Lens Materials Explained

The material used in your glasses or sunglass lenses will affect their clarity, durability weight, and cost. Below are the main advantages and disadvantages of the various lenses available on the market.

 

Glass Lenses

 

Originally, all lenses were made of glass, which while it made them very resistant to scratching, also made them very heavy and prone to breakage. These lenses are now becoming obsolete with the exception of toughened lenses for industrial safety glasses.

Advantages: 

• Exceptional scratch resistance (don't need scratch resistant coating).
• Excellent optical clarity.
• Anti-reflective (AR) coating adheres to glass very well.

Disadvantages:

• At least twice the weight of plastic or polycarbonate lenses.
• About 25 to 40 percent thicker than polycarbonate and high index plastic lenses.
• Can shatter or chip easier than lenses made of other materials.
• Require a special coating to provide 100% UV protection.

 

Plastic Lenses – CR39

 

CR 39® is the most common optical polymer in the ophthalmic field. It refers to the type of plastic that is used to make prescription lenses. The lens comes from pouring the liquid polymer into a mould of optically treated glass. This enables to obtain a lens with the same characteristics of the originating glass. It provides very little distortion (more than glass but less than polycarbonate), it is more lightweight and thinner than glass making it comfortable to wear and it is more impact resistant though it may still break and shatter upon impact.  As such, it is not recommended for active sports.

Advantages:

• Good optical quality.
• Lightweight - About half the weight of glass lenses.
• More shatter resistant than glass lenses.
• Plastic lenses accept tints easily with uniform grade colours.

Disadvantages:

• About 20 to 35 percent thicker than polycarbonate or high index plastic lenses.
• Require scratch-resistant coating for added durability.
• Require a special coating to provide 100% UV protection.

 

 Polycarbonate Lenses

 

Polycarbonate is one of the strongest and safest materials on the market. They have exceptional strength and resilience. They will survive the impact of a steel ball travelling at 160 km/hr without shattering which meets EN/EU grade for Low Energy Impact Resistance (small particle).

Our entry level glazing packages offered for our rimless and semi-rimless glasses are polycarbonate lenses. Due to the nature of these types of frames where the lenses have to be drilled or clipped into the frame, polycarbonate will greatly reduce the possibility of damage.

Advantages:

• Excellent impact-resistance - 10 times more impact resistant than other lenses.
• Thin - About 20 to 25 percent thinner than plastic or glass lenses.
• Lightweight - About 20 percent lighter than plastic lenses.
• Blocks 100 percent UV rays without needing a special coating.

Disadvantages:

• Requires scratch-resistant coating for durability.
• Peripheral vision may be slightly less clear in strong prescription powers.
• More lens reflections than glass or plastic lenses (AR coating recommended).

 

NXT  Lenses

 

NXT is a relatively new material developed for the US military to provide bullet proof windshields for helicopters and vehicles. It is a unique material that combines the advantages of CR39, Mid-Index lenses and polycarbonate, but leaves out all their inherent weaknesses. In fact it is a lens without weakness! It is the best material for mild to moderate prescriptions.

Advantages:

• Superior impact-resistance. Passes all impact resistance standards (ANSI Z-87.1 Impact Resistance Standard and High Speed Impact Test, EN 166) and its impact resistance is higher than polycarbonate.
• Excellent optical clarity even for higher powers.
• Flexible and ultra-lightweight. About 10% lighter than polycarbonate and 16% lighter than CR-39.
• Solvent resistant.
• Blocks 100 percent UV rays without needing a special coating.
• Thin – Ideal for those with lens powers of between -4 to +5.00.
• Not sensitive to stress cracking.

Disadvantages:

• Expensive.

 

 Mid to High Index Plastic Lenses –Thin and Lite and Super Thin and Lite

 

Mid and high index lenses are for people with a high prescription. They allow spectacle lenses to be made thinner, lighter and flatter than previously possible. They are compressed lenses that offer better optical viewing through them. For higher prescriptions they not only reduce the weight by up to 50% but also reduce the thickness by up to 60% making them more attractive cosmetically. In addition to making the glasses look cosmetically better, they will feel lighter on your nose, and the flatter shape of the lens will make things look less distorted towards the edges of your lenses.

Advantages:

• Lightweight - Between plastic and polycarbonate lenses in weight.
• Thin - About 15 to 30 percent thinner than plastic lenses.
• Better peripheral optics than polycarbonate lenses (for prescription sunglasses).

Disadvantages:

• Requires scratch-resistant coating for durability.
• More lens reflections than glass or plastic lenses (AR coating recommended).

 

Aspheric Lenses

Aspheric or Free Form Lenses are available in all lens materials except glass and are a cheaper compromise to mid to high index lenses. They provide a more accurate prescription than standard lenses because they are not made with moulds. Each design starts with a spherical lens which is digitally surfaced to the exact design and prescription creating a personalized lens especially for you.  Digitally surfaced lenses are superior to conventional lenses in every way.  Their lightness, increased visual clarity and cosmetic differences are especially noticeable in higher prescriptions. They virtually eliminate unwanted astigmatism, swim and peripheral distortions by proving maximized fields of vision (20% wider fields of view in all zones).  They offer the highest RX precision (up to 0.01 dioptres) and virtually eliminate peripheral distortions. Recommended if your prescription is between 1 and 3 spheres.