# Why Do Plastic Headlamp Lenses Yellow? **URL:** \ **Category:** Factual Questions\ **Created:** [July 30, 2008, 1:22am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179 "2008-07-30T01:22:38Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [July 30, 2008, 1:22am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179/1 "2008-07-30T01:22:38Z")
I am wondering why the environment, light, etc. turns clear plastic yellow. My Dad’s 9 year old car has one headlight that is all cloudy, the other is clear. Why the difference? and, can you restore the ellowed plastic? ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [July 30, 2008, 2:17am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179/2 "2008-07-30T02:17:02Z")
> [@](#): > > Are the surface of your headlamps fogged? Plastic lenses are covered with a special UV-resistant coating. After years of exposure to pollution and UV-containing sunlight, it can fog. Using rubbing compound to remove the haze is a short-term solution. With the coating polished off, the lens will yellow and haze. your only solution is to replace the entire assembly. From the [_Popular Mechanics Complete Car Care Manual: Updated and Expanded_ p. 169-170](http://books.google.ca/books?id=39xxTCsBjUAC&pg=PA170&lpg=PA170&dq=popular+mechanics+headlight+yellow&source=web&ots=00EqH3OBcU&sig=XLK5P-KhkOVH-XB1IsLCDnY1lDU&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA170,M1) ---
**Author:** ![ignis\_glaciesque](https://avatars.discourse-cdn.com/v4/letter/i/5f8ce5/32.png) [@ignis\_glaciesque](https://boards.straightdope.com/u/ignis_glaciesque)\ **Post date:** [July 30, 2008, 2:52am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179/3 "2008-07-30T02:52:59Z")
Err, shouldn’t this be in General Questions? ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [July 30, 2008, 2:59am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179/4 "2008-07-30T02:59:44Z")
[QUOTE=ignis\_glaciesque] Err, shouldn’t this be in General Questions? [/QUOTE] Yep. And now it is. ---
**Author:** ![Full\_Metal\_Lotus](https://avatars.discourse-cdn.com/v4/letter/f/df705f/32.png) [@Full\_Metal\_Lotus](https://boards.straightdope.com/u/Full_Metal_Lotus)\ **Post date:** [July 30, 2008, 7:24am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179/5 "2008-07-30T07:24:33Z")
Ask the Musuem Conservator… Transparent plastic (especially polycarbonate (ie Lexan)) tends to yellow because the UV wavelength breaks the long chain molecules into smaller segments. These products are generally non transparent, and over time tend to accumulate and discolour the plastic. Reconditioning is possible, but generally not worth the time/effort/cost. The discolouration tends to occur in the first few micrometers (1000ths of a millimeter) of the surface. It involves sanding off and repolishing of the surface. A cheap, fast and temporary fix to things like head lights is to sand down the surface with a fine grit wet paper, and then spray with a clear solvent based gloss coat. This will make the surface look clear, but due to its own yellowing from UV (a lot faster than polycarbonate),and the chemiocal break down of the poly carbonate by the bonding/solvent reaction (which yields faster UV breakdown in the orginal polycarbonate molecules). This “fix” however, will only be good for about opne summer worth of UV exposure… Regards FML ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [August 2, 2008, 1:59am UTC](https://boards.straightdope.com/t/why-do-plastic-headlamp-lenses-yellow/458179/6 "2008-08-02T01:59:32Z")
FML has a nicely plausible answer. Lots of things turn yellow with time. More precisely, pretty much anything transparent has some cutoff wavelength below which it is no longer transparent, and for a great many things this cutoff wavelength increases with time. We often observe things “yellowing” with age. Our own eyes do, the transparent parts I mean. This phenomenon is happening in the ultraviolet, too. Not that it’s caused by it, though it’s reasonable that it would be, but that things become less transparent in the ultraviolet before they start doing it with visible blue lights. It is a headach in the world of ultraviolet technology finding materials that are transparent enough at very short wavelengths. Many things in nature have colors that are characterized by this cutoff, which is why there are so many reds and yellows and browns and oranges. If it weren’t for chlorophyl, there would hardly be any greens in nature, except maybe a few metal oxides. If it weren’t for the scattering of sunlight in the atmosphere, blue would also be very rare in nature.