Our eye is a complex structure of various minute components each responsible for perfect vision ideally known as 20/20 or 6/6 vision. However, there are various refractive conditions that lead to discrepancies in the vision for which external correction is required. This led to the invention of the spectacle lens.
The idea of eyeglasses can be traced back to 2000 years ago in the time of Seneca a Roman philosopher who said, that the letters could be magnified with the help of a crystal ball. This is because of the simple convergence and divergence principle of light. Basically, light that travels from one medium to another bends to some degree.
Let us understand two basic classifications of ophthalmic lenses.
Convex lens
The property of a convex lens is to converge the light rays traveling through it. It has center thickness but is thinner towards the periphery. In other words, it has outward curvature. A convex lens with curvature on both surfaces is called biconvex and a lens with only one convex surface is called plano-convex.
These types of lenses are typically used for Hyperopic correction where distance vision is clear but near vision is blurred. This is because the lens is less powerful and hence plus power correction is required.
Concave lens
The property of the concave lens is to diverge. These are thinner at the center and gain thickness moving towards the periphery. It has inward curvature on one side known as Plano concave and concave on both surfaces is known as biconcave.
This is typically when the power is the crystalline lens is more and needs minus power correction to focus on the retina, a condition of Myopia.
Types of lenses
Based on your eye condition you need a refractive correction. The design of the ophthalmic lenses comes from the above classification. However, there is a whole variety of lenses to choose from, depending on how much you are willing to spend.
Single vision lenses
This type of lens can have Plano plus (Hyperopia), Plano minus (Myopia), or cylindrical power correction (Astigmatism). It can either be used to correct near vision refractive errors or distance vision refractive errors.
An unusual type of single vision lens is lenticular lenses that are prescribed in case of extremely high power corrections.
Multi vision lenses
These are special lenses that are used to correct both near and distance vision. People after the age of 40 start to lose their ability to view near objects; a condition is known as Presbyopia. Hence a plus power correction is given with the help of multifocal ophthalmic lenses. There are different types commonly found of multi-vision lenses:
- KT Bifocal
- D Bifocal
- Trifocal
- Exe-Bifocal
- Progressive lenses
Aspheric lenses
Aspheric design lenses also known as variable curvature lenses are a step ahead of conventional spherical lenses. However, spherical lenses are more commonly used in eyeglasses.
The aspherical lens is a simple positive or negative meniscus design with thickness more than normal. As a result, the magnification or minification effect is seen of the eye. This type of spectacle lens has a symmetrical surface meaning, the power is the same on the entire lens. Further, as we move from the center towards the periphery the curvature changes and so aberration is seen.
On the other hand, as the Aspheric lens has an asymmetrical surface that changes at every spot on the lens, it gives center vision results from every angle of the lens. The curvature of this lens aligns with that of the Cornea; hence, aspheric lenses are thin and flatter. Here are some benefits of aspheric lenses.
- Better cosmetic appeal
- Decrease in aberration increases field of view
- No prismatic effect
- No aberration
- Optically correct vision at every angle on the lens
- Lightweight & thickness is reduced by 10%. It is especially needed in high power lenses
For dispensing opticians, it is important to take monocular PD when prescribing aspheric lenses. Also, note they fit tight in half-rim eyeglasses.
For eyeglass buyers, note that these are not an economic range of lenses but if you desire superior vision there are no other such advanced lenses at disposal.
Absorptive lenses
As the name suggests, ophthalmic lenses that absorb light are known as absorptive lenses. These are tinted glasses ranging from SP1 to SP10 or B1, B2… etc. For measuring the amount of tint, numbers 1 to 10 are denoted.
In high power tinted lenses more darkness is observed. The purpose of these lenses is to filter light transmission as well as protection from UV light.
Here, let’s rundown on different types of absorptive lenses are:
- Photochromic lenses
- Gradient lenses
- Mirror coat/color coat lenses
- Polarized lenses
While polarized, gradient, and mirror coat lenses aim at eliminating glare; absorptive lenses such as photochromic glasses or photochromic sunglasses reduce the amount of UV transmission due to the presence of Silver Halide Crystal in the lens material. Hence, they are also commonly referred to as UV glasses or UV protection glasses.
There are two types of photochromic lenses: Photogrey and Transition. The difference between the two lies in the process of making. Overall, transition glasses adapt to light more quickly, ensure 100% UV A & B protection, and more that makes them a preferred choice amongst consumers.
Summary
Being aware of the types of lenses available for use can help you make an informed decision for your prescription eyeglasses. Help your optician by sharing your medical history, working requirements, lifestyle, or anything else so that they can guide you with your selection.
