37

II. Lighting Strategy and Practice

PETER WILSON

The Lessons of the Recent Past

Designed by the architects Llewelyn-Davies, Weeks, Forestier-Walker and Bor, the last major extension to the Tate Gallery was opened in 1979.’ It was supposed to represent the ideal compromise between the requirements of display and the preservation of paintings. Yet when it opened there was immediately considerable critical disquiet about its extravagant superstructure of lighting control hardware.* Soon, there was equally vigorous criticism not only of the hardware itself but also of its efforts to deal with rapidly changing conditions of weather, 3 One point which seemed largely to have escaped notice, even in the technical press, was that, in most circumstances, the building delivered superbly well those display conditions which the Brief required of it. The Brief had been drawn up in the climate of the received wisdom of the early 1970s (for the most part emanating from the conservation lobby-a group to which the present author then unashamedly owed primary allegiance). It had attempted to mitigate the worst effects of the National Gallery solution where daylight merged inextricably with artificial Light behind a translucent diffusing ceiling. The 1979 building had skylights of clear glass so that in theory the weather could be experienced and the sunscreening was programmed to ignore small variations of natural light. But, nonetheless, the Brief had decreed that the lighting should be uniform, constant (i.e. varying only between narrowly defined limits) and should maximize the use of daylight whilst excluding all trace of direct sunlight from the building envelope. All this was satisfactorily achieved and yet the result was, in terms of display, a failure. Why?

Towards a New Policy

When the chance came to reassess our lighting brief, and to think afresh for the Clore Gallery, it was necessary to take a critical look at our existing lighting control philosophy. This was as important for the conservator as for the curator because the rigid standards which had previously been applied were in danger of being abandoned since they produced a visually unsatisfactory result in a daylit gallery. At the time, David Loe of the Bartlett School of Architecture and Planning at University College London (who had worked on the lighting design for the I979 building) was doing some very relevant work which suggested that the illuminance (light level) chosen in the existing standard needed to be adjusted upwards. 4*5 Were there other factors which needed reconsidera- tion? First of all the reasons for providing daylight at all were re-examined. Sir Norman Reid, the Tate Gallery’s Director throughout the period of the design and construction of the previous building, had, we later discovered, expressed the need succinctly in an interview given to the press:

. . . we had a strong liking for the variation which natural light produces. If the environment is dead level you do get a sensation, after a bit, almost of boredom, because nothing ever changes. Whereas, in this country, natural light has extraordinary variations, which can change in ten minutes.6

38 The Clore Cullery: II

There were, however, obvious advantages in excluding natural light altogether, since without a glazed or partially glazed roof the building would be both better insulated and subject to much lower solar gains. As a result the air-conditioning plant could be smaller, as could its ducts. Even with a sophisticated and variable artificial lighting system to replace daylight, the building would be less costly, and more display space could be bought for the money available. However, neither the curators nor the Trustees were prepared to consider this possibility seriously: they were instinctively against it. Yet they were, in general, equally unable to articulate clearly just what it was in daylight, as opposed to artificial light, that was required for the good viewing of art.

This needed investigation. We took as our starting point the last Brief, because clearly we had removed whatever quality it was that was needed for a successful daylit gallery. Did the light need to be brighter ? Conservation considerations apart, this could be achieved with artificial light, so higher illuminance alone could not be the essential quality. Did the light need to be less uniform? This also could be achieved with artificial light. So did it need to vary with time? Yes it did: at last we were beginning to establish a quality of daylight that we must include. Could we not vary the artificial lighting with time? No, everyone agreed that the quality of natural light most prized was its unpredictability and unevenness! This realization seems in retrospect rather obvious, but it was very hard to formalize at the time, in spite of Sir Norman Reid’s clear statement. There remained one puzzling question: daylight is not always varying in an obvious way and yet it was generally perceived to be superior in all weathers, times and seasons. It seemed that there was an element of ‘psychological set’ in the visitors-when looking at paintings under natural lighting, however modified, they seemed to remember the exterior conditions far longer than any period of adaptation of the perceptual system of eye and brain would explain. It was a question of expectation: in midsummer on a brilliantly sunny- afternoon the light in the gallery ought to be brighter than on a rainy November afternoon.

An informal analvsis of written comments received at the Information Desk supported this view. Most &icism of the 1979 building’s lighting came on sunny summer days when the light levels in the galleries were actually very similar to those achieved in the winter. In summer the louvre system in the 1979 building has to close partially to exclude direct sunlight, but it is actually often possible for this louvre configuration to require concurrently artificial lighting as back-up because on clear days relatively little light comes from the north sky. The subjective effect is of louvres unnecessarily closed and artificial light needlessly applied. In fact, it was the very changeability of English daylight which had beaten the designers of the 1979 building-the narrow range of illuminances permissible inside the building could only be achieved with rapid, sometimes noisy and always noticeable, response from the external louvres. The desired variation of daylight Was, despite the Tate Gallery’s aspirations, being suppressed.

From Illuminance Control to Annual Exposure Quotas

Clearly the Clore Gallery Brief had to encourage the design of a building which would allow the full range of variation of natural lighting to be reflected in the galleries. The light on the gallery walls would have to be constantly changing its colour, intensity and distribution in response to the exterior conditions, though this was not difficult in itself, since a greenhouse does precisely that! However, we had to meet not only the requirements of display but also of conservation. The problem for the conservation scientist was to reconcile science with conservation. It was obvious that the recognized

PETER WrLsoN 39

illuminance standards were arbitrary, whilst Loe’s work had demonstrated that they

were inadequate, and since those standards were founded on display criteria and not

conservation considerations, we could justify an increase based on his work.

Nevertheless, the system now proposed would seemingly lead to even higher illuminances and we had to be sure that we would not be needlessly exposing works of art to excessive light. The natural variation of daylight is large, but how could we preserve its character without a tremendous increase in annual exposure? One fact is obvious and also crucial: every hour spent at, say, 100 lux above the target average illuminance needs to be compensated by an hour spent at 100 lux below that average illuminance. This whole strategy could be achieved only if, apart from the exclusion of daylight at all times when the galleries were closed, the periods spent at below-average illuminances were acceptable to the public, and so the building had to look brightly lit at the lowest possible illuminance. The times when below-average (and this does not mean inadequate!) illuminance would be psychologically most acceptable would be when there is little or no daylight at all: i.e. at the latter end of days in the winter months. Under these circumstances the galleries would rely wholly on artificial lighting. It was crucial to ensure that this would be effective even at relatively low levels of illuminance.

We had to be able to set a criterion for acceptability (in conservation terms) for a lighting solution which had the potential of continuous variation. Spot readings of illuminance would tell us nothing as the value might, for example, have been half as great two minutes beforehand and double a minute later. The present author had, in 1979, tried to lay the ground rules for such a new approach by ‘entering a plea for the replacement of an adherence to arbitrary [light] 1 evels with an approach based on a consideration of total exposure” in a review of Garry Thomson’s The Museum Environment8 which had appeared the previous year.

Thomson, whilst championing his long-established illuminance standards, knew only too well that alternative strategies based on total exposure were possible and recommended them to those who could not afford a stricter regime of automatic control. He also acknowledged the shortcomings of the then current generation of control

systems :

. . . sometimes a work of art may be caught in a light that reveals unexpected or rare beauty. This is the nearest we may be able to get to identifying the peculiar quality of daylight-its infinite variability . . . out of doors everyone has felt exhilaration when the sun, emerging from cloud, exerts its pervasive influence . . . such opportunities are lost . . . under heavily controlled daylight.’

In retrospect, it is possible to affirm that our insight into our problem owed much to Thomson’s book. Our much-needed criterion was also provided by him:

‘Instead of aiming to achieve a steady illuminance . . . we can work out the total annual dose of illumination, or exposure, equivalent to this.”

We judged that our criterion for exposure should be that the potential damage by light would be no greater than it would be in a gallery operating at a constant ZOO lux over an 8 hour day, 61/2 days a week, every week of the year. Such a control regime would allow an exposure of:

200 x 8 x 6.5 x 52 = 540 800 lux hours per annum

So we had a little over 0.5 million lux hours as an annual target. We could be confident that exposure control was practicable only if we could have a

The Clove Gallery: II

method of recording or logging the total exposure on each display wall. This requirement could be met: again we were indebted to Thomson, whose pioneering experiences with data logging were available to us,” and The Clore Gallery would certainly need an extensive data-logging system. The Brief was being worked up at a time of rapid and significant development in the field of electronics and microcomputing. Equipment was becoming vastly more sophisticated and costs were falling all the time. It appeared that we would be able to have a photocell monitoring the illuminance on each major display surface in the building and be able to integrate the results of frequent spot readings to provide a running exposure total for each wall. About this time it became obvious that an exciting possibility had emerged: if we extended the logging specification to include position monitoring of the sunscreening and control of the motors driving it, the lighting control strategies applicable to the building would then be limited only by the size of the computerized logger and the sophistication of its software. Such a computer could control the artificial lighting as well. Furthermore, we could proceed with as innovative a strategy as we liked, and not only could we monitor its performance and modify its control system in the light of experience, but also mimic earlier strategies, or adopt entirely new ones, as yet undefined. All this was a far cry from the hard-wired logic circuitry of the control system of the building we had only just completed, in which a simple modification to a time delay in the lighting switching programme had necessitated the removal and replacement of no fewer than 42 small electrical components!

The Design Phase: Practical Refinements

Our initial briefing for the design team, though not formalized into a document, was roughly as follows: the galleries would be naturally lit, and they would have external sunscreening, probably using wide louvres of the type used on the 1979 building, There would be no need for total uniformity, whatever the external conditions; rather the interiors should echo the variability of daylight. The louvres would need to close tightly to exclude daylight during non-public hours. It also made sense to specify that louvres could be partially opened to reduce overall exposure on any wall which was tending to receive excessive illumination over a significant period (a month, say). Artificial lighting needed to be even, but not dull. The Architect rejected the notion of imitating daylight and instead offered the solution of ‘uplighters’ to counter the problem of the oppressive darkness of the ceiling above the lighting. This was a problem experienced not only in the 1979 building but also in recent relighting schemes in the older galleries of the Tate Gallery.

The Design Team came up with a system of vertically glazed roof sheds which reflected the light down onto the ‘picture zone’ of the gallery wall through a slot via a suitably angled central baffle. As Poul Hansen describes elsewhere in this issue, the design process moved rapidly to an intensive period of model testing. It was of great benefit to the Tate Gallery to be treated as a ‘technical’ client by the consultants and thus to be able to contribute directly to the model-testing phase. Many valuable ideas were formulated during the course of these discussions.

The model was tested under a standardized artificial sky which mimics an overcast day. Under these special conditions we needed a uniform internal illumination so that we could be sure that the design did not light one wall excessively rather than another. If this were the case, there would be scant chance of achieving similar annual exposures on all walls. Our aim was that variations of intensity round the walls would result from the

PETER WILsoiv 41

14. Detail study by James Stirling of a natural lighting system for the Clore Gallery.

directional variations of daylight and not be an artifact caused by the building itself. The testing process revealed that, under overcast condi- tions, there were optimum louvre positions which maximized the ratio of internal/external illumination (the so-called ‘daylight’ factor). These optimum positions involved angling the louvres above the horizontal so that their upper surfaces were reflecting additional light onto the baffle. This effect was dubbed ‘scooping’.

The daylight factor associated with any particular louvre position could be calculated. Moreover, in combination with available statis- tical data, l2 it could be used to calculate those louvre positions which would achieve our target exposures. Th e optimum position could be reduced for times when the target would be likely to be exceeded. The louvres on any aspect could, in addition, be closed or partially closed to exclude direct sunlight from the picture zone. It should be emphasized that it has always been intended that such positions would be prepro- grammed. There was no question of having a system which responded to changes as they occurred: this was positively undesirable as it would damp out precisely those unpredictable changes of natural light that we had been at such pains to preserve.

Realization: ‘In-service’ Refinements

At the time of writing (December 1986), the commissioning of the logging system lies before us, and the performance specification for sunscreening control and data logging finally produced by the Tate Gallery in March 1982 has undergone substantial revision. We are to have a computerized system13 which allows on-line monitoring (current data being available to the system operator in coloured mimic diagrams on screen). This is designed to calculate and store hourly, weekly, monthly and annual exposures for each display surface, to monitor temperature and relative humidity in each gallery, and to control each louvre bank from its own ‘look-up table’ of positions, as well as to monitor and record those positions so that the causes of apparent anomalies in the recorded exposure data can be resolved. This mass of data will be put to good use: real ‘daylight factors’ will be available to replace those of the model. Long-term ‘high spots’ and ‘low spots’ can be identified and corrected. It also allows the construction of complex, user-definable, artificial lighting control algorithms which remove the possibility of the on-off flashing Christmas-tree light syndrome to which the 1979 building is prone. The system is furnished ‘with a ‘fourth generation’ control language14 which allows a

42 The Clore Gallery: II

technically aware user to generate new control routines and modify old ones without being a computer programmer. The system has the capacity to take over control of the 1979 building, using the new strategies, and the Tate Gallery is well advanced with’its plans to provide similar louvre and lighting systems for the older display galleries.

Future Developments

The Tate Gallery changes its displays frequently (although in fact the Turner

oil-paintings in the Clore Gallery will not be moved very often) and the system described above offers the potential of an object-by-object rather than a wall-by-wall quota. This is a longer-term aim which is not yet realizable, but the integration of data from the Tate Gallery’s projected mainframe-based collections management system15 with logged environmental data, using a spreadsheet software, is under active c,onsideration.

All this affordable high technology has been applied to give the visitor a seemingly low-technology space in which to view the works of art. Indeed, they should not be conscious of the intervention of technology. It is perhaps appropriate to temper euphoria with a note of caution. We have relaxed an arbitrary lighting rule which had been imposed to preserve our collection. Our reasons for doing so are empirical but the justification is at best informed supposition. Conservators and conservation scientists are increasingly called to account for and justify their strictures-should we not also ask curators and the viewing public to demonstrate the genuineness of their affirmed desire for naturally lit galleries? Many questions remain to be asked: maybe views out are more important than natural light from above ? The Clore Gallery has such a view, too, onto

the front gardens and the riverbank. The control and monitoring systems we will have in the new building will give us a perfect experimental tool to vary subtly the lighting conditions, or to exclude daylight altogether, and to keep records of exactly what are the conditions experienced. The opportunity to test public response by questionnaire” will enable us to separate fact from myth: the conservator may yet strike back!

Notes

1. D. E. Church, ‘Picture gallery lighting’, DOE Construction, 16. 2. Kenneth Robinson, ‘Tam ah Tate!‘, The Listener, 31 May 1979, p. 746.

3. William Feaver, ‘The endurance test’, The Observer, 27 May 1979, p. 14.

4. David Lawrence Loe, Appropriate Lighting Conditions for Vzewing Works of Graphic Art, MPhil

thesis, University College London, June 1981. 5. D. L. Loe, E. Rowlands and N. F. Watson, ‘Preferred lighting conditions for the display of oil and

watercolour paintings’, Lighting Research and Technology, 14 (4), 1982, pp. 173-192.

6. Dennis Barker on Monday column, ‘Tate officials had to consult lists to remember who painted

what abstract’, The Guardian, 12 March 1979. 7. P. Wilson, ‘Review of: “The Museum Environment” by G. Thomson’ The Conservator, pp. 46-47. 8. Garry Thomson, The Museum Enwivonment (Butterworths, London, 1978).

9. Ibid., pp. 31-32. 10. Ibid., p. 30.

11. An account was later published: Garry Thomson, ‘Control of the environment for good or ill?

Monitoring’, National Gallery Technical Bulletin, 5, 1981, pp. 3-13. 12. D. R. G. Hunt, Availability of Daylight (Building Research Establishment, Department of the

Environment, Watford, 1979).

13. MICROFAST 2 from Lee Micromatics Ltd.

14. FLO-GEN from Lee Micromatics Ltd. 15. Provided by STIPPLE Database Services Ltd. 16. A suitable technique is described in J. A. Vielhauer, The Development of a Semantic Scale fey

Description of the Physical Environment (University Microfiims, 1965).