Color Separation Techniques in Printing
UNIT 13: THEORY OF REFLECTION
The pigmented color image is expressed in our eyes and by recruitment teams based on the colored light that is able to reflect, in the case of opaque media (pigmented areas), or transmitted in the case of transparent filters.
Color vision is always made by additive synthesis, so we must turn to their principles. Any pigmented surface will tend to absorb light that reaches those who do not have RGB components and reflect the existing ones, which, being appreciated by the feedback system (eye, scanner, camera) determine the chromaticity of the same. A green area will see an impact because the white light on it, reflecting the green component of this through and have it stand and absorb the blue and red do not have.
SEPARATION PROCESS for an original whose surface is green:
TONE SEPARATION
SHORTCOMINGS IN THE SEPARATION OF COLORS AND TONES
The current system of ink manufacturing does not allow for pure ink, i.e., ink C is not only composed of C, but has small amounts of M and Y. This causes an error in the separation of colors and tones. On the other hand, filters are not pure, i.e., an RC filter consists of not only R but also has a small dose of G and B. As in the inks, this causes an error in the separation of colors and tones. The errors of pigmentation of the filters for separation and printing inks produce two types of deficiencies in the four-color color: the presence of abuse and lack of proper color.
▪ Error color by the presence of abuse. The former leads to the presence of complementary colors in each primary color channel and therefore a loss of overall brightness of the playback. The signal is corrected by subtracting the other two color channels.
▪ Error due to lack of color. Causes a loss of color saturation. Its effect is noted where the three inks (CMY) would have to generate black and consequently it is necessary a complementary ink, which is the black (K) to compensate for this error.
An IDEAL INK would fully reflect the appropriate color light and absorb the rest. But in reality, the inks absorb some of the light they should reflect (giving due to the lack of color) and reflected, in part, those that should not reflect (the presence of improper color).
THE TOTAL INK COVERAGE, also known by its acronym in English as ICT refers to the sum of the percentages of different inks in the darkest areas of the image.
ADVANTAGES AND DISADVANTAGES OF TECHNIQUES OF REDUCTION
_ Reduction in the cost of ink due to smaller contribution.
_ Reduction in the problems of ink drying.
_ Increase in the speed of the roll.
_ More stable printing conditions.
_ Very stable gray balance.
_ Increases the adhesion of inks.
_ Printed results are cleaner, with more saturated colors.
_ Reduction in the possible problems of repainting.
_ Reduction in print trapping problems, since the lower the tones of color ink, the less ink is accumulated on the support.
DISADVANTAGES
_ Depending on the degree of reduction, it can result in a plot point so fine that it may be lost when making a generic way, of course, errors in printing color.
_ It requires more control over the ink-water balance in offset printing.
UCR
UCR, or Under Color Removal, is a selection technique that eliminates achromatic color inks in the gray areas of an image and replaces them with a black frame. Equal values of cyan, magenta, and yellow produce a neutral gray, and the UCR technique replaces them with an equivalent amount of black. This produces less ink and deeper into the shadows. Because it uses less ink, UCR is used for newspapers and uncoated paper, which typically have higher dot gain than coated paper. It should be noted that this technique is only applied on neutral tones and not the saturated ones. It is therefore a useful technique for images in which gray abound, and to help maintain neutral tones to where they are, while not altering the image areas with saturated colors. This technique eliminates problems of trapping and drying of the ink during printing.
GCR
GCR, or Gray Component Replacement, also called PCR, acts not only in the neutral colors of an image, but in all colors. You can consider any color as consisting of a gray component of an achromatic component. GCR replaces the gray component, obtained by equal amounts of cyan, magenta, and yellow, and replaced by black ink.
Like the UCR, it can be applied to different degrees, it can be eliminated only a small component of gray color inks or remove the maximum amount of color inks without affecting color balance.
The benefits of applying the GCR technique will be revealed during the printing process: less ink, trapping reduction, faster drying inks for printing, which allows faster roll, reduced Retinto, etc. Therefore, the GCR is applied at the request of the printer. GCR is applied also when scanning pictures to comics, with nearly flat colors defined by black lines, as this prevents strokes in those involving black color inks, as these are replaced by black ink
UCA
UCA, or Under Color Addition, consists of adding percentages of cyan, magenta, yellow, and black to neutral areas previously removed with UCR. It is a necessary process to compensate for the loss of intensity where the black ink does not adequately cover.