Luminous efficacy h is ratio of the Luminous flux emitted by a light source in lumens to the electrical power in watts needed to generate the light. The unit is [lm/W].

Luminaire efficiancy h_LB is the ratio between the luminous flux that is available for lighting and the total luminous flux emitted by the lamp or lamps. The unit is [%].

The life of a lamp is defined in various ways depending on the type of lamp. The "average rated life" of a group of lamps is average of the lives of all the lamps in the group. Accordingly, 50 % of the lamps will have failed by this time. The "service life" is the number of hours after which the system luminous flux has reached 70 % of its initial value, taking into consideration the available luminous flux of the lamps still operating at this time.
 
Generally speaking, a compromise between quality of light and economy has to be found when selecting a lamp.
 
We now have the following four criteria for evaluating a lamp:

• luminous efficacy
• life
• color temperature
• color rendering index