How to Choose the best lenses for your Eyeglasses(spectacles)

 

Summary of eyeglass lenses

Your eyesight, comfort, and safety while wearing eyeglasses are greatly impacted by the type of lenses in your frames. It's also difficult to find the optimum lenses for your glasses. Eyeglass lens materials and designs come in a wide variety, and each one has unique qualities, advantages, and prices.

 

Your choice of eyewear lenses affects four things:

 Vision

 Comfort

 Appearance

 Safety

You may purchase eyeglass lenses more carefully if you use the following information. Astigmatism, nearsightedness, and farsightedness are all addressed with prescription eyeglass lenses. If you have presbyopia, it will also assist you in selecting the finest multifocal or progressive lenses.

 

Materials for eyewear lenses:-

Glass Lenses-

 All eyeglass lenses were made of glass in the early days of vision correction. Glass lenses provide excellent optics. However, they are heavy and brittle, which increases the risk of a severe eye injury or possibly the loss of an eye. These factors have caused to a decrease in the usage of glass lenses in eyeglasses.

Plastic lenses-

The 1940s saw the creation of the first plastic eyeglass lenses. These compact glass lens substitutes were known as CR-39 plastic lenses. The weight of plastic eyeglass lenses is approximately half that of glass lenses. They offer superior optical properties and are quite affordable. They are also more resistant to impacts than glass lenses.

Polycarbonate-based lenses-

The 1970s saw the introduction of the first lightweight polycarbonate safety glass lenses. Polycarbonate lenses have increased popularity significantly since then. In comparison to standard plastic eyeglass lenses, they are significantly more impact resistant and lighter. Children's eyeglasses, safety glasses, and sports eyewear all benefit from polycarbonate lenses.

Trivex lenses-

In 2001, Trivex, a brand-new, impact-resistant, lightweight lens material for glasses, was unveiled. A decent substitute for polycarbonate lenses is trivex. In addition to being lightweight, they also have slightly differing optical and impact resistant properties.

High-index plastic lenses-

Several types of high-index plastic lenses have been created over the last 20 years. These lenses are much lighter and thinner than typical plastic lenses. They come with an aspheric lens design and have a higher index of refraction.

Index (refractive index) of Lenses

The refractive index (or index of refraction) of an eyeglass lens material is a measure of how well the material bends light. The speed at which light passes through the material affects its efficiency.

The ratio of the speed of light in a vacuum divided by the speed of light inside a lens material is known as the refractive index of that material. For instance, the CR-39 plastic has an index of refraction of 1.498. This means that compared to a vacuum, light moves through CR-39 plastic at a speed that is around 50% slower.

A material's higher refractive index causes light to pass through it more slowly, which causes more light to be bent or focused. Therefore, less lens material is needed to bend light to the same degree as a lens with a lower refractive index, the higher the refractive index of a lens material.

In other words, a lens made of a material with a high refractive index will be thinner than a lens made of a material with a lower refractive index for a given eyeglass lens power.

The most common types of eyeglass lenses used today have refractive indices range from 1.49 (CR-39 plastic) to 1.74. (a type of high-index plastic). A lens made of CR-39 plastic will be the thickest and a lens made of 1.74 high-index plastic will be the thinnest for lenses with the same prescription power and design.

Aspheric  Lenses

Aspheric lenses  eyeglasses have a thinner,Lighter,Flat and more appealing profile than conventional lenses.

The curvature of aspheric lenses progressively changed from the centre to the edge. When making eyeglass lenses, this makes it possible to utilize flatter curves.

Aspheric lenses don't magnify as much as conventional lenses since they are flatter. The wearer's eyes appear larger and more natural as a result. Aspheric lens designs can enjoy helping the wearer's peripheral vision.

aspheric lenses are used most often in high-index plastic lenses.To optimize  appearance and optical performance,  An aspheric design is optional and chargeable extra for polycarbonate and CR-39 lenses.

Abbe value

The amount of chromatic aberration that a lens produces is determined by the Abbe value of the lens material. This optical flaw results in colourful halo rings surrounding lights.

Chromatic aberration can be observable and irritating when lenses are made of materials with low Abbe values.

The most pronounced chromatic aberration is seen when gazing through the edges of eyeglass lenses. When gazing directly through the lenses' middle optical zone, it is least perceptible.

The Abbe values of different types of eyeglass lenses range from 59 for conventional glass to 30 for plastic (polycarbonate). The possibility that a lens material will produce chromatic aberration increases with decreasing Abbe numbers.

The German physicist Ernst Abbe (1840–1905), who developed this helpful metric of optical quality, is honoured by the name of the Abbe number.