Introduction

The electromagnetic spectrum is comprised of a series of waves arranged in order of wavelength. The various types of radiation forming this spectrum differ widely, and only a very minute part of this spectrum has any relevance to photography. Usually photography is confined to the visible part of the spectrum, those waves that the eyes see as "light'. This part of the spectrum is comprised of wavelengths from approximately 400 - 700 nanometers (nm), which can be seen by the eye as a change of colour. The shorter wavelengths are blue, the longer ones red. At either end of the visible spectrum lie two "invisible" spectra: the ultraviolet which extends from x-rays to the blue end of the visible spectrum, and the infrared which extends beyond the red and into heat (Figure 1). One important function of photography is to extend the range of spectral visualization of the human eye and record these "invisible" spectra. Infrared and ultraviolet photography therefore acts as investigative tools that are capable of discovering new facts about the subject. In some fields of investigation, extensive work has been reported on the use of invisible radiation photography. Other areas of application, however, remain unexplored and await the attention of the research-oriented photographer.

Figure 1 (above). At either end of the visible spectrum lie two "invisible" spectra: the ultraviolet which extends from x-rays to the blue end of the visible spectrum, and the infrared which extends beyond the red and into heat.

Both ultraviolet and infrared photography offer a visible interpretation of an invisible state - no one has ever seen what the subject looks like under these radiations because the retina is insensitive to them. There is, therefore, no "correct" density to print to. It is also sometimes very difficult to interpret the infrared or ultraviolet record. It is for these reasons that one should always include a control photograph taken with visible light to provide an exact comparison of the subject. It is also worth pointing out that clinicians and scientists will often have an incomplete understanding of the value of infrared and ultraviolet techniques. The competent photographer, however, will always be alert to the possible application of these techniques, and may indeed need to correct misunderstandings about their use.

There are two distinct techniques of ultraviolet photography: the reflected or direct method, and the ultraviolet fluorescence method. Reflected ultraviolet photography requires the subject to be lit with ultraviolet radiation, and filtration used so that only ultraviolet radiation is allowed to reach the film. The ultraviolet fluorescence technique requires that only ultraviolet radiation is allowed to fall on the subject, and the camera (if there is any fluorescence) records the emitted visible light. The reflected ultraviolet method is described in this chapter; the fluorescence method is described in 'Ultraviolet fluorescence photography.'

< Acknowledgments

Ultraviolet radiation >