The coefficient of utilization (CU) is a measure of the percentage of lumens incident on the workplane to the total emitted lumens from the lamps in the luminaire. The CU value takes into account the room geometry, surface reflectance, and efficiency and distribution of the luminaire.

The CU table computed by Photometric Toolbox is applicable for rooms with an effective floor cavity reflectance of 20%. Multiplying factors for floor cavity reflectances other than 20% can be found in the IES Lighting Handbook.

The CU Table is used by applying the known Room Cavity Ratio (the space between the effective ceiling cavity and effective floor cavity) and the wall and ceiling reflectances for the given room.

Reading the CU Table:

cavity ratio = 5 x room cavity height x (cavity length + cavity width) / cavity length x cavity width

- for irregular rooms

cavity ratio = 2.5 x (cavity height x cavity perimeter) / area of cavity base

- You may need to interpolate between the given reflectances and RCR values to find the CU for your space.
- For example, for a space with a ceiling reflectance of 0.8 (80%) and wall reflectances of 0.5 (50%), you would look in the first grouping of columns.
- For this space, we calculated an RCR of 2.5. Reading down the 50 column, we calculate a CU of 43 (midway between 46 for an RCR of 2 and 40 for an RCR of 3).