When buying a new pair of glasses you will first decide on a spectacle frame and the type of lenses suited to your needs, under the guidance of your optometrist. You will also be offered various coatings which will enhance the performance and appearance of the lenses as well as prolonging their life.
Types of Coatings
Anti-Reflection (AR) Coating
This is applied to reduce the bright reflections which occur on the surfaces of the lenses (back and front) and also within the lens on the internal surfaces. It has the effect of reducing glare caused by these reflections whether they be from room lights, computer screens or on-coming cars at night. More light can then pass through the lenses into the eyes, so increasing contrast and making vision sharper. The lenses are virtually invisible to the onlooker making the eyes more visible, but the main benefit is that wearers will find their eyes less strained at the end of the day.
Anti-reflection coated lenses do, however need more regular cleaning. This is not because they get any dirtier than uncoated lenses, as is a common belief, it is because smears and dirt are much more apparent as there is no glare to hide it. Nowadays good quality coatings include a hydrophobic (water-repellent) layer and an oleophobic (grease-resistant) layer, making them easier to clean and keep clean. Other factors such as improper cleaning, extreme heat and certain chemicals can cause an anti-reflection coating to scratch, crack, craze or peel. But all in all, if you follow advice on proper glasses care, your coating should last at least as long as your frame.
This is an invisible silicone-based layer applied to the back and front of lenses to prevent everyday hair-line scratches. It is highly recommended for anti-reflection coated lenses. It is scratch-resistant, not scratch-proof, so it will not prevent deep scratches from sharp objects.
A UV coating, also invisible, is applied to lenses to protect the eyes and orbits from the sun’s harmful rays which can cause cataracts, macular degeneration and skin cancer.
Experiments have shown that coatings have a weakening effect on lenses, making them fracture more easily. However this is not usually a problem in normal day-to -day use unless lenses come into contact with sharp or pointed objects or high-speed missiles. (Clin Exp Optom 2006 March)
Caring For Your Lenses: Do’s and Dont’s
- Never clean your lenses on clothing or other fabrics, or tissues of any kind. These are abrasive. Tissues and all paper towels are made from wood pulp (like paper), and contain tiny wood particles that will scratch lenses over time. Even Kleenex themselves advise against using them to clean lenses.
- Don’t use very hot water on your lenses. It can cause the coating to crack or craze.
- Don’t use household chemical cleaning agents as these are strong solvents and can destroy coatings.
- Avoid contact with hairsprays, perspiration, juices and cosmetics as these can destroy coatings.
- Do not expose lenses to high temperatures as this can cause cracking/crazing, eg. Leaving glasses in a hot car, opening an oven door, standing over a barbeque.
- Resting your glasses on their lenses can cause scratches.
- Do clean your lenses with recommended lens sprays, impregnated microfiber cloths or premoistened cleaning wipes.
- Do wash your microfiber cloth regularly to get rid of dust and dirt that gets embedded in its fine fibres, as these will scratch the lenses. It should be washed in lukewarm water with a few drops of mild dishwashing detergent or liquid laundry detergent. Harsh detergents, such as bleach, will damage the cloth fibres as well as the lens coatings if not washed off well. Fabric softeners will leave a waxy layer on the cloth which will prevent it from picking up dirt, and will also leave a film on the lenses. Lens cloths should last several years or 200-300 washes.
- Do use lukewarm water to rinse away any abrasive particles before wiping.
- Do use a few drops of dishwashing liquid that has no lotions or conditioners in water and rub with fingers.
- Do store in a case to avoid scratches and dust particles landing on them.
In my experience, 90% of coating failures are caused by the wearers themselves, and sometimes by the technician glazing and assembling the glasses. This is because nowadays coatings are of such an advanced quality (you get what you pay for, obviously, so the more expensive ones are far superior in quality) that they will not be damaged easily through wear and tear or aging. These are the various faults that can occur and how they are caused:
- Crazing /cracks: a network of microscopic random cracks in the anti-reflection coating at right-angles to each other. This is caused by extreme heat, possibly during the assembling or adjustment of the glasses with a frame heater, or exposure to heat by the wearer as previously mentioned. It can also occur if pressure is applied to the lenses as when rubbing them too hard, or if exposed to certain chemicals like household cleaners, hairsprays, acids or alkalis.
- Delamination/cracking: if a lens is cut too big for the frame and is squeezed in, the coating can buckle up (delaminate) and crack in lines.
- Scratches: these occur through wear and tear or cleaning with an abrasive material, and will show up more on an anti-reflection coated lens. In extreme cases they will cause a lens to appear cloudy.
- Peeling: this shows up as large patches of the anti-reflection coating missing, and was more common in the past when coatings were not so high-tech. Nowadays it will generally only occur in the cheaper versions which are thinner, and will be easily damaged by the chemicals mentioned above.
Isopropyl alcohol is the mildest form of alcohol and is a common solvent and disinfectant. It is frequently used in optical practices to sterilize equipment, such as contact tonometer heads and chin rests, and is also a very effective lens cleaner that is found in most lens sprays and impregnated cloths. (It should not be confused with ethyl alcohol, which is the one found in your favourite alcoholic drink. This type should not be used on lenses as it can be damaging to plastics and coatings with prolonged use.) Isopropyl alcohol has the advantage that it contains less water and therefore evaporates quickly from lenses and does not leave streaks or residue. It is also effective at removing oils, gums and resins without damaging lenses or coatings.
Despina Christoforou is an optometrist born and raised in London, UK. She studied Optometry at the City University, London, and is a member of the British College of Optometrists. She ran her own practice within her family business for 13 years before moving to Cyprus with her family. She now works part-time in an optical company and draws on her expertise to write optometry articles for various web publications.