A Few Eyeglass Questions, Please

[Robert11]

Hello,

Two questions, please:

a. The Anti-Reflective (AR) coating they use on eyeglasses these days:

How durable are they ?

-O.K. to wash with water ?

- How about with Alcohol (many "wipes" come impregnated with Alcohol)

b. Is the coating typically on the outside or the inside of the lens ?

c. Is Polycarbonate "meaningfully" heavier than regular plastic for a
lens ?

Thanks,

Bob

[Mark A]

a) The good ones are quite durable. The are fairly expensive ($75 or more) and done at manufacturer
labs (not in store). Ask for the brand of AR coating such as Crizal, Caret Advantage, etc. If only costs
$25, then probably in-house cheap coating that will not last. OK to wash with water, mild soap, or
alcohol.

b) typically both

c) Polycarb is lighter and thinner (due to higher 1.59 index than 1.50 regular plastic). However polycarb
has the worst optical properties (measured as abbe value) of any commonly available lens. But not
sure what you mean by regular plastic, since these are the common materials available in some form
of plastic (with index of refraction):

- Regular Plastic (CR-31) 1.50 (very high quality optics)
- Trivex 1.54 (safety lenses with high quality optics)
- Polycarb 1.59 (safety lenses with very low quality optics)
- 1.60 plastic (good optics)
- 1.67 plastic (fair optics)

Regarding optical quality as measured in abbe value, some people with a relatively mild or moderate
Rx will not be bothered by low abbe values (poor optical quality). If you have a moderate to high plus
Rx (far-sighted), don't even touch polycarb with a 10-foot pole.

[Salmon Egg]

In article <joc758$vh$1...@dont-email.me>, Mark A <ma...@nowhere.com>
wrote:

a) Although not up-to-date on the subject, I do not believe that there
are ANY truly durable AR coatings for plastic optometric lenses when
comparerd to what used to be available for glass. I have heard that the
best were really good because they would last at least two years. Well
deposited coatings on glass could not be effectively removed except by
repolishing.

b) It hardly makes any sense to coat one surface. Reflective loss for an
uncoated lens is about 8%. Coating both sides with a simple coating will
reduce that to about 2% in the most visually important portion of the
spectrum. Coating only one side will give you a loss of 5%. Coating both
surfaces With a fancier design and fabrication can reduce loss down to
about 0.5%. Do not expect to get that at a reasonable cost.

c) Abbe number is a measure of dispersion (chromatic aberration)--not
optical quality. It can definitely reduce performance in a spectacle
lens.

--

Sam

Conservatives are against Darwinism but for natural selection.
Liberals are for Darwinism but totally against any selection.

[Mark A]

On Wed, 09 May 2012 09:54:41 -0700, Salmon Egg wrote:

> a) Although not up-to-date on the subject, I do not believe that there
> are ANY truly durable AR coatings for plastic optometric lenses when
> comparerd to what used to be available for glass. I have heard that the
> best were really good because they would last at least two years. Well
> deposited coatings on glass could not be effectively removed except by
> repolishing.
>
> b) It hardly makes any sense to coat one surface. Reflective loss for an
> uncoated lens is about 8%. Coating both sides with a simple coating will
> reduce that to about 2% in the most visually important portion of the
> spectrum. Coating only one side will give you a loss of 5%. Coating both
> surfaces With a fancier design and fabrication can reduce loss down to
> about 0.5%. Do not expect to get that at a reasonable cost.
>
> c) Abbe number is a measure of dispersion (chromatic aberration)--not
> optical quality. It can definitely reduce performance in a spectacle
> lens.

a) My Zeiss Caret Advantage AR coating has lasted almost 5 years so far without any problem. I am not
especially careful about my lenses, and I clean them daily, so I have been pleasantly surprised.

c) That is correct for optical professionals, but in layman terms, anything that affects the quality of
vision (including chromatic aberation) would be considered to fall under the umbrella of "optical
quality." The fact is that chromatic aberation is the number one factor that differinetiates the "optical
quality" of the various lens materials most commonly used. However, people with weak and sometimes
moderate Rx's may not notice chromatic aberation at all, or not enough to worry about. For myself,
with a high plus lens, I prefer 1.60 with 42 abbe value. When I had my Zeiss Id progressives made, the
Zeiss lab in Germany (at that time they were only made in Germany, not sure about now) upgraded me
to 1.67 for free, but I sent it back and told them I only wanted 1.60 (which is what I originally
requested).