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Automotive bulbs – how they work and why they degrade

Replacement car bulbs remain one of the fastest moving and profitable product lines, leading Rob Marshall to question Osram and Philips Lumileds about how they work and why they degrade.

It is quite remarkable that the humble light bulb has become so central to our modern way of life that we take the technology for granted. While conventional bulbs were banned from domestic use from September 2021, they remain popular within an automotive context. The principle on which a halogen bulb works extends to the early 1800s. Humphrey Davy discovered that a metal wire heated up, when electricity passed through it. Unfortunately, the wire reacted with the oxygen in the air and burnt away; enclosing the wire within a glass tube containing inert gases posed the solution.

Behind the scenes

Modern automotive bulbs employ coiled tungsten filaments, because the metal possesses the highest melting point. The use of halogen gases extends the filament’s life, because any evaporated metal redeposits itself onto the filament, increasing its life. Naturally, the bulbs do not last forever, chiefly because the filament thins and, eventually, fractures. While all automotive replacement bulbs, used in exterior lamps, must be Type-Approved, quality still varies between manufacturers. Some bulbs possess thinner filaments, less expensive gases and lower quality glass. The gas fill pressure can also be too low and poor quality production can also introduce impurities into the glass envelope.

Low grade bulbs can also damage the lamp to which they are fitted. Should the bulb glass lack an ultra-violet filter, the UV can damage the reflector and plastic cover. Some bulbs can also shatter, due mainly to thermal shock, especially if water enters the lamp unit for any reason, although those featuring quartz glass should be immune. Yet, technicians can also facilitate failure. Touching the bulbs with bare skin can impart an impurity that creates a hot spot. In addition, badly-fitted bulbs can increase their exposure to vibration and, possibly, overheating.

External issues…

High voltages are a notable bulb killer. Just raising the voltage by 10% can halve filament life. Therefore, any vehicle faults that increases voltage need to be remedied. Some lamp designs also possess poor airflow, which overheats the bulb, courting early failure. Driving regularly on deteriorated roads expose the bulbs to more vibration, as do certain aftermarket suspension upgrades.

Some technicians question the wisdom of carmakers’ thin wiring. However, this ‘thin wall’ cabling uses a more efficient higher temperature rated insulation. As a standard 55W headlamp bump draws 4.6 amps at 12 volts, the fact that these cables are rated at 11 amps means that such concerns are unfounded. However, should high wattage bulbs be desired, consider that a bespoke loom may be necessary but this advice is only really relevant for off-road bulbs that are not Type-Approved for on-road use. Even so, melted connectors can occur, as pictured, although this is due to a poor connection between the bulb terminals and those within the plug. Fortunately, replacement connectors are available in many cases.

HID bi-xenon headlamp bulbs

Rather than relying on a glowing filament, High-Intensity Discharge (HID) bulbs share their working principles with arc-lights, where light is produced by a glowing, high-voltage arc within a glass case. Like a domestic strip light, however, additional components are required. In a typical automotive setting, the ballast is a separate part that converts the 12V DC vehicle voltage to >20,000V AC to create the plasma. Once this occurs, the voltage is reduced to the bulb rated voltage (42V for a D3 bulb for example). An igniter is also required, although it is incorporated as part of D1, D3 and D5 HID bulbs. All these extra complications notwithstanding, HIDs produce, approximately, twice as much light for half the power consumption of filament bulbs. They also possess a far longer lifespan.

Yet, early HID D1 and D2 burners contained mercury vapour. Responding to the banning of the heavy metal, D3 and D4 mercury-free HID bulbs appeared in 2012, which are incompatible with the earlier types. To improve energy efficiency further, lower wattage bulbs have since appeared, such as D5S, D6S and D8S types.

The issue of quality

Counterfeits have been more of a problem for HIDs than filament bulbs. Fake items have been known to catch fire and Lumileds reports consequential vehicle fires. Illegal HID ‘xenon’ kits, which allow an HID burner to replace a halogen bulb, were popular for a while, until MOT enforcement discouraged their installation. The same situation involves LED conversion kits for halogen exterior lamps for cars registered from 1986.

A different failure pattern

Unlike filament bulbs, HIDs rarely cease to work. Instead, over time, the light output reduces. New HID burners produce a crisp, white output, possessing a colour temperature of around 4,100K. As they degrade, the colour temperature drops to 3,000K or below, resulting in a yellower output. Therefore, while the bulbs still work sufficiently to pass an MOT test, their condition may have fallen below that required for Type Approval. Therefore, advising the driver of an older vehicle to replace both bulbs is likely to improve the lighting significantly.

Wear and tear on the burner is higher, dependent on the frequency that they are switched on. This explains why certain vehicles, such as taxis, have HIDs that last around 2,500 hours, instead of the more typical 3,000+.


Valeo is a major supplier of LED technology to car manufacturers. Speaking with AT, it reveals that, unlike halogen, or HID bi-xenon, LED headlamp light sources are designed to last over 16,000 hours, which is the equivalent to over 15 years’ use.

With a desire to reduce emissions, car manufacturers have embraced LED headlamps, because they offer a 70% energy consumption saving for the same light output, compared with halogen. The headlamps are sealed, because the typical LED light source module is designed to last 20 times longer than a halogen bulb. This means that the LEDs, heat sink, fans, optics, and any self-levelling systems are enclosed. Yet, the main control unit, situated on the rear of the headlamp moulding, can be dismounted and replaced. Valeo reports that it stocks this driver for its lamps as a spare part.

Should the LED lamp develop a fault, a dashboard warning tends to be displayed. A replacement lamp also requires diagnostic programming. Given that a complete replacement headlight may be the only option, it is interesting to note that certain remanufacturing specialists, such as ACtronics, are adding LED headlamps to their ranges.

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