Series editors' preface - Wood Tools

Table 6.1 UV Component of various light
sources
Light source UV (micro watts per
lumen)
Blue sky @ 15 000 °K 1600
Cloudy overcast sky (north light) 800
Direct sun 400
Fluorescent lamps 40–250
Tungsten halogen (iodine) up to 130
Normal tungsten 60–80
atmosphere. This still leaves potentially
damaging radiation in the waveband
300–400 nm and this must be removed by a
filter. Ordinary window glass absorbs only the
shorter wavelengths of UV and is transparent
to UV in the region 300–400 nm. However,
certain chemicals, such as the substituted
benzophenones, have the property of being
able to absorb almost all of the UV without
absorbing visible light. Such materials can be
added to a varnish or a transparent plastic
sheet or film to make an effective UV filter.
With currently available technology, it is possible
to reduce UV levels to the threshold of
measurement and this should be the goal. UV
filters must be placed between the light source
and the object illuminated. Complete protection
can be provided by placing filters over
windows, skylights and light fittings as necessary.
Filters can also be used in display cases
and in the glazing of pictures. However as
increasing amounts of blue light are removed
by the UV absorber, films appear increasingly
yellow. This can be overcome by slightly
reducing the strength of the UV absorber to
the point where a colourless material is
obtained or by adding a complementary dye
or pigment to the film to produce a neutral
grey. A yellow cast will usually only be noticeable
if the plastic material is close to an exhibit
and not if a whole room is so treated. Where
possible, it is in any case better in principle
to remove UV at source rather than on an item
by item basis.
The choice of UV filter varnish or sheet is
influenced by cost and fitting considerations.
Sheet can be invisible if it is behind diffusing
glass but may be too apparent behind plain
glass. Varnish may show if applied by brush
or spray but can be applied by special flow
techniques to make it practically invisible. It is
General review of environment and deterioration 249
also cheaper though less durable than sheet.
These materials are more liable to scratching
than glass and are also liable to attract an
electrostatic charge which will result in the
accumulation of dust. An antistatic coating is
required to reduce this effect.
Most UV filters may be expected to last
about ten years but their effectiveness should
be checked from time to time using a UV
meter. They can conveniently be replaced
during redecoration of rooms. Under conditions
of very strong sunlight the filter may
degrade in an uneconomically short time.
Under these conditions, it may be possible to
arrange that all light reaching a sensitive object
has been reflected at least once from a wall
which has been coated with a white pigment
having a high absorption in the UV range.
Titanium dioxide and zinc oxide are both
suitable for this purpose.
Most fluorescent tubes are coated on the
inside with a material which absorbs some of
the UV and re-emits radiation of a longer
wavelength. However, most fluorescent lamps
still emit levels of UV that require filtration.
Quartz halogen lamps (e.g. tungsten iodide)
have a quartz envelope which is transparent
to UV and take advantage of higher operating
temperatures than ordinary tungsten lamps to
obtain more light per watt of electricity
consumed. The small but powerful amount of
short wave radiation emitted by these lamps
makes filtration a necessity.
Control of visible radiation All objects, unless
known to be insensitive to UV radiation,
should be protected in the ways outlined
above. In considering control of visible radiation,
it is necessary to find levels of illumination
which will be satisfactory for both
conservation and display purposes. A certain
amount of damage will be caused by the very
act of display and yet it will not be worthwhile
to exhibit an object at all unless an
adequate level of detail can be perceived by
the viewer. Just where this level of detail
should be fixed is a matter of judgement
taking into account the form and surface qualities
and susceptibility to light of different
objects. Therefore, when an object is not being
viewed there is no need to illuminate it.
Objects should be illuminated only when it is
necessary for display or study and the illumin-