Halftone Printing Screening Technology

Foreword

In the more than 100-year history of the modern printing industry, half-tone screening technology has taken a big step forward without knowing it. AM (Amplitude Modulation: commonly known as the amplitude modulation network, as shown in Figure 1) as a traditional network point, point distance is equal, the screen angle is strictly set, tune reproduction is stable. However, there are well-known defects in the AM network, and prone to moiré and screens sometimes have color jumping and broken lines. Moreover, the accuracy of registration during printing is very strict. From AM network point to FM network point (Frequency Modulation: called FM network, Figure 2) has new features. The FM outlets have the same size and are randomly configured to express the tone in terms of the number of outlets in the unit area. The FM network does not require a high level of registration accuracy. Because of the smallness of the dots, the reproduction of the details and the richness of the tone, it is the first choice for fine prints. The FM outlets solved all the problems of the AM outlets mentioned above, but FM also had its shortcomings. First of all, due to the small size of the FM network, its printing is much more difficult than that of the AM network, and its stability is also inferior to that of the AM network. Moreover, due to the poor uniformity of the dots, the tune of the tone of the flat image has an unbalanced feeling. Therefore, at present, AM outlets are still the mainstream.

On the other hand, with the advancement of the digitalization process of platemaking and printing (CTP), the printing technology has also been continuously improved, and the demand for fine prints from 200 lines to 300 lines has become apparent. However, the difficulties of the AM outlets and the FM outlets have hindered the development of the technology.

The FAIRDOT (Figure 3) introduced in this article is a hybrid network point that combines the advantages of both AM and FM. It is also aimed at the introduction of high-resolution printing.

Different dot shapes

FAIRDOT technology can form different dot structures in different tone regions.

First, the highlights and dark areas

FM outlets can occur in highlights and dark areas. That is, in these two parts, the tone is represented by the number of dots with the same size and randomly distributed. However, even though it is randomly distributed, its configuration is very optimized, and there will be no problem of too dense network space. This also inhibits the appearance of particles in the image.

In addition, FAIRDOT technology deals with the minimum dot size. In general, the smallest dot that can be printed onto a plate in a 2 400 dpi output is 1/2 400 in. (=10.5 μm). However, because the 10.5 μm dot is too small, it is difficult to print, thus affecting the stability and reproducibility of printing.
The smallest dot of FAIRDOT consists of a collection of one or several light spots (the minimum dot size of a 2 400 dpi output machine is expanded to 21 μm or 32 μm) to improve stability and reproducibility (Figures 4, 5, 6). ).

Second, intermediate tone

The midtones are randomly distributed as FM, but the tone representation method is irreqular clustereel dots. That is to say, it is based on the random distribution of a certain number of dots of different sizes to express the intermediate tone (Figure 7).

If attention is paid to the phenomenon of convergence at the outlets, it can be found that in the AM outlets, the outlets that are close to the intermediate tone are almost connected together because of the specific tone. Therefore, the color shifting phenomenon easily occurs in the tune output. In the FAIRDOT technology, when the nearby meshes are joined, each joint is different, which prevents the jump. Moreover, as a difference from FM, it is also the midtone density (line number) that can be defined. Since the number of outlets is fixed, the density of outlets can be calculated from the number of outlets per square inch. For example, if there are 40 000 outlets per unit area, then the density equivalent to 200 lines can be reproduced. However, this is limited to the midtone range.

What is the range of the so-called midtones in the entire tone region? It can be known from the following calculations: Calculate the output accuracy at 2 400 dpi, set the minimum dot size to 2 x 2 dots, equivalent to 400 lines of FAIRDOT = (2 × 2) ÷ (6 × 6) = 0.111, which means that the halftone dot area ratio is a midtone region in a large range from 11.1% to 88.9%.

Analysis of Spatial Frequency Characteristics of 50% Tone

Now from another angle, what is the characteristic of the spatial frequency of 50% dot image? (Figure 8) is a circular spectrum.

The center of the ring represents the low-frequency area, and the closer to the low-frequency area, the coarser the picture is and the grainy. On the contrary, the outer ring represents a high-frequency area, and the more components there are, the better the reproducibility of the details, but the poorer the printing stability. FAIRDOT features: fine, good stability, no moire, indicating FAIRDOT between low and high frequency areas. The radius of the circle in FIG. 8 represents dot density (number of lines).

Dot density comparison

FIG. 9 is a dot density (conceptual view) of AM (175 lines), FM (21 μm), and FAIRDOT in each tone reproduction. The horizontal axis represents the dot area rate, and the vertical axis represents the dot density. From this figure it can be seen that FAIRDOT blends the properties of both AM and FM.

The most suitable prints As mentioned earlier, FAIRDOT is a solution for high-precision printing. especially:
● Design of maps, rooms, etc., and performance of hair and other details ● Reproduction of delicate patterns such as mesh and wood of stereo microphones and fashion cloth ● Reproduction of products based on black (dark) ● Shading, CG Reproducible smoothing tone ● The reproduction of fabric stripe, CTR screen, etc. can exert quite good results.

On the other hand, in the production of fine prints of 300 lines or more, the accuracy of the output device is required to reach 4 000 dpi. However, as a result, the efficiency is inevitably lowered, and the requirements for print management and registration are high. The other major feature of FAIRDOT is that, with low resolution, such as 2400 dpi output machines, high line count output can be achieved at the original management level while maintaining the same production efficiency.

Print test

So, how does the stability of FAIRDOT technology be used for high-precision printing on existing equipment? This requires actual printing tests. After the AM (175 line) and AM (300 line) test printing, the picture quality and printing stability results are as follows:

image

The line pattern of AM (175 line) clothes is moiré, so the reproduction of details is not very good. In the FAIRDOT technology, all the above-mentioned disadvantages did not occur at all, and therefore, the reproduction of lines such as wood grain furniture, texture details, and hair, lines, and the like was very good. We printed four-color maps with AM, indicating that the roads, routes, etc. have significant and significant disconnections. However, these print lines represented by FAIRDOT technology are quite smooth.

So you can get the same printing quality as the AM outlets.

stability

For every 500 sheets of paper, the area percentages of the image points in highlights, midtones, and shadows were evaluated. The evaluation order was AM (line 175)> FAIRDOT> AM (line 300). The stability of the highlights is better than that of the AM (300 lines) because FAIRDOT dots are easier to print.

However, the higher the dot density, the more susceptible it is to the amount of the wetting water, the amount of ink emulsification, etc. Therefore, the stability is slightly inferior to that of the AM (line 175), and if there is little attention, there should be no problem.

The above tests prove that the general equipment (= maintain productivity), using FAIRDOT technology, can be fully high-precision printing. (Translated by Hong Shenwei)