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How to make a UV photograph


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#1 nfoto

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Posted 20 January 2013 - 10:12

Rørslett, B. 2013. How to make a UV photograph. http://www.ultraviol...-uv-photograph/

In this article I will deal with the topic of reflective photography in the UV range of the spectrum. The spectral range 300 nm - sub 400 nm is commonly taken to be the area of ultraviolet photography and is the one used here. Fluorescence induced by UV light (UVIFL) is treated separately (ref) and also is observed in the visible range.

UV photography in principle is not different from the approach in visible light. A lens captures light that is focused onto a film or digital sensor to capture a persistent photographic record. What is different is that the human eye cannot directly observe reflections off subjects in UV light, so we have to rely on indirect means to make a photograph of these reflections. To this end a digital camera, or silver-halide film in a non-digital camera, that is sufficiently sensitive to UV rays is required together with a lens that transmits at least some UV in the spectral band of interest. A source of UV light also has to be used to make the exposure.

Attached Image: P0704154505.jpg
Figure 1. Shooting in UV requires the same paraphenalia as any other field of photography: A camera, lens, subject, and sufficient light. The main difference is the requirement for blocking off all spectral bands apart from the UV range itself.

Let us examine these requirements one by one.

Firstly, we need an appropriate UV-transmitting optical system that either can be focused in visible light with negligible focus shift for UV, or at least has a predictable magnitude of such focus shift. This is to ensure we can obtain an output image with sharpness located in the plane of interest. Alternatively, with a digital camera, we can sometimes do focusing directly under UV light conditions by employing a "LiveView" capability if this feature has adequate response to UV. Do note that this still is an indirect method in the strict sense as the camera "translates" UV energy impinging on its sensor to an electronically based real-time view of the subject. So we are not seeing UV per se, only a translated manifestation of it. Using Liveview might require shining a UV torch onto the subject in order to get sufficient response from the camera to allow real-time focusing. This is almost always required for studio work because UV levels indoors are very low.

The lens has to be able to transmit UV preferably down to 350 nm, the lower into UV the better, and to make a photographic image with adequate contrast and detail sharpness. Most lenses are not designed with UV operation in mind and may either attenuate UV to the extent that their use is impractical or impossible, or render an image with great loss of sharpness and contrast so even though a picture can be taken, it is for all purposes useless. Once in a while lenses are encountered, however, which exhibit good UV characteristics and thus provide useful images. If a lens specifically designed for UV usage is put into service, these earlier considerations are moot of course. Such lenses typically have quartz or fluorite elements, transmit UV almost unimpaired to around 300 nm or even lower, and in general are very expensive.

Non-specialist lenses for UV:
  • Petri 35 mm f/3.5
  • Novoflex 35 mm f/3.5 Noflexar
  • Nikon 50 mm f/1.8 SE
  • Enlarger lenses (many models)
  • Petri 135 mm f/3.5
The list above can be made much longer, of course, and only serves as an indication that useful lenses can be found in many places. Do read the caveats below, though.

One should try to pick an older lens with simpler coatings as modern, multi-coated optics tend to transmit very little UV. Frequently a lens without cemented elements in its optical design gives better performance because the cement substance blocks some UV. Fast lenses or lenses with thick high-index glass inside often transmit poorly in UV as well. An enlarger lens has to be mounted on a bellows or similar focusing device and may not reach infinity focus. However, as that isn't a design criteria for such lenses, image quality is likely to suffer anyway.

A focus shift between what we observe through the lens/camera finder and what the camera actually captures is very common. If the shift is small enough, stopping down will take care of the difference, otherwise some kind of focus adjustment is required. Comprehensive testing may be necessary to make a calibrated focusing scale to correct from visible to actual focus in UV. If some kind of real-time in-camera focusing ("LiveView'") capacity exist, the task is far easier though.

If the aim is to demonstrate UV marks on flowers, many lenses can indicate the presence of these in UV because they start to appear just below 400 nm, but unless a suitable lens is used the UV record will be rather low quality. The intended application will dictate what level of quality is required.

Some well-know specialist lenses for UV:
  • Coastal Optics 60 mm f/4 APO (to 300 nm)
  • Nikon 105 mm f/4.5 UV-Nikkor (to 200 nm)
  • Rodenstock 60 mm f/4 UV-Rodagon
  • Carl Zeiss 60 mm f/4 UV-Planar
With the specialised lenses, outstanding image quality in UV captures can be attained. It will be evident that UV, because of its shorter wavelengths, makes for even sharper images than we are accustomed to in visible light photography. If the lens is parfocal (same focus in UV as in visible light), no further focus correction is of course required. However, even these specialised lenses might not be perfectly parfocal over their entire spectral range of transmission, so preliminary testing is advised. Some, like the UV-Nikkor, might be parfocal for UV-visible light, but exhibit a focus difference between visible and IR.

Some of the lenses listed above come in various fittings, others are brand specific. Adapters can make non-native lenses work with other camera brands, or in some cases, the entire mount can be replaced.

Secondly, the camera should record UV to an adequate degree. That requirement is not as simple to accomplish as one might think.

For film systems, that usually means one should use a black-and-white film. Colour films tend to have a UV-absorbing protective coating so are much less efficient for UV photography. If colour films are to be deployed, tungsten-balanced versions might show a better UV response than daylight material.

A digital camera has an anti-aliasing (AA) filter pack in front of the imager, and UV- and IR-absorbing filters are integrated into this pack. Thus, the stock camera will have very low sensitivity to irradiance outside the visible spectral range. In order to be useful for UV, or IR, photography, the entire filter pack must go. To keep the register distance (lens flange to film plane), it is customary to replace the AA filter pack with a neutral quartz glass window of similar dimensions. This will provide some protection of the imager proper as well. The camera now can record in a broad range of the spectrum and is designated a "broad-spectrum" modified model. One governs what part of the spectrum to use by mounting filters in front of the lens. Alternatively, the inside replacement can be a UV- or IR-transmitting filter to give a dedicated UV- or IR-enabled camera, respectively.

The camera mount and possibilities of using adapters to encompass lenses of different makes can be an important decisive factor as well. Some systems, for example the mirrorless models, can readily be adapted to many different lenses. Others are less easily adaptable if one needs to preserve infinity focus.

A modification of a digital camera is mandatory since all current models have far too low response to UV as delivered from the factory. Do note the low sensitivity to UV (and IR) is by design from the maker's side so is not a flaw of the equipment as such. If an appropriate replacement filter is used, infinity focus is preserved. Be aware of the possibility of internal monitor LEDs in some models, as these emit IR and will contaminate any UV captures unless the UV bandpass filter is seated closer to the imager than the LED.

There are a wide range of digital cameras in use which all can do useful UV work. Economy, versatility, and availability are factors to help select an appropriate system.

Thirdly, when a suitable camera and lens are found, one has to consider adequate filtration of spectral bands reaching the film or digital imager. The importance of the UV bandpass filter cannot be stressed enough. It should leak less than 10-3 (transmission) anywhere outside the UV passband. Only a few filters will be up to this challenge. At present, the latest incarnations of the "Venus" Baader filter can be recommended. Do note that the earliest version, in 1.25" size, leaks too much IR for photographic recording of UV, so the more recent 2" models have to be deployed. Read more on the filter requirements here: http://www.ultraviol...uv-photography/ .

Fourthly, a good source for UV illumination is required. The sun itself is a major albeit a trifle unreliable source of UV. If more predictable illuminants are required, Xenon-based flash units are probably the better alternative. The flash tube should not be coated so as to allow a maximum of UV to be emitted. The presence of coating can be seen as a shimmering of gold, either on the tube itself or on the protective window outside it. Studio flash devices often have, or can be fitted with, uncoated flash tubes. Xenon flashes have an output spectral characteristic quite similar to that of the Sun itself, so are moderately rich in UV. Nikon offered the SB-140, a special flash for UV (and IR) where the output could be controlled by dedicated fiters. Unfortunately, this unit is no longer in production, but sometimes can be found on the second-hand market. Some smaller flash units, from Vivitar amongst others, can be modified to give better UV output too.

LED-based devices can be useful if the LEDs deliver sufficient energy in the UV band. Do note such devices normally are narrrow-band often with a peak of 365 or 390 nm, and being able to deliver continuous output may be dangerous to the eyes, so never use them without wearing UV-protective goggles. The LED devices are perhaps most useful for UV-induced fluorescence photography.

Blacklights, or Wood's lamps, are similar to LED lights capable of continuous output, so serve the same purposes. In terms of output they are much weaker than the better flash units so exposure times tend to be longer using them.

Figure 2. Field equipment for UV reflective photography: A UV-modified DSLR (Nikon D200 in this case), a UV-capable lens, UV bandpass filter (in the cell on front of the lens). a UV flash (SB-140), and colour/reflection standards for a calibrated colour profile.
Attached Image: U1104211615.jpg

Figure 3. Under studio conditions, flash heads with uncoated Xenon tubes can be used to provide an excellent source of UV light. Their large ring-shaped tube and reflector ensure an even lighting of the specimens under study.
Attached Image: T1010141578.jpg

Figure 4. Using portable battery packs, smaller studio flash kits can be brought into the field and used to provide UV illuminance for improvised studio setups.
Attached Image: U1105095233.jpg

[Published 25 Jan 2013 Last update 28 Apr 2013]