As police officers, we are used to living a dichotomy. At almost every call for service, we can be sure that there is someone who wants us there (normally, the caller), and someone that does not want us there (normally, the suspect). We take control of dynamic scenes at work, then let our spouses decide what is for dinner that night. Police work is a yin and yang proposition; everything has two sides, and we consider this normal.
As part of our double-sided work, we look for people. Sometimes, we are looking for suspects: criminals who have fled or are hiding, people who want to avoid detection. Balancing these people are the lost and disoriented; children who have gotten lost in a park or elderly people who have become disoriented and wandered from home. Just as we can use the thermal imager to find fleeing felons, we can also use it to assist in search and rescue efforts.
The keys to using a thermal imager (TI) in search and rescue are not significantly different from using the TI to track a suspect. In both situations, we are looking for a heat source that might indicate where the person is located. Using the TI to detect heat can make our job much easier, especially in poor visibility. The primary difference in search and rescue is that the person wants to be found.
As a result, the victim will not be taking active steps to hide himself from you and your thermal imager. However, that does not mean that the process is automatically easier. There are a number of complications related to search and rescue, any of which could subvert your efforts if you are not aware of them.
First, remember that the objects preventing the person from being found may also block his body heat. That means that all of the trees and bushes that keep the child lost in the woods may keep his heat from reaching your TI. The best way to overcome this is to scan areas from several different angles. This may mean that you have to move around and view not only from different angles, but also different elevations. Specifically when foliage is the problem, attempting to view the area from higher or lower elevations can improve your imager's performance.
Trees with a thick canopy can make higher scans difficult. Getting down to ground level can limit the amount of material blocking body heat, improving the imager's ability to see the victim. Conversely, a corn field will make horizontal scans difficult. The stalks and leaves are oriented in the row, but offer little canopy. Elevating your viewing position can make it much easier to confidently scan a corn field. Because many areas have a combination of vegetation (and other obstructions), be sure you are scanning from as many angles as possible.
Second, don't overlook the effects of any precipitation. Rain and snow will cool the outer garments of your victim. This will reduce his heat signature, making it harder to detect him with the TI. Also, since infrared energy doesn't penetrate water, if you get rain or snow build up on your TI lens, you will dramatically reduce the quality of your thermal image.
The effects of weather can also turn your victim into a hider. In this case, the victim is not trying to hide from you, but from the elements. As a result, you may not see the large heat signature that you would expect from a person. The victim may be trying to stay dry (or warm) and present a very small heat signature from his face or hands. Keep this in mind when you are looking for a victim in inclement weather.
The effective range of a thermal imager will vary based on a number of factors. The two most important are the size of the heat source you want to detect and the background temperature of the objects around the target (in this case, a victim).