Human eye has the ability to see 'invisible' infrared light

For the first time, researchers found that under certain conditions, human eye can sense "invisible" infra-red light.

By analyzing cells from the retinas of mice and people and powerful lasers that emit pulses of infra-red light, the team found that when laser light pulses rapidly, light-sensing cells in the retina sometimes get a double hit of infrared energy.

"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, PhD, associate professor of ophthalmology and visual sciences at Washington University. "We hope that ultimately this discovery will have some very practical applications."

The findings are published Dec. 1 in the Proceedings of the National Academy of Sciences (PNAS) Online Early Edition. Collaborators includescientists in Cleveland, Poland, Switzerland and Norway,

Human eyesight depends on photons of light triggering cells known as photoreceptors in the retina. Generally, the light-sensitive pigments in photoreceptors are activated only by visible wavelengths of light. However, the scientists found these pigments can also be activated by longer infrared wavelengths of light if they absorbed them two photons at a time. "Two longer-wavelength infrared photons can achieve one shorter visible-wavelength photon does," Palczewski says.

"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.