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Introduction to Indirect Lighting

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“Indirect lighting” has come to represent a family of fixtures in which 90-100% of the light output of the fixture is bounced off of room surfaces before reaching the workplane. It is available in a broad range of configurations, mounting options, materials, shapes, colors and distributions. Typical indirect fixtures include pendant-mounted linear fluorescent, workstation-integrated/mounted, disk-shaped pendant-mounted decorative compact fluorescent, and others.

Indirect lighting for open office plans began to grow in popularity with the rapid adoption of computers in the early ’90s. The computers feature a video display terminal (VDT), a vertical workplane and a highly reflective surface. Since lensed recessed fluorescent troffers in the ceiling of many open offices are brighter than the surrounding ceiling space, they can be reflected on the VDT screen. The eye tends to drift towards the brightest spot in our field of view, making these reflections distracting and visually fatiguing. In many cases, veiling reflections from VDT screens can obscure tasks on the screen.

In 1992, the Illuminating Engineering Society of North America published RP-24 (RP-1), which offers guidelines designed, in part, to minimize glare on VDT screens. Afterwards, indirect lighting sales began growing 15% per year, showing up in offices, computer classrooms and other environments. Indirect lighting, properly designed and installed, can reduce glare on the screen by reflecting light from ceilings and walls. By 1996, said one manufacturer, indirect lighting began to be considered in 40-50% of projects where visual comfort was an important design goal. As a result, the industry flourished, reducing prices, increasing volume and promoting research and development.
One of the most exciting new developments in indirect lighting is the introduction of the T5/HO lamp, which reduces required fixture profiles, increases possible fixture spacing distances, and reducing the number of fixtures required in a space.

Indirect Lighting Types
Indirect lighting is generally classified as indirect (“pure indirect”) and direct/indirect. Indirect fixtures bounce all light off of room surfaces, using these surfaces as reflectors (see Figures 1, 2). The result is a soft, uniform distribution of light with minimal shadowing and glare on VDT screens.
Direct/indirect fixtures are indirect fixtures that incorporate a downlight component. These fixtures provide some direct lighting to the workplane while reducing contrast between the fixture and the surrounding bright ceiling plane (see Figures 2, 3). As a result, direct/indirect fixtures can be more energy-effective than pure indirect fixtures. A number of methods are used to distribute and shield the downlight component, including baffles, wire-mesh, silk-screen and lenses.

Task-Ambient Lighting Systems
In many office workspaces, the overhead lighting system was designed and specified separately from the workstations, which provide high supplementary light levels at the task. The result is very high light levels that may be unnecessary and waste energy.

Indirect lighting and task lighting are often combined in a task-ambient lighting system, providing layers of lighting with light output concentrated closer to the task. In such a scenario, the indirect fixtures provide sufficient illumination for orientation (about 20-30 fc), while fixtures located close to the task provide higher light levels for the task. As a result, fewer lumens from the overhead lighting system may be required, resulting in energy cost savings.

Task-ambient systems may also improve the quality of the visual environment. A May 1997 study was conducted in tandem with the renovation of the offices of the Norwegian Trade Council in New York City. The renovation included installation of indirect lighting that provided ambient illumination for orientation, etc. and task lighting that provided task illumination (“two-component” office lighting). The Council’s 140 employees were interviewed. Dr. Alan Hedge, Professor of the Department of Design and Environmental Analysis at Cornell University, reported the results in a white paper, “Evaluating the Benefits of Two-Component Office Lighting,” in January 1998:

Results showed that most interviewees found the two-component lighting performed well to support their visual tasks. None of the interviewees reported experiencing glare from the lighting, although glare from the windows was a problem for some employees. Most interviewees reported being highly satisfied with the office lighting. Ratings of lighting quality showed several statistically significant before-after changes. The two-component lighting was rated as significantly better, less bright, attractive, pleasant, calming, likable, soft, relaxing and interesting … All interviewees thought that the two-component lighting improved the office appearance. Eighty percent of interviewees reported being satisfied with their lighting and all thought that lighting was an important office feature.

Completely direct distribution luminaires can create glare. Completely indirect distribution luminaires may create flat, uninteresting spaces. Direct/Indirect luminiares can offer a balance between the two extremes.

Advantages/Disadvantages

Indirect lighting’s advantages include:

• Provides soft, uniform light distribution
• Minimizes harsh shadowing
• Minimal glare on VDT screens
• Good for lighting vertical surfaces such as bookshelves in libraries
• Reveals faces well
• Makes the space appear brighter and more spacious by adding more light to ceiling and upper walls
• At peak performance, direct/indirect fixtures are much more efficient than direct parabolic fixtures, with efficiencies as high as 95% for direct/indirect vs. 50-70% for lay-in fixtures
• Works well to provide low ambient illumination while task lighting provides most of the task illumination, reducing energy costs

The disadvantages of indirect lighting include:

• Possibility that details in three-dimensional objects will not be rendered as well as direct lighting
• Higher installed cost in many cases than direct lighting schemes
• Fixture reflectors and room surfaces must be cleaned regularly to ensure maximum performance

In addition, care must be taken to balance relative brightness and contrast ratios within the space.

Interaction With Room Surfaces
Room surfaces are used as an integral component of all indirect lighting systems. The reflectance of the surface material, and other room surfaces, directly affects the delivery of light to the workplane. Other characteristics of these surfaces, including texture, material and color, will directly contribute to overall perception of the visual environment

In response to a growing emphasis on surface reflectances, Armstrong World Industries now manufactures high-reflectance acoustic ceiling systems with reflectance values up to 89%. A study performed by the Weidt Group used computer simulations to predict that the use of these high-reflectance ceiling tiles can produce increased light levels of up to 25% compared to standard ceiling tiles with reflectance values that typically range from 75% to 82%.

Pendant-mounted indirect lighting works best for higher ceilings, about 9.5 ft. or greater. With lower ceilings, the fixture must be mounted closer to the ceiling, resulting in a “hot spot” that in itself can become a source of reflected glare on VDT screens.Indirect fixtures should be mounted at least 1.5-2 ft. below the ceiling plane. For lower ceiling heights, recessed direct/indirect fixtures can be considered for use or fixtures can be integrated/mounted onto office furniture (see Figures 5, 6, 7). When using recessed indirect fixtures, the designer must note the contrast ratios between the ceiling plane and the reflector to avoid reflected glare in VDT screens.

Cost
According to one manufacturer in an article in a 1996 issue of Architectural Lighting, costs for indirect lighting had come down from about $60-70 per linear foot in the early ’80s to $20-30. In a June 2000 article in Design Share, Randall Fielding, editor, estimated the cost of a 4-ft. steel pendant-mounted, pure indirect fixture to be about $80 and a similar pendant with direct/indirect distribution to be about $140 (compared to a recessed parabolic fixture starting at about $50). However, the specifier should be cautious about comparing these fixtures apples-to-apples, as a different number of fixtures may be required for a direct vs. indirect solution to an office space. Because of the high level of interest in indirect fixtures, costs will continue to come down with competition and technological progress. Productivity studies involving direct-indirect fixtures are driving sales in an upward trend, which should also decrease cost due to volume.

Conclusion
Indirect lighting, if properly designed and matched to the right application, can provide a pleasant visual environment that appears more spacious, minimizes glare on computer screens, and can save energySource: www.lighingdesignlab.com