How Low Can You Go?

By Craig DiLouie | Sep 15, 2008




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In 1972, the Illuminating Engineering Society of North America (IESNA) recommended a light level of 70 foot-candles (fc) for typical tasks in open offices, which was typically provided by magnetic-ballasted F40T12 2-by-4 troffers, creating a lighting power density (LPD) as high as 5.5W per square foot. By 1990, IESNA had refined its recommendation to 15–50 fc. LPD was reduced to about 1.5W per square foot.

The general trend is obvious. As energy costs have increased over time, the industry has responded by lowering light levels and producing more efficient technology.

The bar has now changed. Since 1999, energy codes have become more widespread and aggressive, setting LPD limits based on the day’s leading technology and challenging the industry to become more efficient. For example, LPD in open offices was capped at 1.3W per square foot by ASHRAE 90.1-1999/2001 and 1.1W per square foot by ASHRAE 90.1-2004/2007. And energy-strapped states, such as California, want more.

But how low can you go—in terms of watts per square foot—in office lighting without jeopardizing quality?

The California Energy Commission and the state’s largest utilities tasked the California Lighting Technology Center (CLTC) to find out.

“We were directed to explore opportunities that could result in reduced lighting energy use while maintaining or increasing lighting quality within typical workstation geometries,” said Michael Siminovitch, director of the CLTC, associate director for the Energy Efficiency Center and a University of California, Davis, professor. “The concern now is that significant reductions in power density could start compromising lighting quality, particularly in juxtaposition to an aging population.”

The CLTC research team first surveyed office buildings to create a snapshot of the typical open office: a configuration of cubicle-type workstations, including various furniture, undercabinet linear fluorescent task lighting (often) and integrated partitions (occasionally), with no occupancy-based automatic shutoff. The general lighting is typically a regular grid of lensed or parabolic troffers.

The CLTC research team identified the following ways to improve lighting quality and reduce energy waste:

• General lighting should provide only ambient light—uniformly distributed light that provides a general level of visibility in the space, typically about 25–30 fc.

• Use direct/indirect general lighting fixtures to improve light on walls and ceilings and thereby improve visual comfort and reduce glare on computer screens.

• Supplement the general lighting with task lighting, producing 40–50 fc and sufficient light on the back walls of workstation partitions to eliminate shadows.

• Avoid undercabinet fixtures that blast surfaces with light and produce glare.

CLTC then tested its ideas by developing and demonstrating a personal lighting system (PLS) as part of an integrated task/ambient lighting system in a series of beta site tests, ranging from single offices to large open-office environments.

“The system is made up of an array of freestanding LED task lights that can be positioned by the occupant to provide a wide distribution on the task plane within a cubicle environment,” Siminovitch said. “The task-lighting system also includes a series of LED undercabinet lights in different lengths/wattages to provide vertical illumination across the back wall.”

Designed by CLTC, PIER and manufacturer Finelite, these task-lighting systems are intended to reduce glare while producing sufficient vertical and desktop illumination. The lights can be installed and relocated easily, plugging into a single power supply. The systems can individually configure 3W, 6W and 9W units to produce a balanced luminous environment as long as the total connected load is not greater than 21W. The power supply also features an occupancy sensor for automatic shutoff.

To optimize energy savings within an integrated task/ambient lighting system, general lighting should either be specified as direct/indirect producing 25–30 fc or, if an existing building, replaced or delamped to that level. Because delamping can affect lighting quality, do so with caution.

Siminovitch said the system has demonstrated 0.5–0.7W per square foot power density, 36–55 percent less than ASHRAE 90.1-2004/2007, and 40–50 percent energy savings compared to a typical California office. The use of the occupancy sensor, in turn, was found to reduce energy consumption by an additional 20–30 percent. Because occupants do not need the ambient lighting system to operate in order to work, Siminovitch added, integrated task/ambient designs suit utility demand response programs.

Meanwhile, occupants expressed satisfaction with the approach. Shadows were eliminated, glare was reduced, lighting on partition walls was improved, and occupants appreciated the task-light aesthetics and the ability to configure.

“Certainly, in new construction and major renovation, the argument is very compelling, and we don’t see many problems with the installation of the PLS task/ambient approach. It’s cost-effective, easy to do, and people like it,” Siminovitch said.

Acknowledging that the integration level required by task/ambient systems is increased, he said CLTC is working on implementation guides for new construction and retrofits.

“The study suggests strongly that integrated lighting approaches are the way to address the office of the future. ... I believe Title 24 will address this quite explicitly in the next few rounds,” he said.

DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at

About The Author

DiLouie, L.C. is a journalist and educator specializing in the lighting industry. Learn more at and





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