Lumens (symbol: lm) are the unit of luminous flux in the International System of Units (SI), measuring the total amount of visible light emitted by a source as perceived by the human eye

. In simple terms, lumens indicate the brightness of a lamp or light source- the higher the lumens, the brighter the light appears

. Unlike watts, which measure energy consumption, lumens measure actual light output, making them a more accurate way to compare the brightness of different bulbs, especially with modern energy-efficient lighting like LEDs

. For example, a 6.5W LED lamp can produce the same brightness (lumens) as a traditional 50W halogen bulb, using much less energy

. Lumens quantify the total visible light output regardless of direction, but for directional bulbs like spotlights, a "useful lumens" rating may be used to indicate light emitted in a specific direction

. The lumen is defined as one candela-steradian, representing the luminous flux emitted in a solid angle of one steradian by a source with a luminous intensity of one candela

. In summary, lumens measure the perceived brightness of a light source, providing a standardized way to understand and compare how much light different lamps produce