During a normal day in the lab, while lots of research equipment occupied the room and the computers were full of various experimental data, the scientists worked with a laser. It was nothing like the laser pointers used in lecture halls or as toys, but a carefully engineered and calibrated research device, emitting light waves in an infrared spectrum, invisible to the human eye. Everything went smoothly, except for one thing: the researchers noticed occasional flashes of green light, that shouldn’t be there.
Our visual perception is most responsive to stimulating light in the 400 to 700 nm range (and a bit beyond that), also called the visible spectrum. Next, already invisible to the human eye, there is the ultraviolet (10 to 400 nm) and infrared (700 nm to 1 mm) lights. Why we see this spectrum and no other, like various animals, is well studied and known, so there was no known reason for these green flashes to appear. They weren’t part of the experiment and weren’t an expected or even assumed result or side effect. So, what were they?
“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.
Many people could ignore it and move on with the planned research. Many probably did, but not this group. An international research team was assembled, and a series of experiments were conducted. In addition, the research group examined the scientific literature and revisited reports of people seeing infrared light. This phenomenon isn’t new, but not yet thoroughly studied.
The experiments had shown that under certain conditions infrared light is visible, and its color would appear as about half the wavelength of the used infrared laser. This means that a wavelength of 1200 nm, way beyond the visible spectrum, would be seen as orange at about 600 nm. A 1000 nm wavelength would appear to us almost as green 500 nm wavelength.
The required conditions are quite unique. The team tried different techniques and found that short pulses of light increase the chances for two photons to be absorbed at one time by a single photopigment, a type of pigment that undergoes a chemical change when they absorb light and allow us to see. In light, the higher the wavelength of the photon, the lower its energy. Infrared light doesn’t have enough energy to activate the photopigment, but the combined energy of the two photons is enough to activate the photopigment and allow the eye to see what normally is invisible.
While sharing the news of this exciting discovery, we received one response with a very interesting thought: One has to be highly skeptical to discover something like that; to know that something is impossible, like seeing infrared light, and still to put it in doubt and research it, in case it’s possible after all.
The researchers hope this discovery to allow some practical applications, like more advanced analysis of the vision. We are sure that it will eventually happen, but even before that, we are more than happy to discover another sense in the human body, to learn something new about ourselves, and to gain a new “super-power”, long thought to be impossible: Infrared vision.
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https://source.wustl.edu/2014/12/the-human-eye-can-see-invisible-infrared-light/
Palczewska G, Vinberg F, Stremplewski P, Bircher MP, Salom D, Komar K, Zhang J, Cascell M, Wojtkowski M, Kefalov VJ, Palczewski K. PNAS Online Early Edition, Dec. 1, 2014 http:www.pnas.org/cgi/doi/10.1073/pnas.1410162111 – https://www.pnas.org/content/pnas/early/2014/11/25/1410162111.full.pdf
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