Guideline 15.5: The lighting plan makes effective use of artificial lighting and hardware to achieve the light exposure recommended by the Conservation Requirements[edit | edit source]
What role does artificial lighting play in meeting exhibit Conservation Requirements for light exposure?[edit | edit source]
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?[edit | edit source]
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?[edit | edit source]
[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?[edit | edit source]
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?[edit | edit source]
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?[edit | edit source]
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?[edit | edit source]
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]