The EI library in London is temporarily closed, as a precautionary measure in light of the ongoing COVID-19 situation. The Knowledge Service will still be answering email queries via email@example.com, and is available for live chats on this page during working hours (09:15-17:00 GMT). Our e-library is always open for members here: https://knowledge.energyinst.org/services/elibrary, for full-text access to over 200 e-books and millions of articles. We are sorry for any inconvenience.
List of abbreviations and technical terminology
Ballast: a device used in order to regulate the amount of electric current that is supplied to a light source. It is essential to all types of discharge light sources. Typically, a ballast is placed in the lamp fitting.
Colour rendering: the Colour Rendering Index (CRI) is a measure of the ability of a light source to accurately portray the colours of the objects and surfaces it illuminates. The higher the CRI value, the more accurately the colours are being reproduced.
Colour temperature: a term that describes the overall colour of a light source. For example, diffused daylight on a cloudy day is perceived by the human eye as being white in colour.
Daylight Factor (Fd): a variable which takes into account the amount of energy that can be saved when daylight dimming control systems are in place.
Glare: the sensation produced by intense light in the field of view, which can prevent the task from being performed, reduce visibility and lead to visual stress and fatigue.
High-pressure sodium lamp: a type of discharge light source, which uses sodium, mercury and xenon to produce electric discharges. Their efficacy levels are relatively efficient, typically around 100 lumens per Watt. They are mainly used for outdoor application such as street lighting and security lighting.
Illuminance levels: a factor that indicates the amount of light required for performing a task in a safe and efficient manner. There is prescriptive guidance available for illuminance levels. Examples of these levels include 150 lux for loading bays, 300 lux for offices where mainly screen based work is carried out and 500 lux where paper-based work is carried out.
Illuminance uniformity: in general lighting applications, it is recommended to illuminate the task areas in a uniform way as much as possible. A qualitative expression of illuminance uniformity is the uniformity ratio, which is defined as the ratio of minimum illuminance levels to average levels. Typically, a ratio of 0.7 is required for creating a comfortable visual environment and avoiding glare. However, the ratio varies depending on the type of task. Illuminance uniformity is often achieved by installing light sources spaced in a regular grid over the area.
Illuminance: essentially, a measure of how much light illuminates a surface. Technically, the amount of luminous flux that reaches a given surface. This is the amount of light per unit of time and per unit area of the lit surface. The SI unit of illuminance is lux (lx), which equals to lumens per square meter.
Induction lamp: a lamp that produces electric current within the discharge gas tube through the use of electromagnetic fields. They are characterised by relatively high energy efficiency and a long lamp life (more than 100,000 hours). They are used in hazardous environments (such as the oil and gas industry) since they can be safer than other types of discharge light sources.
LED: a Light Emitting Diode, uses a different technology to emit light than a discharge light source. The technology is based on a scientific phenomenon known as electroluminence. It refers to the emission of light through a material that is activated by an electric current. In the case of LEDs, the materials used are semiconductors, e.g. germanium. Energy is released in the form of photon. LEDs provide a wide range of advantages over other lighting technologies, such as lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. LEDs are expected to eventually dominate the lighting industry.
Light Output (luminous flux): the measure of the power of light, as perceived by the human eye.
Light Intensity (luminous intensity): the measure of the power of light that is emitted by a light source in a particular direction. The SI unit of luminous intensity is the candela (cd).
Lumen: the SI unit of light output, or luminous flux.
Luminaire: a complete electric light fixture, including the lamp(s), mechanisms for inserting or holding the lamp(s), wiring, socket and other protective components.
Luminance: it describes the amount of light per unit of time (i.e. power of light) that passes through, is emitted or reflected from a given surface in a particular direction. It refers to light that “goes away” from a surface (either through emission, reflection or transmission). The SI unit for luminance is candela per square metre.
Luminous efficacy: is a measure of how well a light source produces light. It is defined as the ratio of the light output to the electricity required either for just the lamp (i.e. light source) or the whole lighting system to operate. In the first case, this will give us a unit of lumen per Watt (lm/W – referring only to power of the lamp) whereas in the latter case, it will give us a unit of lumen per total circuit Watt (lm/TCW – referring to the whole lighting system). The higher the value of the efficacy, the greater proportion of energy that is converted into light.
Lux: the SI unit of illuminance, which equates to lumens per square meter.
Metal halide lamp: a discharge lamp that uses a blend of vaporised mercury and other metals known as halides to produce electric discharges. Halides are mainly comprised of halogens. In the EU, a halogen bulb phase-out came into force on 1 September 2018.
Unified glare rating (UGR): the luminance from lamps divided by the background luminance from a room's walls and ceiling. It varies depending on elements such as the spatial dimensions and the technical properties of lamps and luminaires. According to the ‘SSL Lighting Guide 07: Offices’, the limiting unified glare factor for office spaces should not exceed 19.