COLOUR MANAGEMENT (material derived from Epson Scanner Help files)

1. What is colour ? : Light entering the eye is converted to signals in the retina and sent to the brain via the optic nerve. The eye reacts to the three additive primary colours of red, green, and blue, and the brain perceives colour as a combination of these three signals. The perception of colour varies greatly according to the external environment. The same colour appears different when seen by sunlight or candlelight.

2. Colour & wavelength : Light is a type of electromagnetic wave, similar to radio waves. The characteristics of light change according to the length of the electromagnetic waves, ranging from radio waves, through visible light to gamma rays. The energy carried by waves, which are approximately 400-700 nm (a nanometer equals one millionth of a millimetre and is the unit used to measure wavelengths of light), will stimulate the receptors in the human retina, producing colour stimuli. Humans perceive the light of the noonday sun as "white light," a mixture of visible light ranging from 400 nm (blue) to 700 nm (red).

3. Types of coloured object : Humans can perceive two types of colour. The colour of a self-luminous object is called self-luminous colour, and the colour of an illuminated object is called object colour. A self-luminous object may be natural, such as the sun, or artificial, such as computer displays, incandescent light bulbs, mercury lamps, and the like. Object colour is the colour reflected from an illuminated object, and is comprised of both the light reflected from the object's surface as well as light reflected and scattered from beneath the object's surface.

4. Additive mixing : The human eye perceives wavelengths of 400-500 nm (nanometers) as blue, 500-600 nm as green, and 600-700 nm as red. These are known as the three primary colours in the computer industry, abbreviated as RGB. All colours encountered in nature can be reproduced by combining light of these three wavelengths in varying intensities. Mixing 100% of all three colours will produce white light. Reducing each component to 0% will result in the absence of light, or black.

5. Subtractive mixing : White light is produced by mixing 100% of all three primary colours. Subtracting red produces cyan (a mixture of blue and green). Subtracting green produces magenta, and subtracting blue produces yellow. When an object absorbs red and reflects blue and green, we perceive the colour to be cyan. Paints or dyes reproduce colour by the subtractive method: when the dye or pigment absorbs red and reflects green and blue light, we see cyan. When it absorbs green and reflects red and blue, we see magenta. When it absorbs blue and reflects red and green, we see yellow. Cyan, magenta, and yellow are the three primary colours used in subtractive mixing.

6. Colour models and terminology :

Hue refers to the attribute of colours that permits them to be classed as red, yellow, green, blue, or an intermediate between any contiguous pair of these colours. Hue differences depend primarily on variations in the wavelength of light reaching the eye. Hue can be represented visually by a hue circle going from red to green to blue and back to red. Brightness refers to the relative lightness or darkness of the colour. It is determined by the degree of reflectivity of the physical surface receiving the light. The higher the brightness, the lighter the colour.

Saturation refers to how vivid a colour appears. It is measured in terms of the difference of a colour from a colourless (neutral) grey with the same degree of

brightness. The lower the saturation, the greyer the colour. When saturation is zero, the colour is grey.

Colour Wheel : (see next page)

The colour wheel demonstrates the relationship between the three primary colours of red, green and blue and the three primary lights, cyan, magenta and yellow.

For example, magenta can be made from the two adjacent colours of red and blue. Similarly, yellow and cyan, when mixed together, produce green.

Colours opposite each other on the colour wheel are called complementary colours. The complementary colour of green is magenta, for example. If the photograph you've taken has too much green in it, you can suppress this effect by adding in its complementary colour, magenta (red and blue according to the RGB

model). Conversely, you can make red brighter by reducing cyan (green and blue, according to the RGB model).

7. Achromatic colours are white, black, and the grey in between. They lack the attributes of hue and saturation.

Chromatic colours are everything that we perceive as having "colour"; everything other than white, black, or grey.

8. Colour temperature : Colour is intimately related to temperature. When a flame burns at a high temperature, the colour is blue; at a low temperature, the colour is red. The measurement criterion of "colour temperature" is used to assign objective numeric values to the condition of light when we see a colour. Colour temperature is expressed in Kelvins, based on an imaginary object called a black body. The colour temperature is the temperature at which a black body emits radiant energy competent to evoke a colour the same as that evoked by radiant energy from a given source (such as a lamp). The sun at noon is 5,000 degrees Kelvin; at morning or evening it is 4,000 degrees Kelvin. A daylight fluorescent lamp is 6,500 degrees Kelvin, and the average computer screen is also 6,500 degrees Kelvin. The lower the colour temperature, the closer the colour is to red; the higher the colour temperature, the closer to blue. This explains why the same red item of clothing will appear differently out of doors than under a fluorescent lamp indoors.

9.Colour Calibration : Devices such as scanners, computer monitors, and printers sometimes have a colour bias. Calibration refers to an operation designed to stabilize the colour of each interconnected device by adjusting its colour temperature, gamma colour balance, and other characteristics. If the calibration data is saved as a profile, it can be conveniently output to other devices. When other devices receive the profile data, their colour settings can also be adjusted. This is called characterization. Examples of convenient colour management systems that have been designed for this purpose are Apple's ColorSync and Kodak's KCMS (see section 13).

10.Gamma : Gamma is a numeric indication of the relation between input and output. If the total gamma value from input through output is "1", the colour gradations that are output will be identical to what is input. If the gamma value is changed (either for all colours or for one or more component colours) then the relationship between input and output is different (see images from PaintShop Pro previous page).

11.Colour gamuts : The human eye, colour film, computer monitors, and colour printers all have a different range of colours (gamut) that can be reproduced. Gamut mapping is a technique for adjusting the colour across different devices so that the image seen by the human viewer will be as consistent as possible when reproduced on devices with different ranges of reproducible colour.

For example, the gamut of colours that can be reproduced by a CMYK colour printer is smaller than what can be shown on an RGB display. As a result, the vivid green seen on the display will turn hazy when

printed. There are several different methods for dealing with colours that are outside the gamut of different devices.

12. More on Colour models : A colour model is a method for defining colours. The printing industry uses the CMYK model. Applications that use computer monitors, such as graphics for the Internet, use the RGB model. It is important to select the right model for the job.

RGB : The first colour model that comes to mind is RGB (red, green, blue). It is used in self-luminous display monitors, television sets, and similar devices. RGB is widely used but extremely device-dependent. When the device is changed, the colour will change too. It is not suitable for colour reproduction when a number of devices, such as a scanner, monitor, and printer must be used together. Since it uses the three additive primary colours, it is not suitable for paints or for the dyes and pigments used in printing, which use a different set of primary colours (cyan, magenta, yellow).

CMYK : The CMYK (cyan, magenta, yellow, black) model is another typical model, used in the printing industry. The definitions are based upon the absorptive properties of ink. The three RGB primary colours, when mixed, produce white, but the three CMY primary colours produce black when they are mixed together. Since actual inks will not produce pure colours, black (K) is included as a separate colour, and the model is called CMYK. With the CMYK model, the range of reproducible colours is narrower than with RGB, so when RGB data is converted to CMYK data, the colours seem dirtier.

HCV (hue, chroma, value) : This colour model is based upon the human eye and perception. The CIE colour model (Commission Internationale de l'Eclairage) is a typical example. It is the most important model for understanding computer graphics. These models are used to define so-called device-independent colours, which can be reproduced faithfully on any type of device, such as scanners, monitors, and printers. They are widely used because they are easy to use on computers and describe a wide range of colours. The best known models are the CIE XYZ and CIE L*a*b (which is an improved version of the CIE XYZ model) .

13. Colour management systems : a colour management system (CMS) is used to adjust colour between different peripherals such as a scanner, monitor and printer. The CMS aims to reproduce device independent colours, and uses a standard colour model, such as one of the CIE models.

Calibration & characterisation : Devices such as scanners, computer monitors, and printers usually have a colour bias. Calibration refers to an operation designed to stabilize the colour of each interconnected device by adjusting its colour temperature, gamma colour balance, and other characteristics. If the calibration data is saved as a profile, it can be conveniently output to other devices.

When other devices receive the profile data, their colour settings can also be adjusted. This is called characterization. Examples of convenient colour management systems that have been designed for this purpose are Apple's ColorSync and Kodak's KCMS.

This is how the Help system in PhotoShop (version 6) describes Adobe's approach to Colour Management (based on the conventions laid down by the ICC : the International Colour Consortium).

Colour numbers (colour codes) Each pixel in an image document has a set of colour numbers that describe the pixel's location in a particular colour mode—for example, red, green, and blue values for the RGB mode. However, the actual appearance of the pixel may vary when output or displayed on different devices, because each device has a particular way of translating the raw numbers into visual colour. When you apply colour and tonal adjustments or convert a document to a different colour space, you are changing the document's colour numbers.

Colour profiles An ICC workflow uses colour profiles to determine how colour numbers in a document translate to actual colour appearances. A profile systematically describes how colour numbers map to a particular colour space, usually that of a device such as a scanner, printer, or monitor. By associating, or tagging, a document with a colour profile, you provide a definition of actual colour appearances in the document; changing the associated profile changes the colour appearances. Images without associated profiles are known as untagged and contain only raw colour numbers. When working with untagged documents, Photoshop uses the current working space profile to display and edit colours.