Contact lenses

Contact lenses (or contact lens) are a type of optical lens, designed to correct faulty vision such as short sightedness (myopia) and astigmatism. More than 125 million around the world wear contact lenses and there are many different types, colours and designs, e.g. novelty lenses, to choose from.

Many people prefer them to wearing glasses as they are quick and easy to use and convenient. They are particular favoured by sports people and those who have an active social life. Two other important considerations are practicality and aesthetics.

But what are contact lenses and how do they work?

This section discusses in four subsections the general function of contact lenses and the three main roles as follows:

• How do contact lenses work?
• Corrective lenses
• Cosmetic lenses
• Medical lenses

Visit any of these in turn to find out more about the different functions these lenses perform.

But before you look at these sections why not find out more about the structure and brief history of contact lenses. This is discussed below.

What is a contact lens?

A contact lens is a small, plastic disc which is worn over the surface of the eye. This disc is placed over the cornea (clear part of the front of the eye) and helps to correct a visual impairment such as myopia (short sightedness).

What are contact lenses made from?

To start with, all contact lenses are produced from plastic.

But there are variations in this depending upon the type of lens. By this we mean hard lenses, gas permeable lenses and soft lenses.

Hard lenses

These are made from a substance called polymethyl methacrylate or ‘PMMA’for short. This substance is also known as acrylic glass and has a strong, durable quality. All contact lenses were made from this substance up until the early 1970’s.

The main problem with hard lenses is that they do not allow oxygen to reach the eye which is important to keep it healthy and fully functional. Plus many people find hard lenses uncomfortable to wear.

Hard lenses are rarely used nowadays.

Gas permeable lenses

Gas permeable lenses are named as such due to their ability to allow more oxygen to reach the eye compared to hard lenses or soft lenses.

These lenses are produced from a silicone plastic which is more flexible than PMMA and has the added bonus of allowing oxygen to be transmitted to the eye. This improves the health of the eye as well as ensuring the comfort of the lens.

Gas permeable lenses are long lasting, easy to look after and noticeable improve the vision. In many ways they are considered superior to soft contact lenses although it is a matter of personal preference.

These lenses are an ideal choice for people who are unable to wear soft lenses for any number of reasons.

Find out more about this type of lens in our gas permeable lenses section.

Soft lenses

These lenses have a gel-like quality and are the most popular of all the contact lenses. These tiny discs are produced from a water-containing type of plastic which covers the entire iris (coloured part of the eye) as compared to the gas permeable lens.

The daily disposable type of soft lens is a highly popular lens as it can be thrown away after a single use. This lens appeals to people with busy lives or who prefer not to have to clean their lenses on a daily basis.

Soft lenses were introduced in the early part of the 1970’s and largely replaced hard lenses. The latest soft lenses are made from silicon hydrogels which enable a greater amount of oxygen to reach the eye and, reduce any drying out of the lens.

There is discussed in more detail in our soft contact lenses section.

Contact lens design

Contact lenses are designed in such a way to correct faulty vision. They are either spherical (round) or toric (combination of a cylinder and a sphere) and the one which you use will depend upon your visual impairment.

Spherical contact lenses

These are the most common type of contact lens. They are rounded in appearance and both the inner and outer surface resembles a section of a sphere (ball).

If you place a spherical lens on your finger you will notice that it is almost half a sphere in shape and soft to the touch. The soft lenses are just that: they are very soft and almost flimsy which is why you need to take great care when handling them.

Spherical lenses are worn to correct myopia (short sight) and hypermyopia/hyperopia (long sight). They have the same strength throughout which remains the same even if the lens moves around.

Toric contact lenses

This is less well known type of lens but is nevertheless, still as important in terms of vision correction. It is produced from the same material as spherical lenses but differs in terms of the design.

A toric lens has two types of strength or power which is manifested as two curvatures at different angles. One curvature corrects myopia or hypermyopia and the other curvature corrects astigmatism.

Confused? One way to think about this is to imagine the spherical lens as a single curvature lens and the toric lens as a double curvature lens.

Plus a toric lens remains in place on your eye. It does not move around unlike the spherical lens.

There are two other designs which are:

• Bifocal
• Orthokeratology

Bifocal are worn to correct problems with close (near to) and far vision.

Orthokeratology lenses can reshape the cornea without the need for eye surgery.

How long are contact lenses worn for?

In regard to soft lenses: this ranges from daily use through to two weekly and even monthly wear. There are lens which can be worn without a break (and includes overnight) for up to 7 days. And then there are lenses which can be worn for up to a month without needing to be removed.

But there are issues with wearing lenses for a long period of time which includes the risk of eye infections.

The traditional gas permeable lenses can be worn, cleaned and reused for several months at a time.

Basically, gas permeable lenses last longer than soft lenses so do not need to be replaced as often which can be a money saver.

Contact lenses are prone to the build up of dirt, debris and deposits over time which increases the risk of an eye infection. This is why it is important to care for your contact lenses and to replace them whenever possible.

Important factors with contact lenses

There are two main issues with any type of contact lens:

• How well they fit onto your eye and comfort level
• Whether the contact lens corrects your eye impairment, e.g. corrects your myopia (short sight)

Your optician will decide the best and most suitable type of contact lens for you. He/she will use your eye prescription to help him/her decide as well as the anatomy of your eye and your general state of health.