Color characterization of infrared two-photon vision

Abstract

Humans have the ability to perceive pulsed near-infrared (NIR) light as visible light with about half the wavelength through a process known as two-photon (2P) absorption. Although it has been known for several decades, color perception in 2P vision remains uncharacterized. In this study, we conducted color matching experiments between pulsed NIR light and continuous visible light. We investigated seven NIR wavelengths ranging from 880 to 1100 nm, along with three radiant power values at the pupil plane, varying from 10 to 30 µW. Through these experiments, we obtained chromatic coordinates, chromaticity diagrams, dominant wavelengths, and average spectra. We found a pronounced correlation between perceived hue and wavelength, with hues shifting from reddish purple at 880 nm to blue, green, and yellowish green at 1100 nm. Moreover, we observed a relationship between hue and power for the wavelengths closer to the visible end of the spectrum. This phenomenon appears to be a consequence of the intensity-dependent ratio between the single photon (1P) and 2P absorption efficiencies of the visual pigments.