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.