Museu da Luz.

The Fundação Alma Clara holds around four thousand works on paper and cloth, most of which may be shown for only a few months each decade. The brief asked for a building that could hold the strictest light limits without ever feeling like a vault.

We started with a single number, fifty lux, and spent the first half-year drawing nothing but sections through a roof.

Two hundred and eight precast coffers, each 1.2 metres deep, form the entire ceiling of the galleries. Their north faces are glazed; their south faces are solid and finished in a pale, sanded concrete that bounces light down without a single direct ray reaching the floor.

Through the day the coffers shift tone from cool grey to warm sand, so visitors read the hour in the ceiling while the works below stay in an even half-light.

Visitors arrive through a walled concrete courtyard, open to the sky, then pass into a stone stair hall and a curved corridor lit from a single skylight. The sequence is planned as a gradual dimming, so that eyes adjust before reaching the first gallery.

Galleries are linked enfilade, with deep reveals between rooms. Each threshold is a moment of shadow, a pause between pieces.

Thick walls of board-marked concrete and local stone, with a buried services floor, keep temperature and humidity steady with very little machinery. The building uses a third of the energy of a conventional gallery of its size.

We specified every material for a hundred-year life, and wrote a maintenance guide the foundation's staff can use without us.