Room Cavity Ratio (RCR) for CU Lookup

ScopeThe 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 or tableroom_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.

Source and editionIES 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.

Free accessThe RCR relation is the public IES zonal-cavity formula; the room dimensions and cavity height are the design geometry, and the CU it feeds is on the manufacturer's photometric report.

What governsEstimate. AHJ and licensed professional govern.

API field namesroom_length_ft, room_width_ft, cavity_height_ft, room_cavity_ratio

• IES zonal-cavity RCR RCR = 5 x cavity height x (L + W)/(L x W); cavity height = luminaire plane to the work plane (~2.5 ft above floor), not floor-to-ceilingIES Lighting Handbook
• Feeds the CU / lumen method high RCR (tall/narrow) -> lower CU; enter RCR with the surface reflectances into the fixture CU table; that CU feeds lumen-methodmanufacturer photometric report