UV filters (Ultraviolet blocking filters) typically have a designation letter (or number) following the printed letters “UV”. You will find: UV(A), UV(B), UV(C), UV(W) and UV(0) , (and that’s a zero, not the letter O). The A, B and C simply indicate that the glass blocks UV radiation down to one of the UV regions A, B or C. The Ultraviolet C region is furthest from the visible part of the spectrum, and almost all of the UVC within Sunlight is lost due to absorption as the light passes through the atmosphere. UVC is of little interest in general digital photography. The A and B likewise refer to bands within the UV spectrum, with the A band being closest to and adjoining the visible spectrum. UV(C) will just block UVC. UV(B) will block UVC and UVB. And UV(A) will block UVC, UVB and UVA.
As for W and 0 (sometimes mistaken for “O”), the W actually stands for wide, indicating that the lens blocks a wide band of Ultraviolet light, which includes C, B and A (in other words, it blocks practically all of the UV). 0 stands for zero, meaning “does not pass any significant amount of UV”. W and 0 have effectively the same meaning – they block practically all of the UV. To get maximum UV blocking, choose an A, W or 0 filter.
This represents the methods that some of the major manufacturers have used to designate their different classes of UV filters. If you are simply using a UV filter as a lens protector on a digital camera, then the differences may be of little consequence. However if you are using a film camera, or a digital camera adapted for photographing in UV, then this may help you to choose appropriately. Bear in mind that their is no accross-the-board industry agreement on the use of designators, and there is nothing to stop a manufacturer from arbitrarily assigning designators, simply as catalogue references, such as, series 1, series 2, and so on.