and timely, it is impossible to get three identical numbers, but the eye can define a neutral ash very accurately, and then you can put the densitometer in this ink - paper combination The neutrality was calibrated.

The significance of neutral gray for color separation during color separation: whether it is a silver salt film slide ruler or a photosensitive paper reflection gray scale ruler as a manuscript, it must be properly controlled so that it can still be used after multicolor printing. Neutral gray reproduction. Currently, color separation adjustments have been very successful in the total reproducible color space. Recently, there have been some new cases of multicolor reproduction. For example, in the non-colored structure, the black in the image is no longer synthesized with three color inks, but is printed with black ink. At this time, the original manuscript for color balance is unnecessary. Because the black in the original will only be copied with black ink in the future, the balance control of the combined colors is no longer necessary. What people need is a multicolored test strip.

The determination of the solid density of two and three primary inks

Determining a solid density for each trichromatic ink is not sufficient because a printing color is a combination of the color of the ink itself, the thickness of the ink layer, the absorption of the ink by the paper, and the gloss. If the ink is a cover color, determining the density on the ground is very simple, because there is a certain influence in addition to the gloss, and other problems no longer exist. This definition can be given for transparent colors and covered colors:

Transparent color: These colors allow a portion of the visible spectrum to pass through almost unimpededly (ink allows 2/3 of the visible spectrum to pass through).

Cover color: In the visible spectrum, no light reaches the printing surface through the ink layer.

If it is a transparent color, when the thickness of the ink layer increases, a certain gray level will be generated. When the density measurement is measured with a densitometer, the measured gray level variation is inconsistent with that seen by the human eye, and the color measurement can be used to obtain a visually consistent result. . Increasing the thickness of the ink layer is not only reflected in the improvement of the saturation, but also in the shift of the entire chromaticity value. When the chromaticity parameters are used to express the color characteristics of the ink, besides the hue and the saturation, the brightness is also an important parameter. .

When the ink is printed on different papers, the gloss will be different. As for the evaluation of gloss difference, modern colorimetry is not completely understood and glossiness analysis can only be performed by changing the geometric condition of measurement (0° to 45°). In the case of diffused lighting and directional lighting, the difference in glossiness can be visually judged.

The color change of an ink depends on the following parameters: ink layer thickness, printing material, printing method (handprint, embossing, waterless bucket solution, whether heat drying is used).

There are two ways to determine the color in the field: one is to judge according to the vision of the chromatogram, and the other is to establish the standard field density, and the field color density has a range. Within this range, the printing contrast has the maximum value, and the solid density at that time is used as the standard solid density.

The method of seeking a standard field based on relative contrast is summarized as follows:

The proofs are printed with different ink layer thicknesses. The proofs have solid blocks and dot pieces. Then the i-th paper has a solid density Di and dot density DRi. The color sequence of the printing experiment should be pointed out. Then look for the greatest contrast in which field density and site tone values. When the thickness of the ink layer is gradually increased, the contrast gradually increases until the dot starts to increase. In the extreme case, the contrast is reduced again to zero.

The so-called contrast refers to the relationship between the two brightness, such as the relationship between the actual printed surface and the white paper reflectivity:

K=R1/R2

People are accustomed to expressing the contrast using the above formula:

U=D1-D2

D1 in the above formula may be the density of the printed solid surface, and D2 may be the density of the paper surface.

Let D = -lgR

U=D1-D2=-lgR1+lgR2=lg(R1/R2)

U=-lg(K)

The concept of relative contrast is different from contrast. Its definition is:

Kr=(Ds-Dr)/Ds

The maximum value of Kr can not be obtained by differentiating the above formula, because when the density of solids Ds increases, the change of Dr is unknown, so it can only be determined through experiments.