What measurement indicates the luminous efficacy of a light source?

The luminous efficacy of a light source is measured in lumens per watt, which indicates how effectively a light source converts electrical power into visible light. Lumens measure the total amount of visible light emitted by a source, while watts measure the amount of energy consumed. Therefore, the correct way to express luminous efficacy is through the relationship between lumens and watts, not simply by identifying one or the other.

In this context, luminous efficacy provides valuable information about the efficiency of light sources: higher lumens per watt means more light is produced for every watt of energy consumed. This is crucial for evaluating lighting systems in terms of energy efficiency and performance, particularly in multifamily buildings where energy costs and environmental impacts are important considerations.

Other choices, such as Color Rendering Index (CRI), watts, and Kelvin, refer to different properties of light and do not relate to luminous efficacy in the same way.