Which parameter is used to quantify the effectiveness of a light source in generating visible light?

The parameter used to quantify the effectiveness of a light source in generating visible light is measured in lumens. Lumens represent a measure of the total amount of visible light emitted by a source per unit of time, providing a direct understanding of how much light is actually available for illumination in a particular area. This value factors in the human eye's sensitivity to different wavelengths, making it a more relevant measure for evaluating light sources that are intended to provide usable light for various applications, such as in homes, workplaces, or outdoor settings.

In contrast, watts measure the total energy consumed by a light source, not directly the light output; thus, it doesn't provide useful information about visible light effectiveness. Candelas are a measure of luminous intensity in a specific direction, which reflects the power emitted by a light source in a particular beam, but again, this doesn't equate to the total light output in all directions. Foot-candles measure illuminance, which quantifies how much light is hitting a surface, but this too is not a direct measure of the light source itself. Therefore, lumens stands out as the most appropriate choice for quantifying the effectiveness of a light source in generating visible light.