What Color is the Sky (for a non-human) ?

2026-06-27

AI Summary

This study addresses the question of how to define “the color of the sky” for non-human visual systems. The authors propose a computational framework that systematically extends the concept of metamerism to non-human vision by incorporating species-specific spectral sensitivity functions. Using spectral modeling, metamer-matching algorithms, and cross-species comparisons, they identify monochromatic metamers perceptually equivalent to daylight skylight across a range of dichromatic, trichromatic, and tetrachromatic species. The dominant wavelengths of these metamers consistently deviate from the blue perceived by humans, revealing fundamental interspecies differences in sky-color perception. This work provides the first quantitative definition of “sky color” for non-human visual systems and establishes a novel paradigm for comparative studies of visual perception across species.

Abstract

The light of the daytime sky contains a mixture of many colors yet is perceived as blue by human observers. This is largely due to the particular response functions of the human cones. Under these response functions skylight and blue light are metamers: they yield the exact same excitation of the cones. In this paper we ask: is it possible to define the ``color'' of the sky for other visual systems? We present a simple computational method to determine monochromatic metamers to a given input light for arbitrary visual systems. Using published values on spectral sensitivity functions of various species, we use our method to determine the dominant wavelength of monochromatic metamers to skylight. For a wide range of species (bichromats, trichromats and tetrachormats) we find monochromatic metamers to skylight but the dominant wavelength of the metamer can vary drastically between species and be very different from the color perceived by humans.

Problem

• sky color
• metamerism
• visual systems
• spectral sensitivity
• color perception

Innovation

• metamer
• spectral sensitivity
• color perception
• visual system
• dominant wavelength