Colorimetric space

A colorimetric space or space of color associates numbers with the visible colors. Taking into account the limits of the human vision, these numbers are generally appeared as triplets. Each color of light can thus be characterized by a point in a space with three dimensions. During an impression, for reasons related to the quality of the pigments, space used comprises generally at least four dimensions then.

There are spaces which one can regard as elementary spaces . These elementary spaces, adapted to the description of a particular apparatus, present a disadvantage: they cannot constitute absolute references, which resulted in defining spaces of reference . The latter are used in particular like spaces of connection of profile to ensure the translation between two apparatuses.

The possibilities of a screen of computer or a printer, expressed in such spaces, are far from covering all the visible colors. The field of the realizable colors, or Gamut of the apparatus, is called also profile of color.

These concepts of space of color, profile of color and translation of a space to another were standardized by the International Color Consortium (ICC). Partial standardizations defined workspaces adapted to particular problems.

The human spirit having difficulties in reason in a space with three dimensions, the comparisons between spaces are carried out on sections of these spaces by plans corresponding to brightnesses given.

Elementary spaces

The human eye distinguishes the colors through receivers called cones. There exists in all and for all three types of cones, which means that any discernible color by the man is characterized by a point in a space with three dimensions. More precisely, the values “measured” by the three types of cones represent primarily the intensities of the red, of the green and blue, space RVB being defined starting from these three primary colors.

With this space, which translates the physiology of the eye, one prefers sometimes a space, perfectly equivalent, which corresponds more to the intuitive concepts. A color can be characterized by its “color” within the meaning of the language running (yellow or blue, for example), its purity and its intensity. This led to the system dyed, saturation, value (TSV).

Preceding spaces, and of many others, which describe the colors of the lights (additive Synthèse), are not adapted to the description of the colors reflected by an object (subtractive Synthèse). In this case, one does not leave more than one black reference (darkness) but of a white reference (maximum light). It is thus natural to use pigments which withdraw white the three primary educations preceding, i.e. which add the complementary colors cyan, magenta, yellow; it is system CMJ.

This stage, a problem practices arises: in additive synthesis, it is possible to use rather pure lights primary, which is not possible in subtractive synthesis. The gray and the blacks, obtained in theory by a mixture of equivalent quantities of the primary colors, are translated by rather dirty colors. This result led to replace the Trichromie by the Quadrichromie by adding a black pigment, which defines the system CMJN.

Spaces of reference

Space RVB is convenient to describe the colors of a screen of computer while space CMJN is well adapted to the impression. None of both is appropriate nevertheless for a universal description of the colors. The International commission of lighting (Co) studied the problems involved in the description of the colors seen by the human ones. For that, it was devoted to many experiments of comparisons of colors by a great number of observers statistically gathered so as to define an average observer.

After the first studies which related to the precise definition of space RVB, the Co defined the space of reference xyY. A comparison resulted in defining the Luminance Y, intensity subjective of a light of pure color, independently of this color. Then the color was characterized independently of the intensity by the couple (X, there) which represents the Chrominance.

This system, which generated a great number of practical systems, suffers from an imperfection: in the diagram of chrominance the distances between two close points badly represent the differences in perception by the human brain. In particular, the distances in the zone of the greens correspond to differences in perception weaker than those which are associated with the same distances for the other colors. This remark resulted in defining space YUV particularly used in Vidéo. Y is same brightness as previously but the two parameters of chrominance are selected so as to reduce the zone of the plan reserved to the greens. This space should not be confused with L*u*v* supplanted by the following system.

The system L*a*b* differs from the precedents in the sense that he results from xyY by non-linear relations. To imitate the answer of the eye, the parameter L is roughly proportional to the cubic root of brightness Y. In addition, the useful zone of the plan of chrominance closed by a horseshoe curve corresponding to the pure colors and a line corresponding to crimsons is transformed into a square reported to axes which go from the red to the green and the yellow to blue.

Conversion of spaces of color

The data processing of the images can be selected like example. The colors of a screen of computer are colors of lights, in general synthesized starting from the red, of the green and blue, which naturally results in using system RVB. For a similar reason, system CMJN is adapted to the description of the colors on the printer. To obtain best possible coherence, it is necessary to pass by a space of reference, independent of the two apparatuses, which is generally L*a*b*.

Each apparatus having only a limited gamut, this one must transmit to the space of destination its profile, subspace of this one. In general both gamuts are not identical. A module of management of the color must then carry out approximations to optimize the result. Those depend on the intention on returned which can be is Perceptif, relative, by saturation or absolute.

Workspaces

Under - whole of space RVB was defined to answer practical problems. sRGB is a space suggested by HP and Microsoft according to the most common screens in data processing; it is the space of standard color for the posting of the images on Internet but it is too narrow for the impression. Adobe RGB 1998 was thus designed by Adobe Systems Inc. to optimize the results of the printers.

See too

Management of the color
Description of various spaces

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