Originally Posted by
lensmanmd
Chris, I too, viewed this article and an interesting quote was "I would suggest applying your AR coating on a Sola CR39 -2.00 lens and putting it through the test" Now, we all know that the Sola FSV that is used for the salt water boil has, well, a ridiculously soft factory HC. This is the exact same test, with the same materials that we conduct our shock test with. Routinely, the lenses score 4s and 5s. When compared to the UV87 or even the AST, the Sola Permaguard is akin to the lizard oil that we use to treat CR39 with and overcharged the patients. Dip coating is a better method, but can be fairly cost prohibitive for a small lab, hence the spin coating that most labs use. That said, today's spin coatings are so much more durable as compared to what was available even 5 years ago. They are harder and protect the lenses better. This may be the primary issue with temperature related crazing. The article did point this out. For AR applications without a primer layer, softer HCs may be the answer.
Expansion and contraction is a known thin film killer, be it AR or Mirror. Expansion occurs more rapidly than contraction. That much is known. The question here is, which is more damaging? Expansion then contraction, or contraction then expansion? My hypothesis is expansion to contraction, as it is a more rapid and destructive process. If you take a thin water glass, heat it to 160F then shock it with cold (ground temp) water, chances are, the glass will crack. However, if you take that same water glass, freeze it and then shock it with the same ground temp water, the glass will survive. This is what most of us in the real world deal with, so this would make sense. Now, we are not talking about extremes here.
Winter climates north of the border are a bit more extreme, granted. Here in the Mid-Atlantic region, our winters are not as extreme, though there are days when it can get that way. My experience with AR crazing in this region during the winter months is basically nil. However, I see a lot more issues in the summer months, as temps can average between 90F and 100F with fairly high humidity very frequently. Again, this happens more to those that are lazy and leave their 'clear' pair in the car for extended periods of time.
Back to the article, it does not mention pressure and its effects. Happylady had an issue with her AR. Hers was an issue that included heat to cold and back to heat, but also introduce pressure variables into the equation. An odd thing about this, thin films are applied in a vacuum which approximates the cruising altitude of a jet, but a much shorter duration of time. A room temperature lens is introduced to a vacuum and a hot plasma, then the heated lens with the thin film applied is reintroduced to cooler ambient temps and 'sea level' pressure, generally without any side effects. Happylady's lenses were a more extreme example, and did create a crazing issue.
If there are any of you out there that can shed light on this, I would greatly appreciate it. One always learns from smarter persons.
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