Seeing the invisible: The limits of two-photon vision

Near-infrared light is invisible to humans. And yet, under the right conditions, the human eye can perceive it. Researchers from Poland's International Center for Translational Eye Research (ICTER) have now shown that the efficiency of this phenomenon depends not only on the laser pulse itself, but also on two highly specific factors: the beam diameter and the precise focusing of light on the retina. The research is published in the journal Optics Letters.