In pitch darkness, where you can barely see a thing, you might be aware that there are animals that can see perfectly. Low-light vision seems like a superpower, and indeed for us humans, it is! Part of this is how certain animals with good night vision, like large cats and owls, can perceive a greater spectrum of light than we can. This includes infrared light. But you might have seen the faint reddish glow of an infrared surveillance camera; in fact, it may have even caught your eye. Isn’t this supposed to be invisible? Can humans see infrared light, then?
Fascinating stuff, right? In this post, we’ll explore the subject. But to understand whether we can “see” infrared, we have to learn a little bit about how light works and how the human eye perceives light.
Explaining Light: Wavelength, Radiation, and Sight
Light is part of the electromagnetic spectrum, a range that covers all types of electromagnetic radiation. This includes everything from common AM and FM radio, which have the longest wavelengths, to microwaves used to heat your food and make cell phones communicate with each other. Next up are infrared waves, which lie just before the visible light spectrum begins. Ultraviolet is on the other end of the visible light spectrum, followed by X-rays and gamma rays. They’re all measured in units of length, ranging from kilometers long for radio waves all the way down to nanometres for light and picometers for X-rays.
The visible band of light lies within the 390-720 nm (nanometre) range. Infrared light, on the other hand, exists on the very edge of this band. It ranges from 700 nm to 1 mm in wavelength. The human retina has special photosensitive cells within it that are particularly sensitive to the visible light spectrum.
So getting down to business: can humans see infrared light?
Can Humans See Infrared Light?
Well, the human eye can detect the visible band of the electromagnetic spectrum.
It should make sense, then, that we cannot see infrared light. However, under certain conditions, the human retina can sense infrared light after all. It’s important to note that there isn’t an absolute cutoff. The retina only becomes less sensitive to light as it goes beyond the visible light spectrum, and it can sense infrared light, especially if it’s bright enough.
Researchers led partly by scientists at the Washington University School of Medicine in St. Louis used retina cells from mice and humans and used powerful, speedy lasers that emit infrared light pulses. They discovered that the retina sometimes interprets multiple, high-speed pulses of infrared energy as a single pulse of half the wavelength. For example, a pair of 1000 nm photons might deliver the same amount of energy as a single 500 nm photon. This is well within the visible spectrum of light, and the eye can see it potentially. The findings from the researchers, while focused on medical applications, are nonetheless fascinating.
So that’s why human beings, under certain conditions, can see infrared light – especially if it’s bright enough. Manufacturers of infrared lights for common applications, such as surveillance and trail cameras, are aware of this. Quality manufacturers make covert and semi-covert illumination solutions for applications where naked-eye visibility is a concern.
Iluminar makes custom, dedicated infrared illuminators for surveillance and license plate capture applications. All of our infrared illuminators feature 850nm or 940nm wavelength options. Waterproofing, AC/DC power options, and pressure relief valves are standard for rugged outdoor use. To upgrade your illumination setup, call us at (281) 438-3500, and our experts will be able to help you with any inquiry.