Just in case if you're interested:-
http://www.lenstip.com/113.1-article-UV_filters_test.html
The Spectrophotometer, i.e., light spectrum analyser:-
A typical result:-
Btw, they also tested the flare introduced by the filters by taking real photos, with and without the filters side-by-side and the result pictures are posted, like this:-
Last but not least, the visible light transmission rate, UV light blocking rate and vignetting were also measured in the test. An overall mark is then given for all the measurements and field tests carried out. You could also read the final conclusion for each filter in each test and could compare different filters further. A good comprehensive set of tests, professionally done, highly recommended!
Besides, it seems that the Hoya regular HMC filters are still the best deal out there, with consistent good overall performance and lower prices. However, I've noticed that many users have been confusing about the notations and actually difference of UV(0), UV(N) and UV(C) (amongst which the (N) and (C) filters were not tested above) used by Hoya, the chain of Q&A emails in this forum post may clear things up!
http://www.thestudentroom.co.uk/showthread.php?t=1007047&p=29671078#post29671078
Related:-
UV Protector Measurbation and Shootout (14 Make/Models Compared)