Reflected or direct ultraviolet photography

The reflected ultraviolet photographic technique records only ultraviolet radiation, in the region 320nm to 390nm, reflected from the subject. All other radiation is prevented from reaching the film. A source of ultraviolet is directed at the subject which will then reflect this radiation back into the camera. In some instances the subject may be excited by this high-energy radiation and emit fluorescence in the visible spectrum. With the reflected technique, it is necessary to fit an ultraviolet transmission filter over the lens to prevent any visible radiation from impinging on the standard black-and-white film. Visible radiation, either in the room or reflected from the subject, can thus be ignored because it is absorbed by the filter over the camera lens. Figure 5 illustrates a general arrangement.

Figure 5 (above). A generalized arrangement for reflected ultraviolet photography.

For most photographic purposes ultraviolet is a problem - it is scattered easily by haze in the atmosphere which often ruins the appearance of landscape pictures, and causes very blue shadows in colour photographs taken with daylight or with flash. Figure 6 demonstrates this effect; when the author took this colour photograph of the Toronto waterfront the extent of low wavelength atmospheric haze was not visible to the eye but fairly effectively ruins the photograph. Photographic manufacturers have responded to these problems, and most films now have an ultraviolet absorbing overcoat and most electronic flashguns an ultraviolet absorbing filter over the flash tube. As professional photographers we are so accustomed to ultraviolet being a problem that we even advocate fitting a "skylight" or ultraviolet absorbing filter permanently over the lens (Figure 7). To turn the situation completely around and intentionally use this ultraviolet radiation to make the photograph, therefore presents quite a challenge.

Figure 6 (above). The Toronto waterfront on a sunny summer's day - effectively ruined in this photograph by the ultraviolet dispersion in the atmospheric haze.

Figure 7 (left). The 'UV Filter' or 'Skylight' filter routinely fitted to the front of camera lenses as physical protection is actually a UV blocking or barrier filter - useful for preventing the unwanted effects of haze in landscape photography and blue colour casts in flash photography. It does not enable reflected ultraviolet photography but prevents it.

Many subjects have very unpredictable reflectance or absorption under ultraviolet. Figures 8 and 9 show examples of how different some subjects appear under reflected ultraviolet radiation.

	Figure 8 (left). Many subjects have a dramatically different appearance when viewed via reflected ultraviolet photography. This colourfully patterned blouse seen recorded with visible light onto panchromatic film in the top "control' photograph turns into a candy striped material when recorded with reflected ultraviolet radiation.
	Figure 9 (left). These photographs of an orchid also demonstrate the marked difference between the reflected ultraviolet record (top) and the visual appearance (below). Flowers have often been the subject of reflected ultraviolet photography in an attempt to identify markings or 'guides' for insect vector pollination.

Some materials that are black in visible light reflect ultraviolet so effectively that they record as white using the reflected ultraviolet technique, the fur of white seal pups, for example, records black against the white background of snow, and has been used in aerial surveys of arctic seal populations (Lavigne, 1976). Most biological subjects react less dramatically but the principle is the same. Tone and colour differences so slight they are barely discernible to the eye, often become very clear when imaged with the reflected ultraviolet method.

References

• Lavigne, D., 1976, "Counting Harp seals with ultraviolet photography," Polar Record 18:(114):269-277.

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