Abstract
Purpose : Humans can detect pulsed near-infrared light as visible light due to a process known as two-photon (2P) absorption. Although it has been known for decades, a quantitative characterization of 2P vision as a function of the various adjustable parameters in the illumination source and its wavelength has not yet been completely achieved. The purpose of this work is to determine the characteristics of the perceived color when vision is elicited through 2P mechanism with pulsed infrared light.
Methods : A dedicated apparatus was used for the experiment (Artal et al., Optica, 2017). A ~1 ns-pulse laser in the near-infrared with a repetition rate of 2 kHz was used to project a 2×2 arcmin square on the fovea of 3 subjects next to another square of equal dimensions produced by a visible-light OLED display. Seven quasi-monochromatic wavelength bands of IR light were used (central wavelengths: 880, 900, 920, 950, 1000, 1050, 1100 nm; FWHM: 10-50 nm). For each wavelength, subjects had to adjust the color (hue, saturation, brightness) of the OLED display using a keypad to match the color of the square in 2P vision. A spectrometer was then used to measure the chromatic coordinates displayed by the OLED at the plane of the eye. Ocular exposure remained below the exposure limits of the ANSI Z136.1-2014 standard.
Results : The color obtained for 880 nm corresponds to a non-spectral purple in which the red hue predominates, presumably due to the effect of continuous absorption of the IR tails. For 900-950 nm the prevailing hue is blue, with dominant wavelengths of 452 nm for the color perceived at 900 nm, 488 nm for that at 920 nm and 487 nm for that at 950 nm. For 1000-1100 nm green is the prevailing hue, with dominant wavelengths of 505 nm for 1000 nm, 517 nm for 1050 nm and 536 nm for 1100 nm. CIE 1931 coordinates (x, y) have also been obtained (Δx, Δy ≤ 0.06): (0.388, 0.316) for 880 nm, (0.32, 0.31) for 900 nm, (0.25, 0.31) for 920 nm, (0.23, 0.30) for 950 nm, (0.266, 0.40) for 1000 nm, (0.266, 0.45) for 1050 nm and (0.302, 0.44) for 1100 nm.
Conclusions : Shifting the wavelength of a pulsed near-infrared laser source used to stimulate the retina causes a change in the perceived hue that can be quantitatively characterized. A precise explanation of the dependence of visual perception on wavelength and other adjustable parameters gives us a better understanding of 2P vision and its potential clinical applications.
This abstract was presented (poster) at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.
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