I remember being told that as a newborn, my parents would have the baby monitor volume turned up so high that they could hear cars driving outside my window through the speakers. So it was no surprise to me when I saw some new parents with a fancy baby monitor that wirelessly broadcasted video to a tablet, which they carried around with them. Since babies generally sleep in the dark – you know like most sane humans do – the video used infrared night vision to see into the unlit room. This made me think a lot about how all of the science that goes into making that work was probably being taken for granted.
To discuss how night vision cameras see in the dark, let’s first talk about how our eyes see in the light. Light, or more precisely visible light, are waves with a particular range of frequencies. When the light comes from our nearest star (which is commonly referred to as the sun) this entire range of light waves is represented, and appears as the color white. White is not technically a color, but you get the idea. It appears as white, because when we sense these light waves with our eyes, our brains interpret it as such. Remember though, white light consist of the entire range of visible light, so let’s say that this light is traveling towards us, but before it reaches our eyes it runs into a transparent medium. Oh I don’t know, how about a rain drop. Then this white light refracts causing all of the different light frequencies to bend and make what is technically referred to as a “rainbow.” Occasionally, someone films a “double rainbow” and if it goes “all the way across the sky” that person becomes an internet sensation. The point is our brains interpret each of these frequencies as their own color.
However, the visible light range is only a small part of the entire light range and it is the only part our eyes can see. Gamma rays, Radio waves, and infrared light waves are all other parts of the light range that our eyes cannot see. So I know what you might be thinking: “Does my brain interpret the color green the same way that someone else interprets the color green?” But that question is a whole different can of worms. You might also be thinking: “So if infrared light waves exist, why can’t we make some type of camera that can see them?” Which is exactly how infrared night vision works. You are so smart!
In the case of the baby monitor mentioned before, imagine shining a flashlight on the sleeping baby, but the only thing that can see this light is the camera for the monitor. Most infrared night vision cameras come surrounded by infrared LEDs. Take a look at most security cameras and you will see a ring of clear LED lights around a central camera. Now a little “computer” changes that infrared signal into a single color on a tablet screen, kind of like how your own brain does visible light. Thus, these parents can see their newborn, or a Tom Clancy character can see a mission target.
What is stopping us there? Why not look at the entire range of light? Well many space telescopes actually do this when looking at interstellar bodies like stars, nebulas, quasars, and more. Here the measured light is given an artificial coloring so that we can see it on a poster or computer screen.
So next time you see some night security film, a horror scene taken in the dark, or any one of the other night vision applications, take a moment to appreciate the physics and engineering at work and the smart people like you who made it a reality.
Until next time,
-T
Back To First Principles 