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Reflected ultraviolet photography with colour
As mentioned in the section on films all silver halide films - including colour negative and colour reversal - are sensitive to ultraviolet . There is no significant advantage to using colour film for pure reflected ultraviolet work because only the blue layer of the tri-colour pack will be exposed by the ultraviolet (the integral yellow filter effectively preventing exposure of the red and green layers). The widespread use of digital (CCD) cameras for imaging has spurned a new interest in infrared photography and to a lesser extent reflected ultraviolet work. The web abounds with personal accounts of experiments in invisible radiation and false colour systems. Some workers have designed systems that use source/filter combinations and colour film to give false colour renditions. Their unpublished, and often incomplete, work is difficult to interpret but most interesting and worth therefore noting here.
Rørslett basically uses three techniques - each with combinations of sunlight and/or supplementation from the SB-140 flash fitted with the ultraviolet transmission filter. He reportedly uses the Nikon UV Micro Nikkor. His experiments are summarised as follows:
Rørslett has experimented with many landscape and natural history subjects and his flower photographs - particularly specimens of Taraxacum officinale and Potentilla erecta - show most interesting effects consistent with the theory that many flowers have insect 'guides' for pollination which are only 'visible' in ultraviolet radiation. Figures 60 and 61 show Rørslett's results for these flowers. The interested reader is urged to visit Rørslett's own web site for further details of this work.
Figure 60 (above). Taraxacum officinale - the common dandelion photographed with visible radiation on colour film (left) and with Fuji RTP tungsten balanced transparency film and an ultraviolet transmission filter(right). This is effectively a 'false colour' record with ultraviolet radiation exposing the blue layer and far red radiation exposing the red sensitive layer of the film. Images © Rørslett.
Figure 61 (above). Potentilla erecta photographed with visible radiation on colour film (left) and with Fuji RTP tungsten balanced transparency film and an ultraviolet transmission filter(right). This is effectively a 'false colour' record with ultraviolet radiation exposing the blue layer and far red radiation exposing the red sensitive layer of the film. Images © Rørslett.
Yokozawa, working independently from Rørslett in Japan has achieved equally interesting results but the details of his methodology are so sparse as to be completely unintelligible. Figure 62 shows one of Yokozawa's images. He apparently used an interference filter which reportedly passed ultraviolet, green and red in order to create a 'false colour image on a variety of colour films. Yokozawa's web site may reveal further information in the future.
Figure 62 (above). H. annuus L recorded conventionally (left) and using a false colour ultraviolet method (right). No technical details are available. Images © yokozawa .
Hogan reports using Fuji RTP tungsten balanced colour slide film with a Schott UG1 ultraviolet transmission filter to photograph a number of plant species. He also experimented with Kodak EIR infrared sensitive film and an ultraviolet transmission filter. Some of his results are similar to Rørslett - deep blue, through magenta to red. Miksik has also used Fujichrome RTP film in his work in Czechoslovakia; with no details of the filtration it is again difficult to interpret what his images are showing but they are almost exclusively confined to the blue layer of the film - suggesting a tighter - more narrow cut - ultraviolet transmission filter than that used by Rørslett et al.
Atlanta Camera - a US based equipment manufacturer supplies a 'ready made' camera system for 'office' ultraviolet photography, targeted mainly at the dermatologist. No technical details are available for their system but the results are basically reflected ultraviolet recording in the blue layer with the usual red 'leakage' of the UV transmission filter slightly affecting the red layer of a standard tri-pack colour emulsion. The manufacturer's claims about what the images show however are very questionable. (See our section on invisible radiation and the skin for a more reasoned interpretation.)
Still others have assembled separate black-and-white images filtered for different parts of the spectrum using Adobe Photoshop. http://home1.gte.net/jww6/uv/technique.html (Anon) has an explanation of the technique where essentially green, blue and ultraviolet records are combined in the red, green and blue channels respectively to achieve a false colour reflected ultraviolet image. In an interesting web published paper on insect vision "Eddie" (currently down) - a final year biological imaging student from Derby College in the UK - used this same principle to simulate a variety of insects' views of each other and of nectar guides in British flora.
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Last modified: 3 May 2002