Pin hole lens for thermal imaging?

[Vipitis]

So I thought about lens options again. Germanium and even ZnSe aren't very accessible for me so I resulted to different techniques. Mirrors were one option but highly complicated and in need for high quality.

Pinhole lenses has been the cheap toy to cameras for a while. In theory they should work.

A quick search has thrown that someone build a pinhole scanner.

My concerns are that pinholes have a really replay tiny Aperture and therefore need a lot my light coming in. I don't know alot about exposure theory in thermal cameras tho and how much exposure time matters.

My plan is to read up on the topic and use a pinhole calculator to see what could be achieved. Then 3d model and print a lens bracket and use paper or thin Metalsheet for the pinhole.

Has anyone tried this stuff?

[Cat]

My concerns are that pinholes have a really replay tiny Aperture and therefore need a lot my light coming in.

While not exactly the same I placed a sheet of paper with a 2mm hole in front of the lens and observed a running soldering iron. As you can see a lot of energy is blocked by the small aperture. Things at room temperature are nearly impossible to see.
Unfortunately I don't have a freely accessible FPA at the moment to test what happens with a pinhole lens.

Germanium and even ZnSe aren't very accessible for me

ZnSe and Ge-Lenses can be bought from chinese sellers for a few $. They are commonly used in CO2-Laser cutter machines. They might not have the best properties for thermal imaging but are probably much better than the pinhole aparture.

[Spirit532]

A pin hole lens will never work on an uncooled camera.
The smallest aperture lenses designed for midwave cooled cameras only really reach into the f/5 to f/6 territory(and that's pushing it), whereas a pinhole would be on the order of... well, "effectively" around f/20 to f/30(not a real measurement though).
You would need an extremely intense light source(the sun, for example), and a cooled camera that operates way below the standard 77K to see anything at all.

[Bill W]

f/32 is 1/500 of the energy passed by the typical f/1.4 lens.

When fire cameras had a dynamic range of 50°C or so, f/32 was the typical closure to cope with looking at fires.  On that basis a pinhole should see a fire OK.

I'll have a try.....

Bill

[Fraser]

Interesting discussion.

As has been stated, the pinhole 'lens' is an inefficient beast. Experiments with them suggest that any form of 'real' lens outperforms the pinhole in thermal imaging. As has also been stated there are good options for a simple lens that are inexpensive. Thanks to the popularity of CO2 laser etching and cutting systems we are seeing excellent availability of ZnSe close focus lenses. The FL of these commonly ranges from around 50mm to 100mm but I have seen 25mm FL as well. Such a simp,e lens will never win any prizes for image quality and distortion, but they will work. Other available options from the Laser engraver/cutter market are GaAs and Germanium lenses. These tend to be more expensive but a simple 12mm diameter Germanium lens has recently become very affordable thanks to China's mass production.

You could even try making a NaCl (common salt) crystal lens !

https://www.crystran.co.uk/optical-materials/sodium-chloride-nacl

Fraser

[Fraser]

I just checked on my favourite ZnSe lens supplier, Laserlands.

A 12mm diameter Germanium lense with FL of 50.8mm costs less than $15 delivered to the UK 

There are plenty of CO2 engraver lens suppliers on eBay. The 1" (25.4mm) FL lenses are more common than I remembered. A fellow forum member is selling some very nice Thorlabs (high quality) 12mm Germanium lenses that have an FL of only 15mm  They are not crazy expensive and a message to him may elicit a favourable direct purchase price. I have some of his Thorlabs Germanium lenses and they are excellent.

I will add links here in a minute.

Fraser

[Fraser]

Laserlands on eBay ..... loads of interesting lenses and optical stuff from China at good prices.....

https://www.ebay.co.uk/itm/ZnSe-GaAs-Focal-Lens-for-10-6um-CO2-Laser-Engraver-Cutting-Machine-12-25mm-1-4/382231394962?hash=item58fec43a92:m:mBYIu-9Du9tMohxBSlFzNdg

The lovely Thorlabs Germanium lenses from a member of this forum.......

https://www.ebay.co.uk/itm/Germanium-Lens-PCX-Thorlabs-LA9410-F-1-2-Dia-f-15-mm-IR-AR-8-12-m-CO2-Laser/111231059741?epid=1937210724&hash=item19e5e3471d:g:UAoAAOSwhOxZ8gp4


Fraser

[mrflibble]

Not sure about the requirements, but maybe laser etching PMMA (or similar material) fits the bill?