What Blocks Thermal Imaging? What Thermal Cameras Can and Cannot See
TL;DR
- Thermal cameras read infrared radiation from visible surfaces; they usually do not see directly through solid barriers.
- Walls, concrete, metal, ordinary glass, and water generally block direct long-wave thermal imaging.
- A barrier can still reveal indirect clues if heat changes the temperature pattern on its surface.
- Smoke, fog, rain, and foliage can reduce image quality depending on density, distance, moisture, and thermal contrast.
- Thermal images can be distorted by reflections, emissivity, sunlight, wind, distance, and incorrect camera settings.
Walls, concrete, metal, even ordinary window glass or water usually obstruct a thermal imager’s direct view. But that does not mean the camera cannot obtain any information about what lies beyond such a barrier. The surface itself may warm up or cool down because of what is happening on the other side. By analyzing and correctly interpreting these changes, it is possible to learn quite a lot about something that remains hidden from ordinary sight.
Let us look at what thermal cameras can see through and which materials genuinely restrict long-wave infrared radiation.
Thermal cameras read surface temperature
Thermal imaging devices generally show the thermal state of a visible surface, rather than the contents of an object behind it.
If the device is pointed at the exterior wall of a house, its sensor receives infrared radiation from the wall’s surface. It does not look into the room through brickwork or timber cladding. At the same time, temperature variations caused by processes inside the building may become visible on that surface. This can reveal, among other things, poorly insulated areas, the position of utility lines, or zones of elevated moisture. Much of building thermography relies on precisely this principle.
How thermal imaging works
Every object with a temperature above absolute zero emits electromagnetic energy. Part of this radiation lies within the infrared spectrum, a region invisible to the human eye. A thermal imaging sensor detects incoming infrared radiation and constructs an image from differences between individual areas within the scene.
It is important to understand that a material transparent to our eyes is not necessarily transparent to a thermal camera.
Take glass. We instinctively regard a window as transparent because it transmits visible light very effectively. In the long-wave infrared range used by many thermal imagers, the situation is entirely different. And if you are wondering whether thermal cameras can see through glass, the short answer is: no. At least, an ordinary LWIR thermal imager will not show the scene beyond a window in the way a conventional camera does.
Let us consider the principal factors that affect thermal imaging cameras.
Thermal contrast
Detecting an object depends not only on its temperature as such, but also on how different that temperature is from its surroundings.
An animal standing on cool ground during a cold night creates pronounced thermal contrast. On a hot sunny afternoon, however, the same animal may be set against heated soil and almost merge into the background on a thermogram.
Emissivity and reflection
Some materials emit their own thermal radiation efficiently, while others reflect a significant share of the surrounding radiation. Smooth metallic surfaces are particularly deceptive in this respect.
Polished metal behaves almost like a mirror. And not only for visible light, but for infrared radiation as well. A bright patch on a metal panel therefore does not necessarily mean that the area itself is unusually hot. The camera may simply be detecting the reflection of a warmer object located somewhere else entirely.
Thermal mass and heat transfer
Different materials absorb, conduct, and release heat at different rates. A concrete slab, a wooden wall, and a thin sheet of metal will respond very differently after identical external heating. As a result, a concealed heat source can sometimes alter the temperature of an outer surface.
What thermal cameras can and cannot see through
| Material | Can thermal see through it? | Practical note |
|---|---|---|
| Walls | Usually no | Temperature differences may indicate air leaks, missing insulation, moisture, or heated structural elements |
| Glass | Usually no | The operator’s own thermal reflection often appears in the image |
| Metal | No | The temperature pattern of the metal itself may sometimes reveal the indirect effect of its contents or a heat source |
| Wood | No | A temperature gradient may appear through a thin section, but this is not an image of an object behind it |
| Concrete | No | Pipes, underfloor heating, or other sources can alter the temperature of its surface |
| Clothing | Usually no direct image of the skin | Body heat may create warmer areas in clothing, but the camera does not reveal anatomical detail through the material |
| Smoke | Sometimes partially | Smoke density, hot gases, and the characteristics of the scene still affect image quality |
| Fog | Limited | Contrast generally decreases as distance increases |
| Light foliage | Partially | Visibility depends strongly on vegetation density and viewing angle |
| Heavy brush | Usually severely limited | A thermal object may appear only in fragments or fail to separate from the scene at all |
| Water | Virtually none for typical LWIR systems | The camera mainly shows the temperature of the surface itself |
| Thin plastic sheets | Depends on the plastic and its thickness | “Plastic” cannot be treated as a single optical category: composition and thickness are crucial |
Can thermal imaging see through walls?
No. Walls are opaque within the operating infrared range of typical thermal imaging cameras. Brick, wood, concrete, sandwich panels, and other wall structures block direct thermal radiation from objects on the other side.
Yet the idea that thermal imaging can see through walls did not emerge entirely without reason. Thermography really can detect signs of processes occurring within or behind a structure.
Imagine an interior wall with a heating pipe running through it. If the pipe releases heat into the surrounding materials for long enough, the section of wall above it may become warmer than adjacent areas. A line or diffuse band then appears on the thermogram. Its shape may even resemble the concealed element, making it easy to draw the mistaken conclusion that the device has somehow “seen” through the wall.
Can thermal cameras see through glass?
Ordinary window glass creates an almost paradoxical situation. It is transparent to the eye, but not to a typical thermal imager.
Most devices intended for observing objects at ordinary temperatures operate at wavelengths of approximately 8–14 μm. Ordinary glass does not transmit this range in the same way that it transmits visible light.
If a thermal imager is pointed from a warm room through a closed window at a person standing outside, the expected outdoor scene may be replaced by the temperature of the glass surface and reflections of objects inside the room. Sometimes the operator is among them.
This is also a good example of why the phrase what blocks infrared cameras should not be interpreted too literally. Glass does more than simply create a barrier. It also emits its own thermal radiation and reflects part of the radiation arriving from the surrounding environment. The display may therefore show a perfectly convincing thermal image — only it will not correspond to the objects beyond the glass.
Special infrared-transparent materials are used in optics and technical windows for infrared systems. Their properties, however, should not be conflated with those of ordinary household glass, or vice versa.
Can thermal imaging see through clothing?
The device primarily registers the temperature of the fabric’s outer surface.
But clothing does not exist in isolation from the heat source beneath it. The body warms both the air and the material, so different areas of a jacket, shirt, or trousers can have different temperatures. Fabric thickness, the number of layers, wind, moisture, and the amount of time spent outdoors after leaving a building all alter the resulting pattern.
A thin shirt, for example, warms fairly quickly to a temperature close to that of the body. With a thick, well-insulated winter jacket, the same logic no longer applies. On a thermal display, it may differ very little from the cold surroundings.
In any case, this is nowhere close to “seeing through” clothing.
Can thermal see through smoke, fog, rain, or foliage?
Thermal imaging can indeed remain much more useful in smoke than ordinary vision or a visible-light camera. This is one reason the technology is widely used by firefighters. Particles that sharply reduce visibility for the human eye do not necessarily block long-wave infrared radiation with the same effectiveness.
However, the density and composition of the smoke, hot combustion products, distance, and the thermal background can all degrade the image.
Fog is different, largely because of the presence of water. Microscopic droplets absorb and scatter part of the radiation, and this effect accumulates with distance. A thermal imager may sometimes retain an advantage over an ordinary camera in light fog, but a crisp, highly detailed image should not be expected.
Rain also introduces water into the path between the object and the thermal imager. Yet another effect comes into play as well: rain changes the temperature of the surfaces themselves. Wet clothing or a vehicle body cools down and acquires different thermal characteristics.
Now let us consider can thermal see through trees. A thermal imager cannot see through a tree trunk or a continuous mass of foliage. It can, however, receive radiation from a more distant object through the gaps between leaves and branches.
Sparse vegetation creates numerous small “windows.” Dense brush gradually closes them off. Instead of a clear outline, the camera may therefore show only a few isolated warm fragments.
Viewing angle matters a great deal here. Moving only a few steps sideways changes the relative positions of the leaves and branches. An invisible object can then become visible.
What can distort a thermal image
| Factor | Effect | What to keep in mind |
|---|---|---|
| Polished metal, foil | Reflect infrared radiation and create false hot or cold areas | The apparent temperature may not correspond to the actual surface temperature |
| Moisture | Changes the temperature and emissive properties of the surface | A cold area does not necessarily indicate a defect |
| Paint, contamination, oxide layer | Alter emissivity | Identical materials may appear differently |
| Sunlight | Heats the surface unevenly | Solar heating of a facade may be mistaken for an internal heat source |
| Wind | Reduces thermal contrast | Outdoor measurements become less representative |
| Long distance | Conceals small thermal details | Object size and camera resolution matter |
| Oblique viewing angle | Increases the effect of reflections | For measurements, it is preferable to position the camera closer to perpendicular |
| Incorrect emissivity setting | Distorts the calculated temperature | Particularly important for shiny surfaces |
| Low thermal contrast | Causes the object to merge with the background | Poor visibility does not mean that the heat source is absent |
Common thermal imagers myths
Some misconceptions about thermal cameras arise simply because their images look unfamiliar. A bright silhouette against a dark background can easily be mistaken for something approaching “X-ray vision.” Let us examine the most persistent myths.
“Thermal imagers can see through walls”
When the warm outline of a pipe, beam, or another element appears on a wall, it may seem as though the camera is showing the object itself. In reality, it is only registering a temperature pattern on the surface.
“If a material is opaque, a thermal imager will show nothing”
Not quite. Opacity in the infrared range means that the direct signal from an object behind the material does not pass through it. The surface of the material itself, however, continues to participate in heat exchange and may still provide useful information.
“A thermal image shows the exact temperature of any object”
Not necessarily. The final measurement is influenced by surface emissivity, reflections, distance, atmospheric conditions, and device settings. Particular care is required with smooth metals and polished coatings.
“The brighter the object, the hotter it is”
Only if the palette and temperature range are configured accordingly.
Color in a thermal image is a method of visualizing data, not a physical property of the object. The same area may appear white, red, yellow, or black depending on the selected palette and scale.
“If an object is visible to a thermal imager, it can be identified with confidence”
Detection and recognition are different tasks.
A warm object may stand out clearly against the background while occupying only a handful of pixels. The camera can show that a heat source is present in the scene, but this may still be insufficient to determine exactly what it is.
“A thermal blanket makes an object invisible to a thermal imager”
Not quite. It may temporarily reduce or alter thermal contrast, but the blanket itself gradually warms up and becomes visible on the thermogram.
“All thermal cameras perform the same”
No. The result depends on numerous parameters: sensor resolution, sensitivity, spectral range, optical characteristics, and frame rate. In addition to hardware specifications, image-processing algorithms also matter. Cameras in different classes can differ dramatically in detection range and image detail.
What to consider when choosing a thermal imaging device
Understanding thermal imaging limitations makes it easier to assess device specifications deliberately.
Sensor resolution
Resolution indicates how many sensitive elements make up the thermal image. Common 384×288 sensors, for instance, contain about 111,000 elements, while 640×512 sensors contain roughly 328,000 — nearly three times as many.
640-resolution sensors perform well in open terrain and when digital zoom is used frequently. A 384 sensor, meanwhile, can be entirely sufficient and more economical for shorter distances and wooded areas. You can read more about the difference in resolution in our article 384 vs 640 thermal resolution.
NETD
NETD indicates how small a temperature difference the sensor can distinguish. In this case, a lower value is generally preferable.
Current AGM product lines include devices with NETD below 20 mK. Under low-contrast conditions, this specification becomes genuinely significant, allowing the sensor to distinguish extremely subtle variations within a thermal scene.
Lens
The focal length of the lens is directly related to magnification, field of view, and how large a distant object appears in the frame.
A shorter focal length generally provides a wider image. This is useful when a large area needs to be scanned quickly, when working at shorter distances, or when following a moving object.
A longer focal length narrows the field of view, but makes a distant object occupy a larger portion of the sensor.
Take the AGM Taipan V2 thermal monoculars as an example. The version with a 10 mm lens has a field of view of approximately 18° × 13°, while the 15 mm version narrows it to about 12° × 9°.
Refresh rate
This specification indicates how many times per second the image is updated.
At 50 Hz, the camera generates a new image 50 times every second. This refresh rate is used, for example, in a number of current AGM Taipan and ReachIR monoculars.
The advantage becomes particularly noticeable when panning or observing a fast-moving object. The image appears smoother, and the object’s position is easier to track.
Detection range
A range figure may look impressive in a specification table, but it is often misunderstood.
Detecting a heat source means noticing that an object is present. Recognizing it requires enough information about its size and shape to determine what kind of object it is. Confident identification demands still more detail.
An object can therefore be detectable at a considerable distance as a few warm points, even when it is not yet possible to determine exactly what is shown on the display.
Once again, the outcome depends on a combination of factors: resolution, lens, field of view, sensor sensitivity, and weather conditions.
Different tasks require a different balance. A handheld device used for rapid terrain scanning and thermal weapon sights rely on the same physics, but their requirements for magnification, field of view, and device positioning differ.
If the primary task is nighttime observation, it is also worth considering the fundamental difference between the technologies. A thermal imager creates an image from infrared radiation, whereas a night-vision device amplifies available light. The same conditions may therefore pose very different levels of difficulty for the two technologies. This distinction is examined in more detail in our article night vision vs thermal.
Thermal imaging FAQ
What blocks thermal imaging?
Direct thermal radiation is usually obstructed by solid materials, whether they are opaque or transparent to visible light. Yet the word “blocks” itself can be misleading.
Even if the camera does not receive radiation from an object behind a barrier, it still sees the temperature of the barrier itself, which may reveal certain information about concealed objects.
Can thermal imaging see through walls?
No. Consumer and professional thermal imagers do not show a person, furniture, or equipment through an ordinary wall as though the wall were transparent.
Can thermal cameras see through glass?
Usually not. Window glass transmits visible light well, but behaves very differently in the long-wave infrared range used by most thermal imagers.
Can thermal see through clothing?
No. The camera registers the outer surface of the clothing. Body heat may warm the fabric unevenly, creating warmer and cooler areas, but this is not an image of the skin or anatomical details beneath the material.
Can thermal see through smoke?
Sometimes a thermal imager does retain a useful image where an ordinary camera shows almost nothing. Smoke, however, does not become completely transparent, and image detail is influenced by numerous factors.
Can thermal cameras see through fog?
In light fog, a thermal imager may sometimes provide a more useful image than a visible-light camera, but water droplets still attenuate infrared radiation.
Can thermal see through trees?
Not through a trunk, thick branch, or continuous layer of foliage. In less dense vegetation, however, the camera may partially detect thermal radiation from various objects.
Can a thermal imager identify a person?
A thermal silhouette on its own usually does not contain the kind of information we rely on to recognize someone in an ordinary photograph.
A thermal imager can show a person’s figure, movement, relative temperature distribution, and occasionally certain broad features.