There are two processes by which exposure to light causes damage to museum exhibits: photochemical action and radiant heating effect (Cuttle, 1996). Photochemical action occurs when the activation energy for a chemical change is derived from the absorption of a photon, and as photon energy is inversely proportional to wavelength, photochemical action is associated with short-wavelength radiation, which includes non-visible ultraviolet (UV) radiation and short-wavelength visible (blue) radiation The most common symptoms are loss of color (fading), and loss of mechanical strength as evidenced by the fraying of textiles and paper becoming brittle. The absorption of radiant energy also causes a heating effect, and the daily switching of display lighting results in cyclic expansions and contractions of the exposed portions of exhibits, coupled with moisture migrations, which can result in cracking of wood, surface crazing of finished materials, and separation of varnish layers. This is particularly associated with incandescent display lighting, where the irradiance due to non-visible longwavelength infrared (IR) radiation may exceed the irradiance for wavelengths within the visible spectrum.
Current recommendations for museum lighting practice are broadly in agreement that UV should be severely restricted or eliminated, and that exposure to light should be. limited both in intensity and duration (Rea, 1993, CIBSE, 1994) For exhibits that are categorized as highly susceptible to exposure damage, the recommended maximum illuminance is 50 lux, which is recognized as the lowest practical level for exhibits for which color discrimination is an important factor (Thomson, 1986). Even when these recommendations are strictly applied, display lighting still causes permanent damage to exhibits (Feller, 1968; Michalski, 1987). While there are no specific standards for IR control, dichroic reflector spotlights are generally recommended. The mirror that forms the beam for this type of spotlight has a wavelengthselective reflecting surface that directs light into the beam, but not IR.
It used to be supposed that incandescent light sources are safe because they are weak sources of short-wavelength radiation, but recent studies of exposure of artist’s pigments have confirmed that it is the spectral absorption characteristic that determines the wavelength susceptibility of the pigment (Saunders and Kirby, 1994). An artist’s palette that covers a full color range inevitably includes pigments that are selective absorbers for every waveband in the visible spectrum. For this reason, the damage potentials of the visible radiation provided by alternative light sources may be compared in terms of irradiances at equal illuminances (Michalski, 1987).
Irradiance is the measure of incident radiant power density in watts per square meter (W/m2), and illuminance is the measure of the density of incident light in lux, where one lux is one lumen per square meter. The basis for comparison used in this report is to evaluate sources in terms of radiant luminous efficacy measured in lumens per radiant watt (lm/Wr). This term should not be confused with the luminous efficacy values quoted in lamp catalogs, which are measures of lumens emitted per watt of electrical power input.