Catalysis

Isomerization

Biology

Photochemistry

Materials science

Retina

Infrared

Retinal

Two-photon excitation microscopy

Photoisomerization

Biochemistry

Visible spectrum

Photon

Physics

Optics

Fluorescence

Chromophore

Chemistry

Rhodopsin

Biophysics

Visual phototransduction

Optoelectronics

Proceedings of the National Academy of Sciences

Significance This study resolves a long-standing question about the ability of humans to perceive near infrared radiation (IR) and identifies a mechanism driving human IR vision. A few previous reports and our expanded psychophysical studies here reveal that humans can detect IR at wavelengths longer than 1,000 nm and perceive it as visible light, a finding that has not received a satisfactory physical explanation. We show that IR light activates photoreceptors through a nonlinear optical process. IR light also caused photoisomerization of purified pigments and a model chromophore compound. These observations are consistent with our quantum mechanical model for the energetics of two-photon activation of rhodopsin. Thus, humans can perceive IR light via two-photon isomerization of visual pigment chromophores.

Human infrared vision is triggered by two-photon chromophore isomerization