Six-color printing process theory and description (2)

 Two-tone printing also reduces the appearance of textures in overprinting. On the one hand, it reduces halftone dot patterns and on the other hand enlarges the range of reproducible tones. With six-color printing, coarse halftones appear less coarse, 50 lines/inch halftone looks like 100 lines/inch, and 65 lines/inch looks like 120 lines/inch or better.

进行 Printing with the duotones of cyan and magenta also extends the dynamic range of the copied image. The dynamic range is the difference between the brightest area and the darkest area. In short, it makes the darker tone darker and brighter.

Most prints have a maximum dynamic range of no less than 2.0. This standard is rarely met in screen printing and is generally between 1.6-1.8. A 2.0 dynamic range means that 1% of the bright dots can be restored, and the copied ink can absorb 99% of the light and reflect only 1% of the light (compared to the screen-printed black, which can only fall to 97.5%. -98.4%).

But the dynamic range of the human eye can reach 4.0 or more (1/10000 light perception, or 99.99% black feel). 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.

Using 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 so on. 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.

You can also benefit from the darker tone. 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 tones can compensate for difficulties in copying lighter and darker colors, and the color drift phenomenon is also well 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 deep water is bluer than the shallow water, that is, the shallow water is yellow. drift. 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 involved in the problem of color reduction, a certain wavelength of light is absorbed, and the reflected wavelength constitutes the color we feel. Light colors tend to be yellow, while dark colors tend to be blue.

The challenge faced by printing 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.

The theory and description of six-color printing process

First, let us 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 exactly 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-degree diagonal dots from the film to the substrate.

 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 the 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 midtone. 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.