For Spectacles Users
Types of Lenses
Single vision lenses: These are used to correct common vision defects such as myopia, hypermetropia and astigmatism.
Reading glasses: People with presbyopia, or nearsightedness associated with age, are prescribed these. They correct near vision but objects at a distance may be blurred. Some users of reading glasses may use two pairs of spectacles. One for reading and the other for seeing distant objects.
Bifocals: These lenses combine both near and distant requirements. There is an obvious line in the middle that shows where the two lenses blend. They are quite thick and indicate the age of the wearer. Until recently, however users of spectacles had to be satisfied with these as the best solution to presbyopia.
Progressive Lenses: Like the bifocals, progressive lenses combine both near vision and distant vision requirements. But unlike bifocals they are divided into three zones: near, distant and intermediate and this division is seamless with the power computed for each zone of the lens and the transitional areas as well. Of late these lenses are available in compact sizes, enabling users to have smaller, smarter spectacles frames.
For 600 years the traditional lens material has been glass. Today we have a range of materials to choose from:
- Poly carbonates
- High Index plastics
Glass: The earliest documented use of spectacles has been in the 13th century in Italy. Since then to the mid-twentieth century glass has been the material from which lenses have been made. The properties that make glass a good material for making lenses are:
- It has excellent optical qualities
- It is scratch resistant
The disadvantages of using glass for spectacles are:
- It is heavy
- It shatters easily
- It gives the soda bottle look and is not
preferred for aesthetic reasons.
Plastic: Conventional plastic lenses, first introduced in the 1960s, seemed to have many advantages over glass lenses. But their usefulness has been challenged by the arrival of (polyurethane) high index plastic.
Advantages of the traditional plastic lenses are:
- They are lighter than glass
- They do not shatter like glass
The disadvantages, however, are:
- They are as thick as glass lenses
- They are not scratch resistant
Polycarbonate lenses: Polycarbonate material was made by the American aerospace industry for use in helmet visors and these were worn by American astronauts. This material is also used in bulletproof windows and in aeroplane windows. This is about 10 times as impact resistant as normal plastic. Lenses made from this material were the early high index lenses.
Advantages of polycarbonate lenses:
- They have a high impact resistance
- They are thinner and lighter than ordinary plastic
- They have built-in u-v filters.
The disadvantages are:
- Their optical quality does not match the new high index lenses
- They have more distortion than ordinary plastic
These lenses are advised for kids, sportsmen and those with eye injuries who may need strong shatter proof glass.
High index lenses: These can be made from glass or plastic. They are thinner and lighter than conventional glass or plastic lenses. They can be made for medium or strong prescriptions.
The advantages of high index lenses are:
- They are thinner and lighter than conventional plastic or glass
- They can fit into neat and fashionable frames
- They have a higher scratch resistance than
normal plastic and higher impact resistance than glass.
The disadvantages are:
- They are expensive
- They are not as scratch resistant as glass.
Only in the last century has attention been drawn on the fact that spectacles can do more than just correcting vision. Various coatings are used to avoid unwanted reflections, glare and excess sunlight.
Here are some of the coatings for spectacles that are very effective and increase vision comfort.
Anti-reflective coatings: Also called multiple coatings, these give clear vision by cutting out unwanted reflections. They also improve appearance by letting people see more of your eye rather than reflection bouncing off from your glasses.
U-V coatings: U-v coatings protect you from a portion of the u-v radiation that can damage your eyes. It would be advisable to buy sunglasses that also have a u-v coating.
Scratch resistant coatings: The main drawback with plastic lenses seems to be their low scratch resistance. The durability of plastic lenses can be increased with scratch resistant coatings, preferably on both sides of the lens.
Photochromatic lenses: Apart from the many disadvantages spectacles wearers suffered in the past, one was that they could never wear sun glasses.
Ever since the arrival of photochromatic, or light sensitive glasses, the possibility of power glasses also serving as sun glasses opened up. Todays photochromatic lenses can darken in bright light and automatically lighten when the user is indoors. Many cosmetic tints are also available.
Tips on choosing the kind of lenses suitable to your lifestyle
Computer users: Looking at a computer screen eight hours a day can cause visual stress. It can cause headaches, blurred vision and sore eyes. Choose a glass with an antireflective coating.
Motorcycle riders and people who take up sports: Squinting in bright light can be avoided by use of photochromatic lenses. Driving at night is especially demanding since you have to prepare yourself for ghost images, reflections and glaring headlights from the opposite lane.
The obvious choice should be plastic lenses that are shatter proof,with an anti reflective coating. Polycarbonate lenses are ideal.
For Children: You cant stop a child from being playful, even though he is prescribed glasses. Children will have their quota of games, fights, rough and tumble activity each day. It is best to give children spectacles with polycarbonate lenses with scratchproof coatings on each side. Choose durable frames.
Five rules for spectacles users
- Do not leave your glasses in such a position where the lenses come into contact with a rough surface. The glasses must rest on the frame and not on the lens.
- Do not leave your spectacles on hot surfaces like kitchen counters or car dashboards.
- Use a stiff case or spectacles box rather than a pouch. This must be remembered by those who carry their spectacles in their handbags. When there is some pressure on the bag the soft case might not protect the glasses from breakage.
- Clean the lenses with a soft cloth regularly.
- Keep spectacles dust free and free of scratches
as much as possible.