MATERIALS OF OPHTHALMIC LENS

Materials of Ophthalmic Lens

    All materials used during manufacturing  of basic Ophthalmic Lenses. The end  result is specific lens geometry with specific topographical features. 

    All ophthalmic lenses currently available in Ophthalmic industry are 

    I. Glass Material

    II. Resin Material

I.GLASS MATERIAL

    Raw glasses for spectacle lenses possess all necessary optical features, but it is not delivered in form of ready-to-use lenses.

    They are designed to day-to-day use under all conditions.

    It demands certain mechanical and chemical properties for comfort, safety and long-lasting usage.

    And also, it calls for special manufacturing techniques which makes not only qualities demanded by spectacle lenses and for economical production.

    The components used for glasses are vitrifiable mixture which is put into melting tank in the form of extremely pure raw materials to obtain perfect quality glass.

    The basic components such as silica and alumina are introduced in respective forms of iron-free sand and feldspar.

    The alkaline oxides (sodium or potassium) are generally added in form of carbonates or nitrates. Lime and magnesia are sometimes replaced by limestone and dolomite.

    These basic components are accompanied by numerous other ingredients such as rare earths. The role of different components is:

1. Oxides

2. Refiners

3. Colorants

1.Oxides

Oxides used in lens compositions into 2 major categories:

1. Network Formative Oxides

2. Modifier Oxides

a.Network Formative Oxides

    Network formative oxides are used at the base of any composition which could almost form a lens on their own.

For Eg:

        1. SiO2

2. B2O3

3. P2O5

b.Modifiers Oxide

    It is used to modify basic properties contributed by formative oxides. These may range from viscosity to electric properties or from chemical resistance to coefficient of expansion.

Eg. K2O, Na2O, BaO.

    According to composition, some oxides behave either as formatives or modifiers. They are called intermediary oxides. 

Eg. AI2O3, ZnO, TiO2.

Refiners

    Refiners have basic function of eliminating gaseous inclusions (bubbles) in molten glass.

Eg. Antimony Oxide, Alkaline Nitrates.

Colorants

    Complete absence of colorants is essential to obtain clear white glass. When added deliberately, it leads to selective absorption of each wavelength in spectrum and so determine the tints.

Eg: Cobalt oxide gives rise to blue, Nickel Oxides are brown.

The most common glasses currently popular in Optical industry :

1. Crown

2. Photo-Chromic Glasses

3. High-Index Glasses

1.Crown

    Crown is most commonly used glasses for spectacle lenses. It belongs to the family of window glasses. It differs essentially through the choice of raw materials and certain additions of ingredients to adjust optical and physical properties.

    Their composition is characterized by high silica, lime and sodium. The extra white crown is very pure silica-based glass which is extremely transparent. Some crown glasses through addition of metal oxides like nickel and cobalt show specific tints and adsorbent properties.

2.Photo Chromic Glasses

    Owing to the need for glass to have special properties of reaction to certain light radiation, the composition is more elaborate.

The vitrifiable structure is extremely stable, ie, a borosilicate with high boron content to which is added series of elements introduced to develop microcrystals of silver halide which are responsible for photochromic phenomenon.

3.High-Index Glasses

    Lead oxide has been traditionally used in glass composition to produce high index glasses. This produced flint glass.

    Today, for high refractive index glass lead oxide is replaced by titanium oxides, thus preserving high -index while reducing glass density.Certain other elements like niobium, zirconium and strontium are also included for adjusting optical properties. To obtain improved range of segment glasses for fused multifocals, lead oxide is replaced by barium oxide.

II.RESIN MATERIAL

    Resin lenses are made up of small molecular units called monomers which link together to form long chain known as polymers and the process of linking monomers together known as polymerization. There are 2 processes to make resin lenses:

1. Thermosetting - Eg. CR39

2. Thermoplastic  - Eg. Polycarbonate

1.THERMOSETTING

    Thermosetting are crossed linked molecular structure, resembles a ladder with extra rungs.

    They do not melt or flow when heat is applied and makes the material less flexible. Therefore, allows for superior optical processing.

    CR39 is the good example. Most hi-index resin lenses are made by thermosetting process. The monomer liquid is poured into a mould where chemical process of polymerization takes place to form a solid material.

2.THERMOPLASTIC

    Thermoplastic are not cross-linked molecular structure. Molecular chains are independent of each other, looks like a ladder without rungs.

    Optical, they are not so stable during process. They soften under heat and therefore, good for injection molding process.

    They are very sensitive to abrasion. Polycarbonates are good example for thermoplastic. Following resin materials are popularly available in ophthalmic lens industry.

a.CR39

    CR39 is thermosetting resin, ie., in its basic form it is liquid monomer which is hardened by polymerization under the effect of heat and catalyst.

    Diethylene glycol bis, better known as CR39. It is the material most widely used to manufacture currently marketed plastic lens.

    It was discovered during forties by chemists from “Columbia Corporation” hence its name Columbia Resin #39. 

    The refractive index of CR39 is 1.498, which is close to standard crown glasses, specific gravity is 1.32 gram/cm3 and abbe value is 58, ensuring low chromatism.

    The material is highly impact resistant and carries high transparency.

    Multiple tinting and coating is possible. Only drawback is weak resistant to abrasion.

b.POLYCARBONATE

    Polycarbonate is relatively old material, but its use in    ophthalmic optics has grown over past few years.

    It is linear polymer thermoplastic with an amorphous structure. It is atleast ten times more impact resistant than CR39. 

    A high refractive index of 1.586 makes it comparatively thinner. Specific gravity of 1.20g/cm3 makes it very light in its weight.

    The material has efficient protection against ultra violet rays. Abbe value is 30, which is significantly low for good optical performance.

    The material is very soft in nature and hence prone to scratches. 

    Tinting is relatively difficult with polycarbonate lens material which is usually obtained by impregnating a tintable scratch-resistant coating.

c.TRIVEX

    Trivex is partially a cross-linked material and uses the best attributes of thermoplastic and thermosetting processes. 

    The result is superior impact resistance and superior optical performance. 

    The refractive index is 1.53 with a specific gravity of 1.11 gm/cm3 makes the lens thin and light. 

    Probably, this is only lens which is best for rimless frame. Abbe value is 45 and the material having 100% UV protection property. It can be easily tinted and coated with AR coating.