Lumen (lm) is the international standard unit for measuring the total light output of a lamp or luminaire. Unlike watts (which measure power consumption), lumens measure actual visible light. Key relationships: 1 lux = 1 lumen/m². A typical LED downlight produces 100 lm/W, meaning a 10W LED outputs ~1,000 lumens. For reference: 60W incandescent ≈ 800 lm, 100W incandescent ≈ 1,600 lm. When specifying lighting, always compare lumens (not watts) for brightness.