The history, design, technology & metrology of vehicle headlamps
By Robert Yeo, May 2020
This article was first published in the April 2020 issue of the ‘Lighting Journal’, published by the Institute of Lighting Professionals (The ILP).
Introduction: Dim and Dimmer
Vehicle headlamps were once rated in units of candle power, which is an apt description for my first car, a 1969 Morris Minor. The designer of the Minor (also of the ubiquitous Mini) was Sir Alec Issigonis, who once opined that in the interests of safety and staying alert behind the wheel, he should make his cars as uncomfortable to drive as possible. To the extent of fitting a spike to the steering column to impale the drive in the event of a collision. I’m not sure how serious he was about this particular “passive” safety feature, but the dim illumination provided by my Morris’ headlamps lead in no small part to the observance of speed limits, at least at night.
Today, vehicle lighting technology has advanced out of all recognition, with my current car luxuriating in the excess of “active pixel LED headlamps with laser assisted main beams”. Short of the light output from a thermonuclear explosion (2 x 10^12 candelas per square meter if you’re interested), you won’t find a brighter source of artificial illumination, with night almost being turned into day.
In this article, I will review the development of vehicle headlamp technology (from candles to LEDs, lasers and the very latest digital matrix LED) and explain the goniometric and projection photometric methods used to measure them.
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Evolution of Lamp Technology: from Candles to Incandescent Lamps
The very earliest vehicle lighting was based on candles or burning oil (e.g. acetylene), but light output was less of a concern because at that time vehicle speeds were strictly enforced by a man walking in front of the car carrying a red flag. Throughout most of the 20th Century, vehicle headlamps were based on incandescent lamps. The invention of the incandescent lamp is popularly credited to Thomas Edison in 1879.
Incandescence is the emission of (visible) light as a result of resistive heating of a metal (filament of tungsten). The more current that flows, the greater the heating effect, the greater the incandescence. With an efficacy of about 10 lumens per Watt (electrical to optical efficiency), incandescent lighting will never set the world alight. The lamp was positioned at the locus of a reflector (usually parabolic) so as to collimate the beam, while the desired beam distribution was achieved using prismatic optics moulded into the headlamp lens.
Complex Reflector Headlamps (1983)
The 1980s saw the introduction of complex reflector optics in headlamp design, and the gradual shift away from simple, parabolic reflectors. The optics required to shape the headlamp beam were designed into the reflector, rather than into the lens itself. Complex reflector optic headlamps provided an improvement in light collection from the incandescent lamp and better control of the beam shape.
The 1983 Austin Maestro is credited with having the first production complex reflector headlamps. The headlamp cover serves only to protect the headlamp from dirt and debris, and does not alter the beam distribution.
Projector Headlamps (1986)
The 1980s also saw the development of projector headlamps. This design places the lamp at the focus of an ellipsoidal reflector and uses a condenser lens to collimate the beam. This type of headlamp offers the vehicle designer greater flexibility due to the reduced cross-sectional area of the headlamp, albeit with greatly increased depth.
The 1986 BMW 7-Series (E32) is cited as being the first volume production car with polyellipsoidal projector low beam headlamps.
Halogen Lamps
Separate from developments in headlamp optical design, the simple tungsten incandescent lamp was being replaced by the quartz tungsten halogen (QTH) lamp. First developed in 1955 by Elmer Fridrich and Emmet Wiley at the General Electric company, the so-called “halogen” lamp improves upon the original tungsten lamp by burning brighter and extending the life of the filament. A halogen gas (typically iodine or bromine) is placed within the quartz (fused silica) vacuum envelope of the lamp. As the tungsten filament evaporates, rather than condensing on the glass envelope of the lamp and darkening it (as occurs with a standard incandescent lamp), it reacts with the halogen gas to form a halide, which doesn’t deposit on the glass. Instead, the tungsten halide dissolves back to tungsten when in close proximity to the filament, returning the metal back from whence it came. This process is known as the halogen cycle and it has the major benefits of keeping the glass of the lamp clear and extending the life of the filament. The luminous efficacy of quartz tungsten halogen lamps varies in the range from about 12 to 24 lumens per Watt.
Spiral Compact Fluorescent Lamps (CFLs)
As an aside, spiral compact fluorescent lamps (CFLs) are unsuitable for vehicle lighting. It is more difficult to efficiently collect and collimate light from an extended (larger) source area, and the sheer size of the emitting area of CFLs alone precludes them from automotive lighting applications. The spiral format CFL was developed by Edward Hammer at General Electric in 1976 in response to the 1973 oil crisis.
High Intensity Discharge (HID) Lamps, a.k.a. “Xenon” (1992)
During the 1990s, high intensity discharge (HID) lamps started to become popular, referred to as “xenon” in the vernacular. The very first dipped beam HID headlamps were developed by Hella and Bosch and launched on the E32 BMW 7-Series in 1992. HID lamps produce light as a result of an electric arc that is struck in a metal halide gas containing xenon; the addition of xenon reducing the warm-up time compared to an argon-only gas mixture that would otherwise take several minutes to reach full output. The spectrum of light emitted from an HID lamp contains a distinct blue peak, which compares with relatively little blue content from an incandescent lamp. Thus, the correlated colour temperature (CCT) of an HID lamp is much higher (4000 to 5000 Kelvin versus 2800 to 3200 for tungsten or tungsten halogen). The presence of a blue peak from the HID lamp and the use of a condenser lens in a projector headlamp can result in a characteristic blue glint (colour fringe) at certain angles of view, caused by chromatic aberration from the lens. A biproduct of the higher CCT and blue-rich output of HID lamps is their higher perceived brightness as a result of the semi-dark adapted state of the human vision system. The scotopic response of the eye favours blue light over longer wavelengths. HID lamps typically exceed a luminous efficacy of 100 lumens per Watt, an order of magnitude higher than an incandescent lamp.
Light Emitting Diode, LED (2006)
The new millennia saw the start of the next automotive lighting revolution: the light emitting diode (or LED). The first series production car with LED-based dipped beam headlamps was the 2006 Lexus LS (courtesy of Koito), with the 2007 Audi R8 V10 featuring the first all-LED headlamp (manufactured by Automotive Lighting, the joint venture between Magneti-Marelli and Bosch). An LED produces light by a process called electroluminescence, a phenomenon discovered in 1907. The recombination of electrons and electron holes in the semiconductor produces photons of light, with the bandgap of the semiconductor determining the wavelength (colour) of the light emitted. LEDs provide a unique set of advantages over all other lighting technologies in automotive applications. They are directional, compact, efficient (>100 lumens per Watt) and long lasting (if driven and thermally managed correctly) and present the automotive lighting designer with new opportunities for innovative design concepts based on their compact form factor. An LED itself does not produce white light on its own. White light LEDs are in fact blue LEDs with a phosphor coating through which the blue light is transmitted. The phosphor absorbs some of the blue light and through a process called fluorescence, converts this to light of longer wavelengths. The combination of blue LED light with green to red phosphor converted light creates the white light output that we desire. The high brightness indium gallium nitride (InGaN) blue LED was invented by Shuji Nakamura at Nichia in 1995.
Matrix & Pixel LED (2013)
Bringing us almost to the present day, “matrix” LED or “pixel” LED headlamps employ an array of LEDs. A forward facing camera determines the presence of oncoming vehicles, or a vehicle travelling in the same direction in front and dims individual LEDs in the array so as avoid dazzling the other traffic. The headlamp beam is adjusted in real time so as to illuminate the road around or to the side of the other traffic, thereby maximising the illuminance for the driver of the vehicle. Matrix or pixel LED headlamps operate on full main beam outside of urban environments and above urban speed limits, with the headlamps automatically dimming in bands or zones to avoid blinding other traffic. Matrix LED headlamps were first introduced on the 2013 Audi A8 that featured 25 LEDs that could be individually switched on, switched off or dimmed.
As an example of the current state of the art, consider the 2018 model year Range Rover (L405) – pictured above – and Range Rover Sport (L494) model ranges that were introduced with matrix LED and pixel LED headlamp technology. The land Rover matrix LED type features 52 LEDs splitting the beam into vertical strips. The Land Rover pixel LED type incorporates 142 LEDs that allows the main beam pattern to be split vertically and horizontally. This provides for more precise control of the beam with improved illumination while at the same time minimising dazzle to other vehicles.
Laser Assisted High Beam (2014)
Supplementing some high-end LED headlights is laser-assisted high beam. This operates on a similar basis to how white light is generated from an LED. In this case, an intense blue laser “pumps” a remote phosphor, which converts some of the blue light to longer green and red wavelengths. As with white LEDs, the residual blue laser light is combined with the fluorescent green and red to create white light. As with LEDs, the laser light is also directional, except even more so. The near-collimated white laser beam has a much further reach than standard LED beams, so is activated outside of urban environments above a threshold speed (typically 50 mph) so as to illuminate the road further ahead of the vehicle (claims of useful illumination out to 500m are made). The first production vehicle equipped with laser-assisted high beams was the BMW i8 in 2014.
Digital Matrix DMD LED (2019)
The very latest innovation in headlamp technology was announced in November 2019 by Audi. The Audi E-Tron Sportback features “Digital Matrix LED” headlamps which employ three LEDs and a DMD (Digital Micromirror Device) chip with one million micromirrors, each of which can be tilted up to 5,000 times per second. The DMS chip is similar in concept to the DLP (Digital Light Processing) technology developed by Texas Instruments and used in projectors. Each of the micromirrors on the 2D mirror array either allows the light from the LEDs to pass through to the headlamp lenses (and so illuminate the road), or to deflect into a beam dump which absorbs the light. The resultant “high definition” headlamp illumination pattern improves upon the beam steering and selective illumination capabilities of matrix and pixel LED headlamps. One of the more novel aspects to the Audi DMD technology is the ability to project a “carpet” of light in front of the vehicle, indicating to the driver which lane they should be in and their position within the lane. Selective illumination of hazards (e.g. pedestrians) is also highlighted as a benefit of DMD lighting, as it the ability to perform car-to-car communications to warn of hazards.
Goniophotometry & ECE Headlamp Regulations
In the context of vehicle headlamps, the relevant photometric metric is the luminous intensity that the headlamp emits. Headlamps must be designed to illuminate the road surface and kerb and they must not dazzle the drivers of oncoming vehicles (this is a critical safety requirement). Nor should headlamp beams shine upwards (which would be both a waste of light but also lead to problems of glare in adverse weather conditions). It is for these reasons that international regulations exist that govern the performance of headlamps, such as those published by UNECE (the World Forum for the Harmonisation of Vehicle Regulations). The figure to the right shows the dipped and main beam headlamp beam illumination patterns respectively (right hand drive vehicle).
In order to comply with ECE regulations for vehicle type approval, the illumination performance of a headlamp must be tested. There are two basic methods used when measuring the photometric performance of vehicle headlamps. The first is to project the headlamp onto a screen and measure the illuminance as a function of position within the beam, while the alternative is to mount the headlamp onto a motorised, two-axis goniometer and measure the luminous intensity as a function of angle as the headlamp rotates and tilts with respect to a fixed light meter. I’ll refer to these methods as the “Projection Test Method” and the “Goniometric Test Method” respectively. Both methods require that the headlamp is measured at a distance of 25m for regulatory compliance testing.
Why measure at 25m? The short answer is that this is the measurement distance specified in applicable ECE regulations. However, it is helpful to understand the physics of why 25m is chosen and not some other distance. In the photometry of light sources, we refer to near-field and far-field measurements. In the photometric far-field, the light beam can be considered to be “fully formed”, the source itself behaves as a point light source, the luminous intensity (candela value) of the beam remains constant and the illuminance (in lux) decreases in proportion to the inverse square of the distance travelled. In the photometric near-field, the beam is not fully formed, and the size of the light source and photodetector input aperture both influence the value of illuminance and luminous intensity recorded. In addition, the beam illuminance does not follow the inverse squared relationship and the luminous intensity is not a constant.
It may be perfectly reasonable to want to measure illuminance in the near-field in some instances (for example, you may wish to know the lux level up close to the lamp), but the problem with near-field measurements is that the readings are to some extent dependent upon the actual size of the photodetector and the intensity value will be lower than the true far-field reading. In other words, a near-field illuminance (lux) measurement cannot be used to calculate the far-field luminous intensity (candela) value as the inverse squared relationship does not hold.
For simple, diffused, wide angle light sources, the photometric far-field is typically at a distance of five times the luminous aperture on the lamp. This would be the case for automobile side and marker lamps, with the ECE regulations requiring their measurement at 3m. For narrow angle sources and those with complex beam structure (such as vehicle headlamps), the far-field distance is much greater. To ensure that headlamps are correctly measured in the far-field, ECE and other regulations stipulate a measurement distance of 25m. While in reality the true far-field distance for a given headlamp may be less than 25m, this distance is specified to ensure that all conformity measurements are made in the far-field irrespective of the design of the headlamp optics.
Goniophotometric Test Method
Goniophotometers intended for testing vehicle headlamps need to be very precise, with the beam sampled every 0.1°. To avoid undesirably long measurement times, high-end automotive goniometers commonly measure “on-the-fly”, meaning that they don’t stop and measure at each angle. Instead, the photometer records the intensity as the headlamp is in motion. As a consequence, goniometers designed for testing automotive lighting tend to be quite expensive. If you are prepared to live with a longer measurement time, or perform lower resolution scans for indicative testing, “stop-and-go” type goniometers are a lower cost option. In either case, the photometer must be placed at a distance of 25m from the headlamp mounted on the goniometer platform. The reason for this is to ensure that the headlamp beam is sampled in the photometric far-field, in which the beam can be considered to be “fully formed”. The complex beam shape of headlamps would yield significant measurement errors if sampled closer to the source, in the near-field. Generally, the goniometer control software will perform a scan, and then automatically analyse the beam shape and intensity values so as to determine whether the headlamp satisfies the requirements of the relevant ECE regulation.
Projection Test Method
Provided that you have a sufficiently large dark room as your lighting laboratory, the projected illuminance using imaging photometer method of testing holds many advantages. This technique employs an imaging photometer to record a 2D illuminance (lux) pattern of the headlamp beam as it is projected onto a vertical surface (wall or screen) at a distance of 25m. The lux versus XY position data set can be mathematically transformed into an intensity versus angle data set, often performed automatically though instrument software.
The projection method is the method cited in regulations. The simplest approach is to manually position an illuminance photometer (lux meter) on the projection wall facing the headlamp and record the beam illuminance as a function of position. However this is a very time consuming and laborious process and requires manual computation of the ECE test point compliance matrix. Manual illuminance measurement of headlamp beams using lux meters is not considered practical due to the time required to sample all of the required beam spots.
The more recent development is to replace the simple lux meter with an imaging photometer. The imaging photometer is used to take a photometrically-calibrated, high dynamic range digital picture of the beam pattern and automatically align the required test point matrix to the headlamp HV point (datum). The ECE test point compliance matrix will then be automatically generated based upon the best possible alignment of the test points within the beam that the software can find. The figure shown to the right shows a grid of ECE R20 test points superimposed into a dipped beam headlamp beam pattern that was recorded using an imaging photometer.
Goniometric Versus Projection Testing
Compared with goniometric measurements, the projection test method using an imaging photometer has the following attributes:
- It is faster, measurements taking seconds to perform.
- It is simpler, the headlamp can remain mounted on the vehicle (assuming that the laboratory is large enough that the car can be driven into it).
- It is less expensive, the typical cost of an imaging photometer being a fraction of the cost of an automotive goniophotometer.
- The imaging photometer can also be deployed for other kinds of testing, for example analysing the lit area uniformity of side or marker lamps or of DRLs (daytime running lights).
- On the down side, you do need a 25m long dark room to perform projected beam tests, however, indicative tests at shorter working distances (e.g. 10m) have been shown to correlate closely with 25m measurements. In other words, a headlamp that passes the ECE test points at 10m is very likely to also pass at 25m, and vice versa. Thus, even if space is limited, projection headlamp testing will still serve as a useful developmental and benchmarking tool for vehicle manufacturers and for those manufacturers of vehicle headlights.
In Conclusion
Headlamp technology continues to develop, with the very latest matrix LED, laser assisted high beams and digital matrix LED designs providing safer, less stressful illumination for the driver, while at the same time avoiding dazzle, glare and discomfort to other road users. Yet the basic metrology of headlamps remains the same, regulations requiring that one measure the luminous intensity of the headlamp as a function of angle (or, equally, one can measure illuminance as a function of XY position within the beam).There are two basic approaches to the testing of vehicle headlamps, one using a goniophotometer, the other being to project (shine) the headlamp beam onto a vertical surface or screen and record the 2D illuminance pattern using an imaging photometer. The goniometric technique is probably the one which most engineers would be more familiar with, the headlamp being mounted on a 2-axis motorised rotary stage (the “goniometer”) which is viewed by a fixed light meter (the “photometer”) that records the intensity for each angle of view as the lamp rotates. The more recent, and significantly lower cost approach is to employ an imaging photometer to record the beam pattern as it illuminates the screen. The projection method of testing is not only cheaper, it is faster, but you do need a large dark room to project the headlamp down (headlamps being tested at 25m, per regulations).
