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. There is another band of radiation that is also included in the complete spectrum, and extends from about 10 nanometers to 400 nanometers, known as ultraviolet (UV) radiation. Ultraviolet light is divided in three bands: Long wave UV that extends from 320 to 400 nanometers. Medium wave UV that extends from 280 to 320 nm.. Short wave UV that extends from 200 to 280 nm.|
In archaeological photography, we use ultraviolet radiation when we want to provide information about an object which cannot be obtained by other photographic methods. With UV radiation we can reveal the coloring, the images and many times the writing that have been rendered invincible with the passage of time.
Another application, is to collect information about the condition of the ancient subject. We can collect information about the level of moisture deterioration and how deep it has penetrated. We can reveal hidden cracks in marble artifacts. We can also verify the authenticity of an artifact, as well as any additional work or restoration done on it at a later time.
In archaeological photography, long wave UV is mostly used. A big deciding factor is that long wave UV radiation can pass freely from most modern lenses, in contrast with the short bands that are absorbed by normal glass lenses. There are some special quartz crystal lenses made by Zeiss for Hasselblad and other companies, which allow wavelengths from about 200 to 280 nanometers to pass. These short UV wavelengths are used mostly in medical research, will cause "sunburn" of unprotected eyes or skin, and of course the price of the lenses is very high.
There are two ways to use UV radiation to take photographs. The reflected ultraviolet and the ultraviolet fluorescence photography.
The reflected ultraviolet photography has no practical use in archaeological photography (other than a few cases), as it has 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 than a cloudy or rainy day.
Other suitable sources are electronic flash, and "black light" fluorescence tubes. Electronic flash, in combination with an aluminum reflector can be used efficiently. Some flash units have a special UV absorbing glass over the flash tube, which must be removed before the exposure.
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.
In reflected UV photography the subject is illuminated directly by 2 lamps (radiation sources). A filter is placed on the lens, that allows ultraviolet to pass and absorbs all visible light. Kodak Wratten 18A, B+W ….. are some of this kind of filters. These filters are made from glass. Most types of glass will allow long wave UV to pass, but absorb all the other UV wavelengths.
Ultraviolet fluorescence photography on the other hand, has better results in archaeological photography. We use the same fluorescence 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. 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 color clothes.
In case a layer of varnish is used over a subject to protect it against the sun's ultraviolet radiation, this layer must be removed before taking a photograph.
Due to the very low intensity of light in practice, it is very difficult to use a common exposure meter to take measurements. Correct exposure time can be found only with a series of test exposures, depending on the lighting equipment and the size of the subject. The lights must be placed as close to the subject as possible, while being able to illuminate the whole surface, i.e. from 20 to 50 cm. Using a Polaroid B&W film, type 667 3200 ASA or equivalent as a starting point it help to determine the correct exposure time.
All B&W Panchromatic films are sensitive to ultraviolet radiation and are suitable for ultraviolet photography, but a faster film like 400 ASA is better than 100 ASA. Using Kodak T-Max 400 ASA as 800 ASA is a good solution without needed pushing proccessing. Due to the very long exposure -from 2 to 15 min.- reciprocity effect must be taken in mind during development. Image contrast decreases as the wavelength of radiation becomes shorter. It is better to increase development time a bit, to get a higher contrast B&W negative.
With color films, daylight type film is preferable. Using the new Kodak E100S or Fuji Provia you would not have big problems with reciprocity effects and color changes. You might need a second color correction filter for better overall color balance. This filter must be fitted between the barrier filter and the lens.
Focusing for ultraviolet light is another problem. In practice you must focus a bit farther than the surface of the subject, lighting the subject first with a tungsten lamp for focusing, switching it off before shooting, and finally decreasing the lens aperture at least 1 stop, to obtain adequate depth of field.
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Sample of UV application