The luminous efficacy of emitting lamps is a characteristic value given as the ratio of the luminous flux emitted in lumens( lm) to the electrical power consumed in watts (W): Lumen per Watt (lm/W). The luminous efficacy depends on the light source and essentially corresponds to its efficiency.
The luminous efficacy value is decisive for lighting efficiency; the less power a lamp consumes and the brighter it shines, the more efficient it is. The values for luminous efficacy vary considerably and, with CO2 savings, are the focus of energy efficiency and the European energy label.
As already mentioned, the luminous efficacy depends on the light source and whether it is a thermal or a non-thermal radiator. The classic thermal radiation source is the incandescent lamp, which also has the worst luminous efficacy, which is also highly dependent on the temperature of the tungsten filament. Its luminous efficacy is between 5 lm/W and 15 lm/W. The remaining power is emitted as infrared light and perceived as heat radiation.
Non-thermal emitters such as gas discharge lamps and fluorescent lamps have a luminous efficacy that is 5 to 10 times higher. The efficiency scale is headed by the light-emitting diode or the power LED, whose luminous efficacy is over 250 lm/W.