The required illuminance of a surface depends primarily on the type or nature of the jobs or human activities on the illuminated surface.
Therefore, step 1 is to determine how much illuminance is required. Type of task, duration of usage, age, and number of people working at that area, precision level of the work, available natural lights are all some of the factors that decide the illuminance requirement.
Step 2 is to determine how many and what type of light sources are required to produce the required luminance. Three main factors have to be considered.
The type of the source: cost, colour, and life of the source are the main deciding factors to decide the type of light source.
Incident light energy follows the square law. The illuminance on a surface is indirectly proportional to the square of the distance making the distance from the source an important factor.
The co-efficient of utilization also is a factor that determines how much of the luminous flux from the source is actually available on the required surface. Reflectors, lampshades, and luminaires can increase or decrease the utilization factor. Good industrial fixtures can have utilization factors up to 90 %.