U-360 filter 

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 found a much better (albeit more expensive) alternative in the Schuler UV filter. However, by all means do continue to read this page.

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 only 2% of IR in a narrow peak around 720 nm. If you want to eliminate as much IR as possible (which I do recommend), you should consider the more exotic Schuler UV, Baader U or Baader U2 filters.

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.

These pictures are an experiment with a flower photographed in natural sunlight with a D200, Nikkor 50 mm f/1.4 D at f/8, and (except for the above picture) U360 filter. The filter is much smaller than the front of the lens, but stopping down to f/8 eliminates all visible vignetting. The above picture is the result in visible light, without filter, at ISO 100 and automatic exposure. The pictures below are processed from an image taken at ISO 1250 and a manual exposure of 30 seconds.

The Nikkor 50 mm f/1.4 D is not a good choice for UV photography. Its lens coating can be expected to absorb almost all UV. In addition, sunlight contains large amounts of IR, which is only partially filtered out by the U360. Therefore, the resulting picture with the U360 should be expected to (and does) contain mostly IR information. However, the fact that the three colour channels respond in different ways (in particular, the blue channel versus the red and green ones) does indicate that something other than IR is present. IR passes almost unimpeded through all three colour filters of a Bayer colour mask of a digital camera sensor, and therefore, an IR-only image should look virtually identical (except for different overall levels and noise) in all three channels.

The above pictures represent, respectively, the red channel alone, the green and blue channels combined, and the blue channel alone. Level and contrast in each channel were adjusted manually to give roughly similar luminance (the red channel was preponderant in the original picture). Information in the red and green channels is largely the same. The flower and grass blades are light and the background (moss and soil) dark. The situation is reversed (except for the sun-lighted spot on the top left portion of the flower, which is overexposed in all channels), instead, in the blue channel. Since the blue channel is where most of the UV can be expected to be recorded, I believe this information is contributed largely by near-UV light. In fact, using a proper IR-pass filter, like the Fuji IR 82, that absorbs in the visible and UV does not produce this different information in the blue channel.

Of course, a lens with a better UV transmission can be expected to give much more useful results in the UV range. Nonetheless, even lenses with a low UV transmission do provide detectable amounts of light in this range.