Why not use infrared rays to detect signs of life? (I mean at night)

Today, life detectors are used for useful infrared thermal imaging. Of course, the principle is more complicated, so it can't be said that it will come out with infrared light.

The infrared thermal imaging life detector SM- 1 was specially invented for personnel search and rescue. Its characteristic is to help rescuers quickly find weak life in or around the ruins disaster area, which can be used normally in the dark, unaffected by light, and can also work normally in the absence of light, thus helping rescuers quickly locate the victims buried under the ruins or hidden behind dust and fog. It can withstand the harsh conditions at the rescue site.

Of course there are more advanced ones. Dkl lifeguard life detector is the most advanced search and rescue instrument in the world. Lightweight, only about 1kg. Handmade design, easy to carry, simple to operate and superior in performance to any other high-tech search and rescue products. It has been widely used by the military, customs, sea patrol, fire protection, security, rescue, aerospace and other government departments in advanced countries in the world.

This product was developed by American high-tech companies combined with the world's most advanced biochemical, dielectric, ultra-low frequency conduction and DNA technologies, and applied for a number of technical patents. This model is the most mature product designed by DKL company at present.

Lifeguard life detector provides rescuers with the ability to confirm whether there are survivors when they enter the search and rescue site, so as to reduce the danger of rescuers in search and rescue and detect survivors behind any obstacles at the first time. It detects the direction of the distant weak heartbeat medium field through passive reception, and only detects the surviving human beings without interference from other animals. It can cross various obstacles such as steel plates, cement, composite materials and trees, so that the detection distance can reach 500 meters in the open area.