Damaged lens coating - Page 1
Forum section: electronics / general

author: bostwickenator
author_rank: Contributor
timestamp: November 09, 2022, 01:51:21 pm
content: Guess what, I broke something again!I have a nice little germanium IR lens I was using for macro on a P2 pro. To hold it in place I was using a little plastic case made of PET. To avoid using glue to hold the lens into the case I trapped it in place by melting a couple of spots on the case with a soldering iron. But something on the iron or the heat itself kicked off a chain reaction which has eaten the lens' AR coating. I filmed the effect under a microscope and sped it up 15 times. Maybe someone will find this interesting or informative.Attached is the "wide angle" of the lens (12mm diameter) the next morning.

author: Fraser
author_rank: Super Contributor
timestamp: November 09, 2022, 04:47:47 pm
content: Thanks for sharingI have no idea why this has occurred. Very unusual behaviour of the AR coating.The good news is that the AR coating is still present so the lens may look ugly, but it may well still work. It is the Germanium that does the refractive work and the AR coating is just the anti reflection layer to counter reflection losses.Fraser

author: bostwickenator
author_rank: Contributor
timestamp: November 09, 2022, 07:17:35 pm
content: The process you can see in the video creates features about a hair's width and half a hair's height. It definitely wasn't usable like that even by my standards. Thankfully the failed coating was easily buffed off with a diamond grit polishing compound.Thinking more about it I bet the manufacturer laid down some kind of bonding layer to get the AR material to stick to the Germanium. I think this third intermediate bonding layer is what failed as the coating that flakes off doesn't appear to continue reacting. My bet is tin/silver/or lead residue on the soldering iron tip aggressively alloyed with whatever the intermediate layer is.

author: ArsenioDev
author_rank: Frequent Contributor
timestamp: November 09, 2022, 07:57:41 pm
content: Almost certainly it's a metallurgical reaction of the solder residue and then dendritically migrating on an intermediary layer inside the lens between AR and Ge.Bouncing this along to my thermal optical engineer friend and asking someone from optics div at work.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 10, 2022, 07:46:31 am
content: Sort of reminds me of acrylic crazing when exposed to alcohol - something about it being enough of a solvent to break some of the long chains of the polymer and form fractures in it even though it doesn't fully dissolve it. Sort of eerie to see in motion.Actually, are lenses like this or their coatings commonly heat treated? It actually reminds me a little bit of tempered glass slowly fracturing outward after a small point of impact. I wouldn't expect a coating to be hard enough/thick enough to really do the same thing, but maybe if the adherence to the lens material isn't high and the application/drying of it puts it under tension, maybe it could be a physical relief phenomenon.

author: Bill W
author_rank: Super Contributor
timestamp: November 10, 2022, 11:07:37 am
content: Not had any immediate ill effects from 'durable' grade coatings being flame heatedNot had any immediate ill effects from 'internal' grade coatings being heated to 250°C over a few minutes.These are normal tests for fire cameras.What did seem to go eventually were any metal - lens interfaces where perhaps frequent expansion ground through the coating, and then normal water damage.Bill

author: Fraser
author_rank: Super Contributor
timestamp: November 10, 2022, 01:53:45 pm
content: I wonder whether it could be a plasticiser or hydrocarbon release from the 3D printed lens holder causing this reaction ? When heated, some plate s release all manner of chemicals.Fraser

author: bostwickenator
author_rank: Contributor
timestamp: November 10, 2022, 10:39:46 pm
content: Or rosin flux residue

author: bap2703
author_rank: Regular Contributor
timestamp: November 12, 2022, 10:02:14 pm
content: I wouldn't say it's chemical.I'd opt for stress in the AR coating: by mechanically holding or through heat you created cracks that just provided the starting point to relieve the stress.It's a very common problem in all deposition techniques.

author: amyk
author_rank: Super Contributor
timestamp: November 13, 2022, 02:51:29 am
content: It looks almost as if gas is being evolved from under the coating and pushing it away.