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?
Yes, you are correct, I found this dire warning:
"and you shouldn’t clean the colored (usually yellow) glass or the color is likely to come off and then no more double-image in your rangefinder!"
on Matt's Classic Cameras web page about repairing rangefinders.
I would assume that you would need to ask a professional rangefinder camera restorer to see if it is possible. You can search the internet just as well as we can to find Professional restoration services.
Another site you may find interesting is Photoethnography.com
Well, according to this photo.net discussion thread, it can be renewed, but it's probably beyond most folks' DIY skillz and toolsets. This can be as "simple" as disassembly, cleaning and reassembly, but a dim patch could also require (as John Shriver says in the thread) "uncementing some prisms, re-silvering, and re-cementing", which again, isn't within the typical skill or toolset for someone who doesn't regularly perform vintage rangefinder camera CLAs. :)
Finding someone who does professionally do CLAs for vintage rangefinders and is familiar with your specific model is likely to be your best path. Hanging out on a messageboard, like rangefinderforum.com might net you some referrals.
