In the realm of lighting design, the Coefficient of Utilization (CU) is a critical metric that determines how effectively a luminaire’s light output reaches the work plane. CU tables, often provided by manufacturers, account for room geometry and surface reflectances—specifically ceilings, walls, and floors—to predict lighting efficiency. Understanding these tables is essential for accurate illuminance calculations using the lumen method.

Surface reflectances significantly influence CU values. A white ceiling with 80% reflectance, light walls at 50%, and a dark floor at 20% produce high CU factors, as more light is reflected downward. Conversely, dark surfaces absorb light, reducing CU. For example, a room with 50% ceiling, 30% walls, and 10% floor reflectance might yield a CU of 0.45, compared to 0.70 for brighter finishes.

Designers must match CU table entries to actual room conditions. Tables are indexed by room cavity ratio (RCR) and reflectance percentages. Interpolation is sometimes needed when exact values are absent. Proper use of these tables prevents over- or under-lighting, ensuring energy efficiency and visual comfort.

In practice, adjusting surface reflectances through paint or finishes can improve CU by 20–30%, reducing fixture counts and operational costs. Thus, mastering CU tables is not just technical—it is a strategic tool for sustainable, cost-effective lighting design.