Good diving signaling mirror?

[bleeb]

When I was into amateur telescope making, the best coatings (on a polished glass mirror blank) were silver vacuum deposition with a coating on top to prevent tarnishing. Something like 95% reflectivity. I wonder if anyone makes these with an actually waterproof plastic coating around it. That would make a killer signal mirror, and if it would last a few years of SCUBA use I'd gladly pay $25 or so for it (although basing business decisions on my purchasing preferences is not usually a good idea!).

When I was involved in designing and building optical instruments (professionally) one of the down sides of most of the high performance surface materials requiring coating is that they would start to oxidize 'sideways' from any nick, scratch or imperfection in the coating. Usually acceptable in an instrument you take really good care of or internal surfaces, but not so good on the 'outer' surface or anything exposed to wear.

As Tobin said, more robust but slightly less reflective materials like nickle are often preferred for those surfaces. But practically speaking for this application, IIRC you're talking about the difference between 98+% reflectivity and 99+%. In other words, not a terribly significant advantage. You loose more if the mirror is worn, scratched or has some goop or salt on it. You're probably better off with something scratch resistant, which dirt, salt and water doesn't like to stick to, and which doesn't oxidize much, especially around salt, which I guess brings us back to the original question of this thread.

[ptyx]

You're probably better off with something scratch resistant, which dirt, salt and water doesn't like to stick to, and which doesn't oxidize much, especially around salt, which I guess brings us back to the original question of this thread.

Yup. 99.999% reflectivity when it's brand new and shiny doesn't do help if it's down to 20% after 10 dives. Glass doesn't necessarily scratch either (the glass they use on the front of the iPhone is an excellent example, the one on the back ... not) - and you can put it in a sleeve if needed.

It shouldn't be that hard to pick a good glass, deposit a reflective layer and bond the back and edges (lack of edge bonding seems to be killer #1).

[gsk3]

Ah, cool. Good to know. In case I ever need to make one on a desert island ;-)

Seriously, Tobin, you're a fount of knowledge.