Colour is the end result of light waves interacting with the human visual system. These light waves have no colour themselves, but when they are processed by the visual system they evoke the sensation of colour. Humans can actually only detect three colours: red, green and blue.
All other perceived colours are created by the stimulation of the brain by various mixtures of red, green and blue. For example, yellow is created by a mixture of red and green radiation and the impression of white is created by a mixture of red, green and blue radiation. This is called the 'additive' method of making additional colours.
{2}Colour is thus a subjective experience that is extremely difficult to quantify. It is for this reason that several descriptive 'models' have had to be created to allow colours to be measured to meet differing circumstances. Among users of computer graphic applications, the RGB (Red, Green and Blue), HSL or HSB (Hue, Saturation and Lightness/Brightness) and CMYK (Cyan, Magenta, Yellow and Black) models may be the most familiar. However, for a variety of reasons, colours specified in one model cannot always be translated into a numeric description from another model.
The RGB and HSL models are subjective with a range of colours encompassing the entire visible spectrum. However the CMYK model, used to describe printed colours, has a smaller range and is unable to reproduce some of the RGB spectrum.
{3}The RGB Colour Model
Colour monitors are constructed to make use of the eye's sensitivity to red, green and blue colour. The screen is made up of pixels - minute cells comprising a group of red, green and blue dots. When all are illuminated equally, the pixel appears to be white. If the intensity of each pixel is varied, a wide range of colours can be created, limited only by the viewing condition and the sensitivity of the screen phosphors.
The illumination of the RGB cells in a PC monitor is set by an intensity value expressed in a digital form. The number of tonal values that can be described is limited only by the resolution of the binary number. For a monochrome file, the three coloured dots will all be switched between only two values - on or off. This can be described using a single data bit operating on all the colour channels.
{4}If a range of intermediate levels is required, this needs a more complex binary sequence. With a 4-bit number up to 16 different tonal values can be defined (24) and with 8-bits, up to 256 tonal values (28). Applied to each of the colour channels these values will either result in 16 x 16 x 16 (total of 12-bits of information) = 4,096 different colours, or 256 x 256 x 256 (a total of 24 bits of information) = 16.7 million colours. Program dialogue boxes will allow the user to set values of red, green and blue either in percentage values, giving no idea of the amount of colour information, or as a number (usually between 0 and 255).
Despite the ability of a computer to define colours in precise numeric terms, the actual colour that appears on the screen will vary considerably depending on its manufacturing source, the way that the video card interprets the numbers and on ambient light conditions. The relationship between the number describing a colour and the way this appears on screen is quantified in the gamma curve (see the section on How scanners work for more details). This is why, when accuracy is required of a screen display, the ambient light conditions have to be specified - and the screen gamma curve has to be adjusted using a calibration process.
{5}The RGB colour model is used in scanning or when printing the contents of a colour file to a film recorder loaded with colour transparency or negative film. When printing to colour transparency film, variations in colour rendering will occur because of changes in the film gamma curve from batch to batch, or in the chemical processing used to develop the image.This description of the RGB colour model should make it apparent that precision is not inherent in the model unless a number of external factors, such as calibration of the complete system from scanner to output device, are controlled precisely.
The HSB and HSL Models
The Hue, Saturation and Lightness (or Brightness) models are mathematical representations of the way the eye perceives colour.
{6}Hue describes which colour it is, Saturation the amount of colour, and Lightness or Brightness describes how much white is included in the colour.
A graphic representation of the HSB and HSL models places the colours of the spectrum around the circumference of a circle where the description of a particular Hue is defined as the number of degrees of angle away from a reference point. Thus red is located at 0o, green at 90o, cyan at 180o and blue at 270o around the circle. Saturation and Brightness are usually defined as percentages.
The CMYK colour model
This model is used to specify colour in terms of Cyan, Magenta and Yellow, plus Black (represented as K because the letter 'B' is used for Blue). When printing, two methods exist for creating the required colour range. The colour can either be made by mixing a special ink to the colour required, or by using transparent cyan, magenta, yellow and black inks and overprinting these on paper. Here, they act as filters for the light reflected from the paper's surface. Thus, if a combination of Cyan and Yellow inks are used, the colour the eye sees is the result of the absorption of Green and Blue, leaving just the Red portion of the spectrum.
{7}Colour matching and management models
Pantone, Focoltone and Trumatch systems
These are not colour models in the true sense of the word, but are specifications for the mixing of standard ink colours to produce a single spot colour that has been tested and proved on different types of paper. Reference colours are printed in sample books available to printers and designers. Several printers and software houses have designed computer palettes which define the colour as spot colour and provide an approximation of its appearance on the screen and for desktop colour printing. Some of the colours can be reproduced using process colour separation. When separating a file containing one or more of these colours, a separate film, bromide or plate will be generated in addition to any that may be required in the four process colours of CMY and K.
{8}Colour management systems
It is widely recognised that there is an urgent need for users across the wide colour-publishing marketplace spectrum to be able to use colour images in a common way which provides predictable results. New applications are thus being developed which can manage and specify colour in a device- and application- independent way. These systems include Kodak's KEPS, Apple's ColorSync, Electronic For Imaging's EFI color system and Agfa's Autocolor technology and FotoFlow systems.
This area is a complex one, with the various approaches and applications perhaps seeming to introduce more confusion than efficient management. However, in the not too distant future, we can expect colour management systems to be operating in true open system environments, encompassing various applications and platforms with ease.
Colour calibration
Without device-independent colour, sometimes the user may have to rely on their own 'visual' calibration or quite often on the closed-loop calibration systems offered in many applications programs. These usually involve the scanning and printing of known tonal scales so that a calibration can be made.
