PrimaLuceLab U filter
The most interesting result of this comparison is that the PrimaLuceLab U is the cheapest of these filters by a significant margin, regardless of size and mount. Legacy ionic filters like the U360 and U340 may be even cheaper, but they cannot be used for UV imaging with digital cameras unless steps are taken to remove IR radiation, either at the illumination source or at the camera end.
Historically, the price of the 2" Baader U increased significantly over a few years, since it became the most popular filter for UV photography with digital cameras. We will see if the same is going to happen with the PrimaLuceLab U.
As expected, a year and a half after writing this page, the price of the PrimaLuceLab 2" has increased to € 255. The 2" Baader U, instead, is still € 295.
The amount of UV radiation contained in sunlight at ground level rapidly decreases with decreasing wavelength. Much the same is true of the UV contents of electronic flash emission, because of a combination of lower emission at shorter wavelengths and increased absorption by the glass of the flash tube. The sensitivity of Bayer sensors also decreases rapidly at shorter UV wavelengths. These factors combine together to require a substantially increased exposure time (for continuous UV sources), UV intensity and ISO sensitivity as the wavelength decreases.
For the above reasons, although the Baader U and XRR0340 may not look too different when their transmission spectra are compared, using the latter filter in full sunlight often requires exposure times 2-3 times higher (or 1-1.5 stops more open, or a correspondingly higher ISO) than with the Baader U.
Taking advantage of the full transmission band of the PrimaLuceLab U requires a "real" UV lens (in practice, among full-frame lenses currently marketed, either the Tochigi Nikon UV 105 mm, CoastalOpt Apo 60 mm, or CoastalOpt UV 105 mm) and a converted camera with a replacement window of a suitable material, or no replacement window at all.
The best choice for such a replacement window is a fused silica or Spectrosil plate, optically flat to λ/10 and anti-reflection treated with a UV-optimized coating. Edmund Optics and Thorlabs are typical sources for these optical components. BK7 and similar glass types absorb too much UVB to be useful below 310-320 nm, if you really want to try and record these short wavelengths. For less demanding uses in UV photography, BK7 may be fully adequate.
The following test images were shot with multispectral-converted Panasonic G3 and CoastalOpt 60 mm Apo in full sunlight in central Sweden in August. The approximate time was 17:00. A usable amount of UVA is available in these conditions, although sunlight, as a whole, remains relatively bleak all year round at this location. All filters discussed on this page were used.
All images are straight out of the camera and use the same custom white balance. I initially set the custom WB on the Panasonic G3, after multispectral conversion, with UVIR-cut Baader filter on a sunny outdoor scene, and never changed it afterwards. This guarantees that VIS images are correctly white-balanced (a gray card is correctly rendered as neutral gray), and I use no different in-camera WB for UV and IR images.
The following test images were shot with high-power, UV-enabled electronic flash (0.75 KWs at 30 cm from the flash tube).
Images with the PrimaLuceLab U and Asahi Spectra XRR0340 are obviously different. The XRR0340 records no false-color blue (which seems indicative of longer UVA wavelengths), while the PrimaLuceLab U transmits plenty of these wavelengths. In this respect, the latter filter is more comparable to the Astrodon UVenus than to any other of these filters. On the other hand, some subjects that reflect or emit strongly around 350-360 nm are imaged as yellow with both the XRR0340 and PrimaLuceLab U, while this happens more rarely (i.e., only when the 350-360 nm wavelengths exceed by far the longer UVA wavelengths) with the Baader U, and almost never with the Astrodon UVenus.
The white spots on the first of the flower subjects are mold clusters, not evident in VIS. All four filters are successful in making them very visible in NUV.
Although with PrimaLuceLab U the second flower subject looks washed out, this is caused by the fact that, with some subjects, this filter requires a lower exposure than with the other three filters discussed on this page. The last image shows the effects of correct exposure with PrimaLuceLab U. This filter also seems to provide a slightly lower contrast than Baader U and XRR0340, but there is plenty of dynamic range in the images that can be recovered by moderate post-processing.
These preliminary tests suggest that the PrimaLuceLab U may be an excellent filter to record UV specular reflections and iridescence, which with other filters may be difficult to distinguish from diffuse reflectance. The last of the flower images shows specular reflectance/iridescence as false-color yellow, and diffuse reflectance as false-color blue. The Baader U also works relatively well in this respect, but neither the Astrodon UVenus nor the XRR0340 are successful in discriminating between the two reflectance types.
There is no visible trace of NIR contamination with the PrimaLuceLab U. Some of the other UV-pass filters of comparable UV transmission I tested on other occasions (e.g., Omega Optics 340WB70 and 340WB80, which may have been superseded by newer types) show instead a troublesome red and NIR leak with some subjects. This leak is usually recorded in images as red or pink.
The PrimaLuceLab U is different from other commonly used filters for NUV imaging, and produces significantly different visual results. It is a welcome addition to the quite limited range of UV-pass filters available for digital UV imaging, not the least for its price, currently lower than that of any other comparable filter.