This articled originally appeared in the November/December 2014 issue of SGIA Journal.
The Question of K: A Primer on GCR
For a theoretically perfect inkjet printer, combining cyan, magenta and yellow inks would produce a perfect black; the combined spectral reflectance of each ink would be sufficient to absorb virtually all visible light such that very little would be reflected back for your eye to detect. Reality, however, is not perfect. The substrates we print on are rarely perfect reflectors, they typically reflect certain wavelengths more so than others. Likewise, our inks are not perfect spectral absorbers, the pigments used tend to absorb some wavelengths more than others. When printed in even ratios, C+M+Y rarely produces a neutral gray – more common is a process gray with a sickly green or deep magenta cast.
That alone, however, is not an adequate reason to add black to the printing process; the C, M and Y ratios can be adjusted to produce a neutral process gray. Performing G7 calibration, for example, prior to profiling (characterization) will achieve that and produce a common visual appearance on virtually any four-color process device. So, why not just ditch the K and print with only the three primaries?
The question is a logical one. Three-color chromolithographic prints using photographic separations were introduced in Europe and the US in the latter part of the 19th Century, based on earlier lithographic printing technology invented in Germany over a half-century before. Color lithographs were not uncommon throughout most of that century, but used well over a dozen or more colors that typically overprinted a black base image. Colors were often printed as solids, but as techniques, ink chemistry, equipment and registration methods improved, color tints were overprinted to produce intermediate colors and reduce the number of ink colors actually required on press.
Mechanical separations using photographic plates became a common method of color printing by the beginning of the 20th Century. The earlier methods of color printing which printed solid colors on a base image, employed opaque inks. Color printing using photographic separations required semi-transparent inks that could be printed one atop the other. Rather than a black base image, a white substrate was used and black was produced by stacking the three ink solids. A wide range of colors, including photographic reproductions could be produced from these three primaries.
These were exponential advances in color printing, and their developed in the late 1800s was no accident. The Scottish physicist James Clerk Maxwell had published his electromagnetic field theory in 1865, and made a number of advancements in the field color photography. Physicists at this point were coming to understand both light and color perception in much more intimate terms, which led to ever more innovative technologies to feed the growing demand for color printing (which continues today).
What came to be understood was that the human eye contains cones that allow it to perceive color as three bands of spectral energy; three types of cones each with a unique peak responsivity. These are referred to as long, medium and short; or red, green and blue. When white light passes through a photographic plate, certain wavelengths are absorbed. For example, when red light is absorbed the remaining green and blue light excites the medium and short cones to cause us to see cyan. This can be accomplished by otherwise blocking the red portion of the spectrum as it passes through a glass plate, or by absorbing the red light with a pigment as it reflects off a white substrate.
Cyan, magenta and yellow are the inverse of red, green and blue. Cyan ink printed on white paper absorbs red light, magenta absorbs green and yellow absorbs blue. If you wish to convert white light to yellow, you remove the blue component. This is why incandescent lighting appears more yellow than compact fluorescent – lower color temperature (2800 vs. 5000 K), less blue energy. It is also why the RGB LEDs found in novelty color changing lamps typically cycle red, yellow, green, cyan, blue, magenta, white and back to red. The red, green and blue colors are produced by the LED’s individual chips, whereas the intermediate colors are produced by combinations: red and green yields yellow; green and blue, cyan; and, blue and red, magenta (violet). Turn on all three chips and you see white. If measured with a spectrometer, you would see that these lamps do not produce yellow, cyan, magenta or white light, only red, green and blue. They are the perfect inverse analogy of three-color process printing.
Coming up, Part 2...