B+W's 43mm UV Black #403 Filter passes UV A radiation (320 to 385nm), but blocks visible light and looks pitch-black to our eyes. It is used with appropriately sensitized films in such applications as ultraviolet reflection photography in forensics or in materials research, but also as filters on UV-emitters for fluorescence photography. Depending on the illumination and on the film's sensitization, its filter factor is in the range of 8 to 20.
This filter uses the standard B+W F-Pro filter mount, which has a front accessory thread and is made of brass. Compared to the earlier standard mount introduced in 2001, the F-Pro mount has become thinner. Now it can be used with wide angle lenses, including most 24mm focal lengths on a full frame body, without vignetting. Another advantage of the F-Pro mount is it's modified retaining ring, which is no longer threaded in from the front, but holds the filter glass in place from the back. When removing a filter or lens hood that has been screwed on too tight to the filter, the retaining ring is not at risk of loosening.