Ultraviolet photography

Photographic process using UV radiation

Ultraviolet photography is a photographic process of recording images by using radiation from the ultraviolet (UV) spectrum only. Images taken with ultraviolet radiation serve a number of scientific, medical or artistic purposes. Images may reveal deterioration of art works or structures not apparent under light. Diagnostic medical images may be used to detect certain skin disorders or as evidence of injury. Some animals, particularly insects, use ultraviolet wavelengths for vision; ultraviolet photography can help investigate the markings of plants that attract insects, while invisible to the unaided human eye. Ultraviolet photography of archaeological sites may reveal artifacts or traffic patterns not otherwise visible. Ultraviolet images have no color since ultraviolet radiation is invisible to human eyes. Photographs of dyes that fluoresce under ultraviolet illumination are examples of ultraviolet fluorescence photography.

In reflected UV photography the subject is illuminated directly by UV emitting lamps (radiation sources) or by strong sunlight. A UV transmitting filter is placed on the lens, which allows ultraviolet radiation to pass and which absorbs or blocks all light and infrared radiation. UV filters are made from special colored glass and may be coated or sandwiched with other filter glass to aid in blocking unwanted wavelengths. Examples of UV transmission filters are the Baader U filter or the StraightEdgeU ultraviolet bandpass filter, both of which exclude most visible and infrared radiation. Older filters include the Kodak Wratten 18A, B+W 403, Hoya U 340 and Kenko U 360 most of which need to be used in conjunction with an additional infrared blocking filter. Typically such IR blocking, UV transmissive filters are made from Schott BG 38, BG 39 and BG 40 glass. Filters for use with digital camera sensors must not have any "infrared leak" (transmission in the infrared spectrum); the sensor will pick up reflected infrared radiation as well as ultraviolet, which attenuate the contrast and may even completely obscure the details that would be resolved by ultraviolet radiation alone. Most types of glass will allow longwave UV to pass, but absorb all the other UV wavelengths, usually from about 350 nm and below. For UV photography it is necessary to use specially developed lenses having elements made from fused quartz or quartz and fluorite. Lenses based purely on quartz show a distinct focus shift between visible and UV light, whereas the fluorite/quartz lenses can be fully corrected between visible and ultraviolet light without focus shift. Examples of the latter type are the Nikon UV Nikkor 105 mm f/4.5, the Coastal Optics 60 mm f/4.0, the Hasselblad (Zeiss) UV Sonnar 105 mm, and the Asahi Pentax Ultra Achromatic Takumar 85 mm f/4.5. Suitable digital cameras for reflected UV photography have been reported to be the (unmodified) Nikon D70 or D40 DSLRs, but many others are suitable after having their internal UV and IR blocking filter removed. The Fujifilm FinePix IS Pro digital SLR camera is purpose designed for ultraviolet (and infrared) photography, with a frequency response rated from 1000 to 380 nm, although it also responds to somewhat longer and shorter wavelengths. Silicon (from which DSLR sensors are made) can respond to wavelengths between 1100 and 190 nm.

Photography based on visible fluorescence induced by UV radiation uses the same ultraviolet illumination as in reflected UV photography. However, the glass barrier filter used on the lens must now block all ultraviolet and infrared radiation, permitting only visible radiation (light) to pass. Visible fluorescence is produced in a suitable subject when the shorter, higher energy ultraviolet wavelengths are absorbed, lose some energy and are emitted as longer, lower energy visible wavelengths. UV induced visible fluorescence photography must take place in a darkened room, preferably with a black background. The photographer should also wear dark colored clothes for better results. (Many light colored fabrics also fluoresce under UV.) Any camera or lens may be used because only visible wavelengths are being recorded. UV can also induce infrared fluorescence and UV fluorescence depending on the subject. For UV induced non visible fluorescence photography, a camera must be modified in order to capture UV or IR images, and UV or IR capable lenses must be used. Filters are sometimes added to the UV illumination source to narrow the illuminant waveband. This filter is called an exciter filter, and it allows only the radiation to pass which is needed to induce a particular fluorescence. As before, a barrier filter must also be placed in front of the camera lens to exclude undesired wavelengths.

Ultraviolet photography was used as evidence in court at least as early as 1934. Photographs made with ultraviolet radiation can reveal bruises or scars not visible on the surface of the skin, in some cases long after visible healing has completed. These can serve as evidence of assault. Ultraviolet imaging can be used to detect alteration of documents.