HUMAN EYE CAN SEE INVISIBLE INFRARED LIGHT
Any science textbook
will tell you we can't see infrared light. Like X-rays and radio waves,
infrared light waves are outside the visual spectrum.
But an international
team of researchers co-led by scientists at Washington University School of
Medicine in St. Louis has found that under certain conditions, the retina can
sense infrared light after all.
Using cells from the
retinas of mice and people, and powerful lasers that emit pulses of infrared
light, the 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 light that falls outside
the visible spectrum.
"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 include scientists in
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.
"They were able
to see the laser light, which was outside of the normal visible range, and we
really wanted to figure out how they were able to sense light that was supposed
to be invisible," said Frans Vinberg, PhD, one of the study's lead authors
and a postdoctoral research associate in the Department of Ophthalmology and
Visual Sciences at Washington University.
Vinberg, Kefalov and
their 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. "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."
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,"
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."
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.
The research was made
possible, in part, by the Kefalov team's development of a tool that allowed the
scientists to obtain light responses from retinal cells and photopigment
molecules. That device already is commercially available and being used at
several vision research centers around the world.
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. NIH grant numbers: R24EY021126, R01EY009339,
R01EY019312, P30EY002686, P30EY011373 and R44AG043645.
Comments
Post a Comment