| quotient of the sum of the areas of the working plane and the plane of the luminaires and the sum of the area of the walls bounded by the working plane and the plane of the luminaires, used in the calculation of the utilization factor or utilance Note 1 to entry: The room index is expressed by the formula where 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 to entry: See also "ceiling cavity index" and "floor cavity index". Note 3 to entry: The room index has unit one. Note 4 to entry: This entry was numbered 845-09-55 in IEC 60050-845:1987.
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