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I'm working with thermal infrared (IR) cameras to detect human thermal radiation. I notice I can easily distinguish non-human objects throughout the camera's field of view, though all are at same room temperature.

I assume this is due to reflection of thermal IR 'bouncing' around the room. Indeed, we observe our large white board to be an excellent thermal IR 'mirror.'

CONCERN: Therefore, I'm concerned radiation reflection may affect frequency of thermal IR emanating from a human subject.

QUESTION: When thermal IR gets reflected from an object, does it change its wavelength (frequency) at all?

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Reflecting from an object doesn't change the frequency. However the object may not reflect all frequencies equally - that's why things appear colored in white light.

The human is at a different temperature to the room and so will emit at a different wavelength than the other objects at room temperature. It is possible that a mirror (or something sufficently reflective) will give the same signal as the human if it is reflecting IR emitted from the target.

In practice this isn't an issue because the camera is measuring the power emitted for an area of target. When this IR energy spreads out from the human and is then further spread out when reflected from the mirror it is going to be much weaker.

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  • $\begingroup$ Perhaps not relevant in every day experience, but since photon transfers momentum to the receiver, the re-emitted photon has lower energy than the received photon. $\endgroup$ – HolgerFiedler Mar 24 '16 at 15:57
  • $\begingroup$ Or if the particle it scatters off has high enough energy the reflected photon is blue shifted! Although philosophically - is it the same photon ? $\endgroup$ – Martin Beckett Mar 24 '16 at 16:23

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