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Introduction to Part 2

Since colorimetry was established in 1931, considerable improvements in the metrology of the colour stimulus and immense advances in the knowledge of colour vision have been made.

The colour sensation results from physiological processes, the first of which is the capture of photons by the cones of the retina. The fundamental sensitivities of the cones need to be precisely known to accurately specify a colour stimulus from a given spectral power distribution.

Part 1 of this report provides the scientific community with cone fundamentals, which are the relative spectral sensitivities of the long-wave sensitive (LWS), middle-wave sensitive (MWS) and short-wave sensitive (SWS) cones as measured at the entrance of the eye. The cone fundamentals have been derived from the best set of colour-matching functions experimentally collected on a 10° field. In particular, the 2° cone fundamentals, which have been reconstructed from the 10° data by guidance of psychophysical data for 2° field size, represent the best proposal available today.

Part 2 of the report aims at providing the user with practical colorimetric tools, in the form of chromaticity diagrams. The chromaticity diagram is a two-dimensional representation of colour, independent of the luminance of the colour stimulus. The hypothesis that luminous quantity as measured by flicker photometry (referred to as LM-luminance in this report) relies only on the sum of the activity of the LWS and MWS cones has been recognized, and a conefundamental- based spectral luminous efficiency function is implemented in Clause 7. As outlined in 8.1, this hypothesis offers the possibility of proposing an equi-luminant chromaticity diagram directly based on cone fundamentals, which is a considerable advantage for understanding colour. In addition, to allow for comparison with the traditional CIE procedures, a transformation of the cone fundamentals in the form of cone-fundamentalbased XYZ tristimulus values and of an accompanying chromaticity diagram is presented in 8.2.

Part 2 ends by concluding that a link has been established between colorimetry and physiology. Such a reasoning, which has been developed by several scientists in the past, is a modern CIE approach that will improve the understanding of colour. It will be useful for education and will offer novel opportunities of solving problems of colour measurement and colour perception in everyday life and in industry.

NOTE The clause numbers in this document are continuous with respect to Part 1 of the report, i.e. start with Clause 7.