Why don't tail lights cloud and oxidize?

Good science question. Different plastic for clarity and heat resistance and ruggedness)?
Just cleaned mine off and I am getting better at understanding the fix, so I used the car kit to fix a scratched up acrylic aquarium and now I need to transfer my one fish Nemo back over there. Dory (role played by a Damsel fish in our tank) died a while back.

I'd guess the tailights are more like hard polystyrene, and headlamps more like acrylic?

 

Just for giggles, looked them up, about $122 a side (plus shipping):

Headlamp Components for 2008 Toyota Prius | Toyota Parts

I go into a slow burn whenever I start thinking about these styling-exercises-formerly-known-as-headlights. Maybe count your lucky stars: the fourth gen headlights are $570 a side (plus shipping):

Headlamp Components for 2016 Toyota Prius | Toyota Parts

I would kill for a car that had simple, round headlights. Sure they can be plastic, but simple bolt-on, around $50 max, so you don't need to take out a second mortgage when they haze up.

 

Most headlights use polycarbonate for its high impact resistance and light transmittance. Seems it is also common for taillights, so the difference probably comes down to their purpose and treatments.

The additives and treatments used for filtering out UV light will also impact on visible light to some extent. Polycarbonate is also suspectable to scratching, so there is a protective layer for that, which could impact light transmittance. With headlights, you want as much light getting through the lens as possible, so a minimum amount of those agents are used. Taillights aren't being used to see with, but to be seen. So can use more of those agents, or use a plastic more resistance to UV, but transmits less light. Plus the dyes for the red an amber lenses can have protective qualities.

 

Interesting question.
Speculative answer.

Assuming we are only talking about the clear plastic of the rear lights, and not the red of the brake lights.
My speculative "guess" would be orientation related.
That is most front headlights have a more horizontal orientation, which means more hours of the sun shining directly down on the plastic. It means more possible contaminants raining down on the headlights, and of course driving, the headlights are always exposed to everything head on.
Meanwhile rear headlights are often more vertical in orientation, which I would think would mean less direct sun exposure, and of course they aren't meeting things head on as we drive.

But all this is just my own guess. I have no scientific testing, results or reality to back any of my "musings".

 

I think it has to do with the heat...tail lights rarely get warm, yet head lights are often quite hot.

It could be the colour difference, too.

 

Could it possibly be due to the fact headlights face upwind, whereas tail lights face down wind most of the time?

 

Quality aftermarket headlamp assemblies seem to go for about $60 each for the Gen II. Almost be tempted to replace them at that price, but on the G2 you need to remove part of the bumper to take off the assembly. Although even that isn't that tough.

 

We walk the dog down a block where there's a couple of elder care homes, lots of staff parking all day. The cars facing southward in particular, the older ones very consistently show headlight haze. I think it's that sitting outside, day after day, south-facing in particular. Or north-facing, for @alanclarkeau .

 

You get faded taillights, but not so often - I know one car where the very top, flat surface is quite faded, the rest reasonable - these images from GOOGLE:


But they don't seem as prevalent as headlights.

 

Great question and with two plausible competing hypotheses offered in response.

An excellent science-fair project could expose head and tail 'examples' to different levels of heating and UV flux, alone and in combination. Invent a 'cloudiness' sensor, probably with an off-axis light detector. Cloudiness is a combination of scattering and absorption here.

Clever student gets noticed by high-level engineering school, and Bob's your uncle. Her uncle.

 

Good invention: plastic headlight that doesn't haze over? probably impossible to patent though, so here we are?

 

Why was glass (amorphous silica) abandoned?

 

Weight? Complexity of forming into weird-nice-person shapes?

 

Fuel savings -- glass weighs. Or, that is a guess based on what a Toyota salesman told me around 1979 at a Toyota lot in Pomona, California when I asked why all the door handles were a very cheap plastic instead of "metal."

That was when California had a deal where you could only fill your gas tank on odd or even days, depending on the last number of your license plate, because of the fuel crisis -- and when some stations were advertising price per liter, instead of gallon, to lessen sticker shock.

I noticed "metal" handles never did come back, but the plastic was beefed up....those early model plastic door handles really seemed cheap.

 

I can remember picking up sealed beam replacement headlights (glass); they were not that heavy. In the main because they were as big as they needed to be, not some humongous sculpture.

 

The old round sealed-beams weren't particularly heavy, but I remember the first big rectangular one I picked up (I think a VOLVO?), I was astounded at the weight - much more than 2 sealed beams.

The other consideration is shape - I guess it's much easier to mould in poly of some sort, something like:

 

As I understand it, there really are non-negligible aerodynamic advantages to the way the molded shapes conform to the rest of the hood/fender design (while still leaving plenty of scope for designers to come up with weird-nice-person shapes). The old (whether round or rectangular) sealed-beams were big air-dozers by comparison.

 

In addition to fuel savings, there is a safety advantage to plastic in having less glass shards flying around. Bullet "proof" glass is actually polycarbonate. Shattering it takes great force. Which also means lower maintenance costs as you don't have to replace broken headlight assemblies as often.

The complexity is probably in not getting unwanted refractions of the exiting light in curved glass. With the Prime's double curved rear window, the technology might be at the point of reliably making glass headlight lenses, but the pros of polycarbonate still favor it.

 

Why don't tail lights cloud and oxidize?

D.) All of the above.