There is no doubt that process color printing has developed from an art to a science, from camera and art boards to fully data prepress. But the printer still has to contend with subtle changes in the raw materials - both inks and substrates - which are assembled to form the final printed product. These changes, particularly in substrates containing high recycled content may manifest themselves as major swings in color balance on press.

Improper color balance will appear as an overall colorcast in the print reproduction. This typically occurs when all the colors of the reproduction, including the neutral gray areas, undergo a shift in the direction of the colorcast. The color imbalance can be induced at any stage of the reproduction process, whenever the image is transferred to another format. For example, scanning, converting from RGB to CMYK, output to proof and printing can all result in a color shift as the image changes medium.

The introduction of digital color management systems has, for the most part, eliminated the generation of imbalance in the prepress area by aligning the color gamut of the various devices. The fact remains, though, that overall colorcast is still prevalent in the final press print, particularly on paper products. This article will explain the application of process color management through gray balance control at press side.

Color Communication To understand how colorcast can be controlled we must first look at the color communication system. During charaterization of the reproduction process and subsequent printing, there are two principle operators who determine the overall color balance throughout the tonal ranges in a process color print. First is the press operator, who establishes the trapping sequence, trap values and solid color densities of the primary colors. Second is the prepress operator who sets the halftone percentage mix of the overprint colors in the tone areas of the print, such as highlights and midtones. The overall balance of color in a print reproduction is determined by the strength (density) and color (hue and chroma) of the three process inks when printed. Only when these print factors have been stabilized can the percentage mixtures of the colored tone dots in the rest of the print be determined. Without close control of the inks on press and communication of colors and tone reproduction between press and prepress operators, there can be no guarantees of printing within the control parameters of a color management system. It has been shown through long experience that controlling ink density and gray balance on press may be the single most important function in color management of halftone process color reproduction. Gray balance control is not a new concept. Before the age of computers, when separations were camera based, the standard SWOP (Specifications for Web Offset Publications) densities were established using this very technique. Since then several enures have changed. Flexography has established itself as a major contender in process color printing. The colors of

Additive colors (RGB)     Subtractive color (CMYK)

As a general ruie, the yellow should be 0.40 below your Cyan and Magenta. The cyan and Magenta should run within 0.05 of the press operator can only measure and set the ink densities within the limits, which are dictated by the printing process. These limits are specified in various publications such as FIRST and SWOP (Standards for Web Offset Printing). But these density tolerances allow for a wide variation of the overprint trap colors, and what if the inks are not pure and transparent, or the substrate is not pure white? This current practice is not sufficiently in control of the printed colors for application of prepress color management systems.

According to the specifications, the range of density values shown above may be applicable to any color sequence - YMC, YCM, CMY, CYM, MCY, or MYC. But will each sequence produce the same overprint colors? In a word, no.

Even if the solid densities are controlled to the absolute mean values, each trapping sequence will produce different trapped colors of red, green, blue and three-color black. This is because each ink formulation has specific properties that affect trapping. These include opacity, which affects the "hiding power" of the ink and therefore how much of the previously applied ink will show through; and the surface energy of the dry ink film, which affects how well the subsequent application of wet ink will cover the previously applied ink. Opacity is generally low in the yellow pigments and higher in cyan and magenta, whereas surface energy is generally higher in the yellow-pigmented inks.

The first step in color management at press side is to determine the best trap sequence and density balance that will provide the highest color gamut. Some years ago the Graphic Arts Technical Foundation (GATF) developed a Color Hexagon (see Figure 1) to provide the printer with a simple way to visualize the interactions of the primary colors. Densitometric information from the printed colors including hue error, graynessand overprint trapping, may be displayed and compared to the perfect set of inks.

For example, at the even numbered corners of the hexagon are the three perfect subtractive primaries (YMC) and at the odd numbered corners, the perfect overprint traps (RGB). The center of the hexagon is neutral - white, through gray, to black. The stronger the individual ink color, the more the neutral is pulled toward the respective corner.

When the actual printed colors and overprints are plotted on the GATF Hexagon diagram, the densities, hue error and grayness values of the primary inks (CMY) and the trapped colors (RGB) are displayed. Because the inks are not perfect, the resultant six-sided (irregular hexagon) shape of the plot will not be as large as the full diagram but the shape can be used to optimize the printing process. Therefore, using the GATF method of plotting an irregular hexagon to display the printed primary colors and trapping information, the best densities and trapping sequence can easily be determined. The area covered by the print measurements determines the overall gamut and contrast of colors in the printed piece and should be as large as possible.

Other density values may also be plotted as part of the measuring sequence, e.g. quarter-tone, half-tone and three-quarter-tone values of the respective colors, together with those of the press proof. Concentric plots of these other tonal values will indicate that the process is in control, whereas misshapen plots are an indication that the process is out of control and highlight what needs to be changed.

The Effect of Substrate Color

The range of density values and tolerances specified above does not take into consideration the color of the substrate. Typically, when the solid densities are read at press side the substrate color is included. The substrate color is subtracted from the equation only when trap and tone values are to be determined.

The color of the substrate can and does vary, affecting the final ink densities as well as the overprint colors. For instance a small shift of substrate color from yellow to blue may not be visibly apparent but will cause the measured yellow density to fall and both the cyan and magenta densities to increase, without-any change in the dry ink film thickness. If these changes in the densities of the inks are "chased" by the printer, the higher setting of the yellow density may be sufficient to produce an overall color-cast in the reproduction.

So how should the printer determine the correct ink densities, and how can the printer tell they are in balance and will remain that way when other factors, including the ink and substrate, change? The answer to this question may be found in the center of the GATF Hexagon diagram, where there are no colors, only levels of gray.

To explain the effects of gray balance imagine that the center point of the hexagon is the white substrate and yellow ink is printed. The denser the yellow ink, the further the white point is dragged toward the yellow corner. Similarly, when magenta is printed on white paper the white point is dragged toward the magenta corner. If yellow and magenta are printed over one another in perfect proportions and perfect trap, the white point will be dragged to the corner representing the perfect red.

Now cyan, the gray component of red is positioned diametrically opposite the red comer of the hexagon. If cyan is printed over the red, formed by the previous two colors (Y + M), the strength of the cyan pigment will pull the red spot back toward the neutral point in the center of the hexagon. When the density of the cyan is completely balanced with the density of the red, the resulting overprint color will be neutral and the spot will be in the center of the hexagon.

Measurin Subte Differences in color

The key to balancing the color of the substrate, the three primary inks and the overprint traps, is to maintain aligment of the centers of the hexagons, i,e, the neutral gray points. this must be true for all the gray tones, that are built from the primary colors, both from the overprints of the solid inks and throughout the tonal ranges. If both printer and graphic artist hava done their part toward applying gray balance control then the printer should, in theory, be able to produce a gray-tone stale using only the three primary colors.

By definition gray has no color - that is, it has no hue or chroma - and reflects equal amounts of light in the red, green and blue segments of the spectrum. Therefore, the density of black and also shades of gray should, in theory, produce equal readings in all the color channels of a calibrated densitometer. In practice, not all densitometers are that accurate and some imbalance between channels is inevitable. However, because the objective is to achieve a constant and repeatable value for the gray point, if the same calibrated densitometer is used for measuring the press samples each time a job is printed, a constant imbalance can be as accurate a standard as an absolute gray.

Even if the solid densities are controlled to the absolute mean values, each trapping sequence will produce different trapped colors of red, green, blue and three-color black. This is because each ink formulation has specific properties that affect trapping. These include opacity, which affects the "hiding power" of the ink and therefore how much of the previously applied ink will show through; and the surface energy of the dry ink film, which affects how well the subsequent application of wet ink will cover the previously applied ink. Opacity is generally low in the yellow pigments and higher in cyan and magenta, whereas surface energy is generally higher in the yellow-pigmented inks.

Some densitometers may be coupled to computers equipped with print measurement software (see Figure 2). These measurement systems can compare the densitometric values of the measured gray to a measured black standard. Using integrated color algorithms, these software solutions report the absolute values of the imbalance, giving the printer information as to which ink is out of balance and by how much. To achieve and maintain constant color balance, these computer aided measurement systems make press side adjustment simple and continuous records of print measurements can be stored for future reference.

Compensating for Differences

When compensating for the subtle variations in ink and substrate, the press operator must deviate from the normal practice of setting ink densities within a tolerance range and trying to control overall color cast only at the prepress stage. Mixing percentages of color by printing halftone screens can only be controlled  if first the maximum quantity (density) and quality (hue and chroma) of each printed color is under control and in absolute balance. Any color cast produced in the shadow area of the print, caused by ink or substrate variation, will automatically be reproduced throughout the whole tone scale. Once color balance can been established repeatedly and tone reproduction is controlled, color management can follow.

By controlling the density balance of the CMY primary colors, primary gray balance can be achieved and used as a control to compensate for any slight color cast that may be derived from the ink trapping and, or substrate color. Thus eliminating any overall colorcast.

In order for the printer to control the gray balance throughout tonal ranges he must first establish a standard for the three color overprint, which contains both the density and trap information of all three inks (C, M, Y) plus the substrate. The following procedure should be carried out during characterization to first establish the best density balance and enable prepress to apply the correct color compensation to the halftone overprints.

Ink and Substrate Characterization

Starting with the specified densities, hue error, grayness and tolerances for the inks, the press operator should begin by verifying that the correct ink kits are at the press. Color balance can then be achieved using the following procedure:

• Establish the printed densities at, or slightly above the high limit of each tolerance band.
• Check the hue error and grayness of each ink color against the standards. If the ink is out of specification then it must be changed before proceeding.
• Measure the trapped red, green and blue to verify that the inks are trapping to specification. If not, some adjustment is necessary before proceeding.
• Measure the density of the three-color overprint and check the density in the yellow, magenta and cyan channels.
• Reduce the density of the ink that corresponds to channel with the highest value.
• Repeat steps 3 and 4 until all three channels (C, M,Y) read within 0.05 density units.

When color balance has been achieved, the densities should still be within the specified tolerances but if one ink is slightly out of tolerance this is less important than maintaining color balance. These densities should be recorded to become the aim point for that particular set of inks and particular substrate in subsequent jobs.
The procedure for setting gray balance should then be repeated each time any process color job goes to press. Minimum compensations need then be made to re-establish the reference gray point providing there are no major changes in the inks and substrates.

Advantages of Gray Balance Control

Using only a densitometer, the press operator can set and control the shadow gray balance and also control the tone reproduction of the individual colors. By following a simple procedure, color management on press is achievable using the densitometric data.

By controlling the balance of the printed primary colors and establishing a repeatable reference point that reflects the interaction of inks and substrates, the printer has control of his part of the color reproduction process. This enables the prepress operator to apply color management to every stage of the reproduction process. One distinct advantage to using gray balance to control color balance is when non-standard densities or ink colors are used, as in modified process printing.