Installing bright 2-foot by 4-foot, three-lamp, T12 fluorescent fixtures 8 feet apart in dropped tiled ceilings was the common method of office lighting design in the 20th century. Lighting designers believed it was best to give people a lot of light, much more than is now recommended by ANSI and IEEE standards.
Back then, a recommended level of horizontal footcandles served as the most important criterion for evaluating an office lighting system. However, the ninth edition of the Illuminating Engineering Society of North America's (IESNA) Lighting Handbook expands considerably on what constitutes good lighting and is the best reference until the re-release of IESNA's Recommended Practice for Office Lighting (RP-1). It's currently being updated to include the latest research in vision and technology, and it should be available later this year.
Office work involves a variety of visual tasks. The ease of each task depends on several factors, including an employee's ability to see and read office documents, the contrast of the task with the background, the luminance of the background, and how long the documents must be viewed.
Luminance differences. An undesirable ratio of the amount of light delivered on the work area to the amount of light adjacent to the area and in the general field of view of the worker can cause problems ranging from annoyance to noticeable glare. Thus, RP-1 recommends that the luminance within the immediate area should maintain a maximum bright-to-dark ratio of 3:1 with respect to the task. Greater contrast away from the immediate task area but within the field of view may be desirable to enhance visual clarity, depth perception, and a sense of spatial orientation. In these areas, a maximum ratio of 10:1 with respect to the task area is recommended.
One way to provide suitable bright-to-dark ratios with respect to the task — also called nonuniform lighting — is to restrict the lowest illuminance to a circulation area or a corridor adjacent to the area of general illumination. A task-lighting fixture installed at each workstation would provide even higher illuminance levels on the work surface.
Applying controls. Today's control technology helps to minimize unnecessary lighting use in an office throughout the day and to comply with various codes and standards. All applicable codes should be considered in the design, but they're never a substitute for a thorough analysis of proper energy use and economic tradeoffs.
Consider these general provisions for both manual and automatic controls:
Each separate office or area should have its own control switch(es).
In large open spaces, each defined work area should be grouped and switched independently.
When using single- and two-lamp fluorescent luminaires, adjacent luminaires should be placed on alternate circuits.
When using three-lamp fluorescent luminaires, the inside lamp should be connected to a separate circuit from the outside lamps.
When using four-lamp fluorescent luminaires, the inside pair of lamps should be connected to a circuit that's separate from the outside lamp pair.
When using electronic ballasts, consider using ballast models with high/low circuiting and/or with dimming capabilities.
Two of the most common automatic control systems used by lighting designers in offices today are based on the concepts of occupancy and daylight harvesting. In the former, infrared or ultrasonic motion detectors automatically turn lights on and off depending on activity in the space, thus saving on energy consumption. In the latter, a harvesting system continuously senses the level of daylight coming through the windows and skylights and turns off or dims the lighting system accordingly.
Numerous firms are applying a dimming system that allows each employee to use a wireless handheld remote control, computer commands, or a coded telephone message to customize the lighting level supplied to his or her work area by nearby ceiling fixtures.
Lighting design concerns. For the purpose of illuminating the work areas of an office, the general lighting in the ceiling can serve as both ambient and task lighting. This is best for private offices or special situations where separate task lighting is inappropriate. It's also possible to use localized lighting for task lighting fixtures and then provide a lower level of general, or ambient, illumination. A 30-footcandle to 35-footcandle level of ambient light is a typical design goal, but some employees prefer even less — on the order of 15 footcandles to 20 footcandles. A general illumination level is appropriate for most open plan offices that use vertical partitions and cabinets.
The work areas of an open office can be bullpen-like desk arrangements, desk and credenza combinations, floor standing panels that support work surfaces and storage components and partially enclose a space, and free-standing screens or panels between desks.
These kinds of layouts can present problems, though, because the panels and storage shelves above work surfaces may create undesirable shadows on the tasks and adjacent surfaces. Depending on how many are in the room, these panels will change the distribution of light and reduce the luminance on the work surface. As the number of vertical partitions increases, their reflectance becomes more significant. It's also important to note that dark finishes absorb more light.
General lighting luminaire types. The following types of suspended, surface, or recess-mounted luminaires for general office area illumination are usually installed in a pattern that provides completely uniform lighting throughout the space:
Open direct luminaires. Designed without any type of shielding, and unless equipped with reflectors, these systems radiate light in all directions. This type of illumination system offers very efficient lighting with high coefficient of utilization (CU) values, but it can cause visual discomfort and disabling glare.
Shielded direct lighting luminaires. These systems are equipped with some type of louver to prevent direct viewing of the lamps at normal angles of view. Infrequently used today, the opaque louver consists of flat vanes — sometimes called an eggcrate louver — made of plastic or aluminum in a longitudinal and lateral design. A louver with a depth equal to its width has a shielding angle of 45°.
The vanes of the more popular parabolic louvered troffers are manufactured with a parabolic surface. Light rays that strike the sides of the cells are redirected downward out of the fixture, optimally providing a cutoff angle of 45°. Thus, a minimum of reflected brightness from the fixture is seen on nearby CRT screens. The quality of a deep-cell parabolic louver depends on the type of metal finish and how well it cuts off light source glare.
However, the low source brightness of parabolic louvered troffers can create the appearance of a dark ceiling and dark upper part of walls. This “cave effect” can be reduced or eliminated with perimeter fluorescent “wall-washing” units and other methods.
Lensed direct lighting luminaires or troffers. Although these systems typically have higher efficiency and CU values than parabolic louvered troffers, they provide less precise glare control. Diffuser, clear refractor (usually a prismatic pattern), pigmented refractor, and hemispherical refractor lens types are available. The actual photometric performance of a lens is also determined by other factors, such as the reflector type and the number and type of lamps used in the luminaire.
Indirect lighting luminaires. These systems radiate light up to a reflective ceiling where it's redistributed downward. Such systems generally employ luminaires suspended from the ceiling, though cove lights and lights mounted to walls and furniture can also be used. Indirect lighting systems that use well-designed and properly spaced luminaires can provide excellent illumination, uniformity, and freedom from glare.
Recent designs that use T8 and T5 lamps in fluorescent indirect systems use lenses or imaging reflectors to achieve high luminaire efficiency by producing a broad batwing light distribution while allowing for close-to-ceiling mounting. This raises the CU of the indirect system to nearly that of a lensed troffer system.
Direct/indirect luminaires. These systems direct light both down and up to the ceiling in a suitable distribution pattern that reduces or eliminates the possibility of creating glare on computer screens in the office.
Recessed direct/indirect luminaires. Either rectangular or square in shape, these systems include lamps that filter output through a perforated lamp shield to provide soft, glare-free down lighting. At the same time, light is also evenly distributed and reflected off the matte finish of the main reflector to provide uniform indirect light distribution in the vicinity of the luminaire.
Like modern control equipment, lamp technology has improved significantly. Both the T12 fluorescent lamp and the magnetic ballast are now considered obsolete and have been replaced by the T8 lamp and electronic ballast. The latter offers reduced energy/operating costs, eliminates the annoying flickering common to previous generations of fluorescent lamps, and the T8 lamps also greatly improve color rendering. The T5 lamp, which is smaller and more efficient than the T8 lamp, is particularly suited for indirect and indirect/direct luminaires in offices and similar spaces.
Environmental components. The reflectances and textures of room finishes affect the light level and apparent brightness of the room. Dark finishes and heavy textures absorb light so it isn't reflected back into the room. Light finishes help to reflect more light into the space, thus making the lighting system more efficient. Lighter wall and ceiling finishes increase the measured light level and make the space appear even brighter.
Room boundaries have a strong influence on how a space is perceived and on the effectiveness of a lighting system. Room cavity ratio, which is the ratio of the surface area of walls to the floor area, is often used in lighting calculations to determine luminaire performance in an office space.
Additionally, natural light that comes through windows and skylights has an effect on a worker's perception of time and place, which is psychologically beneficial. However, the potential problem of distracting glare that comes from windows must be considered in the lighting design.
In the business world, so much time is spent trying to find ways to cut costs through improved efficiency, but oftentimes the effect the office itself can have on employee performance is overlooked in that equation. Lighting is one of the best ways to improve the working environment. Savvy building managers are discovering that a properly designed lighting system that takes employee comfort into account by reducing glare and increasing natural light may not ultimately be noticed by the workforce, but it will have a definite effect on the bottom line by making the office a pleasant place to work.