The ability of the human eye to perceive infrared light at around 1,000 nm in wavelength has now been investigated in detail by scientists in Spain and Poland. Pablo Artal and co-workers from the Universidad de Murcia, Nicolaus Copernicus University, Baltic Institute of Technology and Polish Academy of Sciences studied the two-photon absorption properties of the retina. Six volunteers ranging in age between 26 to 55 years old with healthy eyes took part in the experiment. Visual stimuli in the form of continuous-wave green HeNe laser light at 543 nm or short (435 fs) infrared (1,043 nm) pulses from a femtosecond laser were delivered to the subject’s eye at a power level well below safety limits. Interestingly, in both cases of infrared or visible exposure, the colour perception reported by the volunteers was almost indistinguishable and the visual acuity was comparable. The results not only support the hypothesis of two-photon vision but also indicate that it is indeed a retinal phenomenon. The findings may also be useful for developing an optical instrument for early detection of retinal disease or one that can evaluate the condition of the underlying visual system in the case of someone with nearly opaque eyes.
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Graydon, O. Two-photon vision. Nature Photon 12, 2 (2018). https://doi.org/10.1038/s41566-017-0081-4