Thermal Imaging reflection?

Question

I was viewing a youtube video that was about thermal imaging and saw something that caused me to ponder but I couldn't figure out an answer and the one assumption I have I don't know if it's correct. The video was of a man and while he was doing an experiment I noticed his reflection in the glass door behind him about five or more feet away. How is that possible? The thermal camera should only pick up heat released from objects but the door shouldn't release a perfect reflection (not blurred or fuzzy -- looked just like him just a little dimmer). I thought maybe the heat from his body is bouncing off the glass but that doesn't sound right. A moment later I noticed it was picking up his cat's reflection as well and it was father from the door than he was.

I'm not going to pretend that I understand all the ins and outs of thermal imaging, I don't, so can someone please explain to me how this is possible?

Answer 1

I think a great misconception has been perpetrated (I had it too, don't worry) by people talking about "infrared" as if it were interchangeable with the word "heat." "Thermal radiation" comes off of all bodies at finite temperature in a phenomenon known as "black body radiation." If the body is hot, this radiation becomes higher energy, which is why objects tend to glow when heated. The only thing a thermal imaging camera does is capture the IR radiation that comes off of objects around 30-40 C. If you're an astronomer interested in objects that are much colder because they're in space, you look at radio waves or microwaves. The light from the sun actually looks a lot like a black body at 5500 K, so that is also thermal radiation.

My point here is that you should erase the close association in your head between "IR" and "heat." Infrared light is light. Period. It's just light that (a) you can't usually see and (b) it happens to be the kind of light given off by objects at earthly temperatures. If it hits a mirror, it bounces off, just like the light you're familiar with. Now, it's possible that this won't hold for all the optical properties of a material--certainly some materials might absorb IR when they reflect visible, for instance. But many optical responses will be the same, and there's no a priori reason to assume a reflection won't appear. (When the reflection doesn't appear despite the fact that you might expect it to, then you have an interesting situation.)

Answer 2

I was wondering the same thing, after doing some research I understood the answers that are written here and my answer supplements them.

All objects emit electromagnetic radiation in the form of photons. Photons can have different wavelengths, very long wavelengths like radio waves (low frequency) to very short wavelengths like x-rays and gamma rays (high frequency). In physics, light refers to electromagnetic radiation of any wavelength. Visible light that humans can see is between 400-700 nm, in between ultraviolet light (shorter wavelength than visible light) and infrared light (longer wavelength than visible light). The kind of electromagnetic radiation emitted from an object depends on its temperature; this radiation is due to the vibration of its molecules and is called thermal radiation. Thermal radiation from objects can be emitted at any wavelength; objects around room temperature emit infrared radiation, objects with hotter temperatures emit higher frequency visible light, and even hotter objects emit ultraviolet light. A surface can be heated by an electromagnetic radiation of any frequency as long as it is able to absorb that radiation. A material can reflect, transmit, absorb and/or emit radiation. I thought that the glass absorbed radiation emitted from your body which changed its temperature, and that's why we can see a person's reflection (probably did occur but too minuscule for the thermal camera to detect or for it to be of relevance). But yeah I understand now that it's just the infrared light being reflected off of the glass and detected by the infrared camera.

Answer 3

Thermal imaging is just seeing far-infrared radiation emitted by warm objects. This is just another form of electromagnetic radiation, like light.

As with light, it will pass through some substances (but not normal glass), be absorbed by some surfaces, and reflect off other surfaces.

Answer 4

I don't know if an answer was correctly given to the question:

I have a FLIR camera mounted on the front of my car, and sometimes I see perfect reflections of heated objects off of objects that don't look reflective from reasonable distances.

For example a metal box on a telephone pole or sometimes even a mailbox can give perfect reflections of cars passing them from distances of up to 15 feet

I understand how the radiation is released and how it reflects off the surfaces splitting off in all directions but to get indirect information off these reflection points leads my brain down So many holes....