One of the chief influences on the design of the National Gallery extension was the lighting to the galleries. They appear substantially daylit, but in fact receive less than 1 percent of the daylight available
The external walls and the structure from ground level down in the Sainsbury Wing are concrete, but the upper parts of the galleries and the walls between them are framed in steel. The configuration of the steel was dictated by the specific lighting requirements.
The constraints included not only the now stringent environmental requirements for paintings, and the architects’ own decision that the rooftight ridges should line through, but also included the planners’ requirements that the rooflights be kept down to a certain level, as a result of which 250 mm had to be lost from the height of the rooflights at a relatively late stage.
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The galleries are always artificially lit. Daylight is secondhand: you are aware of the intensity of sunlight, or of the sun being obscured by clouds, but you should never see a sunbeam. The filter to effect this, and to provide the complementary artificial light as necessary, is a complex construction, referred to as the lightbox.
The angles and sizes are all dictated by what will produce the best lighting effect internally. The roof glazing, which Venturi wanted to be legible, is a proprietary double glazed system, the inner sheet of which cuts out ultraviolet light. The louvres directly underneath this are operated by externally mounted sensors to regulate the level of light penetration. The louvre positions are reviewed every two hours. When the building is unoccupied the louvres are closed. Internal sensors were tried on the mock-up built at Shepperton film studios, but locating them, and ensuring relevant readings were obtained, was less satisfactory. Each sensor covers a zone – not just an individual gallery.
Daylight enters the galleries through vertical etched glazing. Much effort has gone into eliminating the appearance of shadows through the glass, but they can still occasionally be seen. The lightbox is by no means an empty space merely allowing the passage of light, but also contains substantial steel structure and services. Air conditioning supply ducts run above the angled parts of the ceilings and air is extracted through a slot at the back of the lower cornices.
By the time the daylight reaches the galleries it has already been mixed with artificial light from batteries of colour-corrected fluorescent tubes fixed just below the rooflights. Within the galleries themselves are two tracks of low voltage spotlights which can each be directed at individual paintings. Where these back on to the lightbox a flap is provided for maintenance access. Within the lightbox there are two access walkways, and externally a sliding cradle allows maintenance of the rooflights.
This articled appeared with the original building study in AJ 21.08.91
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