Headlights go cloudy mainly due to Ultra Violet (UV) damage from sun light, which is particularly harsh here in NZ. The plastic (ie. polycarbonate) used in headlights naturally goes cloudy very quickly due to oxidisation on the surface. To stop this car manufacturers coat the headlight lens with a clear coat coating. This however breaks down over time (several years) and will itself become hazy and cloudy, and sometimes starts yellowing as it degrades.
So the cloudiness you see on your headlights is the result multiple processes:
- The degradation of the protective clear coat layer
- Oxidation of the exposed polycarbonate surface now that the clear coat layer has failed, and...
- The 'roughing up' of the headlight surface due to ballistic particles, dust and stones that strike the surface as the vehicle speeds along.
The affect of this is a considerable reduction in the performance of the headlights: a significant drop in amount of light output due to the filtering affect of the opaque 'cloudiness' on the lens surface, and a decrease in the quality of the light output caused by light scatter, de-focusing the beam from where it should be directed - on the road ahead. (Fyi. this scattering causes the halo affect you see when a car with cloudy headlights drives towards you at night).
And of course cloudy headlights make you car look terrible.