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Lighting Source Locations
There are two methods of lighting exhibit cases: external lighting from the room's ceiling (or walls) and lighting that is integral to the case. Ceiling lighting is preferred from a preservation perspective because electrical fixtures and heat generating lamps are kept at a distance from the display objects.
Ceiling lighting, including recessed and surface mounted lights, is generally installed as angled lighting, and as down lighting. The primary disadvantage of angled lighting are glare, reflection and shadows from visitors. The 30 degree angle shown below is suggested for optimal lighting.
Down lighting avoids glare but shadows from objects can be problematic. Case ceilings can be fabricated to include protective diffuser panels or films. Advantages of integral lighting include:
- light source concealment;
- object by object adjustment of illumination levels;
- flexibility to customize lighting for changing content.
Lighting Duration and Intensity Controls
To minimize exposure of light sensitive objects to case lighting, reduce the duration and intensity of exposure. There are five major control methods available: manual switching, manual dimming, pre-set switching, pre-set dimming and use of motion sensors.
- Manual switching- involves manual activation of lights by visitors or staff: on-off switching of either entire circuits or individual fixtures. Inexpensive remote switches can be added to cases and operated by staff from a central desk.
- Manual dimming- involves the adjustment of light levels for a single case or for an entire exhibit area. Consider installing fixtures with individual dimmers so that individual lamps can be turned down- dimmer levels should be fixed and made tamper-proof.
- Pre-set dimming- involves more sophisticated, programmable dimmers. Specific lighting levels are consistently obtained and gradual ramping of light levels is possible. Systems are usuallyemployed with occupancy or automatic sensors.
- Pre-set programming- involves the use of a simple time clock or more complex, electronic timing device to automatically turn on lighting according to a predetermined schedule.
- Occupancy (motion) sensors- involve sensors that determine exhibit visitor movement and switch lighting on-off accordingly. They are linked to manual or pre-set dimmers and usually rely on ultrasonic or passive infrared technology.
Integral Lighting Access
Lighting compartments must be isolated from object display chambers: no air exchange between the chambers should occur. Lighting fixture adjust- ment and lamp maintenance should be accomplished through an independent access door which has its own security features.
The same key should not permit access to the display chamber and objects. Doors should be designed to provide sufficient space to install and manipulate fixtures and lamps-large enough for a technician to reach all fixtures. Special attention should be given to the support for larger or heavy doors in order that they be held safely open during maintenance procedures. Hinged doors should open "up" rather than "down". All lighting compartments require ventilation; those located under or to the side of the display chamber require insulation to reduce heat transfer.
Lighting Compartment Heating
A significant disadvantage of integral case lighting (besides the risk of electrical fire) is over-heating of the display chamber by poorly designed lighting compartments. Adequate ventilation and insulation of lighting compartments are critical to prevent heat transfer. Fan cooling designs should also include convection cooling for periods of mechanical failure. To ensure adequate heat dissipation consult a mechanical engineer to determine the BTU's produced, the size of the vent area and air flow requirements. Key factors include, lamp wattage, compartment size and room temperature.
Light Modifying Devices
Light modifying devices can easily be introduced into light compartments between the light source and the objects. Applications includes layeri materials and directional screens. The selection of light modifying devices should be matched to the lighting system being used. Lighting compartment dimensions must be large enough to accommodate the devices, and to prevent over heating of flammable materials.
Damaging radiation can be efficiently minimized by filters which control both the intensity of light reaching the objects and its UV and IR components. Glass or synthetic (plastic) filter systems can be used to eliminate UV radiation below 400 nm: both soft, thin plastic film or rigid plastic sheet is available. Lighting diffusers, louvers and baffles can be used to direct or diffuse light; reduce hot spots and glare; and hide fixtures, insects, and debris.
Currently, low voltage (quartz halogen) incandescent lighting is the most common light source for exhibit cases. Both traditional incandescent and quartz halogen lamps produce a desirable 2800° to 3000°K color temperature and have good color rendering. A great range of fixtures, lamps and beam varieties are available, however, they have several disadvantages:
- lamps are often too bright at the close distances;
- lamps produce more heat than light.
Design incandescent lighting systems to avoid excessive visible light levels and overheating:
- whenever possible, use low voltage systems with low wattage lamps (ie. 20-35 watts);
- use the correct beam width for the application;
- adequately vent the lighting compartment;
- keep at least 4 feet between lamps and objects;
- set lamps as far from transparent plastic panels as possible within the lighting compartment;
- use adjustable fixtures that accept filters.
Although fluorescent tubes have frequently been used for case lighting they have 2 important draw- backs: the light is difficult to direct and lighting levels are difficult to control.
Objects in the upper regions of cases are overex- posed while limited light reaches objects at lower levels. Lamps which are centrally located do not illuminate the fronts of displayed objects and shadows are difficult to control. Other associated problems are:
- designs frequently call for too many fixtures, and lower illumination levels cannot be obtained;
- heat from the fluorescent fixtures and ballasts heat objects in the display chamber, and destabilize the interior climate unless well vented;
- most tube lamps emit unacceptable levels of UV.
If conventional or compact lamps are used the lighting compartment should be carefully designed and tested. Design options include:
- selecting lower UV lamps and introducing UV filters;
- venting the compartment adequately and locating the ballast outside of the compartment to reduce heat
- using blockout masks, louvers and baffles to control light.
Fiber Optic Illuminator Locations
Fiber optic illuminators generally consist of an incandescent lamp and a focusing lens in a metal box. The illuminator housing relies either on natural convection cooling or forced air cooling. The location of this equipment necessitates careful planning because illuminators require:
- electrical power (usually 110 volts);
- efficient cooling/ventilation systems;
- proximity to the lens (less than 30 feet);
- a clear path for optical cables to the lenses;
- physical access for maintenance.
When enclosed within the lighting compartment caution needs to be exercised to avoid heating of the display chamber. Adequate air flow is required to dissipate heat that is generated. Insulation will be required for illuminator locations under the display deck.
Fiber Optic Lighting Options
The benefit of using a fiber optic system is not only the contol of UV and IR, but the light source can be located remotely and the light distribution locations can be hidden. A wide range of lenses(luminares) can also be used. Lenses vary in quality, beam spread and beam distance. Eyeball (swivel) luminares offer more flexibility than fixed down lights; mirrored lenses augment the angle of illumination.
Fibers, made of either glass or acrylic, can be fed into exhibit structures to allow lighting from the ceiling, walls, case corners or floor. Light can be focused onto objects with accuracy; from a fine pin point to a wash of light, offering a high degree of illumination control. Illumination levels can be controlled by:
- choice of lens;
- the use of neutral density lens filters;
- size and length of cable;
- lamp wattage;
- light reducing screens at the lamp.