Every object above absolute zero emits infrared radiation, and the amount and wavelength depend on its temperature. Objects at everyday temperatures radiate most strongly at 8 to 14 micrometers, far beyond visible red. A thermal camera has a detector array sensitive to that band, a lens made of germanium (glass is opaque there), and electronics that convert the radiation each pixel receives into a temperature and a color. The picture is a map of surface temperatures, made from the objects’ own glow rather than reflected light.
Thermal radiation
A body at 20 °C radiates about 420 watts per square meter of infrared, peaking near 10 µm. Warm it to 40 °C and the output rises about 30 percent and the peak shifts slightly shorter. This relationship (the Stefan-Boltzmann and Planck laws) is what lets a camera infer temperature from radiation. The visible glow of a red-hot object is the same physics at 600 °C and above, when the peak reaches wavelengths the eye can see.
The detector
Consumer and building thermal cameras use an uncooled microbolometer: an array of tiny elements, typically 80 × 60 to 640 × 480, each of which warms slightly when infrared falls on it and changes its electrical resistance. Reading the resistance of each element many times a second gives a frame. The elements are made of vanadium oxide or amorphous silicon, isolated from their surroundings to hold the tiny temperature change. The number of elements is the native resolution; see thermal camera resolution explained. How small a temperature difference the array can register is its thermal sensitivity; see what NETD is.
The lens
Ordinary glass absorbs long-wave infrared, which is why a thermal camera cannot see through a window and why its lens is germanium, a metal-like element that is opaque to visible light and transparent from 2 to 14 µm. Germanium is expensive and the lens is a large part of a camera’s cost. Some low-cost cameras use chalcogenide glass or silicon.
From radiation to temperature
The radiation a pixel receives is not only the object’s emission. It includes infrared reflected from surroundings and, for distant targets, some from the air in between. The camera calculates temperature assuming values for the object’s emissivity (how efficiently it radiates compared with a perfect emitter), the reflected apparent temperature and the distance. For matte, non-metallic surfaces emissivity is high (0.9 to 0.98) and the assumption holds. For shiny metal it fails badly; see what emissivity is and why shiny metal misleads a thermal camera.
Making the image
The temperature at each pixel is mapped to a color palette (ironbow, rainbow, grayscale) across a span the camera chooses automatically or the user locks. The span matters: a room with a 3 °C spread looks dramatic with auto-scaling and bland with a wide manual span. Many cameras overlay visible-light edges (FLIR’s MSX, others’ fusion modes) so shapes are recognizable.
What the image shows
- Surface temperature, not internal temperature and not what is behind a surface. See can a thermal camera see through walls.
- Patterns: a cold stripe on a wall where insulation is missing, a warm breaker, a cool patch of damp drywall evaporating.
- Relative differences more reliably than absolute values; the absolute value depends on emissivity and reflection settings.
Cameras for home and building use are compared in best thermal cameras for home inspection.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
Does a thermal camera need light?
No. It detects infrared radiation emitted by objects because of their own temperature, so it works in total darkness.
What wavelength does a thermal camera use?
Most consumer and building cameras use long-wave infrared, about 8 to 14 micrometers, where room-temperature objects emit most strongly and the atmosphere is transparent.
Why is the lens not made of glass?
Glass absorbs long-wave infrared; a thermal camera looking through a window sees the window, not what is behind it. Lenses are germanium or chalcogenide glass, which transmit at those wavelengths.
Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.
Related Guides
Thermal Camera vs Borescope: Which Inspection Tool Does What?
One finds where to look from outside; the other looks inside. They are a sequence, not a choice.
Read More→
What Is a Borescope and When Is It Useful?
A camera on a cable that goes where your eyes cannot: cavities, ducts, drains, engines.
Read More→
Can a Moisture Meter Detect Mold?
No. It detects the water mold needs. High readings say where mold could be, not where it is.
Read More→