Can people ever see Infrared without a filter?
Yes. Until 2014, it was thought that IR was not ever visible to the naked human eye. Many birds and other animals can see it, but we believed we could not. Under special circumstances, however, humans can see IR without a filter. In 2014 the Washington University School of Medicine in St. Louis, MO discovered that quick pulses of infrared could be detected by the human eye.
"We experimented with laser pulses of different durations that delivered the same total number of photons, and we found that the shorter the pulse, the more likely it was a person could see it," Dr. Frans Vinberg explained. "Although the length of time between pulses was so short that it couldn't be noticed by the naked eye, the existence of those pulses was very important in allowing people to see this invisible light." The image someone sees during this "double pulse" is a green flash.
Normally, a particle of light, called a photon, is absorbed by the retina, which then creates a molecule called a photopigment, which begins the process of converting light into vision. In standard vision, each of a large number of photopigments absorbs a single photon.
But packing a lot of photons in a short pulse of the rapidly pulsing laser light makes it possible for two photons to be absorbed at one time by a single photopigment, and the combined energy of the two light particles is enough to activate the pigment and allow the eye to see what normally is invisible.
"The visible spectrum includes waves of light that are 400-720 nanometers long," explained Kefalov, an associate professor of ophthalmology and visual sciences. "But if a pigment molecule in the retina is hit in rapid succession by a pair of photons that are 1,000 nanometers long, those light particles will deliver the same amount of energy as a single hit from a 500-nanometer photon, which is well within the visible spectrum. That's how we are able to see it."
Washington University in St. Louis. "Human eye can see 'invisible' infrared light." ScienceDaily. ScienceDaily, 1 December 2014. www.sciencedaily.com/releases/2014/12/141201161116.htm.