14.02 Glossary of the most important lighting terms As with any technical or scientific discipline, lighting technology has its own special terms and concepts for defining the characteristics of lamps and fixtures and for standardizing the units of measurement. The most important terms are described here. Luminous flux Φ Unit of measurement: lumen [lm] Luminous flux Φ is all the radiated power emitted by a light source evaluated with the spectral sensitivity of the eye and the photometric radiation equivalent km. Luminous intensity Ι Unit of measurement: candela [cd] Generally speaking, a light source emits its luminous flux Φ in different directions and at different intensities. Luminous intensity is the luminous flux radiated in a particular direction (solid angle Ω). Illuminance E Unit of measurement: lux [lx] Illuminance E is the ratio between the luminous flux and the area being illuminated. An illuminance of 1 lx occurs when a luminous flux of 1 lm is evenly distributed over an area of 1 m2. Luminance L Unit of measurement: candela per square meter [cd/m2] The luminance L of a light source or an illuminated area is a measure of the impression of brightness. Luminous efficacy η Unit of measurement: lumen per watt [lm/W] Luminous efficacy η indicates the efficiency with which the electrical power consumed is converted into light. Light and radiation Light is taken to mean the electromagnetic radiation that the human eye perceives as brightness, in other words that part of the spectrum that can be seen. This is the radiation between 380 and 780 nm, a tiny fraction of the known spectrum of electromagnetic radiation. Ω Φ Luminous intensity Ι is a measure of the luminous flux Φ emitted in a solid angle Ω. Illuminance E 0° 4000cd 6000 cd 8000 cd 10000 cd 0° 70° 50° 30° 0° 10° 10° 70° 50° 30° Luminance L Observer Viewed surface Luminous intensity Illuminated area Polar diagram The most important photometric formulae: Luminous intensity Ι [cd] Illuminance E [lx] Luminous flux [lm] Solid angle Ω [sr] Luminous flux falling on area [lm] Illuminated area [m2] Luminance L [cd/m2] Luminous efficacy η [lm/W] Luminous intensity [cd] Viewed luminous area [m2] Generated luminous flux [lm] Electrical power consumed [W]
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