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Vital Signs
Project: Daylighting in Three Libraries
(Overview) (Method) (Survey) (Mt. Airy) (Michigan) (Exeter)
(Syllabus) (Exeter Intro) (Exeter
Model Study) (Exeter Survey) (Exeter Student Evaluation) The Exeter Library (1972) is among the renowned masterworks of Architect Louis Kahn. It evidences an unassuming contextualism in a traditional colonial-style campus and a strong focus upon centrality of space and clearly defined materials (concrete, brick and wood) revealed by light. Daylighting is thus one of its inspirations. The lighting model tests of the Exeter Library presented a counter-intuitive result, at least one not anticipated. The level of light at the center of the Library's famous skylit atrium remains relatively even and at a very low level of illumination (01% daylight factor) irrespective of time of year and direction of sunlight. The daylighting in the central space is very subdued and diffuse, quite independent of sun angle. Illumination at the bottom of this famous space is provided as much by lateral daylighting and electric lighting as by the overhead daylighting.
The field visit did raise questions of underlighting of some areas within the library,
including critical areas affecting stairway safety (tripping due to poor visibility). Like
the glare at the librarian's desk at Mt. Airy (from a similar source . . . horizontal
sunlight entering through the entry portico), this instance illustrates that the ideal
disposition of daylighting and electric lighting remain elusive objectives of
architectural design. Hypothesis prior to field testing The lighting model analysis revealed an extremely subtle - in fact hardly measurable - variation in light in the central atrium from the roof skylight. This was accountable due to the relatively small and removed clerestory windows but also due to low-reflectance of the concrete surfaces. Kahn obviously had choices in designing the clerestory lighting. Whatever options were considered, he concluded with a design that makes the windows hardly noticeable. The question or hypothesis raised via model testing was whether the daylighting is all too subtle and not effective, especially under low light, that is, cloudy conditions. Methods As with the other libraries studied by our research/design studio, we first constructed a lighting model and followed this with a site visit and field evaluation. In this particular case, the most revealing step was provided by daylighting measurement itself. Our measurements documented the relatively low light level and insignificant variation therefrom within the atrium, irrespective of the sun's orientation and angle of penetration. This was in some ways a counter-intuitive result, explained only by the relatively small aperture, the low reflectance of exposed concrete and the relative number of times the light is reflected and thus diffused. The occupant survey provided additional information related to user concerns, some related to too much light at the reading carrels at the periphery, but resolved by the choice of reading areas. Results and conclusions The hypothesis, that the quantity and variation of lighting from clerestory windows at the top of the atrium is not significant, was borne out by field measurement. But that does not negate the design intent. Low quantities of daylight do not mean low quality...a significant lesson to compare with the opposite approach at University of Michigan Law Library where there is a surfeit of daylighting. In the case of Exeter, the design intent to create a serene central space is achieved, especially evident during sunny hours in which a small band of sunlight does penetrate the interior, mostly noticeable by reflection on the major beams that form a diagonal cross at the atrium ceiling. This tends to draw one's eye to the height, the scale and space defined by the structure of the atrium. The effect, however, is somber during cloudy and evening hours, when electric light provides the principle or only illumination. In these instances, the result is debatable. Looking up in the atrium, one's eye is drawn more to the bare florescent bulbs or the adjacent bookstacks, hardly complementary and for the most part detracting from the overall design. The building was repaired a few years back due to problems with leaking roofs, a problem that reportedly continues. According to user reports, a few areas have poor lighting, specifically the entry stairs where bright contrast and shadows make the stairs difficult to see. There have been some incidents of visitors tripping at the stairs. In some corridors, lighting level is very low, which some users described as "like being in a night-club." This example thus presents an instructive and contrasting case to most daylighting approaches, and provides partial if debatable evidence that not a lot of light is needed to achieve a significant result. The concern that the building is dark and unrelieved during predominantly cloudy hours is open to discussion. This was not explicitly included as a question in the user survey. It should be noted that none of the users commented on it in the survey, whereas other positive and negative concerns were noted. The class discussed alternatives to improve the daylighting effect and level of illumination by painting the ceiling surfaces and/or exterior roof surfaces white. However, it was argued that this would violate the long-developed aesthetic approach that Kahn established, of unadorned surfaces that reveal their material and method of construction. The many considerations that must be balanced in this building illustrate the complexity of designing with light, worthy of lifelong study of a thoughtful architect. The next section presents the results of daylight model testing for the Phillips Exeter Academy Library. |
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author: vitalsigns@ All contents copyright (C) 1998. Vital Signs Project. All rights reserved. Created: 05/13/97 |
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The Phillips Exeter Academy Library was designed by Louis
Kahn. The building was completed in 1972.
