Michael Lane LC

archived articles

Private Office Lighting
January 1997

So what is the best lighting system for private offices? Recessed lensed, recessed parabolic, indirect, direct/indirect? The most common solution, the recessed parabolic luminaire, may actually be the worst option.

A few years ago, at the Lighting Design Lab, we did an extensive mock-up for one of our sponsoring utilities. The mock-up had four identical closed offices, but with different lighting systems. Office A used four Columbia 1-lamp asymmetric 1-by-4s located next to the wall, acting as wall washers; office B used four Zumtobel RC 1/36 to simulate indirect lighting without changing the ceiling height; office C used two 4-lamp lensed luminaires; and office D used two 3-lamp parabolic luminaires with the center lamp disabled. Office C with the 4-lamp lensed luminaires was used to simulate standard office design of the '70s and '80s. All four of the lighting systems were connected to a dimming system that adjusted the light level from 20 to 60 footcandles on the desk with the initial level set at 35 footcandles; office C's light level ranged from 20 to 100 footcandles with the initial level set at 100 footcandles. The study involved both computer and paper tasks, and participant spent either 2 hours (short-term test) or about 8 hours (long-term test) in the four rooms.

The results were, that office A and office B were the highest rated with office B slightly preferred due to the style of the luminaire. Participants were allowed to tune the lighting level and preferred 45-55 footcandles instead of the 35 footcandle initial setting. Office C with the lensed luminaires were preferred over office D, but both lagged well behind office A and office B. Many of the participants complained that office D was shadowy and dark feeling.

So are you using the wrong luminaire for lighting private offices? If you are using parabolic fluorescent luminaires, the answer would have to be yes.

How does the lighting system in offices A and B differ from the lighting in offices C and D? The big difference is that in offices A and office B the luminaires light the walls along with lighting the work surface. Then why didn't office C perform as well as offices A and B since it also lights the walls and work surface? Some of that difference has to do with the physical placement of the luminaires. In office C the lensed 2-by-4s, as with any direct lighting system, are sensitive to furniture placement . A typical layout would place the luminaires symmetrically in the center of the space. With this layout, the luminaires tend to be over the occupants, cause shadows, and can produce glare in eyeglasses. This last point is why the parabolic luminaire performed so poorly in the test as it does in real life.

In a perfect world, with parabolic luminaires, the luminaires would be located at each end of the desk so the light would fall perpendicular to the worker. Seldom does the lighting layout respond to the furniture layout. (How often does the client provide you with a furniture layout anyway!) The beauty of a indirect lighting system is that it is not furniture sensitive. With office B, the indirect lighting system, the ceiling and walls become the luminaire and light comes from all directions. With office A, the luminaires are located along and are lighting the walls which keeps the lights away from the occupant, and reduces shadows and glare.

So why aren't you doing lighting as described in offices A or B? Probably because of cost. In both offices A and B there are four luminaires as compared to only two lensed or parabolic luminaires as in offices C and D. Additionally the 1-by-4 luminaires in office A and the indirect luminaires in office B tend to be more expensive than the lensed luminaires in office C and the standard parabolic luminaires in office D . These two facts could make the lighting system cost three to four times more expensive. So you're stuck; offices A and B provide a better lighting because the lighting system is not furniture sensitive, but cost more than your client is willing to pay.

One solution to the above problem would be to use a single pendant mounted direct/indirect luminaire. I know you're thinking, "One luminaire in a private office will never work." Well, I have been living in a 10 ft by 12 ft office for the last four years with a single 2-lamp luminaire. The luminaire is nearly 86 percent efficient with a 60/40 indirect/direct distribution. From a furniture placement standpoint, a single luminaire placed in the center of the office is not ideal, but the indirect component really fills the room with light. The direct component provides adequate light on my desk--about 40 footcandles. The best part is, that because there is only one luminaire, the cost of the lighting system is not extravagant and may meet the client's budget. Oh by the way, my office uses only 0.5 watts per square foot, that should also make the client happy.

Now, I will be the first to admit that about half of the people who come through my office think it is slightly underlit . Not me though. The best option to increase the light level would be to use a three lamp direct/indirect luminaire which would increase the light levels to over 50 footcandles while using only 3/4 of a watt/sq.ft.

The important thing to remember is quality over quantity. As designers we need to balance footcandles, energy consumption and cost of the lighting system, but providing the best lighting quality possible should be your primary concern.