The Importance of Gray Balance
by Dennis Cheeseman
Gray balancing is a scanner-calibration procedure in which the operator programs specific dot-percent ratios of yellow, magenta and cyan. When film is output to these percentages and proofed or printed, it should reproduce a neutral-gray color.
Gray-balance agreement occurs when the proper proportions of yellow, magenta and cyan that were blended together at the scanner are reproduced on the press. Gray balance is a significant factor in determining what the overall color gamut will be.
We need to measure for gray balance because each person's color perception is different. The human eye can detect any shift in neutrality when neutral areas are compared side by side. Comparing two gray scales is much easier than comparing reproduced process inks with a color in the original. The eyes can quickly detect if there is any color cast in the neutral area. In this respect, the eyes become the most sensitive instrument for checking gray balance.
Differences between monitor and press colors create their own limitations. The color we view on a computer monitor is made up of red, blue and green spots, and can amount to as many as 16.7 million shades, of which we can see approximately 8 million.
The printing process, on the other hand, uses mixtures of yellow, magenta, cyan and black. These colors filter out various portions of the electromagnetic spectrum in an attempt to "simulate" the red, blue and green monitor colors. Due to ink and stock limitations, this is an imperfect process right from the start and can only reproduce approximately 5,000 shades of color.
The gray scale is a common device for checking both gray balance and tone reproduction. For gray balance, only the neutrality of the reproduced gray scale is compared with the original gray scale. To check for the accuracy in tone reproduction, the reproduced tone values of the gray scale are compared with the original tone values.
A midtone three-color gray can be composed of 50 percent yellow, 50 percent magenta and 60 percent cyan, rendering a neutral gray tone. All other midtone process-color values will be dependent upon this setting. If the gray-balance settings are altered, then the color balance will also be altered.
The second-most important control is the choice of gradation, which dictates how "full" a job prints. In other words, the scanner determines or influences the amount of dot gain a job will have (TechNews, March/April 1998, p. 24).
During the adjustment of gradation in a scanner, the tone values of gray are independently set for each color. If these settings do not compensate for individual characteristics of each ink and do not take into account the press and paper variables, there will be an uneven gray balance. The tone values for one or all three of the colors may have to be readjusted to properly reproduce the gray scale.
The third-most important control is the percent-UCR setting. This dictates how much yellow, magenta and cyan are printed in order to make up a three-color shadow neutral (grays and browns). This will affect how well inks will trap or overprint and what shadow colors can be reproduced.
With scanning having this much influence over color, it is imperative that the pre-press and press operations attempt to reproduce the scanner gray balances. Otherwise, we are only guessing at correct color during a makeready on press.
Dennis Cheeseman is director of customer services for US Ink. Phone, (201) 935-8666; fax, (201) 933-2291.