with this flexibility in order to specify a level that is suited to the visual task, keeping in mind the lighting
design issues listed at the beginning of this chapter and in Chapter 10, Quality of the Visual Environment.
In determining an appropriate illuminance level, the designer must also consider how the illuminance is to
be delivered, and to what locations. It is essential to differentiate between general lighting for the space
and the illuminance specifically on the task or at the task location. In open plan offices, providing tasklevel illumination only at specific task locations and at a lower illuminance level throughout the space is
typically appropriate. In private offices with free standing desks, it is more likely that the general
illumination of the room provides the task level illumination.
The general illumination level of an office facility should be determined by several factors. The
reflectance values of surfaces surrounding the task area should be considered to create a visually
comfortable environment. Luminance levels surrounding the task should not be greater than three times
the luminance value of the task, or less than one-third the luminance value of the task. If the offices
contain VDTs, the general illumination should meet the guidelines established for that specific type of task
(see the section "Offices with Video Display Terminals" in this chapter). Additionally, the general
illumination should meet the psychological need for light of the occupants of the space. It should be
remembered that room reflectance values and the distribution characteristics of the luminaire may be as
important as illuminance level.
Figure 11-3. In a VDT screen, veiling reflections from bright objects, which reduce contrast, are
prominent on a white-on-black display (right side of screen). These reflections are less
noticeable on a black-on-white display (left side of screen).
If there is more than one task in the space, with each requiring a different illuminance, the designer must
choose among them. There are several alternative methods for combining different target values.
The illumination requirements of different tasks may be satisfied by providing different task lights. A
flexible lighting system, individual dimming controls, and multilevel switching are other available
alternatives, depending on furniture layout and architecture. For locations with multiple tasks, designers
can design for the task requiring the highest level of illumination and provide dimming capabilities that
allow the user to adjust the lighting level in various areas to suit different tasks. Multilevel lighting
systems also may be appropriate. If flexibility is not possible, the designer may be forced to choose one
criterion over another for the entire system. However, it should be noted that most task lights provide more
than enough illuminance.
Often, office buildings are built on speculation, so that the visual tasks and the occupants are unknown. A
building in which the lighting has been thoughtfully designed for today's typical office tasks is more
attractive for prospective tenants. A logical recourse is to design for the modern electronic office in which
a combination of paper and VDT tasks will be perfor
performed. Ambient illuminances throughout the office
space should not exceed 500 lx (50 fc), where VDTs are used, and extreme care should be given to
providing a general lighting system that does not create disability glare, or reflected glare off of VDT
screens (see the section "Offices with Video Display Terminals" in this chapter). Higher illuminances at