The following is copyright 1999, Northeast
Document Conservation Center. All rights reserved.
Protection from Light Damage
Section 2, Leaflet 4
PROTECTION FROM LIGHT DAMAGE
by Beth Lindblom Patkus
Light is a common cause of damage to library and archival collections. Paper,
and media (inks, photographic emulsions, dyes, and pigments, and many other
to create words and images) are particularly sensitive to light. Light damage
itself in many ways. Light can cause paper to bleach, yellow, or darken, and it
and embrittle the cellulose fibers that make up paper. It can cause media and
dyes used in
documents, photographs, and art works to fade or change color. Most of us
as a form of light damage, but this is only a superficial indication of
extends to the physical and chemical structure of collections.
Light provides energy to fuel the chemical reactions that produce deterioration.
most people know ultraviolet (UV) light is destructive, it is important to
all light causes damage. Light damage is cumulative and irreversible.
THE NATURE OF LIGHT
Light is a form of electromagnetic energy called radiation. The radiation that
from medicine and nuclear science is energy at wavelengths far shorter than the
spectrum; radio waves are much longer wavelengths. Visible light, the form of
that we can see, falls near the center of the electromagnetic spectrum.
The visible spectrum runs from about 400 nanometers (nm, the measurement applied
radiation) to about 700 nm. Ultraviolet wavelengths lie just below the short end
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.
HOW DOES LIGHT DO ITS DAMAGE?
Light energy is absorbed by molecules within an object. This absorption of light
can start many possible sequences of chemical reactions, all of which damage
general term for this process is photochemical deterioration. Each molecule in
requires a minimum amount of energy to begin a chemical reaction with other
This is called its activation energy. Different types of molecules have
If the light energy contributed by natural or artificial light equals or exceeds
activation energy of a particular molecule, the molecule is "excited," or made
for chemical reactions. Once this happens, the molecule may behave in a variety
The excess energy may show up as heat or light; the energy may break bonds
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
molecule. One of the primary photochemical reactions is oxidation, in which the
molecule transfers its energy to an oxygen molecule, which then reacts with
to initiate damaging chemical reactions. While the sequence of events can be
complex, the end result is always deterioration.
Shorter wavelengths of light (UV light) have a greater frequency (that is, they
closer together) as well as more energy than longer wavelengths. This means that
bombard an object with more energy in a shorter time, and that their energy is
meet or exceed the required activation energy for many different types of
they cause photochemical deterioration to happen more quickly, and they are
damaging. As wavelengths become longer, toward the red end of the spectrum, they
energy, less frequency, and reduced capacity to "excite" molecules.
It is important to remember, however, that even longer wavelengths of light
and other materials. The energy absorbed from infrared light raises an object's
temperature. This in turn increases the speed of damaging chemical reactions
occurring within the paper.
ULTRAVIOLET LIGHT VS. VISIBLE LIGHT
Since UV radiation is the most energetic and destructive form of light, we might
that if UV light is eliminated, visible light is of minimal concern. This is not
and it is believed that all wavelengths of light do significant damage.
In practical terms, UV light can be easily eliminated from exhibit, reading, and
areas, since our eyes do not perceive it and will not miss it. Visible light is
problematic, but it should be eliminated from storage areas as much as possible
carefully controlled in other areas.
SOURCES OF LIGHT
Light has two sources: natural and artificial. Libraries and archives should
light. Sunlight has a high percentage of ultraviolet. Daylight is also brighter
intense, and therefore causes more damage, than most artificial light.
The two primary artificial light sources currently in use in libraries, museums,
archives are incandescent and fluorescent lamps. (The term "lamp" is used by
and engineers to refer to the various types of light bulbs, rather than to the
containing the bulbs.) Driven by the need for energy conservation and cost
manufacturers continue to refine lamp technologies to produce longer-lived lamps
consume less energy and provide better light. Compact fluorescent,
intensity discharge (HID), and electrodeless lamps have all been developed in
Conventional incandescent lamps produce light when an electric current is passed
tungsten filament, heating it to about 2700 degrees Celsius. Incandescent lamps
only a small percentage of this electricity into light; the rest becomes heat.
Conventional incandescent lamps emit very little ultraviolet light and do not
filtering. Examples of conventional incandescent lamps include the ordinary
light bulb and a variety of lamps used for exhibition lighting, such as the
(R), Ellipsoidal Reflectorized (ER), and Parabolic Aluminized Reflector (PAR)
Tungsten-halogen lamps (also called quartz lamps) are a variation on the
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
Filters can be expensive and special housings designed to accept the UV filters
necessary. Tungsten-halogen lamps are also used in exhibition lighting, and
include the Halogen PAR and the Mirrored-Reflector (MR) lamp.
Fluorescent lamps contain mercury vapor inside a glass lamp whose inside surface
painted with white fluorescent powder. When electricity is passed through the
lamp (via a
filament), the mercury vapor emits UV radiation which is absorbed by the
powder and re-emitted as visible light. Some UV light passes through most
lamps, however, so they are more damaging than incandescent lamps. The newest
fluorescent is the compact fluorescent lamp; these are smaller, last longer, and
more pleasant color than traditional fluorescents, and they can usually be used
incandescent sockets. These lamps must still be filtered, however.
Like fluorescents, high intensity discharge (HID) lamps contain a vapor inside a
lamp coated with a fluorescent powder, but they are much more intense than
fluorescents. There are two types. Mercury or metal halide HID lamps should not
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
good color 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
UV emissions, which can be further reduced by painting the ceiling with white
dioxide paint, a UV-absorber. Sodium HID lamps generate little heat, are very
and have low operating costs.1
Fiber optic lighting is an energy-efficient means of providing display lighting,
particularly in exhibition cases. In a fiber optic system, light is transmitted
light source through glass or acrylic fibers. The fibers do not conduct infrared
ultraviolet light, and unlike fluourescent lamps, fiber optic lighting does not
buildup of heat within the case (provided the light source is mounted outside
The electrodeless lamp is the newest type of light source. A normal incandescent
subject to the eventual failure ("burn out") of its electrode, which is a piece
(usually tungsten) that is heated until it produces light. Electrodeless lamps
light in other ways, including the use of radio frequencies to excite a coil or
energy directed at the element sulfur to produce visible light. Electrodeless
produce a lot of illumination, so thus far they have only been used as sources
light (the light produced by one electrodeless sulfur lamp equals more than 250
100 watt incandescent lamps). They are more energy efficient than HID lamps, and
provide excellent color rendition, low infrared and ultraviolet light, and long
is expected that this technology will eventually be miniaturized for use in
spaces and in exhibit cases.2
HOW MUCH LIGHT IS TOO MUCH?
Do we have to eliminate all UV light? Since all visible light cannot be
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 śW/l. Any light source with a higher UV emission must be
Control of visible light is obviously more problematic. It is essential to
light damage is cumulative, and that lower levels of illumination will mean less
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
same amount of damage as long exposure to a low-intensity light. For example,
100 lux for 5 hours would cause the same amount of damage as exposure to 50 lux
For many years, generally-accepted recommendations in the preservation community
limited visible light levels for light-sensitive materials (including paper) to
55 lux (5
footcandles) or less and for less sensitive materials to 165 lux (15
footcandles) or less.
In recent years, however, there has been some debate about these
have argued the importance of aesthetic concerns: older visitors need more light
exhibited objects well, and any visitor will find that more fine detail is
colors appear brighter as light levels increase. In addition, the assumption
paper objects are equally sensitive to light has been challenged.3 Scientists at
Canadian Conservation Institute (CCI) and others have begun to gather data on
light fading for specific media and colors in an effort to begin developing more
guidelines based on the International Standards Organization (ISO) Blue Wool
fading standards (see "Practical Tips for Estimating Light Damage", below).
In the absence of universal guidelines, it is recommended that each institution
its own limits on exhibition for its collections. Factors to consider include:
of time the lights are turned on in the exhibit space (this may be more than
since lights are often turned on for housekeeping or other purposes when the
closed to the public); the sensitivity of the items or groups of items being
desired lifespan of these items or groups of items; and the importance of
concerns in exhibition. Ultimately, every institution should decide on an
limit of exposure (i.e., a certain number of lux hours per year), which may
different parts of an institution's collection. Publications by CCI and the
policy developed by the Montreal Museum of Fine Arts for works of art on paper
helpful in estimating the sensitivity of various types of paper-based
Using the law of reciprocity, an exhibition limit can be achieved in different
example, a limit of 50,000 lux hours per year could be achieved by keeping the
for 10 hours per day, either at 100 lux for 50 days or at 50 lux for 100 days.
important to remember that even with such guidelines, some fading will occur.
The goal is
to achieve a workable compromise between exhibition and preservation.
HOW DO YOU MEASURE LIGHT LEVELS?
Visible light levels are measured in lux ("lumens per square meter") or
footcandle equals about 11 lux. A light meter measures the level of visible
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
object does in order to get an accurate reading.
If you do not have access to a light meter, you can measure the approximate lux
using a 35mm single-lens reflex camera with a built-in light meter, using the
- Place a sheet of white board measuring 30 cm x 40 cm at the position
light level is to be measured and at the same angle as the artifacts.
- Set the camera ASA/ISO rating at 800. Set the shutter speed at 1/60
- Aim the camera at the white board and position it just close enough so
field of view is filled by the board. Be sure not to cast a shadow on the
- 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
relates to the aperture setting as follows:
F4 represents 50 lx
F5.6 represents 100 lx
F8 represents 200 lx
F11 represents 400 lx
F16 represents 800 lx5
A light meter measures only the level of illumination; a UV meter must be used
the UV component of light. UV light is measured in microwatts per lumen (SYMBOL
śW/l). The most common UV meter is the Crawford monitor, but all UV meters will
the proportion of ultraviolet in visible light. Again, this should not exceed 75
A word of caution regarding UV meters: some older UV meters (ranging from $500
in cost) may not be adequately sensitive to UV light; they may indicate that
safe when in reality they are not. Newer, more expensive ($3000 to $5000) meters
designed to measure UV levels more accurately.6
It is possible to estimate the damage that might result to an artifact from
intensities of light and lengths of exposure. This can be done using the ISO's
standards cards, available from TALAS, and the light-damage slide rule,
the Canadian Conservation Institute (CCI).
The Blue Wool standards can clearly demonstrate the destructive powers of light.
cards provide a standard against which subsequent fading can be judged, and
be used to convince skeptics that light really is a problem.
Each Blue Wool standard contains eight samples of blue-dyed wool. Sample 1 is
light sensitive, while sample 8 is the most stable dye available (although not
Sample 2 takes twice as long to fade as sample 1, sample 3 takes twice as long
2, and so forth.
To demonstrate the degree of fading caused by the intensity of light in a
location, cover half of the card with a light-blocking material to protect it
from light damage. Write the date on the card, and set it out in the desired
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
corresponds to light sensitive materials such as paper and textiles, the results
you a general idea of the amount of damage you might expect if materials were
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
types, light levels, and exposure times to predict the fading of a blue wool
these conditions. For example, it shows that an artifact displayed at 150 lux
years will fade at the same rate as an artifact displayed at 5000 lux for 3
above-mentioned exposure of 150 lux for 100 years would cause significant fading
Wool standard 4 and below. The slide rule also compares damage that would be
UV-filtered and unfiltered light. In the above case, standards 4 and below are
more faded when exposed to unfiltered light.
The tools described above can be useful in demonstrating the effect your
will have on exhibited materials. In most cases a general correlation between
sensitivity of the artifact and the Blue Wool standard's scale will be
sufficient to allow
informed decision-making. If more detail is needed, the publications from CCI
Montreal Museum of Fine Arts cited above may be helpful.
CONTROLLING ULTRAVIOLET LIGHT
UV light can be filtered by passing the light through a material that is
visible light but opaque to ultraviolet. The ideal filter would prevent all
UV below 400 nm from passing through, but this is difficult to achieve. There
products available that do the job adequately. In setting priorities, it is
important to deal with natural light first, and then fluorescent light.
Ultraviolet-filtering plastic is available to cover windows and skylights. It
the surface completely so that all light passes through it. This plastic is
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
existing windows, or hung inside the window from hooks (the panel must be cut
the window glass, so that all light passes through it). Tinted panels are also
to reduce overall light.
Varnishes that absorb ultraviolet light are also available. A supplier applies
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
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
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
sleeves should also be properly sized for the lamp. If necessary, two sleeves
overlapped to extend the length of a single sleeve. Whatever type of filter is
maintenance staff must be trained to transfer the filter when they change lamps.
If the fluorescent lights are housed in recesses that are completely covered by
shield, however, UV light levels should be tested before an institution spends
UV-filtering sleeves. Experience has shown that these plastic shields often
filtering, reducing UV levels to 10-20 śW/l.
Some fluorescent lamps produce significantly less UV than others. To insure
protection, one suggestion is to use lamps that produce relatively low UV in
with UV filters. This will further reduce UV levels, reduce damage caused by
installation or failure to replace filters, and extend the lives of the filters
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
be taken not to replace a custom UV-filtering lamp with an ordinary one.
Another option available for protecting against UV light is the use of white
containing titanium dioxide. While this method is not as effective, it will cut
UV light significantly. Titanium dioxide paint absorbs ultraviolet light, and
painted directly on windows or skylights, if they do not provide the only source
HOW LONG DO UV FILTERS LAST?
At this time there is no definitive data to indicate how long UV filtering
retain their effectiveness. In a CCI Note published in 1984, the Canadian
Institute reported that both soft plastic filtering sleeves and hard plastic
tubes retain their UV-absorbing properties for at least 10 years. UV-filtering
films may also have limited life-spans; some manufacturers quote a life of 5-15
these films.8 In climates with intense sunlight these filters may not last as
The only conclusive way to determine the continued effectiveness of UV-filtering
is to measure the UV levels emitted using a UV monitor (see cautions on UV
accuracy given above). Since these monitors are very expensive, smaller
should make arrangements to borrow one every few years from a nearby large
museum or other institution.
CONTROLLING VISIBLE LIGHT
It would be ideal to keep collections sheltered from all light, but this is
impractical. Even collections stored away from light must sometimes be used.
fact, storage and research areas cannot be separated. Materials must be
particularly in a museum setting. A difficult balance must be maintained between
desire to protect materials and the need to make them accessible. Any reduction
light reduces long-term damage.
Storage areas that are not routinely occupied by staff or researchers should be
they should be windowless, or the windows should be blocked. Lights should be
in such areas except when needed. This can be done with timers, but at the very
staff should be trained to turn off the lights when the space is unoccupied.
sensors can also be installed that turn off lights when no movement is sensed in
Lighting should be incandescent (tungsten) rather than fluorescent wherever
Many situations are not ideal and space is often at a premium. If you cannot
object out of the light, keep the light from reaching the object. Boxes from
suppliers made by professional box-makers to fit the exact dimensions of
are useful. While boxes will prevent damage from direct light exposure, it is
whether they will protect objects from the fluctuations in temperature and
may be caused by solar heating.
The specifics of determining guidelines for exhibition lighting of objects have
discussed above. All windows in exhibit areas should be covered with drapes,
blinds, in addition to being filtered for UV. Skylights should be covered to
sun. Light levels should be low, and materials should never be exposed to direct
Never display objects permanently unless they are expendable.
Very fragile and vulnerable objects should not be displayed, and research use
limited. If materials must be exhibited, great care must be taken to minimize
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
whenever possible for both display and research.
Spotlights should never be trained directly on an object. Indirect and low
spare the object, and it will also require less adjustment of the eye from areas
light to those of relative darkness, allowing the use of lamps with a lower
throughout exhibit spaces. A gradual diminution of light levels through a series
may accustom viewers' eyes to lower exhibition light levels. Strategic placement
explaining the reason for low light levels can be used to educate patrons.
All light contributes to the deterioration of library and archival collections
providing energy to fuel destructive chemical reactions within the paper. Light
damages bindings, photographic emulsions, and other media, including the inks,
pigments used in many library and archival materials. Institutions should follow
guidelines given above for measurement of light levels and control of light
sources of ultraviolet light illuminating collections should be filtered, and
of collections to visible light should be strictly controlled.
1 William P. Lull, with the assistance of Paul N. Banks, Conservation
Environment Guidelines for
Libraries and Archives (Ottawa, ON: Canadian Council of Archives, 1995), pp.
2 See Florentine, Frank. "The Next Generation of Lights:
Electrodeless," in WAAC Newsletter 17:3
(September 1995), for more information and details on the use of electrodeless
lamps at the Smithsonian
Institution's Air and Space Museum.
3 Michalski, Stefan. "Towards Specific Lighting Guidelines," in
proceedings of "Museum Exhibit Lighting -
Beyond Edison: Lighting for the next Century," a workshop presented by the
National Park Service and the
Washington Conservation Guild, March 6-8, 1996.
4 See Michalski, Stefan, "Towards Specific Lighting Guidelines"; "A
Light Damage Slide Rule," CCI
Notes 2/6, Canadian Conservation Institute, Ottawa (1989); and Colby, Karen M.
Exhibition/Exposure Policy for Works of Art on Paper" (available at The Lighting
5 Taken from: "Using a Camera to Measure Light Levels," CCI Notes
2/5, Ottawa: Canadian Conservation
6 Lull, p. 19.
7 Lull, p. 44.
8 Abbey Newsletter 16.7-8 (December 1992): 114.
SUGGESTED FURTHER READING
Anson, Gordon. "The Light Solution." Museum News (September/October 1993): 27.
Canadian Conservation Institute. A Light Damage Slide Rule. CCI Note No. 2/6.
Ottawa: Canadian Conservation Institute, December 1988, 10 pp.
Canadian Conservation Institute. Ultraviolet Filters for Fluorescent Lamps. CCI
No. 2/1. Ottawa: Canadian Conservation Institute, June 1983, 1 p.
Canadian Conservation Institute. Using a Camera to Measure Light Levels. CCI
No. 2/5. Ottawa: Canadian Conservation Institute, 1992, 1 p.
Colby, Karen M. "A Suggested Exhibition/Exposure Policy for Works of Art on
July 1993. Available at The Lighting Resource web site:
Feller, Robert L. The Deteriorating Effect of Light on Museum Objects. Museum
Technical Supplement No. 3. Washington, DC: American Association of Museums,
1964, 8 pp.
Florentine, Frank. "The Next Generation of Lights: Electrodeless," in WAAC
17:3 (September 1995).
Lull, William P, with the assistance of Paul N. Banks. Conservation Environment
Guidelines for Libraries and Archives. Ottawa, ON: Canadian Council of Archives,
Museum Exhibit Lighting, An Interdisciplinary Approach: Conservation, Design,
Technology. proceedings of a workshop presented by the National Park Service and
American Institute for Conservation at the 1997 AIC Annual Meeting. Available
AIC, 1717 K St., NW, Suite 301, Washington, DC 20006; 202-452-9545.
Nicholson, Catherine. "What Exhibits Can Do to Your Collection." Restaurator 13
Thomson, Garry. The Museum Environment. 2nd edition. London and Boston:
association with The International Institute for Conservation of Historic and
Works, 1986, 308 pp.
SOURCES OF SUPPLIES
This list is not exhaustive, nor does it constitute an endorsement of the
listed. We suggest that you obtain information from a number of vendors so that
make comparisons of cost and assess the full range of available products.
A more complete list of suppliers is available from NEDCC. Consult the Technical
Leaflets section of NEDCC's website at www.nedcc.org or contact NEDCC for the most
up-to-date version in print.
3M Product Information Center
3M Center, Building 304-1-01
St. Paul, MN 55144-1000 U.S.A.
Toll Free: (800) 3M HELPS or (800) 364-3577
Telephone: (651) 737-6501
Fax: (800) 713-6329 or (651) 737-7117
UV-filtering window film (call for local distributor)
Canadian Conservation Institute (CCI)
1030 Innes Road
Ottawa, Ontario K1A OC8 CANADA
Telephone: (613) 998-3721
Fax: (613) 998-4721
Light-damage slide rule, CCI Notes and Technical Bulletins
7425 North Oak Park Avenue
Niles, IL 60714-9930
Telephone: (800) 323-4340
Light meters - regular
The Cooke Corporation
600 Main Street
Tonawanda, NY 14150
Telephone: (716) 833-8274
Fax: (716) 836-2927
Light meters - regular
P. O. Box 4901
Syracuse, NY 13221-4901
Toll Free: (800) 448-6160
Toll Free: (800) 428-3631 (Help Line) Toll Free Fax: (800) 272-3412
UV filters, light meter - regular, light damage, slide rule
439 Monroe Avenue
P.O. Box 940
Rochester, NY 14603-0940
Toll Free: (800) 828-6216
Telephone: (716) 271-8960
Fax: (800) 828-5539
Littlemore Scientific Engineering Co. - ELSEC
Phone: 01865 747437
Fax: 01865 7477E-mail: email@example.com
Rohm and Haas
Independence Mall West
Philadelphia, PA 19105
Telephone: (215) 592-3000
Fax: (215) 592-3377
53-11 105th Street
Corona, NY 11368
Telephone: (718) 592-8222
Fax: (718) 271-0891
New York, NY 10012
Telephone: (212) 736-7744
Fax: (212) 219-0735
Blue Wool standard cards
Thermoplastic Processes, Inc.
1268 Valley Road
Stirling, NJ 07980
Fax: (800) 874-3291
Arm-a-Lite UV-filtering tubes (hard plastic)
517 Main Street
P. O. Box 101
Holyoke, MA 01041
Toll Free: (800) 628-1912
Telephone: (413) 532-3372
Toll Free Fax: (800) 532-9281
Fax: (413) 432-9281
UV filters, light meters - regular and UV
Copyright 1999, Northeast Document Conservation
Center. All rights reserved.
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Reviewed December 20, 2003