Purpose: It has been shown that humans can detect pulsed infrared light perceived as visible light of the corresponding half wavelength due to a two-photon process. However, little is known about the role played by the different variables involved in the process. To better understand this phenomenon, we used a supercontinuum laser to evaluate the effect of wavelength and pulse repetition rate on the light power thresholds (PT).

Methods: We used as pulsed light source for our instrument a supercontinuum laser (SuperKCompact, NKT Photonics, Denmark) covering the range between 450 and 2400 nm. To select the exciting IR wavelength, a set of bandpass filters with a spectral bandwidth of 40 nm were used (850, 880, 900, 920, 950, 1000, 1050 and 1100 nm). The visual stimulus was projected onto the retina by a 2-axis galvo mirror system which delivered a raster scanning beam through a 1.5-mm aperture. The subjects perceived a Tumbling E letter subtending 2 minutes of arc. The light source provides a constant flux of pulses in the order of 1 ns at selectable repetition rates (1.98 and 9.91 kHz). Four volunteers participated in the experiment. They were asked to set a variable neutral density filter to find the power threshold for each wavelength and pulse repetition rate. The procedure complied with the Declaration of Helsinki and was approved by the Ethics Committee of the University of Murcia.

Results: Participants experienced purple, blue and green color sensations as the wavelength of the IR light was increased. PT was measured at each of the selectable wavelengths for 2 different pulse repetition rates. On the one hand, the average across participants shows that PTs at the higher pulse repetition rate are always greater than those for the lower one for all wavelengths, being the ratio between both PTs 2.0±0.3 (average across wavelengths). On the other hand, the wavelength dependence of the PT shows a similar trend at both pulse repetition rates: it exhibits a drop in the shorter wavelength range up to 920 nm and stays roughly constant passed this wavelength (avg. 3.7 μW at 9.91 kHz, 2.1 μW at 1.98 kHz).

Conclusions: We successfully demonstrated the use of a supercontinuum laser producing infrared pulses in the range of nanoseconds to produce two-photon vision. This source allowed us to test the effect of different parameters on the visual threshold.

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

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