FAR SIGHTED
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Department of Transport figures reveal that 70% of all road traffic casualties occur between 0&00 and 18:00hrs, but despite those statistics it's a lot more dangerous driving at night than during the day: the rate of accidents at night is up to three times greater than during the day.
Much of this is down to impaired vision at night. After-dark accidents are often accompanied by the classic: "I didn't see the other bloke until it was too late." Conventional headlights can only do so much and vehicle speed is the key factor when it comes to avoiding an accident.
At night vehicles drive on dipped headlights most of the time: a good dipped beam can reach 65 metres. But at 90km/h (56mph) the safe stopping distance is considerably more than this.
POORLY LIT
The Highway Code quotes the stopping distance at 50mph (80km/h) as 53m, comprising 15m thinking distance and 38m braking distance. But that's for an average saloon car, not a fully laden 38-tonner travelling on a poorly lit road.
By the time many truck drivers see a night-time obstruction, and register it, they've gone beyond their safe stopping distance. Result: accident.
Suppose a truck driver could see further than conventional headlights currently allow? Under the Prometheus European road safety programme two systems are being evaluated which could give truck drivers longer-range night-time vision.
Saab Scania is investigating the potential of ultra-violet vehicle lighting in collaboration with Volvo and Swedish lighting manufacturer Ultralux.
"Improvements in lighting are important for two reasons," says Scania's vice-president for chassis development and production Kaj Holmelius. "First there's the matter of safety. Then there's reliability-90% of all electrical faults happen in the lighting system.
"Improved and more reliable lighting can be achieved by the use of ultra-violet lighting based on a new system involving central light sources, optic fibres and 'solid' lenses," says Holmelius. The advantages of using ultra-violet are certainly impressive. In the right conditions a driver's night-time vision can be extended from the 65m he currently has on dipped beam headlights to 150m—well within the safe stopping distance running at 90km/h.
WIRE ELEMENT
How does it work? Conventional headlamps have a wire element through which a current is passed. When the element heats up it glows producing visible light and we see the reflection of this light when it strikes an object ahead of the vehicle.
With ultra-violet light the bulb is replaced by a gas discharge lamp where the conductive element is normally mercury vapour, although other elements can also be used. When a current is passed through the mercury vapour its atoms are excited, giving off invisible UV light, Unlike the conventional "white-light" headlamp, CV illuminates by creating fluorescence as it strikes a particular object which reacts to UV light. It is this fluorescence that is visible to the eye.
Because some CV wave lengths are harmful the rays have to pass through a filter which removes those elements. But what's left is more than useful.
"As UV is invisible to the human eye the lights cannot dazzle and the beam can be optimised," says Holmelius. "Also in fog the visibility of materials with fluorescent properties is nearly as good as in daylight as there is no visible light for the moisture in the air to reflect back. Thus if vehicles carried fluorescent reflectors back and front the need for fog lights could disappear."
For UV to work it has to hit material that is fluorescent. But as the chart shows, many articles of everyday clothing work surprisingly well. Holmelius explains: "Fluorescent materials are not always those we automatically think of. Denim jeans, for example, have excellent qualities "If vehicles were equipped with UV lights, and road markings signs, vehicles, bicycles and so on were marked or painted with fluorescent materials, road safety would be significantly improved. The infrastructural changes are not so great—the increased cost of suitable paint is estimated at 10% above today's prices."
FIBRE OPTIC
A UV lighting system could also mean lower maintenance and better durability While UV lighting is still some way away Scania is currently working on what Holmelius describes as "a new system for normal lighting", again using a discharge lamp but with another gas, producing visible light. A central lamp is enclosed in a unit and surrounded by reflector lenses coupled to fibre optic cables. These carry the white light into "solid" lenses (basically blocks of hard plastic with good optical qualities) set in the grille or bumper.
"The advantages are that the lenses are almost indestructible, the light source is protected, there are minimal electrical connections and the lamp is more efficient," says Holmelius."It consumes only about 30 watts but produces significantly more light. It's posible that only two such light sources can be used for all the vehicle's external lighting needs. And the life expectancy is about: 10,000 hours, compared with 800 hours for a lamp today."
11 by Brian Weatherley