Ultraviolet photography

Ultraviolet photography is a photographic process of recording images by using light from the ultraviolet (UV) spectrum only.

Overview

Ultraviolet image of the globular cluster NGC 1851 in the southern constellation Columba.

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 180 nm. 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.

Equipment and techniques

Reflected UV photography

A portrait taken using only UV light between the wavelengths of 335 and 365 nanometers.

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. Examples of these filters are Kodak Wratten 18A, B+W 403, Hoya U-340, Baader U-Filter and Kenko U-360. 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 350 nm and below. Only special developed lenses made of quartz (fused silica) or quartz and fluorite can be used. Using these lenses allow the camera to reach the range 180..200 nm. Lenses based purely on quartz show a distinct focus shift between visible and UV light, whereas the later developed fluorite/quartz lenses are fully color corrected without focus shift. Examples of the latter type are the Nikon UV Nikkor 105 mm, the Hasselblad (Zeiss) UV Sonnar 105 mm and the Asahi Pentax Ultra Achromatic Takumar 85 mm.

Ultraviolet fluorescence photography

Fluorescent materials lit by UV light. No filter is used to absorb violet visible light. Photo taken with daylight film by Beo Beyond.

Photography based on (visible) fluorescence induced by UV radiation has better results in archaeological photography. The same ultraviolet light sources as in reflected UV photography, except that a glass filter is used to absorb all the visible light and allow only the desired ultraviolet to pass. 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, by 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 excited in the sample by the UV source. 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. The photographer should also wear dark-colored clothes for better results. (Many light-colored fabrics also fluoresce under UV.)

Suitable digital cameras for reflected UV photography have been reported to be the (unmodified) Nikon D70, D40 and Fuji Film S5 cameras, but 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.

The Fujifilm FinePix IS PRO digital SLR camera is purpose-designed for ultraviolet (and infrared) photography, with a frequency response rated from 1000-380nm, although it also responds to somewhat longer and shorter wavelengths. Silicon (from which the sensor is made) can respond to wavelengths between 1100-190nm.

See also

• Ultraviolet video
• Infrared photography
• Fujifilm FinePix IS PRO UV- (and IR-) sensitive DSLR camera

External links

• UV digital photography
• UV photography of plants, buildings, landscape
• Ultra-violet kite and pole aerial photography