Optical fibers illuminate miniatures display at Warsaw's National Museum
Miniature paintings are special exhibits, since they are both very precious and extremely sensitive to the environmental conditions present in museum exhibition halls. One of the most important factors that cause such museum exhibits to deteriorate is the illumination:1 ultraviolet (UV), visible (VIS), and infrared (IR) radiation. Ultraviolet radiation causes the greatest damage to museum exhibits. Infrared radiation, though generally less harmful, can heat the surface of museum exhibits and lead to surface cracks.
The National Museum in Warsaw has a collection of 1100 miniature paintings and required a lighting system for an exhibit of 350 in five vertical display cases. The construction of the cases meant that the lighting system had to be as small as possible while offering many lighting points, and the average illuminance value at the surface of the miniature paintings was assumed to be 100lux. The VIS radiation used to light the miniature paintings could not have any UV or IR components. Additionally, the lighting system could not cause a temperature increase inside the display case of more than 3°C.
Optical-fiber lighting systems are perfectly suited to meet these lighting requirements. Each display case was equipped with an illuminator powered by a 50W tungsten-halogen lamp and a dichroic ellipsoid reflector. A double-absorption IR glass filter, together with an interference UV filter, was applied to the optical-fiber illuminator, making the VIS radiation safe for the exhibits. The common belief that optical radiation guided by optical fibers is perfectly friendly to museum exhibits is erroneous. The base material of the optical fibre greatly influences its technical parameters.2,3 Plastic optical fibers (POF) exhibit weak attenuation for shorter wavelengths and are good guides for UV components. Glass optical fibers are good at guiding IR radiation. It was necessary to apply a double IR filter to protect the exhibits, although it resulted in a color temperature increase to 3400K and reduced the color rendering index to 87. The filter also protected the end of the POF bundle against melting, since it was not cooled otherwise: fanned illumination systems cause significant noise, which is inappropriate in a museum.
Two lighting solutions were tested: one with single optical-fiber terminals located uniformly around the perimeter of the case, and one with optical-fiber bundles located only in the top and bottom. The first solution was unsatisfactory because individual optical fibers ended up with highly-differentiated luminous flux levels. The final system consists of nine optical-fiber bundles located at the top and five at the bottom of each display case. Each optical-fiber bundle was composed of seven 1mm-diameter plastic optical fibers made of polymethylmethacrylate, each with a numeric aperture NA=0.5 and a lighting half-value angle of ±18°, which suited the lighting requirements. Additional small changes in the luminous-intensity distribution were possible by processing the end surfaces of the fibers.4 Of the nine optical-fiber bundles at the top of the display case, five illuminate the top section of the display area while the remaining four (located between the aforementioned five bundles), also illuminate the center of the display case.
The lighting effects obtained can be seen in the photograph of a display case (see Figure 1). The average, pre-defined illuminance value of 100lux was obtained in all display cases. The light beam illuminating the miniature paintings was devoid of harmful UV and IR radiation. The temperature levels inside the display cases increased by just 1°C as a result of using the optical-fiber lighting system. This met the pre-defined lighting requirements. Satisfactory lighting uniformity was made possible by locating the small optical-fiber terminals as far from the illuminated surfaces as possible, and by being able to adjust the lighting direction of each bundle separately.
