Looking at light in the Kimbell art museum

The one building I can write about without having visited it in person is the Kimbell Art Museum in Fort Worth, Texas. My appreciation of the building — its aesthetics, its proportions and above all its emanating light — has only grown with time.

The Kimbell Art Museum is a creation by Louis Kahn, located in Texas. I first encountered it when I analysed the daylight it captures and transforms, as part of a course at University. To give a bit of context, I will quote the words of Wendy Lesser describing both the daylight experienced in Texas as well as its composition within the museum.

“Somehow the harsh Texas sunlight which prevails outdoors has been converted into a cool, silver-tinted beam that bathes the concrete and the paintings and the people who stand in front of them, making everything seem as if it exactly belongs here.” — Wendy Lesser in You Say to Brick: The Life of Louis Kahn

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Interior of the Kimbell Art Museum. Photo by Michael Barera

The effect of bringing daylight from the outside and changing the nature of its perception is the power of architecture, perhaps best embodied in the works of Kahn who wrote on the subject:

The vault ceilings of the Kimbell Art Museum, often described as “pearly gray”, “moonlit” and, in the words of Kahn himself, “a touch of silver to the room”, are the result of three specific design strategies — the geometry, the material, and the opening through which the light is brought in. But how Kahn prepares the visitors to enter the “silent light” of the museum from the harsh Texas sun outside, by means of a transition, is instrumental to appreciate its effect.

The Transition

Before Kahn designed the Kimbell, the space was an empty site with nothing around, so he had to prepare the visitors to transition from the hot Texas sun to the moonlit interiors where the artworks are displayed. An intentional lighting strategy deployed by Kahn is the transition from the Entrance of the Trees to a deeply shaded portico leading to the museum interiors. Kahn designed the West side of the building as its main entrance, with a grove of ordered trees aligned in parallel creating dotted shades on the lawn. The dappled dynamic shade of the trees slowly merges to the solid static deep shade of the portico.

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Exterior of the Kimbell Art Museum. Left: Shaded Portico. Right: Entrance of Trees. Photos by Christopher Martin

Our eyes are capable to adapt within a phenomenal range of sensitivity, from 0.2 lux representative of bright moonlight to 100,000 lux of bright sunlight, but they need time for the adaptation. As Stephen Palmer describes in his book Vision Science, “when we enter a dark cinema hall from a bright outside, we cannot see much except the images from the bright screen. After a few minutes, we see people seated near us and after 20 minutes we see the whole theatre surprisingly well”. This so-called dark adaptation is our eyes becoming ten thousand to one million times more sensitive due to chemical changes, than when exposed to full daylight.

The design strategy of transition at the Kimbell does two things to prepare the eyes for dark adaptation: One, it reduces the pace of the visitors by means of the Entrance of Trees which provides partial respite from the unshaded lawn and increases visual interest by creating patterned light. Second, it also reduces the light levels gradually from the brightly lit outside without shade, to partial shade, and finally full shade of the portico into the “silver” light of the inside.

The Geometry

The roof (and ceiling) of the museum is a concrete cycloid vault. A cycloid vault is an extrusion of a cycloid curve, which is a curve traced by a point on the rim of a circular wheel as the wheel rolls along a straight line.

The cycloid vault is not continuous but split into two symmetric halves at its apex with a two and a half foot gap through which the light is brought in. The gap is concealed by a wing-shaped reflector making the vault look mysteriously continuous, the genius of this detail.

Geometrically speaking, the cycloid vault smoothly distributes the light throughout its surface. Irrespective of the time of the day or the position of the sun — high up in the sky or lower down at eye level—the cycloid vault maintains this flat distribution of light.

Take a look at the false colour simulations of the interiors of the museum below. The false colour images indicate the light levels with yellows and reds being a lot of light (>370 cd/m2) and the blues and purples indicating low levels of light (<70 cd/m2). It is not the numbers but the patterns of these false colours that illustrate how the cycloid vault works. Whether it is summer or winter; morning, afternoon or evening, observe how the colours on the vault do not dramatically change from one end to the other end.

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Daylight analysis of the interior of Kimbell Museum for Texas [Priji Balakrishnan]

If you take the Kimbell Museum away from Texas, which lies 32 degrees North of the equator, and drop it in Singapore, which lies right at the equator (1.4 degrees North), the cycloid vault still works its beauty — it takes the daylight from the outside and distributes it smoothly onto its surface.

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The Material

“The pearly gray surface appears to glow from within, especially as it reaches its peak, where the arch disappears under the winglike aluminium brackets…” — Wendy Lesser, You Say to Brick: The Life of Louis Kahn

The material used to build the cycloid vault is grey concrete with a smooth texture. Kahn, with the help of architect Fredrick Langford, would go to great lengths to get the right colour and texture of the concrete envisioned in his mind. While the colour of concrete can range from tints of blues to greens to yellows, Kahn chose a “soft gray with lavender tone” colour for the Kimbell museum vault. To obtain the smooth texture, they conducted numerous tests on the drying time of concrete under the Texas sun.

Now, imagine this colour and texture of concrete as a canvas and light as your paint. With the geometry of the cycloid vault, this light gets brushed down smoothly from its source to the two ends of the vault — and right there you get the “pearly gray” or “moonlit” effect.

The Opening

An opening determines the directionality of the light source within buildings. Thus, irrespective of the sun’s position in the sky, an architect can decide the direction and nature of the source of light brought into the building via modulations of this opening.

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East-West section of the Kimbell Museum

At the Kimbell, the opening is a slit, slicing the cycloid vault in two symmetric halves. This central slit is covered by winglike aluminium brackets that diffuse the sunlight and hold the electric lighting. The effect of this opening and the calculated intervention to conceal it are best described by Wendy Lesser when she writes:

“The sunlight coming through the central slit […] turns into a silvery glow on the uppermost reaches of the finely textured concrete. The effect is glorious, but it is definitely an effect, a calculated invention to make you feel that light is being shed from an unseen source.”

The central slit, aluminium brackets and the electric lights combine to form a complete light source for the artworks displayed in the Kimbell. With such minimal but precise devices for combining daylight and electrical light, experts say, the resulting colour temperature (3500K to 3800K) at Kimbell is the “sweet spot” for viewing colour. And once again from Wendy Lesser who describes the colours of the artworks displayed at the museum:

“Here, landscapes have the feel of being actually outdoors. Interiors and portraits glow. Pastel colors gain an added strength, and white highlights leap forward.”

The light of the Kimbell is a synthesis of synergic textures and simple techniques with calculated detailing and a thoughtful design process, mindful of how people move and experience space.

My analysis of light at the Kimbell is informed by simulations, testimonies of visitors who observed the light in the museum, my engagement in the growing discussions of qualities of light and my years of working towards a PhD in daylighting. But it is remarkable that Kahn, without any simulation tools to quantify or visualise light, and working against the styles of his period, brings these profound and timeless qualities of light and space in his works. Kahn’s experienced intuition to design, remarkable observations of historical buildings and a team of experts who believed in his vision made it possible to create a building like the Kimbell Art Museum.

Flâneuse, Urban explorer, Building scientist

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