All About Lenses, Part I - Function and Materials
If you wear glasses, then you know that your glasses are composed of two key items: the frame and the lenses. The frame, in essence, is the structural unit that allows the glasses to rest upon the face. The frame ensures proper fitting, so that an individual can see through corrective lenses, which are housed by the frame.
So what about these lenses? How do they work? What kind of materials are used to make these lenses? And progressive, bifocal, or trifocal; what is the difference?
Well it is time to answer some of your endearing questions.
What are the functions of lenses?
The primary function of a prescription lens is to correct vision. Secondary functions of lenses include UV protection, cutting down glare, and cosmetic changes.
What do lenses do and how do they work?
In order to correct vision, the lens must bend light in order to create a focal point on the retina. The bending of light is a process known as refraction. A lens gains its refractive abilities through curvature, which is ground into the front and back surface of the lens.
- Creating a spherical concave lens (thin in the center and thick on the outer edges) results in a lens with minus power and is used for individuals with nearsightedness/myopia.
- Creating a spherical convex lens (thick in the center and thin on the outer edges) results in a lens with plus power and is used for individuals with farsightedness/hyperopia, as well as presbyopia.
- Creating a lens that is non-uniform in curvature and has two different meridians, results in a lens that is cylindrical in nature, rather than spherical. This type of lens is used to correct astigmatism.
What are lenses made out of?
Traditionally, lenses were made of glass. However, due to the dangers of glass shattering as well as weight and coating considerations, it is rare to find glass lenses these days.
Most current lenses are composed of high tech plastics. Here is a list of the most common types along with their advantages:
- Standard Plastic (CR 39) – a common lens used with a variety of prescriptions, it offers a lighter weight than glass and decent optics, however it is not nearly as resilient as other materials
- High index – a lens used for larger prescriptions; by increasing the index of refraction in the plastic material, this enables the lens to be thinner – thus avoiding extremely thick edges and coke bottle effects often seen when higher prescriptions are made with standard plastic
- Polycarbonate – an impact resistant lens that is used in a variety of prescriptions, including children’s glasses and protective sports wear
- Trivex – thought of as an “upgraded” polycarbonate, this material shares its impact resistant quality with polycarbonate, but in a lighter, thinner, and sometimes optically superior package
- Photochromic – lenses that change from light to dark by activation of UV light from the sun. The common brand name that these lenses are marketed under is Transitions.
Next week, we will explore types of lenses such as single vision and progressive, as well as lens coatings such as anti-reflective. Make sure to tune in.
If you have any other questions about lenses or are looking into getting some new lenses, do not hesitate to contact our office.
-Dr. Aaron Neufeld
Dr. Aaron Neufeld is the Chief Optometrist at Los Altos Optometric Group and primary author and editor of The EYE Digest.
Contact him with questions or ideas for future articles at (650) 948-3700 or firstname.lastname@example.org