Inkjet Color Management

The human eye can distinguish around a million colours, the precise number depending on the individual observer and viewing conditions. Colour devices create colours in different ways, resulting in different colour gamuts.

Colour can be described conceptually by a three-dimensional HSB model:

  • Hue (H) refers to the basic colour in terms of one or two dominant primary colours (red, or blue-green, for example); it is measured as a position on the standard colour wheel, and is described as an angle in degrees, between 0 to 360.
  • Saturation (S), also referred to as chroma, refers to the intensity of the dominant colours; it is measured as a percentage from 0 to 100 percent – at 0% the colour would contain no hue, and would be grey, at 100%, the colour is fully saturated.
  • Brightness (B) refers to the colour’s proximity to white or black, which is a function of the amplitude of the light that stimulates the eye’s receptors; it is also measured as a percentage – if any hue has a brightness of 0%, it becomes black, with 100% it becomes fully light.

RGB (Red, Green, Blue) and CMYK (Cyan, Magenta, Yellow, Black) are other common colour models. CRT monitors use the former, creating colour by causing red, green, and blue phosphors to glow; this system is called additive colour. Mixing different amounts of each of the red, green or blue, creates different colours, and each can be measured from 0 to 255. If all red, green and blue are set to 0, the colour is black, is all are set to 255, the colour is white.

Printed material is created by applying inks or toner to white paper. The pigments in the ink absorb light selectively so that only parts of the spectrum are reflected back to the viewer’s eye, hence the term subtractive colour. The basic printing ink colours are cyan, magenta, and yellow, and a fourth ink, black, is usually added to create purer, deeper shadows and a wider range of shades. By using varying amounts of these process colours a large number of different colours can be produced. Here the level of ink is measured from 0% to 100%, with orange, for example being represented by 0% cyan, 50% magenta, 100% yellow and 0% black.

The CIE (Commission Internationale de l’Eclairage) was formed early in this century to develop standards for the specification of light and illumination and was responsible for the first colour space model. This defined colour as a combination of three axes: x, y, and z, with, in broad terms, x representing the amount of redness in a colour, y the amount of greenness and lightness (bright-to-dark), and z the amount of blueness. In 1931 this system was adopted as the CIE x*y*z model, and it’s the basis for most other colour space models. The most familiar refinement is the Yxy model, in which the near triangular xy planes represent colours with the same lightness, with lightness varying along the Y-axis. Subsequent developments, such as the L*a*b and L*u*v models released in 1978, map the distances between colour co-ordinates more accurately to the human colour perception system.

For colour is to be an effective tool, it must be possible to create and enforce consistent, predictable colour in a production chain: scanners, software, monitors, desktop printers, external PostScript output devices, prepress service bureaux, and printing presses. The dilemma is that different devices just can’t create the same range of colours. It is in the field of colour management that all of this colour modelling effort comes into its own. This uses the device-independent CIE colour space to mediate between the colour gamuts of the various different devices. Colour management systems are based on generic profiles of different colour devices, which describe their imaging technologies, gamuts and operational methods. These profiles are then fine-tuned by calibrating actual devices to measure and correct any deviations from ideal performance. Finally, colours are translated from one device to another, with mapping algorithms choosing the optimal replacements for out-of-gamut colours that can’t be handled.

Until Apple introduced ColorSync as a part of its System 7.x operating system in 1992, colour management was left to specific applications. These high-end systems have produced impressive results, but they are computationally intensive and mutually incompatible. Recognising the problems of cross-platform colour, the ICC (International Colour Consortium, although originally named the ColorSync Profile Consortium) was formed in March 1994 to establish a common device profile format. The founding companies included Adobe, Agfa, Apple, Kodak, Microsoft, Silicon Graphics, Sun Microsystems, and Taligent.

The goal of the ICC is to provide true portable colour that will work in all hardware and software environments, and it published its first standard – version 3 of the ICC Profile Format – in June 1994. There are two parts to the ICC profile; the contains information about the profile itself, such as what device created the profile and when and the second is colourmetric device characterisation, which explains how the device renders colour. The following year Windows 95 became the first Microsoft operating environment to include colour management and support for ICC-compliant profiles, via the ICM (Image Colour Management) system.