Luminous efficiency

Luminous efficiency is the measure for efficiency with the generation of light, and is specified in lumens per watt (lm/W). Lamp luminous efficiency designates the ratio of emitted light quantity to electrical power consumption of an open distribution lamp in standard ambient conditions. Among the classic light sources, energy efficiency is especially high for gas discharge lamps, and here especially with fluorescent lamps with 16 mm diameter (so-called T5 lamps).

Control devices are needed to operate LEDs and discharge lamps, and these also cause power consumption. The system luminous efficiency of a lamp switching is here defined as the ratio of lamp luminous flux to the consumption of a lamp and control device.

System luminous efficiency is therefore determined by the luminous efficiency of the lamp and the power loss of the required control units (ballasts). This often has significant influence on the cost-efficiency of a planned refurbishment project. Lamp tables specify the luminous efficiency of lamps according to the type of control unit.

For LED light sources, lamp or system luminous efficiency can only be specified in the case of retrofit lamps. Only these lamps can be operated with open light distribution in defined conditions. In addition, optical losses occur in the luminaire due to reflection and absorption  on optical materials, as well as thermal losses because of heat generated in the luminaire.