What Is a Lumen?

The lumen (symbol lm) is the International System of Units derived unit of luminous flux, the measure of the total light emitted by a source as weighted by the sensitivity of the human eye. It is defined as the luminous flux emitted into a solid angle of one steradian by a source of one candela of luminous intensity, so that one lumen equals one candela steradian. Because the lumen is a photometric rather than a radiometric quantity, it does not measure raw radiant power in watts. Instead it weights each wavelength by the spectral luminous efficiency function, which peaks near 555 nanometers in daylight-adapted vision and falls to negligible values in the ultraviolet and infrared.

Photometry ties directly to the SI base unit of luminous intensity. The candela is defined by fixing the luminous efficacy of monochromatic radiation at a frequency of 540 terahertz to exactly 683 lumens per watt, a value chosen for continuity with the older candlepower standards. That single defining constant makes the lumen, the lux (one lumen per square meter), and the candela per square meter all traceable to a radiometric measurement. The SI Brochure published by the BIPM sets out these definitions and the photochemical and photobiological conventions that accompany them.

Relationship to the Other Photometric Quantities

Luminous flux describes a source, not a scene. Luminous intensity in candelas describes flux per unit solid angle in a particular direction, illuminance in lux describes flux arriving per unit area on a surface, and luminance in candelas per square meter describes the brightness a viewer perceives from an emitting or reflecting surface. A bare lamp rated at 1,600 lumens therefore says nothing on its own about how brightly it will light a desk: that depends on the luminaire optics, the distance, and the geometry of the room. Lighting design software works with all four quantities together, using photometric data files that record the intensity distribution of a luminaire across angles.

Realizing and Measuring the Lumen

National metrology institutes realize the lumen either by integrating measured luminous intensity over solid angle or by absolute sphere methods. The NIST realization of the lumen uses an absolute integrating sphere technique developed in 1995, in which a known flux from an external beam is introduced into the sphere and compared with the flux from the lamp under test, removing the need for a previously calibrated flux standard. Goniophotometers provide the alternative route, scanning a photometer head over a sphere of directions around the source and integrating the result. Detector-based scales anchored to cryogenic radiometers give both methods uncertainties in the range of a few tenths of a percent for well-behaved sources, though lamp aging, self-absorption, and spectral mismatch of the detector remain the dominant error terms.

Luminous Efficacy and Solid-State Lighting

Dividing luminous flux by electrical input power gives luminous efficacy in lumens per watt, the figure of merit that drives lighting policy and product regulation. Incandescent lamps deliver roughly 15 lumens per watt because most of their radiated power falls in the infrared. Fluorescent sources reach 60 to 100, and commercial white light-emitting diodes now exceed 150 lumens per watt at the package level, with laboratory devices higher still. The theoretical ceiling for a white source of usable color rendering sits near 300 lumens per watt, well below the 683 figure that applies only to monochromatic green light. Reporting efficacy honestly requires measuring flux under standardized junction temperature and drive current, which is why photometric test methods for solid-state lighting are written as formal standards.

Applications

The lumen is used across fields that specify or measure light, including:

• Architectural and roadway lighting design, where fixture output and illuminance targets are set in lumens and lux
• Energy efficiency regulation and product labeling for lamps and luminaires
• Projector and display engineering, where projected output is quoted in lumens
• Horticultural and photobiological research, alongside photon flux measures better matched to plant response
• Automotive headlamp and signal lamp qualification
• Machine vision and photographic system design, where scene illuminance budgets determine exposure