Propuesta del procedimiento de Gestión de Cambio para el SIGEP v2.0

The light received by objects on display may cause them to deteriorate. In addition to creating the visual conditions needed to study and enjoy the exhibits, the designer must consider conser-vation. This imposes a minor constraint with temporary exhibitions of low-sensitivity items, but is a matter of overriding importance with national collections in museums and galleries.

Lighting is not responsible for the most dramatic damage that occurs to museum exhibits, but there is a clear link between exposure to light and deterioration. This can be divided into two main areas: thermal and photochemical. Both occur primarily in organic materials.

Thermal damage is the result of local heating from absorption of radiation. This causes a loss of humidity, which in turn causes warping and splitting in moisture-absorbing materials such as wood and animal skins. The infrared content of a light source’s spectrum makes the greatest contribution to thermal damage, and this can be controlled by the use of dichroic reflectors and other methods.

Of more concern is photochemical damage, the visible effects of which include colour change and physical deterioration. It is irreversible, and cannot be corrected by conservation processes. Damage to light-sensitive exhibits cannot be totally avoided, but it can be minimized by taking protective measures. These are based on controlling the energy received by the exhibits, and this is related to three main factors: the spectral composition of the light source, the illuminance on the object, and the length of exposure.

The most damaging part of the spectrum is ultraviolet (UV) radiation. Because this plays no part in the visual experience (except on fluorescent materials), it should be eliminated from the gallery—by UV filters on windows and, where necessary, with UV filters on electric lighting. It is normally considered that, except for tungsten halogen sources, incandescent lamps do not emit sufficient UV to demand filtering.

Deterioration caused by light increases with light exposure. This is a product of the illuminance (lux) and the length of time of the exposure (hours). For example, if the illuminance is doubled, the time in which a given amount of damage occurs will be halved. This is known as the reciprocity law.

Materials vary in their sensitivity to light; internationally agreed illuminance recommendations to limit deterioration are given in Table 11.4. In good conservation practice these are associated with a policy of reducing periods of exposure, such as excluding light when a museum is closed, and of presenting the most delicate objects only in temporary exhibitions. Higher illuminances than given in the table can be adopted if the total light exposure is not greater than the annual lux-hours specified. This is important to the use of daylight in museums.

Table 11.4 Typical maximum illuminances and light exposures recommended for Metal, stone, glass, ceramics, enamel No limit, subject

only to thermal damage Oil and tempera painting, fresco, undyed leather,

horn, bone, ivory, wood, lacquer

In museums many exhibits are displayed in glass cases. This provides the opportunity to control the environment within the case and to physically protect the exhibits.

Wherever possible, the lighting equipment should be contained inside a separate compartment from the display. This allows the lighting equipment to be maintained without disturbing the exhibits, and to limit the effect on the display of heat from the lamps. Uniform illumination can be provided with concealed fluorescent lamps;

low-voltage spotlamps can highlight features; a combination of the two is often the most pleasing. Fibre optic lighting is ideal for display cases. With this, a relatively powerful lamp is positioned remotely, and the light is transmitted to the display area via bundles of glass fibre. At the end of each fibre bundle there are optics that can focus the light and direct it to where it is required. The light is both cool and UV free, but the efficiency of the system is low, so it is often useful only for modest light levels. One lamp can serve a number of fibre outlets, but colour distortion can occur if very long fibres are used.

Reflected glare can be a problem with display cases. External lights are reflected in the outer surfaces; sources inside the case are reflected by the inside faces. This, again, places a constraint on possible luminaire positions. In low-light conditions it may be impossible to eliminate unwanted reflections completely, but every attempt should be made to minimize them.

Many museums stage experience displays, which aim to provide more than the viewing of a static exhibition. They may, for example, illustrate an historical period by transporting the visitor through a series of exhibits. Often called ‘dark rides’, these employ both sound and light; and although the objects on display are often genuine they are presented in a theatrical setting. Colour and projected light patterns are commonly used. The lighting may be animated to create the effect of time changes (such as sunrise

Type of exhibit

110 The design of lighting

to sunset) or of a fire or of sunlight reflected from moving water. The approach to lighting is very much that of the theatre, but there is an important constraint: lighting equipment must be concealed so that the illusion of the experience is not destroyed.

Furthermore, several aspects of safety must be considered—both visually, as in sufficiently illuminating hazards, and technologically, such as avoiding the proximity of high-temperature luminaires and flammable materials.

Windows in a museum or gallery tend to be used for two separate purposes: to give people a link with the outside, and to provide illumination of good colour. The use of daylight must be considered at the earliest design stages, because it is dependent on the basic geometry of the building. If there are areas where very low illuminances are required, there must be transitional spaces for adaptation between outdoor levels and the gallery; if daylight is to give picture illumination, window locations are constrained by the need to avoid unwanted specular reflections.

Most old pictures were originally painted in daylight and intended for daylit spaces;

many exhibits in museums were originally outdoor objects. It is in relation to daylight that the opposing requirements of display and conservation become most clearly apparent: for their long-term preservation, many precious items cannot be continuously shown in a lit environment that is historically accurate. But clever design can give a good simulation of the appropriate lighting. In particular:

• Consider the adaptation level of the viewer. The sense of brightness in a space is primarily related not to the absolute level of luminance but to the level of brightness adaptation. A room with moderately low illumination seems bright to a person who has a spent a few minutes in a darker space.

• Consider object lighting separately from general room lighting. The illuminance range in the room as a whole can be far greater than that falling on the object, and small areas of high brightness on floor or ceiling can have an insignificant effect on either object illuminance or viewer adaptation.

• Ensure that the natural variation for daylighting is not lost. Significant change can be permitted in the ambient lighting while the display lighting remains steady.

• Use windows for low-brightness views. Vision out into courtyards or gardens can be provided with little effect on interior lighting. Windows are especially appropriate in circulation areas and cafés; they also provide a means of orientation for the visitor.

Daylight varies continuously in spectral balance, changing greatly in colour appearance while retaining good colour rendering. It is this variability of colour that cannot be matched by electric sources, and should be the first argument for its use for illuminating works of art. Daylight is expensive to provide if its intensity must be regulated, and a complex gallery ceiling system of photoelectrically controlled louvres and blinds often fails to give natural-appearing daylight because the dominant characteristic—brightness fluctuation—can be lost. A modern automatic system can allow interior daylight to vary and still maintain recommended exposure levels, but the more rigorous the constraints, the less natural the lighting appearance. It is a dilemma that the finest paintings, which reveal most under the changing colour and intensity of daylight, are those that must be most carefully preserved.

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