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 20.6 Light delivered with a high Coefficient of Utilization (left) and light delivered with a low Coefficient of Utilization (right).
There is a very tedious way to accurately calculate the coefficient of utilization that takes into account room geometry, surface reflectances and luminaire placement. For our purposes, we will rely on a few ballpark numbers that will simply add to the usefulness of our lumen method calculations. Like the light loss factor or any other safety factor, we include these numbers in our calculations to make sure that we are over designing and accommodating for non-ideal conditions. The list below shows Coefficient of Utilization values that would be appropriate in a calculation where we are concerned with an illuminance level on the floor or a horizontal work plane. The values also assume that the space in question consists of fairly reflective surfaces:
Lumen Method Calculations with Safety Factors
Both of the factors discussed here simply get inserted into our equations to convince us to use more light or, conversely, to expect less light from our lamps and luminaires.
This creates a more accurate and useful equation for predicting lighting
effect. With the addition of the light loss factor and the coefficient of utilization, our lumen method calculation goes from:
Illuminance = Lumens ÷ Area
to
Illuminance = (Lumens × LLF × CU) ÷ Area
Our formula for determining how much light we need to introduce into a space goes from:
Lumens needed = Illuminance target × Area
to:
Lumens needed = (Illuminance target × Area) ÷ (LLF
× CU)
Oftentimes, when we are after a very basic calculation and we don’t want to numerically include these two factors, we refer to the lumens in our equation as “functional lumens,” that is, lumens that we know are being utilized to put light onto the task with which we are concerned. In this manner, we can use the basic versions of our equations, without the safety factors, but describe our results in terms of how many “functional lumens” we would need, or how many “functional lumens” we are providing.
Regardless of whether we use the simplified equations, or whether we
include the two safety factors, it is important to recognize where lumen method calculations are viable. In order for an “average illuminance level” to be meaningful, the space or surface in question must actually have an “average illuminance level.”
So the lumen method calculation is really only valid for large, open areas with even lighting layouts. This means that if we have a space where the luminaires are concentrated and directed to one area, knowing that the average Illuminance level on the floor is 25 foot-candles tells us very little since we can plainly see that the room is exceptionally bright in one area and very dark in another area.
Good candidates for lumen method calculations are open office spaces,
classrooms, sports arenas, warehouses, public hallways… any rectangular space with an even layout of luminaires and few obstructions.
When we are interested in lighting effects onto objects and specific
surfaces, we use the other form of calculation: the point calculation method.