The human eye can see ‘invisible’ infrared light - possible applications in retinal disease diagnoses

All this while, we have been under the impression that our eyes cannot 'see' infrared light. Like the other waves in the invisible spectrum, X-rays and radio waves, infrared light waves were considered to outside our capacity to visualize...that is, until now.

Light wavelength. @Sara Dickherber, Wash Univ SoM

An international team of researchers, along with scientists at Washington University School of Medicine in St. Louis has found that under certain conditions, the retina can sense infrared light after all.

The researchers have 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. This finding will potentially help 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 its functioning.

The above findings have been published in the journal, the Proceedings of the National Academy of Sciences (PNAS), in its online early edition. The other collaborators in this work include scientists from Cleveland, Poland, Switzerland and Norway,

The research was initiated after scientists on the research team 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.

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 visible spectrum includes waves of light that are 400-720 nanometers long. The scientists found that 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. And hence seen by the human eye.

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.

This research has been funded by the National Eye Institute (NEI) and the National Institute on Aging (NIA) of the National Institutes of Health (NIH), Research to Prevent Blindness, the Norwegian Research Council, the TEAM project financed by the European Union and the Foundation for Polish Science.

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