A Daylighting Pattern Language: Bilateral Lighting

February 9, 2011 / no comments

In Christopher Alexander’s book A Pattern Language he points out: “When they have a choice, people will always gravitate to those rooms which have light on two sides, and leave the rooms which are lit only from one side unused and empty.” He touts that “this pattern, perhaps more than any other single pattern, determines the success or failure of a room.”

Why is this? Alexander goes on to point out that his experiments had been rather informal and drawn out over many years. But the trend is very real. He also recalls that light on two sides was a tenet of the Beaux-Arts design tradition. What is it about certain patterns of light that attract people or enhance space and volumes effectively? Looking to the past may be the best way to design daylighting for the future.

In last September’s entry, ‘A Daylighting Pattern Language’, Robert Osten used Le Thoronet abbey as an example of how well small apertures were designed for introducing daylight into the interiors of large masonry constructions. Before the use of sophisticated computer models and analytical studies, architects based their designs on common sense, common practice, and a basic understanding of the relationship between architecture and the sun. Why are some spaces much more successful than others? When it comes to light, and especially daylight, it’s a human response: what feels good.

Let’s examine some reasons why a room or space with bilateral light is more successful than one with light only from one side.


The first, seemingly obvious, reason is brightness balance and contrast ratios. For example, sitting in a space and talking to a person in front of a window without the benefit of light on their face (from another window or skylight on the opposite side of the room) is not only uncomfortable, but since their face is in silhouette, it is difficult to read their expression, giving us little or no information about their mood or response.

In work environments, the contrast ratio could cause discomfort, and result in eye fatigue. R. G. Hopkinson in his book Architectural Physics: Lighting, published back in 1963, discusses the issues of glare and contrast in great detail, showing how it affects much more than simply the quantity of light. Today, from simple evaluations to exhaustive studies, we continuously find proof that high contrast and glare will affect productivity in the workplace.

Not only is bilateral lighting preferred for being more comfortable, but it instinctively feels more natural. Balanced light – light coming from more than one direction – is more akin to ‘natural’ light. In the great outdoors we have both direct sunlight and light from the sky itself – light coming from all different directions. This helps provide depth, giving us clear information about shapes and forms. A strong light source from one direction tends to flatten our views, providing less visual information. Dramatic, but a bit unnatural, and uncomfortable over long periods of time.


Notice how the unlit wall, relative to the illuminated walls on either side,
looks dim and cavernous in contrast with the daylit atrium above.

Since many large or multi-story buildings tend to introduce daylight through sidelighting, it is critical to balance the overall lighting in the space. The more daylight introduced from one side, the more light it will take from the opposite side to offset cavernous effects. Of course, introducing more daylight through an additional window or clerestory is the most effective approach since it will maintain the same exterior intensities. If daylight cannot be provided, supplemental electric lighting should be designed to fill in the gradients where daylight is lowest. Washing walls and surfaces with light and using light colors will increase the perception of balanced light in the space.

The relationship between daylighting apertures and electric lighting is key. There is a lot to be learned from historic, daylit buildings. We should not ignore those patterns throughout the history of great architecture that have always met with positive responses from the humans who use them.


Photos: © Lam Partners

A Daylighting Pattern Language: Deep Apertures

September 27, 2010 / no comments

Le Thoronet is one of three wonderful Cistercian abbeys in Provence, built around 1170. In the mid-twelfth century this part of southern France was not a major tourist destination. The monks who built Le Thoronet were avoiding the political intrigues and feudal power struggles of the cities by locating in a remote area, and they weren’t necessarily welcoming company. And they were building for eternity, too, so the walls are thick, sometimes over three feet thick. As with a lot of ancient masonry construction, this has a salutary effect on the way daylight works in the interior. Why?

In contrast to today’s vogue for all-glass buildings, how is it that massive masonry construction can result in wonderful daylighting? This has a lot to do with contrast control, which is related to the deep apertures created through the thick walls.

Because sunlight is such a powerful light source, a major challenge with daylighting is to moderate the contrast between very bright exterior views and the relatively much darker interior surfaces. In particular, the interior face of the wall containing the window often tends to be the darkest surface in the entire space, since it may receive no direct daylight at all. This can result in very harsh contrast at the apertures. But, when the aperture has depth, the sides of the opening provide extensive surfaces with a brightness that is intermediate between exterior and interior, graduating the contrast.

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Without this kind of buffer, the contrast is often more than the eye can comfortably accommodate. If splay or architectural ornamentation is present in that zone, the contrast gradient is even more improved; the ornamentation itself is beautifully rendered by the raking light and brightness gradient from exterior to interior.

In addition, those surfaces, especially the sill, diffusely reflect daylight into the interior – for example, illuminating the ceiling even though most of the original daylight source is heading for the floor.

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These are daylighting principles we would do well to emulate in our designs today. We’re rarely going to have walls three feet thick to work with, but we can accomplish similar effects by, for example, positioning our apertures against flanking walls or piers. In this house by Tadao Ando, the room surfaces perpendicular to the apertures have a brightness intermediate between the view outside and the darker interior surfaces. In addition, they diffuse daylight back onto the inside surface containing the aperture, which further softens contrasts.

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The deep aperture approach lies in stark contrast to just treating daylight apertures like simple holes in the wall. Besides improving contrasts, the deep aperture uses daylight as a powerful expression of the extension of space.

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Photo Credits: Betina (1), Nicola Comodo (2), Glen Craney (3), Lam Partners (4-5)