First, the concept of print quality
Print is a visually arbitrated commodity and artwork. When people comment on the quality of prints, they always think of aesthetics, technology, and consistency. This way of thinking about the problem is to consider the human visual psychology factor and the physical factors in the replication project, that is to say not only consider the commercial value or artistic level of the printed matter, but also consider the influence of the printing technology itself on the quality of the printed matter. . However, practice has proven that the technology for evaluating the quality of printed matter from the perspective of commodity value or art is not yet perfect. Such evaluations often fail to reliably express the reproduction quality characteristics of prints. Only by assessing them from the perspective of printing technology, can we correctly evaluate the quality of printed matter and achieve unanimity. This view has been endorsed by most experts at home and abroad.
A•C•Zettlemeyer et al. once defined this “quality of printed matterâ€; the quality of printed matter is the combined effect of various appearance characteristics of the printed matter. From the perspective of printing technology, the so-called appearance characteristics of printed products is a more general concept and has different connotations for different types of printed products.
For line or field prints, the ink should be thick, uniform, shiny, textless, high definition, overprint accuracy, no overprint and back convex overweight, no back pods.
For color dot prints, the tone and color reproduction should be faithful to the manuscript. The ink color is uniform, the gloss is good, the dots are not deformed, the overprinting is accurate, there is no ghosting, through-printing, all kinds of bars, the back of the dirty and mechanical traces.
The combined effects of these appearance characteristics reflect the overall quality of the printed matter. In the evaluation of print quality, various appearance characteristics can be used as the basis for comprehensive quality assessment, and of course, it can also be used as the basic content and requirements for the quality management of printed matters.
Second, the concept of printing image quality
The connotation and extension of the above-mentioned print quality concept is very rich. For example, for books, the binding quality should also be included in the category of “print qualityâ€. However, considering the quality of prints so widely, it is often inconvenient and unnecessary from the perspective of the print image reproduction technology. G.W. Jorgensen et al. pointed out that the above definition of print quality is not accurate enough. From the perspective of copying technology, they defined print quality as “faithfulness of original copyingâ€. This method of definition is convenient for conducting research on the print reproduction process and evaluating the quality of the various stages of the print reproduction. Based on this consideration, this book distinguishes the quality of the printed matter from the quality of the printed image into two different concepts, that is, the quality of the printed image is defined as “the fidelity of the printed image to the copy of the originalâ€.
Compared with the definition of print quality, the definition of printed image quality narrows the scope of the discussion question, so that the printed image can be regarded as a set of information for a certain number of pixels with brightness and darkness and color change on a two-dimensional or three-dimensional image (note Text can also be treated as graphic information, so text can also be considered as an image). The quality of printed images at best includes two aspects: image quality and text quality. The feature parameters expressing image quality and text quality are now described as follows. These quality feature parameters can be used as evaluation parameters for quality evaluation, and can also be used as target parameters in quality management.
1. Image quality characteristic parameters. Can be divided into the following four, tone and color reproduction, image resolution, tortoise and other fault graphics and surface characteristics. The following description is given in this order.
Tonality and color reproduction refers to the case where the tone balance of the printed copy image and the color appearance correspond to the original. In the case of black-and-white copying, the reproduction of gradation is generally represented by the density correspondence between the original and the copy (copy curve). For color reproductions, the hue, saturation, and lightness values ​​are more practical.
The tone and color reproduction capabilities of printed images are not only affected by the inherent characteristics of the inks, substrates, and actual printing methods used, but are also often subject to economic constraints. For example, in multi-color printing, high-fidelity printing processes can achieve relatively high copy quality, but that will be at the cost of increased costs. So for the prints with the theme of screen, the best reproduction of the tone and color is within the various constraints and capabilities of the printing device. Quality printing image technology and technology. The question of the best tone and color reproduction will be described in more detail later.
The problem of image resolution in the best reproduction includes both resolution and clarity. The resolution of printed images depends mainly on the number of screen lines, but the number of screen lines is limited by the printing materials and printing methods. The number of mesh lines that can be distinguished by human eyes can reach 250 lines per inch. However, in actual production, the maximum number of lines can not always be used. In addition, the resolution is also affected by the change in registration. Sharpness refers to the contrast on the edge of the tone. On the color separation machine, the sharpness of the image can be adjusted by the electronic enhancement method. However, people still do not know what the best level of clarity is. If too much enhancement is made, images such as landscapes or portraits may not look like actual ones, but images like fabrics and mechanical products can improve performance and appeal.
Moire, bar, grainy. Water marks, mottled spots, etc. all affect the uniformity of the appearance of the image. In the dot image, some moiré patterns (such as rosettes) are normal, but when the mesh angle is deviated, bad moire patterns will be generated. There are many factors that affect the granularity of the image, and the paper smoothness and printing The grain size of the plate is related to the graininess of the image. From a technical point of view, in addition to the moiré and grain patterns, one can make most of the spots that cause non-uniformity and the fault pattern close to zero.
Surface characteristics of the printed image include gloss finish, texture, and flatness. The requirements for gloss are based on the nature of the original and the end use of the printed image. In general, the use of high-gloss paper is better when copying photographic originals. In the 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 the surface light scattering, thereby enhancing the color saturation and darkness. However, when using high-gloss paper to reproduce watercolor or pencil drawings, the effect is not very good. Use of non-coated or matte coated paper can produce better reproduction. The texture of the paper will damage the image to some extent, and copying the photographic original should usually be avoided using textured paper. However, when using non-coated paper to copy fine art, the original texture of the paper will make the print feel closer to the original.
2. Text quality feature parameters. The definition of the best text quality is very clear. They must not have the following physical defects: ink blocking, broken characters, white spots, unclear edges, excess ink marks, etc.
The density of text images should be high. In fact, the density of the text image is limited by the thickness of the printable ink layer. On the coated paper, the maximum density of the black ink is about 1.40 to 1.50; on the uncoated paper, the black ink has the maximum density of 1.00 to 1.10.
The stroke and literal width should be consistent with the original font drawn by the designer. The strokes and literal width of the font are also affected by the thickness of the ink layer. When the ink layer is relatively thick, the resulting deformation will be relatively large. Under a certain thickness of the ink layer, the distortion generated by the small character is much more pronounced than the distortion produced by the large character. In order to obtain the best reproduction effect, the variation of the stroke width should be kept within 5% of the specification set by the font designer or manufacturer; the character size should be kept within 0.025 to +0.050 mm of the original specification.
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