As you know, silver was originally used as the reflective coating surface on headlights, turn signals and other lights. The modern process, which I use in the restoration of H1 headlamps, is to use vapor deposited aluminum. But auto lights are not the only place where this process is used. It's used on a variety of scientific instruments including almost all large telescopes. I just read a thread at a physics website titled "Why Do High Quality Mirrors Use Aluminum Instead of Silver.". Here's a chart from that thread showing the reflectivity of aluminum (Al) and silver (Ag) (and gold (Au)):

Name: image.jpg Views: 141 Size: 39.5 KB

As discussed here previously, aluminum is slightly less reflective than silver at most wave lengths. But it's more consistently reflective over the spectrum of wave lengths which eliminates a blind spot for telescopes. And the biggest advantage aluminum has over silver with telescopes is the same as with our headlights - it doesn't tarnish.

I'm still hoping to develop a way to test headlight brightness. It would be interesting to compare: NOS H1's, restored H1's and non-restored H1's. I have read (Daniel Stern as I recall) that NOS lamps sitting in a box lose reflectivity over time. It would be interesting to have some empirical data.

Cheers,
John