Let There Be Light
Let there be light
TDC
Kevin Cameron
MOTORCYCLE ELECTRICAL SYSTEMS have come a long way since I used to drain the water out of my Lucas “wading magneto” before attempting to start my AJS 500 Single.
Back then, headlights and taillights used simple bulbs like something from a flashlight, with vibration-sensitive tungsten filaments supported on whippy wire stalks. The hope was that the natural frequency of these filaments or supports would not come into exact step with the engine’s firing frequency, causing rapid fatiguing and breakage of the wire. That result would transform the cone of yellow light extending optimistically forward from the headlight into a sudden wall of darkness. Lightbulb plus vibration equaled darkbulb.
This process was often accelerated by subsidiary vibrations of whatever the light happened to be attached to-for example, a rear fender waggling on slender struts. It was wise to carry spare bulbs, but wisdom was not always equal to the ingenuity of failure. A ride on a brand-new Bultaco Metralla ended in silence when wiring routed through drilled holes in the frame was shorted by the aggressive cutting action of the ungrommeted and very sharp edges of the holes. The rider, and not careful equipment design, was expected to be the main guarantee against failure.
The coming of Japanese motorcycles in the 1960s pretty much put an end to such adventures. Lighting advanced beyond flashlight bulbs and dull reflectors that looked as though they needed a good polish. The nice thing about sealed beams was that the reflector was inside a glass envelope, safe from the dulling effect of sulfur-rich city air.
A lightbulb filament’s life is supposedly inversely proportional to the fifth power of its temperature, so even a small temperature increase, undertaken to increase the brightness of the light, drastically shortens filament life. Anyone who’s worked with old-time photofloods knows how short the life of filaments becomes when their temperature is pushed up to give a more nearly “daylight” spectrum. Home lightbulbs are good for about 600 hours, but those hot “white” photo lamps only lasted 1 percent of that. As the fluorescent light-makers never tire of telling us, incandescent light bulbs convert only about 5 percent of the energy supplied into light. The rest becomes heat.
As the hot filament operates, metal evaporates from it and plates itself onto the inside of the bulb, causing it to dim progressively. Once the filament loses 6-10 percent of its diameter, its temperature rises to burn-through values. With a flash, the filament vaporizes at its hottest region and goes out. Filament life is also shortened by the shock temperature changes of on-off switching.
The halogen bulb attacks these problems by filling a high-melting-point quartz bulb with a halogen (chlorine, bromine, etc.) gas. This type of bulb can be operated at a higher temperature, giving a whiter light and more of it per watt of power supplied. Where a nonhalogen filament lamp gives 10-18 lumens per watt, H-bulbs give 22-26 lumens per watt. The presence of the gas prevents evaporating filament metal from coating the bulb. Being unable to settle on the very hot quartz bulb, metal atoms have only one place to go-back onto the filament.
At some point it seems silly to go on trying to get more and more light out of a very hot solid object (the metal filament), for everything we do to get more light tends ever more powerfully to convert that solid into a gas. Lightning produces brilliant light because a gas is ionized by an intense electric field, and a powerful current travels along the ion channel so produced. The free current electrons, accelerated to high velocities by the electric field between cloud and ground or cloud and cloud, collide with gas atoms. These collisions excite the bound electrons of the gas atoms to high energy levels. As they fall back to lower energy states, they emit light that can briefly make night seem like noon.
The new high intensity discharge (HID) lamps operate on this same principle. An HID lamp consists of a tiny quartz bulb containing xenon gas, with molded-in conductors to pass current through the gas. Like a fluorescent lamp or natural lightning, the HID requires a pulse of high voltage to initially ionize the gas. Once the lamp is started, a current-controller supervises its warm-up, keeping brightness constant. In steadystate operation, the HID light source is much more efficient than filament lamps, giving more like 85 lumens per watt. The higher energy of these lamps gives the resulting output more green and blue, which is how you can recognize late-model vehicles with this expensive option. Tailoring of light color and intensity is achieved by varying the pressure of the xenon gas and the current density. This results in most of the light generated coming from certain preferred electron energy transitions.
HID lamps are small and use relatively little current, which gives new options for accommodating adequate lighting into swoopy modern body shapes. Their high operating temperature (quartz envelope at approximately 1000 degrees C) places limits on how close they can be mounted to temperature-sensitive parts. Users are cautioned not to touch the quartz envelopes of such lamps, for surface contaminants can cause failure.
For lower-flux lighting applications such as taillights, the cumulative increase in light-emitting diode (LED) brightness over the past few years has now made this kind of light attractive. Low operating temperature allows close packaging. Previously, LEDs were used mainly for such things as instrument lights. LEDs produce light when charge carriers within the semiconductor diode (electrons and “holes”) recombine. Yamaha has chosen LEDs for the taillights on the R6 sportbike and there will soon be many more such applications. These new LEDs are amazingly bright. Hot, vibrating, breakage-prone wire filaments are no longer the only way.
