Human eyes can detect light at wavelengths in the visual spectrum; other wavelengths, such as infrared and ultraviolet, are supposed to be invisible, but an international group of scientists from Poland, Switzerland, Norway and the United States, has found that under certain conditions, it’s possible for humans to see infrared light. Using cells from the retinas of mice and people, and infrared lasers, the group found that when laser light pulses rapidly, light-sensing cells in the retina sometimes get a double hit of infrared energy. When that happens, the eye is able to detect light that falls outside the visible spectrum.

The new study was initiated after the scientists reported seeing occasional flashes of green light while working with an infrared laser. Unlike the laser pointers used in lecture halls or as toys, the powerful infrared laser they worked with emits light waves thought to be invisible to the human eye.

“They were able to see the laser light, which was outside of the normal visible range, and we really wanted to figure out how they were able to sense light that was supposed to be invisible,” said Dr Frans Vinberg of Washington University School of Medicine in St. Louis, who is a co-author of the paper published in the Proceedings of the National Academy of Sciences.

Dr Vinberg and his collaborators examined the scientific literature and revisited reports of people seeing infrared light.

They repeated previous experiments in which infrared light had been seen, and analyzed such light from several lasers to see what they could learn about how and why it sometimes is visible.

“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. 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,” Dr Vinberg said.

Normally, a particle of light, 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 nm long,” said co-author Dr Vladimir Kefalov of Washington University School of Medicine in St. Louis.

“But if a pigment molecule in the retina is hit in rapid succession by a pair of photons that are 1,000 nm long, those light particles will deliver the same amount of energy as a single hit from a 500-nm photon, which is well within the visible spectrum. That’s how we are able to see it.”

Although the team is the first to report that the human eye can sense light through this mechanism, the idea of using less powerful laser light to make things visible isn’t new.

The two-photon microscope, for example, uses lasers to detect fluorescent molecules deep in tissues.

And the scientists said they already are working on ways to use the two-photon approach in a new type of ophthalmoscope, which is a tool that allows physicians to examine the inside of the eye. The idea is that by shining a pulsing, infrared laser into the eye, doctors might be able to stimulate parts of the retina to learn more about its structure and function in healthy eyes and in people with retinal diseases such as macular degeneration.

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Grazyna Palczewska et al. Human infrared vision is triggered by two-photon chromophore isomerization. PNAS, published online before print December 1, 2014; doi: 10.1073/pnas.1410162111