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 hLB 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:
| Light source | Luminous efficacy [lm/W] |
Life [h] |
Correlated colour temperature [K] |
Colour rendering index Ra |
| Incandescent lamp | 14 | 1000 | 2750 | 100 |
| Tungsten-halogen lamp | 20 | 2000 | 2850 | 100 |
| Xenon lamp Short-arc lamp |
15-50 | 1200 | 6000 | >94 |
| Compact fluorescent lamp | 87 | 12000 | 2700-6000 | 85 |
| Fluorescent lamp | 104 | 12000 | 2700-6000 | 85 |
| Metal halide lamp HQI | 100 | 9000 | 3000-6000 | 85 |
| Metal halide lamp Short-arc lamp HMI, HTI |
80 | 300 - 1000 | 5500 | >90 |
| High-pressure sodium lamp |
130 | 9000 | 2000 | 20 |
| Low- pressure sodium lamp |
197 | 10000 | 1800 | -20 |
| Hypothetical black body at 555 nm |
683 |
