Transcript INTRODUCTION • Light is a common cause of damage to collections.

INTRODUCTION
• Light is a common cause of damage to collections. Paper, bindings, and media
(inks, photographic emulsions, dyes, and pigments, and many other materials
used to create words and images) are particularly sensitive to light. Light
damage manifests itself in many ways. Light can cause paper to bleach, yellow,
or darken, and it can weaken and embrittle the cellulose fibres that make up
paper. It can cause media and dyes used in documents, photographs, and art
works to fade or change colour. Most of us recognize fading as a form of light
damage, but this is only a superficial indication of deterioration that extends to
the physical and chemical structure of collections. Light provides energy to fuel
the chemical reactions that produce deterioration. While most people know that
ultraviolet (UV) light is destructive, it is important to remember that all light
causes damage. Light damage is cumulative and irreversible.
What is light
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THE NATURE OF LIGHT
Light is a form of electromagnetic energy called radiation. The radiation that we
know from medicine and nuclear science is energy at wavelengths far shorter
than the light spectrum; radio waves are much longer wavelengths. Visible light,
the form of radiation that we can see, falls near the centre of the
electromagnetic spectrum.
The visible spectrum runs from about 400 nanometers (nm, the measurement
applied to radiation) to about 700 nm. Ultraviolet wavelengths lie just below the
short end of the visible spectrum (below 400 nm). The wavelengths of infrared
light lie just above the long end but our eyes cannot see them. This type of light
also damages collections.
The Chemistry of Light
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Light energy is absorbed by molecules within an object. This absorption of light
energy can start many possible sequences of chemical reactions, all of which
damage objects. The general term for this process is photochemical
deterioration. Each molecule in an object requires a minimum amount of energy
to begin a chemical reaction with other molecules. This is called its activation
energy. Different types of molecules have different activation energies.
If the light energy from natural or artificial light equals or exceeds the activation
energy of a particular molecule, the molecule is “excited,” or made available for
chemical reactions. Once this happens, the molecule may behave in a variety of
ways. The excess energy may show up as heat or light; the energy may break
bonds within the molecule (this will create smaller molecules and weaken the
paper); the energy may cause a rearrangement of atoms within the molecule; or
the energy may be transferred to another molecule. One of the primary
photochemical reactions is oxidation, in which the “excited” molecule transfers
its energy to an oxygen molecule, which then reacts with other molecules to
initiate damaging chemical reactions. While the sequence of events can be
extremely complex, the end result is always deterioration.
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Shorter wavelengths of light (UV light) have a greater frequency (that is, they
occur closer together) as well as more energy than longer wavelengths. This
means that they bombard an object with more energy in a shorter time, and that
their energy is likely to meet or exceed the required activation energy for many
different types of molecules. Thus they cause photochemical deterioration to
happen more quickly, and they are extremely damaging. As wavelengths
become longer, toward the red end of the spectrum, they have less energy, less
frequency, and reduced capacity to “excite” molecules.
It is important to remember, however, that even longer wavelengths of light
damage paper and other materials. The energy absorbed from infrared light
raises an object's temperature. This in turn increases the speed of damaging
chemical reactions already occurring within the paper.
Visible Light versus UV
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ULTRAVIOLET LIGHT VS. VISIBLE LIGHT
Since UV radiation is the most energetic and destructive form of light, we might
assume that if UV light is eliminated, visible light is of minimal concern. This is
not true; all wavelengths of light do significant damage.
In practical terms, UV light can be easily eliminated from exhibit, reading, and
storage areas, since our eyes do not perceive it and will not miss it. Visible light
is far more problematic, but it should be eliminated from storage areas as much
as possible and carefully controlled in other areas.
Light Sources
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SOURCES OF LIGHT
Light has two sources: natural and artificial. Libraries and archives should avoid
natural light. Sunlight has a high percentage of ultraviolet. Daylight is also
brighter and more intense, and therefore causes more damage, than most
artificial light.
The two primary artificial light sources currently in use in cultural institutions are
incandescent and fluorescent lamps. (The term "lamp" is used by architects
and engineers to refer to the various types of light bulbs, rather than to the
fixtures containing the bulbs.) Driven by the need for energy conservation and
cost savings, manufacturers continue to refine lamp technologies to produce
longer-lived lamps that consume less energy and provide better light. Compact
fluorescent, tungsten-halogen, high intensity discharge (HID), and electrodeless
lamps have all been developed in response to these concerns.
Incandescent
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Conventional incandescent lamps produce light when an electric current is
passed through a tungsten filament, heating it to about 2700 degrees Celsius.
Incandescent lamps convert only a small percentage of this electricity into light;
the rest becomes heat. Conventional incandescent lamps emit very little
ultraviolet light and do not require UV filtering. Examples of conventional
incandescent lamps include the ordinary household light bulb and a variety of
lamps used for exhibition lighting, such as the Reflectorized (R), Ellipsoidal
Reflectorized (ER), and Parabolic Aluminized Reflector (PAR) lamps.
Tungsten
• Tungsten-halogen lamps (also called quartz
lamps) are a variation on the traditional
incandescent lamp; they contain halogen gas
inside a quartz bulb, which allows the light to
burn brighter and longer. These lamps emit
significant UV light and do require filtering.
Filters can be expensive and special
housings designed to accept the UV filters
may be necessary. Tungsten-halogen lamps
are also used in exhibition lighting; examples
include the Halogen PAR and the MirroredReflector (MR) lamp.
Fluorescent
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Fluorescent lamps contain mercury vapour inside a glass lamp whose inside
surface is painted with white fluorescent powder. When electricity is passed
through the lamp (via a filament), the mercury vapour emits UV radiation which
is absorbed by the fluorescent powder and re-emitted as visible light. Some UV
light passes through most fluorescent lamps, however, so they are more
damaging than incandescent lamps. The newest type of fluorescent is the
compact fluorescent lamp; these are smaller, last longer, and have a more
pleasant colour than traditional fluorescents, and they can usually be used in
incandescent sockets. These lamps must still be filtered, however.
High Intensity Discharge
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Like fluorescents, high intensity discharge (HID) lamps contain a vapour
inside a glass lamp coated with a fluorescent powder, but they are much more
intense than normal fluorescents. There are two types. Mercury or metal halide
HID lamps should not be used, since they have a dangerously strong UV output
and filtering can be difficult. High-pressure sodium HID lamps are too intense for
direct lighting (and do not provide good colour rendering), but they can be used
for indirect lighting (i.e., bouncing light off the ceiling) in large storage spaces
with high ceilings. Sodium HID lamps have very low UV emissions, which can be
further reduced by painting the ceiling with white titanium dioxide paint, a UVabsorber. Sodium HID lamps generate little heat, are efficient, and have low
operating costs.1
Fibre Optic
• Fibre optic lighting is an energy-efficient
means of providing display lighting,
particularly in exhibition cases. In a fibre optic
system, light is transmitted from a light source
through glass or acrylic fibres. The fibres do
not conduct infrared or ultraviolet light, and
unlike fluorescent lamps, fibre optic lighting
does not cause build up of heat within the
case (provided the light source is mounted
outside the case).
Electrodeless lamp
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The electrodeless lamp is the newest type of light source. A normal
incandescent lamp is subject to the eventual failure ("burn out") of its electrode,
which is a piece of metal (usually tungsten) that is heated until it produces light.
Electrodeless lamps produce light in other ways, including the use of radio
frequencies to excite a coil or microwave energy directed at the element sulphur
to produce visible light. Electrodeless lamps produce a lot of illumination, so thus
far they have only been used as sources of ambient light (the light produced by
one electrodeless sulphur lamp equals more than 250 standard 100 watt
incandescent lamps). They are more energy efficient than HID lamps, and they
provide excellent colour rendition, low infrared and ultraviolet light, and long life.
It is expected that this technology will eventually be miniaturized for use in
smaller exhibit spaces and in exhibit cases.2
Control of UV
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HOW MUCH LIGHT IS TOO MUCH?
Do we have to eliminate all UV light? Since all visible light cannot be eliminated,
particularly in exhibition areas, how low should the levels be?
Control of ultraviolet light is relatively straightforward. The standard limit for UV
for preservation is 75 µW/l (see below). Any light source with a higher UV
emission must be filtered. Control of visible light is obviously more problematic.
It is essential to understand that light damage is cumulative, and that lower
levels of illumination will mean less damage over the long term. Another
important concept in controlling visible light is the law of reciprocity. This says
that limited exposure to a high-intensity light will produce the same amount of
damage as long exposure to a low-intensity light. For example, exposure to 100
lux for 5 hours would cause the same amount of damage as exposure to 50 lux
for 10 hours.
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For many years, generally accepted recommendations in the preservation
community have limited visible light levels for light-sensitive materials (including
paper) to 55 lux (5 foot candles) or less and for less sensitive materials to 165
lux (15 foot candles) or less. In recent years, however, there has been some
debate about these recommendations. Some have argued the importance of
aesthetic concerns: older visitors need more light to see exhibited objects well,
and any visitor will find that more fine detail is apparent and colours appear
brighter as light levels increase. In addition, the assumption that all paper
objects are equally sensitive to light has been challenged.3 Scientists at the
Canadian Conservation Institute (CCI) and others have begun to gather data on
rates of light fading for specific media and colours in an effort to begin
developing more specific guidelines based on the International Standards
Organization (ISO) Blue Wool light fading standards (see "Practical Tips for
Estimating Light Damage," below).
Guidelines
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In the absence of universal guidelines, it is recommended that each institution establish its
own limits on exhibition for its collections. Factors to consider include: the amount of time
the lights are turned on in the exhibit space (this may be more than first thought, since lights
are often turned on for housekeeping or other purposes when the exhibit is closed to the
public); the sensitivity of the items or groups of items being exhibited; the desired lifespan of
these items or groups of items; and the importance of aesthetic concerns in exhibition.
Ultimately, every institution should decide on an acceptable upper limit of exposure (i.e., a
certain number of lux hours per year), which may differ for different parts of an institution's
collection. Publications by CCI and the exhibition policy developed by the Montreal Museum
of Fine Arts for works of art on paper may be helpful in estimating the sensitivity of various
types of paper-based collections.4
Using the law of reciprocity, an exhibition limit can be achieved in different ways; for
example, a limit of 50,000 lux hours per year could be achieved by keeping the lights on for
10 hours per day, either at 100 lux for 50 days or at 50 lux for 100 days. It is important to
remember that even with such guidelines, some fading will occur. The goal is to achieve a
workable compromise between exhibition and preservation.
Using a camera
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HOW DO YOU MEASURE LIGHT LEVELS?
Visible light levels are measured in lux ("lumens per square meter") or foot-candles. One foot-candle equals
about 11 lux. A light meter measures the level of visible light. The meter should be placed at the spot where
you want to take a reading (for example, close to the surface of an object being exhibited). The meter
should face the light just as the object does in order for it to get an accurate reading.
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If you do not have access to a light meter, you can measure the approximate lux level using a 35 mm
single-lens reflex camera with a built-in light meter, using the following procedure.

Place a sheet of white board measuring 30 cm x 40 cm at the position where the light level is to be
measured and at the same angle as the object.
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Set the camera ASA/ISO rating at 800. Set the shutter speed at 1/60 second.
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Aim the camera at the white board and position it just close enough so that the field of view is filled by
the board. Be sure not to cast a shadow on the board.
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Adjust the aperture until the light meter indicates a correct exposure, and note the aperture setting.
The approximate level of light in lux at the white board relates to the aperture setting as follows:
F4
represents 50 lx
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F5.6
represents 100 lx
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F8
represents 200 lx
F11
represents 400 lx
F16
represents 800 lx5
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Light and UV meters
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A light meter measures only the level of illumination; a UV meter must be used
to measure the UV component of light. UV light is measured in microwatts per
lumen (SYMBOL -µW/ l). The most common UV meter is the Crawford monitor,
but all UV meters will measure the proportion of ultraviolet in visible light. Again,
this should not exceed 75 µW/l.
A word of caution regarding UV meters: some older UV meters (costing from
$500 to $1500) may not be adequately sensitive to UV light; they may indicate
that levels are safe when in reality they are not. Newer more expensive ($3000
to $5000) meters are designed to measure UV levels more accurately.6
Blue Wool Standard
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PRACTICAL TIPS FOR ESTIMATING LIGHT DAMAGE
It is possible to estimate the damage that might result to an object from
particular intensities of light and lengths of exposure. This can be done using the
ISO’s Blue Wool standards cards, available from TALAS, and the light-damage
slide rule, available from the Canadian Conservation Institute (CCI).
The Blue Wool standards can clearly demonstrate the destructive powers of
light. These cards provide a standard against which subsequent fading can be
judged, and therefore can be used to convince sceptics that light really is a
problem. Each Blue Wool standard contains eight samples of blue-dyed wool.
Sample 1 is extremely light sensitive, while sample 8 is the most stable dye
available (although not permanent). Sample 2 takes twice as long to fade as
sample 1, sample 3 takes twice as long as sample 2, and so forth.
CCI light damage slide rule
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To demonstrate the degree of fading caused by the intensity of light in a
particular location, cover half of the card with a light-blocking material to protect
it completely from light damage. Write the date on the card, and set it out in the
desired location. Check the card periodically (every couple of weeks) to
determine how long it takes for the various samples to fade. Since the sensitivity
of the first few samples on the card corresponds to light sensitive materials such
as paper and textiles, the results will give you a general idea of the amount of
damage you might expect if materials were exhibited for the same period of time
at the current light level in that location.
CCI's light-damage slide rule is a sliding plastic scale that aligns projected light
types, light levels, and exposure times to predict the fading of a blue wool card
under these conditions. For example, it shows that an object displayed at 150
lux for 100 years will fade at the same rate as an object displayed at 5000 lux for
3 years. The above-mentioned exposure of 150 lux for 100 years would cause
significant fading of Blue Wool standard 4 and below. The slide rule also
compares damage that would be caused by UV-filtered and unfiltered light. In
the above case, standards 4 and below are noticeably more faded when
exposed to unfiltered light.
The
Figure 2. Schematic Blue Wool Standard
• The tools described above can be useful in
demonstrating the effect your lighting choices
will have on exhibited materials. In most
cases a general correlation between the
sensitivity of the object and the Blue Wool
standard's scale will be sufficient to allow
informed decision-making. If more detail is
needed, the publications from CCI and the
Montreal Museum of Fine Arts may be
helpful.
Controlling UV
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CONROLLING ULTRAVIOLET LIGHT
UV light can be filtered by passing the light through a material that is transparent
to visible light but opaque to ultraviolet. The ideal filter would prevent all
wavelengths of UV below 400 nm from passing through, but this is difficult to
achieve. There are many products available that do the job adequately. In
setting priorities, it is usually important to deal with natural light first, and then
fluorescent light.
Ultraviolet-filtering plastic is available to cover windows and skylights. It must
cover the surface completely so that all light passes through it. This plastic is
available either in self-supporting sheets of acrylic or in thin film (usually acetate)
that is cut to shape with a knife or scissors and adhered to the glass. The acrylic
panels can be used in place of window glass (if fire regulations allow), mounted
as secondary glazing on existing windows, or hung inside the window from
hooks (the panel must be cut larger than the window glass, so that all light
passes through it). Tinted panels are also available, to reduce overall light.
Varnishes
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Varnishes that absorb ultraviolet light are also available. A supplier applies these
coatings on window glass with a special tool. Currently, varnish is not
recommended; it is very difficult to apply uniformly, and it deteriorates over time.
Plastic is more convenient, lasts longer, and does the job better.
UV filters are normally needed on fluorescent lamps. Filters are available in the
form of soft, thin plastic sleeves and hard plastic tubes. The tubes are generally
several times more expensive, and do not provide any more protection than thin
sleeves. If hard tubes do not fit the lamp exactly, unfiltered light can slip by at
uncovered ends. The thin plastic sleeves should also be properly sized for the
lamp. If necessary, two sleeves can be overlapped to extend the length of a
single sleeve. Whatever type of filter is used, maintenance staff must be trained
to transfer the filter when they change lamps.
UV Sleeves
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If the fluorescent lights are housed in recesses that are completely covered by a
plastic shield, however, UV light levels should be tested before an institution
spends money on UV-filtering sleeves. Experience has shown that these plastic
shields often provide UV filtering, reducing UV levels to 10-20 µW/l.
Some fluorescent lamps produce significantly less UV than others. To insure
maximum protection, one suggestion is to use lamps that produce relatively low
UV in combination with UV filters. This will further reduce UV levels, reduce
damage caused by improper installation or failure to replace filters, and extend
the lives of the filters themselves.7 Some manufacturers now make fluorescent
lamps with UV-filtering glass, but these can be much more expensive than
standard lamps. Replacements must be kept on hand, and care must be taken
not to replace a custom UV-filtering lamp with an ordinary one.
Titanium Dioxide
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Another option available for protecting
against UV light is the use of white paint
containing titanium dioxide. While this
method is not as effective, it will cut
down on UV light significantly. Titanium
dioxide paint absorbs ultraviolet light,
and can be painted directly on windows
or skylights, if they do not provide the
only source of light.
How long to UV filters last?
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HOW LONG DO UV FILTERS LAST?
At this time there is no definitive data to indicate how long UV filtering products
retain their effectiveness. In a CCI Note published in 1984, the Canadian
Conservation Institute reported that both soft plastic filtering sleeves and hard
plastic filtering tubes retain their UV-absorbing properties for at least 10 years.
UV-filtering window films may also have limited life-spans; some manufacturers
quote a life of 5-15 years for these films.8 In climates with intense sunlight
these filters may not last as long.
The only conclusive way to determine the continued effectiveness of UV-filtering
products is to measure the UV levels emitted using a UV monitor (see cautions
on UV monitor accuracy given above). Since these monitors are expensive,
smaller institutions should make arrangements to borrow one every few years
from a nearby large museum or other institution.
Controlling Visible Light
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CONTROLLING VISIBLE LIGHT
It would be ideal to keep collections sheltered from all light, but this is clearly impractical.
Even collections stored away from light must sometimes be used. Often, in fact, storage and
research areas cannot be separated. Materials must be exhibited, particularly in a museum
setting. A difficult balance must be maintained between the desire to protect materials and
the need to make them accessible. Any reduction of visible light reduces long-term damage.
Storage areas that are not routinely occupied by staff or researchers should be kept dark;
they should be windowless, or the windows should be blocked. Lights should be turned off in
such areas except when needed. This can be done with timers, but at the very least staff
should be trained to turn off the lights when the space is unoccupied. Occupancy sensors
can also be installed that turn off lights when no movement is sensed in the area. Lighting
should be incandescent (tungsten) rather than fluorescent wherever possible.
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object out of the light, keep the light from reaching the object. Boxes from
archival suppliers made by professional box-makers to fit the exact dimensions
of individual objects are useful. While boxes will prevent damage from direct
light exposure, it is uncertain whether they will protect objects from the
fluctuations in temperature and humidity that may be caused by solar heating.
The specifics of determining guidelines for exhibition lighting of objects have
been discussed above. All windows in exhibit areas should be covered with
drapes, shades, or blinds, in addition to being filtered for UV. Skylights should be
covered to block the sun. Light levels should be low, and materials should never
be exposed to direct sunlight. Never display objects permanently unless they are
expendable.
Fragile
Exceptionally fragile and vulnerable objects should not be displayed, and research use
should be limited. If materials must be exhibited, great care must be taken to minimize
damage. Books that are opened for display should have the pages turned weekly so that
one page is not constantly exposed. Photographic and photocopy facsimiles of objects
should be used whenever possible for display and research.
Spotlights should never be trained directly on an object. Indirect and low lighting will spare
the object, and it will also require less adjustment of the eye from areas of intense light to
those of relative darkness, allowing the use of lamps with a lower wattage throughout exhibit
spaces. A gradual diminution of light levels through a series of rooms may accustom
viewers' eyes to lower exhibition light levels. Strategic placement of labels explaining the
reason for low light levels can be used to educate patrons.
In Conclusion
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All light contributes to the deterioration of library and archival collections by
providing energy to fuel destructive chemical reactions within the paper. Light
also damages bindings, photographic emulsions, and other media, including the
inks, dyes, and pigments used in many library and archival materials. Institutions
should follow the guidelines given above for measurement of light levels and
control of light exposure. All sources of ultraviolet light illuminating collections
should be filtered, and the exposure of collections to visible light should be
strictly controlled.
With grateful thanks to the NEDCC where much of this information came from.