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Short answer: A thermal camera does not see light. It measures the infrared energy that every object gives off, using a grid of tiny heat-sensitive pixels, and turns that energy into a temperature map.
What is the camera actually detecting?
Everything above absolute zero gives off electromagnetic radiation, and the amount rises steeply as the object gets warmer. At everyday temperatures most of that energy is long-wave infrared, at wavelengths of roughly 8 to 14 micrometres. That is far beyond what the human eye can see. Air lets this band through fairly well, which is why most handheld thermal cameras are built to work in it.
A thermal camera is therefore a passive instrument. It needs no light source and works just as well in complete darkness, because the scene is its own source.
What happens inside the camera?
- The lens. Ordinary glass blocks long-wave infrared, so thermal lenses are made from materials such as germanium that let it through. The lens focuses the incoming energy onto the sensor.
- The microbolometer. Most affordable thermal cameras use an uncooled microbolometer array. Each pixel is a tiny suspended membrane. When infrared lands on it, the membrane warms very slightly and its electrical resistance changes.
- Readout and processing. Electronics measure the resistance change in each pixel and convert it into a signal. The camera then applies a calibration that relates that signal to temperature.
- The display. Those temperatures are mapped onto a colour palette, such as ironbow, rainbow or greyscale, to make the image. Many cameras also blend in a visible-light photo so you can see edges and labels.
The number of pixels in the array, for example 160 × 120 or 320 × 240, sets the thermal resolution. More pixels let you resolve smaller targets from further away.
What is emissivity and why does it matter?
Emissivity describes how well a surface gives off infrared compared with a perfect emitter, on a scale from 0 to 1. The camera has to assume a value to calculate temperature.
- High emissivity, about 0.9 and above. This includes painted surfaces, most plastics, rubber, skin, water, brick and electrical tape. Readings are reliable.
- Low emissivity. This includes polished or shiny metals such as aluminium, copper busbars and stainless steel. These surfaces give off little energy of their own and reflect the surroundings like a mirror. The camera may show your own reflected body heat instead of the real surface temperature.
Common fixes are to set the correct emissivity in the menu, to measure a strip of high-emissivity tape stuck to the surface, or to aim at a painted or oxidised area nearby.
What other things affect accuracy?
- Reflected temperature. Hot lights, the sun or a person standing nearby can reflect off a surface. Change your viewing angle to check.
- Distance and spot size. Each pixel covers a larger area the further you are from the target. If the target is smaller than a few pixels, the reading averages in the background.
- Glass and windows. Long-wave infrared does not pass through ordinary glass. You will see the temperature of the pane, not what is behind it.
- Focus. A blurred image spreads heat between neighbouring pixels and lowers peak readings.
- Ambient conditions. Wind cools surfaces, and direct sun heats them. For building surveys, a reasonable temperature difference between inside and outside makes faults much easier to see.
What are thermal cameras used for?
- Electrical maintenance. Spotting loose or overloaded connections in distribution boards. Only competent people should work near live equipment.
- Buildings. Finding missing insulation, air leaks, damp patches and underfloor heating pipes.
- HVAC and plumbing. Tracing pipes, checking radiators and finding blockages.
- Mechanical. Finding hot bearings, motors and belts.
- Agriculture and livestock. Checking stores for hot spots and spotting inflammation, under veterinary guidance.
Is a thermal camera the same as night vision?
No. Night-vision devices amplify small amounts of visible or near-infrared light. Thermal cameras detect emitted heat and need no light at all. They work in a different part of the spectrum and suit different jobs.
FAQ
Can a thermal camera see through walls? No. It shows the surface temperature of the wall, which can reveal pipes, studs or damp behind it.
Why does a shiny pipe look cold when it is hot? Shiny metal has low emissivity and mostly reflects its surroundings. Put tape on the pipe and measure the tape.
What resolution do I need? For home surveys, 160 × 120 is often enough. Electrical and long-range work benefits from higher resolution.
Do thermal cameras need calibration? Yes, periodically. Many manufacturers recommend a factory check about once a year for measurement use.
Sources
- "Infrared Thermography for Temperature Measurement and Non-Destructive Testing", Sensors (PMC open access): https://pmc.ncbi.nlm.nih.gov/articles/PMC4168422/
- "Thermal cameras and applications: a survey", Machine Vision and Applications: https://doi.org/10.1007/s00138-013-0570-5
- "An improved electrical and thermal model of a microbolometer", Advances in Radio Science: https://ars.copernicus.org/articles/10/183/2012/ars-10-183-2012.pdf
- Manufacturer explainer on emissivity and thermal imaging: https://www.flir.com/discover/professional-tools/how-does-emissivity-affect-thermal-imaging/
Related products: Thermal & Temperature
