Refocusing IR thermal gun.

[Sceadwian]

Guess physics is the best forum to put this under as it's an optics question.

I have a thermal gun that has a distance to spot ratio of 9:1 which is actually moderately decent as far as thermal guns go, but I've run across the problem than even right at the outlet of the gun it's not focused properly for anything less than that 1 square inch area, making it completely useless for checking individual transistors or surface mount packages which is really what I had in mind when I got it.

I've seen close focus (.1 inch spot size at 1inch) IR guns, that are EXACTLY what I need but I've also noticed that the aperture of my thermal gun is threaded, which I'm sure they meant for coupling of a focusing element of some kind.

Can anyone provide any tips on if I can use standard glass optics as a focal element to try to refocus to the smaller spot I need?

 

[Sceadwian]

I now know standard glass optics are out of the question. For 5-14 micron IR, anyone have a suggestions?

 

[Nigel Goodwin]

I thought standard glass optics were fine?, BUT the focal range isn't the same for IR, which makes it difficult as you can't see it to focus.

 

[Sceadwian]

From what information I've been able to dig up Nigel thermal guns are sensitive in the range of **broken link removed**. From what I can find standard optical glass exhibits a brick wall cutoff effect right around 7 microns, I have some lenses laying around that might work to test, finding the focal point is simply a matter of scanning across a piece of paper with a small dot of tin foil, takes quiet a bit of time but it's doable. I'd just hate to spend another 80 dollars on a close focus gun, especially considering the one I have has a serial output header.

 

[Sceadwian]

I just confirmed my suspicions Nigel, it's worse than I thought. I have a plastic eye loupe, wouldn't read anything over ambient even pointed at a surface that was 200 degrees. The only glass lens I had was the same thing, completely opaque to IR, and I wasn't able to get any reading from the reflection of a mirror either.

 

[ericgibbs]

hi Sc,
What freq is the IR, nmtrs?
Edit:
See that its 5 to 14uM

 

[alec_t]

Worth trying an IR-opaque narrow-bore tube (e.g. old ball-point pen casing) as an extension of the barrel? That should reduce the incidence angle (at the expense of sensitivity).

 

[Sceadwian]

Ahh a simple collimator, that might work thanks for the suggestion alec. I'll rig something up and see what kind of results I get.

<edit>
It did work, but the sensitivity loss was so great that it's not useable. It measured room temperature okay, but a 110degree object measured only 2 degrees over room temperature, an open flame that usually measures 250-300 degrees was reading only 100, so it became HIGHLY non linear as well. Worth a shot though.

 

[Sceadwian]

Heh, found the perfect material, germanium. Gonna have a hell of a time finding anything on the hobby level that's practical though, commercial lenses seem to run about 300 dollars. Obviously aimed at high end thermal optic imaging systems, I just need crude refocusing, meh. Probably less hassle at this point to buy a close focus unit.

 

[Joe G]

just a thought, have you tried or thought about the small optic focus lens thats used in a cd rom, they do have a really small focal length, it can't hurt to try,
and it'd be a freebee if you have an old cd player/writer laying around.

 

[Sceadwian]

CDROM's use 780nm optics, DVD's 650nm... How would this be able to focus 5-13 micro meter radiation? The media you're describing is typical optical media, totally useless in far IR.

 

[jpanhalt]

For 5-14 micron, the cheapest window/lens is NaCl (salt) crystals. It has been used for decades for IR spectrophotometer windows. KBr is an alternative, but is more sensitive to water. Sapphire might also work (i.e., heat-seeking missile nose cones) -- might get one on surplus. It would be much harder to grind into a lens, though.

If you have a hydraulic press, you can make KBr windows, just by pressing the powder between two anvils.

John

Edit: Just checked, and sapphire probably wont work. I feel silly that I forget to mention diamond. I am attaching table from a large pdf file I have. Sorry, I can't give a link to the file. If you need the whole file, because the png is not readable, I can attach that.

 

[alec_t]

Would an offset parabolic polished metal reflector do the job?

 

[Sceadwian]

That's a bit more data than I found, but pretty similar. I didn't find NaCL lenses, but that's probably because they're hygroscopic, and the Knoop hardness of 15 makes them practically VERY difficult to use without careful handling and coating. Sapphire is only responsive down to 5nm, and based on my experiments with clear optical glass it looks like the range for thermal imagery is closer to the 10-15nm range. Zinc Sulfide, Silicon, Germanium.. expensive..

The real shocker in that data to me is high density polyethylene! Sure the Knoop hardness is crap, but it's freegin polyethylene! Dirt cheap and about as common as such. It's right at the limit of thermal imaging limits at 16nm. I'm wondering now if the lens cover to my thermal gun is HDPE, it's slightly milky in appearance and a flexible plastic material. I'm sure has heck not going to dissect my gun to find out what the primary lens is made of but as I said it looks like a 1 inch fresnel lens. The pain in the butt part is the 'interesting' section of the thermal gun is a module that's nothing more than a 1.5 inch long by 1 inch in diameter cylinder, all the practicals are encapsulated.

Please definitely attach the full file, I'd like it for future reference.

 

[Sceadwian]

Alec, it might possibly, but the setup would be ungainly and still be subject to incident reflection and emission, I rank it a few notches higher than the collimator for practicality but I certainly won't try to implement that one knowing what I know now. I'm pretty much resigned to eventually purchasing a close focus non contact meter, but the information I'm getting by looking into this is priceless to someone as purely curious about physical materials as I am.

I mean really, of all the materials you'd think of, Glass, Quartz, Zinc Sulfide, Silicon, Germanium, HDPE. Seriously, who would start off the bat thinking that one of the most common industrial plastics on the planet can be used for focusing far infrared!

 

[jpanhalt]

My friend, I have been using NaCl windows for IR since 1963 as have tens of thousands of other chemists. They are not at all hard to handle. Remember, this is 5-15 microns, not a few hundred nanometers. A few surface imperfections have almost no effect. Of course, the real question is whether you want to get it done now and relatively cheaply, or just dream about it. Sorry if all the data confuses you. Diamond works too. It is not hygroscopic and is readily available.

John

 

[Sceadwian]

I'm not asking about windows, I'm asking about lenses, and if you can't suggest better than an 80 dollar element, a close focus IR meter is cheaper in whole.

 

[alec_t]

If HDPE is, surprisingly, transmissive in the 5-14u range is it worth doing some original research by experimenting with other readily-available plastics in front of your gun to see which pass/block the IR? For example, how about hot-melt glue (which could be easily moulded to lens shape)? I doubt you'd find published info on that!