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CIE chromaticity diagram. This diagram (developed in 1931) shows all the colours that can be perceived by the normal human eye, and is the international standard for defining colours. The colours are defined as a weighted sum of 3 primary colours, although in reality no 3 primary colours can combine to give all possible colours, so virtual primary colours are used. The curved edge of the plot corresponds to the colours of the visible spectrum (wavelengths are shown in nanometres), while the straight edge corresponds to non-spectral shades of purple. Pure colours are at the edges; as colours move towards the centre they become less saturated. White is at the centre of the diagram.
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