Quote:
Originally Posted by T-Cut
Of course, in the real world environment, plastics and their coatings are affected by abrasion from dust, etc. Frequent freeze/thaw conditions also pay their part in fatiguing the lenses.
The UV that degrades polycarbonate headlamp lenses comes from sunlight, which is absorbed by the sacrificial coating. Compounds in the coating react preferentially with UV radiation preventing it getting at the lens itself. The reactive compounds are permanently changed into non-absorbing chemicals. As their activity drops, the underlying polycarbonate will start to degrade. There's little if any UV generated by the headlamp itself, so lenses don't require an internal coating.
I recently repolished my headlamps for the second time after not giving them any protection the first time. They're still off the car (SORN) until the weather warms up a bit. The expensive UV protectant I intend to spray on needs somewhat milder ambient temperatures than we have at present.
Some polishing kits do come with a wipe-on type lens coating which I suspect is next to useless. They will give a very short lived UV mitigation, but you really do need a solid plastic film rich in UV absorbent to do a proper job. Even so, none of this is permanent unless you keep the car in perpetual darkness.
TC
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The reason I was asking, particularly about the reflection of the UV, is some years ago I can recall reading about a lens that had various coatings on it. Not referring to any UV coming from the bulbs themselves. It was supposed to be a high end lens, and indeed it was, except for bright light. There was a lot of flare with this lens, and after heavy investigation, it was explained that one of the glass coatings caused an internal reflection between the coating and the glass it was coating (talking microns!). Am probably thinking too much into it lol.
I have had those kits with the 'UV' protectorant. I have no doubt that it washed off lol. The ones I have used reminded me of Autoglym's extra gloss protection sealant, in its application.