Interior Lighting for Designers

the luminaire; sometimes either a third
plane at 45° or three planes at 22½° inter-
vals are added (figure 10.2).
Reflector lamp sources or luminaires
with directional distributions and abrupt cut-
offs, where the light intensity changes rap-
idly within a small angular area, give values
that are difficult to read on a polar graph.
Consequently, a rectilinear or Cartesian
graph is substituted to portray the candela
distribution (figure 10.3).
On this graph, the horizontal scale rep-
resents the degrees from the beam axis and
the vertical scale represents the intensity in
candelas. In the graph on the right of figure
10.3, approximately 3,000 cd are produced
by the light source at 10° from the beam
center.
When selecting luminaires for a lighting
application, be sure that the proposed
luminaireand its sourceare precisely those
shown in the manufacturer’s photometric
test data. It is inaccurate to extrapolate from

one source or reflector finish to another

unless the photometric report includes mul-

tipliers for various tested sources and reflec-

tor finishes.

Recommended Illuminance Values

Illuminance value recommendations are

published as footcandles (fc) at thework-

plane. For almost all commercial and indus-

trial activities, this surface is considered to

be a horizontal plane 2 ft 6 in AFF (above fin-

ished floor)—standard desk height—even

though the space may be a corridor or a bas-

ketball court with no desk in sight.

Remember that these values refer to

illuminance on the horizontal work surface

only; they have limited significance to us

when we interpret the actual environment.

Such factors as wall lighting, brightness

accents, shadow, sparkle, and color have a

greater influence on emotional reaction.

These factors are particularly important in

areas involving casual seeing where low

INTERIOR LIGHTING FOR DESIGNERS

Figure 10.2Polar graph for a fluorescent luminaire.