Room Cavity Ratio (RCR) for CU Lookup
The shape number a lighting designer needs to read the coefficient of utilization (CU) off a luminaire's photometric report before the lumen method: RCR = 5 x cavity height x (length + width) / (length x width), where the cavity height is from the luminaire plane down to the WORK plane (~2.5 ft above floor), NOT floor-to-ceiling. A 40 x 30 ft room with an 8 ft luminaire-to-workplane cavity has RCR 2.33. A tall, narrow room (big cavity vs floor area) has a HIGH RCR -- more light hits the walls, less reaches the work plane, so a lower CU; a low, wide room has a low RCR and higher CU. Enter this RCR with the ceiling/wall/floor reflectances into the manufacturer's CU table, then that CU feeds the lumen-method fixture count. A design input; the fixture's IES photometric file and the actual surface reflectances govern the CU.
Formula and source
room_cavity_ratio = 5 x cavity_height_ft x (room_length_ft + room_width_ft) / (room_length_ft x room_width_ft). Cavity height = luminaire plane to the work plane.
IES zonal-cavity method room cavity ratio (IES Lighting Handbook), by name; the fixture's photometric (IES) file and the actual surface reflectances govern the coefficient of utilization it reads.
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