Sunglasses are designed to protect the eyes both from excessive light and from damaging UV light rays.
They were originally made to dim excessive glare from bright sunshine, and early models were just tinted glass, with dark tints thought to be better than light hues as they screened out more light.
Two developments changed this perception; one was our understanding of the nature of light, especially light at the ultraviolet end of the spectrum which led to a revolution in lens manufacturing and the other was the improvements in plastics which brought about a sea change in making the frames.
It is the rapid progress in creating new metals and plastics that helped shape the nature of designer eyewear. Frames could be both light and strong while lenses could be moulded into almost any curvature without loss of visual clarity.
Metals like silver-grey titanium became popular for the lightweight, durable, and corrosion-resistant properties as did titanium-based alloys like Flexon.
Frames made of aluminium and stainless steel have excellent resistance to corrosion, scratching and heat.
But it is development in plastic that made the biggest impact. Multi-coloured creations like zylonite (a cellulose acetate) became the rage while sports performance eyewear preferred the nylon-based acetates.
Most modern sunglasses are composed of a pair of light filtering lenses held in a frame of coloured plastic. Metal frames and glass lenses are now much less common as cellulose acetate and polycarbonate compounds have improved.
The cellulose acetate used to manufacture most sunglasses frames is derived from cotton or wood pulp. The natural cellulose is combined with acetic anhydride to make flakes of cellulose acetate.
The flakes are ground into a fine powder and blended with plasticisers to create granules which can be mixed with coloured pigments and melted down into thermoplastic blocks.
The result is a material that is both flexible and strong. Other useful properties of these thermoplastics include high impact resistance, excellent machinability, hypoallergenic properties and aesthetic appeal – in fact all the qualities needed to manufacture performance eyewear.
There are many different grades of cellulose acetate however and to achieve a consistent polymer of the highest quality requires a combination of technology, experience and quality control.
Some luxury manufacturers now also use cellulose acetate propionate, a nylon enriched plastic that is lightweight, strong and hypoallergenic but also more glossy than other acetates.
Most plastic sunglasses frames are punched out from cellulose acetate sheets using sharp metal dies. The acetate sheets are heated to soften them before the die punches out the design using several tonnes of pressure.
These frame blanks are then cooled, and grooves are cut to hold the lenses. Any rough edges are smoothed and polished with abrasive brushes before nose pads are attached to the frame.
Much the same process is used to manufacture the side arms or temples, and after smoothing, they are connected to the frame with a metal hinge. Some temples are heated before hot core wires of steel are inserted for extra strength.
While cheaper frames usually have hinges glued into place, more expensive sunglasses can be treated with ultrasonic vibration which heats the frame and melts the acetate around the metal hinge.
The completed frames are often polished several times by tumbling them inside a drum of ever finer pumice dust. Frames may also be 'bent' into shape between each polish by being heated slightly and pressed into dies to give the required 'face' curvature.
The temples may undergo similar operations to add a final sheen. A high measure of quality control is vital here, especially for luxury sunglasses brands like Ray-Ban and Oakley. The luxury eyewear industry is highly competitive and quality control is essential.
Metal is still used to make sunglasses frame, particularly the 'aviator' style eyewear made fashionable by Ray-Ban and much emulated by other manufacturers.
Metal or 'wireframe' sunglasses are often manufactured from spools of wire or flat threads with each lens frame shape with thinner spooled brazed onto the main frame to create groves for the lenses.
Metal frames, particularly those in the luxury end of the market, are also often made of alloys of nickel and other metals such as silver.
Titanium and beta-titanium are also popular materials for sunglasses. Titanium is lightweight, durable, flexible and corrosion-resistant and can now be manufactured in a variety of subtle colours.
Sunglasses made from titanium are usually very thin for a clean, stylish look but the metal comes with a hefty price tag. Titanium frames these days often contain an alloy blend to help reduce the price with nickel and copper the favourites, although nickel can irritate some skin types.
Beryllium is another steel-grey metal that is a lower-cost alternative to titanium being lightweight, strong and flexible.
One of the most important qualities of metal sunglasses frame is that they are hypoallergenic, so they do not irritate the skin. They must also not corrode when wet.
Other useful metals used in sunglasses frame manufacture are stainless steel, an alloy of steel and chromium and aluminium. Sunglasses made of aluminium are lightweight and corrosion-free but, like titanium, they come at a premium price. Pure aluminium is weak, so it is blended to silicon and iron to make modern sunglasses.
New materials are always being developed, and carbon fibre is a super-tough, super-lightweight material that has been heavily promoted by Ray-Ban. Carbon fibres are twisted and bonded with resin to produce a durable, but expensive model range.
The 20th century saw dramatic improvements in the manufacture of sunglasses thanks mainly to the rise of plastic-based material. Frames made from thermoplastics made sunglasses cheaper and offered a broader range of styles, colours and designs.
The fashion for wearing sunglasses has kept the industry afloat despite the rise in the use of contact lenses and laser eye surgery. Improved technology has made modern sunglasses more lightweight and durable, and the fashion industry has kept the market buoyant.
It is rare these days to find a fashion house that does not have its range of accessories, and they will inevitably include a range of designer eyewear.
So sunglasses look like they are here to stay and the manufacturing process has grown in sophistication as fashion houses and designer eyewear companies try to outdo each other to catch the fashion market high quality, stylish glasses.
For anyone who is interested, both Ray-Ban and Oakley have online videos on how their designer sunglasses are made. You can view them here.