Plastic lenses (known as CR39 lenses in the trade) are now available in most forms and generally can offer a wider range of lens sizes to suit the modern fashion frames. The big advantage of plastic lenses is that of weight. They are approximately half the weight of any equivalent glass lens. This is a big benefit, but the disadvantage is that they are slightly thicker than the equivalent powered glass lens.
To achieve better cosmetic appearance modern lens designs can produce thinner and flatter lenses in the form of aspheric and higher index plastics. Aspheric design lenses are designed to give a flatter curved surface which improves the cosmetic appearance of the lens. Mid and High Index Plastic gives a thinner lens by basically making the lens material denser. so a thinner flatter lens can be made using a high index aspheric plastic lens.
A major disadvantage of plastic lenses is that the plastic material is softer than glass so they are more prone to scratching than glass, however, modern technology has come to the rescue in the form of various types of coatings which can be applied to the lens surfaces to prevent scratching.
The main advantages of plastic are weight, high impact resistance, better size and power ranges available. They can be obtained in all lens forms, i.e. Single Vision, Bifocal, Trifocal, Executive Bifocals and Progressive's.
Polycarbonate is a plastic like resin, which deserves a mention for it's merits.
It is a very strong , lightweight and thin alternative to plastic and glass. Polycarbonate is often used in industrial safety spectacles, because it is nearly shatterproof. In fact Polycarbonate is one of the components of bullet proof glass.
Polycarbonate is the worlds strongest prescription lens, and eliminates any breakage problems completely.
Polycarbonate does have a downside though. A polycarbonate lens has been known to break down if exposed to certain subtances such as petrol, methonol, benzene, toluene, xylene and especially acetone. If the lenses are coated with an anti-scratch or anti-reflection (see below) they should resist these chemicals, but not always.
All the types of Glass lenses are available in CROWN GLASS form, which is the standard glass material used for production of spectacle lenses, Whilst CROWN GLASS lenses are still prescribed by opticians today they have taken a back seat to Plastic lenses.
If your glass lenses are of a fairly high power then they can become quite heavy and thick. If the lenses are for shortsightedness then they will be thicker at the edges of the lens than at the centre, and also the larger the lens is the thicker the edge will be. A point to be remembered when selecting frames.
HIGH INDEX GLASS
However, there are some disadvantages with this material. For example there can be a slight dispersion of colours towards the edge when looking through the lenses. This is however a minor disadvantage when you consider the greater benefits of weight saving and cosmetic appearance of these types of lenses.
These lenses are usually supplied with an anti-reflection coating, which will be explained later.
Another disadvantage is cost, as high index glass lenses are approximately double the cost to the patient of normal glass lenses. However, the cost may be high, but the benefits are high also.
A patient with high minus lenses can now choose a fashionable frame and still have lenses of less weight than was previously available.
Also see Aspheric Lenses
Most types of lenses can be cosmetically improved by the addition of an anti-reflective coating. This is a coating which when applied to the front surface of a lens reduces the reflections and the glassy appearance of the lens.
This coating can be an additional benefit to someone who has two lenses of widely differing powers, as the difference will not be as noticeable. Anti-reflection coating also allows more light to pass through the lenses and this is obviously a major benefit to drivers and people who work under artificial lights.
All lenses coated in this manner look cosmetically far superior to untreated lenses thereby giving that expensive look.
Tints are available on both glass and plastic lenses and are now available in many different colours and shades. They can be solid tints. i.e. uniform in colour throughout the whole area of the lens, or graduated tints. These can also be of different density to cut out more light.
Tints should now always be supplied with an ultra violet protection built into the tint to cut out ultra violet light rays.
Graduated tints are also dispensed but mainly for cosmetic purposes or for drivers. These are tinted darker at the top than the bottom to cut out brighter light from overhead, yet leaving a lighter tint or even a clear area for reading or for viewing the instruments in a car whilst driving.
Many colours are available but the most popular and most sensible for use in England are the grey or brown tints as these tend to distort other colours less when viewed through the lenses. Rose coloured tints may look good but rose coloured rain does not.
These are lenses which go darker when exposed to sunlight. These lenses are available in glass or plastic but a different type of technology is used in the case of plastic lenses.
Photochromic lenses are available in a wide range of forms, i.e. Single Vision, Bifocal, Executive Bifocal, Varifocals or Multifocals.
The Glass reactolight lenses are heavier than the equivalent plastic lenses.
These lenses are available in a fairly wide range of powers but are not recommended for high power lenses, or where a high power cylinder is also shown on a prescription. The reasons they are not recommended in these cases, is that the higher the power the longer they take to darken or lighten. In the case of high cylinders a stripe effect may be created where the area of the lens of least power will darken more than the rest of the lens.
PLASTIC PHOTOCHROMIC or TRANSITIONS reactolight lenses are now widely available in all forms and these offer the advantage of the light weight of plastic lenses.
They are always supplied with a Hard coating to aid the anti-scratch properties needed to protect the lens surfaces. They can also be supplied with Anti-reflective lenses with all of the affore mentioned benefits.