Profile connection space (PCS) – Part 2 of ICC profile series

Posted: September 24, 2012 in cie system, Color, FOGRA, ICC, ICC profile, PCS
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A key component of the specification is a well-defined profile connection space. This standard color space is the interface which provides an unambiguous connection between the input and output profiles as illustrated in the diagram below. It allows the profile transforms for input, display, and output devices to be decoupled so that they can be produced independently. A well-defined PCS provides the common interface for the individual device profiles. It is the virtual destination for input transforms and the virtual source for output transforms. If the input and output transforms are based on the same PCS definition, even though they are created independently, they can be paired arbitrarily at run time by the color-management engine (CMM) and will yield consistent and predictable results when applied to color values.

The profile connection space is based on the CIE 1931 standard colorimetric observer. This experimentally derived standard observer provides a very good representation of the human visual system color matching capabilities. Unlike device dependent color spaces, if two colors have the same CIE colorimetry they will match if viewed under the same conditions as those defined for the colorimetry.

Profile connection space

Because images are typically produced for a wide variety of viewing environments, it is necessary to go beyond simple application of the CIE system. The profile connection space is defined as the CIE colorimetry which, in the case of the perceptual rendering intent (defined later), will produce the desired color appearance if rendered on a reference imaging media and viewed in a reference viewing environment. This reference corresponds to an ideal reflection print viewed in a standard viewing booth conforming to ISO standard viewing conditions.

The default measurement parameters for the profile connection space and all other color spaces defined in this specification are based on the ISO 13655 standard, “Graphic technology – Spectral measurement and colorimetric computation for graphic arts images.” Essentially this defines a standard illuminant of D50, the 1931 CIE standard colorimetric observer, and 0° /45° or 45° /0° measurement geometry measured with a black backing behind the print for the reflectance measurements. The reference viewing condition is that defined in ISO 3664 as viewing condition P2 using the recommended 20% surround reflectance. This is a graphics arts and photography print viewing environment with a D50 illumination level of 500 lux.

One of the first steps in profile building involves measuring a set of colors from some imaging media or display. If the imaging media or viewing environment differ from the reference, it will be necessary to adapt the colorimetric data to that appropriate for the profile connection space. These adaptations account for such differences as white point chromaticity and luminance relative to an ideal reflector, maximum density, viewing surround, viewing illuminant, and flare. Currently, it is the responsibility of the profile builder to do this adaptation. However, the possibility of allowing a variable illuminant in the PCS is under active consideration by the International Color Consortium.

More to continue….

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