Architect/theorist Christopher Alexander conceptualized his Pattern #159: ‘Light On Two Sides Of Every Room’ in his 1977 book, A Pattern Language. Alexander identified that “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 further maintained that, “this pattern, perhaps more than any other single pattern, determines the success or failure of a room.”
Why is this? Alexander does point out that his own experimentation, from which he drew his conclusions, was rather informal. But the phenomenon is very real. What is it about certain patterns of light that attract people or enhance space and volumes effectively? There are several probable reasons why a room or space with lit on two sides (bilateral daylighting) is more successful than one with light only from one side.
Firstly, bilateral lighting 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.
Another reason could be related to brightness and contrast ratios. Take for example the case where one is engaged in conversation in a room lit by a single window, without the benefit of natural side lighting. Because that person’s face is in harsh contrast, it’s more difficult to see facial expressions, giving us considerably less information about their mood or response. This in turn is thought to induce some amount of psychological discomfort, which we in turn project into feelings about the room itself.
The third culprit is glare. The Illuminating Engineering Society of North America [IESNA] identifies glare as two sensations, disability glare and discomfort glare. Disability glare is defined as “the effect of stray light in the eye whereby visibility and visual performance are reduced”. Discomfort glare is defined as “glare that produces discomfort. It does not necessarily interfere with visual performance or visibility”.
Researchers know that the relative location of a light source is a factor contributing to discomfort glare. In the present example, the high contrast between the window aperture and the surrounding surfaces is much higher in a unilaterally lit room than one which is bilaterally lit. The direct impacts of glare are eyestrain and discomfort. Indirect physiological impacts of glare can include red and itchy eyes, headaches, gastrointestinal issues, and fatigue.
Here are excerpts from A Pattern Language, Pattern #159: Light On Two Sides Of Every Room:
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.
Locate each room so that it has outdoor space outside it on at least two sides, and then place windows in these outdoor walls so that natural light falls into every room from more than one direction.
The arrangement of daylight in a room, and the presence of windows on two sides, is fundamental. If you build a room with light on one side only, you can be almost certain that you are wasting your money. People will stay out of that room if they can possibly avoid it. Of course, if all the rooms are lit from one side only, people will have to use them. But we can be fairly sure that, they are subtly uncomfortable there, always wishing they weren’t there, wanting to leave – just because we are so sure of what people do when they do have the choice.