Abstract
Purpose :
In the two-photon vision, a non-linear optical effect in the visual pigments, namely the two-photon absorption, causes the human eye to perceive pulsed infrared light as visible light with approximately halved wavelength. The phenomenon is characterized by highly localized stimulation of the retina with no glare, which can be a very useful feature in developing novel devices for ophthalmic diagnostics. In this work, we focus on longitudinal spatial properties of two-photon stimuli and study the influence of beam defocus on visibility thresholds upon two-photon excitation with infrared light (1040 nm) compared with visible light at 520 nm. The findings may be useful for the interpretation of two-photon perimetry results as well as for the design considerations of future two-photon ophthalmological devices.
Methods :
We employed a custom-build, dual-wavelength (1040 nm and its second harmonic at 520 nm) microperimeter to measure visual thresholds (in two-photon and one-photon vision, respectively) by using the 4-2-1 staircase strategy. We tested 3 healthy dark-adapted volunteers with dilated pupils and accommodation blocked by administration of 1% Tropicamide drops. The stimuli were scanned circumferences of 1.0 deg diameter presented with varying degrees of defocus (-5 D to +5 D). The diameters of both beams at the pupil plane were equal to 1.1 mm. Experiments were conducted in scotopic and photopic (4.28 log phot Td) conditions obtained by Maxwellian view illumination of the retina with 520 nm light. The study was approved by the Ethics Committee of the Collegium Medicum, NCU.
Results :
We observed no changes in the effect of defocus on normalized visibility threshold in the range (-1 D to +1 D) for both wavelengths. For larger values of beam’s defocus (> 4 D and < -4 D), there are differences between 520 nm and 1040 nm beams (more pronounced increase in IR thresholds). The observed differences are statistically significant for scotopic case.
Conclusions :
The results are consistent with the phenomena resulting from diffraction. For a given numerical aperture, the effect of two-photon excitation is similar to the one-photon case, as the properties of nonlinear excitation (quadratic dependence of the signal on intensity) are largely compensated by diffraction (i.e., twice longer Rayleigh range). The results highlight the leading role of optical, rather than neuronal or physiological factors in two-photon infrared vision.
This is a 2021 ARVO Annual Meeting abstract.