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

THE POINT CALCULATION METHOD
When we are interested in illuminance levels that exist at a very specific point on a surface, we use a simple calculation that takes into account how the delivery of light spreads out as the distance between the light source and the object increases. We generally use this calculation for considering object, accent and task light levels that are a product of a few specifically-placed luminaires that are directing their light immediately onto the surface in question. Because of this, the point calculation method requires that we be able to at least estimate the location of our lighting equipment in reference to the surface and the distance and aiming angle between the two. One of the most common situations to use the point calculation is in accenting artwork or other objects of interest.
The equation we use for the point calculation method is, again, not really
an equation at all, but rather an expression based on the unit definition of the candela. Candelas are a measurement of light density exhibited from a specific source in a specific direction. Candle-power (expressed in Candelas) is a very common way to describe the way light leaves directional luminaires.
When we consider a glowing globe, it is fairly obvious that the source
distributes light evenly in every direction.
Figure 20.7 A candela distribution diagram (left) for a diffuse source (right).
Architectural luminaires like downlights, accent luminaires, spot-lights and wall-washers have optics and reflectors that drive light in a very specific manner. The Candle-power measurement from a particular luminaire is a measure of the light density that a luminaire produces in a specific direction.
Figure 20.8 A candela distribution diagram (left) for a directional source (right).
There really is no way to estimate the candela values coming from a light source by observing the source, so this information must be given to us by the manufacturer of the lamp or luminaire. Luminaire literature often includes a candela distribution diagram. This diagram gives specific candela values that occur directly beneath a source and at various angles.
Figure 20.9 Literature for a high-performance luminaire may provide a candela diagram.
Point calculations usually involve very directional luminaires that create pools or spots of light. In these situations, we can usually count on the highest concentration occurring at the center of the pool of light. The candela value from this center point has a special designation: we call it the “Center Beam Candle- Power” or CBCP. Some luminaire and lamp literature will forgo a complex candela distribution diagram and simply publish this CBCP value, assuming that the user is concerned with the bright area of light at the center of the beam.
Figure 20.10 The highest candela value from a light source is commonly found at the center of the light output.
The point calculation method is based on the principle that light spreads out as it travels away from a source. This spread of light is exemplified by the pool of light created by a directional luminaire. As the distance from the source increases, the pool of light gets larger, but also gets less “bright” because the same quantity of light is spread out over a larger area. Using this principle, we can determine the illuminance level at a specific point by knowing the candela value of the source in that direction and the distance and aiming angle from the source to the point. Candelas are an expression of light density or lumens per area where the area is a piece of spherical space (see ch. 18). The most important step in putting the point calculation equation to use is determining the distance value “D” from the source to the illuminated point in question. This value must be determined and always expressed in feet before it is squared in the equation.
The equation for a straightforward point calculation looks like this: