PrimaLuceLab U filter
for UV imaging  

PrimaLuceLab is an Italian company marketing astronomical and radioastronomical equipment, mostly addressed to advanced astronomy amateurs and comparatively low-budget professional use. Among other equipment, they offer a number of 1¼" (31.8 mm) and 2" (50.8 mm) filters mounted in threaded metal cells.

While most of the PrimaLuceLab filters are designed for quite ordinary astronomical applications, for which several alternative filter sources are available, they also offer a U filter, apparently named to present it as an alternative to the Baader U filter for UV imaging. It was pointed out to me that a "U" prefix or suffix is commonly used to indicate UV-pass astronomic filters, and indeed it is also used in legacy UV-pass glass materials like U360. Astronomy filter sets are also commonly called RGBU (for Red, Green, Blue and UV). However, until recently the Baader U was the only modern UV-pass astronomy filter named just "U".

As far as I am aware, in addition to the Baader U and PrimaLuceLab U, there is only another currently available UV-pass 1¼" and 2" astronomical filter, i.e., the Astrodon UVenus. Therefore, for UV photographers, the PrimaLuceLab U filter is potentially very interesting.

Spectral transmission

A comparison of the transmission spectra provided by the respective brands of these three filters shows significant differences. These three filters are not equivalent to each other. Instead, they transmit different, although largely overlapping, UV bands, and this, in turn, means that the visual results with these filters are substantially different.

The Astrodon UVenus transmits longer wavelengths, with a flat peak between roughly 330 and 390 nm (according to the transmission spectrum available on web sites marketing this filter). In practical use, sunlit scenes produce mostly blue and some violet tones. The transmission diagram shows the top of the transmission curve at or immediately below 100%, which is not realistic if transmission is compared with the absence of any filter. Therefore, I assume that the transmission curve is calibrated to display transmission as a percentage of the transmission peak instead, and in the absence of more significant data, we cannot state exactly how well this filter transmits. Ordinarily, broad-band UV-pass filters with well-designed coatings transmit between 75% and 85% at their transmission peak.

The Baader U transmits a peak at 365 nm (where absolute transmission is around 78%) and a 50% relative transmission band (defined as the band where transmission ≥ 50% of the peak transmission, i.e., not where absolute transmission ≥ 50% ) of approximately 320-380 nm.

The PrimaLuceLab U transmits a rounded 85% absolute transmission peak at approximately 330-350 nm, with a 50% relative transmission band at 290-375 nm.

Among the filters I often use for UV imaging, the Asahi Spectra XRR0340 is, at least on paper, the closest to the PrimaLuceLab U. The XRR0340 has a 74% rounded absolute transmission peak at 337 nm and a 50% relative transmission band at 295-365nm. At least according to these diagrams, the XRR0340 has therefore a narrower bandwidth than the PrimaLuceLab U. This filter is available only as unmounted 25 mm round and unmounted 50 mm square, and therefore is not immediately usable "out of the box" for photography or astronomy with ordinarily available equipment.

Filter construction

"Front" surface of filters
 
"Rear" surface of the same filters
 
UV-pass filters. Top left: Baader U. Top right: Astrodon UVenus. Bottom left: PrimaLuceLab U. Bottom right: Asahi Spectra XRR0340. All re-mounted in standard photographic filter carriers.

The Baader U is an ionic glass substrate (probably U360) with different multi-layer dielectric coatings on either side.

The Astrodon UVenus is a dielectric-only filter on a substrate (probably fused silica) transparent to both VIS and NUV. The NUV-pass, VIS- and NIR-cut coatings are on one side only, and (unlike any other UV-pass filter I have used, with the exception of specialty ones) silver-colored. The other side of the filter is anti-reflection coated only.

The PrimaLuceLab U is an ionic glass (probably U340) substrate with different multi-layer dielectric coatings on either side.

The Asahi Spectra XRR0340 is an ionic glass substrate (U340) with dielectric coatings on one side and anti-reflection coatings on the other.

Because of the similarities of the PrimaLuceLab U and XRR0340, it is likely that both filters employ a hybrid pigment-dielectric construction. Like in the XRR0340, the filter glass is most likely U340 with different dielectric coatings on either side to reflect both NIR and red VIS radiation. This is made necessary by the U340 substrate, which transmits significant amounts of red in addition to NIR.

With a a filter that has different dielectric coatings on either side, it may be asked which side is best oriented toward the subject. This has been a matter of discussion on web sites. I have not seen convincing evidence that the orientation has any effect on images, at least if one considers only the transmitted radiation. From theoretical principles alone, the orientation does not matter (assuming that there are no odd phenomena like UV-induced fluorescence of some of the glass layers or coatings, which is unlikely). It might matter slightly, or might not matter at all, if one considers the effects of off-axis illumination on the formation of flare or multiple reflections between filter and sensor, between multiple filter layers, or between coatings on either side of the filter. This is difficult to prove since UV flare seems to be an elusive phenomenon, ready to spoil images shot casually in the field but difficult to reliably elicit in laboratory tests under controlled conditions. Therefore, I am writing "front" and "rear" (i.e., in quotation marks) in the above figure caption. Use whatever orientation seems to work best for you, but keep in mind that which face of the filter is outermost may not matter, so don't loose any sleep on it. It is more important to use a lens shade narrow enough to cut out as much off-axis illumination as possible without causing a visible vignetting.

Prices

I found the following lowest prices by searching online, whenever possible on EU web sites. These filters are available from multiple sources, albeit often at substantial price mark-ups.

Where the items were not available in the EU at reasonable prices, I converted US$ prices to € according to the August 17, 2015, official exchange rate of 1.11 USD/€.

All prices quoted in € include sales tax. In most EU countries, sales tax is between 20% and 25%, so I arbitrarily added a 22.5 % tax in the table below when converting from US$ to €. Actual prices for US orders shipped to the EU may be higher than computed below, because of bank or exchange fees, import duties and shipment costs.

Filter type standard 1¼" mount standard 2" mount special size/mount
Baader U € 225 € 295  
Astrodon UVenus € 195 (not found) 2" unmounted round:
US$ 385 + tax = € 425
PrimaLuceLab U € 139 € 199  
Asahi Spectra XRR0340
N/A N/A 25 mm unmounted round:
US$ 220 + tax = € 243
50 mm unmounted square:
US$ 360 + tax = € 397

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.

Practical use

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.

Test images

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.

Baader U
 
Astrodon UVenus
 
PrimaLuceLab U
 
Asahi Spectra XRR0340
 
Test: sunlit scene, automatic exposure.

The following test images were shot with high-power, UV-enabled electronic flash (0.75 KWs at 30 cm from the flash tube).

Baader U
 
Astrodon UVenus
 
PrimaLuceLab U
 
Asahi Spectra XRR0340
 
PrimaLuceLab U with correct exposure
 
Test: UV-enabled electronic flash, same illumination intensity and exposure in all images except the last.

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.

Conclusions

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.


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