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High-Intensity Discharge (HID) headlights are being fitted on increasing numbers of up-market cars. Some use the HID lights for low beam only while others use it for both beams. Either way, they are much brighter than conventional halogen headlights. HID By PETER SMITH Headlights –– how how they they work work www.siliconchip.com.au May 2003  11 E ven if you haven’t heard of these new headlights, you’ve probably noticed the occasional piercing “bluish” flash on the road at night. HID headlights are already being fitted to up-market European, Japanese and American cars. As you might have guessed, the technology used in these headlights is radically different from conventional tungsten-halogen headlamps. Not only are HID headlights much brighter, they are much more efficient and draw less current from the battery. History in a flash All high-intensity gas discharge lighting is related to the original mercury vapour arc lamp, invented back in 1901 by an electrical engineer named Peter Cooper Hewitt. The original mercury lamps were not very efficient (about 10%) and produced a rather harsh blue-green light. The next major advances came with the inventions of the low-pressure and high-pressure sodium lamps. To this day, low-pressure sodium lamps are the most efficient commercially available lighting source. However, they generate a pure monochromatic yellow light that is 12  Silicon Chip unsuitable for many applications. The high-pressure version retains much of the efficiency (about 50%) and produces a “warmer” light colour, making it an obvious replacement for mercury lamps in street and factory lighting, where it is used extensively today. In a further search for efficiency and whiter light output, the General Electric company experimented with various iodine salts (indium, scandium, sodium, and thallium) in their mercury vapour lamps. The result, born in 1962, was dubbed the “Multi Vapour Metal Hal- A current GE Multi Vapour Metal Halide lamp. The arc tube is suspended inside a familiar bulb-shaped glass enclosure. Overall height is almost 300mm. Notice the third (starting) electrode emerging from the bottom of the arc tube to the left of the main electrode. ide” lamp, after the fact that iodine is one of the halogen elements. Derivatives of the first metal halide lamp can be found wherever an efficient, high-intensity white light source is required. Uses for this type of lamp have until recently been restricted to industrial, high-wattage sizes in the 175W to 1500W range. Now, with a few modifications to lamp chemistry and some electronic circuitry, engineers have been able to adapt them to small, low power applications such as automobile headlights. To understand the need for electron- Sketch of a Philips D2S HID lamp. The arc tube is tiny in comparison to a conventional MH lamp. This lamp is only 76mm high. www.siliconchip.com.au ics, let’s look first at the operation of a conventional metal halide lamp. Metal halide lamp operation A basic lamp consists of two “glass” tubes, one within the other. The inner tube is made from fused quartz or ceramic and houses two main electrodes and a starting electrode. The tube is filled with an inert gas (argon) which has been “spiked” with a tiny quantity of mercury and various halide salts. The outer glass envelope serves a number of purposes. It isolates the hot inner tube (up to 800°C) from the outside world. It also filters out some of the shortwave UV radiation, which if left unchecked is a health hazard and can damage rubber and plastic components. When power is applied, the voltage between the starter electrode and nearby main electrode causes ionisation of the argon gas. Ionisation lowers the resistance between the main electrodes located at opposite ends of the tube, allowing an arc to be struck. Initially, the tube emits a dull bluish discharge but as heat from the arc vaporises the mercury (and other metals) and the pressure increases, it changes to a brilliant white. The heat also activates a bi-metallic strip, which shorts out the starting electrode after about 2-4 minutes. The starting cycle can take up to six minutes. If power to the lamp is interrupted, a cooling-off period of ten minutes or more is required before it can be restarted. All metal halide lamps are designed to be “burnt” in a particular position for longest life. This is generally described as “base up” or “base down”. Typically, high-wattage industrial lamps are powered directly from the 240VAC mains via a simple magnetic constant power ballast circuit. In addition to the starting method described above, some metal halide lamps omit the starting electrode and just use a high-voltage pulse across the main electrodes to ionise the gas and strike the arc. Apart from eliminating the starting electrode, high-voltage starting also allows higher initial gas pressures. This provides faster runup, better burn colour and quicker re-starting. Gas-discharge headlights Engineers had to overcome some major hurdles in order to bring high-intensity gas-discharge lamps to low-voltage, instant-use applications such as automobiles and battery-powered torches. For a start, about 85V is needed for the lamp supply. As well, the lamp needs to start immediately it is switched on and have useable light output within seconds, not minutes. It also needs to be instantly restart-able, with no cool-down period. All this has been achieved by re-engineering the basic lamp, along with some clever electronics. Here’s how. Gassing up In order to obtain higher initial light output, the automotive metal halide arc tube is filled with Xenon rather than Argon. This fact hasn’t escaped car enthusiasts who often use the Fig.1: HID lamp operation is carefully controlled by an electronic ballast. This diagram plots lamp voltage and current against time, showing six distinct phases from turn-on to steady-state operation. www.siliconchip.com.au May 2003  13 How good are HID headlights? These two shots compare conventional halogens with HIDs on low beam. The difference is quite spectacular! Notice how the light/dark cut-off appears about the same, but the view is much whiter and brighter (sounds like an Omo ad!) and there’s a lot more side illumination. (Photo: Hella) name “Xenon” when referring to HID headlamps. Xenon, by the way, is an odourless, colourless, tasteless, non-toxic, monatomic and chemically inert gas. Although having markedly different dimensions, the lamps appear to operate in much the same way as their industrial counterparts. From the diagrams, you can see that the lamps retain all of the elements discussed above. To date, manufacturers have standardised on several lamp styles, code named D1S, D1R, D2S and D2R. All four lamps are rated at 35W but the D1S and D2S versions produce 3200 lumens whereas the D1R and D2R produce 2800 lumens. The “R” versions have lower light output due to a black mask on the outer envelope. This is used to control light dispersion, which we’ll talk about later. To put these figures in perspective, a typical 55W tungsten-halogen lamp develops just 1000 lumens. In addition, HID systems consume less power (about 45W; 35W + 10W in the ballast) than conventional lamps; in other words, about 20% less current drain for three times the light output. The difference between the “D1” and “D2” versions can be seen in the base size. The D1 base is physically larger as it houses the igniter circuitry. In contract, the “D2” lamp requires an external igniter. Lamp life HID lamps are generally expected to last the life of the vehicle. With no filament to burn out, you might expect them to last forever but the arc tube does eventually “wear out” due to several unavoidable reactions. In particular, tungsten from the electrodes gradually blackens the inside of the tube, a process that is greatly accelerated during cold starts. Manufacturers specify tube life at up to 3000 hours, which includes a “typical” number of cold starts. By comparison, tungsten-halogens have a life of between 700 and 1000 hours. Electronic ballasts To power a lamp from a 12V DC Fig.1: HID lamp ballast concept. The controller block generally includes a microcontroller or digital signal processor (DSP) chip. 14  Silicon Chip www.siliconchip.com.au Fig.3: basic igniter circuit. When the breakdown voltage of the switching spark gap (SSG) is reached, it momentarily connects C1 across the primary of the trigger transformer (T1). electrical system an electronic ballast is required. Fig.1 shows the basic layout of a typical 12V DC lamp ballast circuit. The input voltage is first stepped up by a DC-DC boost converter. During normal running conditions, the voltage across the lamp needs to be between about 60V and 110V. However, the open-circuit lamp (no arc) voltage can be as high as 600V. This high voltage is used by the igniter circuit (see Fig. 3) to generate the required 23kV ignition pulse. Two transistor pairs in a H-bridge configuration apply the converter output to the lamp in an alternating fashion, with the resultant drive being a square wave of between 250Hz and 10kHz. Power to the lamp is carefully regulated by the controller during all phases of operation. This is where the “smarts” of the system are to be found. The lamp must be brought up to maximum output in the shortest possible time, while minimising electrode erosion. This is achieved in five distinct phases, as follows: 1) Turn-on. Power is applied to the ballast and the controller commands maximum voltage from the boost converter. Within 30ms, the igniter is ready to fire the tube. 2) Ignition. One or more high-voltage pulses, at 20Hz repetition, are applied to the lamp to ignite the arc. If the arc is not struck after 20 pulses, a serious fault is assumed and the sequence is terminated. 3) Take-over. To maintain the arc but also conserve the electrodes, the controller regulates lamp power to 75W maximum at up to 12A. This high current surge lasts only about 300µs. During ignition and take-over, the H-bridge applies DC to the lamp so as not to “disturb” the arc. 4) Warm-up. The H-bridge performs one switching cycle, first applying a negative half cycle of 10ms duration, then a positive half cycle. Power input to the lamp is regulated to 75W at 2.6A maximum. 5) Run-up. The H-bridge begins switching symmetrically at about 400Hz. Until the lamp voltage reaches 50V, the controller regulates lamp power to 75W at 2.6A maximum. This takes about 6-12 seconds. During this time, lamp intensity rises to near its full rated output. 6) Steady state. Lamp power is regulated to 35W ±2W. Continuing regulation ensures that the light output remains constant, regardless of variations in battery and lamp voltages. Of interest is the need to power the lamp from AC rather than DC. Apparently, applying a symmetrical square wave (ie, average = 0V) prevents electrolysis and other life-shortening effects within the arc tube. A relatively low switching frequency (250Hz-10kHz) ensures circuit efficiency and avoids acoustic reson-ances that can occur at higher frequencies. Igniter To ignite the arc during a cold start, a pulse of about 5kV is required. For a hot start (re-strike), as much as 25kV is required to ionise the highly pressurised gas. This is achieved by a dedicated igniter circuit, as shown in Fig.3. The igniter circuit is positioned in series with the lamp so as not to expose the ballast circuitry to high voltage transients. When power is applied, capacitor C1 charges towards the full open-circuit ballast voltage (up to 600V). When it reaches the breakdown voltage of the switching spark gap (SSG), the SSG “flashes over”, dumping the capacitor’s charge into the primary side of the trigger transformer (T1). The voltage appears on the secondary side of the transformer multiplied many times over, resulting in more than 23kV across the lamp electrodes. Packaging the parts Although the lamps and bases conform to a standard, the same can not be said of the ballast, igniter and wiring harness. Generally, the ballast is (Left): components of a Hella “Mark 4 Xenon” HID headlight system. The large metal box on the left houses the ballast, whereas the smaller box houses the igniter. A PES-type headlight (note the lens) appears at the rear. At right is a complete system, including washer and leveller, ready for installation. (Photos: Hella). www.siliconchip.com.au May 2003  15 sealed in small metal enclosure which is mounted a short distance from the lamp socket. For D2S and D2R lamps, the igniter may be a separate black box or integrated within the ballast housing. Wiring harnesses are fully shielded, usually sealed and include high-voltage connectors for the D2S and D2R lamps. Putting the light on the road Equally important to lamp intensity is the ability to be able to direct the light exactly where it is needed. Conventionally, this has been achieved with large parabolic reflectors and segmented glass lenses. In this simple system, the lens is mostly responsible for light distribution. High beam units also include a metal shield or mask that is used to provide the light/dark cut-off. Also popular is the free-form (FF) reflector, which is characterised by a clear, rather than segmented lens. In this system, a complex-surface (segmented) reflector performs precise light distribution. Highly accurate placement of each individual segment is achieved with the aid of computer design software. PES headlights Recently, manufacturers have team-ed complex-surface reflectors with optical projection technology to come up with the Poly-Ellipsoid System (PES) headlight. This system provides many advantages over other headlight systems. For a start, projection allows precise definition of light/dark cut-offs, transition areas and contrasts with the use of an imaging screen. As well, only a very small light-emission surface is needed in comparison to conventional systems. This equates to smaller headlight enclosures, allowing vehicle designers to weave all kinds of magic with front-end styling. Other tricks, such as signal image enlargement and light rings are used to reduce glare and provide better Fig.4: a poly-ellipsoid reflector and projection lens form the heart of the Bosch PES headlight. Dualbeam systems move the screen up and down with the aid of an electro-mechanical actuator. position marking. HID lamps can be fitted to both reflection and projection systems. The masked HID lamps (D1R & D2R) are designed for reflection systems, whereas the clear lamps (D1S, D2S) go in the projection units. Low/high beam solutions To date, implementation of dual About Lamp Efficie ncy Throughout this article , we’ve listed lamp efficiency in perce ntage points, which is intended as a very rough guide only. The most co mmon measure of lighting efficienc y is calculated by dividing light output (in lumens) by the power input (in wa tts). The result is termed “lumens per watt”. Since the value of lum ens per watt is always greater tha n one, it is a measure of “efficacy”, rather that “efficiency”. beam headlights has varied considerably among manufacturers. In some vehicles, halogen lamps are still used for high beam and HIDs for low beam. However, the trend has been towards more complex systems that use a single HID lamp and some clever mechanical “beam adjustment” devices. For example, the Bosch Bi-Litronic reflection system moves the lamp back in the reflector housing with an electromechanical actuator when low beam is selected. Thus, a completely different projection pattern is obtained for low and high beam positions. Things get even tricker on projection systems. Once again, Bosch have developed a unique electromechanical solution. On their Bi-Litronic system, the position of the imaging screen is shifted to generate low and high beam light patterns. Performance Overseas studies have shown that HID headlights provide considerable safety improvements. In particular, more light to the sides of the road allows drivers to spot pedestrians and potential hazards much earlier, especially during poor weather conditions. The whiter light renders colours better too, making road signs and markings more visible. It seems that drivers are impressed with this new system. A significant (and increasing) percentage of new-car buyers have been willing to part with over $1000 for what has mostly been offered as an optional accessory. In 1997, European research institute Emnid carried out a survey among drivers whose vehicles were equipped with HID headlights. The results of this survey indicate that 94% of all HID users have a positive opinion of the new system. The main features highlighted were brightness (42%) and general illumination (35%). HID controversy? However, some road users have complained about the dazzling effects of these new headlights. Of course, having brighter headlights doesn’t mean that we can “aim them up” to see further ahead; the light cut-off point remains the same. However, up to that point, the light is much brighter and whiter. This means that for on-coming drivers, the familiar gradual fade from dark to Fig.5: the basics of a headlight projection system. Operation is very similar to an overhead projector, with the projected image being a screen used to define the light/dark cut-off. 16  Silicon Chip www.siliconchip.com.au A 3-D model, coloured for clarity, of Hella’s Bi-Xenon projection headlight. The imaging screen (grey, centre) is actuated by the electro-mechanical system in the foreground of the picture. light doesn’t occur. Instead, there’s a sudden jump to “bright” as the cut-off threshold is passed, and this could have a momentary dazzling effect. Doctors have put a slightly different spin on the problem. They say that while the human eye is sensitive to long-wave, red-yellow light during the day, at night the optic nerves are irritated by short-wave light, which is a component of the HID lamp spectrum. European regulatory authorities are aware of the potential dazzling effects and have made automatic headlight levelling and cleaners mandatory on all vehicles fitted with HID headlights. Why cleaners? Well, dirty lenses were found to cause light scatter, another potential dazzler! It appears that local car manufacturers will follow suit and fit automatic levellers and cleaners to Australian vehicles as the technology becomes available on less-expensive mounts. Be warned though – after-market HID headlights are illegal on most on-road vehicles! Factory-approved upgrades to some up-market European cars are possible but the rest of us will have to wait. If you’re hankering to take advantage of this new technology, then you still have a couple of options. HID auxiliary driving lights are available in Australia and can really make a difference to your night driving experience. Check out the Hella web site at www.hella.com.au to see what’s on offer. Still too pricey? The new Xenon-filled tungsten-halogen lamps are a good option for older vehicles. These generate up to 50% more light than the standard parts and are available in plug-in “H” series styles. They’re legal, too. Upgrading halogens to HIDs – is it possible? One of the hottest car upgrades right now has to be HID headlights. A quick search on the net proves our point; there are literally hundreds of retrofit offers and for those that can’t afford the $1000 (or more) price tags, there are cheap HID look-alikes. Even the world’s fastest production car, the Lamborghini Murciélago, gets the HID treatment. (Photo: Hella). More reading? Vehicle lighting is set to become very high-tech. The VARILIS (Variable Intelligent Lighting System) will supposedly enable us to see around corners. Here’s a 3D model of Hella’s VarioX system, depicting how it rotates about its longitudinal axis. Projection optics and special surface contours allow up to five different beam patterns to be projected onto the road. (Photo: Hella). www.siliconchip.com.au If you’d like even more information on discharge lighting, SILICON CHIP has published several articles on the subject in the popular “Understanding Electric Lighting” series. Reprints of these articles are available for $8.80 inc p&p and GST: “HID Lighting” – February 1999 “Metal Halides” – July 1998 “High Pressure Sodium” - June 1998 “Low Pressure Sodium” - April 1998 Credits Thanks to Philips Automotive Lighting and Robert Bosch (Australia) for details of their HID lighting systems; Hella and DaimlerChrysler for photographs. May 2003  17