Unit of measure that expresses the amount of visible light emitted from a light source

Unit of measure that expresses the amount of visible light emitted from a light source

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

Answer: (C)

The key idea is distinguishing photometric quantities: lumens measure the total amount of visible light a source emits in all directions, i.e., the luminous flux. That makes lumens the best unit for expressing how much light a source is putting out. Illuminance (lux) describes how bright a surface appears by measuring light falling on it, which depends on distance and area. Luminous intensity (candela) describes how much light is emitted in a specific direction, not the total amount emitted by the source. So the unit that expresses the overall light output of a source is lumens.

The key idea is distinguishing photometric quantities: lumens measure the total amount of visible light a source emits in all directions, i.e., the luminous flux. That makes lumens the best unit for expressing how much light a source is putting out. Illuminance (lux) describes how bright a surface appears by measuring light falling on it, which depends on distance and area. Luminous intensity (candela) describes how much light is emitted in a specific direction, not the total amount emitted by the source. So the unit that expresses the overall light output of a source is lumens.