Science of Conservation: Ultraviolet Fluorescence Photography using a Digital Camera with Electronic Flash Strobe

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CCI Newsletter, No. 34, December 2004

Science of Conservation: Ultraviolet Fluorescence Photography using a Digital Camera with Electronic Flash Strobe

by Carl Bigras, Senior Scientific Documentation Technologist, Analytical Research Laboratory

The use of digital photography for scientific documentation is quickly making headway, as is its use in ultraviolet (UV) fluorescence photography.

Fluorescence is created when paints and other coating layers, such as varnishes, are excited by long-wave UV radiation. The excited coatings then re-emit the energy in the visible range of the spectrum. However, because the intensity level of the visible fluorescence is much lower than that of the UV radiation that is reflected from the object, effective photography requires that the reflected UV be excluded using a combination of blocking filters.

CCI's standard method of UV photography with a film camera uses electronic flash strobes equipped with 18A-type glass filters (Schott UG1); the strobes provide the excitation energy and the filters block the reflected UV radiation. These strobes are characterized by very high energy output, short flash duration, and consistency. This technique eliminates the problems typically associated with film that is used with "black light" fluorescence tubes (i.e. long exposure, reciprocity failure).

When this standard UV fluorescence photography method was first tested with a Nikon D1X professional digital camera, the resulting images were found to be very red overall. This suggested that there was some interaction between the digital camera and the UG1 UV filters that allowed the infrared (IR) radiation to pass through the barrier filters and lens. Spectrophotometer tests were then done on different kinds of 18A-type glass filters from various manufacturers to see if this problem could be corrected. Results revealed not only the usual peak at 365 nanometres (nm), but also another peak in the IR region between 700 and 800 nm. Some filters even had a higher transmission curve in the IR region than in the UV region.

Although regular film is not sensitive to IR radiation (beyond
700 nm), a digital camera's CCD (charge-coupled device) is generally sensitive to both red and IR radiation. Camera manufacturers are, therefore, incorporating a cyan-coloured, IR-blocking filter on the CCD's surface to exclude IR radiation. To make sure that this is effective, we conducted a spectrophotometer test with a filter integrated to the CCD. Results showed that although the camera recorded a lot less IR radiation, the transmission in the 700 to 900 nm range was still between 4 and 1.25%. Hence the filter decreased the camera's sensitivity in the IR region but did not completely eliminate it.

In an effort to improve the effectiveness of digital cameras for UV fluorescence photography, additional tests were conducted using the same barrier filters as above but with the addition of a #486 UV / IR cut-off filter from B+W filter (Jos. Schneider Optics). In this case, all IR reaching the CCD was eliminated. The final UV fluorescence digital image achieved with this technique is similar to results obtained using colour slide film.