LIGHT BULBS

(From the NZMSC's forthcoming book "Setting The Road Alight - Lighting And The Motorcycle")

Since the days motorists used to have a gas lamp at the front of the vehicle to throw dim splatters of light in the general direction of travel, things have improved vastly.

For the motorcyclist, especially, it needed to! Being balanced precariously on two wheels, a rider has to know what is on the road surface if he is to avoid an unpleasant spill caused by hitting or crossing debris or a slippery surface.

Those riders who have ridden for twenty years or more will no doubt remember the amazing improvement to motorcycle lighting that came with the introduction of quartz halogen bulbs. Suddenly we could see what was on the roads at night.

Even today, however, most of us have little idea exactly how a quartz halogen light differs from an "ordinary" bulb. In fact, the difference lies in the envelope. But let me explain further.

An ordinary light "bulb" (I use that word for convenience only) has a tungsten filament surrounded by a "bottle glass" envelope filled with an inert gas, usually a nitrogen-argon mixture. The reason these bulbs do not produce as much light as is theoretically possible is because, when the tungsten filament is heated, it evaporates and the metal deposits onto the cooler glass envelope. Thus, to obtain a reasonable life expectancy, during operation the tungsten filament is heated to less than the maximum possible.

Enter the quartz-iodine bulb. The filament is still tungsten, but the glass envelope is made of fused quartz (which is not heat-sensitive, is chemically inert, and is expensive!) and filled with iodine vapour. Elements like lodine, Chlorine and Bromine are collectively termed halogens. Hence, a quartz-iodine bulb is often known as a quartz-halogen bulb.

The advantage of this bulb is that the iodine vapour prevents the migration of tungsten to the glass envelope so the tungsten can be heated to higher temperatures, giving a whiter light while still giving a reasonable life expectancy.

If we ignore operational factors that damage a bulb, such as being subject to excessive vibration, the life span of a bulb is almost always decided by either its design or the quality of its production. If any brand of bulb lasts for a shorter time than other brands, then it was built to a lower standard than the other brands or it had been designed to give a whiter light at the expense of long life.

Quartz halogen bulbs do have one disadvantage over "ordinary" bulbs - they tend to blow without the prior warning ordinary bulbs give. With an ordinary bulb, how near it is to the end of its useful life is indicated by whether there is a brown coating of metal on the inside of the glass envelope. No such visible warning comes with quartz-halogen bulbs. However, with most motorcycles having either two headlights or two filaments in the headlight, this is not a real problem as, when one blows, the other still works.

With indicator bulbs, warning of a blown bulb is usually that of an indicator warning light on the instrument nacelle refusing to blink because, with one bulb blown, there is no longer enough electrical resistance to make the flasher unit function.

The main threat to the rider from a blown bulb comes when the tail light blows, since the tail light is an essential piece of equipment with the rider being particularly vulnerable to attack from the rear by half-blind car drivers. That's why, when you carry out your comprehensive check of your bike every month or so, you should always whip off the tail light lens and check whether the inside of the bulb's glass envelope is showing a noticeable brown tinge. If it is, replace the bulb.

After all, replacing the bulb is cheaper and less painful than being rear-ended.

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