Lumen (lm)

The lumen is the SI unit of luminous flux — the total quantity of visible light a source emits in all directions, weighted by the human eye’s sensitivity. It is the primary output figure on any LED luminaire datasheet: a 150 W industrial high bay at 160 lm/W delivers about 24,000 lm. Lumens describe the source, not the surface; how much of that light usefully reaches a work plane depends on optics, mounting height, and room geometry, which is why lighting designs are validated in lux rather than purchased lumens. When comparing fixtures, always compare delivered (luminaire) lumens rather than raw LED-chip lumens, which ignore optical and thermal losses.