By careful planning, daylight can be increased with a few carefully placed windows and appropriately selected interior finishes at a very moderate cost. The energy supply for daylight is free. Keeping windows and interior surfaces clean for better reflectance may add something to maintenance costs. However, because daylight is not available at night, artificial lighting sources must be provided as well.
Through centuries of trial and error and sensitivity to local sites and climates, indigenous builders historically have used daylighting effectively and well. Bill Lam, lighting designer and author of two classic texts on daylighting and architecture, cites Boston’s John Hancock Tower as an example of how modern architects get it wrong. The building is mirrored to keep light out, with the long sides of the building facing east and west. The glazing extends from the floor to about one foot above the ceiling. To lessen glare and control overheating, interior blinds are lowered, and since people tend to be somewhat lazy, the blinds are left down and the lights are left on all the time.

Bill Lam believes that, particularly in North America, we have hardly begun to take advantage of the energy saving potential of exploiting daylight. Now that the California energy crisis has created renewed interest in energy conservation, he believes that sunlighting and good energy design can regain a higher priority. He sets forth several principles for designing pleasant, delightful, luminous environments that are energy conserving and economical.

Basic daylighting can consist solely of making windows and skylights large enough for the darkest overcast days, as in many northern European buildings. True daylighting is more accurately defined as passive solar design. Daylighting involves the conscious design of building forms for optimum illumination and thermal performance. It is most challenging in workspaces such as schools, offices, laboratories, libraries, and museums with varied and demanding tasks, and least challenging in public spaces where comfort standards are less stringent and controlled lighting is less important.

Interior spaces need high ceilings and highly reflective room surfaces for the best light distribution. The light source the sun is constantly changing in direction and intensity. Ground-reflected light is ideal because it is both bright and diffuse, but adjacent buildings or trees often shade the ground. Light bouncing off the ground outside ends up on the ceiling.

Daylighting considerations affect the architecture of the building exterior, determining the amount of fenestration and its appearance on the building facade. The building orientation and shape should be designed with daylighting in mind.

In a daylighting design, heat and light are controlled through the form of the building. For example, in a Middle Eastern mosque located in a sunny climate, limited sunlight enters the building through small windows high in a decorated ceiling, and then is diffused as it bounces off interior surfaces. The large windows in a Western European cathedral, on the other hand, flood the interior with light, colored and filtered through stained glass. To take advantage of sunlight without an excess of heat or glare, the building should be oriented so that windows are on the north and south sides. Frank Lloyd Wright shaded the west and south sides from the most intense sun with deep overhangs. To study where light is coming from in an existing building, look at the shadows (this can be done with photographs). 

Daylighting must be integrated with the view, natural air movement, acoustics, heat gain and loss, and electric lighting. Operable windows offer daylight and natural airflow, but allow in noise. Daylighting doesn’t save any energy unless lights are turned off, so lighting zones must be circuited separately, with lights turned off or dimmed when the natural light is adequate and left on where proper amounts of daylight aren’t available.

To be successful, the daylighting scheme must allow daylight to penetrate into the building, and ensure that daylight will be available whether the sky is overcast or clear. An atrium can be used to bring large quantities of direct or reflected sunlight down into a building interior. In some buildings over two stories high, light wells bring natural light into the interior building core. Light wells are smaller spaces than atriums that can be employed with skylights, clerestories, or window walls.

Sunlight is a highly efficient source of illumination, and a comparatively cool source. Daylight varies with the season, the time of day, the latitude, and weather conditions. More sunlight is available in summer than in winter, and the day’s sun peaks at noon. An overcast day is very different from a day with a clear sky, and conditions can change several times during a day. Of course, sunlight is unavailable until dawn and after dusk. 

On a bright day, sunlight provides illumination levels 50 times as high as the requirements for artificial illumination. Direct sun may be desirable for solar heating in winter, but the glare from direct sun must be managed. Indirect sunlight produces illumination levels between 10 and 20 percent as bright as direct sun, but still higher than needed indoors. Daily changes in daylight controls and seasonal adjustments in the size of daylight openings may help accommodate the changing nature of daylight and the overabundance of sun. Mirrored and low-transmission glass won’t solve glare problems as well as shading does.

We can’t look directly at the sun, and it is almost impossible to carry out fine visual tasks like reading or sewing in the glare of direct sun. The sky is brilliant with scattered sunlight, which is often bright enough to distract the eye. Direct sunshine bleaches colors, and the heat from direct sun in buildings is often intolerable, especially in summer. Clouds often obscure the sun’s glare partly or completely.

The bottom line of daylighting design is to achieve the minimal acceptable amount of natural illumination when the available daylight conditions are at their worst, and to screen out excess illumination at other times. For example, the daylight available at 9:00 in the morning in December is used as the basis for the worst-case conditions. The designer also seeks to provide adequate daylight under average sky conditions, with artificial lighting supplying added light for less than average daylight conditions. The goal is to achieve adequate natural illumination during the majority of the daytime hours the space is occupied. The design is balanced with supplementary artificial lighting in dark areas rather than with an oversupply of daylight in lighter areas.

Daylighting relies mostly on diffused sunlight or reflected, indirect sunlight to illuminate building interiors. The amount of natural light available within a room depends on how much sky is directly visible through windows and skylights from a given point in that room. The amount of indirect light from the sky also depends on how bright the visible areas of sky are. The sky at the horizon is about one-third as bright as the sky directly overhead, so the nearer the window is to the ceiling, the more light it will gather, as long as it isn’t blocked by trees or buildings. Skylights are very effective at collecting the brightest light.

The shape and surface finishes of a space have an impact on daylighting. Tall, shallow spaces with high surface reflectances are brighter than low, deep rooms with windows only at the narrow end and with dark, cold surfaces. It takes fewer bounces off the walls for light to get deep into a room when the windows are high on the wall. High windows distribute light more evenly to all walls and allow light to penetrate into the interiors of large, low buildings. The ceiling and back wall of the space are more effective than the side walls or floor for reflecting and distributing daylight. Remember that tall objects, such as office cubicle partitions or tall bookcases, can obstruct both direct and reflected light.

The level of daylight illumination diminishes as it goes deeper into the interior space. In order to reduce glare, you need to design a gradual transition from the brightest to darkest parts of the space. The amount of light about 1.5 meters (5 ft) from the window should not be more than ten times as bright as the darkest part of the room. In a room with windows on only one wall, the average illumination of the darker half of the room should be at least a third of the average illumination level of the other half with windows. By using windows on more than one side of the room, along with interior light wells, skylights, and clerestories, you can achieve more balanced daylighting. It is best to allow daylight from two directions for balance, preferably with the second source at the end of the room farthest from the main daylight source.
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