I2 cameras try to generate their own reflected light by projecting a beam of near-infrared energy that their imager can see when it bounces off an object. This works to a point, but I2 cameras still rely on reflected light to make an image, so they have the same limitations as any other night vision camera that depends on reflected light energy – short-range, and poor contrast.
All of these visible-light cameras – daylight cameras, NVG cameras, and I2 cameras – work by detecting reflected light energy. But the amount of reflected light they receive is not the only factor that determines whether or not you’ll be able to see with these cameras: image contrast matters, too.
If you’re looking at something with lots of contrast compared to its surroundings, you’ll have a better chance of seeing it with a visible light camera. If it doesn’t have good contrast, you won’t see it well, no matter how bright the sun is shining. A white object seen against a dark background has lots of contrast. A darker object, however, will be hard for these cameras to see against a dark background. This is called having poor contrast. At night, when the lack of visible light naturally decreases image contrast, visible light camera performance suffers even more.
Thermal imagers don’t have any of these shortcomings. First, they have nothing to do with reflected light energy: they see heat. Everything you see in normal daily life has a heat signature. This is why you have a much better chance of seeing something at night with a thermal imager than you do with visible light cameras, even a night vision camera.
In fact, many of the objects you could be looking for, like people, generate their own contrast because they generate their own heat. Thermal imagers can see them well because they don’t just make pictures from heat; they make pictures from the minute differences in heat between objects.