 Posted: Tue Oct 03, 2023 6:53 pm Post subject: |
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SalPro wrote:
| eggplant wrote: |
There is probably an answer I'm missing by not reading the appropriate sources first, but are the anti-reflective coatings on the front and rear of a doublet effected by the process? Submerging in water and boiling etc.
I know the temperatures required to coat a lens are at least double of 200c, but still. |
I will shy from heating doublet due to possible fracture and distortion. Especially with IR. First, glass will reflect most of IR, then absorption by glass is near zero. Assuming whatever get through is absorbed only by glue - this would create thin heated layer against colder glass. Not a good scenario. Stress may, if not crack then pit the glass. Unless thermal flux will even all thermal gradients, but then there is no point to try to heal thin layer only. If absolutely have to heat - I would rather go with slow oven. But then handling glass with cold tools may thermally stress it anyway. Preheating tools, working in the oven - too much complications for me.
I have never seeing coating on inner surfaces of doublets. There reflection reduction is mostly through index matching of optical glue (balm, epoxy, LOCA). Ideally index of glue should be somewhere between, but not precisely in center, of indexes of two pieces of doublet. But I do mostly old, >20 y.o. lenses. I guess theoretically, doublet/triplet inner coating may improve it further. But while index of external coating have to be between glass and air (wide range of selection), coating of inner mating surfaces of doublet need to be between indexes glass and glue, tiny range comparing to external coating. So I guess it is complication that nobody needs, performance improvement yield is too low.
Best of my knowledge, traditional coatings are various metal oxides. They are mostly passive, at room temperature. Never got any damaged by solvents, including Goof Off. Oxidation releases energy. To reverse - need to push energy back, to heat. I'm not sure if heating to 200C as suggested come close to start reversing coating formation process, Maybe some chemist can enlighten us?
Then, googling glass transition temperature: 140 °C to 370 °C. That is when it started to flow. At 200C it may start to flow, causing distortions. Even resting on support during heating may cause problems, I think. Good glass is polished to fraction of micron.
In my head I feel safer to not to heat the doublets. Just an opinion. |
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