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.
(b) Chroma or saturation
(c) Value or Brightness
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.
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
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