During the past twenty-five years the optical field has seen many new lens types and materials enter the marketplace. These new lenses and materials have provided the eye care professional with many more options to offer the patient eyewear that provides the best vision and is also cosmetically pleasing and comfortable.

Lens Material

Years ago, only one main material, called crown glass, was used for lenses. Since then, advancements in manufacturing and laboratory processing technology have produced many more materials.

The lens material chosen for the prescription will depend on the patient's prescription, lifestyle activities, facial measurements, and the recommendation of the eyewear professional.

Lens materials can be differentiated by their weight, thickness, transmission of radiant energy and optical performance.

Weight

The weight of a lens material is referenced by the term "density." Density is a measurement of weight based on a certain amount of material. The same amount, a cubic centimeter, is measured for each lens material for comparison. The weight is referenced in grams. Looking at Figure 7-1, you can make the comparisons based on density.

Figure 7-1 Density

All lenses are not made up of the same amount of materials. Factors such as lens material, size, shape and facial measurements all contribute to the final weight of the lens. A practical way to compare weight for lens materials is to represent the weight based on power for a determined lens size. An example would be a glass lens for a -2.00 D prescription, 60mm round, and weighing 3.9 grams.

Thickness

As the prescription increases in power so does the thickness for both plus and minus power lenses. Plus lenses will be thicker in the center and minus lenses will be thicker at the edge. The factors mentioned above that affect lens weight would also affect the final thickness of the lens. Proper frame selection for facial features and the correct lens material selection can reduce final lens thickness, making the lens cosmetically pleasing.

As mentioned earlier, lens materials are referenced by their bending power. This unit of measure is called the index of refraction or, simply, index. The index of refraction contributes to creating the lens power. The more bending power or the higher the index of the material, the flatter the front and back curves will be to create the needed power for the prescription. The end result will be a lens that is thinner than a lower index material.

Figure 7-2 shows the index of refraction of some of the different lens materials. Remember, the higher the index of refraction, the thinner the lens.

Figure 7-2 Index of Refraction

Transmission of Radiant Energy

There are three regions of the electromagnetic spectrum of concern with ophthalmic lenses. These regions of radiation are ultraviolet, visible light, and infrared. As radiant energy passes through a lens, these areas may have transmission blocked, reduced, or not affected.

The human eye does not see ultraviolet radiation, but the effects of exposure can be harmful. Transmission of ultraviolet radiation through ophthalmic lenses may be reduced, blocked, or not affected, depending on the lens material. It is important to under-stand how lens materials affect the transmission of ultraviolet radiation. Transmission of visible light can be affected by the color of a lens. Different colors can cause overall reduction or specific reduction in the visible region. The infrared area is generally considered not to be harmful to the eye and will not be covered at this time.

In 1998 the OLA published a compilation of the transmittance of many commonly-used lens materials. This book, Special Transmittance of Lens Materials, was sent to each OLA member lab to help answer questions about UV, visible, and infrared transmittance.

Optical Performance

Lens designers design lenses to provide the best visual performance, cosmetic appearance, and comfort. Proper lens selection, correct facial measurements, and correct frame alignment are all necessary to maintain the design intent. The optical laboratory serves as an important link between the manufacturer and the eyecare professional in providing the latest information on lens performance.

Lens Types

Different lens types are used to correct the simple vision errors we covered in Unit 3. There are also types available to correct vision for occupational and recreational uses. Figure 7-3 shows an occupational lens used for regular near vision and overhead near vision. The lower segment would be used for reading and the upper segment for occupational use such as a mailroom person sorting mail in overhead bins.

Figure 7-3 Double D Occupational Lens