Human eye can see 'invisible' infrared light

Any science textbook will tell you that humans cannot see infrared light, but researchers have found that under certain conditions our eyes can actually detect the 'invisible' light.
Like X-rays and radio waves, infrared light waves are outside the visual spectrum.
Using cells from the retinas of mice and people, and powerful lasers that emit pulses of infrared light, the researchers 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.
"We're using what we learned in these experiments to try to develop a new tool that would allow physicians to not only examine the eye but also to stimulate specific parts of the retina to determine whether it's functioning properly," said senior investigator Vladimir J Kefalov, associate professor of ophthalmology and visual sciences at Washington University.

The study was initiated after 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 the scientists 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 Frans Vinberg, one of the study's lead authors.
Researchers examined the scientific literature and revisited reports of people seeing infrared light.

They repeated previous experiments in which infrared light had been seen, and they analysed 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," Vinberg said.
"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," Vinberg added.

The findings are published in the journal PNAS.