Originally Posted by
Drew Wiley
maltfalc - I'm very familiar with dichroic coatings. The biggest manufacturer of these in the world was once nearby, and their former catalog would stun you. No, you don't get them at places like B&H, unless its just a mild or "colorless" UV filter (I'm wearing coated "computer glasses" right now). Nor are these kinds of coatings used for ordinary taking contrast filters. Vacuum deposition techniques apply to both categories, but for different reasons. With camera filters, it's mainly to fine-tune only tiny transmission characteristics and control reflections, just like multicoatings on modern lenses. The kind used in enlarger colorheads are incidence-angle specific in terms of transmission/reflection characteristics, so would not be dependable in an ordinary photographic application where a lens accepts light from a range of angles. I built what is probably the only privately-owned true simultaneous additive halogen enlarger in the world using dichroic "sandwich" trimmer filters, which cut off the bandwidth on both sides of the desired RGB peaks. But I'm no optical engineer. I merely consulted with them.
There is a real engineer responding to this thread, if he wishes to describe the distinctions more precisely than I can, or correct me as per my own definition of "dichroic". But as I already noted, contrast filters for black and white photography, which are the kernel of this aspect of the discussion, are primarily made of either dyed-in-mass glass, or via the colored thermo-foil sandwich method that Tiffen uses. Any optical coatings are secondary. As far as laser applications go, I once sold certain industrial lasers, which doesn't make me an expert in that field either, but knowledgable enough to understand a number of basis issues.
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