Description

Modern libraries are places for learning as well as entertainment. Libraries demand sophisticated lighting systems that are of high visual comfort and flexible enough to respond to the numerous visual tasks taking place in libraries. The daylighting of libraries has some unique physiological requirements that differ from many other building types. The requirements relate to the quantitative and the qualitative aspects of visual performance defined by illuminance levels, distribution of light, glare and visual comfort.

Lighting in libraries is important because of many reasons. Sufficient light levels must be provided for the users to be able to write on horizontal and read on vertical surfaces such as bookshelves. Design consideration may include avoidance of all forms of glare, excessive sunlight penetration and too high or too low contrasts. The dynamic character of daylight must be taken into account in order to maintain the quantitative and qualitative aspects of illumination.

General Lighting Requirement for Libraries

Illuminance Levels

The goal of lighting in libraries is to facilitate the learning experience by providing adequate and comfortable light levels that can be endured for prolonged periods of time. The main visual tasks in libraries are reading and writing texts, differing in size, shapes and contrast levels. Reading tasks may vary from children’s books printed in 10- to 14-point type on matte paper to newspapers printed in 7-point type fonts. Other tasks, such as studying illustrations and handwritten pages varying in contrasts are possible. An illuminance level of 300–500 lux is recommended for reading rooms.

Both the stacks areas and the general reading areas require toplighting. The stacks need toplighting because the book shelves block light coming from the sides. For the general reading area it is also recommended to have uniform lighting in order to allow for flexibility of use of the space.

Light Uniformity

Uniform light is the most widely used form of illumination in libraries. Such a strategy provides ideally the same illuminance level for the entire workplane where a specific visual task is performed. In practice this is not always possible and, inevitably, there is always variation in illuminance levels on the same workplane and between workplanes. To address this issue of light level variations, lighting standards in several countries prescribe maximum uniformity ratios (fig. 1), ratios between lowest light levels to average light levels in the room that should not be exceeded.[1]

1 Recommendations for illuminance uniformity

While it is impossible to obtain uniform illumination through side windows, it may be possible to obtain more uniformly distributed natural light using a number of top daylighting strategies. Due to the dynamic quality of daylight, light levels are constantly changing according to the time of day and seasons. As a result, the distribution may be uniform but the daylight levels are never constant throughout the day.

Daylighting Strategies

Sidelighting

The two available sidelighting devices are side windows and clerestories.

Side windows

Side windows not only supply daylight but also fulfill the other function of a window, i.e. views and often ventilation. As a result they are the most widely used type of fenestration. From a strictly daylighting point of view, side windows are, however, one of the most problematic daylighting strategies because of the rapid degradation of daylight levels as the distance from the window wall to the interior of the room increases. Daylight levels become very low beyond a distance of 4.5–6 m from the window. A well-known rule of thumb, which relates the size of the window to the depth of daylight penetration, suggests that the depth of the effective daylit zone where daylight levels are more or less significant roughly corresponds to twice the height of the head of the window starting from the floor. However, this rule of thumb in truth only applies to a curtain wall situation where the window extends across the entire width of the room and it does not refer to a singular small window within a wall. Beyond the issue of the depth of daylit zone, a problem that is often encountered with a singular side window is discomfort glare, i.e. discomfort caused by excessive contrast between the high brightness of a singular window and an interior object placed within the window range or an excessive contrast between the window and its darker surrounding surfaces. Tall windows allow for deeper daylight penetration and much improved visual comfort (fig. 2).

2 Tall windows at the Grainger Engineering Library (Woolen, Molzan and Partners, 1994), on the campus of the University of Illinois at Urbana-Champaign allow deeper daylight penetration andimproved visual comfort.”

In order to overcome the problem of excessive contrast between the window and its adjacent surfaces, it is often recommended to harvest daylight from two different directions such as from two corner side windows. In doing so, the daylight coming from the second window reduces contrast by increasing daylight levels on the surfaces within the room.

Lightshelves may be used to capture some of the excessive daylight in the front of a room and deflect it deeper into the room by means of a highly reflective ceiling (fig. 3). In such a case both the upper side of the lightshelf and the ceiling need to be of high light reflectance. Lightshelves may be interior only, exterior only or combined. In addition to its role of balancing the daylight distribution across a room, a lightshelf may provide shading and cut on excessive glare by obstructing a portion of the sky seen from a certain vantage point inside a room.

3 Lightshelves deflect light upward and deeper into a room.

Clerestories

Clerestories are side windows placed high in a wall resulting in a deeper daylight penetration within a room (fig. 4). Clerestories may be combined with side windows to provide a more balanced daylight distribution into the stacks area or the reading room of a library (fig. 5).

4 Clerestories in the reading room of the New York Public Library, Carrère and Hastings, 1911

5 Clerestories combined with side windows provide a more balanced daylight distribution inside a room.

Toplighting

Top daylighting applications harvest daylight from the roof of a building and channel it inside, providing henceforth a better daylight distribution throughout a room. Under overcast sky conditions, the sky is brighter at zenith then it is at horizon. Consequently the amount of daylight on the roof of a building is much higher than the amount incident on the side facade of a building. Under sunny clear sky condition, the amount of daylight incident on a horizontal surface depends on the altitude angle of the sun and orientation of the facade. The higher the angle the higher the amount of daylight harvested on the roof. The amount of daylight striking vertical surfaces depends, in this case, on the facade azimuth angle, the angle between the normal to the facade and the projection of the sun on the ground.

Skylights

Skylights are the most popular and simplest of the toplighting devices. They consist of apertures within the roof of a building covered with a transparent material (fig. 6). In addition to the transmittance properties of the skylight cover, the efficiency of a skylight system depends largely on the geometric proportions of the skylight well and the light reflectance property of the walls of the skylight well. Given a certain size of the opening of the skylight at the top, a skylight with a splay angle of less than 90 degrees is more efficient than a straight skylight of the same depth.[2]

6 Round skylights at Pierce College Library in Los Angeles, California, HMC Architects, 2013

In the event of multiple identical skylights, it is recommended[3] that spacing between skylights should not exceed the interior floor-to-ceiling height of a room for a more even daylight distribution throughout the room (fig. 7).

7 Splayed angle skylight (left); Recommended spacing between skylights for light uniformity (right)

Roof Monitors

Roof monitors (fig 8) are openings with vertical glazing within a roof. The solid and opaque part of the roof monitor may be sized to block sunlight when it is not wanted. As such they allow for greater control of the daylighting condition. Roof monitors may be orientated in any direction to suit a particular need. If they are faced toward the north, only diffuse daylight may be collected. Toward the east and west, a low sun angle may be easily deflected in order not to create visual discomfort inside the room.

8 One-sided roof monitor (left); Doubled-sided roof monitor (right)

Sawtooth Toplighting System

Sawtooth roofs, in which opaque modular elements are combined with transparent surfaces, whether inclined or in various shapes, are especially popular in commercial, industrial or educational facilities such as gymnasia, libraries, museums, warehouses etc. (figs. 9, 10).

09 Sawtooth lighting system from the exterior at the Los Angeles County Museum of Art, William Pereira, 1965

10 The South Jamaica Branch Library in Queens, New York, Carl Stein and Elemental Architecture, 2010, is equipped with a sawtooth lighting system.

Sawtooth systems generally harvest light from one direction, are suitable for large spaces and tend to provide a unidirectional daylight distribution. Similar to roof monitors, sawtooth systems may allow sunlight inside or may block it when it is not desired. The sizing of sawtooth systems involves day­lighting and energy considerations. From a daylighting perspective, the spacing between the various openings of the sawtooth system is 2½ times the smallest height of the room for more uniform daylight distribution (ill. 11).[4]

11 Recommended spacing for sawtooth systems

Contemporary daylighting solutions often involve sophisticated design strategies, addressing specific lighting and climate conditions. The following three examples present but a small selection from a wide range of options. They illustrate dramatically different approaches to library daylighting design from different regions of the world. The daylighting options presented in these three case studies range from an emphasis on ceiling height and solar control, to dramatic core daylighting using lightwells and a tensile fabric structure to shade an all-glass building skin. In each case the key aspect is control of daylight penetration and distribution.

Skylight and Daylight Controls

The Public Library in Champaign, Illinois, USA, designed by Ross Barney Architects and opened in 2008, has a total area of 12,260 m², seats 431 persons and includes 87 computer stations, five public meeting rooms and a collection of 285,000 books and AV materials. The building’s 5.5 m ceiling height allows daylight to flood the entire first floor of the library. Daylight controls are incorporated within the overhead lights throughout the ground and first floors and turn off the electric lights when light levels reach 500 lux. A large linear overhead skylight above the stairwell floods the core area of the building with natural light throughout the day. The west side of the building incorporates brick pillars and copper mesh shades that block the sun while allowing diffuse daylight inside the reading rooms of the library.

The glazing on the south side of the building is fitted with a honeycomb structure, an angular selective technology that redirects light rays, providing high diffuse light transmission while keeping direct solar rays from entering the building (fig. 12). This ray redirection technology provides both more light and better light. Up to 70 % light transmission can be achieved, depending on the insert and glass types specified, while reducing glare and improving visual comfort. The honeycomb structure performs like a series of very small louvers, but in cellular rather than linear form. This structure, combined with a proprietary polymer composition, provides the unique advantage of dynamic performance – increased solar heat protection at peak hours – without the dramatic reduction of invisible light that occurs with other dynamic technologies such as switchable glass. Angular selective glazings are designed to attenuate direct solar radiation, the main source of solar heat gains and glare, while transmitting a significant amount of diffuse daylight.

12 Public Library, Champaign, Illinois, Ross Barney Architects, 2008. The glazing on the south side of the building is fitted with a honeycomb structure to partially redirect light rays (left); Diagram of glazing unit with inserted honeycomb layer (right)

Skylights with Customized Shading Membranes

The existing King Fahad National Library in Ryadh, Saudi Arabia, was refurbished and extended by Gerber Architekten in 2013. The cubic shape of the new building completely surrounds the existing building, thus presenting a new structure while preserving the old one (fig 13). The original concrete dome of the old building was replaced by a steel and glass dome, allowing more light into the central portion of the old building. The former roof of the existing building now provides a reading landscape, and the open-access sections on the third floor of the new building can be accessed via bridges from the reading area. Everything is covered by a new roof, punctuated by skylights under which white membranes provide shade and gently distribute the light throughout the entire interior. The exterior of the new facade consists of customized cladding made up of concave and convex rhomboidally shaped textile awnings that filter and soften the harsh sunlight of the region.

13 King Fahad National Library. The refurbishment and extension of the existing library involved a completely new lighting concept. Textile awnings cover the facade and provide shade (top left); Interior of the new library space (right)

Large Lightwell

Central Library, TU Delft, the Netherlands (Mecanoo, 1998), is the largest technical-scientific and the central library of the Netherlands. The green roof of the library makes the building a landscape feature, sloping gradually from the ground upward and bearing a cone-shaped structure. This structure provides cover for a large lightwell (fig. 14) that permits daylight to enter the middle section of the building. Daylight fills the building through the central lightwell and the glass walls on three sides.

14 Central Library, TU Delft, the Netherlands, Mecanoo, 1998. The lightwell in the center of the building admits daylight to the core of the building, even reaching the lower floors.

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Originally published in: Nolan Lushington, Wolfgang Rudorf, Liliane Wong, Libraries: A Design Manual, Birkhäuser, 2016.