Apparatus and Method for Measuring Individual Colour-Matching Functions

Abstract

This article presents a compact colorimeter for the purpose of measuring colour-matching functions (CMFs) of individual observers through psychophysical experiments. Constructed from 3D-printed parts, optical elements, and LED-based light engines, the colorimeter facilitates the juxtaposition of two fields to create a bipartite field. The system underwent characterisation to evaluate factors that may impact colour-matching experiments, such as LED-light stability, spatial homogeneity of the bipartite field, and potential stray light leakage. A pilot study involved three observers in a maximum saturation-type colour-matching experiment. Using near-spectral lights with peak wavelengths ranging from 400 nm to 720 nm as test lights, observers were tasked with adjusting the intensities of three instrumental reference lights, produced by red, green, and blue LEDs, to achieve a colour match between the two juxtaposed fields. When a match was reached, the spectral radiance of each light-emitting half-field was measured. The difference between the two spectral distributions was obtained, and the tristimulus values for the latter were computed through least-squares estimation. The study aimed to assess the accuracy and performance of the system in measuring individual observer CMFs. Results indicate that the system is stable enough to measure intra-observer variations in CMFs. Through statistical analysis, regions where there are significant inter-observer differences in CMFs become apparent. Furthermore, the measured CMFs were compared with the Stiles and Burch 1955 2° CMFs, after being transformed to the same reference colour stimuli basis. While the measured CMFs exhibited comparable characteristics to those of the Stiles and Burch 1955 CMFs, a significant shift in the function referring to the short-wavelength reference colour stimulus was observed. This discrepancy may be attributed to age-related yellowing of the lens, as the average age of our observers is much higher than that of the Stiles and Burch 1955 study.