Detailed explanation of the six-color overprint scheme for CD copy (3)

Six-color ink supplement

Separating the magenta and cyan hues allows us to make the screen printing tone curve more stable without being too crooked. We can control the loss of high-profile nodes and expand the tone levels to enhance detail. Since the size of the 1/2 standard dot is used at the shadow tone, or four times the dot size is used in the highlight tone to obtain the same hue as the four-color print, this results in a greatly reduced color shift.

Since we have already explained the basic theory of this color reproduction method and discussed its advantages in screen printing, the following is the time when it is officially used. In the second part we will look at all the issues in production and the different technologies we can use to make copies. There are other issues such as gray balance, color channel separation, color separation, selection of halftone screen lines and screen angles, proofing, output using DCS 2.0, color sequence printing, ink, and register lines.

Theory applied to practice

It is the dream of every screen-printer to accurately reproduce a wide range of colors in the color gamut. However, as can be seen from the content of the first part, due to the limitations of the traditional four-color printing process in the printing method, this dream has been limited.

In order to solve the problems contained in the four-color printing, it is necessary to find a non-traditional no-profiling scheme, a small amount of halftone dot enlargement, and a tone compression scheme, and thus there is a six-color overprint replication scheme. Expand the ink unit to six colors, that is, add two shades of magenta and cyan in addition to black and yellow. This method does increase the cost, but at the same time we must see the benefits of the program for the image.

Rubber can only be made into a road if it is made into a tire. The six-color overprint scheme is also the same. Only practical practice can prove its usefulness. Here we must evaluate the color separation technology and look at some important issues. The issues include gray balance, color separation channels, halftone screen lines and screen angles, proofing, output using DCS 2.0, printing color sequence, and inks. Consumption and registration, etc.

When to use six-color overprint

Before dealing with color separation problems, it is best to determine if the processed image is not necessary or suitable to use a six-color replication scheme.

There are two types of images that are best suited for a six-color copy scheme (also known as two-tone copying). The first is images with darker details and full, rich colors that are difficult to obtain with traditional printing techniques such as black feathers, dark wines, black bottle glasses, walnut, reddish-brown and other dark woodwork. In the traditional printing, in order to obtain these colors, black must be added in the color separation, but the black color suppresses the fine color tone of the image, and the effect sought will be obliterated.

The other image types are those that have tertiary color components in high-detailed details or high-profile regions. This kind of image appears in POP panels and similar extreme images. Examples of such images are water-colored soap bubbles, shades of white eggs, details of lace and white clothes, white clouds or barbecue foods. Natural color images such as flowing water, rocks, stones, sand, feathers, bark, shells, leather, sculpted gemstones, and ice are also images that use two-tone reproduction.

Note that the very high light and dark shades in a single image are unlikely to occur at the same time. In general, either very high light conditions or very dark shadows occur. Also keep in mind that the two-tone process is not a panacea. It has the same limitations as other printing methods. The more detailed or dark areas of an image, the more careful and serious it is. Since the trapping method is also used in the six-color reproduction, many human-perceived colors are still outside the color gamut range or the reproducible range, especially the very bright green, purple, and violet/purple colors.

For simplification, focus is placed on images with a large amount of shadow detail and a large number of highlight and tertiary colors. These images are often the problem of screen printing, and there will usually be dark tone and high-profile dot gains and excessive moire. The color separation object used for this test is the familiar Kodak Q60 test image.

Color separation

Photoshop is usually used for color separation. Before working with six colors, you first need to make some basic settings in the Separation Settings dialog box. In Windows and Mac OS by opening the File>Color Settings>CMYK Setup dialog box.

There are several separation settings in image separations that require adjustment for six-color separations. First, adjust the ink color and black generation values ​​in the CMYK Setup dialog box.

If you have a colorimeter or spectrophotometer, measure the L*a*b* value for each printed color and record it in the Ink Options window in Photoshop's Ink Options box. For the purposes of this article, no measurement is made, but the default setting is SWOP Uncoded. Highlight only high-profile colors without darkening the normal separation ink. The focus here is to improve the fine soft tones in the high profile area while maintaining the dark image density.

One of the key adjustments here is to set up the black version in the separation. In order to make this setting, you need to open the Separation Options window and select the black version to enter the black generation curve. The next step is to click the 0% point (lower left corner) with the mouse and drag it to the right to the 50% point. This means that there is no black print in less than 50% tone. At the same time, in the upper right corner, drag the drop down to 75% so that in the darkest area of ​​the image, the black version will not have a value greater than 75%.

The purpose of this is to make the image generated by the trapping achieve gray balance at all shades. This is a typical Under-Color Removal (UCR) process that is particularly suitable for the brighter part of the image. It also reduces the amount of blackplates attached to the darkest areas of the image.

One of the advantages of color reproduction using the double cyan and magenta inks is that it reduces the amount of black ink required to reproduce high and gray ash and rich colors. By adjusting the curve, the black version is applied in the 3/4 tone region, and the other five colors are used to obtain clear gray and shadow details.

(to be continued)