Louis Kahn's Kimbell Art Museum is the building that most elegantly shows that the true material of architecture is not only concrete or stone but light. For Kahn a space is only truly designed once it has been decided how natural light will enter — and in an art museum light is both vital and dangerous: it makes the works visible but, striking directly, harms them.

Kahn's solution lies in the shape of the vault. The museum is roofed by a series of long, low reinforced-concrete vaults; but these are not semicircles, they are a softer, wider arch drawn with a cycloid curve. At the very crown of each vault runs a thin slit, end to end. Harsh daylight entering directly strikes a curved, perforated aluminium reflector set beneath this slit (what Kahn called a 'natural-light fixture'); from here the light bounces up onto the curved inner surface of the concrete and descends into the gallery as a soft, diffused, silvery glow. The works are never struck by direct sun; the space fills all day with a light that breathes with the changes of weather and hour.

At Kimbell structure (the vault), light (the slit and reflector), and space (the gallery) are designed together in a single decision — one cannot be conceived without the other. Kahn keeps the concrete and travertine on a quiet, sober ground and gives light the leading role. The Kimbell is the canonical example of using natural light as a structural and spatial element, and one of the most studied buildings in contemporary museum architecture.