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Lec 3 OPTO 225 (Ophthalmic Optics and Dispensing I)
Transcript of Lec 3 OPTO 225 (Ophthalmic Optics and Dispensing I)
Lec 3 OPTO 225 - Ophthalmic Optics and Dispensing I Plastic materials
Optical and physical properties of plastic ophthalmic lenses Corlon lenses ??? Manufacturing processes of plastics The plastic is separate to two groups depend on the physical properties of the finished product
1- Thermoplastic Materials
these soften when heated and therefore can be remolded.
2- thermosetting Materials
Once hardened, these plastics can not be softened, even at high temperatures.
compare plastic and Glass lenses plastic lenses
1/2 weight of glass
With a CT of 3.0mm w/o special hardening process. thicker profile than glass,
get scratches more easily
No UV protection unless properly tinted.
Glass lenses unlike plastic, must be treated to resist breakage (chemical or heat) Plastic materials
Original plastic lens made
(The primary commercial product is known as organic glass.)
WW II glass was used for submarine and airplanes.
The spectator protection in ice hockey rinks is made from PMMA.
1- CR-39 has a half weight of ophthalmic crown glass with similar prescription and of identical size
2- they are fairly impact resistant in their natural state
3- CR-39 are very inert and are resistant to almost all solvent and chemicals except perhaps, highly oxidizing chemicals
4- CR-39 is less prone to fogging than glass
5- CR-39 has much lower thermal conductivity than glass
6- Cr-39 lenses can be tinted to almost any color
7- reduces the manufacturing price than glass forms as a result of the material nature.
1- it is soft and easy to scratches
2- thicker than crown glass the has the same power
3- although it have fairly low refractive index
They have inferior photo-chromic qualities when compared with glass lenses
1- it is a specific type of lens made from plastic and glass lens
2- the front is glass and the back is plastic
3- these lenses are manufactured as C-Lite lenses.
1- it is 25% thinner than conventional lenses.
2- it is 25% lighter than all-glass lenses.
Take home msg (PMMA) (CR 39) Polycarbonates
Optical and physical properties of plastic ophthalmic (CR 39) Corlon lenses
Manufacturing processes of plastics Thermoplastic Materials Thermosetting Materials made from two different materials 1. poly(methyl methacrylate) (PMMA) which is harder and more resistant to scratches than other plastic lens materials.
Plastic lenses are made from a very high quality material as glass. 2. Modern hard resin lens from ally1 diglycol carbonate
1- they are vastly superior to ophthalmic glass and CR-39 in term of impact resistant
2- it have high R.I (1.586)slightly higher than the ophthalmic crown glass.
3- it have a specific gravity of 1.20 (as compared with 1.32 for CR-39) and therefore lenses made of this material is the lightest spectacle lenses in production
4- these lenses are easily tented and an abrasion resistant coating which also absorbs UV radiation is applied to both front and back surface.
1- it has a low Abbe value and hence gives rise to a higher amount of chromatic dispersion than glass or Cr-39
2- the surfaces of polycarbonate lenses are difficult to mold to completely eliminated imperfections.
it is called the injection molding
When the thermoplastic heated, it is stretched, pressed or molded into complex shapes with no apparent chemical change
When it is cooled, it is retain the shape of the container in which there were heated
Example: Plexiglas, Lucite, PMMA, cellulose,…etc
1- manufacturing of Thermoplastic
The manufacturing process distinguishing factor from the injection molding processing is that the hardening of thermosetting materials is an irreversible process.
Example: epoxy resins, phenolics (Bakelite) and CR-39 (diglycol carbonate).
2- manufacturing of Thermosetting Video CR39 type Polycarbonates