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Beam theory

In the beginning, there was darkness; then the driver switched his headlamps on.

india Updated: Aug 28, 2009 11:24 IST
Grease Monkey

In the beginning, there was darkness; then the driver switched his headlamps on. He doesn’t have to do so much as flip a switch to do that today — cars are now rapidly moving towards light sensors that switch the headlamps on as soon as it’s dark. We’re getting ahead of ourselves.

When the motorcar was invented, I don’t imagine there was much of nightlife, so headlamps weren’t really thought of as important. Besides, with a top speed that an octogenarian would be proud of, there wasn’t a real need for lighting up the road ahead. When it became necessary, however, manufacturers turned to the solution that was already present then - a lantern mounted on the sides of the car.

Light bulb moment
With cars getting more rapid, the necessity for light that not only helped others see and avoid a car, but helped the car’s driver see far enough ahead to maintain a respectable speed even at night. With the development of auto-motive electrical systems, things
got easier. Tungsten-filament bulbs did duty then as they do now — but they have a few problems of their own.

You probably know already that the bulbs you use at home have a vacuum in them to prevent the filament from melting and breaking. There’s an added problem: the filament boils, even in the vacuum, and deposits a film on the inside of the glass of the bulb.
This decreases the output of the bulb during its life. Higher efficiency can be achieved by utilising a halogen-filled bulb. This means that for every amp of electricity going through the bulb, more light will be emitted if the glass is filled with a halogen gas.

The power of Xenon
The next step that bulbs underwent was the ‘HID’ treatment. ‘HID’ stands for ‘High-Intensity Discharge’, which, as the name suggests, has quite a high output. You might know these lamps as ‘Xenon’ lamps.

The formal name for these headlamps is ‘gas discharge lamps,’ as they have no filament. They have an electric arc that produces light, much in the same way that a welder’s tools produce light when welding.

The xenon gas can be replaced with argon as well, which is another inert gas, but then the headlamps would take considerable time to attain their optimum output.

Highs and lows of beams
What you see in expensive cars is a concave lens — this helps eliminate the need for two arcs, to replicate the dual filaments that make up high and low beam. What is does is form a kind of ‘negative’ of the light from the headlamp unit.

For example, the light that comes from the right side of the headlamp is focused onto the left side of the road. The same applies to up and down, so the light that enters the lens at the top exits it to light up the road. This makes for a simple solution for high beam — a shield is used to cut off the light entering the lens at the bottom.

When you turn the high beams on, the shield is retracted, which means that the high beam adds to the low beam, instead of conventional filament bulbs, where the road nearest the car may not be lit up as well when the high beam is turned on.

If you can, do upgrade your headlamps to HID units — bulbs that can mount on almost all cars are available. Of course, it will void your electrical warranty, but massive gains are to be had if you drive regularly on highways at night.