|
|
|
|
|
|
|
|
 |
|
 |
|
 |
|
 |
|
|
|
 Lens Types • Glass or Plastic Lenses? • Anti-reflection Coating • Anti-scratch Coating • Multifocals • Lenses that change Colour • Sunglasses and UV Protection |
|
WHAT TYPES OF LENSES ARE AVAILABLE? Lenses are classified into GLASS and PLASTIC and these can be further sub-divided into categories of thickness. PLASTIC lenses start at the basic, or CR39 lens: this is the thickest type of lens available The thickness is represented by the term “index” and this basic lens has an index value of 1.49. At 20/20 all the other plastic lenses come with a standard anti-scratch and anti-reflection coating. Their thickness levels start to decrease and lens indices of 1.50, 1.60, 1.67, 1.71 and 1.74 are found. The thinnest lens is the lens with the highest index : the 1.74 lens. At 20/20, GLASS lenses have indices of 1.53, 1.70, 1.80 and 1.90. Should your prescription be fairly high, your optometrist or dispenser may advise that you go for glass. The 1.90 lens is currently the thinnest spectacle lens available. Your 20/20 optometrist or dispenser will advise if you need a high-index lens for your spectacles. Whilst glass lenses are thinner and more scratch-resistant, they are also more fragile and heavier. Plastic lenses are thicker and less scratch-resistant, but their lightness and impact resistance far outweigh their disadvantages. Plastic has 3 x the impact resistance of glass and is therefore a lot safer to wear.  WHAT IS ANTI-REFLECTION COATING? This is a coating which cuts out useless rays of light bouncing off your spectacle lenses. The end result is that you will see more clearly as fewer reflective distractions exist and greater image contrast is obtained. Furthermore, you will look much better when you wear your specs as the traditional “glassy” appearance is eliminated. Photos of you will allow the viewer to see your eyes, as camera flashes will stop light “bouncing” off of your lenses! As this is a standard coating that is worked into the lens whilst it is being made, it cannot be “added to” a normal lens surface nor can it “top up” an old surface where the coating has come off. WHAT IS ANTI-SCRATCH COATING? This toughens the outer surface of plastic lenses but at its best, it is still a coating. One must take care not to confuse this with the term “scratch-proof” – in time all coatings come off and the same goes for anti-scratch. WHAT ARE MULTIFOCALS? This term encompasses everything from bifocals to trifocals and varifocals. The Multifocal is a special type of lens which is required mostly when the eyes get past 40-45 years of age. It allows the wearer to see things clearly both far away and close up. These individuals would possibly have both a distance and near prescription, or they may only possess a near prescription. If their distance vision is perfect (or zero) they would use multifocals so that they don’t need to keep taking their glasses on and off. This would be determined by their lifestyle requirements and working distances. Your 20/20 optometrist will advise if your lifestyle will benefit from the use of a multifocal. Bi and tri-focals have lines across the lenses which are visible when you look at them. They are still used, but in this aesthetically conscious millennium, more and more multifocal users are switching to varifocals -especially since bifocals can be perceived as a sign of ageing. Varifocals have no lines across them and look exactly like single focal lenses. Since there are no lines it does take longer to figure out how to use them, but your optometrist and dispenser will advise you on the best way to adjust to them. Unlike the bi and tri-focals which work by changing powers the instant your eye crosses the line, varifocals progressively change power as one looks up or down the lens. LENSES THAT CHANGE COLOUR? These are lenses which darken when outdoors and lighten when indoors. Originally they were only available in glass form and were known as photochromatic lenses. Modern day types are available in both glass and plastic form, with the latter known as Transition lenses. These have all the advantages of plastic lenses in that they are lightweight and safe. Thinner lens versions (high index) are now also available. SUNGLASSES AND UV PROTECTION The sun's ultra-violet (UV) rays trigger premature ageing of the eyes so cataracts and macular degeneration become an accelerated risk. This is made worse by the continuing depletion of the ozone layer in our atmosphere. Other conditions associated with UV exposure are pterygia and pingueculae- these are areas of raised, discoloured conjunctival tissue. Over-exposure to UV can cause “sunburn” to the eyes. Snow-blindness and Arc Eye due to welder's flash are examples of eye “sunburn”. The surface tissues of the eye will literally peel off and like skin sunburn, this is a fairly painful process. Always invest in a decent pair of sunglasses and a hat to protect your eyes from the sun's UV. Wraparound frames are the best design for ultimate protection. As UV is an invisible light ray it is not the tint in your sunglasses protecting you, but the plastic or glass lenses in them. This means that even by wearing normal prescription glasses you are still affording your eyes some UV protection. Extra coatings are available to further enhance this protection. Those with blue or light-coloured eyes are more likely to suffer from the effects of UV and all radiation as the coloured part of the eye (iris) acts as a filter. A simple analogy would be to compare the eye to a window where the iris is the curtain. Those with lighter eyes have thinner 'curtains' than those with darker eyes. As a result much more radiation penetrates through and these individuals will experience more sensitivity to light and relatively greater risk to the harmful effects of the sun. |
|
 |
|
 |
|
|
|
 |
|
Monthly Disposable Contact Lenses | Flexible Continuous Wear Contact Lenses | Coloured Lenses | Varifocal Lenses | Astigmatic Toric Lenses |
Terms & Conditions | Site Map |
Featured Article:Daily Disposable Contact Lenses
Site Content:Vanessa Ho Yan | Site Design & Manager: Hassan. K. Bana