Polycrystalline Germanium is actually metal, this metal in the form of film and this gets shaped accordingly so to serve as lens inside of Infrared Thermometer.
The main property of such Germanium lens (by using a simple language) is that it does not aloud regular light to pass thru, but only thermal radiation.
Shape and Crystalline orientation and also purity of material, all these forming together the Wavelength specification and this can be for example 2-14 μM (um).
Tips about cleaning:
Unfortunately most tips found over the internet are very generic, even the ones who come by vendors who are producing low-cost Infrared Thermometer devices, for example one of them proposed cleaning with cotton swab and water.
Water is definably not an enemy about cleaning things, but distilled water is more appropriate due it clearness, as this is actually totally clean from salt.
Other vendors who offer optic lens and Germanium lens, they suggest isopropyl alcohol or Ethyl Alcohol (Denatured).
And finally all agree that prior any attempt for cleaning, the first step is a blast with Gas duster, also known as canned air or compressed air.
Compressed air can be found in every factory, as there is many huge air compressors all over the place, but the problem with them is the purity of the compressed air.
Badly filtered air it can include a portion of water or even other elements as for example particles of dust or of other unknown substances, or even metallic ones, which those got absorbed by the compressor when this was in the form of smoke that came by near-by welding machines.
What I would do if I have to clean such a Germanium lens would be:
Step A, Start with canned air
Step B, Cotton swab and distilled water, so most dirt to be removed.
Step C, isopropyl alcohol or Ethyl Alcohol (Denatured).
My own Infrared Thermometer Lens has a diameter of just 10mm which this is just one centimeter wide, this lens it is placed deep inside of the supposed gun barrel, in a depth of five centimeter ( two inch ).
While my lens looked sparkly clean after 19 months of ownership, I thought to go at step C right away.
My Infrared Thermometer lives in my workshop and this is a regular place only contaminated by regular dust.
What is interesting as note, is that when touched this lens (Germanium metal) with my cotton swab, this it did wobble about 1mm left or right from the center.
I did investigate this a bit more, and now I am convinced that this part is not glued or 100% centered, this is rather installed inside in plastic slot.
Now with out taking this thing apart I can safely speculate that the actual thermal sensor it is few mm (distance) further behind.
I do not know if this soft wobble is a sign of poor manufacturing? But from the other hand I do not think that this can cause any true impact at a such narrow beam.
I think that a thermal imager which this using several beams as measuring points, this is justified to have more strict manufacturing process, compared to a single spot Infrared Thermometer.