All About Lenses, Part II – Types and Coatings
In last week’s edition of The EYE Digest, we discussed how lenses work through a process called refraction. We also delved into the different materials that lenses can be made from.
Now that we know the basis for ophthalmic lenses, let’s take a look at some of intricacies of lenses that commonly present themselves to us in an optical. When an individual thinks of lenses for glasses, often they think of a type. Is it a bifocal? Or maybe a progressive? And how about anti-reflective coating or tint?
Well, let’s get to the bottom of this.
Types of Lenses
Single Vision Lens – A single vision lens by definition corrects for one refractive error. A single vision lens can be used to correct vision out in the distance, up close, or on a computer. For most individuals under 40 years of age, a single vision lens is all that is needed to ensure adequate vision correction at all distances.
Bifocal Lens – A bifocal lens, in contrast to single vision, corrects for two refractive errors. Most commonly the top section of the bifocal lens corrects for distance and the bottom portion, known as the segment, corrects for near. There is a visible line separating these two sections. Bifocal lenses have clear distinction between their two corrective areas, and thus create a noticeable difference between the two.
Trifocal Lens – A trifocal lens, corrects for three refractive errors. Most commonly the top section of the lens corrects for distance, the middle segment corrects for intermediate (computer), and the bottom segment corrects for near (reading). Visible lines separate the three sections. A trifocal lens is normally used for individuals that have such a large Add power for reading, that a lesser power is needed for intermediate vision.
Progressive Addition Lens (aka Progressive or PAL) – a lens with multiple points of correction without visible line separation. Many refer to a progressive lens as a “blended” bifocal, the lens allows seamless translation from distance vision to intermediate vision to near vision, rather than a jumping transition from distance to near. However creating a lens like this does involve the drawback of having unusable space on the peripheral edges of the lens. Thus a corridor is created where distance transfer to intermediate and intermediate transfers to near.
So now we know the different types of lenses and understand what the purpose of each is, let us take a look at the different types of coatings that we can place on lenses.
Types of Lens Coatings
Scratch Resistant Coating – Just as the name implies, Scratch Resistant Coating is a silicon resin coating that helps a lens be more resilient when it comes to scratching. The scratch resistant coating does not allow a lens to become scratch-proof, however it gives it an added level of protection.
Anti-Reflective Coating – Anti-Reflective Coating is a coating made of magnesium fluoride that can be placed on both sides of a lens. It is useful for reducing low angle glare, such as that comes off of car headlights. Additionally, it can help with eyestrain caused by light emitted from computer screens.
Tint – Tint refers to any color that may be placed on the lens. The intensity of the chose color is normally measured with a percentage. There are two major types of tints:
- Solid tint – provides a uniform tint on the entirety of the lens
- Gradient tint – provides a gradual tint with darkest part of the lens normally being the top
Mirror Coating – Mirror Coating is a coating that is applied to the front of the lens and enables a mirror look so that the front of the lens casts reflections.
And there you have it, a quick and complete guide to lens types and coatings. If you have any questions about lenses and coatings, or wish to find new lenses and glasses to wear; do not hesitate to contact us!
-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