There is - just - a possibility that the sun is going to shine throughout August. So if you haven't got your sunglasses sorted out yet, act pronto, and think of performance as well as pose. Revo Exec sunglasses, at pounds 199 from Bausch & Lomb, are certainly pricier than pounds 2.99 fake Ray Bans, but are they worth the investment?

Ask Nasa. The Execs' high-grade, durable plastic lenses are covered in the same multi-layer film lens coatings which enhance and protect Nasa space equipment. This makes them endearingly scratch-resistant, but more importantly they filter out potentially damaging UV, infra-red and short- wave blue light. Nasa tests show they cut out 99 per cent of polarised glare. Good health sense when you consider that excessive exposure can lead to cataracts and retina damage.

They also offer enhanced clarity, depth perception and lack of distortion under differing light conditions: in other words, you can see better through them than you would through those fake Ray Bans.

But what about style? Well, we think they're chic in a kind of "I've just finished a double triathlon" sort of way, but it really depends on whether you like lightweight aluminium frames and space-age blue/purple reflective lenses.

Well, do you?