 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Medical Forum / General / Vision / October 2006How are bifocals made?
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 service? Bill -- Fermez le Bush One-hour places are equipped to grind and polish the back surface. They stock "semi-finished" lenses with the front surface already made, and grind the back surface to provide sphere and cylinder "in an hour." Bifocals and progressives are cast-molded into the front surface, at the factory, so the lab stocks a range of bifocal adds and front curves. -MT > 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 [quoted text clipped - 7 lines] > Bill > -- Fermez le Bush On 10/13/06 5:27 AM, in article fe6dnXAH1vNKG7LYnZ2dnUVZ_qednZ2d@giganews.com, "Mike Tyner" <mtyner@mindspring.com> wrote: > Bifocals and progressives are cast-molded into the front surface, at the > factory, so the lab stocks a range of bifocal adds and front curves. 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. Bill -- Fermez le Bush 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. >On 10/13/06 5:27 AM, in article >fe6dnXAH1vNKG7LYnZ2dnUVZ_qednZ2d@giganews.com, "Mike Tyner" [quoted text clipped - 19 lines] > > On 10/13/06 11:47 AM, in article cbRXg.16430$7I1.13614@newssvr27.news.prodigy.net, "William Stacy" <wstacy@obase.net> wrote: > 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 [quoted text clipped - 6 lines] > greater precision is possible with glass than with any plastic, which is why > it is so often used in precision optics. 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. Bill -- Fermez le Bush 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. countries... w.stacy, o.d. >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. > > > 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. 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 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. >>Either way, dress safety (min 2.2 mm thickness) must >>withstand a 5/8 in steel ball dropped from 50 inches, [quoted text clipped - 7 lines] > > -Quick |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.