Rick's Free Auto Repair Advice

When to replace headlights

How long do headlight bulbs last?

when to replace headlightsMost people don’t replace headlights until they burn out. That’s a big mistake. All incandescent headlight bulbs (with a filament) dim with use and that means you’ll have a harder time seeing (see filament deposits below). So it pays to replace headlight bulbs BEFORE they burn out to get maximum light on the road. In fact, by the time your bulbs reach the two year mark, light output has already been reduced by about 20%. More than half of all drivers never replace their headlight bulbs at that point and keep driving with dim bulbs. They’re risking an accident at night.

What type of headlight do you have?

There are three types of vehicle headlights: sealed beam, halogen capsule or high intensity discharge. Sealed beam and capsule bulbs use a filament. When power is applied, the filament heats up and that heat creates light. However, the heated filament also evaporates and that evaporated tungsten can deposit on the lens or the interior of the capsule, reducing the brightness. Tungsten evaporation is greatest when the bulb is built with just a vacuum. So manufacturers inject a gas into the bulb.

A sealed beam headlight is a single unit that contains the reflector, filament and lens. When it burns out, you replace the entire unit.

sealed beam headlight

Rectangular Sealed Beam Headlight

headlight sealed beam

Round Sealed Beam Headlight

Sealed beam headlights usually contain a mixture of 88% argon and 12% nitrogen. The gas reduces the rate of tungsten evaporation. Lower evaporation rates mean the bulb experiences less blackening. The gas also allows the filament to operate at a higher temperature, giving off more light.

A halogen capsule bulb also contains a filament, but doesn’t have a reflector or lens. When a halogen headlight burns out, you replace just the bulb and reuse the car’s reflector and lens.

Halogen is also an inert gas. But halogen offers a significant advantage over argon and nitrogen. When a halogen bulb operates, the tungsten molecules evaporate and cling to the halogen molecule. When you turn off your headlights and the filament cools, more tungsten molecules return to the filament. So a halogen bulb filament lasts longer than a sealed beam bulb and experiences less darkening because more molecules return to the filament.

PHILIPS HID bulb 85122

HID bulb

A high intensity discharge (HID) bulb doesn’t have a filament. Instead it has two tungsten electrodes inside the glass envelope. Xenon gas and metal or mineral salts fill the glass envelope between the electrodes. When you turn on HID headlights, an ignitor creates a high initial current

HID ignitor

HID ignitor

that causes an electrical arc to jump between the two electrodes. Once the arc is established, a ballast maintains the proper current to keep the arc burning. The metal or mineral salts conduct electricity and glow to create the illumination. So an HID bulb is

HID ballast

HID ballast

more like a neon sign or fluorescent tube.

Unlike a filament bulb that creates a single “hot spot” of light, an HID creates two hot spots, one near each electrode. That’s why you can’t convert a standard halogen headlight assembly into HID simply by replace the halogen bulb with an HID bulb. See this post for more information on why this conversion is not only illegal, but actually throws less light on the road.

HID bulbs are more energy efficient than either sealed beam or halogen capsules, so they create more light with less power. Plus, HID bulbs last much longer than a filament style bulb. No filament means no evaporation.

However, there’s no free lunch with HID bulbs. Their light output deteriorates more than a sealed beam or halogen bulb. In fact, HID bulbs lose almost 70% of their light output over their life. In addition, HID lighting is far more costly to install and maintain. HID replacement bulbs cost around $100 each. A replacement HID ignitor costs around $40 and an HID replacement ballast can run as high as $400.

How long does a headlight bulb last?

The halogen light bulb capsules used in late model cars burn about 4,000 hours continuously under perfect laboratory conditions. But in the real world where they’re subjected to more vibration and hot/cold cycles, you’ll be lucky to get 400 hours out of them. Depending on how often you drive at night, that translates into about two years for the average driver. When one headlight bulbs burns out, the other is right behind, after all, they’re both on for the same amount of time. That’s why you should always replace headlight bulbs in pairs.

Don’t wait until your headlights burn out

Most cars and trucks are equipped with incandescent halogen headlight capsules, bulbs that fit into a headlight assembly that contains the lens and reflector so you only have to replace the bulb itself. All incandescent bulbs work by  passing an electrical current through a filament until it gets “white” hot and emits light. dim headlightsOver time, the high heat evaporates a small amount of the tungsten filament, turning it into vapor. Since the glass portion of the bulb is cooler than the filament, the liquid tungsten settles on the glass, turning it black. Those black deposits reduce the amount of light the headlight bulb casts on the road. By filling the bulb with halogen gas, bulb manufacturers reduce the buildup of molten tungsten on the glass, but they can’t totally eliminate it.

Filament deposits reduce light output by 20%

How fast the tungsten builds on the bulb’s glass envelope depends on how often you drive at night. But in a recent survey typical drivers, most headlight bulbs develop enough tungsten buildup to reduce light output by 20% within two years. The survey also shows that 55% of drivers have never changed their headlight bulbs. That’s an amazing statistic—it means that most drivers have reduced visibility from their headlights, making it harder for them to see you, pedestrians, bikers and road issues.

Replace headlight bulbs every 2 years

If you wait until a headlight bulb burns out, you’ll be driving with reduced light for a portion of the time. You have to replace the bulbs anyway; why risk an accident to saver $20?

What headlight bulbs should you use?

If you take your car to a shop, they’ll install the same type of bulb used by the car maker. Those bulbs meet Federal minimum standards. They may or may not be manufactured by a well know reputable bulb manufacturer like Sylvania, Philips or GE. In fact, shops can buy replacement headlight bulbs in bulk for as little as $2 each. Those bulbs are often poor quality products imported from China that’ll fail in about a year. Shops generally charge more for the labor than the cost of the bulb, so it pays to insist on a quality bulb to save labor over the life of the vehicle.

©, 2016 Rick Muscoplat




Posted on by Rick Muscoplat

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