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

Candela value needed = Illuminance level desired × distance squared
Or
CD = E × D2
We plug in what we know:
illuminance desired
E = 150 Foot-candles
distance squared
D2 = 7 feet squared = 49 sq.ft.
and our solution becomes
CD = 150 FC × 49 sq.ft.
or
Candela value (typically CBCP) needed = 7350 Candelas.
Of course the second part of this type of situation is figuring out what sort of lamp or luminaire is going to provide this candela value. We may even decide to use two luminaires in which case we need only half of the contribution from each of them. To find a specific luminaire or lamp, we simply look at the various candela values produced by different lighting equipment. In the case of luminaires, the candela values are represented in the form of a distribution diagram.
Figure 20.12 Candela distribution diagrams for various versions of a basic downlight.
When we are using one of our many accenting lamps, like an MR-16 or PAR lamp, the candela value will usually be expressed as the center beam candle power (CBCP), tucked in with a host of other information.
Figure 20.13 Accent lamp literature commonly describes the Center Beam Candle-power values of a family of lamps.
Point calculations become much more useful and slightly more complicated when we consider situations in which the object we are lighting is not perpendicular to the light source. In these situations we must include a bit of geometry in our calculation to be more accurate : If we are aiming the light source at any angle other than perpendicular to the surface being lighted, we know our “pool” of light spreads out and is therefore less “intense”. This is exemplified by the shape the “pool” of light takes on. As the aiming angle is increased, a circular pool of light becomes an elongated and widened “scallop” of light.
We modify our point calculation equation to accommodate for how the geometry will spread out the light intensity: