Natural Light in the Library
Lighting in Libraries
The first requirement for library lighting is to provide enough light to accomplish a visual task such as reading. For daylight, this means tuning the aperture designs to minimize solar heat gain while achieving the illumination levels required for visual acuity.
The second requirement is that the contrast brightness of other objects within the field of view must not be excessive, such that the library user can view the task comfortably and not become visually fatigued over time.
The amount of daylight and its direction at the window or roof of a building vary during a typical day as the sun moves, and seasonally as the sun's predominant position in the sky changes. In general, people enjoy variable daylight and the connection it provides to the natural environment. However, it is important in libraries to maintain a relatively constant light level for visual tasks so that short term variability does not become distracting or result in inadequate illumination.
Roof monitors are popped-up extensions of the roof, with vertical glass areas. Large roof monitors often appear to be forms of vertical extensions of the ceiling, and can provide dramatic high internal spaces in special areas.
One advantage of roof monitors compared to conventional skylights is that there is less risk of water leakage. Furthermore, roof monitors avoid the strong solar impact of the overhead sun and can be designed with exterior sun control devices as necessary to diffuse the direct sunlight.
Libraries are ideal building types for the extensive use of roof monitors because of the common one- or two-story configuration of the floor plan. Roof monitors can also provide daylight to the lower floor level in a two-story facility by carefully locating openings on the second floor below the roof aperture areas. Daylight easily penetrates the normal 25-30 feet (7.6 to 9.1 meters) from roof level to first floor level.
Skylights can be successful daylighting roof apertures provided the direct sun is prevented from coming within view by washing down walls or striking floor or table surfaces. In addition, because of the heat content of direct sunlight, the skylights should be relatively small in area and should be accompanied by large adjacent diffusing surfaces.
If deep apertures are not possible in a design, an alternative is to equip the skylight with some type of sun control device, such as exterior louvers or movable translucent shades. Normally, to be effective, the sun control device should be operable and should respond to the position of the sun to reflect all direct sunlight otherwise incident on the glass.
Like roof monitors, skylights can also be used creatively to render dramatic space in a library and replace the normal appearance of a ceiling containing electric light fixtures. The electric light fixtures, required for times of low daylight availability, can be visually hidden within the skylight apertures.
The perimeter spaces of the library can be effectively daylit for approximately 20 feet (6.1 meters) from the exterior wall by using windows and clerestories (high windows). Generally, the taller or higher the window, the deeper will be the daylight penetration into the space.
Clear glass is preferred for daylighting, but this in turn requires carefully designed exterior sun control devices to provide adequate shading. Although internally mounted shades and blinds reduce the high intensity and heat content of direct sunlight, the most effective sun control device is the exterior sunshade.
An internal shade, even a light-colored fabric or blind, reduces solar heat gain by about one-third to one-half of the incident solar energy. An exterior shade will create a reduction of 80 percent of the incident solar energy.
The south-facing window (north-facing in the southern hemisphere) is easiest to protect because the sun is at relatively high angles in the sky for most of the day relative to this orientation. Horizontal sunshades located above eye level easily shade the window and create the least obstruction to view and daylight.
Sun control at north-facing windows (south-facing in the northern hemisphere) should not be ignored in hot climates because late afternoon summer sun will penetrate them. Simple fixed vertical elements are adequate to control this type of direct glare.
East- and west-facing windows are more difficult to shade because the sun is low in the sky in the mornings and afternoons, and the angle of incident sunlight is almost perpendicular to the glass. For these windows, some kind of vertical device or operable shutter is generally needed.
Daylight through east or west windows is always best when the sun is on the opposite side of the building. When it is not, there may be no direct light at window level if a sunshade is screening the perpendicular, low-angle sunlight. This problem can be solved to some extent through the use of clerestories, or window openings placed high in the wall above the normal window location.
Beyond contributing to higher design quality, daylighting has the added benefit that its energy is locally available and renewable, unlike the power provided by conventional electric generation plants. Therefore daylighting design is an important component of sustainable ("green") design in buildings.
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Edward Dean, AIA is a practicing architect in Berkeley, California. He has led project design teams on many academic and public library projects and has written extensively about energy and green design.
This article was excerpted with permission from Daylighting Design in Libraries, provided through the Libris Design Project, supported by the U.S. Institute of Museum and Library Services under the provisions of the Library Services and Technology Act, administered in California by the State Librarian.