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.
Basics
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.
Distance
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).
The surface temperature of a human is about 90 degrees Fahrenheit. If the background temperatures are significantly cooler (or hotter) than 90 degrees, a human target will stand out much more on the TI display. This is why TIs tend to be more helpful in night searches rather than day searches. During the daytime, the sun will heat up the surrounding environment, bringing the temperatures of the background objects closer to the 90-degree mark. This will make it more difficult to discern a human heat signature.
Also, the type of thermal imager can affect detection range as well. While many of the detectors available today have similar sensitivities, there are a wide range of performance characteristics. The one of most concern is field of view (FOV). This defines how wide an area the TI scans and displays on the viewing screen.
A wide FOV (50 degrees or more) portrays more information on the display; however, it makes objects appear smaller than they really are and makes distances appear farther. A narrow FOV (10-20 degrees) displays items at more lifelike size and distance, but you scan a smaller area. There is always a trade off in performance. Since the narrower FOV displays objects closer to their true size and distance, these generally give a greater detection range. For example, a 50 degree FOV will have effective human detection out to 70 or 80 meters. An 11 degree FOV will have effective human detection to roughly 300 meters.
Note that most fire service TIs have FOVs of 50 degrees or greater. Most law enforcement TIs have FOVs of 10-20 degrees. So while a fire service TI can help in a search, and may be better than nothing, its performance in an outside search will be less effective than a narrow FOV imager.
Conclusion
A search for lost or missing people is rarely just a law enforcement function. However, if we are involved as police officers, we can use our thermal imagers to help us with those searches. The imagers will be more effective in poor visibility than during the heat of the day, but this is not a firm rule.
And the nice part about a search and rescue operation is that everyone wants you there and wants you to succeed. That breaks our normal dichotomy of life as officers...and might even make us as popular as firemen.
