How are bifocals made?

Discussion in 'Optometry Archives' started by Salmon Egg, Oct 13, 2006.

  1. Salmon Egg

    Salmon Egg Guest

    While I can understand that it may not be very big deal to edge stock mass
    produced single vision lens blanks for various combinations of spherical and
    cylindrical power, are stock blanks also used for bifocals and trifocals?
    That would seem to require a very large variety of blanks. Is a reading add
    fused into a stock blank? How are bifocals fabricated for a one hour

    -- Fermez le Bush
    Salmon Egg, Oct 13, 2006
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  2. Salmon Egg

    Salmon Egg Guest

    Is that one of the reasons plastic is pushed? Once an expensive mold is
    made, the repetitive cost to stamp out lenses is going to be low.

    Is it faster (and cheaper) to grind and polish plastic compared to glass?
    Certainly the rough grind is going to be easier. My little experience with
    polishing plastic was that it required finding the right combination of
    abrasive and lubricant for the particular plastic being used. I presume that
    that once the right combination is found, the polishing becomes fairly easy.

    -- Fermez le Bush
    Salmon Egg, Oct 13, 2006
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  3. Plastic lenses are cast, not "stamped", and it is quite a technology.
    Actually, in many ways glass is easier and cheaper. The main reasons
    for plastic ophthalmic lenses are the light weight, the safety issues,
    and the thinness (of some plastics). The grinding and polishing is
    certainly quicker, but therein lies a problem. Generating too much heat
    or pressure on the lens as it approaches its final thickness can distort
    the lens, sometimes dreadfully. So I'd say glass is cheaper and easier,
    and more forgiving, but certainly hard on the nose and a threat to the
    eye. Due to its rigidity, greater precision is possible with glass than
    with any plastic, which is why it is so often used in precision optics.

    w.stacy, o.d.
    William Stacy, Oct 13, 2006
  4. Salmon Egg

    Salmon Egg Guest

    I did not me "stamped" literally.

    In my personal situation after implants for cataracts, the required lens
    power has been greatly reduced. Weight is no longer a big problem. In
    California, tempered glass is required for glass spectacles.

    Just how serious is the fragility of glass. I have not had any break during
    my adult life. As a kid, I do remember doing things like stepping on them.
    And of course, I still remember the movie Doctor Cyclops.

    -- Fermez le Bush
    Salmon Egg, Oct 14, 2006
  5. Only serious if they break. It's the sharpness of the fragments that
    can do the bad work on your eye.

    In the U.S. they must be either chemically hardened (by submersion in a
    molten salt overnight) or by heat tempering (heat them up to almost
    melting, and then blow cold air on them). One method works better for
    small high speed impacts, the other for larger more blunt forces. I
    haven't used glass much in the past few years, so I don't remember
    which, but I think it was chemical for small high speed.

    Either way, dress safety (min 2.2 mm thickness) must withstand a 5/8 in
    steel ball dropped from 50 inches, while industrial (3.0 mm min) a 1
    inch steel ball from 50 in. Each and every lens must be tested, and if
    they break, they fail the test.

    You can get thinner and obviously less safe lenses in many non U.S.

    w.stacy, o.d.
    William Stacy, Oct 14, 2006
  6. Salmon Egg

    Quick Guest

    That's amazing. Can't the testing do damage? Do they
    do the polishing after testing? Couldn't the testing weaken
    the structure? maybe not visibly?

    Quick, Oct 19, 2006
  7. You would think so, but apparently not enough to worry about. The
    testing does mar plastic lenses, so those are only tested by random
    sampling or such. No, the lens is tested after complete finishing,
    including edging and polishing, ready for the frame. OTOH my info on
    this subject is old, like 15 or 20 years, so some things may have
    changed since I was involved with lens finishing.

    w.stacy, o.d.
    William Stacy, Oct 19, 2006
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