Human Eye Sees 'Invisible' Infrared Light

Normally infrared light is invisible to the human eye, but researchers have recently found that under certain conditions the retina can sense infrared light after all.

Using powerful lasers that emit pulses of infrared light, and the cells of retinas from both mice and humans, a team of scientists at Washington University in St. Louis realized that we could see outside the visible spectrum, but with a little push.

"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," Frans Vinberg, who was involved in the study, said in a statement. "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 findings were published in the journal Proceedings of the National Academy of Sciences.

The human eye normally absorbs a single particle of light, called a photon, and then converts it into standard vision via a molecule called a photopigment. This process allows us to see only light 400-720 nanometers long, whereas infrared light falls outside this range.

But if the retina were to get a "double hit" of infrared energy with the laser light fast enough, this invisible light would then become visible. Researchers are currently working to create a tool that would make this possible.

"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.

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