Achieving Color Balance on Press

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<p>Achieving Color Balance on Press #1</p> <p>To read other articles in this series click on the installment number below|1 | 2 | The Importance of Gray Balance ControlThe color management link between press and prepress There is no doubt that process color printing has developed from an art to a science, from camera and art boards to fully digital prepress. But the printer is still faced with subtle changes in the raw materials, inks and substrates, which are assembled to form the final printed product. These subtle changes, particularly in substrates containing high recycle 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 is typically due to all the colors of the reproduction, including the neutral gray areas, being shifted in the direction of the colorcast. This imbalance can be induced at any stage of the reproduction process, where the image is transferred to another format, i.e. - scanning, converting from RGB to CMYK, output to proof and printing. 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 still remains 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 at press side through gray balance control. Color communicationTo understand how colorcast can be controlled we must first look at the color communication system. During characterization 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. 1. The press operator, who establishes the trapping sequence, trap values and solid color densities of the primary colors 2. The prepress operator who sets the halftone percentage mix of the overprint colors in the tone areas of the print, i.e. highlights, midtones etc.</p> <p>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 pre-press operators, there can be no guarantees of printing within the control parameters of a color management system. It has been proven 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 densities were established using this very technique. Since then several things have changed: - Flexography has established itself as a major contender in process color printing The colors of the basic subtractive primaries have changed due to new ink pigment technology Digital artwork has come into being The art of gray balance control on press seems to have been lost during the transition. Color management systemsColor Management Systems rely on controlled printing, and in turn, controlled printing can only be achieved if substrate color, process ink colors, overprint traps etc, are stable and in perfect balance. In reality, very often the balance of these critical parameters will change from press run to press run due to batch variations in the basic materials, ink and substrate. The pre-press artist, using all the controls, color management and measurement systems at his disposal, may still be unable to supply color corrected artwork that is guaranteed to print consistent, balanced halftone process color reproduction. This is due to subtle changes in substrate color, ink opacity or trapping, that generally go undetected by the printer. Density balance of the three process primary colors On press the three subtractive primaries are mixed together in various proportions (densities and tones) to form the spectral gamut of colors expected by the customer. It is the responsibility of the pressman to ensure that the densities of primary subtractive ink colors printed on the substrate are both correct and in balance. As a general guide to obtaining the correct print contrast, the densities should be balanced according to the published specifications and tolerances. For example (Table 1): Table 1 Densities according to Flexographic Image Reproduction Specifications &amp; Tolerancing ApplicationCyanMagentaYellowBlack</p> <p>Wide web: paper products1.18 - 1.321.18 - 1.320.95 - 1.051.43 - 1.57</p> <p>Wide web: film products1.18 - 1.321.13 - 1.270.95 - 1.051.33 - 1.47</p> <p>Narrow web paper products1.28 - 1.421.18 - 1.320.95 - 1.051.43 - 1.57</p> <p>Narrow web film products1.18 - 1.321.13 - 1.270.95 - 1.051.33 - 1.47</p> <p>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 (Flexographic Image Reproduction Specifications &amp; Tolerancing) 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. The effect of trapping sequenceAccording to the specifications, the range of density values shown in Table 1 may be applicable to any color sequence - YMC, YCM, CMY, MCY, etc, 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 - Re d, Green, Blue and three color Black. This is because each ink formulation has specific properties that affect trapping. These are: - Opacity, which affects the 'hiding power' of the ink and therefore how much of the previously applied ink will show through 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. Determination of the trap sequence 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 GATF (Graphic Arts Technical Foundation) developed a Color Hexagon (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, grayness and overprint trapping etc, may be displayed and compared to the perfect set of inks. Figure 1a: The GATF Hexagon diagram 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 towards 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 three quarter tone values of the respective colors together with those of the press proof. Concentric plots of these other tonal values will determine 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 colorThe range of density values and tolerances specified in table 1, 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 remain that way when other factors, i.e. 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. Gray balanceTo explain the effect of gray balance - referring back to the GATF Color Hexagon in Figure 1. Imagine that the center point of the hexagon is the white substrate and yellow ink is printed. The more dense the yellow ink, the further the white point is dragged towards the yellow corner. Similarly, when magenta is printed on white paper the white point is dragged towards 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 corner 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. </p> <p>The Importance of Gray Balance Control How to compensate for the subtle differencesWhen 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 &amp; 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 procedure 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 proccedure: - 1. Establish the printed densities at, or slightly above the high limit of each tolerance band specified by either your ink supplier of using FIRST density tolerances 2. 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 3. Measure the trapped inks Red, Green and Blue to verify that the inks are trapping to specification. If not then some adjustment is necessary before proceeding 4. Measure the density of the three-color solid overprint and check the density in the Yellow, Magenta and Cyan channels. 5. Reduce the density of the ink that corresponds to channel with the highest value 6. 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 poi nt providing there are no major changes in the inks and substrates. Note! - A densitometer coupled to customized Gray Balance Control software will render a more accurate determination. The advantages of gray balance controlUsing 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, </p>