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

1000 Lumens × 5 downlights = 5000 lms.
So the equation
E = Lms ÷ A
becomes
E = 5000 Lms ÷ 150 sq.ft.
or
E = 33.3 Lms per sq.ft.
or
E = 33.3 Foot-candles
(Similar to the illuminance level we might picture on the desk of a home office or library).
Lumen Method Example 2: Solving for Lumens or Luminaires Needed
We tend to use the Lumen Method Calculation more frequently for figuring out how to deliver light to provide a desired illuminance level. In these cases, we use this same equation flipped around.
Illuminance = Lumens ÷ Area
becomes
Lumens = Illuminance × Area
or more specifically
Lumens needed = Illuminance level target × Area (in square feet)
A basic situation where we need to solve a lighting challenge may look like this two-part process:
Given the same 10’ × 15’ room, we would like to light the floor to an average illuminance level of 60 foot-candles. How many lumens need to make it to the floor surface?
Figure 20.4 How do we deliver the desired illuminance level to a 10’ by 15’ floor?
We use our basic equation
Lumens needed = Illuminance target × Area
or simply
Lumens = Illuminance × Area
or
Lms = E × A
We plug in the values we know.
Our illuminance target is
E = 60 foot-candles.
Our area is
A = 10’ × 15’ = 150 square feet
So the equation
Lms = E × A
becomes
Lms = 60 fc × 150 sq.ft.
or
Lms = 9000 Lumens
We need to introduce 9000 functional lumens of light onto the floor to get the average illuminance level we are after.
How do we get those 9000 functional lumens onto the floor? We need to determine what quantity and type of luminaire can accomplish this goal. Hence, the calculation has two parts. In order to answer the question of how many luminaires we need, we need to know the lumen output of those luminaires. For this reason, most luminaire manufacturers provide cut-sheets that provide this information. For this example, we will assume that we have found a luminaire that emits 550 lumens and that all of these lumens will make it to the floor in the space.
If we were to use a luminaire that provides 550 Lumens onto the floor, how many luminaires would we need to evenly place in the space.
This question translates to a simple equation: