**4.2 Spesific measurement types**

#### **4.2.1 Colour measurements**

RGB and CIELab is widely used as a standard spaces for comparing colours. A set of primary colours, such as the sRGB primaries, define a colour triangle; only colours within this triangle can be reproduced by mixing the primary colours. Colours outside the colour triangle are therefore shown here as gray. The primaries and the D65 white point of sRGB are shown in Fig. 5. (Anonymous, 2011c).

Fig. 5. CIExy1931 sRGB gamut (Anonymous, 2011c).

A Lab colour space is a colour-opponent space with dimension L for lightness and a and b for the colour-opponent dimensions, based on nonlinearly colour space coordinates (Anonymous, 2011c).

Fig. 6. Lab colour space (Anonymous, 2011c).

The CIE 1976 (L\*, a\*, b\*) colour space, showing only colours that fit within the sRGB gamut (and can therefore be displayed on a typical computer display). Each axis of each square ranges from -128 to 128 ( Fig. 6.) (Anonymous, 2011c).
