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 Tech Notes
See: Exhibit Lighting Design
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.