Physiologically relevant metrics for evaluating photoreceptor signal reproduction in visual displays

The colour gamut of a display defines the range of colour signals that it can reproduce. For human vision this range is typically visualized in a colour space that reflects the spectral sensitivities of the cone photoreceptors. While the cones dominate canonical visual function (i.e. perception of colour, form, and motion) under photopic light levels, rods and melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) also respond to light. Three-primary display technologies, with their focus on reproducing colour, are not designed to control the rod and melanopsin excitations. As a result, conventional display metrics do not capture the ability of the display to reproduce the full human physiological response to light. In this paper, two physiologically relevant metrics are proposed for quantifying the reproduction and distortion of the photoreceptor signals by visual displays. An equal-luminance photoreceptor excitation diagram is proposed, extending the MacLeod-Boynton chromaticity diagram, providing three-dimensional projections of the five-dimensional photoreceptor signal space to help with visualization. Each metric and visualisation quantifies a specific aspect of the reproducibility of real-world spectra, analogous to the colour rendering index. To demonstrate their application, we use these tools to assess both real and hypothetical displays across tens of thousands of radiance spectra derived from real-world reflectance samples under various illuminants.