Achromatic colours (white, grey, black) differ only on the brightness defined by the formula
Where In - brightness of a surface, nt; E - light exposure of a surface, lk; r - factor of reflexion,%.
The Various colour combinations defined by colour tone, a saturation and svetlotoj, quantitative and qualitative definition of colours and their parametres were a theme of numerous researches in which result various systems of classifications in the form of tables, atlases, colour circles are created. Systems Ostvalda, Mensella and Maksvella constructed in three-dimensional co-ordinates are most known.
Proceeding from substantive provisions trihromaticheskoj systems tsvetoobrazovanija, according to which any can receive the set colour corresponding displacement of three any way chosen radiations, and it is possible for colour characteristics dattochnuju the quantitative estimation, allowing to compare colours among themselves, the International commission on illumination in 1931 has suggested to use for the characteristic of colour of co-ordinate tsvetnostej (h, at, z), corresponding to shares of red, green and dark blue radiations. Transition from system of co-ordinates to system of the characteristic of colour by means of colour tone, a saturation and svetloty is carried out by means of the colour schedule (fig. 84).
Each point of an external contour of the colour schedule corresponds to certain length of a wave. Point E corresponds to white colour. On the straight lines connecting point E with an external contour, colours with identical length of a wave, but different colour purity from zero (cleanliness of white colour) to 100% (cleanliness of spectral colours) settle down.