WASHINGTON, Dec. 1 (Xinhua) -- It seems to be a common-sense concept that we can't see infrared light but a new study out Monday said under certain conditions, our eyes can sense light that falls outside the visible spectrum.

Using cells from the retinas of mice and people, and powerful lasers that emit pulses of infrared light, an international team of 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 infrared light, the team reported in the U.S. journal Proceedings of the National Academy of Sciences.

The research, conducted by researchers from the U.S., Poland, Switzerland and Norway, was initiated after scientists in the research team reported seeing occasional flashes of green light while working with an infrared laser.

In order to figure out how they were able to sense light that was supposed to be invisible, the team examined the scientific literature and repeated previous experiments in which infrared light had been seen.

"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," said Frans Vinberg of the Washington University in St. Louis, who is one of the study's lead authors.

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 team said.

"The visible spectrum includes waves of light that are 400-720 nanometers long," explained senior investigator Vladimir Kefalov, associate professor of ophthalmology and visual sciences at the Washington University.

"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."

Although the research is the first to report that the eye can sense light through this mechanism, the idea of using less powerful laser light to make things visible isn't new, the team said. The two-photon microscope, for example, uses lasers to detect fluorescent molecules deep in tissues.

The researchers said they already are working on ways to use the two-photon approach in a new type of ophthalmoscope, a tool that allows doctors to examine the inside of the eye as well as stimulate specific parts of the retina to determine whether it's functioning properly.