| quotient of the sum of the areas of the ceiling and the plane of the luminaires and the sum of the area of the walls bounded by the ceiling and the plane of the luminaires, used in the calculation of the utilization factor or utilance
Note 1 to entry: The ceiling cavity index is given by the formula
where a and b are the dimensions of the sides of the room and hc is the distance from the ceiling to the plane of the luminaires.
Note 2 to entry: See also "floor cavity index" and "room index".
Note 3 to entry: The ceiling cavity index has unit one.