Light as Material — Notes on Designing with What You Cannot Touch

Architects speak of light as though it were a finish — something selected from a sample card and applied at the end of the project, the way one selects a tile or a door handle. There is a section of most project briefs that reads, approximately, the space should be bright and welcoming, as though light were a quantity to be specified and then delivered, like insulation or drainage.

But light is not a finish. It is a structural component. It bears load in a way that no other material does: it carries meaning. And unlike concrete or timber, you cannot order it. You can only design the conditions under which it enters.

The North Room

The gallery we completed in Oslo eighteen months ago was built around a single spatial idea: a room that is lit only from the north. North light, in the tradition of artists’ studios and churches and old libraries, is the light that does not change. It does not carry the warmth of the morning or the drama of the late afternoon. It simply is: a cool, consistent, indirect luminosity that falls on surfaces without modelling them, that reveals texture without commentary.

The brief asked for a gallery that could hang large paintings. This requirement drove the section. A large painting needs a large wall, uninterrupted. An uninterrupted wall means no windows in that wall. No windows in the long wall means light must come from somewhere else — from above, or from the ends. We chose the ends, and we chose north.

The result is a room twelve metres long, six metres wide, and four metres high, with a single full-height north-facing aperture at each end. The apertures are framed in raw concrete, their reveals deep enough to prevent direct sky view from the hanging wall, so that the light entering the room is always reflected — bounced off the concrete reveals, off the floor, off the ceiling — before it reaches the paintings.

What this means in practice is that the light in the room is an artefact of the room itself. It is shaped by the section. If you stand in the centre of the room and look at the north wall, you see a pale rectangle of sky contained within a grey frame. The light that falls on the paintings comes from that rectangle but has been transformed by the space it has passed through. It is no longer sky light; it is room light.

Measuring Light

We use several instruments to understand light during design. Heliodon models — physical spinning platters on which you can place a scaled model of the building and observe shadow angles at different latitudes, dates, and times of day — are old-fashioned but irreplaceable. No software gives you the same immediate, tactile understanding of how a facade orientation or a roof overhang changes the quality of light inside a room.

We also use lux measurements. A lux is a unit of illuminance: one lumen per square metre. Natural daylight on an overcast day in Scandinavia delivers about 10,000 lux on a horizontal surface. Inside a well-designed room lit by north light through generous apertures, you might expect 200–400 lux on the hanging wall — enough to read a painting clearly without squinting, but not so much that you are distracted by the light itself. Galleries that are overlaid with fluorescent supplements often reach 800–1000 lux; the paintings become bright but the room becomes clinical, and the relationship between the work and the space that contains it is dissolved.

The target we set for the Oslo gallery was 350 lux at mid-room height, measured on the axis between the two apertures. We monitored this during construction and again after completion, on a series of overcast days in October. The actual measurement was 318–372 lux, depending on cloud cover. We were satisfied.

The Shadow as Element

Designers who are comfortable with light are usually comfortable with shadow, and this is not accidental. Shadow is the proof that light has a direction. Without shadow, light is ambient — ubiquitous, unlocated, uninformative. With shadow, light declares its source, and the space it inhabits becomes three-dimensional in a way that plan drawings never capture.

We think of shadow as an element of the programme — as necessary to the experience of a space as the walls that contain it. In the staircase of the Bergen warehouse conversion, a single clerestory window at the head of the stair throws a bar of direct south light across the risers at midday in winter. The bar moves as the day progresses; it is present for approximately two hours. This was designed: the window was located and its reveal depth calculated to produce exactly this effect at this time of year. The shadow is not accidental.

What shadow does, in this context, is temporalise the space. A room that is always the same is a room that has been removed from the world. Shadow returns the room to the world: it shows the position of the sun, the progression of the day, the season. This is not a sentimental idea; it is a practical one. People who work and live in buildings with temporal variation — with rooms that are different at different times — are less likely to lose track of time. The building itself becomes a clock, or a calendar.

Materials That Receive Light

Different materials receive light differently, and this is one of the most powerful instruments in the architectural palette. Raw concrete, board-formed and uncoated, receives raking side light with enormous generosity: every board mark, every tie-hole, every slight variation in aggregate distribution becomes legible. The same concrete painted white becomes a neutral ground, reflective rather than textured, that bounces light without absorbing it.

Lime plaster — our material of choice for interior walls — sits between these two extremes. It is slightly absorbent, matte without being dead, warm without being amber. Light that falls on lime plaster seems to hesitate before it bounces: the plaster takes a moment with it, and then returns it to the room slightly diffused. This is why rooms lined with lime plaster feel calm. The light is being processed before it is distributed.

Oak, oiled rather than lacquered, receives light as a series of small surface events: the grain of the wood, the annual rings visible in the end grain, the occasional knot or mineral streak. Where direct light strikes an oiled oak panel, you see the specific quality of that panel — its particular history, its particular growth. Where indirect light falls on it, you see the colour of the material in its most neutral state. The panel is the same material throughout, but it presents itself differently depending on the light that falls on it.

This is not a mystical observation; it is a technical one. Designing with these materials means designing for these responses. It means knowing, when you draw an oak panel on a west-facing wall, that at four o’clock on a winter afternoon, the light that enters through the window opposite will model the grain of that panel in a way that is impossible to predict precisely and unnecessary to predict precisely. You only need to know that it will happen, and that it will be good.

The Section as Instrument

All of this — the light, the shadow, the material — is controlled by the section. The section is the instrument by which an architect designs the temporal quality of a space. It shows the angle of sun penetration at different seasons. It shows the height of window sills, the depth of reveals, the relationship between floor and ceiling. It shows whether light enters a room obliquely or directly, from above or from the side.

We draw sections exhaustively — far more than is strictly necessary for construction purposes. We draw sections at the winter solstice, at noon; at the summer solstice, at eight in the morning; at the equinox, at four in the afternoon. These drawings are not to scale. They are instruments of thought, sketched and re-sketched until the section resolves into something that we believe will produce the quality of light we are after.

What we are after, in each case, is a room that is lit in a way that is specific to itself — that could not be confused with any other room. A room in which the light has been designed as carefully as the plan, and in which the shadows are as intentional as the walls.