Prisms in roof binoculars require a mirror coating to properly reflect light in its passage through the prism. Typically those mirrors have been aluminium or silver based coatings.
Dielectric are new coatings applied as a one molecule deep coating at a time. This process allows a designer a lot of latitude in making mirror coatings perform the way they want them to perform. The coatings can be applied in any number, composition, or sequence to accomplish design goals. The process is typically a brighter image and boosts light transmission a little.
EDIT: The process was initially exclusive to the high end expensive binoculars. It is now to the point where the technology is becoming easier to do, less expensive, and you are starting to see it in more mid range glass. The ZEN ED 2 is the first I am aware of. We are now seeing it in the Nikon Monarch and I suspect the new Vortex Viper line, among others, will have it too.
Last edited by SteveC99; 11/23/10.
