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AFAIK, all objects above 0K emit infrared radiation. It is an electromagnetic radiation, like visible light. I guess thermographic and infrared cameras work on the same principles, but their images are very different, the former is false color and very different from the visible image, the latter is black and white and usually correlates to the actual visible image, at least that's what I've seen so far. So physically, how exactly are they different?

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    $\begingroup$ False-color is a post-processing step which can be applied to any collected image regardless of the wavelength range the sensor operates in. "thermographic" is a fancy title for any camera which is sensitive to the temperature ranges the user is interested in. <-- which could vary quite a lot. $\endgroup$ – Carl Witthoft Dec 11 '13 at 12:43
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At or around room temperature objects emit light in the far infrared. Cameras that detect this radiation generally give the false colour images that you describe. Because the camera sees radiation emitted by the objects no external source of light is required.

However a second type of camera operates in the near infrared and requires an external infrared light source to illuminate the field of view. These cameras give the black and white images you describe.

The near IR cameras have much higher image quality because the wavelength of the light is shorter and the intensity of the light is much higher. However it does mean carrying around a near IR light source to illuminate whatever it is you're trying to photograph. Wildlife documentaries tend to use near IR cameras because they give a good image of the wildlife but without scaring the animals by illuminating them with visible light.

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  • $\begingroup$ All true but given the vagueness of the OP's question, hard to say if this is what he's after. $\endgroup$ – Carl Witthoft Dec 11 '13 at 12:42
  • $\begingroup$ Near IR means closer to visible light, right? So it means it works very similar to visible light, with reflections and all? I guess the sun also emits near IR light, that's why consumer "normal" cameras all have IR filters. So an object can reflect near-IR, in any number of different wavelengths, but an object only emits far IR, in a single wavelength, based on its temperature? If I heat an object, will it's radiation come closer to visible light? $\endgroup$ – fejesjoco Dec 11 '13 at 12:45
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    $\begingroup$ Yes, near IR is very similar to visible light and behaves pretty much like the visible light. The Sun deas emit near IR, so streets are flooded with near IR in sunlight. The thermal emission is not monochromatic - see the blackbody radiation. Human body is not a blackbody (rather a grey body), so the spectrum is a little different, but still not monochromatic. Heating an object rises the frequency of the maximum of the emitted spectrum. Hot iron can glow in visible light (even though maximum can be in IR). $\endgroup$ – mpv Dec 11 '13 at 13:14
  • $\begingroup$ On a related note: An interesting trick to test if your TV remote is working is to look at the LED on the front through a digital camera (such as a phone's camera) while pressing a few buttons. You should see a bluish/purple light. $\endgroup$ – OSE Dec 11 '13 at 14:02

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