1. The concept of printing quality Printed matter is a commodity and artwork that relies on visual arbitration. When people comment on the quality of printing, they always involuntarily think of the three factors of aesthetics, technology and consistency. This method of thinking about the problem is a combination of human visual psychological factors and physical factors in the replication project, that is, considering not only the commodity value or artistic level of the printed matter, but also the impact of the printing technology itself on the quality of the printing quality . But practice has proved that the technique of evaluating the quality of printing from the perspective of commodity value or art is not perfect. Such evaluations often fail to reliably express the copy quality characteristics of prints. Only by evaluating from the perspective of printing technology can the print quality be correctly evaluated and the differences can be unified. This view is endorsed by most experts at home and abroad. The above-mentioned printing quality concept is rich in connotation and extension. For example, for books, the binding quality should also be included in the category of "printing quality". However, considering the quality of printed matter so widely, it is often inconvenient and unnecessary from the perspective of printed image reproduction technology. G. W. Jorgensen and others pointed out that the aforementioned definition of print quality is not accurate enough. From the perspective of copying technology, they define print quality as "loyalty to copying manuscripts." This definition method is convenient for researching the printing and copying process and evaluating the quality of each stage of printing and copying. Based on this consideration, this book distinguishes the quality of the printed matter and the quality of the printed image into two different concepts, that is, the quality of the printed image is defined as "the faithfulness of the printed image to the original copy". Color Copy Paper is developed and produced in the office copy machine stained
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A · C · Zettlemeyer and others once defined such a definition as "quality of printed matter"; the quality of printed matter is the comprehensive effect of various appearance characteristics of printed matter. From the point of view of printing technology, the so-called appearance characteristics of printed matter is a broader concept and has different connotations for different types of printed products.
For line or solid prints, the ink color should be thick, uniform, good gloss, no text, high definition, good overprint accuracy, no overprinting and excessive back protruding, no back rubbing, etc.
For color dot prints, the tone and color reproduction should be faithful to the original. The ink color is uniform, the gloss is good, the dots are not deformed, the overprint is accurate, and there are no ghosting, through-printing, various bars, backside stickiness and mechanical marks.
The overall effect of these appearance characteristics reflects the comprehensive quality of the printed matter. In the evaluation of print quality, various appearance characteristics can be used as the basis for comprehensive quality evaluation, and of course can also be used as the basic content and requirements of print quality management.
Second, the concept of printed image quality
Compared with the definition of printing quality, the definition of printed image quality narrows the scope of the discussion, so that the printed image can be regarded as a collection of information of a certain amount of single pixels with light and dark and color changes on two-dimensional or three-dimensional drawings (note , Text can also be processed as graphic information, so text can also be viewed as an image). Print image quality includes at least two aspects: image quality and text quality. The characteristic parameters expressing image quality and text quality are now described as follows. These quality characteristic parameters can be used not only as evaluation parameters in quality evaluation, but also as target parameters in quality management.
1. Image quality characteristic parameters. It can be divided into the following four types: tone and color reproduction, image resolution, moire and other fault patterns and surface characteristics. The following is the description in this order.
Tone and color reproduction refer to the situation where the tone balance and color appearance of the printed and copied image correspond to the original. For black-and-white copying, the density correspondence between the original and the copy is usually used to express the tone reproduction (copy curve). For color reproductions, hue, saturation, and lightness values ​​are more meaningful.
The gradation and color reproducibility of printed images are not only affected by the inherent characteristics of the inks, printing materials and actual printing methods used, but also often by economic constraints. For example, in multi-color printing, a high-fidelity printing process can achieve a relatively high copy quality, but that will be at the cost of increased costs. Therefore, for the prints with the theme of the screen, the so-called best reproduction of tone and color is within the various constraints and capabilities of the printing device, and the various requirements of the original manuscript are integrated, resulting in what most people think is high The process and technology of quality printed images. The issue of optimal tone and color reproduction will be described in more detail later.
The image resolution problem in the best reproduction includes both resolution and clarity. The resolution of the printed image mainly depends on the mesh line number, but the mesh line number is limited by the printing material and printing method. The number of screen lines that can be distinguished by the human eye can reach 250 lines per inch, but in actual production, the highest number of screen lines cannot always be used. In addition, resolution is also affected by changes in registration. Sharpness refers to the contrast on the edges of the tone. On the color separation machine, through the electronic enhancement method, the sharpness of the image can be adjusted. However, people do not yet know what the best level of clarity is. If too much enhancement is made, images such as landscapes or portraits will look different from reality, but images such as fabrics and mechanical products can improve performance and appeal.
Moire, bars, graininess. Water stains, ink spots, etc. will affect the uniformity of the image appearance. In the dot image, some moire patterns (such as rose-shaped) are normal, but when the angle of the mesh is deviated, bad moire patterns will be generated, and there are many factors that affect the graininess of the image. Paper smoothness, printing The grain size of the version is related to the graininess of the image. From a technical point of view, in addition to moire and grain patterns, one can make most other spots and fault patterns that cause unevenness close to zero.
The surface characteristics of the printed image include gloss, texture and flatness. The requirements for gloss depend on the nature of the original and the end use of the printed image. In general, when copying photographic originals, it is better to use high-gloss paper. In actual printing, it is sometimes necessary to use bright oil to enhance the gloss of the subject image. A high degree of gloss will reduce light scattering on the surface, thereby enhancing color saturation and darkness. However, when using high-gloss paper to reproduce watercolors or pencil drawings, the effect is not very good. Using non-coated paper or matt coated paper, but can produce a better copy effect. The texture of the paper will damage the image to some extent, and you should generally avoid copying photographic manuscripts with textured paper. But when using non-coated paper to reproduce art, the original texture of the paper will make the printed product feel closer to the original.
2. Text quality characteristic parameters. The definition of the best text quality is very clear. They must be free of the following physical defects: blocked ink, broken characters, white spots, unclear edges, excess ink marks, etc.
The density of text images should be high. In fact, the density of text images is limited by the thickness of the printable ink layer. On coated paper, the maximum density of black ink is about 1.40 ~ 1.50; on uncoated paper, the maximum density of black ink is all 1.00 ~ 1.10.
The width of strokes and literals should be consistent with the original font drawn by the designer. The stroke and width of the font are also affected by the thickness of the ink layer. When the ink layer is thicker, the deformation will be larger. Under a certain thickness of the ink layer, the deformation of the small size is much more obvious than that of the large size. In order to obtain the best reproduction effect, the change of stroke width should be kept within 5% of the specifications set by the font designer or manufacturer; the character size should be kept within 0.025 ～ ＋ 0.05 mm of the original specification.