Several Screening Techniques of CTP

The printing and copying process is the process of converting continuous tone images into halftone images, and the dots are the most basic elements that make up halftone images. Printing and copying is to control the amount of ink through the density change of the dot area to reproduce the effect of changing the color tone of the original. In the process of printing and copying, the screening technology has always been a core technology for graphic and text output. Its development has experienced three stages: contact screen cine screening, electronic scanning color separation machine electronic screening, and computer digital screening.

With the advent of CTP technology, digital screening technology was applied directly to printing plates. CTP technology replaces the traditional analog technology of plate making with digital technology, thereby reducing the number of image transfer and human factors affecting layout quality. Since the dot on the printing plate is an image forming dot, the image on the printing plate is accurate and clear, and the small dot of the highlight portion is not easy to lose, and the dot of the dark tone portion is not easy to appear in the same level, and the quality of the printing plate is greatly improved. The rapid development of CTP technology has also promoted the update of digital screening technology. Currently, the digital screening technology applied to CTP technology is no longer the situation in the traditional AM network. The FM network and hybrid network are also highly concerned by manufacturers.

AM screening technology

AM screening technology, also known as AM (Amplitude Modulated Screening) technology, which is a traditional screening technology. When the plate-making machine records dots, the recording plane is divided into small squares according to a certain rule. These small squares are called recording grids, and n×n recording grids constitute a halftone cell. For example, if a mesh tune unit is composed of 16×16 grids, then the mesh gray scale of the mesh tune unit is 16×16+1=257. The size of the grayscale value of the image determines how much of the raster exposure is recorded in the mesh cells. The ratio of the number of exposed grids to the total number of grids in the mesh adjustment unit is the percentage of the outlets of the outlet, as shown in FIG. 1 .

For the platesetter, the number of grids in the mesh tune unit is variable and is determined by the output resolution of the device and the number of screens. If the mesh number of the mesh tune unit is n, the resolution of the output device is dip, and the number of lines added is line/inch (lpi), the relationship between the three can be expressed by the following formula: n=(dpi/ Lpi) 2.

It can be seen from the above equation that the number of screen lines is fixed and the higher the resolution of the output device, the more the grid number of the screen adjustment unit is; the resolution of the output device is constant, and the higher the number of screen lines required, the more the screen The fewer cells of the tune cell are.

The number of grids in the mesh tune unit actually determines the gray scale of the pixels in the digital image that the dot can represent. For example, if the recording resolution of the output device is 2400 dpi and the number of screen lines is 150 lines/inch, then the screen tones unit is composed of 16×16 grids, which indicates that it can represent 256 gray levels, plus blanks. Can display 257 gray levels. If the gray scale is represented by G, the gray scale that can be represented by this half tone can be represented by the following formula: G=(dpi/lpi)2+1.

Since the resolution of the plate-making machine usually provides only a limited number of options, the change in the number of squares of the gray scale of the mesh tone is limited for the same output device.

The image layer and color of the AM network can generally meet the requirements. It is also a commonly used screening technology in the CTP process, and printing has long been used to the use of AM screening, but AM screening also has its shortcomings.

First of all, the fine-grained level of the images displayed by the screens of the amplitude modulation and screening is mainly determined by the number of screens, which is at the expense of the accuracy of the direct plate-making machine. For example, if the resolution of the plate-making machine is 3,600 dpi, in order to duplicate the 256 gray levels of the image, the resolution of the plate-making machine at the time of output needs to be reduced to (3600 ÷ 16) = 225 lines/inch.

Secondly, the screening angles of the four color versions of the AM screening are 90°, 15°, 45°, and 75°. When the printing is superimposed on the four printing plates, tiny rose spots often appear. Once the number of screening lines is low, the vision is low. The feeling will be more obvious.

Finally, if the screening angles of the four color versions are incorrect during screen printing, moire will appear during printing, which will seriously affect the quality of printed products.

FM screening technology

FM screening technology, also known as FM (Frequency Modulated Screening) technology, is the use of computer technology, with the formation of hardware and software. FM networks are randomly distributed in space and there is no law to follow, as shown in Figure 2.

The diameter of the FM network is generally between 10.6 and 30 μm. The size of each dot is the same. The density of the dots is changed. That is, the frequency of the dot distribution in the space is changed to adjust the amount of ink on the printing paper.

There are two basic types of FM screening technology: one is the size of each network point, only the spatial distribution of network points randomly changes, called the first-level FM network (usually referred to as the FM network); the other is the size of the network and The frequency of spatial distribution is changing, which is called two-level FM network. The distribution of its outlets is shown in Figure 3.

The FM screening technology solves many problems that may occur in AM sites. For example, there are no screening angles for different colors, no moiré and rose spots, small dots make the picture have higher resolution, and intermediate tone colors can be skipped. Elimination and so on.

Compared with AM screening, under the condition that the resolution of the output device is the same, FM screening has a higher level of expressive ability, that is, a higher precision plate can be output with a lower plate resolution. As shown in Table 1, under the condition that the resolution of the output device is the same, the accuracy of the images of the frequency modulation screening and the output of the amplitude modulation screening output network is compared.

Although FM screening has many advantages, due to the small diameter of the FM dots, the phenomenon of dot enlargement during printing is more serious, which makes many printers unable to correctly reproduce the image hierarchy. In addition, the screens of FM screening are irregularly arranged, so lines and checkerboard structures are locally generated, the weighted dots are rapidly increased, and ink is locally accumulated. These are the main reasons why FM screening technology has not been widely used in actual production.

Staccato's Staccato FM plus screening technology used by Creo in its CTP technology is a secondary FM screening technology. The Stellar FM screening technology provides a variety of dot sizes. The dot size used depends on the requirements of the print product. The best 10μm is the finest dot, suitable for fine print printing. Most commercial printing can use 20μm dots, while newspaper printing is suitable for 36μm dots. SFG.com uses high-frequency random dot insertion technology to display subtle details and improve the color fidelity of the image. After the optimization of its screening structure, not only can the rose spots and moire be completely avoided, but also the structure of the mesh structure can be made more stable, and the accumulation of particles, dot gains and intermediate tone printing ink can be reduced. Creo's vision for the second-degree FM screening and stochastic FM screening is shown in Figure 4.

Hybrid Screening Technology

Hybrid Screening technology is a CTP screening technology that draws on the characteristics of both amplitude modulation and frequency modulation. It not only embodies the advantages of FM network points, but also has the stability and operability of AM network points. Normally, the precision of the plate-making machine is 4000 dpi when AM printing technology is used to make fine prints of 300 lines/inch or more. This will reduce the output efficiency and impose high requirements on print management and registration. A major feature of hybrid screening is the ability to achieve ultra-300 line/inch screen accuracy without affecting the output speed while still using the original output resolution (eg, 2400 dpi), and does not require the traditional high line count. The harsh conditions required by the web process. The printability is the same as the traditional AM outlets, that is, 1% to 99% dot reproduction can be achieved under the existing printing conditions.
Agfa's Sublima screening technology is a kind of hybrid screening technology that is more successful in practical application of CTP technology. As shown in Figure 5.

Sublima screening technology reasonably uses amplitude modulation and frequency modulation screening techniques to express the fine-tuned levels of midtones in images using high-line-density amplitude modulation, and to express high-light and dark-tone levels using frequency-division network points. It is based on the minimum net point at which the printing can be expressed as the demarcation point, the use of amplitude-adjusting dot-screens above the demarcation point, the maintenance of the size of the dot-points below the demarcation point, and the change in the number of dots to express the level. The size of the demarcation point network varies according to the number of screen lines, as shown in Table 2.

Sublima screening technology uses Agfa's patented XM overclocking algorithm. When the AM outlets transition to FM outlets, the FM random outlets extend the angle of the AM outlets, completely eliminating transition traces, and allowing the two frequencies to be subtly blended. ,As shown in Figure 6.

In addition, Sublima screening technology can optimize its calculation method when RIP is performed, so the output efficiency is not greatly affected.

Sublima screening technology enables high-line printing without changing the existing printing conditions and without increasing costs. The printed images are very delicate. The rose spots consisting of halftone dots and halftone dots at 340 lines/inch can hardly be discerned with the naked eye. They look exactly like photos and significantly improve the quality of prints. The main advantages of Sublima screening technology:

1. There is no need to change the printing conditions. Sublima screening technology is based on the smallest printable dots (175 lines/inch, 2% dots) of the press. 340 lines/inch dots are the same size at 8%, 2%, and 1%. By reducing the number of dots The density of the distribution expresses the hierarchy, as shown in Figure 7.

The same method applies to 92% to 99% of outlets. The 340-line/inch Sublima minimum dot is larger than 175 lines/inch and 2% of the AM dots. If the printer can copy 175 lines/inch and 2% of the AM dots, you can copy 340 lines/inch, 1% to 99%. Sublima outlets.

2. Does not increase printing costs. With the use of existing printing equipment and processes, high-line screen printing products can be printed, significantly improving the quality of printed products without increasing costs. Sublima can have up to 340 lines per inch.

3. Does not reduce the output efficiency. When outputting the number of Sublima outlets for each line, such as 210 lines/inch, 240 lines/inch, 280 lines/inch, and 340 lines/inch, the output of the plate-making machine can only be 2400dpi, and the output efficiency will not change. The number of screen lines increases and decreases.

4. The image level is delicate, especially in the flat screen area, which does not have the rough feeling like the FM network.
In addition to Agfa’s Sublima screening technology, Screen’s Rapida Screening Technology is also a hybrid screening technology. The difference between the addition of the screens of the Spiro and the AM screening and the FM screening is shown in Figure 8.

According to the color and level changes in the screen, Rapida can use “class FM network” in a timely manner. It has 1% to 10% of the highlight area and 90% to 99% of the shadow area, like the FM network. The use of fine meshes of the same size, and the degree of density of these meshes are used to represent the level change of the image, but the size of the minimum mesh is larger than that normally used, thus making up for the insufficiency of the difficulty of printing by the FM mesh. In the midtone part of 10% to 90%, the size of the dot will be changed like an amplitude modulation dot, but the position of all dots is random, which means that the angle of the screening does not exist. This technology allows the Rapida to achieve the quality of a superfine screen equivalent to 300 lines per inch or more at conventional 2400 dpi, 175 line/inch production conditions, while avoiding rose and moiré The impact of product quality.