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This is a elementary question that troubles me.

My understanding is that biological tissue absorbs and transmit light in the infrared region.

How is that a sensor is able to detect temperature from infrared radiation? If these wavelength are absorbed by the subject, then what is the light that is reflected and captured by the IR camera?

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How is that a sensor is able to detect temperature from infrared radiation? If these wavelength are absorbed by the subject, then what is the light that is reflected and captured by the IR camera?

Not all infrared light incident on a body is absorbed. Some of if will be reflected and captured by an infrared thermometer. But we are actually not interested in the reflected light, since it does not say much about the temperature of the body.

We are interested in the infrared light radiated by the body, since the intensity of that light is a function of the body temperature.

Ideally, the infrared thermometer would receive no reflected IR light, since it introduces an error in the temperature measurements.

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  • $\begingroup$ Very interesting. Therefore , the heat from the body is the source of the infrared radiation. May you point to the theory that resolves how much and what wavelength is this radiation produced? I mention wavelength because Infrared is a broad region from the 700's and mid 1000,s nm. $\endgroup$ Commented Nov 9, 2018 at 14:29
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    $\begingroup$ @JoseECalderon The theory behind it is black body radiation. You can also check out this or similar posts. $\endgroup$
    – V.F.
    Commented Nov 9, 2018 at 15:17
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An IR camera is not a camera in the sense that it captures an image and shows us (optically). At the same time it is very much a camera by the definition.

An IR camera absorbs the radiation incident on the photodetector and a photocurrent is generated. The amplitude of this current is stored. One version of a detector is where we have an array of photodetectors, let's call them pixels (one photodetector in the grid is a "pixel"). Each individual detector captures the infrared radiation incident upon it, and by drawing an image with colours whose strength/hue/intensity depends on the magnitude of the photocurrent generated. I believe this type of detector is called a CCD-detector.

A typical photodetector is a pin-semiconductor photodiode, where the bandgap of the middle, intrinsic, region matches that of the photon energy.

Did this answer your question?

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  • $\begingroup$ .. Thanks, but it does not answer my question. My question is about the origin of the radiation absorbed by the detector. But it is interesting that you mention about the CCD sensor . $\endgroup$ Commented Nov 9, 2018 at 14:24
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    $\begingroup$ In the case of IR the origin should be the blackbody radiation from the environment which we observe @JoseECalderon . $\endgroup$
    – DakkVader
    Commented Nov 9, 2018 at 15:15

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