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

Illuminance = Candela value ÷ Distance2 (distance in feet squared)
Or
E = CD ÷ D2
The equation is best understood through a simple example.
Point Calculation: Example 1:
Suppose we have an accent luminaire recessed into a 10’-0” ceiling directly above a plate that is on a table at 3’-0” above the floor. If that accent luminaire has a center beam candle power (CBCP) of 10,000 candelas, what illuminance level can we expect at the brightest point on the plate?
This is best expressed as a diagram as in Figure 20.1 .
Figure 20.11 Point calculations are commonly used in situations involving one source and one object of interest.
Using our basic Point Calculation equation
Illuminance = Candela value ÷ Distance2
or
E = CD ÷ D2
plug in the values we know:
center-beam candela value of the source
CD = 10,000 Candelas
distance squared: The “Distance” is the distance the light must travel. In this case, it is the mounting height of the luminaire (10 ft.) minus the height of the table (3 ft.)
D2 = 7 feet, squared = 49 square feet (the units get squared too, which represents the translation of distance into surface area)
Our equation becomes
Illuminance = 10,000 CD ÷ 49 sq.ft.
Or
Illuminance = 204 Foot-candles.
This is a high illuminance level, but certainly within reason for an object we are trying to accent. It is also important to note that this illuminance level occurs only at the bright center of the pool of light on the object.
We tend use this calculation more frequently to deduce what type of
candela value (and therefore source) we need to accomplish a certain lighting task. An example of such a situation may look like example 2
Point Calculation: Example 2:
Suppose we have an accent luminaire recessed into the same 10’-0” ceiling directly above the same plate that is on a table at 3’-0” above the floor. If we want to illuminate that plate to 150 Foot-candles, what kind of center beam candle power value would we need from the luminaire? The diagram is the same, but we use the flipped version our basic equation