Visual Comfort Probability (VCP)
Avisual comfort probability (VCP)rating is
defined as the percentage of people who, if
seated in the least desirable location in an
office work space, will find a lighting installa-
tion comfortable. VCP depends on the size
and shape of the room, the reflectances of
room surfaces, and the location and light
distribution of the luminaires.
A VCP of 70 or more is recommended
for general office use, and 80 or more for
office areas using video display terminals
(VDTs). Originally tested and validated using
lensed fluorescent direct luminaires, VCP is
applicable only for direct lighting systems.
Reflected Glare
Visual comfort is achieved by limiting not
only direct glare but also reflected glare.
Reflected glare is excessive uncontrolled
luminance reflected from objects or surfaces
in the field of view. This includes the reflected
luminance from interior surfaces as well as
the luminance of the lighting system.
Specular surfaceshave reflecting prop-
erties similar to those of a mirror. The lumi-
nance reflected is the mirrored image of the
light source, or of another lighted surface
within the reflected field of view.
These properties make specular sur-
faces useful as reflectors for light control in
luminaires, but polished or specular interior
surfaces such as desks, countertops, floors,
walls, and ceilings introduce problems of
reflected glare. Diffuse surfaces prevent
highlights and are uniformly bright from all
angles of view.
Reflected images on glass and other
transparent materials form a visual barrier.
At night, large areas of glass may become
black mirrors. If surfaces viewed through the
transparent material are higher in luminance
than the reflected images, a sense of trans-
parency is achieved.
Most work surfaces reflect light both dif-
fusely and specularly. Diffuse reflectance is
dependent on the quantity of illuminance on
the surface. Specular reflectance is depend-
ent on the luminance of the source: the
reflected image of the lamp or luminaire. This
reflected image causes a veiling image on the
work surface that obscures surface detail.
In the visual task area, remove glossy
surfaces wherever possible. A glass-covered
or highly polished desk top is quite specular;
reflected images become distracting. Matte
(low-gloss) finishes should always be used
for work surfaces.
It is helpful to think of the work surface
as a mirror when orienting task luminaires
(figure 3.34). Proper luminaire location
reduces reflected glare from the task (figure
3.35). When luminaires are located on
either side of the desk, shadows cast by the
luminaires are filled in and light is reflected
away from the worker’s eyes (figure 3.36).Video Display Terminals (VDTs)
VDTs are glossy vertical work surfaces.
Screen reflections are caused by variations
in luminance being “seen” by the screen sur-
face and reflected into the worker’s eyes.
Screens that are convex and inclined
upward, in particular, reflect into the eyes
large areas of ceiling, walls, windows, and
the surrounding space.
Positioning the screen, adjusting its
angle, low-reflectance screens, blinds on win-
dows, and dark clothing for workers are tech-
niques that relieve many reflection problems.
Reflections caused by the lighting system can
be controlled with properly designed, deep-
cell parabolic louvers to prevent lamp images
from appearing on the VDT screen.
In addition to reflections on the screen
surface, which make viewing the screen
images difficult, two additional lighting prob-
lems that cause concern for VDT users are:
(1) proper lighting for the non-VDT tasks theBRIGHTNESS