The architecture of light: architectural lighting design concepts and techniques. A textbook of procedures and practices for the architect, interior designer and lighting designer pdfdrive com

Figure 20.1 One lumen onto a one square-foot area represents one foot-candle of illuminance
Figure 20.2 Ten lumens onto a 10 square-foot area is also represents one foot-candle of illuminance
LUMEN METHOD CALCULATIONS FOR LARGE, OPEN AREAS
The lumen method of calculating light levels is really not a calculation all, but rather an extrapolation of the unit definition of the foot-candle. When we dissect the foot candle, we see it is simply a measurement of light density in lumens per square foot.
1 Foot-candle = 1 Lumen ÷ 1 square foot
or
1 FC=1 Lm ÷ 1 sq.ft.
Therefore, we can generalize
Illuminance = Lumens ÷ area (in square feet)
or we can express it as mathematical abbreviations
E = Lms ÷ A (sq.ft.)
Putting this together tells us that to calculate the Illuminance level onto a surface, we simply count up the number of lumens falling onto that surface and divide by the area of the surface.
In the case of a whole room, that surface would likely be the floor, or an imaginary work plane above the floor at task height (often assumed to be 30”). A basic situation where we want to predict lighting effect may look like example 1.
Lumen Method Example 1: Solving for Illuminance level
Imagine a room that is 10’ × 15’, with 5 downlights placed evenly around the room. Each downlight emits 1000 lumens. If we imagine that all 1000 lumens from each luminaire end up on the floor of the space, what illuminance level can be expected onto the floor in this room?
Figure 20.3 A 10’ by 15’ floor lighted by 5 downlights.
We know that the solution to a problem like this is to simply count up the lumens that make it to the surface in question and divide by the area of the surface.
Using the basic equation
Illuminance = Lumens ÷ area
or
E = Lms ÷ A
We plug in the values we know.
The area of the floor is
A = 10’ × 15’ = 150 square feet.
The total number of lumens that make it to the floor “Lms”