Gray balance and dot expansion principle (1)
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Gray Balance is the basis for any color separation method to achieve correct color reproduction. If gray balance cannot be achieved when color separation, the gray in the original is no longer gray after copying, and other colors will inevitably produce color cast.
When digital color separation technology is used (ie, when converting from RGB images to CMYK images), the main factors determining the gray balance curve are the ink and paper combination, the black version generation type, the black plate generation degree, and the dot enlargement curve. When the above four factors are determined, the gray balance relationship is determined.
First, the gray balance and its influencing factors
(1) Grey balance concept
The concept of gray balance can be described as: yellow, magenta and cyan three color plates produce neutral gray on the printed matter according to the proportion of different dot area ratio.
According to the color reduction method, the superposition of the maximum saturation of the C, M, and Y primary color inks should be black. Similarly, the same amount of super-saturation of the three primary color inks should also produce gray of different brightness. However, since the ink actually used has defects which are insurmountable in ink production in terms of hue, saturation and lightness, an equal amount of the three primary color inks cannot be obtained to obtain neutral gray. In order to make the three primary color inks overlap and display accurate gray of different brightness, it is necessary to change the dot area ratio of the three primary color inks according to the characteristics of the ink, thereby realizing the gray balance which is essential for color reproduction.
(2) Factors affecting the gray balance
There are many factors that affect the gray balance. It can be said that almost every link in the color reproduction process will cause changes in the gray balance. The main factors are:
1. Ink characteristics
The inks produced by different manufacturers have different gray balance relationships. Before the plate making, it is necessary to measure the basic properties of the ink (such as hue, saturation and brightness), the physical and chemical properties of the ink (such as the drying speed and gloss of the ink). The correct gray balance relationship, these factors are taken into consideration when making plates. In addition, the printability of the ink will also affect the gray balance. For example, changes in the viscosity, rheological properties, and thickness of the ink layer may affect the color reproduction of the original, resulting in damage to the gray balance.
2. Paper characteristics
Different types of paper have great differences in the color rendering ability of the same ink. The offset paper and offset paper commonly used in lithographic offset printing have different gray balance parameters for the three primary color inks. The second major indicator of paper's influence on gray balance is its whiteness. The rest are smoothness, absorption characteristics, gloss, opacity and pH. These changes will affect the correct realization of the gray balance.
3. Printing conditions
The variety of the printing plate, the composition and characteristics of the photosensitive liquid, the thickness of the sand, the characteristics of the light source, and the conditions of exposure and development all affect the final gray balance.
4. Printing conditions
The influence of printing conditions on the gray balance is mainly reflected in two aspects:
(1) Printing machine Factors that can be included in the printing machine and affect the gray balance include the type and thickness of the blanket, the type of the lining, the printing pressure, the printing speed, the type and accuracy of the printing machine, and the like.
(2) Dampening solution Such influencing factors include the formulation of dampening solution, water consumption, surface tension and pH value of dampening solution, water temperature, emulsifying degree of dampening solution and ink, and the like.
Among the many factors mentioned above, although the printing and printing conditions have an effect on the gray balance, they cannot be considered during plate making. If you factor in all the factors that affect the gray balance, you will not be able to work because there are too many variables. Therefore, the main factors to consider for plate making are ink and paper properties, which in turn can be attributed to ink and paper combinations. This is why Photoshop first needs to select the ink and paper combination when setting the separation parameters, and then select other conditions to determine the cause of the gray balance.
Second, the ink and paper combination
Different ink and paper combinations have different gray balance relationships that will produce different color separation results.
Image processing software represented by Photoshop usually uses SWOP (Coated) as the default setting for color separation when performing digital color separation. It refers to the use of inks conforming to the specifications of the Offset Offset Publication (Specifications for Web Offset Publication) on coated paper. print. In most cases, good separation results can be obtained with this method, although actual printing process conditions may differ, but for DTP operators who are unfamiliar with color separation theory and printing processes, this It is really a good choice. Although Photoshop simply refers to ink and paper combinations as Ink Colors, it actually refers to the type of ink, and also contains the paper that will be used during printing.
Paper used in the printing industry can be broadly classified into three categories, Coated (coated paper, also known as coated paper), Uncoated (non-coated paper, also known as offset paper), and Newsprint (newspaper). The smoothness, gloss, ink absorbing, water immersion, and ink transfer characteristics of the three types of paper surfaces are different, and the tone characteristics, ink amount, and ink transfer amount which can be reproduced at the time of printing are also different.
In addition to SWOP (Coated), there are three combinations of ink and paper for newspaper printing: AD-LITHO (Newsprint), Dainippon Ink, European ink and paper combination (coated paper, newsprint and offset paper), SWOP In addition to coated paper, there are four types of newsprint and offset paper, Toyo Inks and paper combinations (coated paper, matt coated paper, newsprint and offset paper). Each ink sheet combination has a different default midtone dot gain value, which is an empirical number that has been calculated over a long period of time.
If the ink and paper combination used by the user is not found in the list, you can use the proof of the ink and paper combination you actually use to obtain the relevant parameters and create a custom ink paper combination.
In most cases, the ink characteristics used for printing will not differ much between different printers of the same type (printers). For example, the same Tektronix Phaser II printer uses the same ink, but the dot gain may vary significantly when used on another printer. Therefore, for different printers of the same type, the main need to modify is the dot gain value obtained according to the actual use situation, instead of changing the ink type. For images to be used on digital presses, the same problem may be encountered when separating colors.