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The instrument is a modification of the ordinary trichromatic colorimeter. The three matching stimuli of the ordinary instrument, the red, green and blue, have been increased to six by the addition of an orange, yellow-green and blue-green. The spectral energy distribution of the colour being measured is first approximately matched by means of a mixture of all six colours before the final colour match is made by varying three of the colours only. This eliminates to a large extent the personal error of the observer, and allows a large field to be used with a resultant gain in sensitivity.
R Donaldson (Tue,) studied this question.