Interior Lighting for Designers

For a rough estimate of the average
illuminance on a horizontal surface, the
abbreviated version of the lumen method
described below will suffice. It considers
both the interreflections of light from room
surfaces and the contributions of several
light sources. It corrects for maintained
illuminance, accounting for typical deprecia-
tion in lamp lumens over the life of the
source and for dirt accumulation on the
luminaire surfaces.
This shorthand method is not a substi-
tute for more precise illuminance calcula-
tions. It is merely a guide to be used as a
quick analysis during the design process.
To calculate the average maintained
illuminance falling on a horizontal surface,

fc =Number of lamps Initial lamp lumens LLF CU

Area

×××

Number of lamps. For single-lamp lumi-
naires, the number of lamps equals the
quantity of luminaires in a given area. If
the luminaires contain more than one
lamp, multiply the number of luminaires
by the number of lamps per luminaire.

Initial lamp lumens. The initial lamp lumens
are published by the lamp manufactur-
ers in their large-lamp catalogs.

LLFis an abbreviation forlightlossfactor. As a
system ages, a natural depreciation in
light output occurs. (With tungsten-
halogen lamps, the loss is negligible.)
Also, dust and dirt accumulate on room
and luminaire surfaces. Other light loss
factors such as ambient temperature,
actual input voltage, ballast factor, HID
lamp position, and lamp burnouts influ-
ence the illuminance in a space; for this
quick method, only two are considered:
LLD and LDD.

• LLDis an abbreviation forlamplumen
  depreciation. This is the amount of

light output that is reduced over the

life of the lamp because of filament

evaporation, tungsten deposits on the

bulb wall, and phosphor degradation.

A list of lamp lumen depreciation

for many sources is found in table 11

in the Appendix. When mean lamp

lumens (sometimes called “design

lumens”) are listed in the lamp cata-

log, this value may be used directly in

the formula without the LLD factor.

• LDDis an abbreviation forluminaire
  dirtdepreciation. This is the reduction
  of light output over time owing to the
  accumulation of dust and dirt on the
  reflecting and transmitting surfaces of
  the luminaire. This figure is dependent
  on the cleanliness of the space, the
  frequency of luminaire cleanings, and
  the luminaire’s tendency to collect dirt
  (for example, open-top luminaires
  have a greater ability to collect dirt
  than closed-top luminaires; some
  luminaires have a ventilation pattern
  designed so that the flow of air slows
  the accumulation of dust). See tables
  12 and 13 in the Appendix.

With this method, LLF = LLD × LDD.

Typical light loss factors for open light-

shielding systems (such as louvers) are

0.85 for very clean spaces, 0.75 for

clean spaces, 0.65 for medium spaces,

and 0.55 for dirty spaces.

CUstands forcoefficient ofutilization. The

CU is an expression of the percentage of

light output that is expected from a spe-

cific luminaire in a room. It accounts for

the efficiency of the luminaire: its ability

to deliver light to the work surface com-

pared to the lumens supplied by the

lamp(s). This is the amount of light that

is not trapped and lost inside the

luminaire.

INTERIOR LIGHTING FOR DESIGNERS