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Credit: Jameson Simpson

Understanding how light will affect an architectural space can be a tricky business. Beyond figuring out how much is needed and how it will behave in a given built environment, light has other variables such as code requirements and energy targets, glare and reflectivity, comfort and controllability, and the often unpredictable element of daylight, that add up to create a complex puzzle for designers hoping to deliver a building that is both energy-efficient and well-lit. Luckily, there is also a myriad of readily available lighting-calculation tools—both software-based and old-fashioned hand calculations and rules of thumb—that can help to predict the wide range of factors that play into a project’s lighting scheme and can help you to zero in on the right balance of utility and quality.

Measuring Light

All lighting-calculation tools use two metrics to quantify light. One is illuminance: the amount of luminous flux per unit area, which is measured in footcandles or lux. The other is luminance: the intensity of light reflected from a surface, which is measured in candelas.

"Most lighting is planned with illuminance," explains Christopher Meek, AIA, a research assistant professor of architecture at the University of Washington in Seattle. "You often don’t know what the surfaces in a space are going to be, so you plan to deliver a certain amount of light to a horizontal surface." But, knowing your surfaces and measuring luminance are key to understanding light. Forty footcandles falling on a black surface will appear different from the same amount of light falling on a white surface due to the difference in the amount of light that is reflected.

One important thing to grasp about both illuminance and luminance is that they measure light at a single point in time. That works fine for electrical lighting systems, which do the same thing every time you flip the switch. But this leaves something to be desired when attempting to calculate daylighting, which changes drastically depending on the time of day, the season, the latitude and longitude, and, of course, the weather. Accounting for those variables is the foundation for creating a dynamic lighting scheme that reduces energy use while ensuring adequate light levels.

The Right Tools

"Lighting-calculation tools fall into two basic categories," says Tate Walker, AIA, a senior project director at the Energy Center of Wisconsin, a nonprofit organization in Madison, Wis., that focuses on reducing energy use. "You have your cheap, quick, and dirty ways to get good numbers, and then there’s the high-road option, if money is no object and the goals are lofty."

The low road begins with good-old spreadsheet calculations, using manufacturer specs to figure out your lighting power density and where savings can be made. There are also more sophisticated and free software packages available for download to designers. A new and handy tool for electric lighting schemes is Commercial Lighting Solutions (CLS), a free, Web-based program sponsored by the U.S. Department of Energy that was released in 2009. Located online at www.lightingsolutions.energy.gov, CLS allows you to enter project-specific energy data—watts per square foot—and it then outputs recommendations for technology to use and a generic luminaire schedule that can be passed on to a manufacturer’s representative.

While programs such as CLS offer quick and easy ways to make sure you’re hitting your energy targets with an electrical lighting scheme, there are also free, downloadable daylighting tools. The Sensor Placement + Optimization Tool (SPOT, available at archenergy.com/SPOT), calculates natural light from a given location for how much illuminance is available at a given time of year, allowing designers to optimize window placements and translucency. Skycalc, another daylighting tool available through various websites, does the same thing, but specifically for top lighting, or skylights. Both operate using Radiance (available at radsite.lbl.gov/radiance/HOME.html), a program that former Lawrence Berkeley National Laboratory employee Greg Ward Larson began developing in 1985, which uses algorithms to mimic the physical behavior of light at a single point in time. So in order to predict the range of natural light levels a project may be availed of, you could choose to simulate days in June, September, and March at 9 a.m., 12 p.m., and 3 p.m. on each day, which would give you a good idea of distribution levels throughout the year.

The next level of software calculation tools combines manufacturer photometric data with the highest-quality light-simulating algorithms, outputting presentation-level renderings that show how a given lighting scheme—both daylighting and electric—will work in a space. These include Lighting Analysts’ AGi32 and Autodesk’s Ecotect Analysis and 3ds Max programs. Each of these programs give designers the capability to look at any type of space using any shading method in any location on the globe outfitted with any luminaire. They are highly flexible, but the models take much longer to set up than the quick-and-dirty options.

One of biggest faults of all of the lighting-calculation tools mentioned above is that they capture a dynamic process in single points of time. To really understand how light affects a space over time you would have to simulate every hour of every day under every sky condition. A program by Christoph Reinhart, an associate professor in the department of architecture at Harvard University Graduate School of Design, called Daysim (available at daysim.com) attempts to remedy this limitation by annualizing illuminance calculations using a metric called "daylight autonomy." Daylight autonomy refers to the percentage of occupied time that a space can function with daylight alone. So, if you have a space that will be occupied between 8 a.m. and 6 p.m., and you want to maintain 300 lux during that period, Daysim will tell you how much time you will be at that level with daylight alone. This could provide the foundation for a good energy-performance model.

All of the lighting-calculation tools that we’ve discussed here provide a survey of what is available, but there are more options out there for designers. At the end of the day, though, no single tool is going to do it all. Developers and architects may choose to hire a specialty consultant to handle light modeling, depending on the level of complexity and whether the project’s budget can support it. As Walker says, "There are so many different things you’re trying to zero in on—peak loads, heat loading, glare, views, control, usability, and building-space types—we usually have a quiver of tools we bring to the table."

Aaron Seward writes about architecture and technology from Brooklyn, N.Y.