What is the unit quantity of light output measured in?

The unit quantity of light output is measured in lumens. Lumens provide a direct measurement of the total amount of visible light emitted by a source, which is essential when evaluating the brightness of light fixtures. This unit helps in determining how well a lighting source can illuminate a space, making it particularly useful for applications in residential and commercial design.

It's important to understand the context of other light measurement units to appreciate why lumens is the right choice. Candela measures luminous intensity, indicating how much light is emitted in a specific direction, rather than the total amount of light produced. Lux measures illuminance, which is the amount of light reaching a surface, and is based on the area being illuminated. Foot-candle is also a unit of illuminance, similar to lux but is defined using different measurements (specifically in terms of square feet). While these other units contribute to understanding light and its effects, lumens specifically quantify the overall light output from a source, making it the appropriate choice for this question.