Determine how much light output, expressed in lumens, is
needed from the lighting to provide the required light level. As
1 fc is 1 lumen per square foot, a 100-square-foot room requiring
3 fc of general lighting means the luminaires need to produce
100 × 3 = 300 lumens.
The number of luminaires required is that number divided
by the lumens per luminaire.
We’re not quite done, however. We have to account for light
loss due to the size and dimensions of the room and its finishes.
First, determine the room cavity ratio (RCR), which, for a
typical rectangular room, is [ 5 × H × (L + W)] ÷ (L × W). H is the
vertical distance between the task plane (an invisible horizontal line intersecting the space where the primary task activity
occurs) and the center of the luminaire. L and W are the room’s
length and width. If the room is irregular in shape, RCR is ( 2. 5
× H × P) ÷ A, where P is the room’s perimeter length and A is
its area in square feet.
Once we have RCR, we can determine the coefficient of utilization (CU) using Table 2. This particular table assumes a 20
percent floor reflectance (medium color, such as light brown),
80 percent ceiling reflectance (white) and 50 percent wall
reflectance (pastel paint). A darker finish would reduce CU,
while a lighter finish would increase it.
Therefore, the number of luminaires needed to produce a
given light level is No. of Luminaires = (fc target × room area)
÷ (lumens per luminaire × CU). If our RCR is 8 (100-square-
foot room with an 8-foot mounting height), we would need 731
lumens. If we want to use luminaires producing 450 lumens
each, that would be two luminaires.
The above calculation would be conducted for general lighting (e.g., 5 fc on the floor in a kitchen, 3 fc in a hallway) and
might be repeated for each zone of task lighting. For accent
lighting, the object or area being highlighted should be at least
three times brighter than its surround. For important artwork,
this could be significantly increased.
An alternative approach in residential lighting design is
based on incandescent wattage. For general lighting, multiply
the room’s area by 1. 5. For task lighting, multiply by 2. 5. The
result is the minimum amount of lighting wattage needed. For
a 10-foot-by-12-foot kitchen, we would need at least 180 watts
(W) for the general lighting. For a kitchen island that is 6 feet
by 4 feet, we’d need 60W for the task lighting.
The problem with this technique today is that traditional
incandescent wattages are disappearing in favor of more efficient
lamps that produce similar light output (see Table 3). If you pre-
fer this formula but will be using it in a design that will feature
energy-efficient lamping, make a simple adjustment based on
the actual lamp that will be used. Better yet, convert to lumens
using a benchmark, such as a 60W incandescent. If 120 watts of
incandescent lighting are required, that would be 1,600 lumens,
which could be provided by halogen, LED or compact fluorescent
lamps (CFLs). However, even that lumen output is only roughly
equivalent if we’re talking about a similar type of luminaire.
Wherever possible, dim the lights.
Each lighting layer should be
separately controlled and dimmable,
which allows the owner to set
lighting scenes based on space
function, mood or preference.
Table 2: CU values based on RCR for a space with 20 percent floo ,
80 percent ceiling and 50 percent wall refle tance
(Source: California Lighting Technology Center)
CU 0.88 0.78 0.69 0.61 0.55 0.49 0.45 0.41 0.37 0.34
Table 3: Incandescent, energy-saving incandescent, LED and CFL
wattages (Source: manufacturer’s literature)
Lumens Watts Watts Watts Watts
450 40 28–29 4–5 9–13
800 60 40–43 6–8 13–15
1, 100 75 53 9–13 18–25
1,600 100 70–72 16–20 23–30
2,600 150 — 25–28 30–55