Printing Quality Control--Color Control (3)

Print control strips consist of physical blocks, different dot patches, and signal qualities for visual inspection. In Europe, the Brunner system and the FOGRA system are the most widely used, and China has also published its own system.

In order to control four-color, five-color, and six-color printing based on measured data, Heidelberg, Germany, sells the FOGRA OMS and Brunner system CPC print control strips. These control strips are adapted to the special requirements of the CPC system and are used exclusively in the division ink area of ​​the Heidelberg press.

Table 5-1 lists what a multicolor print control bar based on the density measurement principle can detect. The number of items that can be detected decreases with the increase in the number of printing colors, because in addition to measuring the main evaluation parameters, additional colors also occupy a certain position on the control bar. Therefore, the control strip used must correctly correspond to the number of colors to be printed.

Table 5-1

System Name CPC/FOGRA CPC/Brunner

Color number 4 5 6 4 5 6

Plate Exposure Visual Inspection â—‹ â—‹ - â—‹ â—‹* â—‹*
Specular Control Vision Inspection â—‹ â—‹ â—‹* â—‹ â—‹ -
Solid Density Measurement â—‹ â—‹ â—‹ â—‹ â—‹ â—‹ â—‹
Midtone dot gain measurement â—‹ â—‹ â—‹ â—‹ â—‹ â—‹
3/4 Dot Measure Increase â—‹ â—‹ - â—‹ â—‹ -
Thick and fine mesh comparison measurement - - - - - - - - -
3/4 adjustment relative contrast measurement â—‹ â—‹ - â—‹ â—‹ -
Ghosting and Sliding Measurement â—‹ - - - â—‹ -
Ghosting and Sliding Vision â—‹ â—‹ - â—‹ â—‹ â—‹
Gray Balance Visual â—‹ â—‹ - â—‹ â—‹ -
Overprint Rate Measurement â—‹ - - â—‹ - -

* Only suitable for Y, M, C, Bk four colors

There are also many world-renowned systems that are the total optical effects that control many printing characteristics. The following outlines the similarities and differences of these color control systems.

1. Bruner print control bar. The correctness of the Brunel system that is of interest to the whole world has already been proved by practice. Since 1973, Felix Bruner of Switzerland has been working on researching and improving the color control system he invented. He and his colleagues studied thousands of printing machines in Europe and the United States. They built a huge Database, use this database to compare printing conditions and draw useful conclusions.

Bruner believes that: the content of color control should include the emphasis on color accuracy, level and sharpness control, there is an allowable deviation range of the copied color, the lateral deviation depends on the paper characteristics change, and the longitudinal deviation depends on the change of the entire printing status. . The hue, lightness, and saturation of the color may change, and the brightness or darkness may change due to changes in the overprinting hue and the black ink amount; the hue changes due to changes in the ink amounts of yellow, magenta, and cyan, when these three When the primary color inks are increased by the same amount, the result may be acceptable, and if there is a difference of ±2% in dot gain between the three, it is easy to be visually perceived. If this difference reaches 4%, then the limit of the allowable deviation of hue is reached.

Bruner showed several sets of print samples during the lecture, which showed the importance of consistent increase in the number of outlets. One set of cyan ink printing samples has different dot gains but similar field density; the other set of three color ink printing samples have similar dot gains, and there are different ink density in the field; the third set of samples is in various colors. There was a consistent increase in network points and the density of the field was also controlled. His example shows that three-color inks can obtain acceptable results when the same dot size is increased; and each color has a different dot gain, the result is unsatisfactory. This shows that the control of dot gain is the most significant for controlling the hue of colors. The Bruner print control bar is described below. The control strip consists of many color patches, but the microwire label used to control and display the dot gain is the main basis of the system.

The ultra-measurement element is the core of Bruner's system, which is equivalent to a 150-line dot block (50%). It consists of a circular dot with a coverage of 0.5% to 99.5%, a 50% square dot, a 50% horizontal thin line and a vertical thin line, and small positive and negative crosshairs. The average dot coverage for each pair of dots in the element and dot coverage for each part is 50%; because the slippage is a directional dot gain, the parallel line is the detection flag of the slip.

Next to the ultra-measurement element is a 25-line/inch, 50% coarse mesh line. It is difficult to measure the dot coverage with an old densitometer. However, with this coarse mesh, any reflective densitometer is used to measure the dot. The coverage rate has become very convenient, the dot gain rate of the coarse dot is very small, and the 150 dot line dot block has a large dot gain. The density difference can be obtained by measuring the density of the two-dot block with a complementary color filter, and the difference is added to 0.05 to obtain an approximate dot gain.

This measurement and control method can only indicate the increase in the number of 150-line dots. It is not applicable to the dots of other lines. The optical magnification of dots is the finer the cable, and the larger the optical increase, because there are more dots in the unit area. The larger the number of outlets, the longer the perimeter and the larger the increase in outlets. Bruner called it the edge region theory, and the optical increase is a function of the perimeter of outlets.

It is possible to make a dot increase graph that shows the appearance of dots from 0% to 100% under a given printing plate and printing conditions. Dot coverage is a valid dot coverage calculated using the Murray-Davies formula. Rate, this coverage is the total optical network coverage. The amount of dot gain obtained with the Bruner method can be used to compensate for the difference between different densitometers. When measuring, he recommends a narrowband, non-polarized densitometer (with a measurement aperture of at least 3.5 mm). The micro-measurement element can also be used to evaluate the resolution of the printing plate. Observe the positive and negative reticle with a dedicated 25x Brunel scale magnifier. If they have equal dimensions, then the plate exposure is appropriate.

The microwire block can be used to determine the correctness of the exposure of the printing plate. Compared to years of use, the micro-line block can accurately indicate the exposure of the plate and film, but Bruner suggested that this does not apply to plates with low resolution. When the micro-blocks were used, a properly exposed negative version reacted with 11 μm fine lines and lost 8 μm thin lines. According to FOGRA's research, the human eye can recognize a 3 μm change using the micro-block comparison method.

The Brunel system does not yet have 25% and 5% dots, so that it can be combined with 50% dots to draw dot gain curves and limit Bruner's and DuPont's sample curves.

In order to obtain an intermediate tone of neutral gray, the Bruner system consists of a gray weight block consisting of 50% cyan, 41% magenta, and 41% yellow, and a 50% black mesh point abuts. Beside the gray balance block, the dot coverage should be slightly adjusted when printing with different inks.

The Bruner system also includes red, green and blue solid overprint blocks for checking the ink overprint condition.

The Bruner System Test Plate is a full-sheet-size test plate (original film) that can be used to evaluate the printing characteristics of any press. In addition to the print control bar, the plate has an gray balance sheet. , offset information guides, tolerance ranges, and signal bars for visual comparisons. Bruner designed special press control strips for presses with automatic control systems, such as Heidelberg CPC and Roland CCI.

Modern printing presses can be optimised with test plates, and printing presses can also be used to test and inspect printing presses. This is the conclusion drawn from the comparative analysis of experimental samples of printed samples.

The densitometer used for measuring Bruner's system is a narrow-band densitometer. The prescribed aperture is 4mm in diameter. Both the MacBeth manual densitometer and the MacBeth densitometer can be used to measure the Bruner system. Densitometer peripheral screens can display time, date, standard values, field density, dot gain, and overprint.

There are many systems that can be used for printer control, but only the Brunel system is highly praised. He pointed out the most important control variables related to dot gain.

(to be continued)