Headlight restoration or new bulbs?

Two causes that look identical from the driver’s seat

You notice the same symptom either way: the road ahead is darker than it used to be. The cause is in one of two places, and they are not related.

The lens. Modern headlight covers are polycarbonate with a factory UV hard-coat. Sunlight degrades that coat, the plastic underneath oxidizes, and the surface goes from optically clear to a fine matte haze. Light still leaves the bulb at full output and then gets scattered in every direction by the lens instead of being projected down the road.

The bulb. A halogen filament deposits tungsten on the inside of its glass envelope as it ages. Output falls gradually and the color temperature drifts. Nothing about the housing has changed — the source is simply producing less light than it did.

The test, which takes thirty seconds

1. Look at the lens in daylight, from the front, up close. Yellow, brown, milky or hazy means oxidation. Run a wet finger over it: oxidized plastic feels slightly chalky rather than glassy.
2. Then park facing a wall at night. A clouded lens gives a soft, spread, fuzzy-edged patch with no defined cut-off line. A tired bulb gives a beam with its shape and cut-off intact, just dimmer and warmer in color.
3. Compare left to right. Bulbs age at similar rates but rarely identically. A noticeable difference between the two sides usually means a bulb on its way out.
4. If the lens is hazy, stop here. Restore it first. You cannot judge a bulb through a lens that is scattering its output.

What each fix actually involves

Neither category publishes what you would most want: a before-and-after light output figure. The restoration kit page goes through how to judge a kit on its grit sequence and whether it includes a real UV sealant, and the bulb page works through what FMVSS 108 actually permits — which matters more than any brightness claim.

Choose restoration when

• The lens is visibly yellow, hazy or chalky in daylight. This is most cars over about six years old that live outside.
• The beam has no crisp cut-off line against a wall.
• You are selling the car — clear lenses change how the whole front end reads.
• You have an hour and are willing to sand carefully. Rushing the grit sequence is the main way this job goes wrong.

Who should skip it: anyone whose lenses are already clear, and anyone unwilling to work through every grit in order. A kit used impatiently leaves sanding marks that scatter light as effectively as the oxidation did.

Choose new bulbs when

• The lens is clear and the beam is still weak, or one side is visibly dimmer.
• The light has gone noticeably yellow rather than white.
• A bulb has failed outright — replace both sides, because the other one is the same age.
• You want a modest, legal improvement in a housing designed for a halogen bulb.

Who should skip it: anyone with a hazy lens, for the reason above. And anyone expecting an LED conversion to transform a halogen reflector housing — that is a beam-pattern problem rather than a brightness one, and the bulb page sets out why.

When it is neither

Three cases where both purchases are the wrong answer. The lens is crazed with fine cracks rather than merely hazy, which sanding cannot reach because the damage is through the plastic. The housing has a broken seal and is fogging from the inside, where no amount of exterior polishing helps. Or the aim is simply wrong, which is a screwdriver and a wall rather than a product.

There is also a free step worth doing before either: clean the inside of the windshield. The film that builds up there scatters oncoming light into starbursts and is very often what people are actually describing when they say their headlights got worse — how to stop glare while driving covers it.