The invisible ultraviolet (UV) region of the spectrum extends from c.380 nm (nanometres) to less than 100 nm. It is conventionally partitioned into regions corresponding to its biological effects. UV-A (315-400 nm) is the region that produces suntan, and is transmitted by ordinary glass and Wood's glass (‘black’) filters. Using this filter it is possible to take UV photographs on general-purpose film with an ordinary camera, but as modern lenses transmit these wavelengths only weakly, and some films have an anti-UV coating, it is advisable to use a pre-1950 camera lens and an old-established film such as FP4, and to downrate the film speed in direct sunlight by a factor of 10. Indoors the only method is trial and error. UV-B (280-315 nm) causes serious sunburn and possible eye damage. It requires much longer exposures and refocusing of lenses. UV-C (100-280 nm) is lethal to life forms. Although radiated by the sun, it does not penetrate the atmosphere. Photography in this region demands special lenses made from quartz and/or fluorite, and special Schumann emulsions that have the silver halide crystals projecting through the surface, as gelatin is opaque to UV-C radiation. The best light sources for UV imaging are high-pressure mercury-xenon lamps, with appropriate filtration.
Another type of UV imaging is fluorescence photography. When stimulated by UV radiation at 280-350 nm many chemical substances and body fluids emit visible light and can be photographed using an ordinary camera and film, with a Wood's glass filter over the UV source and a UV-absorbing filter over the lens.
UV and fluorescence photography are used in medical diagnosis, forensic investigation (detection of document forgeries, body-fluid stains, etc.), military reconnaissance (camouflage detection), nondestructive materials testing, and the examination of works of art.
— Graham Saxby
Bibliography
- Ray, S. F., Scientific Photography and Applied Imaging (1999)
