Plastic CR-39

Plastic lenses gained popularity during the '70's due to the fashion trend for large-sized frames and the availability of additional lens styles. Plastic lenses are half the weight of glass lenses, however the lens surface is softer and easier to scratch. Scratch-Resistant (S/R) coatings provide a harder surface and should be recommended to the patient. Like glass lenses, plastic lenses do not provide UV protection and must be treated to provide such protection.

Positive points

1. Half the weight of glass
2. Uniform density in tinting
3. Available in a variety of styles

Negative points

1. Older lens material
2. Not good for drilled rimless

Glass

For many years glass was the material most widely used for eyewear lenses, but in the 1970's other materials started capturing large sections of the lens market. Glass still provides excellent optical quality and the most scratch-resistant surface. But glass is twice the weight of comparable plastic materials and is easy to break. Glass lenses must be treated to comply with ANSI (American National Standards Institute) impact resistance standards.

Glass lenses can be coated in color tints or they can be mirror-coated. Tinted glass lenses are produced by adding metal oxides during the manufacturing process.

Positive points

1. Superior scratch resistance
2. Excellent optics
3. No ledge for the multifocal segments

Negative points

1. Heaviest material

Mid-Index Plastics

Mid-index plastic lenses have a higher index of refraction than regular glass or plastic lenses but a lower index of refraction than High Index materials. This means mid-index materials are thinner than regular glass or plastic but not as thin as a high-index materials. Some lens styles that are currently available only in mid-index plastics may expand into the high-index plastics in the future. They can combine mid-index thinness and aspheric design for a flatter profile and provide superior impact resistance.

Positive points

1. Thinner than regular plastic or glass
2. Most mid-index materials provide UV protection

Negative points

1. Not as thin as high-index materials.
2. Not available in all lens styles
3. Some mid-index not good for drilled rimless

High Index Polycarbonate

During the 1980's, polycarbonate became a popular choice for ophthalmic lenses due to it's light weight, thinness, superior impact resistance and UV protection. As the most impact-resistant material for lenses, polycarbonate should be recommended for children, young adults, and others with active lifestyles, and for safety eyewear. Polycarbonate lenses can be up to %30 thinner than regular glass or plastic lenses and provide protection from harmful UV rays by filtering the radiation in the UVA and UVB areas.

Scratch-resistant coatings can be applied to protect the surfaces.

Positive points

1. Superior impact resistance
2. Blocks UV radiation
3. Thinner lens
4. Lighter in weight
5. Hard coating for surface protection
6. Suitable for drilled rimless       

Negative points

1. Can be hard to tint
2. Viewing away from center can be difficult in higher powers

High/Super High Index Plastics

High index plastics have a higher index of refraction. The benefit to the patient is a thinner, lighter lens compared to lower index materials. High index materials are available in index of 1.60. Polycarbonate material is also considered a high index material with an index of 1.586. High index plastics provide a maximum benefit in reduction of thickness and weight, especially for the high-power prescriptions. The materials with indices above 1.60 provide further reduction in thickness due to the higher index of refraction and lens design and are considered to be Super High Index Materials. Your OLA laboratory can help you determine the best choice of high index plastic to meet your patients' needs.

Positive points

1. Thinnest lens material
2. UV protection
3. Suitable for drilled rimless

Negative points

1. Not available in all lens styles

High Index Glass

Glass lenses are available in some styles in high index material. These lenses provide a thinner lens than comparable regular glass but are much heavier due to the higher density of the material. As an example, regular glass has a density of 2.54 g/cc compared to 1.80 high index glass with a density of 3.37 g/cc. As the index of refraction increases, the density or weight increases, resulting in a heavier lens for the patient.

Positive points

1. thinner than regular glass

Negative points

1. Heavier than regular glass

Check with us for lens availability and Rx restrictions.