A trichromatic colorimeter with spectral primaries

The paper describes a trichromatic colorimeter that has been specially designed in order to make an accurate re-determination of the locus of the spectral colours in the colour triangle for as many observers as possible. A spectrometer system is used in which two spectra are formed from the same source. From one of these, three portions to act as primaries are reflected back through a lower part of the dispersing system, so that the mixing of the three radiations is effected by neutralising the prismatic dispersion by which the colours were first separated. From the other spectrum the test colour and a desaturating colour are selected and mixed in a similar manner, and the composite beams are then brought into the two halves of a simple bipartite field. In the process of being mixed with the other colours, each spectral colour is filtered by its return passage through the dispersing system and the instrument thus becomes very effective in removing stray light. Another device has enabled the Maxwellian method of observing the field of view to be adopted without the introduction of rotating parts into the system. The intensities of the three primaries are controlled with photometer wedges.