How a thermal camera sees heat instead of light

HostMost of us are used to the idea that to see something, you need a light source. You turn on a lamp or wait for the sun. But there are these cameras that can find a lost person in a dark forest or tell if a house is leaking heat through the windows, even in the middle of a pitch-black night. I have always wondered what that camera is actually looking at if there's no light to bounce off the trees or the walls.

GuestIt helps to stop thinking of light and heat as two totally different things. In the world of science, they're part of the same big family. Think of it like a piano. The light our eyes can see is just a few keys in the middle of the keyboard. But the piano goes way further than that in both directions. Heat is just the music playing on the lower keys, where the sound is too deep for our ears to hear. The camera is tuned to those deep notes.

HostSo the camera is basically just listening to those lower notes?

GuestThat's a good way to put it. Everything in the universe that's not frozen down to the absolute bottom of the scale is shaking. Even a block of ice has tiny bits inside it that are jiggling around a little bit. That jiggling sends out waves. The hotter something is, the more those tiny bits shake, and the more energy they throw out. A thermal camera has a special sensor that doesn't care about the bright light we see. Instead, it has thousands of tiny little pads that get warmer or cooler depending on how many of those invisible heat waves hit them.

HostWait, so the camera itself has to get hot to see? That sounds like it would get messy if you're trying to look at something really scorching.

GuestWell, it's not that the whole camera gets hot, but those tiny pads on the sensor chip change how they let electricity flow when they soak up that heat. The camera’s brain checks those pads thousands of times a second. It sees that the pads on the left are getting hit by a lot of heat waves, so it colors those spots bright white or yellow on the screen. The ones on the right are hitting something cooler, so it colors them dark blue or black.

HostSo it's basically just a fancy thermometer that draws a picture. But I have seen night vision goggles where everything looks green. Is that the same thing?

GuestNot at all. That's a common mix-up. Those green goggles are just light boosters. They take the tiny bit of starlight or moonlight that's already there and crank up the volume so your eyes can see it. If you put those on in a room with zero light, you still wouldn't see a thing. But a thermal camera doesn't need any outside light. You could be in a sealed box underground, and if there's a warm cat in there with you, the thermal camera will see it perfectly because the cat is sending out its own glow.

HostThat's a bit eerie to think about. We're all just glowing in a way we can't see. But if this camera is so good at seeing through the dark, why can it not see through a wall? People always think it's like x-ray vision.

GuestYeah, that's a big myth. Heat waves are actually pretty weak when it comes to punching through solid stuff. They're much longer and slower than the waves our eyes use. Most solid things, like a wooden door or a brick wall, just soak them up or bounce them back. If you lean your hand against a wall for a minute and then move it, the camera will see a glowing handprint. It's not seeing through the wall to find your hand; it's seeing the heat your hand left behind on the surface.

HostOkay, so it's surface heat. But I have seen videos where firefighters use them to see through thick smoke to find people in a burning building. Smoke is solid, isn't it? Just tiny bits of soot?

GuestThat's where it gets interesting. To those long heat waves, a cloud of smoke is like a thin mist. The waves are big enough that they can sort of sail right past the tiny bits of ash and soot without hitting them. But our normal light waves are small, so they crash into the smoke and get scattered, which is why we just see a wall of gray. It's the same reason why these cameras are used by pilots to see through fog.

HostThat makes sense. But then I saw someone try to use a thermal camera through a window, and it didn't work. Glass is clear to us, so why is it a wall for heat?

GuestGlass is a weird one. To our eyes, it's like it's not even there. But to heat waves, glass is basically a mirror. If you point a thermal camera at a window, you won't see what's on the other side. You'll actually see a heat reflection of yourself holding the camera. The glass is just too thick and dense for those specific waves to pass through. It's a great reminder that being clear is just a matter of which waves you're tuned into.

HostSo the camera can be tricked. If it sees a reflection on glass, or if it sees a shiny metal surface, does it think those things are hot?

GuestThat's one of the hardest parts of using these tools. Shiny things like a polished metal pot are terrible at showing their own heat. They act like a perfect mirror for whatever else is in the room. You could've a pot of boiling water on the stove, but if the metal is shiny enough, the camera might only see the reflection of the cold refrigerator across the room. It would tell you the pot is freezing cold.

HostThat sounds dangerous if you're relying on it to know if something is safe to touch.

GuestIt's why people who use these for work have to learn how well an object gives off its heat versus reflecting it. A piece of black tape or a bit of wood is great at giving off heat, so the camera gets an honest reading. But a shiny silver spoon is a liar. It's mostly just showing you the heat of whatever is bouncing off it.

HostI have noticed that when people use these cameras, they sometimes make a little clicking sound every few seconds. Is that it taking a picture?

GuestThat's actually the camera resetting itself. Because the sensor is measuring tiny changes in heat, the camera’s own parts can start to mess up the reading as they warm up. It's like trying to weigh something on a scale while the scale itself is slowly getting heavier. Every once in a while, a little flat plate drops down in front of the sensor. The camera knows exactly how warm that plate is, so it uses it to zero everything out. It's like a quick palate cleanser for the sensor so the image stays sharp.

HostAnd the colors? You mentioned yellow and blue. Does the camera just decide what looks like what?

GuestThe colors are totally made up. Heat doesn't have a color we can see, so the computer inside just maps numbers to a set of colors. You can flip a switch and make the hottest things look red, or you can make them look white, or even bright purple. Most pros use a black and white mode because our eyes are much better at seeing tiny details in gray than in a rainbow. But for finding a leak in a pipe under a floor, that bright orange and yellow look makes the hot spot jump right out at you.

HostIt's wild that we live in this world that's constantly glowing with all this information, and we just have no idea it's happening.

GuestA shiny metal pot might look ice cold even when it's boiling because it's just reflecting the cold room around it.

HostThe stove might look off to my eyes, but I'll be thinking about those invisible waves of heat the next time I reach for a pan.