Which quantity measures the amount of light emitted by a light source?

• A. Lumens
• B. Lux
• C. Candela
• D. Watt

Answer: (A)

The quantity that describes how much light a source emits in total is lumens. Lumens represent luminous flux, the total amount of visible light leaving the source in all directions, which is exactly what you want when you’re assessing how much light a device produces overall.

Lux, by contrast, measures how much of that light lands on a surface—lumens per square meter—so it depends on distance and area rather than the source’s total output. Candela measures luminous intensity in a specific direction, useful for focused beams but not the overall emission. Watt measures power (energy use or radiant power), not light quantity itself, so it doesn’t directly tell you how much light is emitted.

So, for the total light emitted by a source, lumens is the best descriptor.