Calculations of illuminance for a given interior include the portions of Luminous Flux contributed by all simultaneously illuminated luminaires plus the light reflected from the room perimeter (walls, ceiling, floor). Illuminance can be calculated for any given point from the Luminous Intensity of a light source and the Distance between the light source and the illuminated surface.

Horizontal or vertical Illuminance at a given point (point illuminance) can be calculated from Luminous Intensity and the relevant geometrical conditions from the following formulae: 

Horizontal illuminance Eh

Vertical illuminance Ev

Cylindrical illuminance Ez

Cylindrical Illuminance Ez is the average of vertical illuminance in all directions (usually 4) around a point (on a vertical axis).