Why reduce FoV increase detection range of thermal and optical sensor?

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I looking at the detection range of some infrared sensor and see that, in Narrow FOV mode (zooming) , their detection range is always much longer. But why exactly is that?

In: 4

Because when you zoom in, you can see the target better.

Do you think you could find a target easier with binoculars or with the naked eye?

The computer has a sensor of a given size X. That sensor can take data from a 10 degree sweep or from a 5 degree sweep. If you go with 5 degrees, all the energy from that 5 degrees covers the whole sensor, so while the same amount of thermal energy will reach the lens, it will hit more “pixels” on the sensor.

Think of it this way, the way the sensor works is by having an IR camera look at an area and see how bright it is. The hotter it is, the brighter it is.

If the camera only observes a tightly-focused line, like a laser pointer in reverse, then it would get wild swings depending on where it hits. So it looks at a spot, and averages how much IR it sees; say like a 6” circle on an object 8ft away. The camera can see 80 ft away relatively fine, but you’d be taking the temperature of a spot that’s 60” in diameter, or whatever would be the equivalent field of view. So usually they define the range based on what range the spot becomes ‘too large to be useful’