FOUR
Natural Lighting
Coauthor: Rachel Aljilani
Natural lighting provides a space with a sense of interest, warmth, and excitement besides saving energy and creating healthier interiors.
We were born of light. The seasons are felt through light. We only know the world as it is evoked by light.…To me natural light is the only light, because it has mood—it provides a ground of common agreement for man—it puts us in touch with the eternal. Natural light is the only light that makes architecture architecture.
—Louis I. Kahn
Natural lighting uses sunlight and diffuse radiation from the sky to provide light inside buildings. It should be considered essential for every new building. Natural light adds delight to our lives by providing movement, change, and connection to the outdoor environment. Global greenhouse gas emissions and the environmental impacts of mining, power production, and power distribution can be greatly reduced by integrating natural light; and unlike artificial light, natural light works when the power grid goes down.
Interest in natural light has increased in the last few years as the many benefits of natural light have become clearer. Studies have proven natural light can improve moods, spirit, performance, and health. Students in schools with natural light perform better, with marked improvements in both English and mathematics. Natural light in work settings tends to increase the productivity of workers and reduce absenteeism. Lockheed found productivity at one facility increased 15 percent when they moved from a conventional building into a naturally lit building, and days off work dropped by 15 percent. The Herman Miller Company found even higher productivity gains in its daylit furniture factory. In larger facilities, human resources are often 90 to 95 percent of the company’s operating budget (far exceeding the initial cost of the building and energy costs of operation), so small investments in natural light up front will yield considerable savings over the long run. Surveys consistently show that people prefer natural light and desire a connection to it. In most cases, they want a view to the outside rather than a luminous panel. User control, flexibility, and adjustable controls are also preferred.
Visible Light
Measured in wavelengths, the typical human eye perceives a portion of the electromagnetic spectrum from violet to red light from about 380 to 750 nanometers. Different creatures can see variations of this range; for instance, ultraviolet light can be sensed by most insects and some birds, while some snakes can see the infrared heat given off by warm-blooded bodies. For the most part, natural lighting is concerned with the visible spectrum—although studies have recently identified the number one cause of bird fatalities to be buildings due to the specular reflection from glass window and wall systems (see figure 4.1). These reflections, rather than the usual suspects—cats, vehicles, or pesticides—can be minimized with good design. Since birds can see the ultraviolet end of the spectrum, glass can also be coated with a UV pattern to allow them to see the glass while it remains transparent to humans.
Fig. 4.1. Since humans and birds can see different parts of the electromagnetic spectrum, glass can be coated with UV patterns to reduce bird fatalities from buildings. This chart shows annual bird fatalities by source as a percentage. Annual US bird fatalities are one hundred million and worldwide one billion from buildings. (a) Percent bird fatalities by cause. (b) Wavelengths in nanometers.
Healthy Light
Natural light is necessary for humans. Physiologically, natural light sends a signal to the pineal gland to stop producing melatonin, which the body releases at night to help the body rest. In higher latitudes, short winter days are known to cause seasonal affective disorder (SAD), a depression characterized by lethargy. SAD can be treated with early-morning exposure to light levels of 2,000 to 12,000 lux, depending on the severity of symptoms. Buildings with poor natural light can cause SAD even in the summer, and lethargy is the number one symptom in the majority of sick building syndrome cases.
While exposure to excessive amounts of direct sunlight is not advised, there is a human need to absorb ultraviolet light for the body to produce vitamin D. Bone disease due to vitamin D deficiency has been linked to the lack of exposure to natural light. UVA (380 to 320 nm) and UVB (320 to 280 nm) are known to cause certain types of skin cancer, but this end of the spectrum is not often transmitted through glazed apertures. Most ultraviolet radiation is scattered by the ozone, so a minimal amount reaches the earth’s surface. Changes in the atmosphere are likely to affect UV exposure levels, as has been the case in areas of Scandinavia where ozone depletion has begun to occur. In Canada, students under full-spectrum light attended school an average of three days more per year and had nine times less dental decay than students in rooms with average lighting conditions. The rapid rise in myopia over the past fifty years may also be driven by the increased dependence on artificial lighting systems.
Sources of Natural Light
The ultimate source of all natural light is the sun, yet only part of the light used for natural lighting is received as direct sunlight. The rest arrives as indirect or diffuse radiation reflected off dust or water particles in the atmosphere or reflected radiation bounced off other structures or features of the landscape. The proportion of each varies as a function of both building orientation and design, variation in ground surface, cloudiness and type of clouds, atmospheric clarity, and the sun’s daily and seasonal path and position. The main factors influencing natural light available for use are microclimate and solar altitude (refer to Microclimate for a brief overview of how these affect a specific site). The same tools described on page 54 for solar geometry can be used to determine shading and solar obstructions. North and south light also vary (see figure 4.2), with diffuse and direct sunlight being influenced by the sky exposure angle (see figure 4.3).
Fig. 4.2. This spectral-energy-distribution diagram illustrates the color difference between light available for daylighting under different sky conditions.
Fig. 4.3. The brightness distribution is typically about ten times greater near the sun than at the darkest part of the sky, which is 90° in the other direction.
Surprisingly, even at 46°N latitude, the average illumination level under overcast skies is 7,500 lux, fifteen times as much is needed for average indoor tasks. In the 1950s, electrical engineers increased the use of electric lighting in buildings dramatically to enhance visual performance and increase sales of light fixtures. figure 4.4 illustrates the limits of only increasing the intensity level of light. The abundance of natural light is not an availability problem, but a challenge of planning and geometric manipulation to capture the resource and use it effectively. Once the resources are understood, it is possible to naturally light almost any application, from single-family homes to high-rises and industrial plants.
Fig. 4.4. (a) Increased illuminance beyond 500 lux or 47 foot-candles yields minimal increases in visual performance. (b) The brightness distribution is typically about three times greater at the zenith than the horizon for an overcast sky.
Units Used in Natural Lighting
Fig. 4.5. Concepts and common terms used in natural lighting.
Table 4.1. Common units used in natural lighting.
Terms Used in Natural Lighting
Many concepts are involved in lighting, as shown in figure 4.5 and table 4.1. The first is luminous intensity, a measurement of light in a certain direction, measured as candela. Luminous intensity is analogous to pressure in a hydraulic system and voltage in an electric system and represents the force that generates the light that we see. Since luminous intensity is characteristic of a light source, it is not affected by a viewer’s perception.
Luminous flux is the rate of flow of visible light per unit time and is measured in lumen (lm). Analogous to flow in a hydraulic system and current in an electric system, luminous flux is a measure of photometric power. Unlike luminous intensity, the sensitivity of a viewer’s eye will cause variation in perceived luminous flux.
Brightness is a factor of the level of illumination and the reflectance of the surface we are viewing. A white surface may have a reflectance of 90 percent, while a dark black surface may reflect less than 10 percent. Brightness is known as the sensation by which an observer distinguishes differences in luminance or the quantity of light emitted or reflected in a given direction measured in candela/m2 or candela/ft2.
Lighting is described in terms of illuminance, the density of luminous flux incident on a surface, measured in lux (lumens/m2) or foot-candles (lumens/ft2). Illuminance or illumination is the amount of light spread over a surface or the light output falling on a working area.
Color
Lighting color also matters. Most people prefer natural light color with diffuse skylight perceived as white, while direct-beam sunlight may seem warmer or redder. Lightbulbs also output light in different colors. The quality of the light color can affect the ability to see and understand colors. When objects are white, black, or any shade of gray, the objects are technically colorless. When objects appear colored, they are selectively absorbing pigmentation and reflecting or transmitting to the eye only the objects’ hue. Hue can be defined as the attribute by which we describe colors such as red, blue, or green. When white is added to a color hue, it is a tint; adding black creates a shade.
Reaction to color will vary as much as temperament from one person to the next, so color must be used carefully. Typically, white sources of illumination are used unless a particular mood or atmosphere is desired. The eyes have an amazing ability to adapt to low levels of illumination and still be able to decipher color, even though shaded. On the other hand, high levels of illumination tend to wash out or tint colors, giving them a less saturated appearance.
Fig. 4.6. Color temperature in Kelvin of common light sources.
Warm and cool colors also have a perceived temperature and can actually be used to perceptually heat and cool spaces. A cold climate would benefit from warm-colored surfaces, whereas a warm climate should use cool-colored surfaces to take advantage of this sensitivity. Cool colors are also known to be calming and can change the perception of time; dull, repetitive tasks may be more tolerable in a cool environment. Warm colors make people more alert and have been considered more appetizing than cool colors.
Color choice can be influenced by cultural meaning. For instance, white is appropriate for a funeral in China, while black is considered appropriate attire for a funeral in the United States, and a bride in India is likely to wear red rather than white, as is common the United States. Color choice depends on the activities, microclimate, and people’s personal or cultural preference.
The color temperature scale shown in figure 4.6 was derived by heating a black body, called a light-absorbing body. When a black body is heated, the first color it turns is dark red, then cherry, then orange, and so on, until it glows bluish white. Using a black-body temperature scale, we can assign color temperatures in Kelvin, or K. Color temperature only applies to heated objects, so illumination sources such as fluorescent lamps are assigned a correlated color temperature (CCT). The quality of light can be measured and described with the color-rendering index (CRI), which is a two-part concept based on color temperature and a number that indicates how closely the illuminant approaches the standard (daylight at the same color temperature). Table 4.2 describes the various color characteristics of common illuminants.
Glare
Glare is a subjective response to excessive brightness and can be described as visual noise usually attributed to a contrast problem. Direct glare is caused by light sources that are bright enough to cause discomfort or loss in visual performance. There are both disability glare, which impedes performance, and discomfort glare, which produces physical discomfort. Bright windows or a bright light behind a computer user can create disabling glare on a computer screen.
Table 4.2. CRI, CCT, and color affect of light sources.
Direct glare is also dependent on the proximity from the offending light source to the user’s center of vision. For this reason, there has been considerable research and development of indirect lighting fixtures. figure 4.7 illustrates direct glare caused by a window. Think of talking with someone when the sun is behind them; though you would like to be able to focus on the person, all you see is a dark silhouette.
Fig. 4.7. Direct glare.
Indirect glare is caused by reflections on glossy surfaces (figure 4.8). Specular reflections in particular appear as though someone has put a veil of light over the area you are focused on, and it is simply a mirror-like reflection of the light source. Matte and flat finishes can be used to reduce indirect glare, which diminishes the effectiveness of product displays, signs, and advertising. Placement of computers, registers, projector screens, and other equipment and fixtures should be designed to minimize glare, and then strategies to control glare should be utilized.
Fig. 4.8. Indirect glare.
Peripheral glare is the most subtle and often overlooked type of glare that occurs from the physiological response of the human eye to various light levels (figure 4.9). The amount of opening of the iris is determined by peripheral light, not the object you are focusing on. A bright spot at the very edge of your vision can give you the wrong iris setting for the specific task at hand. This is why the ubiquitous “can” lights are generally problematic. This type of glare is a major cause of eyestrain and can trigger falling asleep at lectures and meetings in response to the excess stress on the eyes.
Fig. 4.9. Peripheral glare.
Control of Glare
Since glare is a contrast problem, it is usually reduced by introducing light from multiple directions, thereby reducing the contrast. Direct sunlight is constantly changing, and owing to its high intensity is difficult to use without being diffused.
Washing a surface with light is a great way to diffuse light. This can be done in the case of wall washing where a tall, thin strip of windows is used near a room corner, thus washing the adjacent wall with light. Several of the shading devices discussed in chapter 3 on pages 90–91 can be used to control glare, including louvered shades, shutters, light shelves, roller blinds, curtains, and many other options, by diffusing direct sunlight. The ceiling can also be washed with light.
Fig. 4.10. Light aperture with rounded splay.
Elements of Natural Lighting
Apertures
Any opening in the building’s envelope is considered an aperture. Some apertures are primarily for circulation, like doors, while windows and skylights are often associated with light and ventilation. The surrounding surfaces of an aperture affect the distribution of natural light in a space. The rounded window splay shown in figure 4.10 helps to reduce the contrast of light and dark areas near the aperture by creating larger areas of intermediate light. Angled apertures are more efficient at increasing the intermediate light areas because the entire angled surface is illuminated by light entering the aperture. They also can increase the acceptance angle.
Reflectance from roofing materials may be used to increase light gain through roof monitors and clerestory windows. Black roofs may reflect 6 percent or less, while white roofs may reflect 70 percent or more. The push toward white, cool roofs to reduce air-conditioning loads in summer is also helping to improve the capability for natural light. Outside ground conditions may also affect lighting. A field of bright fresh snow or a water body to the south may increase light gain dramatically, and the changing leaf patterns or plant canopies over the year may limit daylight and alter light quality.
Glazing
Natural light usually enters a space through a glazed aperture that may have several functions. An understanding of glazing properties is needed to be able to choose the appropriate glazing product. Both the U-value and solar heat gain coefficient (SHGC) are discussed on page 49. The visible transmittance (Tvis) is of great concern when considering natural light. Tvis is a measure of the relative amount of sunlight passing through a glazing assembly, usually ranging from 0.3 to 0.8, with higher numbers representing larger values of light transmittance. Table 4.3 describes attributes of some common glazing assemblies. Choosing the proper glazing for natural light and passive heating and cooling is critical for integrated design. A one-size-fits-all glazing selection typically yields a trade-off between cooling and heating and is unlikely to perform optimally throughout the year.
Table 4.3. Properties of common glazing assemblies.
Natural Lighting Goals
The basic goal of lighting design is to provide a comfortable, healthful, pleasant, productive, and safe visual environment for people. This is not as simple as it might seem because the human eye and brain are complex and adaptable, and personal preferences, visual capability, and task requirements vary widely. The typical human eye can see from around 0.0001 to 10,000 foot-candles, with sensitivity declining with age. Vision can also be affected by corrective lenses, allergies, disease, and injury. But our vision is very flexible, and we can adapt to a broad range of lighting intensities and frequencies. Research has repeatedly shown that low light levels will not cause eye damage, despite what our parents said, but may lead to tired eyes or errors in reading. Although we can adapt to almost anything, it is good to design with the following criteria in mind, knowing that trade-offs and priorities will help determine the final natural lighting solution. These criteria include:
- Visual comfort.
- Healthful levels and frequencies of light.
- Light for efficient task completion and productivity.
- Minimal glare.
- Varied light levels and patterns—connection to outdoors.
- Maximum use of daylighting.
- Integration with other building-systems goals: passive heating, cooling, and ventilation.
- Minimal life-cycle energy use and cost.
- Low initial cost.
- Reduced operating cost and environmental impact.
- Maximum flexibility and user control.
- Maximum desired heat gain in winter.
- Minimal undesired heat gain in summer.
- Minimal use of nonrenewable energy sources.
- Well-developed educational materials and guides for use.
- Automatic control of switching to adjust to varying natural light levels.
A naturally lit office space can operate without artificial light, as seen in figure 4.11. Utilizing many of the techniques described in the following pages, this office incorporates an equator-facing dormer placed over the ridge, light-colored surfaces to help reflect and distribute light, and light shelves on the interior and exterior to extend the reach of natural light into the building; in addition, operable, insulating, translucent blinds are available to reduce unwanted glare if necessary.
Fig. 4.11. Naturally lit office space.
Approaches to Natural Lighting
Natural light was an integral consideration in building design and influenced building orientation and fenestration until the 1950s, when a long decline began. Using natural light is not difficult, yet it has been widely ignored in the United States because of subsidized prices for electricity and the ease of specifying all-artificial lighting and nonrenewable fuel–based HVAC systems. Daylighting books, articles, and tools have often neglected the connections to heating, cooling, and ventilation.
History is full of wonderful architectural achievements where natural lighting was a major consideration. It had to be because artificial lighting was too primitive, expensive, and difficult to have much effect. Some examples are shown in figure 4.12.
400
Fig. 4.12a. The upper clerestory with lower lit side aisles of the Roman basilica was adopted by the early Christian churches.
538
Fig. 4.12b. Large domes, vaults, and sumptuous surfaces with integrated lighting apertures characterize Hagia Sophia in Istanbul.
1300
Fig. 4.12c. Stained glass and stone curtain walls of Gothic architecture evolved as a way of maximizing natural lighting for low light conditions.
1400
Fig. 4.12d, e. The integration of lighting apertures, structure, and modular vaulting allow exquisite light and scale at the Alhambra in Granada, Spain and the chapel in Palermo, Italy.
1600
Fig. 4.12f. Movable translucent walls, grilles, and screens allowed classic Japanese architecture to have wonderful natural light as well as visual privacy.
1650
Fig. 4.12g, h. Natural lighting in Isfahan, Iran, with modular lighting vaults covering the central bazaar. Tiled domes provide a cool but pleasantly lit space.
Fig. 4.12i. The Baroque architecture of seventeenth-century Southern Europe used carefully designed natural light to create highly dramatic spaces. This example is in Salzburg, Austria, where much of the Baroque architecture has been preserved.
1885
Fig. 4.12j. Natural lighting theory of the late nineteenth century was dominated by the Northern European experience with overcast skies. It was felt that north light was best because it was constant and steady. Painters and gallery people had a lot of influence in regard to this approach, which divorced natural light from heating functions. Shown is the Boston City Library with its large expanses of north-facing windows.
We’re just now emerging from a period when natural lighting became a lost art, and artificial lighting became the norm. This inversion resulted in some really strange and perverse situations if we look at them objectively. Artificial lights burned night and day, indoors and out, whether needed or not and never questioned by anyone. Turning on electric lights became an automatic part of entering a space, regardless of any actual need. The use of a once useful tool, electric lights, became an unhealthy, uneconomical, unecological, and unconscious ritual act. How we got to this unfortunate state has a history as well, which is shown here.
1845
Fig. 4.13a. Natural lighting for large covered spaces was achieved by embracing construction techniques developed for use in large greenhouses. The train station in east Paris marks the change from architecture sensitive to natural light to architecture dependent on artificial light, as oil lamps, and then kerosene, gas, and then electric lights became more available.
1920
Fig. 4.13b. The all-glass high-rise was proposed by Mies van der Rohe as a utopian concept. The simplicity and single-function implication of this concept became dogma in the 1950s.
1950
Fig. 4.13c. Section of a typical 1950s corporate tower showing social hierarchy of poor natural lighting.
1950–2000
Fig. 4.13d. The all-glass curtain-wall building created a city of all-glass cubes (New York City) that paid little attention to natural lighting and often had significant glare and overheating problems. Whether built with clear, tinted, dark, or reflective glass, this modernist approach turned out to be a simplistic formula that really only provided rentable square footage made possible by cheap fossil fuel. Ironically, although the original idea was natural light and view, the reality as shown in the section drawings (4.13c) was much different.
1985
Fig. 4.13e, f. The beginning of the return of natural lighting as an architectural consideration occurred in the work of William Lamm, Benjamin Evans, Mike Nicklas, and others. Techniques for extending natural lighting applications started to develop as energy prices started to rise after the oil crisis of 1979. South light was found to be superior to north light if it was related to passive heating and cooling needs. As interest grew, increased computer speed began to make prediction models better and more useful. Studies by William Lamm for the TVA building in Chattanooga, Tennessee.
Techniques for Natural Lighting
Since one of these fundamental needs is for access to daylight and to the sun, the architect, when considering the general form and character of building will have to bear in mind this requirement for daylight…
—R. G. Hopkinson
Planning for Natural Light
Natural light most commonly enters a building through its envelope in the form of top-light and side-light. Since light can only penetrate a building so far, thin buildings and ones that integrate central spaces have been used to allow for natural light and ventilation. figure 4.14 illustrates the relative daylight potential for buildings with thick versus thin footprints. Top-lighting can easily provide light to landlocked interiors on the top floors, and multistory buildings can use variations of central spaces and translucent floors to enhance natural lighting, as illustrated in figures 4.15 and 4.16.
There are two generic approaches to natural light: top-light at various scales (lithium, light well, and light tubes) and side-light.
Fig. 4.15. Variations of central spaces for natural lighting. An atrium shaped to optimize natural light is often referred to as a litrium. (a) Enclosed glazed atrium. (b) Courtyard. (c) Light court. (d) Atrium facing the sky. (e) Atrium facing the equator.
Fig. 4.16. Various scales for top-lighting strategies.
Top-Lighting
Top-lighting strategies vary depending on the scale of the space and structural systems. We will briefly discuss top-lighting systems and the best ways to control these systems for optimum lighting results.
Fig. 4.17. Roof monitors applied to spaces of various scales designed by Mike Nicklas of Innovative Design Architects.
Clerestory Windows
Windows on an upper part of a wall that light a central space are called clerestory windows. These can be integrated in most building designs to provide light to the center or north side of a building. Clerestory windows can be relatively small for a single-story home with a loft, or substantial on a commercial building. They work best for one- or two-story buildings with relatively open floor plans. The clerestory placed in the middle of a typical pitched roof will make it possible for sunlight to reach directly into north rooms. It may be used to bring light into a hallway or open space, which provides indirect daylight to north rooms and the north side of deeper south-facing rooms.
A clerestory might typically be placed at two and a half to three times room height from the south wall. So for a 10-foot ceiling, the clerestory would be placed 25 to 30 feet in. South-facing windows with light shelves can provide excellent illumination up to that point. The clerestory window height might be at 12 to 15 feet for a simple one-story building, and can provide light up to 30 feet to the north. The increasing height difference between entry windows near floor level and the high clerestory window can also improve stack ventilation.
Roof Monitors and Washing Walls with Light
Roof monitors, dormers, and other bump-ups can be added to most roof patterns to provide benefits comparable to clerestory windows. A monitor with south-facing windows is often the best choice. Roof monitors can provide good quality light with minimal glare. They also allow warm air to rise further, and with operable windows or a reversible ceiling fan they can be very helpful for ventilation, heating, and cooling. A roof monitor over a corridor, stairwell, hallway, or central room can provide excellent ventilation flow paths as well as daylighting. Glare can be controlled with vertical baffles, curtains, or other diffusers. In dusty environments, provision should be made for dusting or cleaning these surfaces.
Table 4.4. Aperture area expressed as a percent of floor area needed for natural light. Study by Mike Nicklas.
For low-rise buildings or in conjunction with atria, top-lighting has many advantages. Table 4.4 expresses the amount of equator-facing aperture needed as a percent of floor area for various lighting strategies. The advantages of the orientation and configuration shown in figure 4.17 include:
- Greater natural light that is easily controlled.
- Cooling-load reduction (daylighting can produce lumens with half the heat of fluorescent fixtures).
- Maintenance cost of electric lighting is reduced.
- Solar gain for heating season.
- As effective as other strategies with less glazing.
One of the most effective approaches is the use of monitors facing the equator combined with side-lighting. Courtyards, light wells, and atria can move light to lower floors.
Sawtooth Roofs
Industrial buildings once commonly used sawtooth roofs to bring in light before air-conditioning and electricity subsidies made electric lighting appear more economical. They can still be used, with either south-facing vertical windows (added-advantage solar heating) or north-facing vertical or sloped windows (stable, very little glare). A 60° north-facing sawtooth provides fairly even light distribution, and is steep enough to make leaks less likely. The adaptive reuse of industrial facilities with extensive sawtooth roofs has become increasingly common.
Fig. 4.18. Sawtooth apertures fashion the roof and tall vertical walls of the Tribune building in San Luis Obispo, California.
Skylights
Although skylights are almost 100 percent efficient as the light enters, eventually that light is converted to heat, and added cooling might be required to offset the heat gain in summer. Likewise, infiltration and conduction of air through skylights may call for additional heating or cooling. However, skylights are better than electric lights, which give off more heat; even fluorescent lights require more cooling than properly used skylights.
Skylight efficiency can be increased with appropriate glazing and by using curbs with R-values comparable to roof insulation. Curbs should also be weather-tight with little or no infiltration. Flat, curved, or pyramid skylights can be used with recognition of their impact on summer cooling loads or with proper controls. Skylight size and spacing can be worked out with models. Curb and surround shapes, color, and detailing all affect light capture and contrast. If the ceiling height is low, then an angled aperture should be used to allow light to spread; otherwise glare can be a problem.
Fig. 4.19. This 1960s office building felt like a dungeon deprived of natural light until a major retrofit incorporated extensive natural lighting techniques including skylights with baffles to transform the interior into an inviting and pleasant place to be. Union-Locust building, Santa Cruz, California. Images courtesy of Swenson Technology.
The standard rule is to start by placing the first skylights at one-half the height of the ceiling from walls that have no windows, and then adding another skylight at a distance equal to wall height (1/2H, H, H, H, H, 1/2H). Diffusers or glare busters are needed on most skylights. figure 4.19 shows an existing building retrofit that incorporated a number of skylights with baffles to diffuse light and heat. Refer to chapter 3 for ways to control solar gain. Skylights are most commonly used to light large open spaces or arranged in a manner consistent with the ceiling module. They can also be placed to wash interior walls with light, which helps brighten the inner wall of a deeper room.
Light Tubes
Light tubes or light pipes include a clear acrylic dome at the roof surface and a circular aluminum duct with internal mirror finish and a diffusing-light emitter to channel light into a space below. Light tubes have been the most popular daylight solution in recent years. They have proven to be good lighting solutions for bathrooms, hallways, stairwells, garages, and shops. The small dome is relatively easy to install in sloped roofs, and leaks have not been a common problem.
Fig. 4.20. Light tubes incorporated into the retrofit of a 1960s office building in Santa Cruz, California, provide natural light while also maximizing roof area for solar electric production. Images courtesy of Swenson Technology.
Recently, a publicly owned facility successfully integrated natural light for 90 percent of its occupants with the addition of 101 light tubes. This facility achieved LEED Gold for existing buildings, and occupants have expressed their appreciation for the light tubes in particular. An unanticipated result of introducing natural light into this facility has been an increase in indoor plants, which love natural light and help take up indoor CO2,thereby improving indoor air quality as well.
Another attractive quality of light tubes is that they can have adjustable joints, as seen in figure 4.20, so the roof penetration does not need to align with the interior ceiling surface. Flexible rather than rigid ducts with adjustable joints are available, but the corrugation in flexible ducts reduces light reflectance and transmittance. Operable light tubes are available and can be used for venting. Light tubes can have a fairly short payback period. In 2009, costs for a nonventing 13-inch light tube ranged from $200 to $500 per installation, with smaller-scale residential applications at the higher end.
Light Wells
Fig. 4.21. Light well serving multiple stories.
Light wells differ from courtyards and atria because glazing is usually placed at the top of the aperture, but not present between the areas being served by the light well. Glazing typically reflects 15 percent of the light that falls upon its surface, so a light well can be more effective in some cases than atria. Generally, light wells are used at an intermediate scale between atria and light tubes. figure 4.21 illustrates a light well used to introduce side-light on a lower level.
Side-Lighting
Side-lighting is the most common method for integrating natural light, but it is severely limited by its depth of reach as shown in figure 4.22. Floor-to-ceiling height, light shelves, sloped ceilings (sloping toward the back of the room), interior light apertures, and color can all be used to enhance the quality and quantity of side light.
Fig. 4.22. Side-lighting enhancement techniques and their limits.
Side-Lighting Details
Light Shelves
The light shelf reduces glare in the room and reflects light up onto the ceiling and farther back into the room. A bright white upper surface is usually preferred, as seen in figure 4.23. Light shelves provide fewer benefits on east- and west-facing windows and almost none on the north. In the future, there will likely be retrofit light-shelf systems that can be easily installed on existing windows. Even a few inches of window above a light shelf can make a big difference in light penetration and glare control.
The interior light shelf may be opaque or translucent and shaped to improve performance. Flat light shelves are the least costly, but curved, adjustable, or even shape-changing light shelves are in use today. The width of the shelf can be adjusted to provide glare protection over a longer period or all year. Internal light shelves have also been found to help with ventilation using high room fresh-air vents. The cooler fresh air spills off the light shelf and into the room better than it would in a room where it simply falls down the outer wall.
Construction of interior light shelves does not have to be very costly, as the materials can be lightweight. The windows used may be fixed, but operable sections may prove helpful for ventilation. Access for operation should be considered carefully, especially if room darkening is a consideration and shades or curtains will be installed.
Interior light shelves may be shaped to reflect light differentially. They can be solid with a diffuse reflective upper surface (semi-gloss for easy cleaning), translucent, or a grid or egg-crate-style baffle. Interior light shelves also work well with exterior light shelves to reduce glare and to increase light penetration into the room. Interior light shelves can be used when exterior light shelves cannot be easily integrated in a new or retrofit design. A set of blinds may be installed in the light shelf window, and interior shelves can double as light soffits to provide backup lighting. Some optimized applications even integrate mirrors to improve light penetration and quality.
Fig. 4.23. Exterior and interior light shelves can take many forms, as shown here, but having a light-colored or reflective top surface is critical.
Exterior light shelves should be sloped to direct water where desired. The angle can be selected to increase gain for specific periods of the year. The exterior light shelf not only provides light but also helps with solar control of the window below it and can reduce summer cooling loads. If the exterior light shelf tilts down toward the building, it can increase light penetration with the higher summer sun; if it tilts down to the outside, it can increase summer shading and light penetration in fall and spring. It can help to leave a small gap between the exterior light shelf and the wall for ventilation and drainage. A light shelf can also be louvered or adjustable. An adjustable awning with white canvas could be a very flexible, seasonally adjustable light shelf.
Exterior light shelves need to be robust. They must withstand the weather, wind, and rain. Louvered or slatted light shelves will catch less wind on walls exposed to very high winds. Light shelf supports must be well integrated in the wall system to avoid leaks.
Interior Light Apertures
Natural light from areas that enjoy a surplus can be shared with other spaces that have less, such as hallways and landlocked interiors. Interior windows can move light to the core of a building. These may be in doors, transoms, or walls. figure 4.24 depicts light sharing with transoms and glass doors. For privacy these may be translucent, but if placed high on the walls, clear may be better. Operable windows in interior walls can also be used for ventilation; adjustable metal vents may be much cheaper, but they are typically thermally transparent.
Although heavy, glass has been used in walkways, floors, and sidewalk pavers to allow light penetration. Pavers and translucent panels have a lower light transmittance but can also be effective at diffusing direct radiation. figure 4.25 illustrates a natural lighting scheme with a series of transparent walks and light wells for a structure in Saudi Arabia where 120°F temperatures demand the use of reflected light to avoid heat gain.
Color
Choice of interior surfaces and fixtures also matters, with brighter colors helping. In general, warm colors such as red, orange, and yellow are advancing colors that appear closer to the viewer. Cool colors such as green, blue, and violet are opposite in that they are receding and will appear to be farther from the viewer.
Fig. 4.24. Glass doors can be used to share light through spaces and can be fashioned with translucent or dark curtains or blinds for privacy. Transoms in the wall above share light with the hallway on the other side. The hallway is full of natural light thanks to interior apertures in the surrounding walls and glass doors.
Fig. 4.25. Natural light can be used in extremely hot-dry climates—but solar radiation must be diffused so light enters and heat does not. Interior light apertures are a primary design technique in this case.
Windows
For small buildings, natural lighting with windows is the best place to start. The building will have windows in the walls, so why not make them work for daylighting, natural heating, cooling, and ventilation? The incremental cost may be zero, or even pay a bonus as mechanical backup systems are downsized or eliminated. Windows can provide many benefits: views, good lighting for work, ventilation, cooling in summer, and heating in winter—but only if they are designed and installed correctly. Make the wrong choice, and they will provide unwanted heating in summer and cooling in winter.
Effective use of windows begins with proper building orientation, facing the equator, and a building shape that maintains thin elements that make daylighting with windows possible. The finger plan schools, with a series of very narrow, long buildings, of the 1950s were a good response to daylighting demands. Increasing the building perimeter adds to the cost, but provides the best potential for daylighting and natural heating, cooling, and ventilation. Taller ceilings as shown in figure 4.26 enable windows to be higher, increasing light penetration and facilitating ventilation and summer cooling. They can also help reduce glare and lighting discomfort.
Fig. 4.26. A taller ceiling accommodates higher windows to allow more daylight to penetrate this office and library space.
Windows offer one of the better methods for providing natural light because they are already included in a building, and their use for daylighting is free or almost free. Thoughtful design and careful analysis can maximize the value of light from windows. Orientation, placement, type, and solar control are essential to provide good natural lighting—without glare, excessive heat gain, or great variation in brightness. Good design can also ensure that the windows also provide needed opportunities for cross- and stack ventilation for fresh air and natural cooling—an added bonus.
Windows should have narrow glazing bars and frames. They should be shaded during the summer in hot areas and spring through fall in very hot areas. Translucent insulated shutters or shutters with light ports may be desirable to reduce unwanted heat gain, particularly on east- and west-facing windows.
Effective window placement can yield the best light for a given window area. Higher windows admit more light and are also good for venting hot air. Thin tall windows placed in walls next to wall intersections can wash the walls with light and provide good lighting without excessive contrast. Vertical windows are preferred. Tilted windows are difficult to shade in summer and are more likely to leak because water accumulates on the glazing during heavy downpours, and the seals tend to degrade faster with continuous exposure to solar radiation. Tilted windows are also vulnerable to damage from hail and accumulate dirt faster, so maintenance is necessary for optimum light transmittance. The placement of windows also depends on the intended use of the space and the interior design. More than one window on more than one wall can provide better lighting distribution. A strip of windows can give more balanced light than a series of discrete windows. Window placement may also be limited by the desire to reduce glare for a projector screen, monitors, or computer stations. A dark space may be provided for screens or TVs, and a bright spot for displays or whiteboards if control devices such as vertical blinds as shown in figure 4.27 are not enough.
Fig. 4.27. Vertical blinds on windows are quite effective at controlling direct gain and glare while providing user operability.
The distribution of light in a room is also affected by the glazing, the window covering, and the width of the window (wider is usually better on equator-facing walls). A translucent window or window covering may also improve the quality of light and distribution in the room. Alternatively, a view and ventilation window of clear glass may be included in a translucent window wall.
Seasonally adjustable exterior fins are often the best solution for difficult-to-control east- and west-facing windows (see the Olgyays’* book, listed in the reference section, for inspiration). Fixed awnings or roll-down shades, wing walls, and egg-crate devices can help. And exterior blinds, a shade screen, or a green wall of vines a few feet from the window can also be used.
Design and Prediction
Light Measurement
Hand calculations can help you explore the energy implications of natural lighting design, or you can use a more complex annual calculation or models, perhaps with an energy-simulation program. However, many of these are not very good at integrating natural light or considering night ventilation issues, and they may underestimate performance significantly. Using the schematic analysis described below is a good place to begin.
Physical models (figure 4.28) are particularly useful in lighting-design work because the scale model of a building is a true photometric analogue, and models are easier and more accurate to use than calculations or computer programs. Even larger models are relatively inexpensive to make and easy to construct. They can provide very useful information and are easy for clients to relate to and understand. These can be evaluated through the seasons using the sun-peg diagrams described in Calculating Insolation Patterns photographed with a digital camera. Options can be tested quickly and efficiently, especially if a light meter is used to take readings from the analog model.
A number of daylight design tools have also been developed, including Daylighting Nomographs (Environmental Energy Technologies Division, LBL); University of Washington Graphic Daylighting Design Method (Professor Marietta Millet; see the details in Stein et al., 2006*), and the AAMA Skylight Handbook (American Architectural Manufacturers Association). Computer models can also be used to evaluate daylighting, but this can be more costly. They may be required for some applications to satisfy code requirements and to assess energy implications for heating and cooling. Software options include Lumen-Micro (from Lighting Technologies), Superlite 2.0 (Environmental Energy Technologies Division, LBL), DOE-1 (Simulation Research Group, LBL), Radiance 3.4, Energy-10 (Sustainable Buildings Industry Council), and many others.
Fig. 4.28. Tools for daylight analysis—a physical model. Walls and roof can be easily changed to test different daylighting solutions. Camera port on left.
Understanding Light Availability
A number of effective and low-cost light meters are now sold. A good light meter can be found for under $100; a very good one for less than $200. Approximate values can also be checked with an older light meter designed for photographers, with the film speed set at ASA-100. The EV = 5 at 10 fc, EV = 7 at 30 fc, EV = 9 at 50 fc, EV = 10.5–11 at 70 fc, and EV = 12 at 200 fc. Readings are only approximate because the light meter is set to measure the spectrum that film responds to rather than the human eye.
Schematic Analysis—A Good First Step
Start the following process during schematic design. It may be recycled through many times as design trade-offs become more apparent. Each step will ask you to draw or compare—do it at the level you are designing—overall or space specific. Sketches of a typical situation provide general guidance:
1. Draw the general plan view of the area under consideration. Label compass directions with a big equator-facing arrow (south in the Northern Hemisphere and north in the Southern Hemisphere).
2. Where in the plan do you want natural light for stationary tasks, such as reading? Draw in the region with a 45º cross-hatch.
3. Where in the plan do you want natural light to illuminate circulation routes? Draw the region with a horizontal cross-hatch on an unmarked general plan. Don’t redraw step 2.
4. Where in the plan do you desire general background lighting? Draw the region with a vertical cross-hatch on another unmarked plan. Again, don’t redraw earlier steps.
5. Where in the plan do you want visual accent lighting? Draw this region with a dot pattern.
6. Now draw a composite sketch showing all lighting goals.
After you have completed these steps, look carefully at what you have. How realistic is it? Cut down all “excess” light requirements as much as possible. Remember, task lighting and lighting for circulation routes are brighter than general background or accent lighting. Therefore, light for those first requirements might be utilized for less demanding needs as well. After you have reexamined your lighting goals, draw a new composite diagram reflecting the changes. If you were working at the overall scheme level, later as you start dividing up the space you can meet more specific criteria by repeating this process. If you were already at a small-space level, now you can repeat the process for some other areas in your project.
Methods for natural lighting can be categorized as either top-lighting or side-lighting, but advances in interior lighting such as translucent walks and walls have proven useful for extending the distribution of natural light throughout a building. To achieve a good balance of natural light and to avoid unwanted glare, a mixture of top- and side-lighting should be incorporated into the design.
Example of Integrated Design for Natural Lighting
Iconic Buildings
The cathedral of Santa Maria del Fiore in Florence, Italy, started in 1296, stood for 140 years without any enclosure over its central altar. The task of enclosure was too difficult because of the large span involved, which didn’t allow traditional wood centering, and also because of the rejection of flying buttresses by the building commission as too foreign (Northern European).
Fig. 4.29. Basilica di Santa Maria del Fiore, Florence, Italy. The dome was engineered by Filippo Brunelleschi and completed in 1436. Today, it remains the largest brick dome ever constructed and marks the beginning of the Renaissance.
The architect, Brunelleschi, solved the engineering problem of the span by using a double-shell dome and iron tension rings. This allowed construction of the dome that could use internal reinforcing to hold itself in place as it went up. Beyond its engineering innovation, however, the resultant context, scale, and form allowed this building to instantly became the symbol of the city-state of Florence and very quickly symbolize the confluence of new and old ideas now known as the Renaissance. Iconic buildings are those innovative enough and lucky enough in their timing to become symbolic of large changes in the social and cultural context in which these architectural expressions occur.
The Reichstag in Berlin
This building played a significant part in the turbulent twentieth century. Built in a severe Prussian Classic style in 1894, it was supposed to become the expression of a new democratic Germany. But it was shunned by Kaiser Wilhelm II (not inclined toward democracy) when he came to power. It was then used as a government building by the more progressive Weimar Republic from 1919 to 1933. It was badly damaged by arson in 1933, an event that was used as a pretext by the Nazis to begin an era of repression culminating in the catastrophic Third Reich. In historical context, it was a kind of a Twin Towers of its time. Further damage by Allied bombing and the Soviet capture of Berlin at the end of World War II added to its tragic history as well as its near destruction.
Fig. 4.30. Reichstag, Berlin, Germany
Although it was part of the western sector of Berlin during the Cold War, the Reichstag was cut off from access by the Berlin Wall. Some limited restoration work was done in the early 1960s, but it wasn’t until the reunification of Germany and increased recognition—fueled in part by a Christo art wrap in 1995—that the building was slated for a return to its original function.
Fig. 4.31. Reichstag, Berlin, Germany. Photo: Richard Beller.
In 1999, it was the subject of an international design competition to restore it as a legislative building for the new democratically unified Germany. Norman Foster and Partners developed the winning design based on expressing the following goals:
- Providing a democratic focus.
- Emphasizing public accessibility.
- Expressing sensitivity to history.
- Fulfilling a rigorous environmental agenda.
The design uses passive lighting and ventilation utilizing the traditional dome form as a gigantic daylight chandelier and public walkway. It operates with hundreds of mirrors and a movable shading device. Electrical needs of the building are provided by photovoltaic panels and a generator powered by biofuel from vegetable oil. The design has reduced the carbon footprint of the building by 94 percent.
This building rose from the ashes like the phoenix and provides a refreshing vitality with the very strong contrast between the ponderous, heavy old building and its new airy dome, which emphasizes light, openness, and public participation. These characteristics give it power similar to that of Brunelleschi’s dome in Florence, built 560 years earlier to mark a new cultural era. In this case the transition is from an industrially dominated era to a new cultural era of sustainability.
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Fig. 4.32. (a, b, c, d, e) Interior photos by Richard Beller. (f) Exterior photo. (g) Section drawing of Reichstag, Berlin, Germany.
The typical illumination specialist has a background in electrical engineering: he is thus all too prone to rely upon artificial light sources exclusively, paying little or no attention to daylighting.
—J. M. Fitch, 1972
Supplemental Lighting
Although natural light can meet most of the lighting demand for many buildings on a good day, artificial lighting is needed to supplement daylighting at night or on dark, cloudy days. Natural lighting can typically meet about 75 percent of the annual lighting needs for a building used between 8 am and 5 pm. Careful integration of design and controls are crucial to ensure comfort and economy of the combined daylight and artificial lighting systems.
As energy prices continue to rise, the cost of artificial lighting is also increasing, and this is encouraging more investment in daylight design and improved lighting systems and controls. Artificial lighting may be needed to meet the demand for both area and/or task lighting when natural light is insufficient. The design of artificial lighting should also focus on comfort and productivity. Glare avoidance is critical for the computer-screen, PDA-dominated world we live in today.
Light efficiency varies from about 10–20 lumens per watt for incandescent bulbs to 30–100 for fluorescents, and 25–100+ for LEDs. LEDs last much longer and are the most efficient, but their light is very directional. They are also the most expensive lights, and fewer types are available for 110V. In the future, buildings may have a 24-volt lighting circuit for LEDs.
Fluorescent lights are more efficient, economical, and cooler than incandescent. Light quality is good with some types of fixtures and bulbs, and dimmable ballasts are available for some types of fluorescent lamps. Full-daylight-spectrum fluorescents are available and should be considered in cases without natural light. Low-cost bulbs and ballasts can be noisy and provide poor light quality. Fluorescents’ flicker and hum may also be troublesome to people. Ideally, much of the supplemental lighting will be reflected rather than direct light, to reduce glare.
Light placement should also be carefully considered when ceiling fans will be used for supplemental cooling. Downward-facing lights set above ceiling fans can create an irritating flutter from blade shadows. It can be better to wash the ceiling with light for indirect lighting or to use a ceiling-fan-integrated high-efficiency light that is mounted below the blades.
Controls
One of the primary challenges is developing an integrated control system that will take maximum advantage of natural light, provide user control and comfort, and function reliably for many years. Easy-to-understand-and-operate controls are critical. Lights can be photo-cell switched to turn off or dim when natural lighting levels are adequate. Savings of 50 percent or more may be achieved with automatic switching. Occupancy sensors can also be used to turn lights off when no one is in the room. Motion and even sound waves are being used to sense occupancy, so it is important to know which should override the other. Most often the photometric sensors will determine light levels sufficient for movement through spaces. Savings have varied from 20 to 76 percent in various studies. In general, the better the control system, the bigger the savings. However, lighting-control systems can slightly increase energy use for the detectors and controllers.
Lighting Manual
A lighting manual should also be prepared as part of each building’s operating manual to ensure that staff are familiar with maintenance and operation of the lights, windows, blinds, curtains, and artificial lights. This will also help ensure that task and background lighting are properly set when rearrangements take place. This can improve comfort and productivity and will help minimize energy use.
Natural light should be an integral consideration in the design of every new building; it reduces heat gain from lighting to one-fourth of the heat from fluorescent lights and as little as one-sixth of the heat from incandescent lights. This can lead to a substantial reduction in air-conditioning load and substantial energy and financial savings as well as reduction in global greenhouse gases, because lighting adds 30 to 50 percent of the cooling load in many commercial buildings.
The health benefits of natural light are well documented. Businesses and schools are catching on as they realize workers are healthier, more positive, and more productive with natural light. These benefits far outweigh the benefit of lower utility bills. Students have demonstrated fewer behavioral problems under full-spectrum light, which makes the learning environment more pleasant and efficient for everyone, including the teachers.
Interior design must be developed and integrated with the natural lighting system. Office layout, including placement of desks, monitors, and other equipment, needs to consider lighting requirements and lighting opportunities and constraints. The planning should also consider where different activities and tasks will be undertaken. Good interior design can help a poorly developed light system function better, while poor interior design can cripple a brilliant daylight design. Lighting design influences color and material choices, the design of interior partitions and interior decoration, and choice and placement of fixtures and equipment. Everyone must be encouraged to understand and accept responsibility for his or her potential impacts on lighting. High reflectances are usually desirable for the ceiling and walls, partitions, and floors. However, desktops, storage, and partitions in workers’ viewscapes should be chosen for moderate reflectance to reduce potential glare problems on computer screens and other monitors.
In 2007, about 526 billion kilowatt-hours of electricity were used for lighting by the residential and commercial sectors. This was equal to about 19 percent of the total electricity consumed by both of those sectors and 14 percent of total US electricity consumption (US Energy Information Administration). The use of natural light can not only reduce the pollution associated with electricity production, but also save quite a bit in terms of water resources, since 47 percent of US water use is for energy production.
Even with abundant natural light, people have been programmed to reach for the lights as they enter a room. It is time for everyone to think before automatically hitting the switch. After all, even though electricity is subsidized to make it affordable now, that will not be the case in the future. Natural light will play an increasing role in buildings in the future, just as it did before the fossil-fool era.
Fig. 4.33. 1999 US DOE-EIA electricity end use in office buildings.