B + W 60mm UV Black #403 Wide Angle Slim Mount Glass Filter for Type "A" Ultra Violet Transmission

Not to be confused with UV-blocking filters that photographers normally refer to as "UV Filters" this one passes UV A radiation (320 to 385 nm), 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.
The filter factor is in the range of 8 to 20.