Jul 14, 2022
In this book excerpt, the author describes how designers can tame the beast that is daylight

By Lisa Heschong

In the vast majority of lighting design books, daylighting is an afterthought, at best deserving a chapter, more often just a few paragraphs. It is most often portrayed as a problem to be addressed by the lighting designer, especially to avoid glare or too much energy waste, rather than as an opportunity. It is very rarely portrayed as a design element that the lighting designer can purposefully manipulate to improve the lighting quality of a space. 

There are far fewer books to date that attempt to teach the art of daylighting design. Daylight is not as easy to control as electric light; it cannot be turned on at will. Daylight is like a wild animal that follows its own whims, caring nothing for our schedules and habits. If we want to work with daylight as a design element, we must understand its native habits.

One of the most basic design concepts to understand about daylight is that it is big—very big. We can choose to let little bits of daylight permeate our buildings, but there is always more outside, waiting to come in: an atmospheric ocean of light. By adding more holes in our buildings, like a colander, daylight will fill the space within, as big as we want it to be. Actually, the bigger the volume of a space, the easier it is to daylight.

One of my early introductions to daylighting design was via William Lam. I was freshly graduated from architecture school and had the privilege of interviewing him for a 1980 Solar Age magazine article on how daylight could be used to save energy. At the time, Lam was already a famous electric lighting designer in Cambridge, MA, but in the spirit of the times, he had recently become an outspoken advocate for restoring the primacy of daylight in our buildings.

His subsequent 1986 book, Sunlighting as a Formgiver for Architecture, argued that the form of the building should be the essential determinant of lighting quality. This is a challenging paradigm shift change when thinking about daylighting design. What if light determined the form of the building, rather than vice versa? What if electric lighting was considered merely supplemental to daylight? It would be as if you were designing an outdoor patio for family parties—logically you would not expect electric lighting to be used outdoors during the daytime. Rather, electric lighting would be a system that could extend the use of the patio at the end of twilight and through the night, while the party lasted. Interior spaces can be similarly conceived, with the presumption that they will be awash in a sea of atmospheric daylight while it is available, and then supplemented with electric light when needed to extend the period of human activity.

One of Lam’s rules of thumb was that a direct beam of sunlight should always be bounced (at least once) off of a white or light-colored surface before it reaches an occupant’s eye balls in the interior of a building. Thus, he tested the daylighting effects of a variety of overhead baffles, reflectors and windows set in deep reveals. Alternatively, beam sunlight can be diffused optically, by refractive systems like prismatic or Fresnel lenses as used in diffusing skylights. This is much like putting a sprinkler head on a garden hose, transforming a powerful and potentially destructive stream of water into a gentle spray covering a much larger area.

Furthermore, the power of sunlight is most useful when there is the least of it, i.e., early in the morning and late in the afternoon. In the middle of the day, there is generally more sunlight available than needed. It is at the margins of the day that sunlight is truly the most valuable, both for circadian stimulus and for electric lighting energy savings. For example, a diffusing skylight with a three-dimensional shape, such as a dome, bubble or pyramid, has yet another major advantage in delivering illumination to the space below, in that it can better capture lower angle sunlight in the morning and afternoon.

Figure 1.
Figure 1. Vermeer’s painting Girl Reading a Letter by an Open Window, circa 1658, illustrates the flow of daylight from a single “punched” window onto room surfaces. Photo: Wiki Commons via Gemäldegalerie, Dresden

Daylight flowing horizontally into a room from a window mounted in the middle of a wall is most people’s default image of a “daylit space.” This configuration is exemplified by the 17th century Dutch artist Jan Vermeer, in his famous painting Girl Reading a Letter at an Open Window (Figure 1). The artist faithfully portrayed the illumination provided by the windows, set into thick masonry walls, typical of an old Dutch home. The horizontal flow of light first highlights vertical surfaces, such as the girl’s face, the wall behind her, and the curtains hanging between her and the viewer, and then reflects around the room, softening shadows.

Vermeer’s painting illustrates the common problem of contrast glare. In the very same scene, had the artist been looking perpendicular to the window, rather than parallel to it, the girl would have been in deep silhouette and the bright window would have created strong visual contrast with the shadowy wall around it. The window enclosure, sill and jambs receive the most daylight, while the inside wall adjacent to it receives the least. Any illumination from the window has to first bounce all around the room and its furnishings before returning to that wall, greatly diminished in power. Thus, a single window, one hole in a wall, can be a source of visual discomfort, a.k.a. glare, due to the visual contrast between the bright opening and the darker wall surrounding it.

Electric lighting designers fret a great deal about this problem. They tend to think of windows as the source of the problem, rather than the solution. I have seen far too many electric lighting designs where additional light sources are placed on or
above the wall next to the windows to try to combat this excessive contrast. The hope is that the additional electric light will brighten the shadows and reduce the contrast and visual discomfort. The reality is that when the window is at its brightest, creating the most contrast with ambient lighting conditions, it can overpower any electric light source, which looks simply feeble in comparison.

Photo: Lisa Heschong

The problem of extreme visual contrasts created by “punched windows,” i.e., a single window set into the middle of wall, is especially common in sidelit office spaces, where deep spaces and low ceilings reduce interreflections and the amount of daylight which could return to brighten the wall holding the windows. One commonly employed solution is to make the windows bigger. “Strip windows,” i.e., those that run continuously along a wall, avoid the dark corners, since there is no longer any opaque wall between each window opening, as there is with punched windows. All glass walls, often called “curtain walls,” reduce the problem even more: the walls perpendicular to the glass, plus the floor and ceiling surfaces, now act like a giant sill and jamb, reflecting the daylight deeper into the space and reducing the visual contrast of the window surround. The huge glazing areas of curtain walls can create other building performance problems, such as thermal discomfort and resulting HVAC energy use, but they do help to solve the visual contrast problem that punched windows create.

It turns out the best way to balance the visual conditions in a daylit room is usually with more daylight. That is because the daylight is always in sync with itself. Christopher Alexander, the architectural theorist who wrote A Pattern Language, gave each one of the many patterns in his book a succinct and descriptive name. One of the most memorable patterns is called “Light from Two Sides,” which postulates that to be successful, a well-daylit room should have windows positioned on at least two different room surfaces. A classically trained photographer would translate this concept into “fill light,” meaning that a second window, like a photographer’s secondary light, provides illumination from a different direction, gently brightening the shadows created by the first. The magic of daylight is that this always works, because the intensity of the light from the two window directions rises and falls together as the day progresses.

Blinds and louvers can also help to redirect some of the daylight, creating a secondary direction of daylight within the room. With vertical blinds, some daylight can be redirected sideways, while with horizontal blinds, some daylight can be redirected upward or downward. All of these design strategies work to improve the daylighting conditions and visual comfort within a room without significantly reducing the amount of daylight illumination.

There are other design strategies that also take advantage of the power of daylight, rather than trying to fight it. For example, a three-dimensional object placed next to a punched window—not in front of it, but next to it—will pick up and reflect some of the daylight spilling beyond the width of the window. The object, which could be a potted plant or a sculptural form, is then also more interesting to look at and a bit brighter than the shadowed wall behind. Therefore, the eye sees less contrast, and the window becomes less glaring.

Another trick is to visually soften the edges of the window, with elaborately shaped moldings, gauzy curtains or perforated vertical blinds. If the surfaces immediately adjacent to the window are painted white, they will both help to redirect more of the daylight in multiple directions, softening the shadows in the room, and also create a brighter and wider visual frame around the window. A white visual frame around a window view is much like a traditional white mat around a photograph, or the white margins around the edges of a printed page, providing the eye with a place to rest before returning to consider the information imbedded in the view.

The simplest rule of thumb for lighting design is: “put the light where you want people to look.” People are naturally phototropic. Their eyes are drawn to the brightest object in sight. Thus, an electric lighting designer will often focus spotlights to highlight a painting, a sculpture or a gorgeous vase of flowers in a room. In a well daylit room, it is essential to understand that the window view is usually the most attractive thing to look at. Thus, allowing the window view to be brighter than other surfaces in the room is not just acceptable, but expected. A well daylit space with a gorgeous view will bathe the view window in a balance of daylight that perfectly frames it, while allowing the view to shine as the key visual feature of the space.


Lisa Heschong

Lisa Heschong

Lisa Heschong, Fellow IES, is a member of the IES Daylight Metrics Committee and speaks widely about the science behind daylight and views. She recently published Visual Delight in Architecture: Daylight, Vision and View (Routledge 2020) from which this article is... More info »