| number representative of the geometry of the part of the room between the working plane and the plane of the luminaires, used in calculation of utilization factor or utilance
Note 1 – Unless otherwise indicated, the room index is given by the formula
in which a and b are the dimensions of the sides of the room and h the mounting height, that is the distance between the working plane and the plane of the luminaires.
Note 2 – In British practice, the ceiling cavity index is calculated from the same formula except that h is the distance from ceiling to luminaires.
Note 3 – In the USA, the term room cavity ratio is currently used. This is equal to five times the reciprocal of the room index defined by the formula in Note 1. Two supplementary terms are used : ceiling cavity ratio and floor cavity ratio which are derived in the same way as the room cavity ratio except that h is respectively the distance from ceiling to luminaires and from floor to working plane.