Sunglasses sold for general use can be too dark or unsuitable for driving. Blinding glare caused by low sun, light reflecting off snow or your own bonnet can be lethal, especially when driving at speed, but the right pair of sunglasses can prevent it. Sun lenses for driving fall into two main categories, ‘fixed’ and ‘variable’ tint. Fixed tint lenses remain the same darkness regardless of light conditions and are readily available. Polarised lenses normally have a fixed tint and can significantly help to reduce glare – their effect can be very evident on wet roads.
Variable tint lenses or ‘photochromic’ lenses are not suitable for driving because car windscreens filter out UV light which both slows and limits the reaction of the lenses and you could be driving with lenses that are too dark or too light. Tinted lenses are graded according to the density of the tint and all sunglasses should by law be labelled and show the filter category number. Due to the light levels within the car, filter category 2 lenses (medium tint) which transmit between 18 per cent and 43 per cent of light are recommended for daytime driving by the Federation of Manufacturing Opticians. If in doubt, discuss the options with your optician. It’s worth noting that sunglasses with deep side arms can block side, or peripheral vision and are not recommended for driving.