Gray balance is a top requirement for overall color balance in the reproduction. The three process color inks - yellow, magenta, and cyan - most often will not produce a neutral. Nor will they always produce neutral light, medium, and dark grays in screened values. White light is made up of equal parts of red, green, and blue light. Black is achieved when all three have been completely absorbed. The full value range of neutral grays that lie between white and black can be achieved when balanced or equal levels of each of the three light primaries have been absorbed.
Gray balance refers to the ability of the set of process inks to absorb equal amounts of the three light primary colors across the range of light to dark when they are produced on the press. Ideally, this means that a 5% dot value in each process color would produce a very light gray. A 50% dot value in each would produce a medium value of gray, etc. In reality, equal screens of process inks do not absorb equal amounts of the primary colors and so, without correction, do not produce a value range of neutral grays. They are imperfect, out of balance.
An important aspect of color separation is to adjust the screen values or dot sizes to achieve gray balance. If this balance is not achieved throughout the possible spectrum of halftone dots, the neutral grays will be tinted yellow, red, or blue. Improper color balance also can cause the colors on the print other than gray to shift. The cyan ink is overpowered by the magenta and yellow, resulting in a brownish color where a neutral gray or black is desired.
This is controlled in color separation by "imbalancing" the three color printers. The cyan printer is allowed to print larger dot sizes of ink while the magenta and yellow printers are reduced in dot size by a controlled amount. A correct set of separations will be imbalanced to produce neutral values throughout the value scale from highlights to shadows. At that point, gray balance is achieved.
The potential causes of improper gray balance are numerous and include almost every variable in the process color system. Color separations often can be blamed for improper color balance, not necessarily because they are incorrect, but because they usually are adjusted to the reproduction characteristics of another printing process.
The following table is an example of gray balance values for CMY (from FTA's FIRST). This is just an example. These values will need to be figured based on the dot gain, density, color of process inks, and trapping characteristics for each set of printing conditions. Running a characterization can help the prepress supplier determine the best gray balance values for a given set of conditions. Running control targets, such as the one recommended by FIRST (also pictured below), can help the printer make sure nothing has changed in the printing process.
The theory of color printing suggests that acceptable results can be achieved by using the three secondary colors or primary absorbers. This is often true, and the addition of a poor black printer often will hinder more than help. The black's purpose is twofold. First, it allows for a less than perfect color balance because it contributes to neutrality in color areas that are slightly out of balance and which should be neutral. Second, it allows for a darker shadow or contrast, which often isn't possible simply by overprinting the three process colors.
Black does what cyan, magenta, and yellow do independently, that is it absorbs all of the green, red, and blue light that strikes it. Black does this all at once, so it can be substituted where all three primaries need to be absorbed. Black can be used where unequal, but not necessarily dark, colors are needed. Since black does the primary absorption, it can substitute for the other colors.
For example, in a section of a print that is expected to reproduce a specific color on the original, 80% magenta, 70% cyan, and 50% yellow is required. If a black printer were used, the separator would take this into account and reduce the level of the three colors to an equal amount and allow the black to make up the difference to reproduce the same density on the print. Subtraction of each of the three color printers from neutral tones in the reproduction and substituting black for them is called "under color removal" (UCR). When done correctly, UCR reduces the amount of expensive process color ink used and increases the less expensive black ink without seriously affecting the color quality of the reproduction.
Often, a "skeleton black" is used. In this case, only the extreme dark areas of the print receive black ink. The reality of dot gain makes any gain of the black printer three times as troublesome because of its absorption in three different wavelengths. So the restricted area of the skeleton black is very popular because dot gain will occur only where added density is required.
The FTA FIRST organization states UCR is not always the best solution for flexographic printing. It is important to maintain definition, contrast, and shadow detail when using under color removal.
Standard Color Separations vs. Achromatic Color [Gray Component Removal (GCR)]
The standard color separation halftone printing process is based on the combined effects of additive and subtractive color-mixing principles. In chromatic process-color printing, all hues, as well as black and neutral gray, essentially are made with the three basic inks. Black is used to stabilize dark gray and add depth to shadows as indicated above. When three colors are overprinted, the two dominant ones determine the color, with the third, lesser color contributing to the color printing and grayness.
In recent years, a different approach to process-color printing has caught people's attention. This approach has been called achromatic process color or gray component removal, among other terms. In GCR process color printing, the primary and secondary colors are the same as in chromatic printing, but black replaces the gray component of the third color. Tertiary colors* now are composed of two colors and black. Now black is the dominant color in gray and black. Modern electronic scanners allow the unwanted color to be selectively removed from the yellow, magenta, and cyan, and a portion of the removed color is replaced by black (refer to table below).
| Chromatic Process | GCR Process | |
| Primary Color Space | ||
| Yellow, Magenta, Cyan | Darkening is accomplished with equal quantities of the other two primary colors. | Darkening is accomplished with black. |
| Secondary Color Space | ||
| Red Green Blue | Yellow + Magenta Yellow + Cyan Magenta + Cyan | Yellow + Magenta Yellow + Cyan Magenta + Cyan |
| Tertiary Color Space | ||
| Brown Dark Blue Dark Green | All 3 basic colors arranged according to chromatic values. Two predominant primary colors, one darkening color; black is sometimes added for depth. | Two basic colors + Black Brown = yellow + magenta + black Dk. Blue = magenta + cyan + black Dk. Green = yellow + cyan + black |
| Black and Neutrals | ||
| Three basic colors in their respective percentages with the addtion of black for depth. | Theoretically: black only Components of the 3 basic colors, depending on printing conditions. | |
Advantages claimed for GCR include sharper printing (because all detail is in the black), better color consistency, stable gray balance, less sensitivity to color-ink fluctuations, reduced ink usage and fewer trapping problems.
On the negative side: this process is unique and requires a major re-learning of process color for those used to the chromatic method. Proofs will be different and progressions will be obsolete.
Intermixing of standard and chromatic separations is not a good idea. Also, the black ink must have a higher density, must be neutral and must give adequate coverage. Dot-gain control of the black printer is essential.
The FTA FIRST organization recommends GCR be restricted to a single unwanted color. FIRST also states GCR should not be used when the printer is printing line black and process black on the same plate. In this situation a skeleton black (or a short black) may be preferred to allow the printer greater tolerance in setting impression.
*Tertiary Colors – Those obtained by mixing two secondary colors.
- Foundation of Flexographic Technical Association. Flexography Principles and Practices Fourth Edition. Ronkonkoma: Foundation of FTA, 1995.
UCA (under color addition) increases the dot sizes of the three process colors in the dark neutral areas. This compensates when the GCR process reduces cyan, magenta, and yellow dot sizes too much. Without UCA, dark areas may be lighter than desired. On the other hand, UCR (under color removal) may be used to compensate for this same issue.
- Miles Southworth and Donna Southworth. Color Separation On The Desktop. Livonia: Graphic Arts Publishing, 1993.