Understanding EM Spectrum: Emitting vs. Reflecting | IR Camera Examples

[AkshayPitt]

Hello,

I was wondering how one would be able to tell the difference between an object emitting a certain wavelength of the EM spectrum, vs. an object reflecting that same wavelength of the EM spectrum. For example:

1) Suppose I have an IR source at 7 microns. I point this source at water, and capture data via a mid IR camera. Water has great absorption at 7 microns, so I don't see a direct reflection of the source captured by the camera. However, I do see that the water is emitting IR at 7 microns after having absorbed most of this radiation from this source.

2) I point the same source at a sheet of polished steel. I capture the data via a mid IR camera again. Polished steel has very high reflectivity, so I see almost a direct reflection of the source captured by the camera.

Would the signal captured by the mid IR camera look different in the two cases above?

Thanks.

 

[Bystander]

polished steel. I

"Specular reflection." You'll also want to look up "diffuse reflection" and "cosine scattering."

 

[AkshayPitt]

Thanks a lot for your response. After looking up those terms, I understand that specular reflection happens on smooth surfaces, and diffuse reflection happens on rough surfaces. What I am still confused about, however, is the difference (from the perspective of a camera) between emission and reflection. I'm not sure if I'm missing something, but to me it seems like the camera would see emission and reflection the same way.

Rather than taking the example of water and polished steel, let's say there are two pieces of polished steel. Assume one piece of steel was already hot before coming into the camera's vision - the camera would pick up the emission of infrared from this piece of steel. Imagine that the second piece of steel has infrared reflecting off of it - the camera would pick up the reflection of infrared off of this piece of steel. Would these two camera signals not look very similar?

 

[Bystander]

In the case of a specular surface, reflected radiation will give you an image of the original source. and the radiation emitted by the reflecting surface can give you only an image of the emitting surface itself. There will be additional differences in wavelengths of maximum intensities if there are temperature differences between the surface and the source being reflected.