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A PHYSICIST WRITES . . .

(February 2003)

Imagine that your headlights failed on a dark night, but you had an ordinary 100 watt bulb which could be plugged in somehow. If you held it outside through the driverís window, would it light up the road ahead? Not very well, I think. Yet car headlamps (usually rated at 60W each when on full beam) can show up objects far into the distance, and may be seen miles away. This is the benefit we get from simply focusing most of the light into a beam by means of a curved reflector.

Admittedly, headlamps are designed to shine slightly brighter and whiter than ordinary bulbs. Also, when the engine is running they receive a voltage a bit higher than the 12V from the battery, so they radiate more than 60W of power. Incidentally, this means that if you see a slight dimming of the headlights when you switch the engine off, then your battery-charging system is very probably in good order. Conversely, if you donít...

But the astonishing fact about all filament bulbs is that they are hugely inefficient, delivering not much more than 5% of their power as light ó the other 95% is totally wasted in the form of heat!

Vehicle lighting is a source of other surprises too. Why are some drivers apparently unaware that one of their headlights (if not both) is pointing too high and causing glare to oncoming traffic? You would think this might be obvious from seeing how their lights illuminate the road ahead when on full beam, or else the car ahead when dipped.

And why have manufacturers started to fit narrower headlight reflectors, which dazzle me even when they do seem to be adjusted properly? As for the latest xenon blue bulbs, when a car fitted with them comes at you over the brow of a hill, you are blinded.

Why are some cars fitted with brake lights at driverís eye-level, or higher, swamping your view through or past the vehicle? Why are some designs of side and rear lights permitted to be so much less bright than others? Why, in some direction-indicator fittings, is the amber colour put onto the bulb rather than into the plastic lens, with the result that the colour eventually fades to white?

Above all, why is the average driver so ready to light up almost as soon as the sun goes behind a cloud, but apparently so unconcerned about driving with a dead bulb? Why doesnít the law crack down on this widespread night-time hazard (which includes the risk that the other bulb of the pair will fail too, of course)? Manufacturers could easily include a warning system for burnt-out bulbs, in all new cars...

Actually, there already is a built-in warning for direction-indicator bulbs: if one fails on your car, you should notice that the winking and clicking goes twice as fast. But I bet not a lot of drivers know this!

Peter Soul

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