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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Digital Reflected-Ultraviolet Imaging

UV Image
UV Imaging (UV camera and light source in a dark-field viewing mode) detecting scratches on a CD jewel case.
UV Image
UV imaging in sunlight showing the application of fresh paint that was undetectable to the eye.
YAG Laser
Imaging a 355nm frequency-tripled YAG laser scattered off optical components.
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By Dr. Austin Richards

Digital reflected-ultraviolet imaging has long been a specialized area of imaging technology that has only found its way into a rather limited number of applications, in spite of the fact that film-based reflected-UV photography has been around for over a century. This is due in part to the characteristics of electronic imaging devices: CCDs and CMOS arrays generally have a greatly reduced sensitivity to UV light relative to visible and near-infrared. It is critical that these undesirable wavebands be removed to get pure reflected-UV images, while still preserving the UV sensitivity of the system.

There is also a great deal of confusion about the difference between reflected-UV imaging and UV-fluorescence imaging. Many people think that the term “UV imaging” refers to the latter. In fact, reflected-UV and UV-fluorescence imaging are different techniques that see different things in a scene.

UV-fluorescence imaging starts with a UV excitation source that stimulates fluorescence of a material, that is, the re-emission of light at a longer wavelength. The fluorescence signal is often visible light that can be imaged with a conventional camera. The classic example of this is the forensics investigator that looks at a crime scene through yellow or orange filter glasses while illuminating a crime scene with a black light. Many types of trace evidence can be found in this manner, provided that the ambient light background is sufficiently reduced first. In contrast, reflected-UV imaging begins with a UV light source, and ends with the imaging of reflected UV light at the same wavelength by a special UV camera. In the past, this type of photography was done with standard black-and-white film, which sees UV light quite well.

Unfortunately, this method suffers from serious limitations related to composition, exposure control and focus. These limitations stem from the visible-light opacity of the UV pass filter combined with the inability of the eye to see UV light through the viewfinder, as well as the fact that light meters (whether external or in cameras) are not designed to measure UV light levels. These difficulties, combined with a dearth of widely-available information about the proper techniques, have prevented film-based UV imaging from becoming as widespread as it might have otherwise. Digital reflected-UV imaging overcomes all of the limitations of film (albeit with a lower resolution for the time being), and now it stands poised to take off just as digital infrared imaging did ten years ago. The drivers are the availability of low-cost imaging solutions, the emergence of new and interesting applications and the rapidly improving family of high-power UV LEDs and lasers that provide pure, narrow-band UV illumination. As with infrared imaging technology, applications for UV imaging are extraordinarily diverse, and span a wide variety of disciplines, including forensics, laser technology, dermatology, biological research, art conservation and defense.

Ultraviolet Wavebands

The ultraviolet spectrum is divided into various sub-bands, with the most common convention dividing the region of the spectrum between 250 and 400nm (the near-UV band) into the A, B and C bands. In this convention, the A band is between 360 and 400nm, the B between 320 and 360nm, and the C between 250 and 320nm. UV below 250nm is sometimes called the deep-UV band. Below 100nm, UV light is very heavily absorbed by air, and experiments done in this band must be conducted in vacuo, leading to the name vacuum ultraviolet or VUV. The A, B and C bands are of practical interest for scientific and industrial imaging, since they are quite a bit easier to generate and detect than shorter wavelengths. UV-C radiation from the sun is heavily absorbed by the ozone layer, and any UV-C light encountered at sea level almost always comes from a man-made source.

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