Thermal imaging (binoculars or telescopes that detect differences in temperature) have been used successfully in tanks and warplanes for over two decades. In the last few years, the technology has gotten reliable, and light, enough for ground troops to use. A recent example of this technology at work can be found in Kashmir, where Pakistani backed rebels cross the mountain border to enter Indian Kashmir to fight and terrorize non-Moslem civilians. The frontier is rugged, cold and covered in many places with forests. At 10-20,000 feet, there is a lot of mist and fog. For over a decade, it was possible for rebels to sneak into Pakistan to train and load up with weapons, and then sneak back into Kashmir. Then the Indians bought over a hundred portable thermal imaging devices from Israel. Weighing 5-6 pounds, the equipment can detect a human at up to 4,000 meters, although 2.500 meters is more common. Since these imagers work just as well at night (although less well in the fog or mist), they give the Indian patrols and border guards an enormous advantage. Nearly a hundred infiltrators have been killed, and many more driven back, by thermal imaging equipped patrols or observation posts. There are few practical ways to hide your body heat from these devices, especially in the chilly upland border area. Covering yourself with heavily insulated clothing or "space blankets" will hide most of the heart. But apparently the Israeli equipment (whose exact specs are, uncharacteristically for this type of equipment, kept secret) is more precise and reliable than other gear of this type.  One shortcoming the devices have is that their batteries only last 3-4 hours.