Quote Originally Posted by ic-racer View Post
dichroic filters are ... Heat and fade proof
Well, almost...

A dichroic filter is a resonant cavity for light. It resonates at the pass/block wavelength - but also at sub-multiples. A bit like a musical instrument that produces not only the fundamental tone but also higher octave harmonics. A filter that passes at 700nM (deep red) will also pass at 350 nM (deep purple/near UV). This isn't much of a problem for most photo processes - though dichroics would make lousy safelight filters.

To block the higher harmonics the filters incorporate an absorbing filter. This absorbing filter is subject to fade just like any other colored filter.

The resonant cavity of a dichroic is made of very very very thin layers of metal and conductive metal oxides - so thin the metal is transparent. These coatings are subject to abrasion and corrosion if the protective over-coat degrades. Close proximity to a halogen lamp is a good degrader.

Though they last a lot longer than wratten filters in the same duty, they are still subject to fade and degradation. It is not unheard of for them to need replacement. There is no need for precautionary replacement. When they fail it is quite apparent with blistering, peeling and mottling. Storage in high humidity environments, like the 10 years an abandoned enlarger can spend in the corner of a garage, can destroy the filters - but by then most of the enlarger will also be quite rusty and the lens covered in mold.