“Seeing through walls” is the most common promise made about thermal cameras, fed by films and some advertising. It is also the most misleading. Knowing what a camera actually sees through avoids buying a device for an impossible job, and above all avoids misreading an image that is perfectly correct.
At a glance
| Obstacle | Sees through? | What it actually shows |
|---|---|---|
| Wall, partition, ceiling | No | The surface, with the thermal effect of a pipe, stud, insulation gap or moisture |
| Window, glass, windshield | No | The glass temperature and reflections, often including the operator |
| Smoke | Largely | The scene behind the smoke, sometimes with reduced contrast |
| Fog, rain | Partly | The scene, with range reduced by scattering on droplets |
| Very thin plastic film | Partly | An attenuated scene; thicker plastic becomes opaque |
| Clothing | No | The fabric temperature, influenced by the body when thin and tight |
| Water | No | The temperature of the top fraction of a millimetre |
| Polished metal | No | Mostly reflections, with a distorted reading |
| Total darkness | Yes | A normal image: the camera needs no light at all |
Walls: the camera reads the building’s skin
A wall absorbs thermal infrared within its first micrometres. The camera therefore measures plaster, paint or tile, not what is inside. Hidden elements still become visible when they change the surface temperature:
- a hot water pipe or underfloor heating warms the surface by conduction;
- framing studs conduct heat better than the insulation around them and draw regular bands;
- missing insulation or cold air infiltration creates a cold patch or streak;
- a damp area cools through evaporation or holds its temperature for longer.
These signatures only appear when there is a temperature difference between inside and outside, or between the hidden element and its surroundings. A wall at thermal equilibrium shows almost nothing. That is why home thermography is done during the heating season, as explained in our article on when to scan a house.
So a camera cannot show a person behind a partition or the depth of a pipe. It tells you where to look, not exactly what is inside.
Glass: a mirror for infrared
This surprises many first-time users: pointed at a window, the camera shows a uniform surface, often with the operator’s reflection, not the garden. Ordinary glass is transparent to visible light but opaque in the 8 to 14 µm band used by common cameras, and it acts as a fairly efficient infrared mirror.
Two practical consequences:
- you cannot inspect a façade through a window, or measure an electrical panel behind an ordinary glass viewing pane;
- measuring the temperature of a window is skewed by reflections. Stand at an angle and check that a hot spot does not move with you.
Thermal camera lenses are not made of glass but of germanium or chalcogenide, materials that are transparent in thermal infrared. Infrared inspection windows for electrical panels use special materials for the same reason.
Smoke: where the camera truly sees through
Soot particles are much smaller than the wavelength of thermal infrared. They stop visible light but let most 8 to 14 µm radiation pass. That is why firefighters use thermal cameras to find victims and locate the seat of a fire in a smoke-filled room.
Transparency is not total: very dense smoke or smoke loaded with steam reduces contrast, and hot gases can saturate the image. Firefighting cameras are built for these conditions, with extended temperature ranges and dedicated modes. They have their own specialized ranking, separate from diagnostic cameras.
Fog and rain: better than the eye, no miracle
Fog droplets are close in size to the wavelength of thermal infrared and scatter it, so range drops sharply. In light haze, a thermal camera can still pick out a warm figure that the eye no longer sees. In thick fog, it loses most of its range too. Rain also wets the surfaces being observed, which evens out their temperatures.
Plastic, clothing, water, metal: the tricky cases
Very thin plastic
A very thin polyethylene film, such as a bin bag, lets some thermal infrared through. That explains the striking demos where a hand stays visible inside a black bag. Thicker plastic, a box or a housing becomes opaque again.
Clothing
Fabric absorbs infrared, so the camera shows its surface. A thin, close-fitting garment warms up by contact and may reveal warmer areas, but never an image of the body. Ads for cameras that “see under clothes” are scams.
Water
Water absorbs thermal infrared within a fraction of a millimetre. A camera measures the surface temperature of a pool or a river, never the bottom. A fish or a submerged object does not show up.
Polished metal
Shiny metal has a low emissivity: it emits little and reflects a lot. A stainless steel pan can look cold while scorching hot, or warm because it reflects the operator. Our article on emissivity and measurement error explains how to work around this trap.
What this means when choosing a camera
Since the camera reads surfaces, the differences you look for behind a wall are often small: one or two degrees for an insulation defect, sometimes less for a deep pipe. Two specifications then matter more than the rest:
- good thermal sensitivity (NETD), to separate small differences from noise;
- a manually adjustable temperature span, to tighten the display around the wall temperature.
For building inspection, see our home inspection guide, then refine with the selector based on your working distance and the size of the defects you look for.
Frequently asked questions
Can a thermal camera see through a wall?
No. It measures radiation from the wall surface. A hot pipe, a stud or a damp area behind the wall can only change the temperature of that surface.
Can a thermal camera see through glass?
No. Ordinary glass is opaque in the 8–14 µm band used by common thermal cameras and reflects infrared like a mirror. You see the glass temperature and reflections, not the room behind.
Can a thermal camera see through smoke?
Largely, yes. Soot particles block visible light but let thermal infrared through, which is why firefighters use thermal cameras.
Can a thermal camera see through clothing?
No. It shows the surface temperature of the fabric. Thin, tight clothing may reveal warmer areas by conduction, but never an image of the body.
Sources
The behaviour described applies to common thermal cameras, sensitive in long-wave infrared (about 8 to 14 µm). Some scientific cameras work in other bands where materials behave differently.