The use of Ambient/task lighting is a lighting design approach that combines a direct/indirect general lighting layer and a task lighting layer. The general lighting layer provides low-level ambient illumination, and the task lighting layer provides higher light levels required to complete critical visual tasks. Both layers typically combine to produce the recommended 30–50 foot-candles (fc) in offices. The use of indirect lighting makes the space appear visually brighter and more spacious, while the focus of lumens and watts (W) at the supplementary task layer improves efficiency, resulting in energy savings compared to traditional designs.
The reasoning is sound: locating a light source closer to the task results in a gain in efficiency (per the Inverse Square Law) as long as the light source does not overlight the space or use relatively inefficient equipment.
Two recent pilot office-lighting field studies discovered that low ambient/task lighting systems generated up to nearly 50 percent energy savings compared to equivalent buildings designed to California’s Title 24 energy code of 2008, which is considered the toughest in the country. The first was a two-story office in Davis, Calif., and the other was a large, 10-story office building in West Sacramento. In both studies, the lighting was monitored for three weeks before and after a low ambient/task system installation. Daylight and interior light levels also were monitored.
In the small office, the existing lighting consisted of direct/indirect fixtures suspended from a 10-foot ceiling over nine workstation cubicles and 2-by-2 lensed troffers in six private offices, a copy room, a reception area and storage spaces. New suspended fixtures were installed throughout, along with some direct lighting and wallwashers.
In the large office, the existing lighting consisted of three rows of indirect fixtures, which were suspended 2 feet below a 10-foot ceiling and mounted almost directly over three rows of workstation cubicles in the windowed open office. The fixtures were kept, but the ballasts were replaced with dimming ballasts that enabled control of light levels.
In both projects, for the task layer, each occupant received one 6W adjustable light-emitting diode (LED) desktop task light and as many 6W undercabinet task lights as would fit the space based on a minimum 4-foot spacing between units. As a result, each occupant received an average of 1.5 undercabinet task lights in addition to a desktop light (average total of 15W).
Lighting energy consumption dropped respectively by 65 percent and 56 percent for the small and large projects. If the projects had been designed to Title 24, savings would have been 49 percent and 37 percent, respectively. Some occupants left their task lights off due to daylight coming in from windows, which contributed to energy savings.
In the large office building, the dimming ballasts were used to simulate a demand-response event, with a one-third reduction of ambient light level to about 12 fc. Occupants could still use their task lighting during the two-hour event. They were informed about the event and why ambient light levels were being reduced. A survey was administered immediately after the demand-response event; 70 percent did not notice the reduction in light level, and none were “bothered” by it.
The occupants in both projects were given another online survey administered at the conclusion of the study period, which asked for their opinions of the appearance, visual comfort and ease of control of the lighting. In both buildings, occupants indicated they were more satisfied with the new lighting than the old.
The main application for a low ambient/task lighting approach is private and open offices, such as the ones in these studies, in addition to some meeting room, library and other spaces. It is highly suitable for demand-response schemes in which ambient light levels are dimmed. With this scheme, additional task lighting can easily be provided for transient tasks and users that require higher light levels, such as older workers. The approach can be particularly advantageous when high, dark or articulated ceilings or required use of inefficient general lighting (e.g., for aesthetic reasons) reduce light-delivery efficiency to the task. It can also be advantageous where tasks require very high light levels and when tasks requiring high- and low-level lighting share the same space, particularly when the ratio of circulation to high light-level task space is high.
The primary obstacle to adoption is a lack of specific industry guidance for implementation of this type of scheme, as the ambient levels will be lower than the Illuminating Engineering Society recommendations, which may create liability and occupant performance concerns among designers and owners. Application is also still unproven for direct lighting with wallwashing.
Energy legislation that intends to remove the least-efficient products from the market continues to target older, relatively inefficient technologies—such as T12. One consequence of this is the rationale for incentives promoting certain technologies, such as T8 lighting, will soon be harder to justify. As a result, utilities may shift to supporting design approaches such low ambient/task lighting. California, in fact, has already begun development work, integrating this approach into utility incentive programs.
DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.