Daylighting
by Gregg D. Ander, FAIA
Southern California Edison
Design Guidance > Daylighting
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INTRODUCTION
Daylighting is the controlled admission of natural light into a space through windows to
reduce or eliminate electric lighting. By providing a direct link to the dynamic and perpetually
evolving patterns of outdoor illumination, daylighting helps create a visually stimulating and
productive environment for building occupants, while reducing as much as one-third of total
building energy costs.
DESCRIPTION
In large measure, the art and science of proper daylighting design is not so much how to
provide enough daylight to an occupied space, but how to do so without any undesirable side
effects. It involves more than just adding windows or skylights to a space. It is the careful
balancing of heat gain and loss, glare control, and variations in daylight availability. For
example, successful daylighting designs will invariably pay close attention to the use of
shading devices to reduce glare and excess contrast in the workspace. Additionally, window
size and spacing, glass selection, the reflectance of interior finishes and the location of any
interior partitions must all be evaluated.
A. Benefits of Daylighting
Daylighting has the potential to significantly improve life-cycle cost, increase user
productivity, reduce emissions, and reduce operating costs:
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Improved Life-Cycle Cost: At an estimated incremental first cost increase of from
$0.50 to $0.75 per square foot of occupied space for dimmable ballasts, fixtures and
controls, daylighting has been shown to save from $0.05 to $0.20 per square foot
annually [in 1997 $].
Increased User Productivity: Daylight enlivens spaces and has been shown to
increase user satisfaction and visual comfort leading to improved performance.
Reduced Emissions: By reducing the need for electric consumption for lighting and
cooling, the use of daylight reduces greenhouse gases and slows fossil fuel depletion.
Reduced Operating Costs: Electric lighting accounts for 35 to 50 percent of the
total electrical energy consumption in commercial buildings. By generating waste
heat, lighting also adds to the loads imposed on a building's mechanical cooling
equipment. The energy savings from reduced electric lighting through the use of
daylighting strategies can directly reduce building cooling energy usage an additional
10 to 20 percent. Consequently, for many institutional and commercial buildings,
total energy costs can be reduced by as much as one third through the optimal
integration of daylighting strategies.
As with all energy-efficient design strategies, there are some costs associated with the use of
daylighting. Designers must be sure to avoid glare and overheating when placing windows.
More windows do not automatically result in more daylighting. That is, natural light has to be
controlled and distributed properly throughout the workspace. Also, for cost savings to be
realized, controls have to be in proper functioning order. Poor installation, commissioning, or
Operations and Maintenance (O&M) practices can all lead to sub-optimum performance.
B. Daylighting Concepts
It is important to appreciate that the daylighting design process involves the integration of
many disciplines including architectural, mechanical, electrical, and lighting. These design
team members need to be brought into the process early to ensure that daylighting concepts
and ideas are carried throughout the project.
1. An awareness of basic visual acuity and performance issues is essential to an
effective daylighting design.
• Veiling Reflections: Veiling reflections of high brightness light sources off
specular (shiny) surfaces obscure details by reducing contract. They should
be avoided, particularly where critical visual tasks occur.
• Distribution: Introduce as much controlled daylight as deep as possible
into a building interior. The human eye can adjust to high levels of
luminance as long as it is evenly distributed. In general, light which reaches
a task indirectly (such as having bounced from a white wall) will provide
better lighting quality than light which arrives directly from a natural or
artificial source.
• Glare: The aim of an efficient daylighting design is not only to provide
illuminance levels sufficient for good visual performance, but also to
maintain a comfortable and pleasing atmosphere. Glare, or excessive
brightness contrast within the field of view, is an aspect of lighting that can
cause discomfort to occupants. The human eye can function quite well over
a wide range of luminous environments, but does not function well if
extreme levels of brightness are present in the same field of view.
• Variety: Some contrast in brightness levels may be desirable in a space for
visual effectiveness. Dull uniformity in lighting can lead to tiredness and lack
of attention—neither of which is compatible with a productive environment.
Often times a good daylighting solution will integrate a "blast" of beam
daylight in a circulation area for visual interest and to help lead occupants
through a building. The human eye is naturally attracted to this bright area
and can be useful in guiding people down an otherwise banal corridor.
2. Good daylighting requires attention to both qualitative and quantitative aspects of
design. Make sure the combination of natural and artificial sources provides adequate
light levels for the required task.
• The Illuminating Engineering Society of North America publishes an
industry-standard method for determining recommended illuminance levels
(expressed in units of footcandles, or fc) for various tasks.
• For office spaces, the U.S. General Services Administration has interpreted
the IES method to recommend a minimum of 50 footcandles on an
imaginary desk-height horizontal "work surface." Nevertheless, when used
in conjunction with indirect an ambient lighting system and direct task
lighting, a high-quality daylighting design can be achieved with ambient
lighting levels of 30 footcandles or less.
3. To be effective, daylighting must be integrated with electric lighting design. In
particular, daylighting must be coupled with efficient electric lighting controls if net
energy savings are to be realized.
• As part of a daylighting design, consider the use of continuously dimming
fixtures controlled by luminous sensors.
C. Design Recommendations
A number of design strategies should be understood and explored during the design process.
These strategies are briefly described below.
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Increase perimeter daylight zones—extend the perimeter footprint to maximize the
usable daylighting area.
Allow daylight penetration high in a space. Windows located high in a wall or in roof
monitors and clerestories will result in deeper light penetration and reduce the
likelihood of excessive brightness.
Reflect daylight within a space to increase room brightness. A light shelf, if properly
designed, has the potential to increase room brightness and decrease window
brightness.
Slope ceilings to direct more light into a space. Sloping the ceiling away form the
fenestration area will help increase the surface brightness of the ceiling further into a
space.
Avoid direct beam daylight on critical visual tasks. Poor visibility and discomfort will
result if excessive brightness differences occur in the vicinity of critical visual tasks.
Filter daylight. The harshness of direct light can be filtered with vegetation, curtains,
louvers, or the like, and will help distribute light.
Understand that different building orientations will benefit from different daylighting
strategies; for example light shelves which are effective on south façades are often
ineffective on the east or west elevations of buildings.
D. Materials and Methods of Construction
1. Exterior Shading and Control Devices: In hot climates, exterior shading devices
often work well to both reduce heat gain and diffuse natural light before entering the
work space. Examples of such devices include light shelves, overhangs, horizontal
louvers, vertical louvers, and dynamic tracking or reflecting systems.
2. Glazing Materials: The simplest method to maximize daylight within a space is to
increase the glazing area. However, three glass characteristics need to be understood
in order to optimize a fenestration system: U-value, Shading Coefficient, and Visible
Transmittance.
• U-value represents the rate of heat transfer due to temperature difference
through a particular glazing material.
• Shading Coefficient (SC) is a ratio of solar heat gain of a given glazing
assembly compared to double-strength, single glazing. [NB: A related term,
Solar Heat Gain Factor (SHGF), is beginning to replace the term Shading
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Coefficient.]
Visible Transmittance (Tvis) is a measure of how much visible light is
transmitted through a given glazing material.
Glazings can be easily and inexpensively altered to increase both thermal and optical
performance. Glazing manufacturers have a wide variety of tints, metallic and lowemissivity coatings, and fritting available. Multi-paned lites of glass are also readily
available with inert-gas fills, such as argon or krypton, which improve U-values.
For daylighting large buildings in most climates, consider the use of glass with a
moderate-to-low SC and relatively high Visible Transmittance.
3. Aperture Location: Simple sidelighting strategies allow daylight to enter a space
and can also serve to facilitate views and ventilation. A rule-of-thumb is that the
depth of daylight penetration is about two and one-half times the distance between
the top of a window and the sill.
4. Reflectances of Room Surfaces: Reflectance values for room surfaces will
significantly impact daylight performance and should be kept as high as possible. It is
desirable to keep ceiling reflectances over 80%, walls over 50%, and floors around
20%. Of the various room surfaces, floor reflectance has the least impact on
daylighting penetration.
5. Integration with Electric Lighting Controls: A successful daylighting design not
only optimizes architectural features, but is also integrated with the electric lighting
system. With advanced lighting controls, it is now possible to adjust the level of
electric light when sufficient daylight is available. Three types of controls are
commercially available:
• Switching controls—on/off controls simply turn the electric lights off when
there is ample daylight.
• Stepped controls—provide intermediate levels of electric lighting by
controlling individual lamps within a luminaire.
• Dimming controls—continuously adjust electric lighting by modulating the
power input to lamps to complement the illumination level provided by
daylight.
Any of these control strategies can, and should, be integrated with a building
management system to take advantage of the system's built-in control capacity. To
take full advantage of available daylight and avoid dark zones, it is critical that the
lighting designer plan lighting circuits and switching schemes in relation to fenestration.
The following figure shows several control schemes.
Fig.
1
6. Other Lighting Control Systems: In addition to daylight controls, other electric
lighting control strategies should be incorporated where they are cost effective,
including the use of:
• Occupancy controls—Using infrared, ultrasonic or micro-wave technology,
occupancy sensors respond to movement or object surface temperature and
automatically turn off or dim down luminaires when rooms are left
unoccupied. Typical savings have been reported to be in the 10 to 50
percent range depending on the application.
• Timers—these devices are simply time clocks that are scheduled to turn
lamps or lighting circuits off on a set schedule. If spaces are known to be
unoccupied during certain periods of time, timers are extremely cost
effective devices.
E. Analysis and Design Tools
Physical Modeling
The physics of illumination are such that light behaves exactly the same way in a scaled
model as it does in a full-size room. Physical models can be built inexpensively and at
various stages of the design process. A number of issues can be accomplished with physical
models.
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Photographs of the model interior can be taken to record and study various design
alternatives;
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The effect of different glass areas and locations can be studied;
Using photometers, illumination levels resulting from different daylighting schemes
can be compiled and used to project energy savings.
Calculation Tools
Over the past 50 years many daylighting calculation tools have been developed. These
include hand methods, nomographs, and computer models to simulate both daylighting
design and its impact on the overall thermal performance of buildings. The following listing
briefly describes some of the available tools.
Daylighting Nomographs (1984)
Description: A hand-method that assists designers determine potential daylighting benefits
and costs; also offers a checking strategy for energy conservation and load management.
Availability: Environmental Energy Technologies Division, Lawrence Berkeley National
Laboratory
University of Washington Graphic Daylighting Design Method (1980)
Description: Determines daylight patterns for a room based on the proportions of the
window openings, providing illumination level, distribution, and gradient. For more
information see: Mechanical and Electrical Equipment for Buildings (MEEB), Ninth Edition
by B. Stein and J. Reynolds. New York, NY: John Wiley & Sons, Inc., 1999. or Inside Out,
Second Edition by G. Z. Brown et al. New York, NY: John Wiley & Sons, Inc., 1992.
Availability: University of Washington, Department of Architecture
Contact: Professor Marietta Millet
AAMA Skylight Handbook—Design Guidelines (1988)
Description: Skylight design analysis with emphasis on optimizing for energy efficiency,
incorporating both a worksheet and Lotus spreadsheet (IBM PC or compatible).
Availability: American Architectural Manufacturers Association (AAMA)
Computer Software
Lumen-Micro
Description: Analyzes complex interior lighting systems including daylight, direct/ indirect
lighting, mixed- and even-aimed luminaires. DXF file editor, user-friendly input, animated
walk-through. Limited to rectangular spaces.
Availability: Lighting Technologies
Superlite 2.0 (1993)
Description: Updated version of SUPERLITE 1.01. Now analyzes daylight and electric
lighting for various room geometries. Max 5 windows. Tabulated output, no graphics. IBM
PC or compatible with 8087 or better math-coprocessor chip. FORTRAN, MS-FORTRAN 3.2
compiler.
Availability: Environmental Energy Technologies Division, Lawrence Berkeley National
Laboratory
DOE 2.1
Description: Comprehensive hour-by-hour simulation; Daylighting and glare calculations
integrate with hourly energy simulation. IBM or compatible Pentium is advisable.
Availability: Simulation Research Group, Lawrence Berkeley National Laboratory
Radiance (3.4)
Description: A ray-tracing program that accurately predicts light levels and produces photo
realistic images of architectural space in all sky conditions. Sun Microsystems, DEC,
MacIntosh with (AUX), CRAY, or other UNIX machine.
Availability: Environmental Energy Technologies Division, Lawrence Berkeley National
Laboratory
Designing Low Energy Buildings with Energy-10
Description: An hour-by-hour simulation program designed to inform the earliest phases of
the design process. Runs on IBM-compatible platforms. Best operated with Pentium or
higher processor and 32 Megs of RAM.
Availability: Sustainable Buildings Industry Council (SBIC)
APPLICATION
Among the primary types of buildings that can benefit from the application of daylighting are
administrative buildings (e.g. offices), educational buildings (e.g. child development
centers), storage facilities (e.g. warehouses), and maintenance facilities.
Case Studies
Lockheed Building 157, Sunnyvale, CA
Daylighting in Schools
ADDITIONAL RESOURCES
WBDG
Sustainable; Productive; Aesthetics; Section 07900: Joint Sealers
Publications
"A Daylighting Checklist" in Solar Age, p. 84. by McCluney, Ross. April 1985. This one-page
outline contains 15 factors to consider when employing daylighting. It contains good
references to other articles that delve into some of the factors in more detail, such as roof
monitor design and calculator programs for skylight.
Architect's Handbook of Energy Practice: Daylighting by The American Institute of Architects.
Washington, DC: The American Institute of Architects, 1992. Part of a series of
monographs by the AIA on energy-conscious design. The text is supported with case
studies of famous buildings that utilize daylighting.
Building Technologies Program, Tips for Daylighting with Windows—The Integrated Approach
Lawrence Berkeley National Laboratory, 1997.
Concepts and Practice of Architectural Daylighting by Moore, Fuller. New York: Van Nostrand
Reinhold, 1986. This good text on the fundamentals of daylighting is well supported with
graphics. Simpler and more direct to use than other texts on the subject, it covers all of
the major issues pertaining to daylighting.
Daylight in Architecture by Evans, Benjamin H. New York: McGraw-Hill, 1971. This designoriented book is intended as a primer. It is strong on basic concepts and model testings
and is a good place to start for those entering the field.
Daylighting by Hopkinson, R. G., Petherbridge, P., and Longmore, J. London, England:
University College, 1966. This text is an excellent technical resource for daylighting
research and design methods, including sections on sky luminance, daylight photometry,
models, and artificial skies.
Daylighting: Design and Analysis by Robbins, Claude L. New York: Van Nostrand Reinhold
Company, 1986. This is a technical two-part handbook that explores the fundamentals of
daylighting. The first part presents the principal sources, control devices, and analysis
methods used in daylighting. The second part contains reference material needed to
supplement the design methodologies given.
Daylighting for Commercial and Industrial Buildings—EREC Reference Briefs. U.S.
Department of Energy, Energy Efficiency and Renewable Energy Network (EREN),
Consumer Energy Information
Daylighting Performance and Design by Ander, G. D. New York: John Wiley & Sons Inc., May
2003. This is an excellent source for daylighting design issues, tutorials on calculation
technologies, and has many case studies documented.
Recommended Practice of Daylighting by Illuminating Engineering Society of North America.
New York: IES, 1979. This publication is a very good source for daylighting information.
The appendix goes through typical examples of the IES method.
Solar Control and Shading Devices by Olgyay, Aladar, and Olgyay, Victor. Princeton, NJ:
Princeton University Press, 1976. This classic text on designing shading devices begins
with a historical overview of indigenous responses to shading and ends by outlining a
detailed analysis and design process. Photographs of many different shading devices are
used to support the authors' claims.
"Strategies of Daylight Design" in AIA Journal, pp. 68-77, 104, 108, 110, 112 by Villecco, M.,
Selkowitz, S., and Griffith, J. W. September 1979. This is a comprehensive article on
daylighting design. The article is a good introduction to principles and concepts due to its
scope and accuracy. Emphasis is on the qualitative aspects of design instead of the
quantitative.
Organizations and Associations
International Commission on Illumination
United States National Committee CIE/(USA) is a not-for-profit organization formed in
1913 to assist the International Commission on Illumination in achieving its objectives in
the fields of light and lighting.
Illuminating Engineering Society of North America (IESNA)
Organization whose mission is to advance knowledge and disseminate information for the
improvement of the lighted environment for the benefit of society.
Lawrence Berkeley National Laboratory
Research laboratory operated by the University of California as part of the U.S.
Department of Energy's national laboratory system.
Lightsearch.com
Internet source for lighting specifiers
Lighting Research Center, School of Architecture, Rensselaer Polytechnic Institute
The world's largest university-based research and educational institution dedicated to
lighting. The LRC takes a unique and energetic approach to problem solving with a
commitment to bringing together unparalleled resources for lighting research.
Southwall Technologies
Southwall designs and produces thin film coatings that selectively absorb, reflect, or
transmit certain types of electromagnetic radiation.
U.S. EPA Green Lights Program, Energy Star
Green Lights is an innovative, voluntary pollution prevention program.
Updated: 04-06-2005
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