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AsiaOne YourHealth: Various shades of protection: Zooming in on photochromic lenses

In recent years, a type of lenses that darken automatically when exposed to ultraviolet (UV) light has become rather popular.

These lenses, which turn clear in the absence of UV light, are often mistaken for transition lenses due to the popularity of the Transitions brand. But they are, in fact, photochromic lenses, which Transitions Optical said it first commercialised in 1990.

Today, there are other brands of photochromic lenses in the market, including cheaper ones from China and South Korea.

Dr Koh Liang Hwee, president of the Singapore Optometric Association (SOA), said this technology has been around for at least 20 years, but recent improvements have made the self-tinting and fading feature more light-responsive, and such lenses reduce glare too.

They are good for those who prefer not to carry a pair of prescription sunglasses around, said Dr Koh, who works at Pearl's Optical.

However, most photochromic lens brands do not darken significantly in the car. This is because they react primarily to UV light - not visible light - and car windshields are built to block more than 99 per cent of UV light. Drivers may therefore want to consider photochromatic lenses meant for driving or use sunglasses.

Photochromic lenses do not turn very dark in hot weather as temperature affects the degree to which they darken. But they can turn very dark in cold weather, like in winter.

There is also a lag of a few minutes when the lenses are either darkening or lightening, so immediate protection from UV light is not possible, said Dr Eugene Tay, medical director and ophthalmologist at Singapore Vision Centre.


LENSES WITH UV COATING
Someone who does not wish to use sunglasses may get his normal spectacle lenses coated to provide UV protection, suggested Dr Tay.

Indeed, most high-index lenses already come with UV protection, said Dr Koh. Some contact lenses also have this feature.

But lenses with UV coating do not protect the eyes against glare or excessive visible light, which can cause eye strain.

"UV-blocking coatings are close to colourless - or an almost imperceptible yellowish tint - so they do a good job of filtering out UV light, but not glare," said SOA councillor Chui Wen Juan, an optometrist at CC Chui Optical.


LENSES WITH GLARE PROTECTION

For glare protection, particularly when engaging in water sports and other outdoor activities, polarised lenses will help.

These can cut glare from reflective surfaces and the surroundings. They are especially ideal when driving, doing water sports or in environments with reflective surfaces such as snow or sand, and in glass buildings, said Dr Koh.

However, they are unsuitable for those whose work requires them to view LCD displays, such as commercial pilots. They have to be careful when using these lenses as they will cause the screen to appear dim or blacked out, said Dr Koh.

As glare protection and UV protection are mutually exclusive, a darkly tinted lens without a UV blocking coat will still let UV light penetrate, Ms Chui pointed out.

Also, make sure the frame fits properly on the face, she said.

"You could have the best UV protective lenses, but if the frame sits poorly on the face, the eyes will still be exposed to a significant amount of UV light," she added.

UV radiation and its harmful effects

Overexposure to ultraviolet (UV) light can damage the skin and lead to freckling, sunburn and an increased risk of certain forms of skin cancer, said Dr Eugene Tay, medical director and ophthalmologist at Singapore Vision Centre.

UV light is present in sunlight and can also be produced by lighting equipment such as sun lamps.

Of the three types of UV radiation, short-wavelength UVC is the most damaging, but it is filtered by the earth's ozone layer and does not reach the earth's surface, according to the World Health Organisation (WHO). Medium-wavelength UVB accelerates skin ageing and significantly promotes the development of skin cancer.

UVB is the cause of sunburn and snow blindness (photokeratitis), and is absorbed by the cornea, said Dr Tay.

It is also linked to the development of pterygia - benign growths that can cause discomfort and blurry vision.

Long-wavelength UVA, which accounts for approximately 95 per cent of the UV radiation reaching the earth's surface, can penetrate the deeper layers of the skin and is responsible for a tanning effect, said WHO.

It is the most harmful type of UV radiation, said Dr Tay, as it is transmitted to the crystalline lens and retina of the human eye, which may increase the risk of cataracts and macular degeneration.

He offers some tips on how to protect your eyes.

Wear sunglasses that can block 100 per cent of UV radiation, especially when you have to spend long periods in the sun.

Choose sunglasses with large lenses or those that wrap around the face, as these give better protection.

Wear a hat with at least a three-inch brim all around. This helps to shield your eyes and eyelids - where the skin is extremely thin - from the sun.

Seek shade whenever you are outdoors, especially between 10am and 4pm.

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