Theory

Ultraviolet light meets visible light at 400 nm (20). The region of ultraviolet which is to be used in this project is within long wave ultraviolet, Long wave ultraviolet ranges from approximately 320nm to 400nm, but as the lens limits this, the range used is 365nm to 400nm.

All theories of colour vision assume that there are 3 types of photoreceptor involved. However, butterflies can have 2, 3 or 4 types (1, 15, 21), and 5 types have been reported in the Japanese Yellow Swallowtail Butterfly, Papilio xuthus (10, 21).

Materials

The photographs were taken on a Canon A1 camera fitted with a Canon 50mm lens through Vivitar extension tubes. Visible light was eliminated by a 3 inch square Kodak Wratten ultraviolet filter No. 18a. This was mounted on the camera via a Cokin P-system filter holder and a gelatin holder. The ultraviolet photographs were taken on Kodak T-Max 400 film.

Film

All photographic emulsions are inherently sensitive to ultraviolet light to varying degrees, but all sufficiently so for use in ultraviolet photography of long wavelength ultraviolet rays. Another limitation is that some colour films contain filters to cut out ultraviolet light between the gelatin layers, as a result it is considered that the best results will be from the use of black and white film.

Filters

An ultraviolet transmission filter is required in order to eliminate visible light from the image. An appropriate filter for this purpose is a Kodak 18a filter which is available in a 2 inch square or 3 inch square format. The 3 inch square version can be mounted in a gelatin holder with the Cokin P system filter holder, for use on most camera formats. This needs to be sealed around the edges to prevent any extraneous light from affecting the images produced. All ultraviolet transmission filters of this type are made from optical glass, as gelatin absorbs ultraviolet light.

Lenses

Different wavelengths of light focus on different planes when transmitted through a lens. As a result there must be an alteration in focus from what appears correct in visible light. This can be overcome by focusing visually and changing focus by a preset amount (only of practical use with a standardized set-up). Depth of focus can also be affected by the use of short focal length lenses, and use of the smallest possible aperture. The system can be limited by the optical properties of photographic lenses, which tend to prevent transmission of wavelengths below 365nm (for photography below this point quartz lenses become necessary) (20).

Methods

The systems described can be standardized by the production of a gray-scale which is effective in the ultraviolet region (25). This is of greatest use in setting up and standardising a system.

Film processing

Development of the ultraviolet images was in Kodalith developer, at 24 C, for 11 minutes, with constant agitation for the first and last whole minutes, and for 10 seconds in every 30 seconds in between.

Ultraviolet photography outside

From my own tests (using Kodak T-Max 400), there is a 7 stop difference in exposure value between images taken with Kodak's 18a filter and images taken without any filter. There are 2 methods of accommodating for this, the simpler of these is to increase exposure of light to the film, and the alternative is to "push" the film. Increasing the exposure of light to the film increases the density of the negative. Pushing the film (extending the development of the film, either by an increase in developing time, developer temperature, or both), increases the density of the negative, however if the film is being pushed to give the standard density then, it has been under-exposed and there will be a loss of detail, there will also be an increase in contrast. Ultraviolet light produces images with a low contrast, so the film will need to be pushed. The best results come from a combination of 4 stop increase in exposure and pushing 3 stops using T-Max developer.

There is at present no viable method for metering available ultraviolet radiation at any particular point in time (ultraviolet flash units, and ultraviolet light meters are available (22) but are prohibitively expensive). Therefore, once a reasonable estimate has been made, bracketing should ensure that a negative of appropriate density is obtained.

The use of flash as a standard source of ultraviolet radiation was considered, initial attempts did not show any results. This may have been due to a filter which is used on some flash units to cut out ultraviolet. This information is not always readily available.

In the production of the images the background must be chosen as carefully as is possible under the circumstances, as many images produced show a darkened background. This is often not the case in natural situations, with highly ultraviolet reflective backgrounds occurring in nature. Some flowers absorb ultraviolet, but to an animal with ultraviolet vision it will stand out from the rest of the plant, as parts of the foliage often reflects ultraviolet (particularly hairs) (23).

Ultraviolet photography in the studio

An alternative method for producing similar images to those produced from ultraviolet transmission photography, is to collect a flower which is known to have an ultraviolet pattern and to press and dry it. After only a few hours the, previously ultraviolet, pattern will fluoresce under ultraviolet light.

Using a copy stand with ultraviolet light is useful for determining the best exposure range. If a quick release base and platforms are used on the cameras, with the same lens being changed from one to the other, the images should come out in register. Registration marks also help in bringing the images together.

The main problems encountered in the studio are wilting of the flowers and movement of the flowers. This is mostly a risk during the change over from ultraviolet tubes to daylight tubes.

Producing false colour images

Three methods of false colour imaging for mimicking the visual system of a honeybee have been documented (24). In this case a different method to those described has been used. Using a copy stand in the studio and a tripod in the field, a Cullman quick release base was mounted securely. The cameras each had bases mounted in the same place to ensure that the images from the different cameras would come into register when overlaid.

As a quality slide scanner was not available, photographs were taken on colour print film (Fuji super HG). I printed the images of the ultraviolet up to 10 x 8 inch on Agfa Multicontrast paper. Both the black and white and the colour prints were scanned on a flatbed scanner into Adobe PhotoShop (via HP Picturescan). Once the images were on the computer, the images were brought to a standard size.

For the false colour images representing bee vision, the colour image was kept in RGB mode and the image was compiled by copying: the Green information into the Red channel; the Blue information into the Green channel; and the ultraviolet information into the Blue channel.

Subjects

Flowers pollinated by butterflies have the highest proportion of nectar guides, reported at 83%. Therefore, taking photographs at a butterfly house provides both a good source of butterflies and of flowers. Permission was given to take photographs at the White post butterfly house, Nottinghamshire.

The use of a butterfly houses can cause problems due the the high level of humidity. The lenses should be attached to the camera before entering the butterfly house, with the film preloaded, to prevent moisture affecting the inside of the camera. The outside of the camera will mist over, the humidity condensing on the lens and other surfaces, this will go away of its own accord as the camera and lens equal the atmospheric temperature (this may require some patience). Other problems include the tendency for butterflies to move when approached, this tendency seems to increase with both temperature and light. Butterflies need the warmth of the sun to get their wings to a working temperature, with sunlight being one of the best sources of ultraviolet light, the result is that the butterflies are easiest to photograph when it is overcast, but longer exposures are required.