I think what is happening is that the UV light is converted to visible light as it scatters off the white paper.
Actually, the white paper is fluorescing. Modern paper is treated with whiteners to make it appear brighter. That's the same reason your white clothes glow under a blacklight - the whitening agents in laundry detergents remain in the fibers, and they are fluorescent (new, unwashed cotton doesn't fluoresce).
Interesting. The sensors in dSLRs are not supposed to be sensitive to UV light, so for a dSLR "clear glass" filters or "protector" filters are as good as UV filters. ... Since you have the UV lamp, perhaps you could also test the claim of dSLR UV insensitivity by taking pictures of the actual UV lamp through the two different UV filters with identical manual settings, and also without filter. The difference should be negligible.
I did a little test of my own, with a UV transilluminator (used to view gels stained with ethidium bromide, a flourescent dye used to label DNA). The bulbs are essentially the same as blacklight bulbs - substantial output in the UV range, and a smaller output in the visible range. Humans are nominally sensitive to 400-700 nm light, and wavelengths below 400 nm are UV, while above 700 nm is IR. A typical blacklight bulb has a main peak at 370 nm with a 20 nm width, and a secondary peak from a mercury line at 404 nm - it's the 404 nm light that we can see.
I shot in M mode, with and without a B+W MRC 010 UV Haze filter (5DII, 135mm f/2L, 0.5 s, f/2, ISO 100). Identical exposure settings, the only difference was the presence or absence of the filter on the lens. As you can see from the image pair below, there's not a lot of difference with the filter. There are two possible explanations for that:
1) The sensor is sensitive to UV, and the filter is not blocking any UV light.
2) The sensor is not sensitive to UV light.
I'm going to go ahead and trust that when B+W sells UV filter, it actually blocks UV light. Given that assumption, it's safe to say that dSLR sensors are relatively insensitive to UV light (which I was hoping was the case, since I've posted that tidbit plenty of times!)
Actually, if you look closely at the images, you will see that the lower one (with the filter) is slightly dimmer than the upper one. Looking at the luminosity values, the filter is attenuating about 5% of the light. However, if you look at the transmission curve for the B+W 010 UV filter (here
), you see that ~5% attenuation is exactly what you'd expect at 404 nm.
So, I conclude that dSLR sensors are insensitive to UV.
...there is still a strong visible component to a UV bulb, which may mask any noticeable result.
Certainly true...and it does confound the issue a bit. The problem is that there aren't typical bulbs available that emit only in the UV range. Of course, I could definitively answer the question - I've got a 100 mW laser with a 355 nm beamline, and being a laser it emits only at 355 nm. But I'm not about to fire a laser at one of my dSLR's sensors...