The digital printing process is a printing technology that integrates imaging, inking and transfer. Its working methods are very different from traditional offset printing. The printing materials are also different from traditional offset printing. Therefore, digital printing quality evaluation and traditional offset printing evaluation There are also differences, and traditional evaluation methods and evaluation indicators are no longer suitable here.

Print quality evaluation index

1) tone reproduction

Tone-tone copy refers to the correspondence between a group of copied gradation values ​​and a corresponding set of tone values ​​on the manuscript. This relationship is called a tone reproduction curve, and the tone value can be expressed by a density value or a dot coverage value. Levels are defined as visually distinguishable density gradations in the image. The tone reproduction is the soul of the copying project, and is the premise of image level reproduction and color reproduction. The longer the dot pitch on the print, the more layers are reproduced, the better the color change, and vice versa. When the reproduction of the tone is not correct, the printed image appears to be unclear, lacks the natural luster it deserves, the bright tone is not bright or there is no contrast, the important part gives a feeling of “flatness” and the color saturation is not enough.

In the actual printing and copying process, the effect of tone reproduction is related to the operator, the characteristics of the manuscript itself, the printing materials, and the copying methods, such as the quality of the ink, the paper, the dot shape, the number of screens, the density of the ink on the paper, and the related Printing parameters such as dot gain, orientation enhancement, overprinting, and stencil printing are parameters. In the process of reproduction of the tone, people are required to be able to achieve "clear print, medium and dark tone, and clear levels." When printing, you should faithfully copy the levels in the manuscript. The tone of the manuscript is roughly divided into low-key, mid-tone, and high-profile. The low profile increases, and the high and middle notes decrease, and the entire print is dark. The high profile has increased, and the medium and low profile have decreased. The entire printed matter is “floating”. The high and low-key increase, the middle tone decreases, and the entire print is empty. The increase in the tone, high and low-key reduction, the entire print on the plain.

2) Color reproduction

Like color reproduction, color reproduction is also one of the important symbols for evaluating the quality of printed matter. The ideal color reproduction should be the reduction of the original color. Since the color of the original document sometimes deviates, the color reproduction should be aimed at satisfying the copying requirement. The color separation process is the key to color reproduction. The color decomposition is correct and there is a basis for color synthesis. At the same time, color separation conditions, light sources, output materials, and development conditions are affected in the process of color reproduction. Quantitative evaluation of color reproduction is a very complicated issue. Objectively, the closer the physical color is, the better the color of the printed matter and the color of the scene are. However, there are no ideal devices that completely replace the eyes. And people's assessment of color is often based on the memory or impression of the scene. Often the memory of a common object deviates from the actual color, and the color that people like is deviated from the actual color. For example, the blue sky is not blue in many cases. Not necessarily all green, which brings difficulties to color reproduction.

In the process of printing and copying, people use neutral gray balance as a basis for judging color reproduction. In the area of ​​grayscale, grays consisting of cyan, magenta, and yellow dots with different proportions of three primary colors are used in prepress and printing. It is called neutral gray balance data, and Figure 2 is the gray balance curve. Due to the sensitivity of the gray area to the color performance, under normal circumstances, when the neutral gray balance data deviates from the normal state, the gray will be biased to a certain color. Therefore, during the entire image reproduction process, neutral gray balance data is used as an important quality standard for monitoring the production process from prepress to printing. In the process of color reproduction, the color should conform to the original, true, natural, and coordinated, and the color difference of different printed sheets in the same batch of products is less than 5.

3) Resolution and definition

The resolution is the resolving and reproducing ability of the printed image to the original image detail, or the resolving power of the image to the original scene. The sharpness refers to the clarity of the image details, mainly the sharpness of the edges of the image and the edges of the lines. The resolution of the printed image mainly depends on the number of image lines added, but the number of screen lines is also restricted by the printing materials and printing methods. Therefore, it is impossible to always use the highest number of screen lines, that is, it is impossible to always obtain the highest resolution. In addition, the accuracy of printing overprinting also affects the resolution of the image. The resolution will also affect the sharpness of the image to a certain extent. Of course, the definition is also influenced by the printing process, materials, and technology.

4) Surface features

The quality of the printed picture, such as smoothness, graininess, unevenness, overprinting error, glossiness, etc., is not the main reproduction and reproduction quality index of the color image, but it affects the appearance of the printed matter. The smoothness, graininess, moire, spots, and unevenness of the print screen will affect the normal reproduction of the image sharpness, and can be directly visually perceived. The smoothness and granularity of plain clothes are mainly determined by the granularity of the original, and are also conditioned by the magnification and the slight emphasis on the initial action value (particle strength adjustment). Subtle emphasis on proper adjustment of the starting value can improve graininess but affect the emphasis of hierarchical clarity. Dot shape, dot uniformity, and printing dot quality of screening images affect the flatness of printed images. The poor quality or uneven quality of printed dots will damage the smoothness of the garments. Moire is due to the incorrect angle of the screen. From a technical point of view, in addition to the moiré and grain patterns, most other spots and fault patterns that cause inhomogeneities can be eliminated. In the process of printing and copying, the requirements are: the layout is clean and there is no obvious stain; the hue of the printing plate should be basically the same; the allowable error of the fine product size is <0.5 mm; the allowable error of the general product size is <1.0 mm; the text is complete and clear. Accurate location.