Human subjects can detect infrared light at wavelengths over 1000 nm perceived as visible of the corresponding half wavelength. This is due to a two-photon process and requires the use of pulsed light sources well focused within the retina. We have developed an experimental system to measure, for the first time, the visual resolution of the eye when is stimulated with infrared (1043 nm) and compared with visible light (543 nm). Scanner mirrors were used to project letters of different sizes onto the retina in both wavelengths. Subjects performed a visual test to determine the smallest letter size that was distinguishable for each wavelength for a range of defocus values. An additional optical path was used to record the retinal images of the spot after reflection in the retina and double-pass through the optical media. The best visual acuity was obtained at different defocus locations corresponding to the chromatic difference between green and infrared. Although, there was some individual variability, visual acuity was found to be similar both in visible and infrared. The use of two-photon infrared vision may have some potential applications for vision in those cases were the optical media is opaque to visible wavelengths while keeping some transparency in the infrared.
Pablo Artal, Katarzyna Komar, Adrian Gambin, Silvestre Manzanera, and Maciej Wojtkowski, "Resolution in two-photon infrared vision (Conference Presentation)," Proc. SPIE 10073, Adaptive Optics and Wavefront Control for Biological Systems III, 100730V (Presented at SPIE BiOS: January 29, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2252586.5380600101001.
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