Detailed explanation of the six-color overprint scheme for CD duplication (2)

However, the dynamic range of the eyes of the human eye can reach 4.0 or more (1/10000 light perception, or 99.99% black perception). This is why dark details can be seen in the visual world and cannot be achieved in printing. With the two-tone color mode, we can increase the printed dynamic range to 2.2 (99.4% of black). This does not seem to be a big deal, but we can see that the black level obtained is better than the four-color overprint.

With two-tone printing, we have increased the highlights and shadows in the tonal range. In the past, our process often lost some of the layers when printing lighter, softer or darker colors. Duotones have a relatively long tone because of their soft appearance, good depth of detail, and dark details.

With the advantages of two-tone printing for high light color control, we have achieved better control over previously uncontrollable high neutral neutral colors such as beige, sand, ice, crystal reflections, detergents, tan, and more. These colors all have very few third color components. Duotone printing allows us to use larger dots in these color areas. This allows for greater control of the environment and physical variables, more accurate color reproduction, and shorter visible color range.

There is also a benefit in the darker part. By using darker colors than the normal primary colors magenta and cyan, we can better control the third color that needs to enhance the surface color of the dark tone region. Its main advantage is that we don't need to use too much black to compensate for the darkness. By using less black ink in the third color component, rich colors such as red brown, walnut, dark red, chestnut, dark green, indigo, and the like can be maintained. In the four-color mode, the additional black must completely cover the hue components of these colors, presenting a flat, black color.

Using two-tone can compensate for difficulties in copying brighter and darker colors, and the color drift phenomenon is also better controlled. Color drift refers to the color characteristics that change the appearance when the color tone becomes darker.

The swimming pool is an appropriate example of color drift. Although the water in the entire pool is the same, for the observer, the deeper water is bluer than the shallower one, that is, the shallow water drifts toward the yellow. This phenomenon is related to the problem of light absorption and reflection, that is, shallow water and deep water have different absorption and reflection of light. In the same way, printing ink is also the same, so there is also a phenomenon of color drift.

In the example involving the problem of color reduction, some wavelength of light is absorbed and the reflected wavelengths constitute the color we feel. Light colors tend to be yellow, while dark colors tend to be blue.

The challenge in printing is that when we increase the ink color density (Dmax of ink ink layer), the color is somewhat bluish. The higher the density, the darker the color, but it tends to shift away from the soft shades and drift to a distinct blue tone. On the contrary, if we reduce the color density of the ink, yellow drift will not show up in high-profile areas. Reducing the intensity of the color will show up in the dark tone, which shows a relatively flat color and loss of brilliance, with a certain yellow drift.

Six-color printing process theory and description

Let us first carefully study the two-tone process through the tone reproduction curve. People engaged in prepress work may have a deeper understanding of the traditional gamma curve, which reflects how the halftone dots on the film are converted into printed dots. When the printing dots precisely replicate the percentage of halftone dots on the film, that is, when the input is equal to the output, the gamma curve is transmitted in exact 45° diagonal dots from the film to the substrate for accurate copying.

This tone curve is almost impossible to obtain in realistic screen printing. In most cases, we must deal with the loss of high-profile regional outlets, maintain the relative stability of midtone transfer, and compress and expand the network of more than 75% of shadowed regions. The typical curve in four-color overprint screen printing should be an “S” curve. The finer the number of halftone screen lines, the greater the “S” outward expansion.

The typical result of four-color screen printing is an "S" curve, which indicates that the high-profile site is missing, and the dark tone site is expanded. The goal of six-color overprinting is to turn the high-profile and dark-sounding curves into a nearly straight line.

The stable transmission site of the curve is midtones. Here, the input and output of the dot are generally in a straight line, and the unstable parts are the toe (high profile) and the shoulder (dark tone) of the profile. These areas change very quickly and are difficult to adjust in printing. However, we can use this information to develop a more stable and repeatable six-color overprint.

With a six-color overprint scheme, it must be known that the reproduction of tones and colors are two related factors. The first factor to consider is the ink density or strength used. The correctness of this value is directly related to the second factor - how close the visible reflection value is to the value in the actual image.

For example, if we print with a standard magenta ink with a density of 1.15, the 20% printed dot area is very close to the 20% tone optical reflectance. However, if the ink density increases to 1.45, the actual reflectance in the 20% dot area is darker than we expected—close to 40% dot.

Halftone dot values ​​are closely related to ink density, which is one of the reasons why printers spend a lot of energy on density control. If one or both are incorrect, the final color will be biased. The severity of this deviation is proportional to the number of overprint colors used. Therefore, deviations in four-color printing may make the whole product messy.

The use of traditional cyan and magenta halftones in four-color printing has a problem in that when the midtones remain relatively stable, high-profile information is lost and dark tone information is enlarged. Can we compensate by combining the errors at both ends of the tonal range? The only answer is to separate the tonal range.

The purpose of the two-tone printing is to change the density or color intensity of the high- and dark shades of cyan and magenta in order to maximize the stability of the gamma curve to pass straight lines. This sounds very technical, but it is not. To achieve this goal, we only use halftones in different ways.

The principle that we rely on is to adjust the dot size by changing the intensity (density) of the color. We reduced the color intensity of the magenta ink to 25% of the normal case in the high profile area, which means that 100% of the magenta dots have the same reflectance as the 25% regular density ink dot. In other words, in order to get a 25% reflection, we need to increase the size of the high-profile magenta dot to four times the original. (The typical light cyan and light magenta color density values ​​measured by the densitometer are between 0.15-0.25.)

Further thinking, the 5% dots that can be printed have a 1.25% tone reflection, which is a very bright color. It allows us to have more control over the color, because every 1% dot gain is lost and the reflection value changes by only 0.25%. It minimizes the loss of high-profile sites and prevents color drift. In actual screen printing, 16% of the dot area change only caused a 4% change in reflectance. Within this threshold, each person engaged in color overprint jobs can easily work.

Another advantage of using this light-colored ink is that if a 5% print dot appears moiré, it is not easily seen on a white background of the paper. The use of this process by personnel who perform printing on very large areas of very soft colors and very high light areas can clearly feel its advantages.

At the dark end of the tonal range, we doubled the density of cyan and magenta (between 1.45-1.60). This means that 50% dark green or dark magenta printing dots have a dot equivalent to 100% standard ink (density 1.15). In other words, we can greatly reduce the size of the largest dot to the original 75%, and can achieve darkness and depth of darkness. We can also reduce the difficulty of maintaining 75% of the dots, so we can greatly enhance the details and resolution of the shadows, and successfully achieve the original difficult to achieve images, such as black feathers.

If the process is so good for cyan and magenta treatments, why don't we divide the yellow and black into two tones? The reason is simple: Yellow is a color with no contrast, because it is relatively shallow in itself, and its influence is mainly in the bright tone region of the tonal range. The darker cyan and magenta inks used in the six-color overprinting have less yellowish bias than the conventional inks and tend to be blue. Unlike four-color printing, black is used to compensate for yellow and increase dark tone density. Bi-tonal cyan and magenta in six-color overprinting actually reduces our need for black ink and may not be needed under certain circumstances.

(to be continued)