Years ago, all eyeglass lenses were made of glass.  Glass lenses have excellent optical quality but they are thick and heavy.  Plastic lenses - a more comfortable, lightweight alternative - were introduced in the 1940s.  Plastic lenses offer comparable optical performance to glass lenses and are about half the weight. 

Plastic remains the most popular lens material for eyeglasses in the United States, accounting for over 50 percent of all lenses sold.  But though plastic lenses are lighter than glass lenses, they too can be thick - especially in moderate and strong prescription powers.

In recent years, many thinner, lighter lens materials have been developed for eyeglasses. These are called high index lens materials because they have a higher index of refraction than regular glass or plastic lenses (see below).  Lenses made with high index materials are significantly thinner and more attractive than regular glass or plastic lenses of the same prescription power. 

High index lenses are available in both glass and plastic materials: 

• High index glass lenses are thin, but they are much heavier and can shatter more easily than high index plastic lenses. 

• High index plastic lenses are thinner than regular plastic lenses and significantly lighter and more impact-resistant than high index glass lenses.  For these reasons, high index plastic lenses are the fastest growing segment of the U.S. eyeglass lens market.

Index of Refraction and Lens Thickness

The index of refraction (also called the refractive index) is a measure of how efficiently a lens material can bend light.  A lens made of a material with a high index of refraction can bend light more efficiently than a lens made of a material with a lower index of refraction.  Because of their superior lend-bending qualities, high index lenses can be made thinner than lenses made of regular glass or plastic, which have a lower refractive index. 

The index of refraction of eyeglass lens materials currently sold in the United States ranges from 1.50 (regular plastic) to 1.74 (one variety of high index plastic). 

Most high index plastic lenses also have an aspheric design.  Aspheric lenses have flatter curves than traditional glass or plastic lenses, giving them a slimmer, more attractive profile.

Though they are significantly thinner and lighter than regular glass or plastic lenses, high index lenses have more surface reflections.  To enjoy the best optical performance, visual comfort and appearance, anti-reflective (AR) coating should be applied to all high index lenses.

The following table shows some of the more popular high index lenses currently sold in the United States and compares their thickness to regular plastic lenses.  Thickness values are based on the overall front-to-back thickness of single vision lenses in an average-size frame.

Lens Materials and Relative Thickness

Comparative Features/Benefits of Lens Materials

Each lens material has a unique set of features and benefits.  Here are the basics:

High index plastics  

• The best choice if you want the thinnest, most attractive lenses possible.

• Range from 20% to 65% thinner than plastic lenses (depending on refractive index).

• The higher the refractive index, the thinner the lens (and the higher the cost).

• Most high index plastic lenses provide 100% protection from the sun's UV rays.

• Anti-reflective (AR) coating is essential for the best optical performance and appearance.

High index glass

• Over twice the weight of high index plastic lenses of the same thickness.

• Some high index glass lenses do not meet the minimum impact resistance standard required by the U.S. Food and Drug Administration (FDA).

• Excellent scratch resistance. 

• Less than 2 percent of eyeglass lenses sold in the U.S. are made of high index glass.    

Polycarbonate

• The best choice if you want the safest and/or lightest lenses possible.

• Over ten times more impact resistant than regular plastic lenses.

• Used in safety eyewear and sports goggles as well as general purpose eyewear.

• Recommended for children's eyewear and whenever eye safety is a primary concern.

• Up to 40% thinner and 30% lighter than regular plastic lenses.

• Provide 100% protection from the sun's ultraviolet (UV) rays.

• The second most popular lens material in the United States, accounting for approximately 20 percent of all eyeglass lenses sold.

Plastic

• The best choice if you want the most economical lenses possible.

• Excellent optical clarity (comparable to glass lenses).

• Due to thickness, not recommended for higher prescription powers.

• Can be easily tinted to dark sunglass shades.

• The most popular lens material in the United States, accounting for over 50 percent of all eyeglass lenses sold.

Glass

• The best choice if you want excellent optics and scratch resistance, a relatively low price, and you are willing to tolerate heavier lenses.

• Excellent optical performance, providing the widest field of clear vision.

• The most scratch resistant lens material available.  Glass lenses don't require scratch-resistant coatings for added durability.  (All other lens materials do.)

• More than twice the weight of other lens materials.

• Due to thickness and weight, recommended only for mild prescription powers.

• Declining in popularity, accounting for less than 8 percent of eyeglass lenses sold in the United States.

Seek Expert Advice Before Choosing

A number of additional factors can influence the overall thickness, weight, and optical performance of your eyeglass lenses.  Some of these factors include the center thickness of the lens, the edge thickness of the lens, the lens design, and the size and shape of the frame(s) you select.

For the greatest satisfaction, seek the help and advice of a skilled optician.  Opticians who are certified by the American Board of Opticianry (ABO) have had special training in the characteristics and selection of lens materials.

Keep in mind that to satisfy all of your visual and lifestyle needs, it's usually best to purchase more than one pair of eyeglasses.