22 August 2006
large open office, the ceiling may be more prominent in someone's field of
view than the walls.
Uniformity: Luminance uniformity should not exceed 5:1 in immediate work
surrounds, not including accent lighting.
Reflected Glare: With high computer use, the ceiling brightness must be
uniform to prevent reflected glare in computer screens. When viewing tasks
with a glossy finish on a desktop, bright luminaire components such as
visible lamps or lenses reflect in the surface of the task. This situation can
make reading tasks annoying and at times impossible.
Source / Task eye geometry: Task areas and luminaire locations need to
be identified to avoid shadows and direct and reflected glare.
Target Horizontal Illuminance ( 10%): 30 lux (30 fc) ambient, 500 lux (50
fc) on the task.
A task/ambient approach to the lighting in open offices results in separate control over
an ambient system (typically a pendant mounted direct/indirect luminaire) and task
lighting (a desk light or undercabinet luminaire). Design the under cabinet task light to
minimize veiling reflections by directing light away from or to either side of the task. By
providing a high illuminance level on the task only, and not the entire room, energy is
saved in the ambient system, which does not have to produce as much light. This
approach also provides a comfortable and flexible lighting environment. Manual
dimming can also be incorporated with the use of remote controls at individual
workstations that control only the nearby luminaires.
Integrating daylight with the electric lighting system greatly enhances the visual comfort
of the space and can save significant amounts of energy. Depending on the
configuration of workspaces and windows, lighting near the perimeter of the space may
be controlled as a separate lighting zone from the lighting towards the interior of the
space. In such a case, perimeter luminaires may be turned off entirely during the day
while only using some portion of the lighting in the interior of the office.
RULES OF THUMB:
Pendant spacing: When beginning a design, start with 3.0 3.7m (10 12
ft) spacing for T8 luminaires (5.5 6.0 m or 18 20 ft for T5HO systems)
and modify accordingly to meet critical design issues.
Pendant length: Pendant lengths range from 0.5 0.9 m (18 in 3 ft).
High performance luminaires may achieve a minimum of 0.3 m (12 in)
pendant lengths. Specialty luminaires for low ceiling applications may be
mounted even closer to the ceiling.
Lighting Power Density: The lighting power density for open office areas
can range from 0.9 1.2 watts /sq ft.