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Lamp Types


Filament lamps are used mainly for domestic and display lighting. There are many types of filament lamp, the most common being General Lighting Service (GLS) and Decorative. Their finish – clear, diffuse/pearl or coloured – is a significant factor in their application. Reflector lamps are similar but have an envelope with an internal reflective coating. Advantages of filament lamps include low initial cost, simple operation (no control gear required) and good colour rendering. Disadvantages of filament lamps are low efficiacy (measure of the energy efficiency of a light source, ie lumens per watt) and a relatively short life. Certain extended life filament lamps have only about half the efficacy of standard lamps. The light output of filament lamps is particularly sensitive to voltage variations.


The main reason for filling a tungsten filament lamp with a halogen gas is to prevent evaporated tungsten from blackening the envelope. Tungsten halogen lamps also have an increased light output and/or an extended life compared with standard filament lamps. The envelope is of small dimensions and made of quartz or hard glass. Some mains voltage lamps have an outer protective envelope. Lamps that are suitable for use in luminaries without a safety screen should be so marked. Otherwise, tungsten halogen lamps should only be used in suitably enclosed luminaries. Extra low voltage (ELV) lamps are, in general, more compact than their mains voltage counterparts and the small filament size can improve the optical efficiency of integral or external reflectors. ELV reflector lamps make possible compact s.b. luminaires for display lighting.


The light output from a tubular fluorescent lamp comes from phosphors that convert energy from a low pressure gas discharge into visible light. The colour temperature and colour rendering are determined by the phosphor mix coated on the inside of the tube. The argon-filled T12 (38mm dia) tubes are being discontinued. The modern range of krypton-filled triphosphor T8 (26mm) dia tubes should be the first choice for switchstart, quick start and high frequency luminaries. Such lamps have a higher efficacy, longer life, improved lumen maintainance and better colour rendering than earlier types of tube. Triphosphor (or multi-phosphor) tubes offer a wide range of colour temperatures from very warm (2700k), warm (3000k) and intermediate (3500k) through to cold white (4000k), daylight (5000-5500k) and northlight (6000-6500k).


A Compact Fluorescent Lamp (CFL) has the characteristics and advantages of linear fluorescent lamps but its compact size is achieved by folding the discharge path, retaining high efficacy. The two main groups of CFLs are those with external control gear and those with internal control gear. High frequency control gear is now available integrated into the CFL lampholder, making lamp conversion from GLS to CFL very simple. Many modern fluorescent lamps are operated at high frequency (typically at or above 30 kHz) which results in a reduction of energy losses both in the lamp and the control gear. The control gear size and weight are often less, the efficacy higher, dimming where required is easier, and operation is silent.


Light is generated by an electrical discharge in a gas containing sodium and mercury (sodium amalgam) contained in a sintered alumina arc-tube. High pressure sodium lamps are used for road lighting, floodlighting and industrial interior lighting.


Low pressure sodium lamps consist of a U tube containing the discharge, and an outer heat reflecting glass jacket. The monochromatic light is concentrated in the yellow part of the visible spectrum which is close to the maximum sensitivity of the human eye at normal lighting levels. The efficacy is the highest of all lamp types, but with very poor colour rendering. Low pressure sodium lamps are used mainly for exterior applications such as road and security lighting (but are not suitable for repeated on/off (operation).


Metal halide lamps have quartz or sintered alumina (ceramic) arc tubes, generally with an outer glass envelope. Light output is from mercury and other metallic elements introduced in the form of halides. Metal halide lamps of the ‘protected’ type are now available for operation in luminaries without safety screens. According to the mix of elements, there is a wide range of efficacy and/or colour appearance, but colour rendering is generally good. Metal halide lamps are generally used in commercial interiors, industry and floodlighting, and (in smaller ratings) for retail lighting.


The high pressure mercury discharge operates in a quartz envelope. Mercury lamps were used for illuminating road signs and industrial lighting but have largely been replaced by the more efficient lamps now available. Such lamps offer low cost discharge lighting where high efficacy is not important. They often incorporate a third electrode for starting and in such cases the control gear required generally consists only of a ballast and a power-factor corrected capacitor.


Induction is a process whereby electrical power is passed from one circuit to another without the use of physical electrical conductors. It enables lamps to be constructed without the need for wire connections to pass through the glass or quartz envelope. Induction lamps are available as low pressure mercury lamps, using the same triphosphor coating of the inner envelope surface as the familiar fluorescent tubes. The commercially available range of induction lamps is limited.


Light emitting diodes have been used for indicating purposes for several decades and recent developments have created larger diodes and extended the range of colours including white. A dramatic increase in efficacy is predicted in the near future. LEDs have an extremely long life and are likely to be built into the luminaire and will not be a consumable item as far as the end user is concerned.