Ultraviolet photography
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Ultraviolet photography is a photographic process of recording images by using light from the ultraviolet (UV) spectrum only.
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[edit] Overview
Light which is visible to the human eye covers the spectral region from about 400 to 750 nanometers. This is the radiation spectrum used in normal photography. The band of radiation that extends from about 1 nm to 400 nm is known as ultraviolet radiation. UV spectrographers divide this range into three bands:
- near UV (380–200 nm wavelength; abbrev. NUV)
- far UV (or vacuum UV) (200–10 nm; abbrev. FUV or VUV)
- extreme UV (1–31 nm; abbrev. EUV or XUV).
Only near UV is of interest for UV photography, for several reasons. Ordinary air is opaque to wavelengths below about 200 nm, and lens glass is opaque below about 180nm. UV photographers subdivide the near UV into:
- Long wave UV that extends from 320 to 400 nm, also called UV-A,
- Medium wave UV that extends from 280 to 320 nm, also called UV-B,
- Short wave UV that extends from 200 to 280 nm, also called UV-C.
(These terms should not be confused with the parts of the radio spectrum with similar names.)
There are two ways to use UV radiation to take photographs - reflected ultraviolet and ultraviolet fluorescence photography. Reflected ultraviolet photography finds practical use in medicine, dermatology, criminology and theatrical applications. Sunlight is the most available free UV radiation source, but the quality and quantity of the radiation depends on atmospheric conditions. A bright and dry day is much richer in UV radiation and is preferable to a cloudy or rainy day. Another suitable source is electronic flash which can be used efficiently in combination with an aluminium reflector. Some flash units have a special UV absorbing glass over the flash tube, which must be removed before the exposure. It also helps to partly (90%) remove the gold coating of some flash tubes which otherwise suppresses UV. Special UV lamps known as "black light" fluorescence tubes are used for long wave ultraviolet photography. Most modern UV sources are based on a mercury arc sealed in a glass tube. By coating the tube internally with a suitable phosphor, it becomes an effective long wave UV source.
[edit] Techniques
[edit] Reflected UV photography
In reflected UV photography the subject is illuminated directly by UV emitting lamps (radiation sources). A UV transmitting, visible light blocking filter is placed on the lens, that allows ultraviolet to pass and absorbs all visible light. Kodak Wratten 18A, B+W 403, Hoya U-340 are some of this kind of filters. These filters are made from special colored glass. Most types of glass will allow long wave UV to pass, but absorb all the other UV wavelengths, usually from about 350nm onwards. As taking lenses only special developed lenses made of quartz (fused silica) or later quartz and fluoride can be used. Using these lenses allow to reach 180..200nm. Lenses based purely on quartz show a distinct focus shift between visible and UV light, whereas the later developed fluoride/quartz lenses are fully color corrected without focus shift. Examples of the latter type are the Nikon UV Nikkor 105mm, the Hasselblad (Zeiss) UV Sonnar 105mm and the Asahi Pentax Ultra Achromatic Takumar 85mm.
[edit] Ultraviolet fluorescence photography
Ultraviolet induced visible fluorescence photography on the other hand, has better results in archaeological photography . We use the same ultraviolet light emitting tubes as in reflected UV photography, except that a glass filter is placed in front of the light source, to absorb all the visible light and allow the desired ultraviolet to pass only. These filters are sometimes incorporated in the source tube itself, but most of the time are separate elements. These filter glasses usually allow a small amount of violet visible light. This should not be confused with UV light, which is invisible to the human eye. The same kind of filters are made for flash tubes also, for BALCAR and other lighting companies. This filter is called an "exciter" filter and it allows only the radiation needed to excite fluorescence. A second filter must be placed in front of the camera lens, to exclude residual ultraviolet and allow the resulting fluorescence. This filter is called the "barrier" filter. Kodak Wratten 2A - 2B is effective as is the Baader UV/IR Cut filter. Using a 2E filter gives better results in some situations. The same filters are also made by other companies with the same codes. The shooting must take place in a darkened room, with a black background and for better results, you should wear dark colour clothes.
Suitable digital cameras for reflected UV photography have been reported to be the (unmodified) Nikon D70 and D40 cameras, but also others might be suitable after having their internal filter removed. If using a modern digital camera (DSLR), it would be advisable to additionally use an IR blocking, yet UV transmissive filter to avoid IR leakage. Schott BG-38 and BG-40 are suitable for this purpose.
[edit] Equipment
[edit] UltraViolet Camera and Spectrograph
George Carruthers, an African American inventor, has gained international recognition for his work which focuses on ultraviolet observations of the earth's upper atmosphere and of astronomical phenomena. Ultraviolet light is the electromagnetic radiation between visible light and x-rays. George Carruthers first major contribution to science was to lead the team that invented the far ultraviolet camera spectrograph. He developed the first moon-based space observatory, an ultraviolet camera that was carried to the moon by Apollo 16 astronauts in 1972. The camera was positioned on the moon's surface and allowed researchers to examine the Earth's atmosphere for concentrations of pollutants. Dr. George Carruthers received a patent for his invention the "Image Converter for Detecting Electromagnetic Radiation specially in Short Wave Lengths" on November 11, 1969.
He has been the principal investigator for numerous NASA and DoD sponsored space instruments including a 1986 rocket instrument that obtained ultraviolet image of Comet Halley. His most recent on the Air Force ARGOS mission captured an image of a Leonid shower meteor entering the earth's atmosphere, the first time a meteor has been imaged in the far ultraviolet from a space-borne camera.
[edit] See also
- Ultraviolet video
- Infrared photography
