Interior Lighting for Designers

The CU also accounts for the efficiency

of the room in redirecting and inter-

reflecting the incident light that strikes its

surfaces. This is affected by the room’s

proportions as well as its reflectances. A

large, low room is more efficient than a

tall, narrow one. In the large, low room,

little incident light is interrupted by the

walls; almost all of the light is received

directly by the workplane. In a high,

narrow room, much of the incident light

strikes the walls at least once before

reaching the workplane, sometimes

being reflected between several sur-

faces before reaching the task.

These variables produce an infinite
number of CUs for each luminaire. For prac-
tical purposes, they are reduced to a group
of figures for typical room proportions and
reflectances.

Room Cavity Ratio.The CU is found by
checking the manufacturer’s coefficient-of-

utilization table, which is usually published

on the back of product data sheets. In order

to use the CU table, it is first necessary to

calculate the room cavity ratio.

The room cavity ratio provides an

expression of the efficiency of room propor-

tions. To determine this ratio:

RCR=

+

×

5( ) (hl w)

lw

wherehis theheight of the ceiling above the

task surface,lis thelength of the room, and

wis thewidth of the room. The task surface

is usually considered to be 2 ft 6 in AFF

(abovefinishedfloor) (figure 10.9).

A sample coefficient of utilization table

for a compact fluorescent, open-reflector

downlight is shown in table 14 in the Appen-

dix.

Example

Calculate the average maintained illumi-

nance on the workplane in a 15 ft × 30 ft

PHOTOMETRICS

Figure 10.9The room cavity used in the abbreviated lumen or zonal-cavity method.