A thermal camera does not see light. It measures heat radiating from surfaces and turns the differences into a picture. Here is how that works, and what it means for what you can and cannot see.
Infrared radiation, not visible light
Every object above absolute zero gives off infrared radiation, and warmer objects give off more. Thermal cameras detect the long-wave infrared band, roughly 8 to 14 micrometres, which sits well outside the visible spectrum. That is why they work in complete darkness.
The sensor
Most affordable thermal cameras use an uncooled microbolometer: a grid of tiny pixels that each change electrical resistance as infrared energy warms them. The camera reads every pixel, converts the value to a temperature, and assigns it a color. A 256×192 sensor has 49,152 of these pixels. A 96×96 sensor has 9,216.
The lens
Ordinary glass blocks long-wave infrared, so thermal lenses are made from germanium or a similar infrared-transmitting material. The focal length sets the field of view: a short lens sees a wide area, a longer lens sees a narrower area with more detail at distance.
What thermal cameras cannot do
- See through walls. They show surface temperature only. Heat patterns on a wall can hint at what is behind it, but you are interpreting a surface.
- See through glass. Window glass reflects thermal radiation like a mirror, so you will see your own reflection instead of the room.
- Read every surface accurately. Shiny metal and some other materials read wrong unless you account for emissivity. See our emissivity guide.
Why this matters when you buy
Sensor resolution, sensitivity and lens choice decide how much detail you get. Our buyer's guide shows how to match those specs to your use, and you can browse every model in our full catalog.