The Baader U filter
The Baader U filter is one of the two types of filter that are confirmed by my direct experience (and several other UV photographers') to have a high transmission in the near-UV and a very low leak in the near-IR, the other being the Schuler (or Schüler) UV filter. These characteristics make either one of these alternatives, or both (see below), a must-have accessory for UV photography. The alternative, traditionally used before these filters were available, was to stack a generic UV-pass filter like the Schott UG-1 with an infrared-cut filter of the cold-mirror type, hoping that the latter was effective enough also in the near-infrared and did not cut too much UV.
While the Schuler UV filter is ionic colored glass and consists of a sandwich of two cemented optical elements of different glass types (one transmitting UV and IR, the second cutting red and IR), the Baader U consists of a conventional Schott UG-11 substrate (which transmits near-UV and quite a bit of near-IR) coated on one side with tens of dichroic layers that reflect IR (and quite a broad part of the visible spectrum). This side looks like a mirror with a golden-green hue. I don't know how well this coating resists scratches and cleaning, but, like all dichroic filters, I suspect it should be treated with more care than ordinary filters. I suggest to only use a blower (not a blower brush) to remove dust, to touch it only with lens cleaning tissue wetted with a proper cleaning agent if it gets really dirty, and never, ever, to wipe it with dry lens tissue. The opposite side has a more conventional antireflection coating optimized to transmit near-UV, and looks like polished copper (the pictures below show definite green and magenta hues, due in part to the highly oblique angle to the filter surface). The gold-green side should face outward (i.e., toward the subject) in imaging applications of this filter. No visible light passes through. It is optimized for transmission in the 325-369 nm band, i.e., a narrower band of shorter wavelengths than the Schuler UV. Like all dichroic filters, it should not be used with extreme wideangle lenses because its transmission properties are dependent on the angle of incident light.
Because of the highly reflecting properties of the coatings, the Baader U filter cannot be manually held in front of a lens for tests. Doing so is likely to cause some ambient light to be reflected from the coatings into the lens, causing a substantial flare and loss of contrast. Instead, the filter should be mounted onto a lens via a light-tight adapter.
Like the Schuler UV, the Baader U filter is designed primarily for astronomy, and available in 2" (above) and 1.25" sizes. An early, discontinued version of the Baader U 1.25" filter has fewer IR-reflecting layers and a troublesome IR leak that largely prevents its use in UV photography. The 2" version appears to be only of the improved type with negligible IR transmission. There is now also a 1.25" version that is said to be of the improved type. However, it cannot be excluded that some shops still have the old 1.25" in stock. Therefore, it is best you purchase this filter directly from Baader. Price from this source is lower than those applied by many astronomy shops. This filter is somewhat more expensive than the Schuler UV, but not impossibly so.
There is also a Baader adapter for converting the 48 mm mounts of 2" astronomy filters to a more common 54 mm filter mount. Since I standardized my filters at 52 mm, i.e., one of the preferred sizes for classical Nikon lenses, I prefer to permanently epoxy filters with special mounts into adapters of the latter size. It is also possible to re-mount a 2" filter into the metal ring of a cheap 49 or 52 mm UV filter (this is feasible with the Baader U, but difficult with the Schuler UV because of its higher thickness).
The different transmission qualities of the Baader U and Schuler UV result in noticeable differences in UV pictures, a sample of which is shown above. These pictures were taken with the same white-balance to make a direct comparison possible. Therefore, if you are serious about UV photography and/or have a specific use for the ability to discriminate among the two partly different UV bands, you may wish to have both filters available in your toolkit. The Baader U transmits a higher percentage of UV than the Schuler UV at their respective peak wavelengths. However, because of the shorter wavelength peak and narrower band of the former and the decreasing sensitivity of DSLR sensors toward shorter wavelengths, the recorded intensity is sometimes slightly higher with the Schuler, especially if a broadband UV source is used.