Your headlights lose most of their light long before a bulb ever fails
There is no warning light for a headlight that still works and no longer lights very much.
A blown bulb announces itself. A clouded lens never does, and that is the whole problem with it. The clear cover on almost every headlight built in the past twenty-five years is polycarbonate, chosen because it is light, cheap to mould into complicated shapes and does not shatter into a pedestrian. It has one weakness: ultraviolet light takes it apart. The factory answer is a very thin, very hard coating sprayed over the outside, a sacrificial layer that then spends its entire working life being scoured by grit, insects, washer fluid and sun. When it finally gives up, the plastic underneath oxidises and crazes into microscopic cracks, and every one of those cracks takes light that was aimed at the road and throws it somewhere else. Laboratory testing of badly degraded headlights has measured them putting out roughly a fifth of what the same units produced when new, with a perfectly healthy bulb burning behind them. Nothing on your dashboard has the faintest idea any of this is happening.
What you lose is not brightness, it is aim
A dipped beam is not a torch, it is a shape. Nearly all of its light is supposed to sit below a sharp horizontal cut-off, so that the road is lit and the eyes of the driver coming the other way are not. A clouded lens behaves like frosted glass: it does not so much absorb light as scatter it. Part of that scattered light never reaches the road, part of it lands in a useless bright pool three metres in front of the bumper, and a meaningful part of it goes upwards, over the cut-off and into oncoming traffic. Hazy headlights therefore pull off the impressive trick of leaving you with less light and everybody else with more dazzle at the same time. It matters most exactly where the reach of a dipped beam is all you have, on an unlit road at dusk in autumn, which is also the road and the hour where an animal at the verge gives you about one second to decide anything.
Inspectors already treat this as a defect, not as cosmetics
Drivers file cloudy lenses under paintwork and parking scuffs. The people who inspect cars do not. The British MOT manual is the bluntest about it: a compulsory headlamp is a major defect if it is excessively damaged or deteriorated, or has a product on the lens, so that light output is well below what is needed to light the road ahead, and a lens that is only slightly deteriorated is still recorded as a minor defect. Germany rejects heavily yellowed, matt or scratched covers at the Hauptuntersuchung outright. The Dutch APK measures what actually leaves the lamp, so a dull lens turns up either as insufficient output or as a beam that has drifted from white towards yellow, and it is one of the more common lighting rejections there. The wording changes at every border. The principle does not: what is tested is the light that reaches the road, not whether the bulb lights up.
The polishing kit is an ordinary Saturday job in Britain and forbidden in Germany
Every parts shop sells a restoration kit for this, and on a British or Belgian driveway using one is unremarkable. In Germany it is not allowed. A headlight is a type-approved assembly and the protective coating is part of what was approved, so the federal transport ministry's position, which ADAC repeats in plain language, is that reconditioning headlight cover lenses by grinding, polishing and sealing or comparable procedures is not permissible. An inspector who recognises polished lenses can withhold the test sticker, and heavily clouded ones fail anyway, which leaves replacement as the only compliant route: several hundred euros to nearly two thousand for a modern unit, ADAC reckons. Legality aside, there is a physical catch that applies everywhere. Polishing works by sanding away the ruined coating along with the oxidised plastic beneath it, so unless the final step genuinely puts a new UV layer back on, the bare polycarbonate you have just exposed has nothing protecting it and clouds again far faster than the original ever did. That is the part the before-and-after photo never shows, and it is worth knowing both when a hazy pair is quietly costing you money as you sell the car privately and when you are pricing a used one you are about to buy.
The top of the lens goes first, and it tells you where the car has been sleeping
The damage is never evenly spread, because sunlight is not. The upper surface of the lens takes the most direct ultraviolet, so clouding almost always starts as a milky band along the top and works its way down, and a car that has spent years parked nose-on to open sky on one side of a street often has one lens visibly worse than the other. Left outside through southern European summers, the first haze can appear within three to five years of the car leaving the factory. A garaged car in Belgium or the Netherlands can go a decade with barely a mark. That puts headlights in the same awkward category as the date code on your tyres: a part whose life is measured by the calendar and by exposure rather than by mileage, with no service interval and no dashboard prompt attached to it. Nobody replaces a headlight at 60,000 km because the schedule said so. It is either noticed, or it is not.
Five metres from a wall at dusk is the whole inspection
You need no equipment for this. Park square to a flat garage door or wall about five metres back, at dusk rather than in daylight, and switch on dipped beam. You are looking for three things. The cut-off should be a defined horizontal line rather than a soft gradient; both sides should be the same brightness; both sides should be the same colour. A tired lens gives you a fuzzy upper edge with light bleeding above it, and often a warmer, yellower patch on one side only, which is the asymmetry that comes from where the car parks. While you are standing there, look at the lens itself with the lights off, then wet it. Haze that disappears the moment water sits on it is mostly surface roughness, and haze that stays is oxidised plastic deeper in. Misting on the inside of the glass is a different fault altogether, and it belongs to the seals and the breather vents rather than to the coating.
What to actually do
Most of this costs nothing and takes ten minutes on a driveway. The one decision worth thinking about before you buy anything is which country's rules your car is tested under.
Do the wall check at dusk twice a year, and make one of them now, before the clocks go back at the end of October and your commute moves into the dark.
Judge left against right rather than one lamp on its own. This damage is usually asymmetric, and the good side is your reference.
Look at the top third of the lens first. That is where the coating dies first, and where you will see it a year before anyone else does.
Keep solvents and abrasives off the plastic. Bug remover left to dry in the sun, petrol, acetone, thinners and cutting compound all attack whatever coating you have left.
Never fit tinted or smoked lens film. A product on the lens that cuts light output is a test defect in its own right, quite apart from what it does to your night vision.
Do not answer a hazy lens with a brighter bulb, and do not drop LED bulbs into a housing designed for halogen. Both push more light into the scatter, and the retrofit is not type-approved in most of Europe.
If your car is tested in Germany, treat polishing as off the table and budget for replacement instead, either new or a sound second-hand original unit.
If you do restore the lens elsewhere, insist that a real UV clear coat is the final step and ask what is being used. A polish without one is a fix with a season's shelf life.
Park the nose out of direct sun where you have the choice, and check the lenses on any used car before you agree a price. Establish whether the unit is halogen, xenon or matrix LED first, because those are three completely different bills.
Frequently asked questions
Can I not just fit a brighter bulb and be done with it?
It treats the wrong end of the problem. The bulb was never the part that failed; the optics in front of it were, and they are now sending a share of the light in the wrong direction. Feeding more lumens into a scattering lens increases the glare above the cut-off along with the useful light below it, which is why a car with hazy lenses and upgraded bulbs is the one everybody flashes at. A legal high-performance halogen buys perhaps a modest gain in reach on a clean lens, which is nothing against what a badly oxidised one has taken away. Fix the glass, then look at the bulb.
Is polishing headlights really illegal, or is that a myth?
It depends entirely on where your car is tested, which is why the internet argues about it endlessly. In Germany it is documented and specific: the transport ministry's position is that grinding, polishing and sealing headlight cover lenses is not permissible, because it alters a type-approved component, and an inspector can refuse the sticker over it. In Britain, Belgium and the Netherlands there is no equivalent blanket prohibition, and what the test looks at is the result, meaning the output, the beam pattern and the colour of the light. So the honest answer is that the same tin of polish is routine in one country and a failed inspection in another, and if your car is registered in Germany the question is settled for you.
My headlight is foggy on the inside. Is that the same thing?
No, and no amount of polishing will touch it. Headlights are vented rather than sealed, because the air inside them expands and contracts as the lamp heats up, so a light mist after a wash or a cold night that clears within an hour of driving is normal behaviour. What is not normal is fog that stays for days, droplets running down the inside or standing water in the bottom of the housing. That means a failed seal, a cracked housing or a blocked breather, and left alone it corrodes the reflector coating, which is a far more expensive kind of dim than a cloudy lens. On sealed LED units there is often nothing to open and repair, so it becomes a replacement conversation early.
Do LED and xenon headlights suffer from this as well?
Yes, because the cover is the same polycarbonate regardless of what sits behind it, and ultraviolet does not care how modern the light source is. The consequence is worse, though. On a halogen car the lens is the cheap part of an inexpensive assembly. On a car with adaptive matrix LED headlights the lens is the front of a sealed electronic module, and there is no bulb to swap, so a clouded cover puts the entire unit on the invoice, often with aiming and coding at a workshop on top. It is one of the quieter running costs of the technology and almost nobody factors it in when the car is new.
The garage quoted more for one headlight than the car is worth. What are my options?
This is common on older premium cars and it is the reason second-hand original units exist as a proper market rather than a bodge; a sound used unit from a dismantler is the route ADAC itself names alongside a new one. If you go aftermarket instead, the thing to check is the approval marking on the lens, because a unit without a valid E mark is its own inspection failure and you will have paid to fail twice. Whichever you fit, budget for the beam to be aimed afterwards on a proper setting device, since a headlight that is straight in the bodywork is not necessarily aimed where the test expects it.
