Ultraviolet Photography - Overview

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 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.