The Hoya U-360 is a glass UV-pass filter. It is black in visible light, and transparent both in the near-UV and near-IR ranges. The denomination of the filter points to the cut-off wavelength of its UV-transmission (360 nm). This filter type transmits 70% of UV in a rather narrow band centred on 360 nm, and 10% of IR at 740 nm. It transmits smaller amounts of IR in a rather broad range (up to about 850 nm). In practice, the U-360 transmits useful amounts of UV up to nearly 400 nm, i.e., the threshold of visible light. It is, however, completely opaque to the eye.
Note: if you are serious about UV photography, hold on to your wallet and don't order a U-360 filter yet. I discuss several far better alternative filters for UV photography here. However, by all means do continue to read this page. It does contain useful information about UV photography.
Most UV-pass filters transmit also variable amounts on near-IR. Some photographers add a IR-cut filter of the hot-mirror type to a UV-pass filters to reduce the amount of transmitted IR. IR is preponderant in natural scenes, because natural illumination contains larger amounts of IR that UV, the latter is further absorbed by non-specialist lenses, and camera sensors are more sensitive to IR than UV. However, adding a generic hot mirror may cut substantially also the amount of transmitted UV. You should choose a UV-transmitting hot mirror, which is quite expensive (at least double the price of a UV-pass filter). Therefore, you may be better off choosing a UV-pass filter with appropriate IR-transmitting properties instead.
If you are looking for a filter that transmits larger amounts of both UV and IR, the Hoya U-330 may be a good alternative. Although centred on 330 nm, it transmits from 70% to 90% in the range 250-400 nm (in fact, it transmits small amounts of indigo and blue, so it is not completely opaque to the visible range) and over 50% at 700 nm. On the other hand, if you are looking for a filter that transmits only small amounts of IR, you are better off with the Hoya U-340, which has a peak of 80% at 340 nm and is roughly intermediate in performance between the U-330 and U-360 in the UV range, but transmits 2% of IR in a narrow peak around 720 nm (which is still too much). If you want to eliminate as much IR as possible, which is an absolute necessity in digital UV photography, you should consider more exotic filters like the ones I discuss here.
Being unable to find a second-hand UV-pass filter, as well as mounted filters of this type, I bought a round U-360 with a 25 mm diameter from Edmund Optics, and mounted it in the frame of a 28 mm surplus polarizing filter (which happens to accept a round glass of this exact diameter). The size of this filter is sufficient for use (with a step-down ring) on enlarger lenses of shorter focal lengths, like the EL-Nikkors. Depending on the type of filter, 12, 25 and 50 mm round unmounted filters may be available, as well as 25, 50 and 75 mm unmounted square filters.
I found the above mount to be a better alternative for daily use. The U-360 is mounted in a 52 mm - 28 mm step-down ring. This particular ring is more than thick enough to contain the filter and two threaded retaining rings in its female 28 mm thread. The larger diameter makes it easier to handle without touching the filter surfaces. It is also easier to find and use 52 mm adapter rings to mount this assembly in front of lenses.
The above is a sample image recorded with the U360, Optomax 35 mm f/3.5 lens, multispectral Panasonic G3 camera and UV-enabled Bowens 1500 Pro studio flash. Comparison images taken with other filters are shown here and here. The image recorded with the U360 contains mostly IR information, and the UV information is overwhelmed by the massive IR leakage of the filter. In fact,using a proper IR-pass filter, like the Fuji IR 82, that absorbs in the visible and UV does not produce a substantially different image.