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The limits of perception of light by two-photon vision

Purpose: Near infrared (NIR) light can cause isomerization of visual pigments by one-photon absorption resulting in the perception of its color as red. Lately it was shown that for short-pulsed infrared lasers, two-photon absorption also causes isomerization of visual pigments – then perceived color corresponds to half of wavelength of the laser. A range of NIR wavelengths can be perceived by both above described processes. The goal of this study is to explore more this region to find boundary conditions for the perception of light by two-photon vision and aid future clinical use of this phenomenon.

Methods: We measured the scotopic visibility thresholds at fovea and 6.5deg temporal retina for 3 healthy subjects (2 males, 1 female; ages 33, 40, 57) between 850–1040nm. At fovea, for the 900-975nm wavelengths subjects were able to distinguish 2 thresholds: the disappearing of half of the wavelength hue (2phT) and total visibility threshold (1phT). The stimuli was an open circle of 0.5deg diameter projected into the retina by scanners. The light source was tunable OPO beam transmitted to the optical set-up by fiber (Frep=76MHz; τp=2ps at the fiber output).

We also performed visibility tests for fiber laser with λc=1028nm, Frep=19.2MHz and τp=17ps. The study complied with the Declaration of Helsinki and was approved by the Ethics Committee of the Collegium Medicum, NCU.

Results: The 1phT increased from 1.5×1013 to 3.8×1015phot/cm2s (45nW to 10µW of mean power) for 850nm and 950nm, respectively. On the contrary, the 2phT decreased from 2.5×1016 to 7.1×1015phot/cm2s (72nW to 18μW) for 900nm and 1040nm. At 995nm the two-photon process became dominant for all subjects (2phT<1phT) resulting in perception of the beam only as having color half of the laser wavelength. The intersect point of the 1phT(λ) and 2phT(λ) curves was: 975±6nm and 1.3±0.1×1016phot/cm2s (34±3μW).

The visibility thresholds for fiber laser are on average twice time smaller than resulting from comparison of its duty cycle with the OPO beam.

Conclusions: The sensitivity of investigated subjects for two-photon stimuli increased with wavelength of about 0.5 order of magnitude in the range 900-1040nm. Obtained results allowed for the first time for determining the power and wavelength of NIR pulsed laser beam for which two-photon vision become dominant.

The tested fiber laser is well perceived by subjects and seems to be very promising tool to investigate two-photon vision in clinical practice.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 – May 2, 2019.

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