Spacing Ratio Often given by fixture manufacturer. Spill light is viable light, though not enough to consider for lamp spacing, it is enough to provide adequate illumination to wall cabinet fronts in a kitchen, for example. The amount of energy consumed by the lamp. Do not select a lamp based solely on wattage. This is a very common mistake.
Wattage has nothing to do with the amount of light produced by a lamp.
A lamp should be chosen based on its CandlePower or Lumen rating which indicates the light energy intensity of the lamp. Only then should wattage be considered to find the most economical lamp among those lamps powerful enough to provide the proper FootCandle level on the WorkPlane. In any space there is an actual or implied height at which an activity takes place. It is on this WorkPlane that the FootCandle measurement is taken. Whether you are calculating General or Task illumination, the light level is determined in FootCandles measured at the WorkPlane height.