Nonlinear perception of infrared radiation in the 800-1355 nm range with human eye

Results are presented of an experimental investigation of the nonlinear perception of infrared radiation in the 800–1355 nm range with the unaided human eye. In this range, the infrared radiation is perceived as visible radiation at a wavelength close to but not coinciding with the second harmonic wavelength. The eye sensitivity curve for infrared radiation has an irregular profile with the envelope maximum near the 1113 nm wavelength. It is shown that the mechanism for nonlinear perception of infrared radiation is apparently localized in the retinal nerve layers. A comparison of these results with established data on the vision mechanism of vertebrates suggests the existence of a new phenomenon — two-photon photoisomerization of the visual pigment molecules in the retinal cones, as the basis of the nonlinear perception of infrared radiation.