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I’ve read that only cold-blooded animals develop IR vision to spot warm prey because a mammal would blind himself with his own heat and not be able to see anything.

But I’ve seen a FLIR camera, operating at room temperature, able to image warm and cool spots on walls, to find places where insullation is lacking. Clearly it’s not blinded by having its sensor, lenses, and housing all at room temperature, and is able to image cooler patches without any problem.

How does that work? Why is the popular explaination wrong?

Qmechanic
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JDługosz
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3 Answers3

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By comparing the signal to the background.

Suppose you get 10 IR photons from the camera and lens background but an extra 5 from the source then you can still detect the source.

There is a whole science of signal processing to detect signals much fainter than the background. Especially in IR astronomy.

Martin Beckett
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In astronomy, the background from the camera itself is called "dark current" and is removed by first taking an exposure with the shutter closed for, say, half an hour, and then subtract those counts from the real observations, normalized to the exposure time of a given image.

Sometimes, if you're bored at the telescope due to bad weather, you can even take multiple dark exposures and add them, just to get better statistics.

pela
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I'm not sure whether it's best to resurrect this old question, or pose a new one, but here goes:

Is it not the case that the sensor emits IR toward the scene, just as the scene emits IR toward the sensor?

If the sensor is viewing a scene at a lower temperature, that cools the sensor. The sensor emits more energy toward the scene than the scene emits toward the sensor.

The camera body and lens have low emissivity, so they don't radiate enough thermal energy to drown out this effect. The sensor has high emissivity -- it must, because it needs to absorb incoming IR with high efficiency, instead of reflecting it away.

This same reasoning works if you consider an individual sensor pixel, and the corresponding small patch of the scene. That's how a thermal camera can image objects colder than itself.

This is also how the ground cools below air temperature on a clear night. The sky looks very cold, and as the ground radiates heat toward it, the sky doesn't radiate nearly as much heat back. The result: the ground can get cold enough to accumulate frost, even when the air temperature is above freezing, all without breaking a single law of thermodynamics!

jeffB
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