By Craig DiLouie
Over the past 30 years, computers have become a staple in modern offices. According to a 2003 issue of the Quarterly Journal of Economics, between 1984 and 2003, the percentage of people directly using a computer at work more than doubled, from approximately 25 percent to nearly 57 percent.
The proliferation of computers in office buildings transformed these environments and worker productivity; it also had a major impact on best practices in office lighting and, by extension, office lighting technology. As computer technology continues to evolve, another significant shift in lighting technology is occurring.
Traditionally, office general lighting includes lighting fixtures that provide a general level of visibility in a space, with light distributed uniformly on worksurfaces. But older computer screens are sensitive to lighting fixture brightness in directions approaching horizontal. To reduce glare on computer screens in offices, lighting fixtures were redesigned to use shielding at high angles. That workhorse fixture, the lensed troffer, fell out of favor, while parabolic fixtures became popular.
The new generation of premium troffers offers higher fixture efficiencies - around 90 percent, compared to as low as 65 percent for many conventional parabolic troffers. LITHONIA
The problem with parabolic fixtures is that, while they reduce glare on computer screens, they are often said to create harsh shadows and a dim atmosphere. In particular, this type of fixture tends to produce strong scalloping on nearby walls, resulting in an atmosphere commonly called the "cave effect."
"Parabolics create shielding at high angles to reduce glare," says Grant Hukle, senior product manager, fluorescent lighting at Day-Brite Lighting, Tupelo, MS. "The drawback to this approach is that the upper spaces of the room are then dark, which makes the room feel darker and smaller than it actually is, as well as creating significant contrast on the walls."Changing Landscape
The question is not whether parabolics are effective; it's whether they are even needed anymore. Computers have changed over the years. Today's screens have less curvature, as well as improved screen brightness and anti-reflectance technologies. Flat LCD screens are becoming increasingly popular, and software
developers typically rely on positive contrast (dark characters against light backgrounds) to present information. Such advances have virtually eliminated the sensitivity of computer screens to high-angle brightness.
As a result, a new type of fluorescent troffer has been introduced in the past few years and is gaining in popularity, both for its energy story and its potential impact on visual comfort and lighting quality. Possibly better known as "volumetric lighting," a term popularized by Lithonia Lighting after it introduced RT5 in 2004, this fixture type is also known by other terms such as "diffuse lighting" or "full distribution lighting." For purposes here, "premium troffer" will be used as a generic descriptor.
Premium troffers (right) can make a room feel brighter and bigger while reducing energy consumption. Lighting quality is improved over conventional parabolics because soft, uniform illumination is placed on the high vertical surfaces in the space. DAY-BRITE LIGHTING
A premium troffer is a fluorescent troffer with a photometric distribution that includes a small amount of light in the 60- to 90-degree zone, tapering off as the angle gets closer to 90 degrees. "With a better balance of luminance throughout the visual field, the space feels brighter, larger, more open, more relaxing," says Steve Lydecker, vice president commercial fluorescent for Lithonia Lighting, Conyers, GA. "Facial rendering is more natural, consistent, and complimentary. Shadows tend to be softer and less pronounced. The appearance of the space is less defined by sharp, arbitrary transitions in surface brightness. This type of lighting does a better job of rendering architecture, its contents, and its occupants true to form."
Premium troffers may feature (concealed) T5, T5HO, or T8 lamping and are available in different sizes. LITHONIA
Premium troffers compete against older-design parabolic and conventional troffers at the low end, updated parabolics and "basket" recessed fixtures introduced in the 1990s in the middle, and direct/indirect fixtures at the high end. Updated parabolics and basket fixtures provide some of the same benefits as premium troffers, but fall short in the high-angle light control (basket fixtures) and fixture efficiency (baskets and parabolics). Premium troffers are specified in offices, schools, healthcare facilities, retail spaces, such specialized public spaces as airports, and other environments where general ambient fluorescent lighting is used. The architectural aesthetic of these fixtures is well suited to upscale offices and similar commercial applications. While not common in retail environments, the higher vertical uniformity is well suited to applications with racks or shelves.
"These could be new architectural-style products or updated parabolic fixtures that have been designed to boost efficiency and offer a higher portion of vertical light," says Keith Hall, director of marketing and product development for Metalux at Peachtree, GA-based Cooper Lighting. "These units tend to be designed with higher efficiencies and use new lamp/ballast technology. Higher vertical light levels add to a perception of more light in the space, allowing energy to be reduced while still meeting the end-user's needs. The benefit is more light for less energy."
Premium troffers direct a carefully controlled amount of light into the upper spaces of the room - more than a parabolic, but less than a conventional troffer. Their photometric distributions have the highest candlepower directly below the fixture, tapering off smoothly as the angles get closer to 90 degrees. LITHONIA
Premium troffers may feature (concealed) T5, T5HO, T8, or compact fluorescent lamping and are available in different sizes - and, in some cases, depths to accommodate various lighting needs and ceiling types. The latest trend in premium troffer design is a fixed lamp cross-section. In other words, the fixture can be specified only with a specific number and type of lamp offered by the manufacturer. For example, a two-lamp T8 fixture cannot be specified as a three-lamp T8 fixture, and so on; instead, the fixture optics are optimized for a specific number of lamps. The benefit of this is higher fixture efficiencies - around 90 percent, compared to as low as 65 percent for many conventional parabolic troffers - with two-lamp units producing comparable light output as conventional three- or four-lamp troffers. This higher efficiency means a significant percentage of lamps can be eliminated from the design, which means, of course, higher energy savings by leveraging fixture efficiency in addition to lamp and ballast efficiency. If more or less light output is needed, different lamp/ballast combinations can be specified to tune output to the application need.
"Another consideration is that premium troffers generate smooth lighting throughout the space rather than only in selected areas below the fixtures, which makes the lighting system much more adaptable to any changes in the configuration of the space," says Hukle. In some office lighting designs, workstations become focal points in the illumination design, with light concentrated on the desktop and tapering off between workstations. Workstations, however, may have to be moved. "Uniformity is important considering how much portability today's equipment has - things like partitions that move and laptops that are used in many locations, rather than fixed computers on desks."
While still a small part of what is largely a highly commoditized lighting category, Lydecker says premium troffers are the fastest-growing segment of the North American lighting market. As energy codes continue to become more stringent and sustainability continues to grow in popularity in construction, look for premium troffers to proliferate in high-end office, school, and other commercial projects.
|Best Practice Office Lighting |
Although accent lighting can add focal illumination for visual interest, uniform lighting is typically preferred for office general lighting, connoting a public, business-like atmosphere. Lighting that is uniform tends to reinforce impressions of space, alertness, and visual clarity. Basic choices for light distribution include direct, indirect, and direct/indirect. Basic fixture types found in office environments include recessed lensed, parabolic, and basket-type fixtures, as well as premium troffers and direct/indirect pendants.
One theory about what constitutes good general office lighting is to avoid totally direct and totally indirect lighting fixtures.
Recessed direct lighting is typically efficient (in terms of percentage of lumens exiting the lighting fixture), but can create harsh shadows and a dim atmosphere, particularly if the fixtures are spaced too far apart or their light distribution pattern is narrow. Of particular concern is the fact that recessed direct lighting fixtures tend to produce strong scalloping on nearby walls, creating the cave effect. In addition, direct lighting can produce veiling reflections - a type of reflected disability glare that obscures a task - if the fixture is located above and slightly in front of the task/occupant.
|An office lighting field study conducted by the Light Right Consortium (www.lightright.org) found that lighting systems that provide downlight (two by four troffers) were rated as "comfortable" by 69 to 71 percent of study participants, leaving a significant percentage (about three out of 10) in a state of discomfort. |
Totally indirect lighting also presents significant trade-offs as a design approach. On the plus side, indirect lighting scatters light in many directions, diffusing light distribution, which can aid visual comfort and facial recognition while eliminating shadows that are distracting and reduce visibility. Using indirect lighting only, however, can make a space appear flat and empty of highlights and shadows that provide spatial definition.
Lighting design authorities have increasingly identified a combination of direct and indirect lighting as best practice for offices. Options include direct/indirect pendants, premium troffers, and recessed basket-type fixtures listed according to level of initial cost. While premium troffers are not actually indirect products, the distribution of light at high angles results in light traveling through the space in directions close to horizontal, which produces results similar to indirect distribution.
Whatever option is chosen, the fixture should include lamping that provides a mean efficacy of at least 90 lumens/watt and is operated by electronic ballasting. The fixture itself should have a high efficiency; the overall combination of the source efficacy and fixture efficiency should deliver an output of 80 lumens/watt or greater, a metric defined as the Luminaire Efficacy Rating (LER). Additionally, because overall efficiency improves as a light source is positioned closer to a task, supplementing general lighting with task lighting enables lower levels of general lighting, which can produce a 20- to 30-percent energy savings. Generally, open and private offices should be lighted to 30 average maintained footcandles for ambient lighting (assuming an empty room).
Craig DiLouie (firstname.lastname@example.org), a lighting industry journalist, analyst, and marketing consultant, is principal of ZING Communications.