Daylighting design practices are changing at, well, the speed of light. New resources, design tools, and products have improved opportunities in this field in just one short year.

By James Benya

A year ago in this column, I coined a phrase '21st century daylighting' to help differentiate a modern multidisciplinary approach from traditional daylighting designs associated with architectural fashion. Whether or not the title will stick, the practice of daylighting is changing rapidly thanks to significantly increased resources, breakthroughs in design tools and philosophy, and a variety of new products.

Expanding Resources

In the last few years, a considerable amount of daylighting research has been funded by the California PIER (Public Interest Energy Efficiency Research) program and through California's utilities. For instance, the landmark research work by the Heschong Mahone Group that documented improved student test scores in daylighted classrooms was funded by the Daylighting Initiative of Pacific Gas and Electric (PG&E). Since then, additional work in this area and expanded research in other related areas has been undertaken, and the aggregate impact of all of this has placed daylighting on the front burner of energy issues in California. Most recently, a full daylighting lab, classes, and a design assistance program has blossomed at Southern California Edison's new Southern California Lighting Technology Center in Irwindale, matching services available through PG&E at the Pacific Energy Center and at the California Lighting Technology Center in Davis (near Sacramento).

In the Pacific Northwest, the Better Bricks Daylighting Laboratories have grown and expanded their services to the entire region served by the Northwest Energy Efficiency Alliance (NEEA). Fully equipped and instrumented artificial skies and heliodons are now available for public use in Seattle and Portland, with design assistance from trained experts. Classes are offered throughout the region.

At the Lighting Research Center in Troy, New York, the Daylighting Dividends program was recently created to disseminate design and project-related information about daylighting. Related projects, such as work on photocells and other technical elements, are also being conducted there. Established organizations-including the Energy Center of Wisconsin's Daylighting Collaborative, the New Buildings Institute, and Lawrence Berkeley Laboratory's Windows and Daylighting Group-continue to provide free information, tools and publications. Try Googling 'daylighting' and be prepared to spend several hours sifting through the results.

Design Tools

What has made daylighting design so difficult until now is the lack of specific design tools. While large architectural practices with a full team of consultants can undertake complex daylighting analysis, the everyday school, office, or retail store project has no such budget, and design tools that enable rapid assessments are the only way to go. Development of these products is crucial to the success of daylighting. Some of the most important developments include:

SkyCalc Historically, the vast majority of skylights have been much too large, causing overly high light levels, localized thermal discomfort, and increased air conditioning energy use. Whether it is a residential kitchen or the world's busiest airport, poorly designed skylights have turned many energy-efficient projects into energy pigs.

SkyCalc is a Microsoft Excel template, accompanied by climatic data for many of North America's cities. It is actually a mini-energy analysis program, taking into account data about the building, including the cost of energy, the building type and operating schedule, the lighting and mechanical systems, and the proposed skylight design. SkyCalc produces a graph showing the energy savings as a function of skylight-to-floor ratio (SFR). For anyone with moderate Excel skills, SkyCalc is easy to use and provides an extremely accurate result.

Latest Lighting Analysis Programs The latest releases of the major independent lighting analysis programs, AGI32 and Lumen Designer (the replacement for Lumen Micro), have incorporated daylighting in a major way. Both programs are capable of fairly sophisticated daylighting calculations complete with interior furnishings and renderings. Costs are reasonable, and seminars are offered to accelerate the learning curve.

Of course, for those demanding the highest degrees of accuracy and realism, Radiance remains a powerful option. The few people who have truly mastered this software are capable of providing extraordinary images and renditions of unbuilt structures, complete with any level of detail needed to illustrate the design. Either way, the power of the latest software enables responsible daylighting calculations with a reasonable investment in software and training time.

Photometric Data for Skylights The latest skylights use materials that shape and direct light very much like luminaires, such as specular wells and prismatic lenses. While lighting programs do a relatively good job with simple clear or diffuse glazing, complex skylights and other daylighting elements need to be addressed in a more thorough photometric manner.

Considering how skylights are affected by solar elevation and weather, developing a method for creating photometric data presents a unique problem. Under the PIER program, Heschong Mahone Group has developed a methodology for gathering photometric measurements and disseminating the data for use in conventional lighting programs. The results of daylighting calculations are much more accurate than any other method, including scale modeling. Photometric data is already available for some of the latest skylight products.

SPOT The Sensor Placement and Orientation Tool (SPOT) is a computer program developed by the Architectural Energy Corporation, also under the PIER program. It will quantify the electric and daylighting performance on a project and assist in the optimum location of a photoelectric sensor. SPOT handles both sidelighting (windows and clerestories) and toplighting (skylights).

Improper photosensor positioning is one of the principal causes of poor performance of automatic daylighting control systems. The outputs from SPOT include graphical representations of light levels, and overall electric and natural lighting performance throughout the year. SPOT, which is presently in the final phases of development and testing, is expected to improve the performance of typical daylighting significantly. SPOT output also includes the average lighting power throughout periods of use, part of the determination of daylight autonomy.

(A recent 2005 New Product Showcase Energy Award has legitimized SPOT as a promising tool for the development of daylighting design. For an overview and tutorial of the program, visit

Daylight Autonomy Factor (DAF) While there is presently a lot of preaching about the virtues of daylighting, there is very little information about what is or how to design the correct amount of daylight. The US Green Buildings Council (USGBC) LEED-NC 2.2 criterion, the best criterion value until now, requires achieving either a minimum daylight factor of 2 percent (excluding all direct sunlight penetrations) or at least 25 footcandles using a computer simulation model in 75% of all regularly occupied areas. Given that daylight varies and that daylight factor is worthless in a sunny or partly sunny climate, there must be a better way.

As part of the California Collaborative for High Performance Schools (CHPS) program, a nonprofit initiative aimed at promoting increased energy-efficiency and sustainable design in California's schools, a group of daylighting experts has developed a better measure, called the Daylight Autonomy Factor (DAF). Using the SPOT system, all of the relevant factors, including climate, building schedule, room geometry, glazing, electric lighting, and so on, are accounted for, and the percentage of average electric lighting power is determined. The percentage of electric lighting energy saved is the DAF. For example, in a good daylighting design for a school in Boulder, Colorado, the DAF might be as high as 74 percent, but the same design in Seattle can only achieve a DAF of 58 percent owing to climate conditions.

Setting the DAF criterion will be just like addressing any other energy code value. A number of models are already being studied and preliminary criterion values are being proposed for California's school program. If SPOT and DAF are as smart and useful as they appear to be, they should be adopted quickly by USGBC for the LEED series of standards, and may play an integral role in future energy codes.

James Benya is a professional lighting designer and principal of Benya Lighting Design in Tigard, Oregon. He serves on the editorial advisory board of A|L.