Catherine Sease
Abstract
The Field Museum of Natural History recently opened a new permanent installation of its Egyptian collection. A revolutionary lighting system was utilized to illuminate large or unusual cases, such as simulated catacombs. Based on our experience, this system has tremendous potential in exhibit conservation.
The system, referred to at the Field Museum as “light pipes”, employs a prismatic film to transform a point source of light into a linear source. Made by 3M, this film was first marketed as Scotch Lamp Film and is now called 3M Scotch Optical Lighting Film. It is made of transparent acrylic or polycarbonate polymer in sheet form approximately 20 mils thick. It is smooth on one side and grooved on the other. These grooves are actually tiny optical prisms.
As one looks through the smooth side of the film, it will appear shiny and metallic, or reflective, from some angles, but one can see through it from others. Both these reflective and transmissive properties are used in a light pipe. A PVC tube is lined with the film and a light source is placed at one end. In order to transmit light through the tube, the reflective properties are used. At incident angles of less than 30°, the beam will be reflected back to the other side. In this way, the beam of light is bounced from side to side down the length of the tube.
To allow light to leave the tube, the transmissive properties are used. By changing the angle the beam hits the film, it will no longer be reflected down the tube, but will pass through the film to illuminate the case. This can be achieved by placing an opaque white material between the film and the tube wall. Light beams can also be made to go around corners by the use of mirrors.
The film is estimated to have a life of ten years, however, the life will be extended considerably by filtering out the ultraviolet component before the light enters the pipe. Any kind of point light source can be used. The narrower the light beam, the more efficient the pipe will be.
The advantages of the light pipes are many. First, the light source is not located inside the case so there is no heat build-up. Also, with an external light source, maintenance of the fixtures does not entail the opening or disturbance of the case itself. This system is “low tech” requiring no maintenance or moving parts. It also allows for maximum flexibility for creative design in case lighting.