Although light is necessary to view exhibition objects, exposure to light causes irreversible damage to organic materials and some inorganic objects. Types of damage include aesthetic changes (such as fading or shifts in color), as well as breakdown in a material's microscopic structure that results in brittleness and weakness. Light is composed of radiation in three spectral ranges that can cause damage: visible light, ultraviolet radiation, and infrared radiation. Only visible light is necessary for effective viewing.
Damage from light is irreversible and cumulative; objects do not "recover" from light exposure. An understanding of the relationship between the amount (intensity) of light and the length of exposure is critical for responsible lighting decisions. Both light level and exposure time influence light damage.
Lighting is a conservation and aesthetic concern for the entire duration of an exhibition or loan. Changes in the wattage or color temperature of a bulb, or the aim of a lamp can alter the amount and quality of object illumination. Note that LED and fluorescent bulbs will degrade as they age, resulting in changes in light quality.
Develop a lighting plan that responds to the established conservation criteria. Produce the plan early in the process to allow enough time for coordination of the complex issues that determine final lighting choices and levels. Note that loan agreements typically specify light requirements or restrictions.
- Limit total light exposure. Best practices include limiting light exposure (duration x intensity) and removing/reducing ultraviolet (UV) radiation even when using newer forms of light emitting diode (LED) lighting.
- An excellent comprehensive reference, Museum Lighting: A Guide for Conservators and Curators (David Saunders, ISBN 978-1-60606-637-9, Getty Conservation Institute, 2020) can be found here: https://shop.getty.edu/products/museum-lighting-a-guide-for-conservators-and-curators-978-1606066379
- Provide separate lighting for security checks, exhibit cleaning and maintenance, object installation, and other routine work. Turn off lights during nonpublic hours to avoid exposing objects unnecessarily. When possible, use occupancy sensors in the room or at the case to turn lighting on only as necessary.
Evaluate collection items for vulnerability to light damage
- Generally accepted light limits include:
- 5 fc (54 lux) maximum for very light- sensitive materials such as textiles and papers
- 10 fc (100 lux) maximum for moderately sensitive materials such as unpainted or dyed organic objects and paintings with sensitive pigments or glazes
- 15 to 20 fc (150-200 lux) maximum for less sensitive objects such as stone, glass, ceramics, and metals. Note that light can accelerate chemical reactions such as tarnish or corrosion.
- Use a light damage calculator to consider multiple factors including intensity and exposure time.
Filter all sources of ultraviolet radiation. Use commercially available filters on all light sources to eliminate ultraviolet radiation or reduce it to 10 or less microwatts per lumen.
Minimize heat, including infrared radiation. Be aware that heat can come from many sources; consider types of bulbs, and any other electrical or electronic equipment that are near collections or enclosed in the case environment.
Control daylight. Filter daylight that is already present in the exhibit space for UV radiation and control its intensity.
Construct lighting mockups. Evaluate the amount and quality of light provided by the proposed lighting plan. Measure final light levels and adjust them accordingly during installation.
Monitor light levels. Continue to monitor light levels throughout an exhibit, especially for light sensitive objects to ensure light levels remain as expected and defined in the lighting plan.
Include the lighting plan in any operational maintenance manual or document. This plan can document instructions for achieving specified light levels and serve as a reference for future lighting decisions such as system replacement or exhibit duration.
Document cumulative light exposure for significant and sensitive objects in your collections database. This information will inform decisions on future display.
See: Exhibit Case Lighting
Object sensitivity helps determine the selection of an exhibit lighting system and the development of a lighting plan. To be successful, the lighting plan must take shape early in the design. The impact of lighting in the exhibit and adjacent spaces, including light from architectural features such as windows or skylights, must be factored into the plan. Lighting issues are complex, so the lighting plan should be developed in consultation with a conservator and (when possible) a lighting designer experienced in museum exhibits.
Technical information for the exhibit designer is available from lighting manufacturers, who generally supply reference material and may even provide individual technical advice on the use of their products. In some situations, a lighting designer may be consulted on specific issues or may be responsible for the entire lighting design.
Consider these key issues when designing lighting:
- Visitor experience: for example, acclimation to lower light levels is enhanced by moving gradually from more brightly lit areas to those with lower light illumination. Exhibit design that considers the light gradients in a room can also help reduce the need for higher light levels on display objects.
- Develop policies and procedures that lower the total light exposure of objects, by limiting the length of exhibit and lighting display areas only when necessary.
- Install an adaptable lighting system suitable for a variety of installations rather than just one exhibit. Whenever possible, separate circuits and controls for task-specific lighting; security checks, exhibit cleaning and maintenance, and object installation. These should be separate from controls to illuminate the objects, graphic panels, and other exhibit elements.
- Use room or case occupancy sensors that raise lighting levels only when visitors are present.
- Exhibit case dimension and lamp location impact the intensity of radiation falling on an object.
- Evaluate placement and potential effect of house lighting (for example, emergency and security lighting).
- Spend carefully and consider sustainability; limited budgets can be optimized to produce appropriate lighting conditions and energy savings yields cost savings.
Detailed, written documentation in the lighting plan, along with communication with the maintenance staff (in a maintenance manual) will help ensure that the bulbs of the same wattage, beam width, and color temperature are used for re-lamping. The angle of aim for each lamp fixture should also be recorded, and dimmers should be marked or fixed to prevent unintended adjustment.
Exhibit Design and Perception of Brightness
A gradual rather than an abrupt transition between differently lit areas is critical to making objects visible at low, restricted light levels. The human eye needs time to adjust from bright to low light; as visitors approach an exhibit area, the light levels to which they are exposed will influence their response to lower light levels inside the exhibit space. Note that using your design plans can help determine where light fixtures should be placed to achieve appropriate lighting.
Grouping light-sensitive objects away from exhibit entrances and points of bright light such as windows help acclimate the human eye. The color of a wall or case interior, the texture, and the intensity of hue can affect the visibility of objects on display. Avoid sharp contrasts, because any juxtaposition of light and dark will lower viewing comfort.
Effects of Color Temperature on Perception
The type of light influences the way an object's color is perceived. Some exhibits attempt to reproduce a specific color rendering; for example, an artist may have painted under a certain type of daylight or artificial light. In general, however, most exhibit lighting plans try to achieve a color rendering close to daylight. Industry uses the color rendering index (CRI) to rate the color produced by a light source. The higher the CRI, the more balanced the light color. Some lamps produce cool or blue light, while others produce a warmer or yellow light. Note that a new standard, the TM-30 https://www.energy.gov/eere/ssl/tm-30-frequently-asked-questions, is becoming more commonly used as a measure of color rendition.
The chromaticity values of lamps are rated according to color temperature (expressed in degrees Kelvin or °K):
- Lamps with a value of 3000°K and lower provide "warm" light that accentuates reds and yellows.
- Lamps of 4000°K or higher produce a "cool" tone that gives an overall blue-green cast.
- Lamps of around 3500°K are considered more moderate in tone and produce a white light.
Unlike other types of lamps, LED lighting can be tailored to achieve a specific color temperature. However, these lights should be monitored over time for color shifts. See CCI TB 36 https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/technical-bulletins/led-lighting-museums.html for more information.(hyperlink here)
In general, dimming lights can cause a shift in color rendering toward the warmer end of the spectrum. This can make the human eye perceive the amount of light as even dimmer than the measurable level would indicate. Instead, for a lower light level without color distortions, choose a lamp with a lower wattage but a high CRI value and a moderate chromaticity value.
Constructing a mockup of overall lighting conditions and design (including fixtures, lamps, ambient light, and anticipated distances between objects and lamps) minimizes problems during installation. Light levels can be measured, and the designer, lighting specialist, conservator, curator, and other exhibition team members can evaluate the overall lighting impacts. During this trial, measurements of UV and other types of radiation emitted from the lighting mockups will identify the need for filters or other corrections.
During object installation, light levels should be fine-tuned using a light meter. Final lighting adjustments often involve compromise between the conservation criteria and the public's ability to view the objects adequately. Note that measuring light levels during daily and seasonal shifts in galleries with daylight may be necessary to determine if the lighting plan accounts for exposure shifts.
Measuring light exposure
Total light exposure is a measure of light intensity multiplied by the duration of exposure. Light intensity is defined by the measurement of radiation falling on a surface in footcandles (fc) or lux using a light meter. Many institutions develop a “light budget” for sensitive objects to avoid damage incurred by prolonged or repeated exposure to light.
Footcandle, abbreviated as “fc,” is a unit of measure of the intensity of light falling on a surface, equal to one lumen per square foot and originally defined with reference to a standardized candle burning at one foot from a given surface.
Lux is a unit of illumination equal to the direct illumination on a surface, one meter from a uniform point source of one candle intensity or equal to one lumen per square meter. One lux is equivalent to 0.0929 foot-candle.
Total Exposure = fc (or lux) x hours
- Example a: 5 fc x 8 hrs = 40 fc/hr exposure
- Example b: 20 fc x 2 hrs = 40 fc/hr exposure
The following links provide guidance on how to measure light exposure:
Light meters contain a photosensitive cell that converts light energy to electrical energy. The value of this energy displays as a numeric value, in footcandles or lux, or on a scale. Some light meters also measure UV radiation using a UV-sensitive cell to convert UV-radiation to electrical energy.
Many light meters used in cultural heritage provide spot readings--that is they provide they measure light energy at a specific point in time. Some instruments measure light energy cumulatively. These light loggers use photo and UV-sensitive cells to measure the light and UV levels and repeat the readings with enough frequently to create a light and UV trend. Data is downloaded from the logger to a computer application for easy presentation and manipulation.
Dosimeters work on the principle that light will cause a perceptible amount of fading of organic material (usually dyes) over time. The blue wool scale is a long-standing dosimeter system described here: https://en.wikipedia.org/wiki/Blue_Wool_Scale. As an example of a museum application, a blue wool card is placed under exhibit lighting. After a prescribed exposure duration, the card is compared to the standard to evaluate the degree of change. Newer dosimeter technology continues to emerge such as the LightCheck dosimeter: https://hal.archives-ouvertes.fr/hal-01491213/document Regular monitoring (should blue wool discussion go here?) should be accomplished throughout the exhibition period, especially of light sensitive objects, to ensure no unexpected changes occur in lighting levels during the exhibit.
The three ranges of radiation that are of greatest concern for the preservation of cultural heritage include: Visible, ultraviolet (UV) and infrared (IR).
While general standards for illumination levels exist, the conservation criteria for an exhibit establish specific levels for the preservation of particular objects. CCI's Light Damage Calculator is useful for calculating light exposure. Tracking light levels and total cumulative exposure (lux hours) are critical in determining exhibit parameters (link to S. Michalski work…besides CCI light Damage Calculator?) Low illumination levels over a long period are equivalent to high levels over a short period. It is possible to illuminate a light-sensitive object at a higher level if the exposure is short and infrequent. There is less flexibility in light levels when an object is displayed for longer periods.
There are many ways to lower the amount of illumination reaching an object. These methods can be used alone or in combination to produce the appropriate amount and quality of light. To ensure that light levels within an exhibit match the conservation criteria, they must be monitored as the objects are being installed. Options for control of visible light include the following:
- inserting diffusers (grids, textured panels, and films) between the light source and the object to redirect and scatter the light
- using rheostatic control dimmers on individual lamps (as opposed to using a dimmer to control several lamps)
- replacing a lamp with a lower-wattage lamp, producing less light
- changing beam aim or width (diameter of the space that a lamp's light beam will illuminate)
- decreasing the number of lamps
- moving the lights farther from the object
Because ultraviolet (UV) radiation is so destructive to materials and because it is unnecessary for seeing the objects, standard museum practice is to eliminate it from lighting sources. UV radiation is a natural component of sunlight and is also produced by artificial lights, especially incandescent, fluorescent and halogen lamps. Note that some LED bulbs also release a small amount of UV radiation.
In the past, 75 microwatts of UV radiation per lumen of light was set as a conservation standard; this describes the proportion of UV radiation to total light exposure. This relative UV level focus on the characteristic of the light source. New products that prevent the passage of UV radiation through window glass, exhibit case and frame glazing, and fluorescent tubes allow the old conservation standard to be superseded. It is now possible to nearly eliminate UV radiation from any exhibition light source, making 10 microwatts per lumen a practical goal. To assess the amount of UV received on the object surface, the absolute UV measurement is needed. To prevent the effect of UV, it is best to keep the absolute UV level below 10 milliwatts per square meter (mW/m2) (CCI Note 2/2, 2015). When objects are displayed in the open, UV filters for individual light sources are needed. For encased objects, common methods include:
- Using UV-filtered glazing to construct cases
- Glazing frame packages with UV-filtering material
- Installing a UV-filtering layer between lights in a case's lighting chamber and the objects
A variety of filters for all lamp types are available to reduce both ultraviolet and infrared radiation. Some fluorescent tubes come coated with a plastic that eliminates UV transmission. Low-voltage bulbs are also available with an integral filter, or they can be filtered with a treated glass. Architectural glass can be filtered with a laminate film, or a special UV-filtering glass can be used instead.
The choice of lighting systems can be guided in part by the relative amounts of ultraviolet and infrared radiation produced by each type of light source. Further information on UV filters can be found in these following links:
- CCI Note N2/1, 2015. At: https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/ultraviolet-filters.html
- Boye, C., F. Preusser and T. Schaeffer. “UV-Blocking Window Films for Use in Museums–Revisited.” WAAC Newsletter, 32,1 (2010), pp. 13–18. At: https://cool.culturalheritage.org/waac/wn/wn32/wn32-1/wn32-104.pdf
Infrared (IR) radiation is the heat associated with sunlight and artificial lights. This heat can cause an exponential rise in the rate of object deterioration by speeding chemical reactions and drying out organic materials and causing color loss. IR radiation can be controlled by:
- Using special filtering window glass in architectural elements and case construction.
- Applying window films to architectural windows and skylights.
- Dissipating heat generated from exhibit lights to prevent buildup in confined spaces within, above, or next to exhibit cases or objects.
- Avoid locating a fixture too far from an object, or the conical beam of light will be spread too wide to provide effective illumination.
- When more than one light is used and the beams overlap, the amount of light reaching an object may be too high.
- Avoid shadows on objects and on the edges of framed pieces through careful location of fixtures and aiming of the beam.
- All lights should be filtered to remove UV radiation.
- Be aware that your light source can generate significant heat; this can be rectified by moving your objects further from the source. For example, locating objects at least 24 inches (60 cm) from fluorescent lights and at least 36 inches (90 cm) from incandescent or tungsten halogen lights can reduce heat.
- Although LED lamps themselves should not emit infrared radiation, the associated/electronics equipment can introduce heat in enclosed environments.
- Note that including audiovisual equipment in an enclosed environment can also produce heat.
Lighting continues to be a conservation concern even after the exhibit opens. Any change in the wattage or color temperature of a bulb or the aim of a lamp alters the amount and quality of light falling on the objects.
Detailed, written documentation in the lighting plan, along with communication with the maintenance staff (in a maintenance manual) will help ensure that the bulbs of the same wattage, beam width, and color temperature are used for relamping. The angle of aim for each lamp fixture should also be recorded, and dimmers should be marked or fixed to prevent casual readjustment.
Daylight is particularly dangerous because it includes high levels of visible, UV and IR. Daylight in an exhibit space provides light of variable quality and quantity, changing with the time of day, season, and weather conditions. Recently, new consideration has been given for the use of daylight as a sustainable means of lighting museums. Proper filtration of UV entering through windows or skylights must be considered in the lighting plan and light levels should be monitored regularly.
Direct daylight should never fall on light-sensitive objects. It may be necessary to block windows or skylights in the exhibit space or to redirect natural light entering the space from an atrium or adjacent area. Architectural renovation and exhibit design can eliminate, block, or control daylight with exhibit panels, light-filtering blinds, films, shades, curtains, or fabrics. Note that different types of window glass covers allow variable amounts of light egress. Window films, shades or curtains can potentially release unwanted volatile organic compounds into the exhibit space. For more information, see (link to materials testing pages).
Controlling artificial illumination is a complex subject. The type of lighting systems, the wattage and color temperature of the lamps, the location and aim of the fixtures, and the use of diffusing filters and dimmers affect the amount of light falling on any object in an exhibit. The exhibition designer has a wide range of lighting options, including surface-mounted track lighting, recessed lights, light emitting diode (LED) and fiber-optic systems. Incandescent, fluorescent, and halogen lighting were historically used, but all have risks due to wavelength emitted, heat, color temperature and intensity limitations. LED lighting has seen increased use in museums because it is fairly inexpensive to run, with low replacement requirements, reproducible performance, ability to select light level, availability of wide color temperature ranges, and low UV content. [From David Saunders 2015 IIC-ITCC presentation] Note that the selected LED lamps must be compatible with your current electronic system, because some components (such as dimmers) may not work.
Lamp locations directly affect the amount and quality of illumination falling on an object. Modifying the overall lighting plan, not increasing the amount of light, is the way to produce a desired effect. A few general precautions should govern decisions about location:
- Inserting diffusers (grids, textured panels, and films) between the light source and the object to redirect and scatter the light
- Using rheostatic control dimmers on individual lamps (as opposed to using a dimmer to control several lamps)
- Reduce the exposure by using occupancy sensors that activate lights by motion, or by visitor activated buttons.
- Replacing a lamp with a lower-wattage lamp, producing less light
- Changing beam aim or width (diameter of the space that a lamp's light beam will illuminate)
- Decreasing the number of lamps
- Moving the lights farther away from the object
- Note that fiberoptic and small LED heads may be placed closer; consult with a lighting team to determine what is safe.
Guideline: Mitigating Light Exposure
Effective conservation measures [design, controls and policies] must protect objects from harmful light exposure
OR: The exhibit design must provide exhibit objects the required level of protection from damaging light exposure =
Best Practice: The designer creates a lighting plan
What is an Exhibit Lighting Plan?
A lighting plan is a systematic approach to designing exhibit lighting to coordinate the diverse requirements of an exhibit. This plan is developed throughout the design process. In its final form, the plan should ensure that the exhibit lighting satisfies the exhibit’s interpretive and aesthetic concerns while also protecting exhibit objects from damaging light exposure. At the same time, the plan must work with budget requirements and accommodate the constraints of the particular exhibit space.
“Exhibit Lighting Plan” refers both to the document recording the plan’s details as well as to the process of designing the exhibit lighting. Once the plan is finalized, the document should provide a detailed description of the lighting design, including the light sources to be used, their location, and what tracks, fixtures, lamps, filters and other lighting control systems are to be employed. The plan should describe the lighting in the general exhibit as well as the lighting in exhibit cases. Detailed written instructions in this document will help ensure that the conservation elements of the design are correctly installed and maintained.
[Conservation considerations for light exposure are described in Guidelines **.]
What are the characteristics of an effective exhibit lighting plan?
When developing an exhibit lighting plan, ensure the following:
- The finalized plan fulfills the Conservation Requirements. The conservation requirements will have established the amount of light that objects can be exposed to without damage. The exhibit lighting system and lighting design should conform to these requirements.
- The plan is initiated early in the design process. The lighting plan must take shape early in the design to ensure sufficient time to meet the diverse requirements (aesthetic, conservation and budgetary) that should determine final lighting choices and levels.
- The plan is produced in conjunction with the conservator or a lighting designer. Lighting issues are complex, so the lighting plan ideally should be developed in consultation with the conservator. And because lighting technology is developing rapidly, consultation with a lighting designer would also be helpful. Look for one with experience in designing museum exhibitions and meeting conservation lighting requirements. Technical information for the exhibit designer is also available from lighting manufacturers, who generally supply reference material and may even provide individual technical advice on the use of their products.
- All light sources are factored into the overall light exposure. The impact of all light sources in the exhibit and adjacent spaces, including light from architectural features such as windows or skylights, must be factored into the plan.
- The plan is allowed to evolve during the exhibit design phase. Several lighting options may be proposed, and considerable deliberation will be necessary to arrive at a lighting scheme that adequately addresses all requirements.
What information should be included in the Exhibit Lighting Plan document?
The written plan should contain sufficient information to guide the fabricator.
The plan should allow the exhibit fabricator to procure and correctly install the lighting systems. Any change in the wattage or color temperature of a bulb or the aim of a lamp will alter the amount and quality of light falling on the objects.
Although the specific content of a lighting plan will vary with each exhibit, a plan should generally include the following information:
- Placement of light track and individual fixtures
- Lamp types with product manufacturer and number
- Exact location, aim, and beam characteristics of each lamp
- Visible light control methods such as dimmers, diffusers, and filters
- Measures to prevent UV radiation from light sources falling on the objects
- Mitigation techniques for natural light entering the exhibit space
- Projection of light levels falling on each object
- Sketches, elevations, or other forms of visual identification of lighting locations, beam angle, etc.
- Energy consumption projections
- Information on access to all lighting fixtures for installation and maintenance purposes
- Location of other relevant features including HVAC vents, fire suppression and detection equipment
- Identification of task lighting, including maintenance and emergency lights
- Electrical layout, including location of circuits and junction boxes
The written plan should contain sufficient information to serve as a maintenance document.
Information must be sufficiently detailed to ensure that exhibit lighting will be correctly maintained.
- Detailed documentation will help ensure that bulbs of the same wattage, beam width, and color temperature are used for re-lamping. The angle of aim for each lamp fixture should be recorded, and dimmers should be marked or fixed to prevent casual readjustment.
Best Practice: The exhibit design excludes sunlight
Why is it important to exclude sunlight from the exhibit space?
All light sources produce some amount of visible light, Ultraviolet (UV) radiation, and Infrared (IR) radiation, but sunlight is particularly dangerous because it includes high levels of all three. Direct sunlight should never fall on light-sensitive objects.
In addition, daylight in an exhibit space provides light of inconsistent quality and quantity, changing with the time of day, season, and weather conditions. The complete elimination or tight control of sunlight not only serves a preservation function, it is also easier on the viewer.
What methods can be used to exclude sunlight from the exhibit space?
The most dependable way to exclude sunlight is to block any windows or skylights in the exhibit space.
However, when daylight is necessary to create a desired effect (e.g. historical accuracy in a historic room) or when it is not possible to block out daylight, then mitigate or control its effects:
- Eliminate UV light by covering windows with glazing and films that contain UV filters. (For more information on eliminating UV light see Guideline*.)
- Use curtains, shades, shutters, and light-filtering blinds to block windows when sunlight is entering the exhibit space.
- Use exhibit panels to control sunlight
- Redirect natural light entering the space from an atrium or adjacent area [Needs further explanation]
- In those particular spaces, objects less susceptible to light damage should be chosen for exhibition.
Best Practice: The lighting plan eliminates ultraviolet light
Why should the lighting plan aim to eliminate ultraviolet light from the exhibit?
In general, ultraviolet (UV) radiation is defined as wavelengths from approximately 280 to 400 nanometers (nm). Sunlight is high in UV radiation; but it is also produced by artificial lights, especially fluorescent, halogen, metal halide and mercury vapor lamps. The high energy of UV radiation is very harmful to organic objects, particularly fiber and dyes, and can damage both their physical strength and pigmentation. Because UV radiation is so destructive to organic materials and because it does not aid the human eye in seeing objects, filtering UV is easy for both exhibit designers and conservators to agree on. It is standard museum practice to eliminate it from lighting sources.
What are the options for eliminating ultraviolet light from the exhibit?
Light fixtures with a low UV output, such as light-emitting diodes (LEDs). [For more information on artificial lighting, see Guideline 15.5.]
A variety of films, filters, glass, and plastics are available that prevent UV radiation from passing through a product. In recent years, it has become possible to eliminate almost all UV radiation from any exhibition light source, and ten microwatts per lumen has become a practical goal. There are two main approaches to eliminating UV Radiation:
Using products that prevent UV Radiation from passing through window glass and exhibit case and frame glazing
- Architectural glass filtered with a laminate film
- UV-filtering glass.
- UV-absorbent varnishes. These should only be applied by an experienced contractor as the varnishes are ineffective and aesthetically undesirable when applied improperly.
Using products that block UV Radiation emitted by an artificial light source
- A variety of filters for all lamp types to reduce both ultraviolet and infrared radiation.
- Fluorescent tubes coated with a plastic that eliminates UV transmission.
- Low-voltage bulbs with an integral filter, or filtered with a treated glass.
What materials can be used to protect objects in enclosures from UV Radiation?
Exhibit objects inside enclosures can be protected from UV radiation by:
- Constructing the enclosure with UV-filtered glazing to block UV radiation from passing into the enclosure
- Glazing the frame package with UV-filtering material to block UV radiation from passing into the enclosure
- Installing a layer of UV-filtering material between the lights in a case’s lighting chamber and the objects to block UV radiation coming from light sources within the case.
[For further discussion, see Case Standards *.]
How can objects on open display be protected from UV radiation?
For vulnerable objects on open display, a large-scale approach to mitigating the hazard of UV radiation must be used:
- If windows cannot be blocked, UV-filtering films or glazing should be used to prevent UV radiation from passing through.
- UV filters for all artificial light sources should be used.
- Films produced for theatrical productions and those used to control UV, visible, and IR radiation passing through architectural glass can be used to lower the amount of radiation in some exhibit situations. These films are placed between the light source and the viewing area.
What are the different options for filtering UV Radiation?
Use the following three charts to guide your selection of UV filtering products. The charts provide filtering options for:
- fluorescent lamps
- tungsten-halogen lamps
- architectural glass
Best Practice: The lighting plan controls Infrared Radiation
Why should infrared radiation be controlled?
Infrared radiation (IR) should be controlled because it does not contribute to visibility and yet can seriously interfere with efforts to regulate the museum environment.
IR has a wavelength just greater than the light at the red end of the visible light spectrum. Although human beings cannot see IR, we experience it as the main source of heat in sunlight and incandescent lights. Heat can be very damaging to objects. It can cause an exponential rise in the rate of object deterioration by speeding chemical reactions and drying out organic materials. It can also influence relative humidity and the moisture content of objects on display. Research also attributes up to 40% of color loss in dyes to IR radiation.
In fact, more than 90% of the energy from an incandescent lamp is heat, and every watt of light adds 4.15 BTUs (British Thermal Units) to the heat load of the building. The IR produced by lighting is therefore an important consideration in efforts to control temperature and relative humidity in an exhibit. It is also a budgeting consideration: some exhibitions incur air-conditioning expenses year-round as a result of the heat produced by poorly designed lighting systems.
What methods can be used to control Infrared Radiation?
- Utilize lighting systems that do not generate heat: LED lighting, compact fluorescents. [More information required]
- Dissipate heat generated from exhibit lights to prevent buildup in confined spaces within, above, or next to exhibit cases or objects. [How exactly?]
- Locate objects at distance from lights: position objects at least 24 inches from fluorescent lights and at least 36 inches from incandescent or tungsten halogen lights.
- Insulate light box in cases to prevent heat building up inside the case where objects are housed.
[For more information on cases see Standard *]
Best Practice: The lighting plan makes effective use of artificial lighting to achieve the light exposure recommended by the Conservation Requirements
What role does artificial lighting play in meeting exhibit Conservation Requirements for light exposure?
As described in the preceding guidelines, various filtering products and structural modifications can be used to limit or eliminate Ultraviolet radiation, Infrared radiation and sunlight from the exhibit. However, it is the selection of the lighting system and lighting controls—the types of bulbs, the wattage and color temperature of the lamps, location and aim of the fixtures, diffusing filters and dimmers—that provide the designer’s most precise tools for controlling light exposure. Through effective use of artificial lighting rather than resorting to increased light levels, the designer can achieve the desired level of visibility in the exhibit.
What amount of light exposure should exhibit lighting aim for?
If objects are to be on display, then they must be visible to the viewer. However, objects vary as to their susceptibility to light damage. Exhibit lighting should therefore not exceed the light levels required to safeguard the particular objects on display. Levels above the minimum amount necessary to adequately view an object causes unjustifiable damage. (Visible light levels are measured in lux (lumens per square meter) or footcandles (FC). One footcandle is slightly more than 10 lux.)
The Conservation Requirements will normally make the following general specifications:
- Very light-sensitive materials: A Maximum of 5 footcandles (54 lux*) for very light-sensitive materials such as textiles, fugitive dyes and most paper-based materials.
- Moderately sensitive materials: A maximum of 10 footcandles (108 lux) for moderately sensitive materials such as high quality paper with light stable inks and textiles with stable dyes.
- Less sensitive materials: A maximum of 15 to 20 footcandles (161-215 lux) for less sensitive objects such as oil and tempera paintings, bone, ivory, wood finishes, leather, some plastics.
- Least sensitive materials: Dependent upon exhibit location for least sensitive materials such as stone, glass, ceramics, and metals.
The exhibit should always limit the amount of light exposure to that established by the conservation requirements. The specific conservation requirements should always supersede general recommendations.
Exhibit lighting must accommodate the fact that damage from light exposure is cumulative. Light level and exposure time together determine the degree of damage. Therefore, low illumination levels over a long period are equivalent to high levels over a short period. For example, exposure to 5 footcandles for 8 hours will cause the same damage as exposure to 20 footcandles for 2 hours. It is possible, therefore, to illuminate a light-sensitive object at a higher level if the exposure is short and infrequent. There is less flexibility in light levels when an object is displayed for longer periods.
What are the pros and cons of the different lighting systems?
[Information on the pros and cons of the different lighting systems and technologies is needed here.]
The exhibition designer has a wide range of lighting options:
- surface-mounted track lighting
- recessed lights
- fiber-optic systems
How can the lighting system be manipulated to control for light exposure?
Strategies for reducing light include manipulating beam direction and using hardware such as dimmers and diffusers, as well as the simple act of using lamps of lower-wattage or decreasing the number of lamps:
- Lower-wattage lamps: Replacing a lamp with one of lower-wattage will produce less light.
- Decreasing the number of lamps will also produce less light.
- Placement of lights and beam direction: Exposure can be reduced by the simple action of moving the lights farther from the object. (For precautions to observe when positioning lamps and beams see below.)
- Dimmers can reduce the amount of light generated by a lamp by up to 50%. They can also be set to turn lights on and off automatically. Use of dimmers can thus reduce both the amount and the duration of light exposure that exhibit objects receive. Dimmers can also be used to raise light levels in a gallery in advance of visitor traffic and they are a particularly good choice in areas of relatively little activity or where visitation fluctuates seasonally. Domestic-type dimmers are sufficient for most situations, although commercial dimming controls will sometimes be required. Dimmers can be used to control:
- the light in an overall exhibit space
- light within that space that is falling on a particular object
- or light within an individual display case.
- Care must be taken to ensure that simply dimming the lights does not cause a shift in color rendering toward the warmer end of the light spectrum, which will further reduce visibility for the human eye. For more information on the color rendering index see: How can the Color Rendering Index be used to enhance exhibit visibility? In Guideline *
- Diffusers are rigid panels that are inserted between a light source and the viewing area to redirect light. They reduce the amount of light anywhere from 18% to 50% and also soften and even out the appearance of “hot spots” from lamps. Lighting diffusers are a good option for controlling the quantity and quality of light in exhibit cases and from overhead light sources. (For more information on diffusers see below: How can diffusers be used to control for light exposure?
- Filters block the total amount of light that passes through the material. Mini-louvered blinds, woven synthetic fabrics, and plastic and metal scrim materials have been used successfully in exhibits. Filters can be applied directly to case glazing or cut to fit into holders on individual lamps, or placed between the display and the lighting chambers. If located close to lamps, plastic materials and the coatings on metal scrims must be researched to ensure inflammability and that no outgassing or other problems will be caused by exposure to elevated heat.
- Occupancy sensors in the room or at the case turn lighting on and off during visitation hours.
What precautions should be taken in placement of lights and beam direction?
The location of lights affects the quality as well as the amount of illumination falling on an object. The following precautions should be taken:
- Place fluorescent lamps at least 24 inches from displayed objects.
- Place incandescent or tungsten halogen light at least 36 inches away from any object because of heat buildup and illumination levels.
- Avoid locating a fixture too far from an object, or the conical beam of light will be spread too wide to provide effective illumination.
- Ensure that beams from different lights do not overlap or the amount of light reaching an object may be too high.
- Locate fixtures and beam direction to avoid shadows on objects and on the edges of framed pieces that will interfere with visibility.
How can dimmers be used to control for light exposure?
When using dimmers to control light levels and duration, consider the following options:
- Whether to install dimmers on individual fixtures: Although dimmers are often installed at the control source for all the lights (the switch), it is more effective to install them on individual fixtures that will illuminate sensitive objects.
- Whether dimmers will be activated automatically: Dimmers can be set to provide continuous illumination at a pre-determined light level; or they can be programmed to increase light output in response to an activation signal such as a lowering in ambient light levels or a visitor request.
- Type of activation: Dimmers can be visitor-operated (the visitor pushes a button to increase light); ultrasonically controlled (visitor movement activates the dimmer); or triggered by body heat or sound.
- Location of activation sensors: Sensors that will trigger dimmers can be located in the ceiling, wall, or floor. In addition, a photoelectrically controlled beam can be aimed across a visitor pathway. Note that unintended signaling can occur if the sensor is poorly located, for example in the airflow from a HVAC duct.
- Degree of sensor sensitivity: The amount of motion (or other triggering mechanism) needed to activate the sensor needs to be calibrated with the distance a visitor is intended to get from an object before it is illuminated. Sensors of different sensitivity are available and a few can be set for individual applications.
- Area of sensor coverage: Sensors are available to cover both small and large areas.
- Timer settings: A timer can be set to either turn the lights off or lower light levels after a pre-determined amount of time or after a pre-determined length of inactivity in the space.
- Compatibility with lamps: Dimmers are compatible with all incandescent lamps; more care is needed when choosing a dimmer for a fluorescent lamp. Lamp life may be shortened by use of a dimmer.
- Whether to include an override: An override ability may be desirable for special situations.
How can diffusers be used to control for light exposure?
Diffusers can be installed as ceiling panels to affect overhead lighting or can be located in front of track lighting. There are two types of diffusers—louvers and textured panels. Product literature and manufacturer representatives can guide the decision on what type of panel is appropriate for a particular situation.
- Louvers: Louvers can block light from reaching sensitive objects located in close proximity. The simplest louvers, often referred to as “eggcrate” due to their appearance, are made up of a series of straight-sided squares of approximately a half-inch; parabolic louvers have more complex profiles to the grid; these angled or curved surfaces are often coated with a reflecting material. Light is directed as it passes through the louver, and the total amount of light reaching the viewing surface can be decreased with proper selection. Microlouver panels utilize fine embedded louvers to control light scattering. Louvers are available in acrylic, polystyrene and (or) polycarbonate plastic, and aluminum. Any of these materials are safe for use near collection objects.
- Textured or tinted panels: A variety of textured and tinted plastic and glass panels is commercially available. The textured and semi-opaque qualities of the panels cut the total amount of light transmission by up to 50%. Panels are typically pebbled, frosted or hazed, or constructed with internal ribbing similar to a corrugation. Textured panels can be purchased in colored, transparent plastic, and tinted or clear glass. The tinted products can lower the light transmission even more than the clear textured products. However, the quality of light may be affected by the tinted products. Textured panels are available in acrylic, polystyrene and (or) polycarbonate plastic and glass. Any of these materials are safe for use near collection objects.
[Link to “NPS Guidelines”: 4:3 Lighting Control options: Dimmers, Films, Filters]
[Link to “NPS Guidelines”: 4:4 Lighting Control Options: Diffusing Panels]
Best Practice: Design strategies are used to enhance visibility without increasing light exposure
What design strategies can enhance exhibit visibility?
In addition to utilizing controls and hardware to reduce, redirect and block light, the designer can also employ design strategies to enhance visibility in the exhibit space and help the visitor adjust to lower light levels:
- Use a gradual rather than an abrupt transition between differently lit areas to make objects visible at low, restricted light levels. Separate bright public access areas from display areas, and provide adaptation paths between the two. The human eye needs time to adjust from bright to low light. As visitors approach an exhibit area, the light levels to which they are exposed will influence their response to lower light levels inside the exhibit space.
- Group light-sensitive objects away from exhibit entrances and points of bright light such as windows.
- Use colors to enhance object visibility: The color of a wall or case interior, its texture, and its intensity of hue can affect the visibility of objects on display. Avoid sharp contrasts, because any juxtaposition of light and dark will lower viewing comfort.
- Use lamps of the appropriate Color Rendering Index (CRI) value: Choosing a lamp with the appropriate CRI allows lower wattage without lowering visitor comfort. This is because the type of light influences a viewer’s color perception of an object. In general, most exhibit lighting plans try to achieve a color rendering close to daylight.
- Provide separate lighting for Non-exhibit activities: Create separate circuits and controls for task-specific lighting. Controls for security checks, exhibit cleaning and maintenance, and object installation should be separate from controls to illuminate the objects, graphic panels, and other exhibit elements.
How can the Color Rendering Index be used to enhance exhibit visibility?
Industry uses the Color Rendering Index (CRI) to rate a lamp’s ability to accurately represent colors. The reference point is outdoor daylight with a CRI of 100. The closer to 100 the CRI is, the more balanced the light color. Some lamps produce cool or blue light, while others produce a warmer or yellow light.
In general, simply dimming the lights can be a critical design mistake. This usually causes a shift in the color rendering toward the warmer end of the light spectrum, making the human eye perceive the amount of light as even dimmer than the measurable level would indicate.
To provide a lower light level without color distortions, choose a lamp with a lower wattage but a high CRI value and a moderate chromaticity value. The chromaticity values of lamps are rated according to color temperature (Expressed in degrees Kelvin or °K):
- Lamps of around 3500°K are considered more moderate in tone and produce a white light. They are usually desirable for museum lighting purposes.
- Lamps with a value of 3000°K and lower provide “warm” light that accentuates reds and yellows.
- Lamps of 4000°K or higher produce a “cool” tone that gives an overall blue-green cast.
Best Practice: Lighting mock-ups are used to check the light exposure resulting from a proposed lighting plan
Why are lighting mock-ups an important step in fulfilling the Conservation Requirements for lighting?
A mockup of the proposed lighting, especially case lighting, allows a scheme to be assessed and adjusted before final installation. The lighting plan for exhibits often involves a complex combination of light fixtures and controls: lamps of a specific wattage and beam angle, particular lamp placements in relation to exhibit objects, lights modified by filters or diffusers. An important consideration is the interaction between all these different elements of the plan.
Constructing a simple mockup of the proposed lighting design—including the fixtures, the various lamps, and the anticipated distances between the objects and lamps—allows light levels to be measured and the amount of light falling on objects to be assessed. Interactions between different elements of the plan—such as overlapping beams from two different lamps—can also be evaluated. And the amount of radiation emitted during the trial will indicate the need for UV filters. The designer, lighting specialist, conservator, curator, and other exhibition team members can also evaluate the quality as well as the quantity of lighting.
How are lighting mock-ups constructed?
[Brief instructions needed or relevant link]
Best Practice: Architectural modifications are utilized to aid the control of inappropriate light exposure
What policies can protect objects from light exposure?
Ensuring maintenance staff are trained to use service lights only as needed. Limiting the amount of time the most vulnerable collections are displayed. Adequate budget and training to ensure correct relamping in exhibits. Blocking exterior windows.