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There are many interesting and diverse applications for ultraviolet imaging, a technology that is now over a century old. These include imaging tiny scratches and imperfections in surfaces, the visualization of sun-induced skin damage and faded bruises, and the detection of trace evidence and residue on surfaces. I am referring specifically to reflected UV imaging, where a UV illumination source is used in conjunction with a UV-sensitive camera to produce a pure UV image. This technique is different from UV-fluorescence imaging, where a UV excitation source is used to stimulate visible-light emissions from a fluorescent surface, which can then be visualized either by human eyes or color cameras equipped with barrier filters that block the UV excitation.
Recent developments in digital ultraviolet imaging technology have made reflected-UV imaging much more affordable and accessible to a diverse user group. Various systems now are commercially available that allow one to take UV digital stills and video with a live preview mode that makes it simple to dynamically adjust shot composition, exposure and focus. These advances have been a great boon to forensics and medical workers who have been trained in frustrating and slow methods of reflected-UV film photography. Consider that methods using SLR film cameras equipped with black glass UV pass filters will require bracketing the exposures, high f/numbers which increase depth of field to compensate for focus shifts, and severe challenges in correctly framing a shot of a moving target, since the eye cannot see an image through the barrier filter. There also is a mythology that one MUST use very expensive fused silica lenses for UV imaging1. For these reasons, many technical people decided years ago that reflected-UV imaging is too difficult and clumsy and have stopped trying to use it, which is one of the reasons that most photographic filter companies have discontinued manufacturing black glass UV pass filters.
There are many ways to divide the ultraviolet portion of the UV spectrum into sub-bands, and every field of science has its own nomenclature, which I will not attempt to catalog here. The two main wavebands considered in this article are the near-UV band (generally considered to be 300-400nm) and the deep UV, which is roughly 200-300nm. 300nm is a convenient dividing point, since both the atmosphere and standard optical glass both cut off rather sharply around this wavelength.
A reflected-UV imaging system consists of the following components:
For digital UV imaging systems, the most common detector is a silicon CCD array, though there are other technologies (such as gallium nitride) that show promise. The CCD may be back-thinned to increase its response to UV light below 300nm. Figure 1 shows the relative spectral response for the Sony XCD-SX910UV camera, which is a two megapixel machine vision camera with enhanced UV response. As is the case with the SX910UV, silicon CCDs are generally more responsive to visible and near-infrared light than they are to UV. In order to image under lighting conditions that include visible or near-IR light (such as in sunlight), the imaging system must have a filter or filter stack that blocks this undesired light so that the desired UV band will dominate the images' spectral content.
In my experience, conventional color video or SLR lenses seem to work fairly well for near-UV imaging (330-400nm), but fail to transmit well below about 320nm, which is where the BK-7 lens glass material becomes highly absorbing. If the bandpass is narrow, as is the case with near-UV LED illumination, the chromatic aberration of color lenses used in the UV is quite tolerable. Imaging in the deep-UV band below the glass cutoff wavelength requires expensive optics made of fused silica or calcium fluoride. These lenses are commercially available but the selection of focal lengths is quite limited, and I know of no commercially available deep-UV zoom lenses. Many of these lenses are not achromatic, requiring refocusing if the bandpass is varied.