by light and heat: (1) evaluate daylight expo-
sure, (2) evaluate electric light exposure,
and (3) evaluate duration of exposure.
Evaluate the daylight first, because it
contains a much higher proportion of UV
than do electric sources. The highest-quality
UV filters for daylight are made of acrylic and
other plastics formulated to eliminate the
transmission of UV but allow the passage of
visible light. They are available as self-sup-
porting sheets used in place of glass, thin
acetate applied to glass, and varnish.
White paint is also a good UV absorber.
If all light entering a room is reflected at least
once from a white surface, the UV problem
will be solved. Titanium dioxide pigment is
best for this purpose, but zinc white is also a
good absorber.
Second, evaluate the fluorescent and
HID lamps. Although they emit UV radiation
less strongly than daylight, all discharge
lamps require UV filters. Fluorescent and
HID lamps with correlated color tempera-
tures above 3100 K need careful attention,
because both UV radiation and short-wave-
length visible radiation increases with color
temperature. Plastic sheets of UV-absorbing
material are available from manufacturers of
color filters.
Incandescent lamps emit too little UV to
require a filter. UV radiation from almost all
incandescent lamps is less than 0.1 percent
of the input wattage. Tungsten-halogen
lamps emit slightly more UV below 300 nm,
owing to their higher filament temperature.
This is still a small amount; fortunately, ordi-
nary glass, transparent to longer-wavelength
UV, completely blocks this extra-short emis-
sion. (Tungsten-halogen lamps in U.S.
luminaires will already have a lens or cover
glass that provides safety protection in case
of lamp breakage.)
It is more difficult to limit visible radia-
tion than to limit UV radiation because the
artwork will then be left in darkness.
Museum practice suggests that for oil and
tempera paintings, oriental lacquer, undyed
leather, horn, bone, and ivory, maximum
maintained illuminance is 15 fc on the sur-
face. For objects especially sensitive to light,
such as drawings, prints, watercolors, tapes-
tries, textiles, costumes, manuscripts, and
almost all natural history exhibits, maximum
maintained illuminance is 5 fc.
With the quantity of light maintained at
5 to 15 fc, radiant heat is also controlled to
reasonable limits. Lamps should be located
outside exhibition cases and ventilated with
air that avoids traveling directly past the
exhibits. Dichroic cool-beam lamps are
useful. Their color appearance is somewhat
cooler than standard sources; the color ren-
dering, however, is undisturbed.
Third, evaluate the time of exposure.
Exposure is the simple product of
illuminance and time. The same amount of
damage will be produced by a large quantity
of illuminance for a short time or a small
quantity of illuminance for a long time. If the
illuminance is halved, the rate of damage is
halved. The optimal strategy is to reduce
both illuminance and time of exposure.
With low illuminance values, warm
versus cool colors of light are preferred. The
low quantity of light is less important to the
viewer than the balance of brightness
between the works of art and the surround
(the remaining space). Adaptation plays an
important role in the enjoyment of art: the
viewer’s eyes must be adapted to the lower
illuminance before entering the exhibit room.Balance of Brightness
In chapter 2 we learned that lighting design
involves the balance of three elements of
light: ambient light, focal glow, and sparkle.
The balance of these three elements causes
a worker’s task to be easily distinguished
from the background, an artwork to stand
out from its surround, and a conference orDESIGN