Ophthalmic lens materials and Refractive Indexes

Major Ophthalmic lens materials

Glass

Glass has historically been the material of choice for ophthalmic lenses. Glass is most stable, scratch-resistant, and provides the best optical quality of all lens materials. However, since glass is more brittle than most materials, lenses made of the glass must be tempered or heat-treated to give them more strength and make them safer to wear. Glass is available in a number of indices of refraction. As mentioned above, the higher the index, the thinner a lens will be for a given power. However, the specific gravity also increases dramatically with the index making high-index glass lenses much heavier. Chromatic aberration is also more pronounced in the high-index glass.

Refractive Index: Crown 1.52; High index 1.60, 1.70, 1.80

Pros of Glass

• Superior optics
• Stable material
• Scratch-resistant

Cons

• Does not accept tint
• Not impact resistant
• Heavy
• High chance to break

CR-39 Lens

Developed by PPG during WWII, CR-39, also known as plastic or hard resin, serves as a much lighter lens material (approximately 50% lighter) than glass. CR-39, however, is far less scratch resistant and often must be coated to improve it’s scratch resistant characteristics.

Refractive Index: 1.498

 

Pros

• Lighter than glass
• Readily tintable
• Less likely to fog

Cons

• Susceptible to scratching (correctable by coating)
• Lower index of refraction makes it less suitable for higher powered prescriptions

 

Polycarbonate Lens

While its optical characteristics are less than ideal, polycarbonate, the same material used for bullet-proof glass, is the most impact-resistant of lens materials. Consequently, polycarbonate is the material of choice for safety and children’s eyewear. With an index of 1.59, polycarbonate also produces thinner, lighter lenses than glass or plastic. These factors along with polycarbonate’s inherent UV protection and pricing make it a popular material.

Refractive index: 1.59

 

Pros

• Thinner and lighter than glass and plastic
• Highly impact resistant (used for safety glasses)
• Inherent UV protection

 

Cons

• Poor optical quality
• Susceptible to scratching (correctable by coating)
• Susceptible to stress fractures in drill mounts
• Does not readily accept tint.

 

Hi-Index Lens

High index lenses polymers typically refer to products with an index higher than 1.58. High index lenses require flatter curves than their lower index counterparts, resulting in thinner and lighter lenses. Furthermore, aspheric curves come standard in many high index products, particularly 1.66 and 1.70 products, and are available in 1.60. Asphericity reduces spatial distortion, reduces magnification or minification, and further helps maintain a thin and flat lens profile. High index material, however, tends to have a lower Abbe value which could potentially affect patients who are sensitive to chromatic aberration. Also higher index and flatter curves tend to result in more backside and inner-surface reflections. AR coatings are usually recommended for high index lenses to eliminate these reflections.

Index: 1.60, 1.66, 1.70

 

Pros

• Thinner and lighter than glass and plastic.
• Better optical quality than polycarbonate.

Cons

• Susceptible to scratching (correctable by coating)
• Susceptible to backside and inner-surface reflections (correctable with AR)

 

Trivex Lens

Developed in 2001 by PPG, Trivex combines the impact resistance of polycarbonate, exceptional optical clarity, and a specific gravity of 1.11 (the lightest available). Trivex’s tensile strength makes it ideal for drill mount frames. Trivex is available from Younger as Trilogy and from Hoya as Phoenix. All Trilogy products are aspheric and guaranteed for life against stress fractures and drill mount cracking. Some Phoenix products are now available with aspheric curves.

Index: 1:53

 

Pros

• Impact resistance of polycarbonate
• Better optical quality than polycarbonate
• Tintable
• Lightest material on the market
• Inherent UV protection
• High tensile strength (ideal for drill mounts)

Cons

• Susceptible to scratching (correctable by coating)

 

Lens Material Properties
Material	Index	Specific Gravity (g/cm3)	Abbe	Reflectance	Transmittance UVA (286 – 320 nm)	Transmittance UVB (320 – 380 nm)
CR-39	1.50	1.32	58	4.0	10.3	0.0
Poly	1.58	1.21	29	5.2	0.0	0.0
1.60 (MR6)	1.60	1.22	42	5.3	0.0	0.0
1.66 (MR7)	1.66	1.35	32	6.2	0.0	0.0
Trivex	1.53	1.11	46	4.4	0.0	0.0
Crown Glass	1.52	2.54	59	4.3	84.3	30.5
1.60 Glass	1.60	2.60	42	5.3	39.1	0.1
1.70 Glass	1.71	3.20	35	6.7	24.6	0.0
1.80 Glass	1.81	3.66	25	8.2	19.5	0.0