Which term describes the total luminous flux emitted by a light source?

• A. Illuminance
• B. Lux
• C. Lumen
• D. Luminance

Answer: (C)

The total amount of light a source emits in all directions is measured in lumens. This tells you the bulb’s overall light output, independent of where the light goes or what surfaces it hits. Illuminance, measured in lux, describes how much of that light lands on a specific surface, so it depends on distance and area. Luminance describes how bright a surface appears to the eye, combining the light it reflects with the surface’s own properties, and is measured in candelas per square meter (nits). So, when you think of the light source’s total emission, lumens is the right term, because it captures the complete flux of light produced.