In simple terms, automotive front lenses on headlamps where traditionally equipped with a textured glass that would spread some of light in the hotspot to each side of the road.
Around 1990-1995 there was a change of design of reflector headlamps: All OE manufacturers went over to totally clear front lenses, which left simply one function: keeping the weather out and create good aerodynamics. The general stageless U-shaped reflector was redesigned to have multiple U shaped elements that took over job spread and focus beam.
3-5mm optical grade polycarbonate (lexan) is the most used material used in headlamp front glass. They are usually surface treated to increase scratch and UV resistance. The first generation polycarbonate equipped headlamps around 1985-1995 had quality problems compared to earlier real front glass. Some OEM companies are still using real glass. Real glass is not recycleable and weighs more, but so far, nothing beats its longetivety.
Absolutely! Transparency means how much light will go through. Some of the light will reflect from inner surface and some will get absorbed into the lens and will go over into heat. This is typical losses and will always be there in smaller or lesser amount. A high quality front lens would use optical grade polycarbonate. It should have a light transmission of 90-94%. Anything less would be considered average or lower quality. One of the best quality front lenses in plastic comes as OE on the VW Golf IV - it does not look like there are any lens present at all:
They are injection molded at 300C
Extreme cold (below about -30C) will make even high quality tempered glass frigile and it can crack. This is a natural phenonomen and there is nothing we can do about it.
We all know that headlights costs up to USD 1500 each at the dealer.
There are 3 reasons for having this:
1) The protection is soft and will not get pitted like all other headlight after 60k miles.
2) All minor inpacts will not break the front glass.
3) If impact is severe and front lens breaks, plastic foil will still do a great
job of holding water out, until you get front lense or unit replaced. You actually have the
time to do so, because your headlight is temporary functional!
Herman have tested a headlight protective film available from
X-Pel
The film is 40mil or 1mm thick, made by 3M Scotch, and is installed wet. This film has
professional quality and sticks so well to headlights that you have to
use excessive force if left on for more than 1 week. This is good!
The film is of coarse scratch resistant and comes with a warranty. The
warranty is not on the film, but on the headlights! Light loss through
the film are not noticable, which make them compliant with transport regulations.
Prices of a pair is 20 to 80 USD depending on size. Also
Stonguard(untested) out west sells precut 3M film
and puts them into kits for variuos trendy vehicles.
A canadian vendor of Stonguard is
Auto Trim Design Incorporated
which is in Concord, Ontario. A 4x2 ft generic film costs about CAD50 pluss shipping.
Another US based web vendor is
Lamin-X (untested).
EGR Europe also have "Headlamp guard". Their address is: http://www.egrinc.com/
If your headlights is not protected they will get pitted over time and let UV rays in and making them yellow. If harsh chemicals (like window cleaners) is used for washing, they will slowly turn yellow as well over a few months time. In most cases, it is the outer surface that gets damaged. Inner surface can turn yellow as well, but only if bulb emits UV light. Low quality bulbs can emit unwanted UV light. Low quality blue coated bulbs can fume out and slightly fog up inside. There is no economically defendable fix to improve inside damages, so a purchasing a new headlight is recommendable.
Fixing the outside is simpler. There are no fixes that will make them like new, but they can be improved. First of all, we got to get rid of the pitts and yellowing. If an overall severe pitting has taken place, a tideuos sanding process must be performed first. Sanding process is pretty much as you would prepare a car body before painting, except one very important part: plastic will boil and destroy headlamp if you apply too much friction. Using cold water could be a good way to cool it down. In general, motorized tools are too powerful for this job. Selecting 400, 600, 1200 and 2000 grade can be wisely for a heavly pitted headlight. When you have reached 2000 grade you can go over to polishing, but it is probably not neccesary. A clearcoat (see below) will normally create a nice glossy surface when finished with 2000 grade. If you still want to polish, here is how: There are several ways to approach this:
Reports shows that once you start polishing, it will last for 6-9 months and then process has to be repeated. It only lasts a short time because pure polycarbonate is generally volatile in the environment. This is solved by painting with scratch resistant clear coat containing UV inhibitant. A great and usually easily obtainable solution for that is clearcoat used on boats. See Tools and Matrials page for more details.
OEM healights usually have a seal that will soften at elevated temperatures. The main idea here is to soften the seal, but not heat damaging headlight. This might not work on all headlamps. Use a stove to bake headlamp. Toaster ovens are too small for headlamps, but can do it for corner lights. Condo style ovens are big enough fro medium headlights. Large headlights require house size oven. Electric or gas driven does not matter. Baking time of 10 minutes and 400F are general numbers, and will need to be adjusted depending on sealant used in headlight and volume of headlight.
It is NOT recommended use iron, butane torch nor heat gun on plastic. It will warm up areas unevenly and will increase internal material stresses. Example of minor damage would be warping. More severe consequences would be cracking. Long time consequenses would be more prone to cracking during minor impacts by tennis balls, squarrals and so on.
Remove all screws. They can be small and hard to find around the edge. Wash headlamp throughly on the outside to get better view and ito reduce fogging fumes while baking. Let it dry before placing in oven.
Remove bulbs, wires, and connectors - they might deform during baking.
Select some sort of thermal insulator to use under headlamp. Use a few layers of aluminum tinfoil, large silicone baking sheet or ceramic dish, etc. Anything else than new aluminum tin foil should be pre baked to burn off whatever is not able to withstand heat. It is better to get those fumes out now than to have them condensate on headlight reflector (very bad).
Take everything out of oven except for one shelf, which you most likely will place as far down as possible without hitting bottom of oven. Preheat oven until reaching 400 degrees F. Then TURN oven OFF.
Insert the thermal insulator and place headlamp on top of it. Then insert headlight while watching that it doesnt get in direct contact with anything. Do not let headlight directly touch any walls or elements, they will burn ugly marks. Let headlamp sit in there for up until 10 mins. Monitor often - we do not want to start a fire. Put on hot or welding gloves. Remove headlamp. Modern silicone baking gloves works also well. Pry carefully with wide screwdrivers or even better, a butter knifes, so lense comes off of reflector housing gradually, working your way around seams. Starting to pry on bottom is advantagous, so that in case u make marks, it will be in a hidden area. When half of it has been separated, you should be able to slowly and persisitant pull them apart. Luy them with front glass down and rear end down. Long strings of sealant will hang from front lens to back housing. Be ready to catch them from falling onto unwanted surfaces. Let it cool down for at least 15 minutes and store away parts in fresh lastic bags. The longer you wait the less sticky it will be.
Whatch out for sealant mess. Seal is extremely sticky. Anything that is or gets in contact will stick and will leave almost irreversible damage to surface. Surface will not break or crack, but it will be dull. Small spills on front lens can be buffed out using a microfibre cloth while front lens is hot (right out o fthe oven). Screwdrivers (or butterknifes) will get into seal while prying out. It will attract liquid sealant in long strings as well.
The oven trick works successfully on:
Hyundai XG
Honda Civics, Prelude, Accord, Integra.
Ford 87-93 Mustang.
Lincoln Mark VII (84-92)
APC aftermarked for Honda Accord.
2002-2004 Nissan Altima
The oven trick has not worked on:
VW Passat (Hella)
If reusing the old sealant is not possible, there are some alternatives listed on the "Tools and Material" page, which is accessible from this site main page. We have not been able to find an irreversable sealant. Its easy to belive that silicone can be removed later with heat. Shure it can, but fact is that most silicones have a melting point that are way higher than the melting point of the headlight itself.
The most common usage of these is the function of making a car look like it has tinted headlights. They basically clips on in a snap to headlights of many trendy autos. The smoke tinted version is for show only, so I am ignoring them for now, and will discuss clear GTS covers only. The greatest thing about the GTS covers is that they have identical shape to original headlamp, and will cover it entierly. They do block ~5-15% of light output. (That is why they are not legal... another story). Not really an issue whith HID. You may want to give up a few percent to get better total looks. GTS covers are extremely frigile to scratches and should be handled with care. Storing them in a thick plastic bag during R&R is a definate plus. Dont panic now, a thick plastic protectant film from expel.com can be added to protect them during normal driving use. GTS covers are only ~2.2 mm thin acrylic, most OEM glass are 3-4mm anti scratch treated polycarbonate. Thats because GTS covers are supposed to be 100% mechanically supported by the OEM light. When comparing thickness, be aware that Polycarbonate is 1.5 - 3 times stronger than acrylic. GTS covers dont glue well to stiffer materials like polycarb, even when using industry strength plastic welder. Material is too soft. The weakest point is the connection GTS cover to glue. The best way around this is to make glue area at least 2 times bigger.
Condensation is deemed to happen if ambient temperature is lower than inside the headlamp
and humidity inside of headlight is too high.
From a headlight design perspective, you cannot lower
ambient temperature. Not much can be done with temp inside headlight either.
However, humidity levels on the inside can be reduced.
Most designs usually have 2 or 3 air vents to keep humidity below condensation threshold levels.
This might sound complicated, but the phenonomen is exactly the same as
windows in a winter home and humdity levels inside. Too much showering,
and bathroom window fogs up. Venting will reduce condensation.
You might wonder why headlamps are not designed to be 100% air tight, so that no humidity enters?
Well, that would be in an ideal world with no heat losses in bulbs. As the headlights goes through continous
warm-cold-warm-cold cycles as you turn light on and off, the air and plastic will expand and retract.
This creates pressure changes, which will move a slight amount of air in and out. So instead
of forcing the air to be inside, air is moved through controllable passages, so that
minimum of road dirt will enter.
Glass lenses cannot be sanded down. Glass is as tough as rocks, so its almost impossible to work with. Glass is basically made from sand, so basic sandpaper does not cut it.
Plastic front lens can be made clear.
Warning: Do not do this unless you plan to replace entire optical unit. (Example: projector unit.) A textured lense is part of the design and headlamp will not work as intended if texture is remved. See top of the page for more details.
First of all, get a junk yard headlamp to practice on. This will make you establish a good tecnique and not make mistakes on the way.
Front lens needs to be separated from reflector. See above.
There is a 3 alternatives to make clear lenses:
The process is pretty much to start carving with something very rough first. Rough in this sense could be a dremel with round carving bit (see Tools and Materials page) at low speed.
Go through the entire range of sandpaper 60, 80, 100, 150, 300, 600, 1000, 1500, which look like this.
Then go over to plastic rubbing compund (3M) followed by plastic polishing compund on a buffing wheel and it should end up like this:
Plastic will cook if it gets too hot, so you have to go easy - operate machinery on low speeds. If plastic starts cooking, you have to start over with a new headlamp, so its a good idea to practice on an old one first. A way to keep lens cool is constantly moving to new area, maybe even using ice cold water. Keep front lens from getting scratch up on the outside by covering it totally in 3M green painters tape and constanstly make sure it stays on. It is not water proof, so it has to be changed from time to time. Usage of vinyl sheet insead of tape can prove itself to have too powerful adehiesive on the back and will ruin the front of lense. If you are afraid that UV rays may ruin the plastic lens, u can paint inside with UV resistant clear paint - See Tools and Materials page.
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