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SCA-2541 - The Swiss Colour Atlas
The Swiss Colour Atlas 2541 (the number is the number of coloured samples in the atlas) was developed in 1962, and is one in a series of such atlases constructed by Aemilius Müller for practical use in industry and education. The second edition, used here, was issued in 1965 and differs from the first only in that the samples are removable. The atlases consist of dyed papers, 20mm in diameter, meant to be displayed on a white background with Y=86, approximately.

The arrangement of the SCA-2541 system is a double cone of constant-hue planes like that of the Ostwald system and the Color Harmony Manual. In SCA-2541, however, the objective of the creator was to provide regular triangular arrays based on equal visual spacing, rather than on colour mixing. A second major difference between SCA-2541 and most Ostwald atlases is the incorporation of a greater number of constant hue planes, here sixty. Since, in such a cylindrical arrangement, the hue planes get closer together as the neutral axis is approached, only on ten equally spaced major hue planes in SCA-2541 are all the possible samples presented: hues 6, 12, 18, 24, 30, 36, 42, 48, 54, and 60.
SCA-2541 Hue
Each of the constant hue planes in SCA-2541 consists of an equilateral triangle with vertices at black, white, and full-colour content. Note that the location of black and white in the SCA-2541 system is the inverse of that in the NCS system, with white at the base of the neutral axis and black at its peak. Chroma content increases from the neutral axis to the full-colour content point and can be described by a chroma coordinate as:

           SCA-2541 chroma = 10 - SCA-2541 blackness - SCA-2541 whiteness
SCA-2541 Chroma
The plane of constant SCA-2541 Blackness 1 is given below; planes of constant whiteness can also be displayed.
SCA-2541 Blackness
SCA-2541 notation takes the form ‘hue, blackness, whiteness’, for example 6,5,2 defines a SCA-2541 colour of hue 6, blackness 5 and whiteness 2, and, by calculation, chroma 3. The SCA-2541 hue circle comprises sixty equally perceptually spaced hues taking the integer values one to sixty. The colours are arranged clockwise around the hue axis in the order: 6-yellow, 12-orange, 18-purplish red, 24-purple, 30-violet, 36-blue, 42-greenish blue, 48-green, 54-yellow green, and 60-greenish yellow. SCA-2541 blackness and whiteness have the range 0 (no blackness or whiteness) to 10 (black or white). SCA-2541 chroma, although not explicitly identified in the notational system, is implied by its relationship to blackness and whiteness and ranges from 0 (an achromatic colour) to 10 (full chroma point, no blackness or whiteness).
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