According to BRE’s The essential guide to retail lighting:
Lamp efficacy is ‘The ratio of the light output from a light source to the power consumed; measured in lumens per Watt (lm/W). The higher the efficacy value of a lamp or lighting system, the more energy-efficient it is.’
‘For example, the efficacy of a 60 W incandescent light bulb is 12 lm/W, and of an 11 W CFL (compact fluorescent lamp) is 55 lm/W. For a 36 W fluorescent tube it is 91 lm/W. This last figure is a lamp efficacy; it excludes the power consumed by the ballast that is needed to run the discharge lamps.’
Ballast or control gear, is ‘Part of the control equipment of fluorescent or discharge lamps, used to stabilise the current. The older, traditional main-frequency ballast can consume up to 20-25% of the total lamp current. A modern electronic ballast working at high frequency consumes about 30% less current, and can be used to regulate or dim the lamp output.’
 Related articles on Designing Buildings Wiki
- Ballast or control gear.
- Colour appearance.
- Daylight benefits in healthcare buildings.
- Daylight factor.
- Daylight lighting systems.
- Dichroic reflector.
- Discharge lamp.
- Extra-low voltage lamps.
- General lighting v task lighting.
- Light pollution.
- Lighting energy numeric indicator LENI.
- Lighting of construction sites.
- Luminaire efficacy.
- Rights to light.
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