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How Humans Can See Infrared as Visible Light

human eye

Image credit: Sarah Scicluna via Flickr

As we know, the reason why some portion of the electromagnetic spectrum is named as the “visible” is attributed to the range from 400-720 nanometers that could be identified with normal human vision. Some previous studies have demonstrated that in some special conditions, humans are able to perceive near infrared wavelengths as visible light, As a result, Krzysztof Palczewski and his team from CaseWestern ReserveUniversity in Cleveland, Ohio, has discovered a mechanism that could explain such phenomenon. And the results of their research have been recently published in the Proceedings of the National Academy of Sciences

According to the authors of the paper, some reports made previously reports as well as their further psychophysical studies have shown that humans are capable of detection of IR at wavelengths longer than 1,000 nm and then identify it as visible light. However, this discovery has not well-explained in a physical sense satisfactorily.

Thirty volunteers had participated in the study with the beams of near-infrared light being pulsed into their eyes. Actually they could see the light as color. To the eyes of participants, the longer wavelengths were seen as being reddish in color, while shorter ones were perceived seen as green.

Normal vision can be attributed to light in the visible wavelength spectrum. When it enters the eye, it will be focused by the lens onto the rod and cone cells of the retina, thus allowing such impulses to be sent back for processing by the brain. As normal photoreceptor sensitivity of human being decreases past 720 nanometers, it is surprisingly learned by Palczewski’s team that the sensitivity of the cells would increased with wavelengths longer than 900 nanometers, which is dependent on the power of the laser.

In the experiments targeted on chromophore within mammalian eyes, they discovered that typically it was necessary for chromophores to absorb visible light for activating the pigment protein rhodopsin, so the same effect could be obtained with near-infrared wavelengths, in the condition that two photons would work together at the same time.

As Palczewski added in his paper, such observations were consistent with their quantum mechanical model in regard to  the energetics of two-photon activation of rhodopsin, therefore, humans are able to see IR light via two-photon isomerization of visual pigment chromophores.

Apart from being regarded as a pretty fun trick, some useful clinical applications would be implemented with better knowledge of this phenomenon. It is reported by Popular Mechanics that beams of near-infrared light could be applied for detection of the eye damage at its earliest stages, when it would have enough time to intervene and protect the patient’s vision. However, visible light would fail in this application, because it would be much damaging to the human’s eyes.

Source: Popular Mechanics

Image source: Sarah Scicluna via Flickr, CC BY-ND 2.0

Journal reference: Palczewska, Grazyna, et al. “Human infrared vision is triggered by two-photon chromophore isomerization.” Proceedings of the National Academy of Sciences(2014): 201410162.