Interior Lighting for Designers

illuminance values; room surface luminances;
and lighting power density (watts/ft^2 ). Output
is usually a chart of calculated values, an
isofootcandle plot, or a shaded plan with
gray scales representing the range of
illuminance values. This software can model
reflections only from perfectly diffuse sur-
faces, however, although this is adequate for
most lighting calculations.
Computer-generated ray-tracing calcu-
lationsare the most accurate method of
computing illuminance. By tracing each
“ray” of light, realistic depictions of
illuminance patterns on room surfaces, par-
titions, furniture, and artwork are displayed,
including reflection from specular surfaces
and refraction by transparent objects.
Output is in the form of renderings or video
images. Hardware requirements for this kind
of program are greater than for other meth-
ods; at a minimum, a personal computer
with powerful graphics capabilities is neces-
sary.
The IESNA publishes an annual survey
of lighting software in its magazineLighting
Design + Application. Products are reviewed
for analysis features, applications, outputs,
user features, hardware requirements, and
costs.

Surface Reflectance

Although interior surfaces are not light con-
trol devices, their reflectance properties are
fundamental to the lighting design. The
quantity and direction of light reflected from
these surfaces affect both the efficiency of
the initial light distribution and our percep-
tion of surface brightness.
Wall, ceiling, and floor surfaces are
large-area “reflectors” that redistribute light
in the room. High-reflectance finishes, such
as white and off-white, promote maximum

use of the available light; increasingly darker

finishes intercept and absorb increasingly

greater proportions of the light.

Because a useful amount of light reaches

the workplane after reflection from the walls

and the ceiling, the efficiency of the lighting

system depends in part on the reflectance of

room surfaces and finishes. This is particu-

larly true of ambient-diffuse and indirect sys-

tems, where a large portion of the light is

initially directed toward the ceiling or the

walls or both.

In task-oriented spaces such as offices,

factories, or cafeterias (where the “task” is

seeing the food and the people), the follow-

ing surface reflectances are recommended

for the efficient use of light:

20–50% floor

50–70% wall

70–90% ceiling

The illuminance values in tables 15 and

16 in the Appendix presume that commer-

cial reflectances are:

20% floor

50% wall

80% ceiling

and presume that industrial reflectances are:

20% floor

50% wall

50% ceiling

The recommended reflectances for fur-

niture, machinery, partitions, and work sur-

faces are the following:

25–45% furniture and machinery

25–50% work surfaces

Room surface finish reflectances are

obtained from the manufacturers of paints,

PHOTOMETRICS