Color adjustment

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In the color adjustment , a desired color pattern is reproduced as precisely as possible with a given colorant set .

Basics

Color adjustment means defining a color as a color location in a color system and displaying it in the gamut (the feasible color space ): This process is known as inner color mixing . The following examples are intended to illustrate the situation:

  • Color formulation: The textile manufacturer receives the designer's template drawn on paper and production should (within agreed tolerances) color the fashionable color tone by using the available (economically justifiable) dyes.
  • Image processing: The photo provided should have the same color brilliance in the printed advertising catalog, color management.
  • Reproduction: The original that has been scanned and edited on the computer should appear again in the magazine in the correct color.

The basis for color adjustments is the fact that (depending on the task at hand) there is a visible component in the emitted or remitted light. This electromagnetic radiation in the range from 380 nm to 780 nm falls in the cone of the eye onto receptors that convert the incident energy into nerve impulses. According to the three types of cones, there is a color stimulus on the optic nerve, which can be mapped in a three-dimensional measurement space (for more details, see color space ). For the human individual, the optical perception of color arises . The neural trick: from an intensity distribution over the visible wavelengths, a summation over the sensitivity curves of the R, G and B cones is converted into a stimulus, a color stimulus, a color perception. This visual process is used for color adjustment.

execution

The color stimulus of the template is determined with a spectral, a three-range device or simply with the eye and implemented in a first test sample with suitable colorants for the desired substrates (materials such as textiles, printing inks, paper, lacquers, monitor fluorescent materials) or light colors . When measured with the same equipment, this sample should produce a sufficiently accurate reproduction of the original.

Today, spectral devices are usually used which determine 16 or 31 support points from the spectrum of the template and these are converted into the three coordinates {X, Y, Z} according to the standard valences and an agreed type of light . Any other color space is also suitable. CIELAB (currently) has the advantage that deviations in the Euclidean distance almost correspond to the visual perception.

In other words: in order to reproduce a color sample (body color), it is first measured colorimetrically . For practical purposes, 31 reference points in the visible spectrum (400 nm to 700 nm) are sufficient , according to the CIE tables for standard spectral color values. Direction- dependent effects, such as gloss, are eliminated by the measurement arrangement; the gray value is part of this colorimetric measurement.

The original spectrum , as emission spectrum or remission spectrum , should now be reproduced in such a way that the same sensory impression arises from the reproduction , that the color valences are the same. The proportions (concentrations of substances, intensities of lights) are determined from an existing set of colorants in such a way that the same color stimulus as from the original spectrum hits the sense of sight. The difficulty is, on the one hand, that this set of colorants is limited by the intended use. On the other hand, this means that non-original colorants are possible and the lighting must also be defined for body colors , usually a standard light type.

Limits to the reproduction of colors

So the adjustment is only possible for pure subtractive color mixing and autotypical color synthesis . Due to the mostly limited range of technical possibilities, usually only a metameric adjustment can be achieved.

With additive color mixing ( computer monitor , television ), radiators with their respective frequency spectrum are the cause of the color impression, the intensities of the individual elements are superimposed and cause a certain color stimulus. Here, too, what is technically feasible is limited by the range of materials available. What is technically feasible for monitors is limited by phosphors, which on the one hand can be produced economically and on the other hand have a sufficient service life.

Since the eye is always required for color, so that the human being is necessary, the objective for re-enactments is also defined. In principle, the colorist can carry out the adjustment by visual comparison, i.e. by eye. He creates a series of samples, which ideally come closer and closer to the original color. However, individual differences in the perception of the color stimulus of different test persons come to the fore. What is defined as the average normal observer in the CIE color space is replaced by the respective individual observer in the visual comparison . Differences between the trained colorist's eye and the perception of the entrepreneurial client cannot be ruled out. It should be noted that the CIE experiment was based on 17 test subjects. A readjustment or even a machine color formulation is easier to carry out with colorimetric support. Colorimetry has become faster thanks to better computing technology, but the theory of color spaces is not yet complete.

Demarcation

It remains to be added that the aim and description here is only to reproduce the color; the virtual influences (gloss, ambient color) left out of the term color according to DIN 5033 can also have an influence. A design template developed on the PC screen has a different structure than the T-shirt on which the design is to be printed. It may have been printed on paper from the PC to show the seller in the print shop how to set the bright, Mediterranean orange of the shirt printer's logo.

As long as the identical colorants, the identical substrates and the identical light and ambient conditions do not exist for the desired reproduction of a color, complications arise. One difficulty arises from the fact that the visual reality of the intensities measured at 31 support points, which were recorded in the eye by means of three sensitivity curves, cannot be adapted by adjusting to each of these 31 support points. Rather, only real dyes are available for color adjustment, the adjustment of which at one of these 31 support points also influences the other intensities.

Inner and outer color mix

Mixing color D from colors ABC is called improper or external color mixing. To do this, negative A would have to be added.

According to Graßmann's first law , color is a three-dimensional quantity for humans: With just three independent color tones , it is possible to represent any other color by mixing colors (examples in Additive Basic Color ). Color tones are independent of one another if no mixture of two of these valences results in the third.

In the CIE standard color table - a section through the visual color space - these relationships can be clearly displayed. On the straight line between the achromatic point U (the white point is here in the CIE diagram ) and a spectral color, there are colors with the same hue. They are more saturated (brilliant) the further the proportions of the standard color valences are from the achromatic point. The colors of maximum color saturation are the spectral colors and the hues of the purple line .

The extension of the straight line from the achromatic point through the desired color to the edge of the color table determines the wavelength of the same color . On the purple line there are mixed colors between violet and red , such a wavelength cannot be determined for this. By extending a straight line beyond the white point, the wavelength of the same color with a negative (!) Sign results:

The color D in the figure cannot be mixed from A, B and C because D is not in the triangle ABC, but outside.
The corner colors A, B and C (which span the gamut) should be as close as possible to the edge of the color sole so that the number of colors that cannot be displayed remains low. For technical reasons, this is not possible with television screens or computer monitors ( additive base color ).
The color M can be achieved by mixing B with C. Overlaying D with violet light A also creates M. Therefore, one has to mix negative violet A into M to get the light D. This is the external color mix or improper color mix .

If the color to be mixed is in the triangle A, B, C (the gamut ), the color can be mixed. This is the inner color mix , also often referred to as the actual color mix .

CIE standard color valences

The often quoted statement that any color sensation can be reproduced with three independent color valences suppresses the necessary theoretical assumption of (non-existent) negative light colors.

Since negative spectral values ​​can only occur arithmetically, efforts were made to ensure that the CIE ( Commission Internationale de l'Eclairage ) developed a system based on standard color valences with the designations X (= red), Y (= green) and Z (= blue content one color).

The device-independent standard color values ​​XYZ are converted into RGB for output on a device with additive color mixing , such as a monitor , and into CMY or CMYK for subtractive mixing systems (printers) .

There are generally no defined equations (gamut mapping) for these transformations. Even if these are defined between color spaces, a problem due to overrange can occur during conversion, which manifests itself in color distortion.

Previous optimal techniques put the required colors into laborious manual methods. Usually there is a list of assignments that are linearly optimized in the vicinity of these interpolation points. The visual impression on the monitor can be optimized with a color template by adjusting the color temperature . The solution is the specification of standards by the International Color Consortium with which input and output devices can be calibrated.

Using only the simple dimensional reduction of state vectors by mapping into the color space is problematic due to metameric effects.

Technical implementation

In addition to the color and colouristic requirements for the coloring agent, technical, economic and authenticity properties often also come into play during the reproduction. The available colorants have a specific representation on the xy color area. The other technical, physical and chemical requirements for the coloring agent are always limiting. If a high level of lightfastness is required, many possible colors are not required. An example is four-color printing: black is relatively easy to produce using carbon black as a pigment, the red-tinged blue is made with reflex blue and yellow with comparatively stable pigments. For the violet pigment required in terms of color, however, there is a small range of possible pigments available that are technically complex to manufacture. This limits the color palette that can be implemented in the CMYK system.

The machine limits the number of colorants for 4-color printing . The defined scale colors form the gamut and limit the possible results. Improper color mixing is not possible here, there are no negative remission results. Printing inks, such as those required for corporate colors ( corporate identity ), can be outside the gamut and must therefore be made as spot colors with suitable pigments.

Adjustments for high-quality art prints are achieved using 7 colors plus the paper white. This expands the gamut.

There are 2 color cartridges for high-quality photo printers . In addition to the usual cyan, magenta and yellow, a second cartridge with additional color inks is inserted in the black shaft.

In the white area, it is not possible to reproduce (naturally yellowish or yellowed) lead white colors with the bright white (modern and permanent) white colors based on titanium dioxide without further additives.