Luminous efficacy

Luminous efficacy of lamps

Light sources convert electrical power into visible radiation (light). A large portion of the electrical power consumed by a light source is converted into heat. Incandescent lamps emit only 5% of their electrical power as light; in fluorescent lamps the number ranges between 20% and 40%. Luminous efficacy is a measure for the efficiency of light generation. It is measured in lumens per watt (lm/W).

Lamp luminous efficacy is the emitted amount of light in proportion to the electrical power consumption of open-distribution lamps under standardised environmental conditions. Among traditional light sources, the energy efficiency of gas discharge lamps and particularly fluorescent lamps with a diameter of 16 mm (so-called T5 lamps) is especially high.

Additionally, control gear units which also consume power are required to operate LEDs and discharge lamps. The system luminous efficacy of a lamp circuit is defined as lamp luminous flux in proportion to lamp and control gear unit power consumption. Thus, system luminous efficacy is determined by the luminous efficacy of the lamp and the dissipation power of the required control gear units. This often has a great impact, particularly on the cost efficiency of a planned refurbishment project (see chapter “Light and economic efficiency”).

An overview of the different luminous efficacies of light sources is provided in figure 9.6 (control gear unit losses are not considered here).

Regarding LED light sources, information about lamp or system luminous efficacy is only available for retrofit lamps. Only these can be operated with open distribution under defined conditions.

In addition, optical losses occur in the luminaire due to reflexion and absorption of optical materials as well as thermal losses due to heating of the luminaire (see chapter “Light output ratio”).

Luminous efficacy of LED luminaires

LED components built into luminaires must be considered separately. Lamp luminous flux and consequently lamp luminous efficacy cannot be defined for them. Standardised operation of the components is not possible since luminaire and lamp can not be separated from each other. A luminaire’s construction characteristics in particular have a significant impact on thermal management (see chapter “Thermal management”). The choice of control gear unit used in the luminaire and electrical operating conditions can also vary widely. Therefore, rated luminous flux ΦB, rated output PB and resulting luminous efficacy ΦB/PB are defined for such LED luminaires.

Figure 3.253: Luminous efficacy (approx. peak values without control gear units) and service life (related to the applicable definition) of different lamp types.

Rated luminous flux:

ΦB = initial luminaire luminous flux, undimmed


Rated output:

PB = power consumption to generate ΦB

Luminous efficacy:

ΦB/PB = (initial luminaire luminous flux, undimmed) / (power consumption to generate ΦB)

Luminaire luminous efficacy refers to the luminaire luminous flux which emitted by the LED luminaire in proportion to the electrical power consumed by the luminaire.

From this perspective, light output ratio is not applicable or it is represented by the value 1 in order to ensure error-free operation of software programs for light calculation (see chapter “Light output ratio”).

The maximum achievable theoretical luminous efficacy at monochromatic radiation is 683 lm/W; for white light in the visible spectral range between 380 nm and 780 nm, it is only 199 lm/W.

Today, technically sophisticated LED luminaires reach a rated luminous efficacy of over 160 lm/W.