Some materials in LWIR

[Stefano]

I was thinking about germanium being transparent in IR, and I remember some videos on this posted on this forum: https://www.ultravioletphotography.com/content/index.php?/topic/4782-hellow-from-poland-minsk-mazowiecki/&do=findComment&comment=48466

 

 

Or imagine how this big block of germanium would look like in IR:

 

Looks like a metal, breaks like glass (actually gallium, a real metal, also breaks with a conchoidal fracture).

 

I searched on YouTube and found this video:

 

I find it interesting because he shows a variety of materials in visible and IR (I guess LWIR), including fused silica, calcium fluoride, sapphire, germanium and zinc selenide, as well as some metals (lead, tin, bismuth, zinc and silver, plus the non-metal carbon as graphite). As expected, some materials (windows in this case) are transparent only in visible light (for example, fused silica), some only in IR (germanium) and some in both (calcium fluoride).

 

Another interesting thing he notes is how the surface of the metals (and graphite) appears more reflective in IR, as if it was more polished. I believe this is due to the longer wavelengths (20 times longer than visible light if he used a LWIR camera like I think he did), and thus the surface is smoother relative to the wavelength.

 

This is also why the ground looks like a mirror in this 90 GHz microwave image: 

 

And a similar effect happens in the THz band:

In that post I said that the rough side of tinfoil should look smoother in LWIR, and Andy replied that the low resolution of his camera wouldn't allow to see the difference. I actually think that the difference can be seen, perhaps under favourable conditions (or using other materials).

 

UV does the opposite. I remember an image Mark once posted where you could see the scratches on a metal bowl better in UV, but his images are no longer here.

 

This makes me think that, in theory, the same effect should be visible with ordinary visible light, with surfaces appearing slightly smoother under red light than under blue light. Probably the reason we don't see this everyday is that the difference is too small.

 

 

 

[Nate]

10 hours ago, Stefano said:

the rough side of tinfoil should look smoother in LWIR

Unfortunately my LWIR spotter/camera couldn't tell much difference even with the microbolometer at 640x480 with the tin foil, the camera part is just so bad compared to the visual. My spotter's closest focus is 9ft/2.7Meters.(Helion XP50 first gen)

But I did wrap some tinfoil around a tube with the two sides showing. first one is straight out of camera, next is processed in topaz software. I used white hot. The heat reflection is from my kitchen light. Microwave is in the background

 

Camera out

 

 

Processed a little

 

[Stefano]

Actually, the dull and smooth sides look similar, which is what I was expexting (they both act like a mirror). In visible light the difference between the two is probably more evident.

 

 

[Nate]

They do, I couldn't tell any difference in the eyepiece, moving around different locations and other heat sources too.

[Stefano]

So your experiment confirms my expectations, if I am not mistaken?

[Nate]

I think so Stefano, now that I see what you thought the outcome would be.

On another note, I tried stacking my 38mm germanium on the 50mm, and to my surprise, my Helion turned into a low powered thermal microscope. I guess if I wanted to detect the source of heat in electronics I could.

[lukaszgryglicki]

Try to go to elevator and see... (FYI - my elevator is matt aluminium inside but it is no longer matt in thermal IR).

When I first time saw it I was amazed - it's like you are in a mirror box inside - your multiple reflections everywhere... using FLIR E75 camera.

[Andy Perrin]

16 hours ago, Stefano said:

In that post I said that the rough side of tinfoil should look smoother in LWIR, and Andy replied that the low resolution of his camera wouldn't allow to see the difference. I actually think that the difference can be seen, perhaps under favourable conditions (or using other materials).

I don’t follow. I stand by what I said originally and the experiment above (using a camera whose resolution in pixels is twice mine) seems to agree with it. You cannot see a difference. 
 

You also wouldn’t expect to see the bumps themselves in principle because the roughness wavelength (separation between two peaks if the foil looks like this under optical microscope: ^^^^^^) of the foil must be in microns roughly. The wavelength of LWIR is 8-14 microns which is ten times bigger. That means you should not be able to resolve those bumps. You may see a difference in the scattering though, if it were significant, but it’s just not. The bumps don’t scatter the light enough, perhaps because they are not deep. 
 

The situation is similar to Jonathan’s diatom experiment where the visible and longer wave UV had much less detail than the shorter wave UV photos. Even if we had the magnification to see the foil surface with LWIR microscope, it would only show details down to about the 1.22*8 micron size. 

[Stefano]

If you mean that you cannot see the difference between the rough and smooth sides of aluminium foil in the LWIR image, that's exactly what I think should happen. The rough side looks mirror-like in LWIR but not in visible light.

 

Also I am not sure that camera resolution has an impact. If you take a photo of a dull reflection from the dull side of aluminium foil with a normal camera, and then reduce resolution to like 320x240, it doesn't look mirror-like, but still dull.

[Andy Perrin]

You may be right about the resolution not affecting whether you see a specular reflection or not — I think that just depends on the roughness wavelength relative to the light’s wavelength. 
 

I don’t follow why you say, “I actually think that the difference can be seen“ if you meant the opposite of that.

[Stefano]

I see, there was a misunderstanding.

 

I was saying that you said you couldn't tell the difference between visible and LWIR (the difference being the change in the reflection type), while I believed that "the difference can be seen", where by that I meant that the dull side of aluminium foil should look different in visible and LWIR.

 

You probably understood it as "there should be a visible difference between the rough and polished side of aluminium foil when seen in LWIR", which is not the case, as also shown by Nate's experiments.

 

Perhaps I wasn't clear enough. We actually agree on every point.

[Andy Perrin]

Ah, ok that makes sense now.