On Thu, 1 Sep 2005, Rick
(etc.)
You're right. Lens haze sharply reduces seeing light and sharply increases glare light.
Federal Motor Vehicle Safety Standard 108 -- Lamps and Reflective Devices -- specifies UV- and abrasion-resistance performance standards for "plastic" (polycarbonate) headlamp lens materials. They are required to undergo two different 3-year weathering tests. One is known as the "Arizona test", and the other is known as the "Florida test". Very scientific stuff (not); samples of the lens materials are placed on a board at a specified angle to the sky and allowed to age for 3 years, after which no more than 30% haze is acceptable.
Obviously, these tests and standards are inadequate, for there are a great many opacified lenses (you say "cataracts" and it's a valid analogy) on the road, even in areas that aren't as sunny as Florida or Arizona. Specific problems with this test regime:
-It means that the materials used to make any given car's lenses are at least three years behind the state of the art.
-It is entirely decoupled from actual headlamp performance. Some headlamps might still comply with FMVSS 108 beam pattern requirements with 30 percent hazed lenses, but many definitely do not.
What's more, FMVSS 108 severely restricts the use of replaceable lenses. Most types of headlamps in North America are required to have bonded, nonreplaceable lenses.
The international ECE (rest of world) headlamp standard contains different test procedures for plastic materials, involving close exposure to artificial (electric) UV light of a specified and high intensity for a specified time period. The allowable deterioration is less, as well. Even this tougher standard, though, does not prevent ECE-spec lenses from opacifying in the sun.
Manufacturers like polycarbonate lenses because they can be fashioned into more extreme styles than glbutt, they weigh less than glbutt, and they can be truthfully advertised as more impact-resistant than glbutt. Also, they can be cheaper than glbutt.
The impact-resistance of polycarbonate headlamp lenses can be approximated with toughened glbutt, but this of course is more expensive.
So, probably the best kind of headlamp lens is replaceable toughened glbutt. After that is probably replaceable polycarbonate, and then replaceable standard glbutt...all distantly followed by nonreplaceable lenses of any material.
Well...no. It is only very recently that Mercedes has used anything but glbutt lenses. Don't think that glbutt-lens headlamps can't degrade, they can. The degradation, though, is slower and less severe.
There are no gold stars on any Federal refrigerator for headlamps that are better than FMVSS 108 says they have to be.
True, see above.
Good luck on that one. FMVSS 108 scarcely ever changes significantly. The last time a change was made that could be considered major was twenty-two years ago. And the last time a major change was made that consbreastuted a tougher, more stringent requirement for material specifications and durability was exactly never ago.
What's more, the man who was in charge of FMVSS 108 for about a decade retired a few months ago, nobody has replaced him, and the search for a replacement is not being carried out with any urgency. And even when a replacement is found, the position does not carry the authority to make any changes to FMVSS 108 without proof that failure to make the change can be directly and demonstrably linked to a large pile of dead bodies. The automakers have a great deal of veto power over auto safety regulations in North America; all they have to do is say "You can't prove this change is necessary to save lives" and NHTSA is legally hamstrung. The system is broken; it will take a great deal more than a replacement 108-manager to fix it. Congress would have to effectively dissolve NHTSA and comprehensively rework the way auto safety regulations are devised and written, and that's not going to happen.
Sealed beams were good. Their performance wasn't amazingly great, but it was certainly adequate, and it was certainly better than that from a great deal of the model-specific junk that has disgraced US-market cars in the last 22 years. They were all one of four shapes-sizes, they were inexpensively available everywhere, they could be replaced and aimed with simple hand tools, they were resistant to environmental factors, they were resistant to idiots bearing blue and overwattage bulbs, and because they were all standard-sized, old cars got constantly updated to the newest headlamp performance every time they replaced a burnout, and the sealed beams could easily be replaced with European conversion lamps for those who preferred (or were moving overseas).
DS
