Abstract

Abstract Purpose: The perceived brightness of different visible light sources can be compared with photometric units based on the standardized luminosity curves (300-780nm range). As reported previously (PNAS 111(50), pp. E5445-E5454 (2014)), near-infrared (NIR) radiation can cause isomerization of visual pigments by one- or two-photon absorption. The perceived color of the stimulus is red in the case of one-photon vision (1PV) or corresponds to half of the short-pulsed laser wavelength for the two-photon vision (2PV) mechanism. Since no luminosity curves for NIR range, comparison of such sources with photometric units is impossible. This study aims to provide an initial data for the 2PV spectral sensitivity curve (SSC). Methods: We measured the scotopic visibility thresholds at fovea for 4 healthy dark-adapted subjects (2 males, 2 females; 27-43 y/o) between 750-1075nm. Up to 930nm, the total visibility threshold corresponds only to 1PV mechanism. For the 810-930nm, subjects distinguished 2 thresholds: the loss of half of the wavelength hue (2PV threshold) and total visibility (1PV) threshold. Starting from 950nm, only 2PV mechanism caused the visual sensation. The stimulus was a 0.5 deg diameter flickering circle projected in the retina by scanners. Two tunable pulsed light sources: OPO (750-990nm, Frep=76MHz, τp=204fs) and fiber laser (872-1075nm, Frep=51.5MHz, τp=205±9fs) were employed for psychophysical tests. The study complied with the Declaration of Helsinki and was approved by the Ethics Committee of the Collegium Medicum, NCU. Results: The 1PV thresholds agreed with the previous works (JOSA 37(7), pp. 546-554 (1947)). Within the 810-930nm range, a relatively high standard deviation of the 2PV thresholds was observed, as finding the disappearance of half of the wavelength hue is a relatively difficult task. 2PV thresholds slightly differ for OPO and fiber laser, which is explainable by differences in pulse durations and repetition rates. For ~1040nm, the 2PV threshold values agreed to the previous authors’ results. Conclusions: The 2PV sensitivity increased with wavelength of about 1 order of magnitude (810-1075nm range). Starting from 1010 nm, the shape of the obtained 2PV SSC differs from the wavelength-doubled scotopic luminosity curve for 1PV, corrected by the eye media transmittance. The maximum of 2PV SSC was not found. The results allowed us to obtain the first approximation of the 2PV SCC shape up to 1075nm.

Authors (6)