Task Lighting in the Open Office

When designing the lighting in a space, there are two different types of lighting to consider: ambient lighting and task lighting.

When designing a workspace, lighting should be given just as much consideration as the furniture and finishes selected for each area. Studies show that both daylight and electric light have an impact on productivity, error reduction, and the health and well-being of building occupants.
Historically, workspaces have generally been over-lit, and provided very little flexibility for end users. This led to wasted energy and user health issues such as eyestrain, disrupted sleep patterns, and headaches. As more and more building owners seek energy efficient designs and increased productivity, we are now seeing a shift in the industry towards dual-layer lighting solutions that incorporate task lighting at the workspace.
When designing the lighting in a space, there are two different types of lighting to consider: ambient lighting and task lighting. Ambient lighting (also known as general lighting) is the overall lighting within a space. It typically consists of a mix of daylight entering the space through windows and the electric light from general light fixtures throughout. In a well-lit space, ambient lighting should be enough light to see and move around comfortably and safely.
In addition to providing enough general lighting, each space should be lit for the work being performed there. The Illuminating Engineers Society (IES) publishes standard recommendations for the amount of light required for different tasks. Task lighting is measured in units known as footcandles (fc).
For example, the footcandle recommendations for some common workplace tasks are:
  • Reading documents or photocopies: 20 – 50 fc
  • Computer workstations: 10 – 25 fc
  • Filing or photocopy work rooms: 30 fc
  • Corridors / Lobbies / Transition spaces: 10 – 20 fc
Historically, light levels within a space have been measured as an average uniform horizontal illumination (footcandles) for an entire space. This means the general lighting is used to light the entire space to meet the minimum requirements of the tasks that happen there. Today, many companies and organizations designate a minimum footcandle requirement at the worksurface.
Currently the majority of tasks performed in an office workspace are a combination of screen-based tasks and written/typed document viewing. These tasks require significantly different light levels. Consider the workspace of a user that engages in both monitor and hard copy reading. If the average general lighting in that workspace is set at 50 fc to accommodate the task of hard-copy reading, the amount of light will be significantly higher than necessary for the task of monitor viewing in that same workspace, which is only 25 fc according to the IES. Not only is this less energy efficient, but it could also lead to a variety of employee health and productivity issues, including headaches, eye strain, and fatigue.
A different approach, and one that is growing more common, is to design the ambient light in a workspace to be adequate for monitor viewing, safety, and comfort while also providing task lighting that properly illuminates hard-copy documents. This is known as “dual-layer lighting.”
When considering a dual-layer lighting strategy, here are some general rules to ensure an effective design.
  • The lighting should provide an even distribution of light and cover the entire workspace where the task is being performed.
  • Vertical surfaces that are too high in contrast to the task surface may increase eyestrain, fatigue, or inattentiveness. Consequently, the furniture provider and the interior designer should coordinate wall finish colors, furniture panel colors, reflectances, and similar design elements to minimize possible negative side effects.
  • Task lighting should have user control – either a switch integral to the fixture or easily accessible from the workstation/desk. It should also allow movement directly at or away from different task surfaces.
In addition to increased user control and a healthier environment for employees, a dual-layer lighting strategy can lead to energy savings, as well. The EPA estimates that lighting accounts for 20-25 percent of the electricity used in the United States. This is typically the single largest source of electricity consumption in a building. Light fixtures also generate heat which, in turn, leads to additional air-conditioning (and energy use) throughout the space.
There is sometimes an upfront investment in providing task lighting, but the energy savings help to offset this initial cost. In some studies, the savings are as high as 60% energy reduction and 50% reduction for overall life-cycle costs. Other studies have shown that users have a higher tendency to turn lights off when they have individual control of their fixtures. Several manufacturers even offer task lights with built-in occupancy sensors that automatically shut off the fixture if the workstation is empty. All of these attributes contribute to reduced energy costs.
In one of our recent projects, the design guidelines called for 50 fc at each workstation. The high number of work stations, the furniture panel height, and the dark color of the furniture resulted in a determination that the general lighting method was actually lighting the space to 76 fc just to achieve that 50 fc minimum at each desk.
After taking field measurements, we did a quick case-study to see the effect of a dual layer lighting approach. Here is what we found:
The open office area was about 29,200 square feet with 290 employee workstations. In order to achieve the 50 fc using ambient lighting alone, we used 432 two-by-two lay-in fixtures. The result was 0.61 watts of lighting per square foot. By adding task lights for hard-copy document reading to each workstation, we discovered the amount of ambient lighting could be reduced to 28 fc at the work surface – a sufficient amount for monitor-based tasks. The dual-layer method reduced the number of two-by-two lay-in fixtures needed from 432 to 240 fixtures, almost a 50% decrease. The total lighting power density, including task lights, was only 0.42 watts per square foot. That is a 33% reduction in energy usage alone! After applying an operating cost of 12 cents a kilowatt-hour, we identified operating cost savings of $1,665 annually.
We also analyzed the lighting project from an initial investment perspective. The installation cost for the single layer lighting was approximately $133,920. With fewer two-by-two fixtures, the dual layer lighting approach reduced the cost of installing the two-by-two fixtures to just $74,400. Even after factoring in the additional 290 task lights for each workstation at a cost of $58,000, the total cost was $132,400 – this is $1,500 less than the cost of the general lighting approach.
Finally, many of our clients are eligible for energy savings utility rebates. When we plugged the data from each lighting scenario into our local utility application, we discovered the client would receive about $4,138 more in rebates if they selected the dual-layer lighting technique over the general lighting technique.
Lighting design is often a subjective piece of workspace design. The best approach is one that accomplishes the right blend of energy efficiency, maintenance feasibility, aesthetic appeal, and user comfort. In order to achieve the ideal level of user comfort and to also realize the energy savings discussed above, a layered lighting approach must be implemented at the beginning of a project. This will require good communication and coordination among all parties, including the electrical engineer, lighting designer, interior designer, and furniture provider. In most workplace designs, the dual-layer lighting method will lead to improved user comfort, increased productivity, and bottom-line benefits for the business.

Kelsey Rowe PE
Associate Partner, Electrical Engineer