High-definition Printing Technology Features and Applications (1)

The sharpness of printed images can be quantified by two independent indicators - spatial resolution and tone resolution. The resolution of image details, called spatial resolution, is determined by the width of the screen (the amplitude modulation network) or the width of the screen (the frequency modulation network). The smaller the width, the higher the spatial resolution; the greater the number of points per unit area. The higher the spatial resolution. The ability to appear subtle and soft when the color of the image changes, known as the tone resolution, is determined by the composition of the dots (the amplitude modulation network and the FM network) and the distribution of the dots (the frequency modulation network). For the amplitude modulation network point, when the width of the network line is the same, the smaller the exposure area of ​​the exposure light source, the higher the tone resolution. The visual clarity of a printed image is the product of spatial resolution and pitch resolution.

According to the traditional view, the network cable frequency reaches 175 lpi and the cable width is 145 μm, which is equivalent to the limit of human visual resolution when printed at 40 cm. If you continue to increase the screen cable frequency, people will not feel the increase in the clarity of the printed matter and can only increase it in vain. cost. This view created confusion in the printing industry and often led companies to abandon their efforts to improve the clarity of print. After analysis, it is known that the roughness of the AM web print is mainly caused by "rose spots". The roughness of the FM network print is mainly caused by "spotting".

Under normal circumstances, the width of the “rose spot” is several times the width of the cable. In the area where the tri-color ink overprint color tone value is midtone, the “rose spot” can be clearly seen. The printed image appears Rough, lack of luster. Therefore, the use of amplitude modulation netting and screen printing to minimize the size of "rose spots" is a necessary measure to improve the clarity of prints. According to experiments, under the condition of the existing ordinary printing materials, the use of 350 lpi screened printing products can maintain the stability of the quality.

In high-definition printing technology, the method of improving spatial resolution is relatively simple. As long as there are corresponding network cable settings in RIP, and the corresponding output linear adjustment, basically meet the requirements. It should be noted that the width of the edge of the film web site should be as small as possible, which has stricter requirements on the laser beam focusing, film sensitivity, and development conditions of the collator machine.

The analysis and control of tone resolution characteristics should pay attention to two issues: first, recognition of tone characteristics, second, ideal tone level determination, and tone value increment measurement and control.

Question one

The gradation series of the up-tone dots on the film is calculated by the following formula:

Tune=(Laser accuracy dpi/screen number lpi) 2 +1

For example, when the phototypesetter laser has an accuracy of 2800dpi and 175lpi screen output film,

The tone 175lpi= (2800/175)2 +1=257

That is, the maximum dot area of ​​175 lpi is formed by 256 laser exposures, 16 rows and 16 columns.

After the amplitude modulation network has set the number of screen lines and the angle of the screen, the maximum screen area and the center position of the screen are determined. The area of ​​outlets increases from small to medium, and the tone of performance is from light to dark. At present, the original electronic file has 8 bytes of 256 bits for each primary color image beam. Therefore, when the 175 lpi screen is being added, the image-setter laser accuracy should not be lower than 2800 dpi.

In high-definition screening, the imager laser should have a higher precision. For example 350lpi screening, the ideal laser precision should be 5600dpi. In the maximum dot area of ​​175 lpi, four 350 lpi dots are distributed. If you still use the 2800dpi output, the number of tone steps for each dot is 64, and the number of all tone levels for four 350lp dots is 64x4=256. The visual clarity of the printed image is 350lpi for 4x256=1024 and 175lpi for 1x256=256, thus it can be seen that the 350lpi resolution is improved by 4 times compared with 175lpi. If the laser accuracy is increased to 4000 dpi, the equivalent tone scale of 350 lpi can reach 522, and 175 lpi is still 256 due to the limitation of the amount of original electronic data. Under this condition, the print image sharpness was 350lpi 4x522=2088, 175lpi 1x256=256, 350lpi sharpness increased 8 times.

If the original image has an accuracy of 350 dpi and is output at 175 lpi, the basic screening algorithm copies every four pixels in the image to a dot. The average value of the four pixel tone values ​​determines the tone value of the corresponding dot. The difference between the four pixel gradation values ​​cannot be expressed in the gradation values ​​of the dots, which is one of the reasons why the printed matter does not look as delicate and soft as the color of the original. If the image with the same accuracy, instead of using 350lpi screen output film, each pixel can be copied by a dot, in the area where the original color tone change is obvious, the accuracy of tone reproduction will make the print color more delicate and soft. Under normal circumstances, the high-definition outlet tone number should not be lower than 128. When the conditions permit, it should strive to reach 256, in order to avoid the image pixel tone over-compression and leveling in the screen output.

The 2x2 FM network with 2540dpi laser accuracy, with a dot width of 20μm, can arrange 4x4 = 16 dots in a 350lpi dot area. In an FM network, the position of the dot is fixed in the area set by the dot algorithm, so the number of equivalent tone levels is 16, which is the same as the dot number. The clarity of the printed image is 16x16=256 for the 2x2 FM network and 1x54=54 for the 350 lpi AM network. If the 350 lpi amplitude modulation net uses 4000 dpi laser precision, the tone progression is 1x130=130, compared to the visual clarity of the 2x2 frequency modulation net image being nearly 1 times higher. If you use an FM network dot with a width of 10 microns, the visual clarity of the image is 49x49=2401, which is about 18 times higher than 350 lpi.

From the above analysis, it can be seen that the high-definition amplitude modulation network point depends on the improvement of the image-setter laser precision, and mainly improves the tone resolution to improve the visual clarity of the printed image. The FM network can improve the visual clarity of the printed image by increasing the spatial resolution with lower laser precision. However, affected by the screening algorithm, in the brightly tuned area of ​​the FM network print, due to the uneven distribution of dots, the color has a rough sense; in the bright-toned area, due to the “freckle” caused by the non-simultaneous connection of the dots, the Print visual roughness. The secondary FM network, the location of the outlets, is affected by the number of outlets in the reference area set by the outlet algorithm, and the distribution of outlets tends to be even. In this sense, although the tone resolution and spatial resolution of the tone are the same as those of the primary frequency modulation network, the distribution of dots is more reasonable, the roughness caused by “freckle” is significantly reduced, and the visual clarity of the printed image is higher.

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