Thermal imaging technology is becoming more and more popular today due to the various spheres of application. Indeed, it is a powerful means for detecting people and animals, gas or water leaks, and other kinds of objects and substances. However, it is not a universal remedy. It also has its limitations, at least so far.

In close-up, let us study the range of its possible see-through capabilities regarding different types of obstacles that might hinder the view and compare the thermal imagers with the naked-eyed when coping with these challenges.
The most accurate reading will find the term “seeing through” not precisely suitable for describing the exact principle of the thermal imager operation, as, from the point of view of commonly-known physics, it is the process of detecting heat varying in intensity and processing it into user-friendly visible-spectrum color schemes. But to facilitate the apprehension of scientific phenomena, we will try to use more simplified terminology to make complicated issues sound more comprehensible.

The capability of seeing through walls

Seeing through walls and other solid objects is not as easily conveyed by a thermal imaging device as it may initially seem. Walls and similar things provide a thorough screening of the heat radiation as it passes through them. Moreover, these solid bodies are often far from being homogeneous in structure. For example, they may contain different reinforcement and heat-insulation items, varying thickness and materials. All these factors may influence the throughput capacity of the wall or similar obstacles.

But suppose heat-emitting objects behind the wall radiate enough heat to pass through this obstacle despite the partial loss and dissimilation of energy. In that case, the thermal imaging device is powerful enough to catch the remaining radiation. You will be able to see them, though not as brightly conveyed as the similar objects located at the same distance with no wall standing in-between. To be more exact, seeing through walls is possible if the object behind the wall produces enough energy to heat the wall through, so the bright image you will get on the output of the thermal imager is that of the heated part of the wall. The same principle will work vice versa: the objects behind the wall which are colder than the wall may decrease the temperature of the adjacent wall area and, as a result, you will get a darker area of the wall on the thermal imager’s output compared with other warmer wall areas.

In other words, the images you see via the thermal imager when looking at the wall are nothing else but the heated or cooled areas of the outside surface of the wall, not the images of the heat-radiating objects or substances positioned behind this wall.

Similarly, the same works for objects and substances located inside the wall. Such objects and substances will significantly affect the wall temperature in the adjacent area compared to those found at a certain distance behind the wall.

So, the thermal imagers usually fail to detect people or other heat-producing objects positioned behind the walls but are suitable for catching any possible leakage of gas, electric power, water, or other liquids inside solid bodies, given the latter are pretty homogeneous. This makes thermal imagers extremely convenient for troubleshooting buildings without wasting time and human resources for deconstructing the whole areas under suspicion.
Moreover, thermal imagers have proven perfect for detecting such leakages or insulation failures at the early stages of their occurrence, which is money-saving and cost-effective. They may prevent considerable damage caused by long-term utility equipment or system misuse, resulting in expensive repair works. In contrast, the problem could have been spotted early and eliminated with the minimum efforts, preventing more severe consequences.
Furthermore, it works similarly well in the opposite direction. In cold weather, you can go inside the building and use the device to check the wall for the homogeneousness of its heat isolation. The thermal imager will detect the colder areas of the wall in a darker spectrum, thus showing the zones where the isolation layer fails to prevent the air leak or heat escape.

The thermal imagers are also suitable for detecting the location of screws, studs, wires, and other metal objects or structures inside the walls because the heat-conducting characteristics of metals are different from those of non-metal ones.

Thus, thermal imaging gadgets turn out to be highly in-demand in the construction and building maintenance business.

The capability of seeing through a glass

It is surprising, but glass, which is usually transparent for visual-spectrum rays and bears almost no problem for the observer to see through most of the time, becomes a real obstacle when using a thermal imaging gadget.
This is because glass reflects most radiation directly or at a certain angle to its source. Thus, no or almost no heat radiated by the object reaches the thermal imager, positioned outside the glass window. Moreover, all the heat-producing objects and living beings standing outside near the person using the thermal imager will also be reflected by the glass; thus, the observer, using the thermal imager, will receive their reflection as well as the reflection of other people or objects positioned in the glass-reflecting area, instead of the things placed behind the glass.

This fact is also actual for foil, polished sheets of metal, or metal-based insulation that may be highly reflective.

So, this should be taken into account when searching for people inside the buildings, for example, in case of fire or other emergencies, as this may turn out to be highly distracting.

The capability of seeing through trees

Since wood features a heat-isolating property, it is also a real obstacle for heat radiation, and thermal imaging devices usually fail to see through trees. However, it does not make such devices useless when searching for people or hunting or observing animals in the forest-covered areas. These areas are not solid wood, so you can see all the radiated heat passing in-between tree trunks and branches.