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Coefficient of Utilization COU

Used for building lighting to calculate illumination levels, the Coefficient of Utilization (COU) measures the efficiency of a fixture to use the lumens from the lamp(s) therein.

Normally found in table form, it shows a decimal (percentage of lamp lumens) based on the Room Cavity Ratio (RCR) (or zonal cavity, often the same number) on the Y axis and the ceiling and wall reflectances on the X axis. Normally the floor reflectance is assumed to be 20%, and is only stated as a note below the chart.

Williams Downlighting's catalog page has specification sheets showing their COU's and how they relate to the RCR and room reflectances.

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