sometimes people ask how the inside of the headlight can be cleaned best.

Well, nut by using abrasive materials obviously. A trick in the book is this:
Use cleaning pearls. They are commonly used to clean a vase. These 3mm metal pearls go inside the headlight unit (need to take it out of the headlight assembly first) together with hot water and a mild soap. turn the pearls for a while until all looks shiny, take it out, let the water out and let it dry for a while. All the moisture must be out completely !!!
Then re-assemble and enjoy the light.

1992 Porsche 928 GTS Midnight blue
The Netherlands