Gray Balance and Dot Expansion Principles (1)

Gray Balance is the basis for any color separation method to achieve correct color reproduction. If gray balance cannot be achieved during color separation, the gray in the original document is no longer gray after copying, and other colors will inevitably produce color cast.

When using digital color separation technology (ie, when converting from an RGB image to a CMYK image), the main factors that determine the gray balance curve are ink and paper combinations, black generation types, black generation levels, and dot gain curves. When the above four factors are decided, the gray balance relationship will be determined.

I. Ash balance and its influencing factors

(A) The concept of gray balance

The concept of gray balance can be described as follows: yellow, magenta, and cyan three color versions generate neutral grey on prints at different dot area ratios.

According to the subtractive color theory, the superposition of the maximum saturation of the three primary colors of C, M and Y ink should be black. In the same way, the equal overlap of different saturations of the three primary color inks should also produce a different shade of gray. However, due to the fact that the actually used ink has drawbacks in terms of hue, saturation, and brightness, which are difficult to overcome in ink manufacturing, neutral grays cannot be obtained by the equalization of the three primary color inks. In order to make the three primary color inks appear accurate gray with different brightness, the dot area ratio of the three primary color inks must be changed according to the characteristics of the ink, so as to realize the gray balance that is essential for color reproduction.

(b) 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 relations. Before the plate making, the basic characteristics of the ink (such as hue, saturation, lightness, etc.) and the physical and chemical characteristics of the ink (such as the drying speed and gloss of the ink) need to be measured. The correct balance of gray balance is taken into consideration when making the plate. In addition, the printability of the ink will also affect the gray balance. For example, changes in ink viscosity, rheological properties, and ink layer thickness all affect the color reproduction of the original, resulting in destruction of the gray balance.

2. Paper properties

Different types of paper have a great difference in the color rendering ability of the same ink. The gray balance parameters of coated paper and offset paper commonly used in lithographic offset printing are not the same for three primary colors of ink. The second major indicator of paper's impact on the gray balance is its whiteness, and the rest are smoothness, absorption characteristics, glossiness, opacity, and pH. Changes in these factors will affect the correct realization of the gray balance.

3. Printing conditions

The variety of plates, the composition and characteristics of the photosensitive liquid, the thickness of the sand, the characteristics of the light source, and the exposure and development conditions all affect the final gray balance.

4. Printing conditions

The impact of printing conditions on the ash balance is mainly manifested in two aspects:

(1) The printing machine can be included in the printing press and the factors that affect the gray balance include blanket type and thickness, lining type, printing pressure, printing speed, type and accuracy of the printing press.

(2) The fountain solution includes such factors as the formula of the fountain solution, the amount of water used, the surface tension and pH of the fountain solution, the temperature of the water, the degree of emulsification of the fountain solution and the ink, and the like.

Among the many factors mentioned above, the printing and printing conditions have an effect on the gray balance, but they cannot be considered when making plates. If all the factors that affect the gray balance are counted, it will not work at all due to too many variables. Therefore, the main factors that need to be considered for plate making are ink and paper properties, which can be attributed to the combination of ink and paper. This is why Photoshop first needs to select ink and paper combinations when setting color separation parameters, and then select other conditions to determine the cause of gray balance.

Second, ink and paper combination

Different ink and paper combinations have different gray balance relationships and will produce different separation results.

Image processing software represented by Photoshop usually uses SWOP (Coated) as the default setting for color separation when digital color separation is performed. It refers to inks that comply with the Specifications for Web Offset Publication on coated paper. print. In most cases, good separation results can be obtained with this method. Although the actual printing process conditions may differ from this, for DTP operators who are not familiar with color separation theory and printing processes, this It is really a good choice. Although Photoshop refers to ink and paper combinations simply as Ink Colors, it actually refers to the type of ink, but also includes the paper that will be used when printing.

The paper used in the printing industry can generally be divided into three categories, Coated (coating paper, also known as coated paper), Uncoated (non-coated paper, also known as offset paper), and Newsprint (newspaper). The surface smoothness, gloss, ink absorption, water soaking, and ink transfer characteristics of these three types of papers are all different, and the tone characteristics, ink amount, and ink transfer amount that can be reproduced during printing are also different.

In addition to SWOP (Coated), there are also three types of inks and paper combinations: AD-LITHO (Newsprint), Dainippon Ink, European Ink and Paper Combinations for newspaper printing (Coated, newsprint and offset papers), SWOP In addition to coated paper, there are four newsprint and offset papers, Toyo Inks and paper combinations (copper paper, matt coated paper, newsprint paper and offset paper). Each ink sheet combination has a different default midtone dot enlargement value, which is the empirical figure that is calculated after long-term use.

If the user's ink and paper combination cannot be found in the list, you can use the ink and paper combination you actually use to obtain the relevant parameters using the proofing method and create a custom ink paper combination.

In most cases, the ink features used for printing will not be much different between different printers (printers) of the same type. For example, the same ink is used for the same Tektronix Phaser II printer, but there may be large differences in dot gain values ​​when used on another printer. Therefore, for different types of printers of the same type, it is mainly necessary to modify 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 occur when separating colors.