how thermal ir imagers work

Thermal infrared imagers are detector and lens combinations that give a visual representation of infrared energy emitted by all objects above 0  k.  In other words, thermal imagers let you "see" heat.  Depending on the sophistication of your system thermography is capable of providing very detailed images of situations invisible to the naked eye.

Thermal images cannot see through walls, although you can gather much information about  the inside of the wall as well as what is happening on the other side of the wall.  
For example, you would not be able to see people and plants involved in an illegal indoor growing operation from the outside of the building.  You would be able to see and monitor the heat escaping from the building that would be a telltale sign of an illicit growing operation.
You would also be able to see things like studs inside the wall or damaged insulation in roofing applications.You cannot hide from thermal imaging by covering yourself in mud to blend in like that movie tough guy.  This might work momentarily, but infrared "heat"
energy transfers well when objects are touching.  Your body heat would quickly warm your camouflage resulting in a thermal image.  We hope that dispels a few of the most common questions regarding infrared energy.  Thermal imagers in Hollywood and in real life are very different.  

From SPI library