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Re: UV protection needed for glass eyeglass lenses?

Yes, you are right. UV coating is best for glasses used outside. If you are outside a lot, a good pair of sunglasses with UV protection is recommended. As far as antiglare goes, it can be excellent when you are dealing with point sources of light. The internal reflections produce annoying multiple images of the sources. Extended sources such as computer screens hide the effect. Plastic lenses with scratch resistant coatings are very carefree, and are lightweight. Glass lenses can sometimes be burdensome because of the extra weight.

The antiglare coatings are similar to the coatings used on fine optical instruments. They help prevent multiple internal reflections for the central frequencies of light. Unfortunately this makes the lens more prone to damage. If the coating is abraded or scratched, it makes the image very unclear. They are also very sensitive to films of dirt. I have found that dirty regular lenses are fairly usable, but dirty coated lenses are very unclear. Clean unscratched coated lenses are wonderful, but they seldom stay that way. When I switched from coated to uncoated lenses, I noticed the multiple reflections. After a while I accommodated to the new lenses, and stopped noticing the reflections.

From my personal experience, you can accommodate fairly large astigmatism until you reach a certain age. At that point the presbyopia (old eyes) catches up and you need glasses. Eventually they are needed for clear vision at all times, and then only certain distances become unclear. Without the glasses astigmatism produces blurry vision at all distances, but you may still be able to read and see in the distance. This is because no one distance is in focus perfectly. If the astigmatism is not lined up in both eyes the brain can unscramble the messages and produce better apparent focus, but things can pop in and out of focus. As the lens hardens the ability to accommodate progressively worsens, and most people need glasses for at least 1 distance, and possible bifocals. For the vain, most of these problems can be corrected by expensive surgery. There is even a form of surgery for presbyopia, but it is controversial. By the way most everyone I know likes the progressive bifocals, which have an interme
iate distance good for computer screens.
See what you have to look forward to!!!

John M. Clement
-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu]On Behalf Of Jane Jackson
Sent: Monday, November 20, 2000 11:09 PM
To: PHYS-L@lists.nau.edu
Subject: UV protection needed for glass eyeglass lenses?


I haven't been able to read phys-L for 2 weeks - had eyestrain. I went to
the optometrist, and he prescribed computer glasses for me, since I work 4
to 5 hours daily at the computer. My astigmatism has worsened a bit in the
past 4 years - gravity, he said (in addition to heredity). My vision is
fine at this distance; I've never worn any glasses while working at the
computer.

Anyway, I'll get glass lenses, since they're less trouble to care for.

I have 2 questions (especially since I've been out of the classroom for
years and haven't kept up on optics - I need quick answers):
1) Since the lenses will be glass, and hence offer some UV protection, is
there any value in getting a UV coating? Computers don't emit significant
UV any more, right?
2) The optician recommended an anti-glare coating too. I can see the value
of that for driving, but how about at the computer? What's your experience
and insight? (When I look at my computer through my uncoated glass lens
reading glasses, I don't notice any glare. At $40 for the coating, this
question is of significant financial interest.)

If this subject has been discussed in the past few years, please tell me
and suggest key words that I can use in searching the phys-L archives.
cheers,
Jane


Jane Jackson, Co-Director, Modeling Instruction Program
Box 871504, Dept.of Physics & Astronomy,ASU,Tempe,AZ 85287
480-965-8438/fax:965-7331. http://modeling.la.asu.edu
"The ideals which have lighted my way, and time after
time have given me new courage to face life cheerfully,
have been Kindness, Beauty, and Truth." - Einstein (1931)