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

Luminaires needed = Lumens needed ÷ Lumens per luminaire
We solve this by plugging in what we know
Luminaires needed = 9000 Lumens needed ÷ 550 Lumens per luminaire
so our answer
Luminaires needed = 16.36
gets rounded up to
Luminaires needed = 17
So we have determined that 17 of these specific luminaires spaced evenly in the 150 square-foot space will yield us the 60 foot-candles of average Illuminance onto the floor.
Lumen Method Reality Checks
When we perform lumen method calculations for critical situations, it is important to add in two factors that make our calculations much more realistic. The first of these factors is a safety factor, or “maintenance” factor called the light loss factor, or “LLF.” The second factor is the coefficient of utilization, or “CU.”
Light loss factor
Light loss factor is a way to account for the fact that the light performance of our lamps and luminaires will deteriorate over time for a number of reasons.
The light loss factor accounts for lamps getting dirty and thus emitting less light
over time. It also accounts for loss in performance of ballasts and transformers over the life of the luminaire. There is an articulate method of calculating each of these contributing factors, but it is common to use an industry standard generic value of 0.85 for a light loss factor. This means that we perform calculations expecting only 85% of the light from our lighting system to be working in the space as the system ages (In exceptionally dirty spaces we determine this value more carefully). This assumption pushes us to design not for how we want a system to perform on day one, but rather for how we want the system to perform 2 or 3 years down the road. Just as an architect over-designs the key structural elements of a building, the lighting designer over designs to make sure that a space will perform as intended well into the future.
Coefficient of utilization
The other way to bring the simple lumen method calculation closer to reality is to account for how effectively the luminaires are working with the surfaces of the space to deliver light to the target surface in question. If we are concerned with the light level on a floor plane and we are implementing an indirect lighting system that delivers light upward onto the ceiling where it then
is reflected downward, it is safe to say that some of that light will be absorbed by the ceiling and walls and lost before it makes it to the floor. Indeed, it is rare that the majority of the light from a luminaire is directed immediately onto the surface with which we are concerned. It is also important to account for the geometry and reflectances of the room surfaces. Room surface geometry and reflectance will determine how effectively light inter-reflects to reach the surface in question. We accommodate for luminaire orientation, room geometry and surface reflectance by including a factor that takes into account these relationships in each specific situation. We call this factor the coefficient of utilization, or CU. The CU is expressed as a decimal number or a percentage to represent how effectively our lighting system is being utilized. The more directly our luminaires are being utilized and the more favorable the geometry, the higher the CU. In situations where light takes a very indirect path to the surfaces we are calculating, the CU is lower.