Tricks of the Light

First published in BMW Car Magazine March 2018

Chris Graham explains how car headlamps have developed, why they’re so good now and what’s in store for the future.

Back in the 1970s, most car headlights were standard, round five- or seven-inch diameter units; all very straightforward and utilitarian. But the progress that’s been made during the intervening 40 or so years has seen nothing short of a revolution, in both the appearance and functionality of these often overlooked essentials of the modern car.

While we all rely entirely on the performance of our car’s headlights for nighttime driving, the technology that goes into them, and the effort that’s put into their design, are factors that most motorists take completely for granted. And yet, in many cases, the modern headlamp unit is a work of visual art; an array of wonderfully-shaped, attractively-finished almost jewel-like treasures, protected behind a crystal-clear cover.

Fantastic advances

Today’s vehicle lights are a far cry from the simplistic and often woefully inadequate headlamps of yesteryear. For decades, car headlights were rudimentary, metal and glass affairs, with traditional filament bulbs that bounced yellowish light off a shiny, convex reflector and out through a glass lens that directed the beam down on to the road.

It wasn’t really until the late 1970s and early 1980s that car designers started to realise that there was real scope to enhance the look of the headlight units, while improving their performance at the same time. These changes went hand-in-hand with improvements in technology which allowed the format to be changed. At long last, it became possible to break away from the standard approach, and the limitations of weight, fragility and expense that the old-style units carried with them.

The first big breakthrough came thanks to the development of the faceted reflector. Typically made from plastic, these multi-surfaced, mirror-like reflectors were designed to replace the outdated and inefficient glass lens. All the beam-direction work was accomplished by the reflector from behind the bulb, which meant that all that was required in front was a weight-saving covering of clear plastic to keep the light weather-sealed.

This gave designers much greater scope to make a visual feature of the headlights, but it also put greater pressure on the functionality of the lights as many were made smaller. Consequently, light output had to be increased and controlled more effectively to deliver the necessary illumination.

The early 1990s saw the introduction of the projector-type unit, which relied on a much smaller, internal glass lens positioned close to the bulb, to focus the light output on the road ahead. This represented another progressive step forward, although the approach wasn’t universally adopted by all vehicle manufacturers.

Boosted output

Of course, bulb enhancements were key to these performance improvements. The original tungsten filament bulbs, that had been around since the 1940s, were finally replaced by halogen bulbs in the 1970s, when it became necessary to boost light output and reduce the size of the source. However, the halogen bulb was effectively just a harder-working version of the tungsten unit (refer to the ‘Light source development’ panel).

But one of the most fundamental drawbacks with all bulbs that rely on a thin wire filament is that, sooner or later, that wire is going to burn through and fail. What’s more, the harder they’re worked (and the brighter they burn) the quicker that failure will occur. So the industry began searching for an alternative, and the first solution involved harnessing the intense light that’s produced when an electric spark jumps between two electrodes

The result was the high intensity gas discharge (HID) unit, commonly referred to as the xenon bulb. An inert gas – in this case xenon – is contained within a small glass envelope. Metal electrodes extend in from each side of this envelope, and the gap between their tips is what causes the electricity to arc, and the bulb to light. This technology offered a much longer service life (no delicate filament to break), lower power consumption, two or three times the light output of a halogen bulb and a colour temperature that was much closer to that of daylight.

On the downside, HID bulbs are very expensive, require complex electronics to control them, an integral lens washing capability and  an automatic, self-leveling system to prevent on-coming drivers from being dazzled by wayward beams. All of these factors added enormously to the cost of installation, and meant that for a good many years, the use of HID lights was restricted to high-end marques such as BMW.

LED rules, OK?

Now there’s a move afoot to replace the expensive but very effective, HID headlights with LED-based alternatives which, although still very expensive when compared to traditional halogen lighting, are a good deal more cost-effective than HID. Consequently, their use is currently restricted to the more prestigious marques, and often only to the high-end vehicles within various model ranges. BMW, however, is adopting LED lighting technology extensively across its model ranges.

The real beauty of LED as an automotive light source is that it’s incredibly controllable. So, rather than having a single light source in each headlamp unit, individual LEDs can be grouped together to produce the same sort of output, but also offer a tailoring of that output for other functions.

The computer-controlled switching of the various LED elements in a cluster can be used to vary the beam pattern that’s illuminating the road ahead, offering a degree of adaptability that’s never been possible before. These systems can be programmed to vary the output depending on the presence of other vehicles, whether they’re being followed, or are approaching from the opposite direction. So ‘high beam assist’ functionality now allows the headlights to be run permanently on their high-beam setting, with the beam pattern being adjusted automatically to suit the road conditions.

The most recent development involves a technology known as ‘LED laser’; a very futuristic-sounding but, as yet, not fully-functioning system. Somewhat disappointingly, it appears that the term ‘laser’ is a touch misleading, as there aren’t actual lasers – as you and I might imagine them – involved. Effectively, we’re still talking about LED technology, just in a higher-performance version and with a greater output.

But the high outputs are still proving difficult to control so, at the moment, this technology is restricted to the high beam units on a few, top-end vehicle applications. Inevitable, though, manufacturers will crack the problem in due course, enabling the power of LED laser lights to be harnessed effectively to use on dipped beam applications too, at which point it’ll become the most desirable lighting option.

Tasty source!

The Holy Grail for a lighting engineer is to create a unit that’s inexpensive to manufacture, low-cost to run, easy to direct and that produces light at a colour temperature that’s as close as possible to natural daylight. The human eye works best in daylight, so vehicle lighting that gets close to mimicking this is going to offer the safest and most effective option. This is why xenon lighting represented such a remarkable improvement over the halogen-powered systems that went before it.

Traditional, incandescent bulbs produce light with a colour temperature of about 2,700 degrees Kelvin (K), which is actually towards the yellow/orange end of the visible spectrum. A typical HID unit operates at 3,500K, which is a lot less yellow and much more neutral, while a modern LED can output light at 6,000K. This is a lot ‘cooler’ and more towards the blue portion of the spectrum.

I remember reading once that, as we age, the human eye becomes less and less sensitive to light at the yellow end of the spectrum which, of course, is exactly where most of the halogen-powered headlights sit. So, those of us of a certain age, who drive cars with traditional headlights, are actually bathing the nighttime road in a light that we find it increasingly difficult to see with!

Another interesting aspect which helped trigger the industry’s move away from HID lighting technology, is that the colour output from these lights varies during their service life. This will increase (becoming more blue), then peak and start to decrease again as the years pass. This typically occurs over a 3,000-hour period, but there are a lot of service-related variables involved, too.

Durability issues

When HID lights were first introduced, vehicle manufacturers proudly announced that these units would last the lifetime of the vehicle but, sadly, that hasn’t been the case. The life expectancy of xenon lights was relatively quickly modified to a more realistic five years and, most recently, research has revealed that it’s actually nearer three. The situation will be better with LED headlights as there’s no electrode consumption involved, so nothing to burn out.

Of course, there remain a great many cars around that still use conventional, filament-type headlight bulbs and, while most nowadays produce a reasonable output, bulb upgrades can represent an affordable and very worthwhile option. ‘High-power’ replacement bulbs from a quality producer will significantly boost light output to enhance nighttime driving and safety.

The genuine gains to be had nowadays from well-engineered, upgraded bulbs are very impressive. These used to be pegged to an improvement of about 30%, but the painstaking development work undertaken by specialist bulb producers like Ring means that it’s now possible to buy bulbs offering a genuine 150% more light output than the standard unit. What’s more, this has been achieved without affecting power consumption, so there are no potentially damaging, knock-on effects for the vehicle’s wiring, switches or sensitive engine management systems, as sometimes used to be the case. The only downside is a shorter service life but, the sort of performance gains now available easily outweigh this disadvantage.

As far as the automotive future is concerned, it would appear that the days of the filament bulb are numbered. The ‘solid state’ solution offered by LED technology will be progressively enhanced and, with unit costs being driven ever lower, the use of this lighting source is surely set to become increasingly widespread on vehicles of all types.

Light source development

Traditional bulbs operate by passing an electrical current through a thin wire, causing it to heat up and glow. The more current that’s passed, the hotter the wire gets and the brighter it glows. But there’s a balance to be struck. Overdo it and the filament will be consumed, the electrical circuit will be broken and the bulb will stop working.

The switch from conventional, filament-type light bulbs to halogen versions made a significant difference to light output. Greater brightness was achieved by ‘burning’ the filament hotter while controlling the greater evaporation rate of the filament by surrounding it with halogen gas.
When you see an old, filament-type bulb that’s failed, and you notice dark-coloured deposits on the inside of the bulb’s glass, that’s the residue from the burnt (evaporated) filament. Operating the filament in a halogen gas-rich environment triggers a reaction between the gas and the filament, causing the vapourised metallic particles to be re-deposited on the filament, thus extending service life.

High-performance halogen bulbs, which burn even hotter to achieve their greater light output, are able to do so because, as well as a halogen gas, the bulb also contains xenon. These gases are contained within the bulb’s glass envelope at a very high pressure, which acts to preserve the filament to an even greater degree.

Nevertheless, wire filaments remained an inherently weak link, with their gradual evaporation and vulnerability to vibration meaning that service life is always going to be limited. Removing the filament from the equation took vehicle lighting technology to the next level, with high intensity gas discharge (HID) and LED bulb types significantly boosting both light output and quality, as well as overall durability.

Light brightness is measured in units called lumens, and a conventional halogen bulb typically produces about 1,500 lumens. This compares to an HID bulb which outputs about 3,000 lumens while, somewhat surprisingly, the latest LED units are producing about 1,500 lumens. Arguably, they represent something of a backward step in output terms but, in every other respect – cost, efficiency, weight, electrical complexity, service life etc – they are superior.

Anecdotally, drivers are noticing the difference between HID and LED headlight systems, and not in a good way. However, it has to be said that the reduction in brightness is somewhat offset by the fact that LED light has a significantly higher colour temperature than HID light, so the illumination can appear more natural.

Unfortunately, thanks to the vagaries of the European type-approval system, LED lights have to be homologated as a single unit, in contrast to HID and halogen units, the components of which are homologated separately. Consequently, HID and halogen failures can be rectified with a replacement bulb (or other component) but, when an LED headlamp fails, the light unit has to be replaced in its entirety, which is massively more expensive.

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