And is there a difference between the cheaper models and the designer sunnies?

Steven Dane, Professor of Optometry at the University of New South Wales put a range of sunglasses to the test for ultraviolet transmittance, polarisation and overall safety.

"The types of sunglasses that fail are firstly the gradient tints, because there is a tenancy not to put the gradient in the two eyes at the same height, so the person ends up looking at the dark part of one eye, and a lighter part of the other," Professor Dane said.

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The results are disturbing.

"They could be part of a risk of driving and misjudging distance of another vehicle or pedestrian."

All sunglasses sold in Australia must meet Australian standards and must be labelled, clearly stating the degree of protection.

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"When you go and choose sunglasses, check the label. The label might be a swing tag, or it might be stuck on it, and on the label it must tell you which category of sunglasses it meets," Anthea Muir from OPSM explained.

"Category three are the darker sunglasses, with great UV protection, so that is probably the number one. Category two is for somebody who likes their glasses a little bit lighter, but still wants good protection. If it is category labelled it means it has been tested and the company you are buying it from will stand behind that it meets the standards."

Nine million Australians are exposing themselves to eye damage, vision loss, macular degeneration and eyelid skin cancers by not wearing sunglasses.

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A recent study showed 65 per cent of Australians wear sunglasses on sunny days, but that drops to just 35 per cent when there's cloud cover.

"The UV is the same whether it is sunny or cloudy, the cloud just takes the sunlight away but the UV remains. We need to be cloud smart, we need to think 'I am outside, I need to protect my eyes from UV' just like you think to put sunblock on," Muir said.

But, as our scientific lens test showed, some sunnies aren't doing the job. We tested six pairs of sunglasses - three cheap, no name pairs from the markets, and three from a reputable retailer: brands included Ralph Lauren, Burberry and Ray Ban.

Keeping eyes healthy

Out of the sunglasses tested, half failed the Australian standard.

Some of the children's sunglasses, which were bought from the markets for $5 had a right lens visibly darker than the left, which is an automatic fail - and they didn't meet the Australian standard.

"Children's sunglasses transmit too much ultraviolet, they are not giving the protection that the standard requires, so they are not giving the protection against the various ultra violet conditions," Professor Dane said.

A $20 market pair of aviators claimed to be polarised.

"The polarising hasn't been set at the right angle - this again causes problems with perception and judgement of depth. The second thing is one of them has too much refracting power - basically the lens is distorting. It is not so much dangerous, but could make someone uncomfortable, give them blurred vision, or give them a headache," Professor Dane said.

And proving you don't always get what you pay for, $400 Italian Burberry sunglasses failed the tests.

"It failed because the two lenses are not of equal tint, the left lens is somewhat darker than the right. It means someone is looking at the world one eye slightly lighter, and one eye slightly darker, and the consequence of that is it affects our ability to judge depth."

Luxottica undertakes rigorous and exhaustive testing on hundreds of frames every day to ensure they meet global standards, in addition to Australian standards. We are very surprised by the results of the test conducted on this pair of sunglasses. We are investigating with further testing, but want our consumers to know that no damage can occur to anyone's eyes from this difference in tint between the lenses. Today Tonight's test clearly shows the sunglasses have passed on ALL UV and protection requirements as well as everything required for a Category 3 rating. Luxottica stands by the quality of its products and will be looking into this matter further.

• Sunglass Hut - www.sunglasshut.com
• OPSM - www.opsm.com.au

This reporter is on Twitter at @maddykennard