Advancing the effective use of light for society and the environment.

Tuesday, July 12, 2005

Simple Retrofit Options Explored in Office Lighting Field Study
By Jennifer Taylor
Office lighting study
LRC researchers lowered ambient light levels by removing the central lamp from each fixture.

General lighting in large open-plan offices can be a double-edged sword. On one hand, general lighting evenly illuminates cubicles at high light levels and gives the space a bright appearance. On the other hand, this type of lighting can contribute to excess energy use. Typical office light levels (500 lux or more) are often higher than needed for most office tasks. Uniform distribution wastes lighting energy by illuminating walkways and corridors where extra lighting is unnecessary. Therefore, lowering light levels by 30 percent can save energy without impacting visual performance. Yet this approach can create a gloomy appearance throughout the space. Many lighting specifiers and facilities managers face this conundrum: How can we save energy without major renovations or complaints of darkness and gloom from office workers?

The Lighting Research Center conducted a field study to identify simple retrofit solutions that save lighting energy in open-plan offices while providing enough brightness to give workers a cheerful impression of the space.

The field study, sponsored by The Connecticut Light and Power Company and conducted at the offices of the State of Connecticut Department of Public Safety, explored office workers’ responses to lower ambient light levels and two brightness-enhancing experiments.

Design of the experiments

Yukio Akashi, Ph.D., LRC senior research scientist, and Peter Boyce, Ph.D., LRC professor emeritus, designed the study with three stages. In the first stage, they evaluated the lighting conditions present in the office and asked office workers for their opinions of the current lighting.

In the second stage, one fluorescent lamp in each luminaire was removed in some offices, other offices were outfitted with new lamps with a much higher correlated color temperature (CCT), and a few offices received a combination of both techniques. Recent studies have found that higher CCTs contribute to people’s stronger perceptions of brightness, particularly because of the shorter wavelengths found in these lamps. Again, workers were asked for their opinions of the lighting.

In the third stage, “sparkle elements” were added to the luminaires in some offices to provide brightness and interest. Sparkle elements are reflective objects or secondary light sources (such as LEDs) that provide small areas of high brightness to create interest without glare. “The nice thing about sparkle elements is that they can create the perception of brightness with very little additional power, or even no additional power at all,” says Dr. Akashi. This field study used high-reflectance, diffused reflectors as sparkle elements without additional power. During the third stage, workers were asked to complete a final survey of the lighting in their space.

Lighting reduction shows no long-term change in worker satisfaction

Survey results showed that after an initial adjustment period, office workers were generally satisfied with the lower levels of ambient lighting, which were brought down nearly 40 percent from 550 lux to 340 lux. Akashi says, “The reduction of ambient lighting led to an increased use of task lighting at workers’ desks. But the energy consumption from more task lighting was negligible and did not offset the savings we realized from reducing the amount of ambient lighting.”

In the short-term, however, workers reacted negatively to the decreased amount of light in the office. “From the initial response, we learned that the best approach to lowering light levels is to do so gradually, making the change in a series of steps rather than all at once,” says Akashi.

The brightness-enhancing experiments showed some positive results. “The change to a 6500 K CCT lamp was effective at increasing workers' perceptions of brightness in the offices where we lowered the ambient light level,” says Akashi. However, those working in offices where the ambient lighting did not decrease reacted negatively to the cold appearance of the high-CCT lamps.

The introduction of sparkle elements produced some inconclusive results. Some workers initially found that sparkle elements enhanced the lighting, while others thought they were too bright. In the long-term, adding sparkle elements did not significantly change the workers’ perceptions of brightness or gloom, although sparkle elements led to positive opinions in terms of preference and visual clarity. Akashi notes that many factors could have attributed to the results, including the style of sparkle element used, and that more research is needed on sparkle.

Retrofit recommendations

Akashi and Boyce recommend that lighting specifiers and facilities managers take the following steps if they wish to reduce lighting energy use in open-plan offices:

  1. Most offices use 3-lamp or 4-lamp recessed parabolic luminaires. Check whether these luminaires can operate on fewer lamps without modification. If they cannot, the retrofit process is unlikely to be economical.
  2. Assuming that the luminaires can work with fewer lamps, relamp the whole installation with fluorescent lamps rated at 5000 K CCT.
  3. Remove one lamp from every third luminaire, except from those around the perimeter of the office space.
  4. Wait three months. This allows time for workers to adapt to the new lighting. Afterwards, remove one lamp from every second luminaire, except those on the perimeter.
  5. Wait another three months for adaptation time. Then, remove one lamp from all the remaining luminaires, except those on the perimeter.
  6. Have a stock of task lights available to workers who complain of too little light.

About the LRC

The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute and is the leading university-based research center devoted to lighting. Founded in 1988, the Lighting Research Center has built an international reputation as a trusted and reliable source for objective information about lighting technologies, applications, and products. Its mission is to advance the effective use of light and create a positive legacy of change for society and the environment.

Rennselear Polytechnic Institute