Description

1. Requirements

Planning the lighting design for a museum requires that a huge number of factors be taken into account, some of which are mutually contradictory. Not only should exhibits be visible, but they also need to be protected against light. The architectural aspects of general lighting, spatial orientation, and view of the outside are indispensable components of lighting design for museums.

1.1 Visibility

Visibility requires a minimum level of illumination, good contrast without shadows, good colour rendering, and avoidance of glare. The requirements vary enormously depending on the type of exhibit. The three-dimensional microstructure of the surface of a two-dimensional painting needs lighting conditions quite different from those required by a three-dimensional object such as a sculpture that needs to be viewed from all sides. Objects made of light or video installations must be shielded from light that would distract the observer, while large objects such as monuments or excavation sites should be shown in as natural a light as possible, without, however, being exposed to damaging weather conditions. There is no magic formula for these different museum lighting tasks, so that the requirements always have to be determined in accordance with the type of exhibit. Lighting should, moreover, be variable to a certain extent so that exhibitions of different kinds of objects can be presented.

1.2 Protection of objects

Protecting objects is often inconsistent with lighting them to achieve good visibility, which increases with light intensity, for light energy damages objects through absorption of radiation. Signs of aging appear, such as colours bleaching out, discolourings, and destruction of substance. Because the energy intensity of radiation increases as the wavelength diminishes, the ultraviolet or blue spectrum is more damaging than the red or infrared spectrum (see Table 1).[1]
[2].

Table 1: Damage factors in dependence on wavelength

In principle, the damaging absorption of radiation depends on the degree of absorption or reflection of the material and its spectral classification. This means that the damage to a dark surface will be greater than to a light surface, and a reddish surface will be more damaged than a blue one. In addition, sensitivity to ra­diation depends to a great extent on the type of material. Art works on paper are much more sensitive than ceramic or metal artworks, for example. Finally, damage is dependent on the duration of the radiation. Illu­minating an object at 1000 lux for an hour has the same damaging effect as illuminating it for 1000 hours at one lux (assuming the same light composition). Corresponding to the light sensitivity of materials, effective threshold loads according to Günter Hilbert[1] have been established to determine the critical value of the light energy radiation for visible discolorations (for example newsprint 5 Wh/m² or watercolours on paper 175 Wh/m²).

For these reasons, maximum illuminance intensities are frequently set for objects of a specific light sensitivity. As 50 lux is considered to be the lowest level for good visibility, illuminances of 50 lux for paper and textiles, and 150 lux for paintings on canvas have been established. This guideline is not scientifically well-founded, because it does not take into account the spectral composition of light and the radiation energy. For this reason, many museums attempt to protect their objects by requiring the following:

• determination of different maximum values of luminous intensity in dependence on the source of light

• limitation of the maximum duration of exposure

• complete ultraviolet protection and dimming of light outside visiting hours

• classification of the individual artworks according to categories of light-sensitivity

Karen Colby has worked out a detailed proposal for the classification of artworks on paper and recommended annual lighting limits. The Victoria & Albert Museum has developed simplified rules, according to which light-sensitive objects may be exhibited for 20 percent of the base lifespan of 500 years at 50 lux, while ex­hibits from the more durable category can be displayed in permanent exhibitions at 200 lux.

Such exigencies for object protection require absolute UV protection, the facility to dim lights, and precise adjustability of luminous intensity with minimum radiation energy. Finally, the potential damage to objects from the thermal effects of lighting should be taken into account (tension, stretching, crack formation). Heat­ing of objects due to heat radiation that can lead to damage must be avoided,[3] so ‘cold’ light without infrared is recommended.

1.3 Room lighting, orientation, view of the outside world

For the orientation of staff and visitors and for sensory perception of the space, general lighting of the interior is useful as a rule. Special exhibitions in darkened rooms where only the objects are lighted are the ex­ception. General lighting can also serve to light objects as well, but this can also be done independently. Frequently, additional object lighting is provided. If the protection of light-sensitive objects requires limited luminous intensities, the general lighting has to be adjusted correspondingly. For this reason, it is advisable to implement zoning with different illuminances and darkening possibilities when planning buildings and their illumination. In general, it should be borne in mind that light-coloured room surfaces favour even light distribution for room lighting and object lighting.

Visual connection with the outside is indispensable for the well-being and orientation of staff and visitors. Correspondingly, sufficient numbers of view windows should be arranged in such a way that it is possible to see the exterior surroundings including the horizon.

2 Daylighting

Daylight has a special role as a natural light source for many exhibition purposes, because of its spectral composition for colour rendering of objects or because of the characteristic change of intensity and the colour of the light depending on the sky and the time of year and time of day.

2.1 External conditions

In our latitudes, illuminances can amount to up to 100,000 lux outside when the sun is shining, while when the sky is overcast, it reaches at most 18,000 lux and on average, 10,000 lux (cf. DIN 5034). With direct sunlight, the colour composition of daylight is not the same as it is with a blue sky or when it is overcast; and it changes with the time of day and the season. People are familiar with the resulting interplay of light and shadow, and daylight can therefore be advantageously implemented in museum architecture for general lighting and/or object lighting. This is particularly true when an exhibition is to be displayed under lighting conditions that resemble exterior conditions, but the object requires a display case to protect it.

2.2 Distribution of daylight in rooms

Nonetheless, daylight glare that can occur through the reflection of direct sunlight or through high light-density contrasts in the field of vision (shadows, differences in luminance between views through windows, room surfaces, and objects) should be eliminated. In addition, the protection of objects can require strict limitation of the light intensity in a room to a maximum of 50 lux or 200 lux (see above). Basically, the particularly energy-rich ultraviolet spectrum of daylight should be kept out of interior rooms, which is why window glass with ultraviolet filtering characteristics is used in museum buildings today.

The even illumination of rooms and objects with daylight is significantly easier to achieve by means of skylights than with side windows. However, direct sunlight should be avoided, and so the diffuse light of the sky is decisive. A skylight reveals a larger section of the sky than a side window, and correspondingly, the light incidence in the room is higher (up to five times more). When the sky is overcast, the differences are even greater, as the light intensity is much greater at the zenith than at the horizon.

As already suggested, apart from the light intensity, the evenness of the lighting across the depth and the height of the room (horizontal and vertical illuminance) have an important role to play in museums. In addition, casting shadows and the direction of the light are important. Basically, objects should not be cast into shadow by visitors or installations or fittings (fig. 2). For this reason, light openings should be positioned correspondingly high. Directional lighting with corresponding shadowing is advantageous for spatial orientation (general lighting) and for the perception of objects (spatial forms, surface structures). Extremely soft, non-directional lighting, for example through a high proportion of reflected indirect lighting should therefore be avoided, just as overly contrasty lighting should as well.

Arrangement of windows and its influence on shadowing or lighting of walls

2.3 Daylight openings

All these requirements have lead to daylighting via roof openings and light-disseminating ceilings being preferred in many museums. In one-storey buildings or on top floors, this is unproblematic. In multi-storey buildings, other solutions must be sought. Atria, for example, are one possibility of bringing daylight to the exhibition areas (cf. fig. 3). However, lateral lighting via openings in the external walls is easier, although it has limitations as far as lighting deep rooms goes. One rule of thumb is: room depth to be lighted naturally = window height x 2.5.

Brussels, Musée des Beaux Arts, daylighting through the atrium

Correspondingly, high rooms and windows placed high on the walls, immediately under the ceiling, are advantageous (for example New Walsall Art Gallery, fig. 4). Light-deflecting devices[4] in the windows can be helpful as well as ceiling cavities lined with light-reflecting materials that deflect the daylight into the depths of the room by means of multiple reflections through a light-disseminating ceiling underneath. Fritted glass, laminated glass with interlayer or with roughened surfaces as well as stretched film, textile, screens, etc. can be implemented as light-diffusing material for these suspended ceil­ings. It is important that the material transmits a large amount of light (>50%) and distributes the light over a large area in order to achieve good room illumination and an even appearance of the ceiling’s surface without structural elements casting noticeable shadows.

New Walsall Art Gallery, UK (architects: Adam Caruso, Peter St. John), windows set high in the walls for deep, shadow-free room lighting

2.4 Control of daylighting

As a rule, the large fluctuations in daylight require devices to attenuate, adjust, or block the light near the windows. Only in this way can the requirements of object protection, glare protection, directional lighting – and, if required, dimming – be met. Immoveable devices frequently do not have the required facility to adjust to external conditions and users’ requirements. For this reason, adjustable systems that can also serve as thermal solar protection are implemented as a rule. Nevertheless, there are many solutions for immoveable shading systems that are worthy of note, some of which block out direct sunlight on purpose and only let in diffuse light from the sky (fig. 5).

Hanover, Sprengel Museum (architects: Peter and Ursula Trint, Dieter Quast), rigid daylighting system in skylight with direct sunlight shut out, integrated artificial light

Moveable systems are for example roller blinds, sun blinds, Venetian blinds, or large-format lamella. Lo­cated on the exterior of the building, they provide good solar protection, but they are exposed to wind and dirt. For this reason it is recommended that for external areas, robust, large-format lamella be used. Lighter-weight constructions are favoured inside the window construction or on the room-side of the glazing. The solar heat absorbed can be drawn off, in particular by means of natural or artificial ventilation (for example Fondation Beyeler when the shade-provision is integrated into roof or façade openings. An electrically-powered motor with automatic control of the shading system is usual today.

2.5 Room surfaces

The quality of the room lighting – including the aspects of light distribution, object perception and limitation of glare – is dependent to a high degree on the design of the room surfaces. The lighter the surfaces, the better the light distribution by multiple reflections and the general lighting of the room. On the other hand, there should not be too much deviation in the illuminance values between dark objects and their background, in order to avoid glare by contrast. This arises when the degree of reflection of the exhibition object and its background differs, and also when the luminance of surfaces (for example ceiling/wall) varies a lot. If the room surfaces are strongly coloured, the colour rendering for the exhibits will be affected, as certain spectral areas of light will be absorbed while others will be reflected. This may increase the general difficulties of harmonious colour rendering in daylight and/or artificial light.

3 Artificial light

Access to the majority of exhibits presented in museums is visual. Light therefore has to serve the mu­seum in a very conventional sense. As a controllable medium, artificial light is particularly suitable for making exhibits visible to visitors. Museum lighting with artificial light varies as much as museums themselves do. The spectrum ranges from even lighting of the space and the exhibits within to targeted staging (fig. 6). Above and beyond that, unusual ways of seeing can be generated through colour of light, light intensity and directional lighting. The light itself then becomes a component of the presentation. Just as the exhibits are arranged in relation to each other, the lighting too affects museum presentations. The interaction between the exhibition design, the exhibits, and their lighting requires a coherent conception. From this it can be deduced that there cannot be generally valid rules for the right museum lighting independent of the exhibition design.

Museum lighting focused solely on the exhibits

For the lighting of art works the light that is ideal is the one under which they came into being. In this way the viewer gets the same impression of the work as the artist did. The daylight conditions under which many old masters worked in their studios or even outside cannot be entirely recreated in museums and in any case would certainly have entirely destroyed many works. Therefore a solution has to be found for the presentation of art that enables it to be viewed and its colours rendered as well as possible without damaging it.

3.1 Room lighting and object lighting

To understand the most important requirements of museum lighting, the essential terms in lighting technology have to be distinguished. The illuminance describes the measurable incidence of light on a surface, while the luminance, as the lightness of a surface, instead reflects the perceived impression of lightness. For curatorial concerns, the amount of light radiated in a specific period is important.

Because of the complexity of human perception and its interaction with psychological effects of spatial perception, it has not been possible to arrive at a simple description of ‘good lighting.’ However, a few guidelines for museum lighting could certainly be established in order to avoid disturbances (for example through reflections or damage to sensitive materials) or keep them to a minimum. For this generally formulated criteria for good lighting might be helpful.[5]

The level of lighting

Room lighting is used to ensure that visitors can move safely through the museum. For this kind of lighting, 20 to 50 lux is already sufficient, for example, for the corridors of administration buildings. However, in addition to the safety aspect, room lighting also has the task of making visitors’ stay in the museum comfortable, for example for relaxation phases after concentrated observation or stimulation through changing of one’s spatial situation. An evenly lighted exhibition quickly leads to fatigue, and the visit becomes strenuous.

The necessary luminous intensity for lighting objects depends on the minimum luminous intensity for perceptibility and the limiting of maximum lighting according to conservational factors. If the luminous intensity is too low, colour perception – particularly of the darker colours – is not good. While up until a few years ago, absolute upper threshold values (for example 50 lux for graphics) prevailed, now, in addition, the duration of the radiation and – in particular for detailed observation – the composition of the radiation is taken into account. This way, it can be arranged that the exhibitions can be shown in a better light for a limited time. For comfortable perception, one should assume a illuminance (vertical for paintings) of circa 200 lux for most exhibits, whereby detailed pieces require more illuminance.

Limitation of glare

Glare arises when high light densities from light sources or reflections overlap the exhibits in the viewer’s field of vision. Lighting is not completely free of glare until no sources of light are visible in the direct field of vision and neither shine or reflection is apparent anywhere. This is only possible with lamps with low light intensity, or pure indirect lighting and matte surfaces. However, lighting sculptures, for example, like this generates a flat effect, and shiny exhibits actually require reflections in order for the surfaces to be perceptible.[6] In any case, care has to be taken that a direct view into a light source does not lie in an obser­vers’ field of vision and that reflections are not so bright that the perception of the entire exhibition piece is im­paired. Several low-powered spotlights are therefore better for three-dimensional illumination than fewer high-powered ones.

Distribution of luminance

The eye selects the lightest surface in the field of vision (wall, ceiling, curtains) as a reference in order to resolve contrasts of up to ~ 1:100. For an exhibit or a field of vision to be perceived as evenly lighted, how­ever, the proportion of maximum luminous intensity to minimum should be no more than 3:1, so that all the exhibits’ own contrasts are properly recognizable. Harmonious distribution of light ensures balanced relations of light intensities, that there are no shadows, spottily lighted walls, or rapid diminution of light from ceiling to floor. Even if the focus of the lighting is on the exhibits, the space-forming effect of the light should not be forgotten (figs. 7 and 8). With a dark painting on a light-coloured wall, the eye has difficulty in adapting to the painting. The viewing conditions are the most comfortable when the wall and the painting are of a similar luminance. The wall surfaces for the presentation of paintings are therefore often entirely or partly dark (fig. 9). In special cases, individual paintings can be made to adapt to the lightness of the surroundings by means of sharp spotlighting.

Interference in spatial perception with spotlighting alone

Spotlighting can become a space-forming element

Tinted walls facilitate observation of the paintings

A problematic distribution of light can result, particularly in very large rooms, even with indirect lighting via light ceiling surfaces or illuminated ceilings. A very light ceiling surface has the effect that when entering the room, the pupil is contracted. The exhibits appear in contrast to be dark, and it is not until they are ob­served more closely and sometimes even with some difficulty that the eye adjusts to the lightness of the exhibits. Continual readaptation quickly leads to fatigue. The museum visit becomes hard on the eyes. In such cases, by means of additional spotlighting, the level of lightness of the exhibits can be raised, or the light intensity of the ceiling observed at a very obtuse angle can be reduced by means of a grid or a screen.

Colours of lamps

For lighting exhibition objects, white light should be used as a rule. The perception processes in the brain enable us to perceive light of widely varying spectral compositions as white. Light colour is described through its colour temperature. White light ranges from 2000 Kelvin (dimmed light bulbs) to more than 6000 Kelvin (daylight). There is a connection between light colour found to be comfortable and luminous intensity: according to Kruithof’s curve (Weintraub 1990), at low illuminances, only warm tones or neutral-toned light sources are found comfortable, while at higher illuminances light sources with higher colour temperature are found more comfortable. The question of the colour of the light is to be kept in mind particularly in combination with daylighting. The eye cannot compensate for the colour shift be­tween ‘cold’ daylight and the ‘warm’ light from halogen spotlights when their luminosity overlaps in the field of vision. Twilight then results, and this can disturb both spatial perception and colour perception of the exhibits.

Colour rendering

For good colour rendering, it is important that in the spectral distribution of the light sources, regardless of their colour temperature, there should be continuous progression. This is the case with daylight and light-bulb light (colour rendering index Ra=100), but not with discharge lamps (for example fluorescent lamps). For the usual lighting tasks, uneven colour rendering is not critical; however, for paintings, a bad colour rendering index (Ra<90) leads to substantial changes in the impression made by the picture, so that for museums, fluorescent lamps with improved colour rendering have been developed, and they should be used despite their higher price and their diminished lighting power.

Light direction and shadow formation

The light direction determines how an exhibit appears to observers. When using directional light, care should be taken first and foremost to avoid observers casting shadows on the exhibit. With reflecting surfaces (for example pictures under glass or display cases), it is necessary to ensure that light sources cannot reflect in them.[7] Bright window surfaces too can lead to reflections that make observation difficult (fig. 10). For the illumination of three-dimensional exhibits, a combination of direct and diffuse light is ideal. The proportion of diffuse light can also result from the reflection of direct light on the surfaces in the room, otherwise illumination from several sides and of varying intensity is to be provided.

Reflections from light sources on shiny surfaces

An object’s own shadow engenders plasticity and there, it is a sign of the quality of good museum lighting. However, deep shadows have to be avoided as far as possible. They can be moderated by diffuse light, but remain nonetheless disturbingly evident. Particularly disturbing shadows are caused when stripes of sunlight fall directly on the exhibits, for example. Window frames can cause striping patterns totally overlaying exhi­bits. Also, badly positioned lights may cast the edges of rooms into shadow or cause shadows to be cast at the edges of deep picture frames.

3.2 Lights and luminaires

As industrially manufactured products, the lamps for museum lighting are to be selected from the range on offer. In this regard, the type of room lighting determines the category of lighting methods.

• Light bulbs: spot-shaped light sources, directional lighting possible, good colour rendering, warm light colour, small models available, wide performance spectrum.

• Discharge lamps: spot-shaped or linear, limited colour rendering, high performance, models more suitable for general lighting.

• Special case: fibre-optics, very small models available, especially suitable for accent lighting, low proportion of ultraviolet and infrared minimizes the damage potential.

• LED: very low illuminating power and limited wavelength spectrum with correspondingly bad colour rendering limits the use in museum lighting to special cases.

Luminaires can be manufactured for the special requirements of the respective lighting tasks and particularly in museums, the proportion of special lights is very high. In this regard, the lighting geometry can be adjusted precisely to the requirements. The largest market segment for variable spotlights is in museum lighting.

3.3 Conventional lighting systems

The two most important lighting systems for museums with their advantages and some potential problems are only introduced as examples here. Because of the usability of daylight, the illuminated ceiling is a widespread form of museum lighting. Illuminated ceilings offer the possibility of eliminating reflections on exhibits to a large extent and thus enable perception of the space and of the objects. Important conditions for the implementation of an illuminated ceiling suitable for a museum are the height of the room and the type of glazing; with daylight-illuminated ceilings, the type of solar protection and the distribution of light, and with artificially-illuminated ceilings, the lighting power installed and the colour of the light. In low-ceil­inged rooms, the use of an illuminated ceiling leads to a large differential between horizontal and vertical luminous intensities. Because the eye first adapts to the brightest area in the field of vision, exhibits on the walls appear dark. With an illuminated ceiling, the choice of glazing is particularly important, as on one hand high transmission is necessary, but on the other hand, with illuminated ceilings that are too transparent, the patterns of the lights or the daylight openings above them can be seen and thus impair perception of the room. With artificially-illuminated ceilings it must be borne in mind that observers tend to associate an illuminated ceiling with daylight. This means that a corresponding lighting power will be expected and illuminated ceilings that do not give enough light are perceived as dim and uncomfortable. As illuminated ceilings give diffuse light, supplementing them with spotlights at least optionally has to be provided for, for example by inserting contact rails in the grid or at the edge of the illuminated ceiling.

Spotlighting is particularly suitable for creating a very sophisticated lighting situation. The spectrum in the lighting power and distribution of light enables very small objects to be lit up, for example with track lighting, and even outside areas can be spotlighted. When lighting with spotlights, the risk of glare from reflection and the formation of shadows has to be taken into account. When spotlights alone are implemented, there is the danger that spatial perception is only possible with difficulty, and visitors will be disoriented. In arranging spotlights or rather when establishing possible spotlight positions, it absolutely has to be taken into account that they have to be spatially integrated, as otherwise the spotlights themselves are turned into space-forming elements.

3.4 Light and energy

Museum lighting usually gives rise to complex lighting installations requiring high numbers of unit watts per square metre that cannot be compared with other uses. With pictures, for example, it has to be borne in mind that here the wall surface has to be lighted and the specific connected value based on the floor area can be very high. In particular, with high-ceilinged galleries, connected values may rise to 100 W/m². The lighting causes a significant heating load for HVAC systems, and it has to be drawn off in order to avoid the rooms being overheated. The solution often arrived at, of installing additional spotlights for special exhibitions, makes designing the HVAC system for the lighting conditions problematic in general.

The consumption of expensive electrical energy for artificial light can amount to a significant proportion of the operating costs of a museum. In addition to that there is the cost of maintenance and the changing of light bulbs – usually short-lived – of the luminaires. Taking into account ecological and economic factors, optimizing the lighting requirements and choosing the optimum illumination source should therefore be a fixed component of the planning process, also when exhibition plans are changed.

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Originally published in: Paul von Naredi-Rainer, Museum Buildings: A Design Manual, Birkhäuser, 2004.