Polarising lenses

Driving is a highly demanding activity which requires maximum vigilance. Ninety per cent of the information for decisions and actions while driving comes from the eyes of the driver,1 yet visual requirements are quite low. Currently in the UK the numberplate test is used to determine visual fitness to drive. This equates to approximately 6/9-2 on the Snellen Chart.
The numberplate test is normally undertaken twice in a driver's lifetime, initially at the time of the driving test and again when the driver reaches 70 years. The law requires the driver to be able to read in good daylight, with glasses or contact lenses if worn, a numberplate at 20.5m, the letters being 79.4mm high.2 Visual field requirements as required by law state that drivers must have a minimum field of vision for safe driving defined as 'a field of at least 120 on the horizontal measured using a target equivalent to the white Goldmann III4e setting. In addition there should be no significant defect in the binocular field which encroaches within 20 of fixation above or below the horizontal meridian.' These are the basic requirements for Group 1 drivers; full details are available from the Department of Vehicle Licensing Authority.
However, drivers who easily meet the driving test standard can be subjected to conditions which cause them to be momentarily 'blinded' while driving. This is often due to glare. Smith3 describes how drivers tend to be subjected mostly to 'discomfort glare'. Discomfort glare causes the greatest problems when the sun is low in the sky, especially during the late autumn to early spring. The sun reflected from wet road surfaces is typically that which drivers complain about most often. It dazzles the driver, perturbs vision and may lower the performance while driving.4
A wearer test was conducted in Northern Ireland, where the weather conditions are frequently changing throughout the day. The test was undertaken using Essilor lenses to quantify the advantage (if any) of polarising lenses over standard tinted lenses for driving. Thirty professional ametropic drivers were recruited for the trial. They were mostly taxi drivers, large vehicle drivers, driving instructers and reps, only four of whom were female. Of these, 24 were already wearing sunglasses for driving. Among them, 17 were wearing corrective sunglasses and their ametropia lay vetween -4.00D and +3.00D.
During this comparative double-masked test, both pairs of lenses were glazed into identical frames. The wearers had each pair for a two-week period beginning with either the standard tinted lens or the polarising lens. In the final two weeks they had both pairs to try. They assessed the lenses at the end of each two-week period. After the final trial they had to decide on their preference and the reason for their choice.
Twenty-eight wearers completed the test. The results were very positive for the polarising lenses since 21 out of the 26 who had a preference chose this type of lens (Figure 1), mainly because polarising lenses were good in all weather conditions and for comfort of vision due to glare reduction.
More specifically, 19 wearers chose the polarising lenses for global comfort of vision, and also for colour reproduction. Eighteen of the participants considered polarising lenses to be the best option against glare.
The performance of the polarising lenses appears to be better in different situations such as 'on wet road', 'in the countryside' and particularly 'at the seaside', not only in sunny situations but also in cloudy weather (Figure 2).
The differences between both types of lenses while driving are illustrated by Figure 3, in which we can see that the benefit of polarising lenses when the sun is shining is not only perceived while looking at the road but also when looking at the windscreen, or when looking at the car front.
This test, on a specific population of professional drivers, would appear to indicate that many drivers do not wear corrective sunglasses. Many of the drivers had never considered the importance of having corrective sunglasses, yet ultimately all the drivers believed themselves to be safer drivers while wearing them. Polarising lenses for 21 participants would in future be the preferred lens of choice, while 26 of the participants would no more contemplate driving without corrective sunglasses.
Polarising lenses appear to be the ideal lens to wear in a variety of weather conditions and are particularly appreciated for driving. Considering these findings, perhaps drivers and, in particular those who spend a great deal of time 'on the road', should be advised to include polarised sunglasses as an essential piece of equipment rather than an optional extra.

References
1 Vue et conduite, Au Volant la vue c'est la vie. www.chez.com/ophtasurf.
2 Department of Vehicle Licensing Authority. www.dvla.gov.uk
3 Smith G. Disability glare and its clinical significance. Optometry Today, 19 April 2002.
4 Theeuwes J, Alferdinck JW, Perel M. Relation between glare and driving performance. Human Factors, 2002; 44(1): 95-107. 

Kathy Delargy is an optometrist and dispensing optician, a lecturer at the University of Ulster and a visiting lecturer at Varilux University