What is the yellow patch in a rangefinder made of, and how is it applied?
(Maybe other colours are used, but all the ones I've seen have been yellow.)
It's clearly extremely delicate, and I've read numerous dire warnings not to attempt cleaning it.
I have a couple of old rangefinders in which the patch has dulled, and would like to have them brightened up.
Is there a safe way to do this, or a repairer or facility able to renew them?