Series editors' preface - Wood Tools

through which daylight is highly diffused so that
all exhibits are illuminated at the agreed level.
Light can be admitted through blinds which are
automatically controlled by a photocell. As
daylight fails, artificial light is introduced under
dimmer control also operated by photocells.
Although this sounds an attractive proposition,
the constancy of illumination and its degree of
diffusion again results in light which is very like
that from an overcast sky in both colour temperature
and directional quality. There is also an
increased probability of failure in such complex
systems. As an alternative to the above, which
aim at a steady illumination of 150 lux, the
equivalent annual exposure can be worked out
and an average exposure value calculated based
on meteorological data. A level of control can
be set which allows light levels to go over the
150 lux limit during the summer months but
averages out to around 150 lux over the whole
year (i.e. the same equivalent dose measured in
lux-hours). If automatic blinds are used they can
be adjusted hourly on a monthly schedule.
Lighting can thus retain some connection with
the weather outside and some of the essential
features of daylight are retained.
Reducing time of exposure Another important
aspect of the control of lighting is reduction of
the time of exposure. For very sensitive objects,
including some upholstered furniture, an excellent
way to reduce exposure is for objects to
be put on display for a limited period only
before being replaced by another, possibly
similar, object. This is commonly known as
rotation. The period of rotation can be varied
to suit the circumstances. It might be, for
example, one year in ten or it could be one
month per year. Obviously it can more easily
be achieved with a large collection, or where
there is only a small space available for display.
In general, illumination, apart from that
required for security purposes, can be restricted
to institutional opening hours. Exposure can be
further restricted by means of curtains or covers
around or over a display and by the use of time
switches so that very sensitive objects are only
illuminated when they are actually being
viewed. Objects in conservation workshops and
in stores should be similarly protected when
they are not being worked on. Black plastic
sheeting in the workshop or Tyvek covers in
stores may be used for this purpose. Some
General review of environment and deterioration 251
items can be quite conveniently arranged on
racking systems so that they can be pulled out
for observation and then pushed back into dark
conditions when this has finished. The use of
replicas is another possibility.
Artificial lighting Artificial lighting has to be
used both for supplementing daylight and as
a substitute for it, during the winter months,
where a source of natural light is not available
and at the lower level of 50 lux where use of
daylight is not really feasible. Artificial light
offers many advantages in flexibility and ease
of control. A range of lights of different colour
temperatures is available with tungsten and
tungsten halogen lamps in the ‘warm’ category
– around 3000–4000 °K – and fluorescent
tubes in the ‘cool’ category – up to 6500 °K.
A warm light may give good effect to reds and
golds, especially under low levels of illumination
while a cooler light may look more effective
with blues. Once the decision is made, a
lamp of good colour rendering must be
chosen. Although a tungsten lamp can look
yellow in direct comparison to daylight when
the two are in close proximity (say on a white
wall), if the whole area is illuminated by
tungsten it will appear white. Within each
category of colour temperature, lamps are
available which the eye will still read as white
provided that there are no very large deficiencies
of any wavelength in their spectra.
Fluorescent lamps, however, can present more
problems in this respect. Fluorescent tubes
have a greater luminous efficiency than
tungsten and are well suited for general
illumination as they come in a wide range of
colour temperatures and sizes and can be very
conveniently used with various types of diffusing
panels in ceilings and so on. They require
a greater bulk of control gear than tungsten
lights and some of this tends to get quite hot
in operation so that siting of it has to be
carefully considered. Tungsten is more suitable
for spot lights that provide accent lighting.
Lighting and heating
One aspect of lighting which should be remembered
is that most lamps, and of course the sun,
produce not only UV and visible radiation but
also infra red radiation and heat. These long
wavelength radiations in fact comprise by far
the greatest part of the output of tungsten and