Stephen J Manton B.Sc.(Hons.), F.C.Optom.

PRESCRIPTION LENSES

GLASS LENSES:  Crown glass was the most common material from which ophthalmic lenses were and to a lesser extent still are manufactured. Lenses of high power can be made thinner by using a special high density glass. The appearance of these high density glass lenses is cosmetically more attractive although they may be heavier. Plastic lenses are now more widely supplied.

PLASTIC (HARD RESIN) LENSES:  These materials which have largely replaced glass are much lighter and more impact resistant, making them safer to wear. Their quality is equal to that of glass, however they are less resistant to scratching and so a hard coating is recommended. Plastics lenses are available in most prescriptions and they can be tinted to any colour including photochromic material and they can be coated with a scratch resistant layer. Special high density plastics materials are also available to reduce the thickness and improve the cosmetic appearance.

PHOTOCHROMIC LENSES:  These lenses change in colour as the light changes and are available in glass and plastics materials. They go dark in the sun and pale indoors. The speed at which they darken depends on the intensity of the sunlight. After fading indoors or when the sun is covered by cloud there is a slight residual colour.

ANTI-REFLECTION COATING: Irritating reflections that give lenses that "glassy" look can be considerably reduced by this special coating. For people with strong lenses it improves the look of the spectacles and it also improves the vision by reducing glare. It can be applied to glass or plastic lenses and is recommended for those who drive at night with spectacles or who often look at computer screens.

HARD COATING FOR PLASTIC LENSES:  A special transparent coating can be applied to plastics lenses to reduce the risk of scratching in normal use. The coating is usually applied to the vulnerable front surface of the lens and can be combined with the anti-reflection coating mentioned above as well as with any tint. Severe abrasions will of course scratch both plastics and glass lenses.