Scientists have uncovered a strange effect that allows humans to see infrared light, a feat once thought impossible. Infrared light is 30% more red than the reddest color in the rainbow, and not considered to be part of the visible spectrum. However, when exposed to low intensity infrared light, people have reported seeing white and green, as well as other colors.
Light consists of electromagnetic waves organized into packets called photons. Light also behaves like waves that oscillate in time. While all light waves travel at the same speed, they don’t oscillate at the same frequency. This oscillation frequency is what the human eye maps to color. The shortest visible wavelength is approximately 400nm (blue) and the longest is 720nm (red). Conventionally, all wavelengths outside this range are considered invisible to humans.
Seeing infrared light is accomplished through a very weak effect in the eye. Two identical photons of the same frequency produce a single-photon with twice the original frequency: effectively changing the light from an invisible deep red (1000nm), to visible green (500nm).
Exploiting this effect requires a special setup. One must both provide many infrared photons and not blind the viewer. Importantly, this demonstrated sensitivity to infrared light could enable future ophthalmic devices to help patients with eye conditions, such as cataracts, that make the eye opaque to visible light.
Visual acuity in the two photon infrared vision – OSA