Purpose : Humans can detect infrared light at wavelengths over 1000 nm perceived as visible light of the corresponding half wavelength. This is due to a two-photon (2P) process and requires the use of pulsed light sources well focused in the retina. Although this effect was known for several decades, the spatial properties of 2P vision in comparison to normal vision are not yet elucidated.
Methods : We developed an experimental system to measure, for the first time, both the visual and optical resolution of the eye when is stimulated with infrared and compared with visible light. The setup is based on a set of scanning mirrors to project letters on the retina using either pulsed infrared light (around 500 femtoseconds pulse duration) 1043 nm or continuous green light at 543 nm. Considering the diffraction effects, two different artificial pupil diameters were used, 1 mm in visible and 2 mm in infrared, to assure that the scanning spots were equal in size for both wavelengths. Visual acuity (VA) was measured using a standard adjustment method to determine the smallest letter size that was distinguishable for each wavelength for a range of defocus values. An additional optical path allowed us to record the double-pass images of a point source in the retina for both wavelengths using an electro multiplying CCD camera. Both VA and double-pass images were obtained as a function of defocus in 6 normal volunteers. The experimental double-pass images were compared with those recorded in an artificial model eye.
Results : Although there was some variability between subjects the average best VA was found to be similar for both visible (0.00±0.10 LogMAR) and infrared light (0.01±0.06 LogMAR). The difference in focus (1.5 D) between the best VA positions for both wavelengths is in agreement with the longitudinal chromatic aberration (~1.4 D). Numerical simulations and measurements in the model eye show that the double-pass images were similar in size for both visible and infrared light after correction of the diffraction effects.
Conclusions : The spatial resolution of two-photon vision is the same as in normal visible light. Vision triggered by a non-linear process in the infrared does not provide any advantage in terms of visual acuity. However, the use of 2P infrared vision may have some future potential applications for vision in those cases where the optical media is opaque to visible wavelengths while keeping some transparency in the infrared.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.