Michael Lane LC

archived articles

Task-Ambient Lighting
August 1995

Task-ambient lighting systems have achieved a bad name, primarily due to lighting attached to the furniture systems. With open-plan offices and modular office furniture, it just made sense to add the lighting to the furniture. Just add a two- or four-lamp fluorescent strip in a box to the top of the partition, and presto! Instant lighting system.

Unfortunately, many people have eye heigher than the 5-foot partition . So to block the view of the lamps a small cell parabolic louver was added, which reduced efficiency and produced a rectangular pattern of light on the ceiling. These patterns of uplight never overlapped, creating an uneven lighting pattern and disappointed clients.

But task-ambient lighting is back. For those who live and work in states with energy codes, lowering power density requirements have forced designers to task-ambient lighting systems. Much different from 10 years ago, today's task-ambient lighting system can be a recessed parabolic, a pendant direct-indirect, a pendant indirect or even a furniture-mounted indirect. The key to today's task-ambient lighting system is to provide the correct maintained footcandles on the task without overlighting adjacent areas, and adding task lighting where needed.

Lets look at an office example: In the IES handbook Figure 11-1 Currently Recommended Illuminance Categories and Illuminance Values for Lighting Design - Targeted Maintained Levels, under the OFFICES (General and private offices) we would be referred to the READING activity.

If we average the act ivities under READING we find 3 F's, 14 E's, 14 D's, 1 C's, and 2 B's. So the majority of the tasks are either activity E (50-75-100 footcandles) or activity D (20-30-50 footcandles). With task-ambient lighting you would design to middle of activity D, 30 footcandles, and add task lighting where activity E tasks are performed. This may mean you will have to spend a little more time designing the lighting system and calculating illumination levels, but the end result will meet the clients needs and save energy.

In my office I have between 30 and 45 maintained footcandles on the desk from the ambient lighting. The 30 footcandles are on the computer portion of my desk and the 45 footcandles are on the writing-reading portion. The computer portion of my desk may actually be over lighted at 30 footcandles, and the lighting on the writing-reading portion is adequate for most of my tasks.

When I need more light I just turn on my task light, which can easily provide an additional 50-60 footcandles on the work surface. Of course, the task light I use is a compact fluorescent version. There are many nice looking, adjustable, swing arm task lights. By the way the power density in my office is only 0.53 watts/sq.ft. not including the task light. Adequate footcandles, quality lighting, and energy savings have all been achieved by using a task-ambient lighting system.

So how do you design a task-ambient lighting system? First, stop over lighting spaces. Refer to the IES Lighting Hhandbook lighting tables and pick the appropriate lighting level. Second, get yourself a good computer lighting program. The IESNA Lumen Method for calculating average footcandles is good for a quick estimate of the light levels. But, you will overlight your spaces every time if you rely solely on this average method.

Think about it. If your target light level is 50 footcandles and you use the Lumen Method, you will design to 50 footcandles AVERAGE . Many parts of the space will have over the 50 footcandles. This is where a good computer lighting program can fine-tune your lighting system and save you and your client money.