Interior Lighting for Designers

office with an 8 ft ceiling and a regular
arrangement of eight luminaires; each
luminaire uses two 26 W compact fluores-
cent lamps.

fc =Number of lamps Initial lamp lumens LLF CU

Area

×××

1. The number of lamps (2 lamps per
   luminaire × 8 luminaires) is 16.


2. From a large-lamp catalog, find that the
   initial lumens for a 26 W compact fluo-
   rescent lamp are 1,800 lm.


3. LLF = LLD × LDD.

a. LLD: in table 11, underfluorescent,

compact, find that the lamp lumen

depreciation factor is 0.85.

b. LDD: in table 12, find that a direct

downlight with an opaque, unaper-

tured top enclosure and without a

bottom enclosure is maintenance

category IV. In table 13, a very clean

room that will have its luminaires

cleaned every six months yields a

luminaire dirt depreciation factor of

0.96.

c. 0.85 × 0.96 = 0.82.

4. To find the coefficient of utilization, first
   calculate the room cavity ratio:
   a. h= ceiling height of 8 ft 0 in minus
   desk height of 2 ft 6 in=5ft6in
   (5.5 ft).

RCR

5.5

2.75

=

+

×

=

+

=

5

51530

15 30

()( )

()( )( )

()()

hl w

lw

b. In table 14, which is supplied by the

manufacturer of this compact fluo-

rescent, open-reflector downlight,

find that for a room with 80% ceiling

reflectance, 50% wall reflectance,

20% floor reflectance, and an RCR of

3, the CU is 0.68.

fc =

(16)(1,800)(0.82)(0.68)

(15)(30)

= 36

This office, lighted by eight, two-lamp,

26 W, compact fluorescent, open-reflector

downlights, will have an average maintained

illuminance of 36 fc on the desk. Although

the lumen method does not demonstrate it,

in all spaces the illuminance value is higher

in the center of the room and drops off near

the walls because of the absorption of the

perimeter surfaces.

Shortcomings

Whether executed by hand or calculated by

computer, the lumen method fails to provide

therangeof light intensity in a room and

identify where differences in illuminance

values occur. It is, therefore, inaccurate for

nonuniform and task-ambient lighting sys-

tems.

This method is also unable to provide

information about lighting quality, visual

comfort, and luminance patterns. The lumen

method is useful mainly for predicting hori-

zontal illuminance with general lighting sys-

tems.

Computer Assistance

The complete zonal-cavity method is found

in the ninth edition of the IESNALighting

Handbook, Chapter 9, and is available on

disc from several software companies for

use with a personal computer.

Computer-generated point calculations

yield illuminance at selected points through-

out a room. They also provide average, maxi-

mum, minimum, and standard deviation of

INTERIOR LIGHTING FOR DESIGNERS