Detection  To detect a person with a conventional camera, we rely on visual contrast between the person of interest and his surroundings. However, if a person is wearing clothes that allow him to blend with his background, it can reduce the camera's effective detection range. In a thermal scene, a person will normally appear warm against a cooler background, so even at long ranges he is likely to show up.
Classification  Classification is the ability to differentiate a man-sized target from other objects in a scene; an automobile for example. With a thermal imager, this can be done with several pixels, but you should consider systems that provide a comfortable margin for recognition. Most security camera operators expect an object to occupy more of the screen than the minimum requirements for classification.
About the chart  The chart above provides the approximate range for thermal object detection and classification. Under good conditions, the arrow identifies the approximate maximum range you will either detect (in red) or classify (in blue) a person.
How far can you see with a thermal imager?  You can clearly see the moon, or anything else you would see with a conventional camera, and more, day or night.
Focal length and range  You can use the focal length to approximate the field-of-view for your application, or you can visit our website and experiment with our online field-of-view calculator to see how a man-sized target fills your screen at given distances. Some sample images are presented at defined ranges to provide a comparison of how a man-sized target will fill the screen.