All About Digital UV and IR. Page 6: Filters for UV

Again, as in the case of IR filters, we need visually opaque filters to screen off visible light rays from reaching the CCD. The filters are basically made up of silver particles deposited in gelatine and enclosed in protective glass. This principle makes the filters quite thick and expensive to produce. Since there is a fairly low demand for these specialised items, the asking prices tend to be quite elevated and this doesn't make much to further their popularity.

Filters for digital UV photography are offered with a number of options.

From left to right the following items are shown,

Hoya U-360, one of several filters close in spectral response to Kodak's Wratten 18A. These are quite thick glass filters. Alternatives are Schott UG-1 and B+W 403. Hoya delivers additional UV bandpass filters, of which I'm mainly using the U-330 and U-340.
Nikon's proprietary FF filter which is distributed with the UV-Nikkor 105 mm lens. Its spectral response lies between the Wratten 18A and the now obsolete 18B. It is a thick glass filter which transmits slightly in the lower purple range and deep red. The FF is still available from Nikon as a spare part. U-330 from Hoya is quite similar to the FF for practical shooting.

Nikon delivers the FF with a AS-1 gel filter holder and a UR-2 retaining ring, both of which mounts together to form a filter unit that swings away for focusing.
A Tiffen Hot-Mirror filter attenuates IR and should be used for all digital cameras with high sensitivity for IR, in order to reduce IR contamination of the UV images. See the charts below for insight into its spectral response.
A Wratten CC20C filter cuts some IR and could be employed with digital cameras not highly suspectible to IR, as its side effects are less than the Hot-mirror type. I used it on a regular basis with my D1H camera. For my D2H and D70, I'm currently employing a B+W BG-38 or BG-40 IR cut-off filter instead. The BG-38/40 does seem to work very well in conjunction with U-330/U-340 or 403 filters on these two cameras. I would now recommend that a BG-38 or BG-40 always be used with any digital camera to abate IR contamination.

Just recently (2006) I have got a specialised Schott UG-11x into my possession. This filter (derived from Schott UG-11) utilises sophisticated coating technology to further cut down on the IR side lobe of the UG-11. The UG11x produces phenomenal results and is simply the filter to get if you are into UV photography and own a quartz lens such as the UV-Nikkor. It is also known as the "Venus filter" (obtainable from www.baader-planetarium.com) and is quite small in size, but obviously adequate for any UV lens I've tested so far.

Update: As of March, 2007, the "Venus" or Baader "U" filter now can be had in 2" size, which facilitates its use on more lenses than the original 1.25" filter. Concomitantly, its IR suppresion is improved by 5 stops or more compared to its predecessor, which already had cutting-edge performance. This remarkable improvement is achieved by increasing the number of interference coatings from 20 to a staggering 100 layers. If you shot UV, then the new Baader U2 is the filter to get. Price isn't that bad either for such a specialised item. Just remember that the filter is asymmetric in its coatings (designed for astrophotography) and to work at peak performance, you should reverse-mount it on your UV-capable lens. The filter itself sits in a small cell and you simply (and gently) remove the retaining ring, flip the filter, and put everything back. Don't overdo the tightening of the retaining ring since the filter needs to "float" in its cell to ensure perfect optical flatness.

All UV band-pass filters, used to block visible light while UV is transmitted, have an additional region of transmittance. Unfortunately for digital UV photography, this side-lobe is in the worst possible spectral area (near-IR), in which many digital cameras have significant response. This is clearly seen in the spectral response charts given below,

Thus, the dedicated digital UV photographer has to balance on a knife's edge, seeking to maximise UV response of his/hers setup while concurrently trying to reduce IR contamination. It should be noted that anyone extracting visual expressions from the digital UV approach need not worry too much about these scientific facts, because visual impact then takes precedence. However you are warned, UV photography is not for the faint of heart.

If you are not adhering to a careful and systematic approach, however, you could easily end up with "pseudo UV" images. This means the image should portray UV details, while in fact it upon close scrutiny can be shown not to. You get this issue, which few people have been concerned with, because the UV fraction of the impinging light tends to be small, the camera is more sensitive to IR than UV, and the UV "bandpass" filters commonly pass a significant amount of near-IR. Even when using a dedicated quartz lens such as the UV-Nikkor 105 mm, will this problem cause potential and dangerous pitfalls to the unwary UV photographer. See the example below and heed the implicit warning: even using a UV lens with a UV bandpass filter is no guarantee of getting a UV record.

Example of IR contamination

IR contamination of the UV record

Left: Hoya U-330 filter only, right: U-330 with BG-40 IR blocking filter added

Nikon D70 with UV-Nikkor 105 mm f/4.5 lens, identical f/16 setting

Broad-band filters such as the proprietary Nikon FF (shipped with the UV-Nikkor 105 mm lens) and Hoya U-330 pass massive amounts of IR in addition to UV. The IR contribution acts adversely in various manners. Firstly, the image becomes less sharp because UV and IR focus are different (even for the UV-Nikkor), secondly, image contrast is lowered, and thirdly, the UV-absorbing areas don't go as dark and this literally "wash out" the UV signature of this dandelion.