Lumens measure the total amount of visible light emitted by a source; this quantity is commonly known as what?

• A. Luminance
• B. Illuminance
• C. Luminous Flux
• D. Reflectance

Answer: (C)

Think about light output in terms of total light the source emits. That overall amount of visible light is described by luminous flux, and it’s measured in lumens. It captures how much light is produced by the source as a whole, regardless of direction.

Luminance describes how bright a surface appears to the eye, which depends on the surface and viewing conditions. Illuminance measures how much light lands on a surface per unit area (lux). Reflectance is the fraction of incident light that a surface reflects.

For example, a bulb that emits 800 lumens has a luminous flux of 800 lumens; if that light is spread over 2 square meters, the illuminance would be about 400 lux.