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“Invisible” infrared light made human-visible

Dec. 1, 2014
Courtesy of Washington University School of Medicine 
and World Science staff

Any sci­ence text­book will tell you we can’t see in­fra­red light. But a team of re­search­ers has found that un­der cer­tain con­di­tions, the eye can sense in­fra­red light af­ter all.

Like X-rays and ra­di­o waves, in­fra­red light waves are out­side the vis­u­al spec­trum. “Infra-red” lit­er­ally means be­low red—it’s low­er in en­er­gy than red light, which lies at the low-en­er­gy end of that spec­trum, and thus too low-en­er­gy for our de­tec­tion.

The eye can de­tect light at wave­lengths in the vis­u­al spec­trum. Oth­er wave­lengths, such as in­fra­red and ultra­violet, are sup­posed to be in­vis­i­ble to us. (Cred­it: Sara Dick­her­ber )


But us­ing cells from the ret­i­nas of mice and peo­ple, and la­sers emit­ting in­fra­red light, the re­search­ers found that when la­ser light pulses quick­ly, the cells some­times get a dou­ble hit of in­fra­red en­er­gy. Then it’s de­tect­a­ble.

“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 of Wash­ing­ton Uni­vers­ity School of Med­i­cine in St. Louis. “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 on­line early edi­tion.

The re­search started af­ter some of the re­search­ers re­ported see­ing oc­ca­sion­al flashes of green light while work­ing with an in­fra­red la­ser, whose light is nor­mally in­vis­i­ble to peo­ple. “We really wanted to fig­ure out” why, said Frans Vin­berg, one of the study’s lead au­thors and a post­doc­tor­al re­search as­so­ci­ate at the uni­vers­ity.

Vin­berg, Ke­falov and col­leagues re­vis­ited older re­ports of peo­ple see­ing in­fra­red light. They re­peat­ed pre­vi­ous ex­pe­ri­ments in which in­fra­red light had been seen, and they 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,” Vin­berg ex­plained.

Nor­mal­ly, a par­t­i­cle of light, called a pho­ton, is ab­sorbed by the eye’s ret­i­na, which then cre­ates a mol­e­cule called a pho­to­pig­ment. That be­gins the pro­cess of con­vert­ing light in­to vi­sion. In stand­ard vi­sion, each of a large num­ber of pho­to­pig­ments ab­sorbs one pho­ton.

But pack­ing a lot of pho­tons in a short pulse of the rap­idly puls­ing la­ser light makes it pos­si­ble for a pho­to­pig­ment to ab­sorb two pho­tons at once, the sci­en­tists said. Their com­bined en­er­gy is enough to ac­ti­vate the pig­ment and stim­u­late vi­sion.

“The vis­i­ble spec­trum in­cludes waves of light that are 400-720 nanome­ters [bil­lionths of a me­ter] long,” said 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.” (Short­er wave­lengths mean more ener­gy).

Al­though the re­search­ers are 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 two-pho­ton mi­cro­scope, for ex­am­ple, uses la­sers to de­tect flu­o­res­cent mol­e­cules deep in tis­sues. And the re­search­ers 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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Any science textbook will tell you we can’t see infrared light. But a team of researchers has found that under certain conditions, the eye can sense infrared light after all. Like X-rays and radio waves, infrared light waves are outside the visual spectrum. “Infra-red” literally means below red—it’s lower in energy than red light, which lies at the low-energy end of that spectrum, and thus too low-energy for our detection. But using cells from the retinas of mice and people, and lasers emitting infrared light, the researchers found that when laser light pulses quickly, the cells sometimes get a double hit of infrared energy. Then it’s detectable. “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 of Washington University School of Medicine in St. Louis.”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 Online Early Edition. The research started after some of the researchers reported seeing occasional flashes of green light while working with an infrared laser, whose light is normally invisible to people. “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 and a postdoctoral research associate at the university. Vinberg, Kefalov and colleagues 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 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,” Vinberg explained. Normally, a particle of light, called a photon, is absorbed by the eye’s retina, which then creates a molecule called a photopigment. That begins the process of converting light into vision. In standard vision, each of a large number of photopigments absorbs one photon. But packing a lot of photons in a short pulse of the rapidly pulsing laser light makes it possible for a photopigment to absorb two photons at once, the scientists said. Their combined energy is enough to activate the pigment and stimulate vision. “The visible spectrum includes waves of light that are 400-720 nanometers (billionths of a meter) long,” said 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.” Although the researchers are 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 two-photon microscope, for example, uses lasers to detect fluorescent molecules deep in tissues. And the researchers 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.