Updated 29/8/14 to include Baader U 2012.
The current ‘gold-standard’ reference UV-pass filter is the Baader U filter. This filter is essentially a thin UG11 filter with multiple layers of dielectric coating applied to it. It has sufficient IR-block to prevent any IR-contamination of the UV image.
While it is thought to be the best UV-imaging filter out there, it is certainly not a cheap filter, and the price seems to go up with each passing year, making it even more out of reach to many who may be interested in UV-imaging. Hence it is worth considering if there are any more affordable options out there. I have shot the same flower with the different filter stacks, so the resulting UV image can be compared.
Now there are many UV-pass filters available out there, but nearly all of these need to be stacked with an IR-blocking filter in order to be useable for UV-imaging. These include Schott UG1, Schott UG11, Hoya U340 (similar to UG11), Hoya U360, B+W 403 (essentially similar in transmission to the UG1/U360). UG5 and U330 both let through some blue and hence may not be suitable for UV-imaging depending on the subject. I do own many of these filters in various thicknesses.
The more common IR-blocking filters are the Schott BG38, BG39, BG40 and S8612. All these will be able to block IR if you use the correct thickness. The BG38 will need to be thicker in order to block IR sufficiently (I use the 3mm), although it lets through more deep UV than the BG39/40.
Both the BG39 and BG40 will block IR very well but at the expense of blocking the lower UV spectrum as well. The S8612 is probably the best IR-blocking filter available, as it lets through the most UV and is capable of blocking IR sufficiently even with thinner filters. I have this in 1.5mm and 2mm, but tend to use 1.5mm as it blocks enough IR for the system I use (this includes when using all the UV-capable 35mm F3.5 lenses).
Anyway, here are the images of a Rudbeckia fulgida I shot tonight using the various filter stacks. I used my home-made sintered PTFE filter to do custom white balance in Capture One Pro 7. Lighting was courtesy of the Quantum X2D flash, which has enough power to spare for this test (only the 330AF20 filter image required full power from this flash). I used my UV workhorse: Olympus EM-5 and Coastal Optics 60mm F4 APO lens.
1) All the above images do not have any visible IR-leak.
2) With the correct white balance, they look pretty similar (except for the UG11/BG38 stack which looks duller).
3) I included the 330AF20 filter as well which transmits between 320 – 340nm – however it is not possible to correctly white balance with this filter, so this was as close as I could get; the flower would look green and black with Baader U custom white balance.
4) The Baader U 2011 and the U340/S8612 stack images look less yellow than those taken with the UG1 filter stacks. This is due to the peak transmission wavelength being different as the UG1 peak is around 360nm, while the Baader U is around 350nm.
5) The U330/S8612 stack has a brownish appearance in the UV-dark area of the flower.
6) There are some dark patches in the UV-reflective part of the tips of the petals, which are present in the 330AF20 image but not in any of the other images. Perhaps an indication that there is use for deeper UV-imaging.
I hope you do find this filter comparison useful, especially for this who are deciding which filters to get. BTW, the background I am using is black foam board I bought from a craft shop.