Resolving power and grey levels
In order to be able to capture images electronically, it must be divided into very small parts, and each part can be measured and quantified separately. In other words, the image must be “decomposed†and the result is a very, very small image element that we generally call “pixels†(this term is derived from both “images†and “elementsâ€. ). The higher the resolution of the image, the smaller the pixels in the image, and the finer the image, the less visible it is to the naked eye, but the larger the size of the image file. In general, the resolution of the image is required to be sufficiently high that only a certain hue can be seen by the naked eye within a certain distance, and individual pixels cannot be distinguished.
In general, the resolution of a digitized image is determined during the scanning process, such as during the input phase. Therefore, in order to ensure high image quality, the final output must be taken into account. For example, if the image needs to be magnified many times during processing, the scanning resolution must be set higher to ensure that the image is still not visible to the naked eye after amplification. , the output image will be very rough or there is a clear "mosaic" phenomenon. However, the scanning resolution of the image cannot be too high, otherwise the amount of data will be too large, and the performance of the image hierarchy may not be greatly improved. Therefore, a reasonable image resolution should be selected during scanning. In general, the resolution of an image in a halftone printing process should be about twice the number of printing lines, for example, printing an image using a flexographic printing process. If the number of lines is 48 lines/cm (equivalent to 122 lpi), the resolution of the image does not need to be as high as 120 lines/cm (equivalent to 305 lpi), as long as it is between 94 lines/cm to 100 lines/cm (equivalent Between 239 lpi and 254 lpi is enough. In the production practice, the appropriate image resolution should be selected in combination with the actual conditions of the printing process, print applications, and substrate materials, etc.
Paper Type Line Number (lpi) Image Resolution (dpi) Applicable to Coated Paper 200~175 400~350 Album, Sample Coated Paper 150 300 Large Poster Offset Paper 133 266 Color Newspaper, Book Newsprint 100 200 Newspaper
7. Get color gradation through different sizes of outlets
In the output process of a halftone printing process (such as a flexographic printing process), the color gradation of an image is expressed by means of pixels. The image is first processed by color separation, and then the reproduction of the colors and the reproduction of the layers are obtained through the reproduction of the dots. The size of the dot size also changes correspondingly according to the value of the tone value. As we mentioned before, the resolution of the image should be about twice the number of screen lines. If you follow this rule, each dot is composed of a group of pixels, and there are four pixels in each group. Therefore, the word "output resolution" was again introduced. In the case of a certain number of screens, in order to faithfully reproduce all kinds of screens from 1% to 99%, it is necessary to obtain a corresponding number of levels in the exposure imaging process. Because modern laser imagesetters operate on a dot network, different sizes of dots can only be obtained when each dot is also composed of many smaller laser dots. For example, suppose that in the image duplication process, as long as each percentage level value can be reproduced, it means that, as it is, this means that 100 level values ​​are needed. Therefore, in the square grid, An outlet must consist of 10×10 small dots so that it can guarantee the required 100 levels. Each addition or deletion of a small dot will cause a 1% change in dot size. Such a system can only meet the minimum requirements for screening.
However, the resolution required for an imagesetter has been raised to five times the assumed image resolution. Therefore, in order to meet the minimum requirements, currently, 480 dots must be formed within one centimeter of the laser imagesetter.
Eighth, a large scale of gray scale is needed. In fact, the 100 levels in this most basic system (we commonly call it the “gray scaleâ€) are still not enough. Among them, one of the reasons is the printing process. Its own requirements. Taking flexo printing as an example, the most prominent feature of flexo printing is that the highlights of the high-light areas are more serious. Therefore, special attention must be paid to the adjustment of the highlights and strictly controlled. Therefore, the 100% level value is insufficient. of. In addition, it is virtually impossible to reproduce visually smooth continuous vignetting effects with only 100 gray scales. Moreover, if two or more screening areas are simultaneously printed using a conventional screening method and no moiré is generated, the screening angle needs to be changed, which also requires increasing the number of small ideas.
Therefore, the output resolution of modern laser imagesetters must generally be at least 1000 lines/cm or more. Another common measure of resolution is dpi, which stands for "dot per inch." One inch is equivalent to 25.4 mm, so 1000 lines/cm is equivalent to 2540 dpi.
Nine, screening process and dot shape flexographic printing is also one of the halftone printing processes, so in order to represent the level of image color, each color in the printed manuscript must also be broken down into many small dots (for this small The printing plate can accept the ink's idea, we call it "net point", but within a certain observation distance, the human eye can't see the network spot), and expresses the color continuous tone image in the form of a dot network. That is to say, the image after screening is divided into many halftone areas and non-halftone areas, and the ratio between the two determines the gradation value of the image area.
Earlier we introduced that in order to obtain dots of different sizes, the dot on the printing plate must be composed of smaller and smaller dots. The greater the number of small points needed to form a network, the higher the tone value of the network.
This kind of dot-based process has a great advantage, and it can very easily generate a variety of different shapes of dots. The regular arrangement of outlets forms a network cable. Common types of outlets are the following types: circular outlets, square outlets, and chain outlets. Each has its own merits. You can use it for a specific printing process or even for a specific printing press. Select the most appropriate number of screens, screening angles, and outlet types. Practice has proved that the use of circular dots or elliptical dots in flexo printing is most beneficial for copying.
So far, we have introduced and discussed only the traditional AM screening process. In this traditional AM screening process, the dots are always arranged along a regular grid, and depending on the level of shades being reproduced, the size of the dots will also change accordingly. The larger the dot, the tone value The bigger it is, the darker it feels. The tone value is usually expressed as a percentage of the dot area.