In the case of visual assessment the observer is the human eye. If colourimetry is used then the observer is the instrument receiver.

Today there is instrumentation available that can numerically describe visual colour and colour difference. The observed differences in colour between two objects can be described in terms of three variables. These are called colour co-ordinates and form the basis of the language system most widely used in colour description and specification. These variables are known as Hue, Chroma and Value.

(a) Hue
This is the term used to distinguish the difference between two isolated colours, e.g., red, green, purple, beige, etc. The total number of hues that can be distinguished is very large but of these only four are unique. These are red, yellow, green and blue. All other hues are a combination of two of these four with the restriction that it is not possible to have a hue that is a mixture of blue and yellow or of red and green. With these pairs eliminated as mixtures, the remaining pairs can be arranged in a continuous circle.

(b) Chroma or saturation
A second difference between two colours of the same or different hue is the amount of hue seen. This variable is saturation or depth of colour, it is vivid or dull. There is no well-defined upper limit to saturation in colour but there is a zero saturation, at which point there is no hue perception. Since hues can be arranged in a circle and saturation (chroma) starts at zero
and increases, the two can be combined in a circle diagram with zero at the centre.

(c) Value or Brightness
The third difference between two isolated colours is brightness. A colour can vary from being too dim to be seen, to being too bright to look at. Except for difficulties in producing it, as far as perception is concerned, such a change can occur for any hue at any saturation (chroma). If we combine the circle diagram (Figure 5.3) with a vertical axis of value (brightness) it gives us a three-dimensional system by which we can describe colour.

Hue, value and chroma are the three colour characteristics and can be visualised in three dimensions as shown in figure 5.4. Colour hues are around the centre axis with value forming the vertical axis, and chroma the horizontal axis.

In 1976 the C.I.E. (Commission International de l'Eclairage) decided to recommend two colour difference formulas for worldwide use. The one recommended for normal reflective or transparent samples is known as .the CIELAB formula.

The CIE colour space is described by L, a and b where

L corresponds to the Light Dark axis.
a corresponds to the Red/Green axis.
b corresponds to the Yellow/Blue axis.

The total colour difference between the standard and the sample in the colour space is referred to as Delta E (DE).

However this does not describe in which direction the difference lies. To determine the difference in direction, values for L, a and b are obtained for the standard (1), and the samples (2), and are calculated as follows

∆L= L1-L2

where a positive value indicates that the sample is lighter than the standard and a negative value indicates that the sample is darker than the standard;

∆a = a1- a2

where a positive value that the sample is redder than the standard and a negative value indicates that the sample is greener than the standard;

∆b = b1 — b2

where a positive value indicates that the sample is yellower than the sample and a negative value indicates that the sample is bluer that the standard.

The total colour difference is calculated using the formula

∆E = √¯(∆L)² + (∆a)² + (∆B)²

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