The achromatic structure is a possibility of image structure in multi-color printing .
Four-color printing with achromatic structure (GCR)
Harald Küppers’s theory of colors results in the alternative possibility of achromatic structure (Gray Component Replacement, GCR) for four-color printing . Now all achromatic values in the print image are created by subsets of the printing ink black, which interact with the unprinted remaining areas of the white printing paper and thus produce the achromatic values. Subsets of the colored printing inks YMC are now only required to form the chromatic values of the color nuances. In principle, this turned the technology of four-color printing on its head. Because now you work with skeleton color separations for the colorful printing inks CMY and with a full color separation for black.
Of course, this technology requires that the printing ink black actually looks black, i.e. has a neutral density of at least 3.0. Where this is not the case, the missing neutral density of black can be supported by corresponding subsets of the colored printing inks CMY. But this only happens in the neutral image depths. This is called UCA (Under Color Addition).
If you want to paint a picture, there are two extreme options. The painter can mix the desired color nuance for each part of the picture on his palette and then apply it to the picture. But he can also first paint a black and white picture, which he then colorizes. This is how the colored copperplate engravings were made in the past. The first described method corresponds to the chromatic structure, the second corresponds to the achromatic structure. The achromatic structure has advantages: Significant stabilization of the printing process; lower ink consumption; less drying problems; less waste; improved image reproduction.
Seven-color print with achromatic composition
The inks used in four-color printing do not have the ideal properties that they should have. They have spectral errors called misabsorptions and misreflections . You can't construct the printing inks the way you want them to be. You have to work with the solutions that are given in color chemistry. For this reason, it is impossible to reproduce pure, luminous color nuances in the secondary colors RGB in four-color printing .
This deficiency is remedied by seven-color printing. In addition to the colored printing inks CMY, the printing inks RGB are added. In principle, seven-color printing only works with achromatic structure. The process reliability is again increased significantly; the color consumption is reduced again; there is also less waste; There are no problems with drying. And the quality of the image reproduction is enormously improved. Because in the critical secondary areas of RGB, pure, bright colors are reproduced. This of course has its price. Seven color separations have to be made and seven printing plates have to be copied. And the print sheet has to run through seven inking units. But in the case of large print runs, these increased costs are more than compensated for by the resulting savings.