All known light sources emit light as well as radiation that doesn't fall into the visible spectrum, namely ultra violet (UV) and infra red (IR, heat) radiation. This means that this part of the energy is wasted, at least from the point of view of a lighting engineer.
The ratio of light output over input power is known as luminous efficacy. It is measured in lm/W (lumens per Watts). The maximum theoretical values is 683lm/W, however, the modern low-pressure sodium lamps which are the lamps with the highest efficacy today will 'only' achieve up to 200lm/W. Tungsten lamps, in comparision, perform rather poorly with a luminous efficacy of a mere 13lm/W.
Listed below are the luminous efficacies of selected lamps.
| Light source | Efficacy |
|---|---|
| Low pressure sodium | 100...200lm/W |
| Fluorescent | 45...100lm/W |
| Metal halide | 70...100lm/W |
| Incandescant | 13...17lm/W |