Author: Dr Anitha Arvind
This article will help you to know about the different type of spectacle lenses, lens materials and the types of coatings available in the market to help you make an informed decision about your next spectacles purchase.
Figure 1: Choosing the right spectacle is crucial for everyone!
Choosing the proper spectacle for one’s prescription gets overwhelming due to the vast options of spectacle lenses, frames, tints, and coatings available in the market. It is easy to get confused to decide. Advancements in this field lead to newer technology in lens materials and enhancements now and then.
Similarly choosing the correct spectacle lens and material is crucial as it impacts the corrective power and the lifestyle of the wearer. This write-up serves as a guide to help you decide the suitable lens material and coating based on your needs.
Why is choosing the proper spectacle glasses important?
People who purchase prescription eyewear spend a lot of time choosing the spectacle frame and not on the lenses and coatings, which are more important.
Most people are unaware that just like a wide variety of materials and designs available in a spectacle frame, a wide variety of spectacle lens materials, designs, tints, and coatings are also available.
A spectacle frame contributes towards the comfort and appearance of the wearer. The spectacle lens contributes to the vision, safety, comfort, and appearance. The lens material, design, tints, and coatings can be customised based on the wearer’s lifestyle requirements; hence, one must discuss their lifestyle requirements with the optician while choosing their eyewear.
Types of spectacle lenses
The spectacle lens choice depends on the visual needs of the wearer. Spectacle lenses may be classified as –
Single vision lenses:
These are corrective lenses either for distance or near, that correct farsightedness (hyperopia), nearsightedness (myopia) and or cylindrical correction (astigmatism). A single vision distance lens is meant to be worn at all times, while a single vision near lens or often called ‘readers’ are intended to be worn only during near activities. These are worn by people above the age of 40 for correcting their near vision, also known as presbyopia.
These are corrective lenses incorporating prescription for two different powers like – distance and near, near and intermediate, or intermediate and distance. We need to see at least for distance, intermediate and near distances. However, the limitation of bifocal lenses is that they can incorporate correction of only two distances out of three. The most commonly made bifocal is distance and near where there is no correcting power for intermediate distance.
The larger part of the lens is used for distance vision, while the smaller segment is used for near vision. The bifocal lenses have a visible line differentiating the
distance and near segment. Bifocal lenses come with different types of shaped segments such as round segment bifocals, D-shaped bifocals, straight-line bifocals etc. Each shaped bifocal has its own pros and cons, so it is advisable to discuss one’s visual requirements with the optician who will help and guide the wearer in choosing the correct option.
These are corrective lenses incorporating correction for distance and near with gradual change in power from distance to near. As a result, the vision for all distances, including the intermediate, is clear. They do not have the demarcation line or segment, differentiating the various areas of use, hence better cosmetically.
The progressive lenses appear like a single vision lens. They are meant for those requiring a prescription for distance, intermediate and near. Many brands and designs of progressive lenses are available based on the wearer’s visual needs and lifestyle requirements.
Figure 2: Progressive and single vision glasses virtually look the same!
Spectacle lens materials
In addition to providing vision correction, the spectacle lenses also serve the purpose of shielding the eyes from the harmful radiations of the sun. Besides providing vision correction, the material chosen should provide good clarity of vision, eye protection, and durability. Advancements in ophthalmic lenses have led to newer materials and designs having their distinct advantages.
Figure 3: Types of spectacle lenses
Glass material was used to make all spectacle lenses earlier as it has superior optical qualities, but now it’s obsolete since it is prone to breakage and can cause injuries to the eye. Also, glass materials are heavier than plastic and not comfortable to the wearer.
Plastic (or polymer) lenses account for the vast majority of prescription eyewear. The major advantages of plastics over glass are that they are lightweight, impact-resistant, and the availability of high refractive index lens designs that comparable lens optics to glass make them thinner with other properties.
Figure 4: Plastic lenses – lightweight and impact-resistant
Besides, they also can block UV rays, which is not the case with glass lenses. The most common plastic material for spectacle eyewear is CR-39 (Columbia resin batch 39) which is suitable for low to medium prescriptions.
Polycarbonate is a thermoplastic material that is thin and highly impact-resistant. This property makes it the material of choice, especially in children who are tough on lenses and those in sporting activities.
The material also provides 100% protection against UV rays. This material offers thinner lens designs in comparison to CR-39 material. However, the downside of this material is that it is a soft material and prone to scratches affecting its durability despite being highly impact-resistant. Scratch-resistant coatings protect the surface and offer increased durability.
High index plastics are thinner and lighter plastic materials. They are the material of choice for high prescriptions. Besides being thinner than CR-39 and polycarbonate material, they also have inherent UV protection ability. Every lens is specified in terms of the index of refraction.
Standard plastic lenses have an index of 1.5. The higher the index of refraction, the thinner the lens will be. Plastics lenses with an index of 1.74 are the highest available in the market.
1.61 MR8 is an ideal lens material for low to moderate powers as it offers excellent desirable lens properties.
Trivex is a relatively low refractive index material but with excellent properties and is unbreakable. It is an ideal choice for children and factory workers or where safety is a concern.
Several coatings are available that enhance the performance of the spectacle lens based on the wearer’s environment and lifestyle requirements. Each coating comes with its distinct advantages and is an add-on to the existing spectacle lens, hence adding to the lens’s cost but providing excellent benefits. A few of the standard coatings or lens enhancements available are listed below:
- Scratch Resistant Coating (SRC) should be used on all plastic lenses as they are a soft material and prone to scratches. This increases their durability.
Figure 5: Anti-reflection coating (ARC) cuts down unwanted reflections
- Anti Reflection Coating (ARC) cuts down the annoying reflections arising out of the lenses, thus improving vision, clarity and comfort to the wearer. It is recommended for all high refractive index lenses. Also, these coatings enhance the appearance of the lenses on the wearer and prevent the annoying light reflections arising from the lens during photography.
- UV protection is required in most cases as UV radiations cause severe eye damage due to long term exposure to the sun. For few exceptional circumstances wherein UV exposure is beneficial, the ECP may advise wearing non-UV protected lenses. It is best to understand from the option the range of UV protection and opt for the required one.
- Blue light protection is a popular coating. It has increased demands from the spectacle-wearing population with extensive digital device usage. Digital screens emanate a lot of blue light, which may be harmful to the eyes in the long run. The blue light consists of two components, the healthy blue-turquoise and the harmful blue-violet, high energy visible light. Extensive digital screens use leads to exposure to the harmful blue rays causing a range of problems from discomfort and difficulty in focusing to disruption of the sleep cycle. The blue light protection coating protects the eye by filtering the harmful blue rays and promoting visual comfort. A word of caution here – wear only the best quality branded lenses as cheaper lenses could be more damaging.
- Photochromic lenses change the tint based on the amount of light the wearer is exposed to. They get darker in outdoor environments and clear in indoor environments. They are popularly known as photochromic lenses and offer visual comfort to the wearer, especially those switching between indoor and outdoor environments. Photochromic lenses are available in several tints to choose from, the popular being grey and green. Various generations are available, and the optician can help to select the right lens.
- Water-repellant coating prevents water due to rain or condensation to stay on the lens surface, allowing them to slide over the lens and not block vision and not cause discomfort to the wearer.
Figure 6: Photochromic lenses darken when exposed to sunlight (UV radiation)
- Smudge-resistant coating helps prevent smudges and fingerprints from staying on the lenses, thus keeping the surface of the lens cleaner for long.
- Anti-static coating reduces static electricity on the lens surface, thus attracting less dust than non-anti-static lenses. This helps to reduce scratching of the lenses.
- Anti-fog coating prevents the spectacle lenses from getting foggy due to face mask wear or shifting between hot and cold environments. This coating is recommended for all spectacle wearers who are troubled with foggy lenses.
To conclude, vision is a critical part of everyday life, and wearing the right pair of spectacle lenses optimises vision and ensures that the wearer looks, feels, and sees good. An optometrist and optician would be the best guide to help choose the proper spectacle based on the requirement.