Hi I was following a thread about crazing on headlight lenses but it was getting a bit heated and was wisely shut down, so here are the facts as I know them, anyone with a better explanation is more than welcome to put me straight, headlight lenses or covers are made from three possible sources, glass, polycarbonate, acrylic plastic, the glass is rare as its heavy and prone to chips and bullet holes from road debris, polycarbonate is much better as it is less  likely to break or crack on contact with road debris but it suffers with age related yellowing and has a lower light transmission efficiency, acrylic plastic has a higher light transmission and is cheaper therefore the most used and with the correct UV resistant coating will give good service especially on cars equipped with Halogen bulbs.

Now the interesting part with the advent of HID bulbs and projector lenses acrylic plastic works well but you tend to get deterioration of the covers from the inside in the area of the projector lenses, now go again with LED lights where each light has its own projector lenses with is made of glass as are all projector lenses the problem is amplified, LED bulbs do not create the same heat as Halogen or HID it is in fact less until you start using projector lenses which amplify both light and heat, in simple terms its the same as using a magnifying glass on a hot sunny day to burn holes in a piece of paper or wood and if you look at how far the sun is away from the earth it gives you a good idea of how potent projection can be, therefore on headlight covers interior the heat builds up and over time will cause pitting inside the cover which wont actually affect the lights brightness but looks unsightly, so this is the price we pay for super bright LED lights.

Steve.

Yet the Audi technician blamed it on using polish of some kind on the covers .  Headquarters also stated to use a wet cloth and nothing more to clean the covers.  If it's the actual LEDs causing it they obviously took a side step to avoid a warranty claim of sorts ..   I wonder if TFR is a factor in this ??   Obviously driving at night constantly, will accelerate this condition. 

Edited December 23, 2025Dec 23 by Audisq524

Hello Billy,

The reason why the original post was closed was that the OP (reasonably/unreasonably?) took offence at a simple logical point - already made. 
If the damage isn’t on the outside of the lens then how on earth can it be acceptable to be told that that damage has been caused by ‘outside influences’. This simple anomaly should have been addressed at source - at the time. 
Anyway, for what it’s worth, I’m very happy to accept Stevey Y’s logical explanation.

Regards,

Gareth. 

I'm happy for you .   You do an amazing job keeping this forum organised.  Enjoy Xmas 🎁. 

You too Billy, and many thanks for all your contributions. 

21 hours ago, Audisq524 said:

Yet the Audi technician blamed it on using polish of some kind on the covers .  Headquarters also stated to use a wet cloth and nothing more to clean the covers.  If it's the actual LEDs causing it they obviously took a side step to avoid a warranty claim of sorts ..   I wonder if TFR is a factor in this ??   Obviously driving at night constantly, will accelerate this condition. 

Hi yes any consistent use of the headlights day/night will hasten the deterioration, TFR wont do anything as the coating on the cover is tougher than the lacquer on the bodywork to avoid UV rays on a transparent surface which will yellow that surface, its definitely a projector problem on the inside, my Passat has a full row of LEDs in each light and they are well on their way to a pitting fest, what put me on to it was an advert for a special forces torch on the net, this thing has a COB LED element but a variable projector lens on the front which if you polarise the beam to its narrow point will burn through plasterboard wood and a host of other things including skin I suspect, open the variable projector and you get over a mile of usable light forward and about six hundred yards from left to right.

Steve.