but what good does a coating do that you can get light to go through all that well?
My guess is that people don't want to change their headlight units every 5 year or so...... due to this.
So maybe that't the reason the UV protection is there?

I don't know how the technical inspections are done in the US, but in the Netherlands we have to pass a YEARLY technical inspection.
In this inspection, the distortion of the headlights is measured....
Fogged glasses and faded mirrors result in a No Go with the inspection.

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