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close this bookIntroduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)
close this folder4. Electrical Energy
close this folder4.5. Conversion of Electrical Energy into Light
View the document4.5.1. Fundamentals of Illumination Engineering
View the document4.5.2. Light Sources
View the document4.5.3. Illuminating Engineering

4.5.2. Light Sources

The triumphant advance of electrical light sources began with the first carbon-filament lamp made by Edison in 1879. Today, a great variety of lamps with different properties is available for the most diversified fields of application. Table 4.5. gives a selection including some important technical data.

Table 4.5. Important Technical Data of Electrical Sources of Light

Light source

Power steps in W manufactured

Luminous efficiency in lm/W

Service life in hours of operation

incandescent lamp

25 to 1,000

10 to 15

1,000

halogen incandescent lamp

10 to 5,000

16.7 to 22

25 to 2,000

fluorescent lamp

8 to 65

30 to 55

4,000 to 10,000

mercury-vapour high-pressure lamp

50 to 1,000

32 to 54

5,000 to 12,000

sodium-vapour high-pressure lamp

175 to 400

70 to 100

8,000

halogen metal-vapour lamp

175 to 2,000

62 to 87

1,000 to 6,000

The incandescent lamp is the oldest source of light which is still in frequent use today. Fig. 4.11. shows the basic design of this lamp. The lamp cap for general-purpose incandescent lamps is provided with an Edison screw having a diameter of 27 mm or 14 mm (designation of the cap: E27 or E14). In addition, there is a great variety of other cap shapes and diameters for quite different applications. Likewise, the glass bulb is made in many shapes. The glass may be clear, frosted, opal (white enemelled) or coloured; ornamental glass is also used.


Fig. 4.11. Design of an incandescent lamp

1 - Contact
2 - Insulating body
3 - Cap with Edison thread
4 - Glass bulb
5 - Pump tube end
6 - Electrode
7 - Supporting wire
8 - Glow wire (filament)

A disadvantage of incandescent lamps is the low luminous efficiency. About 95 % of the input of electrical power are delivered in the form of heat energy; and this is also associated with problems of heat dissipation in lighting fittings. The short service life does not meet our wishes. It is due to the fact that the incandescent filament is reduced to powder (evaporates) at its surface to some extent. Due to the deposition of the filament material, the glass bulb is blackened. Slight differences in the cross-section of the incandescent filament due to manufacture lead to an increase in the resistance at points of reduced cross-section and thus to a local increase in temperature which, in turn, causes a more intensive evaporation and further reduction of the cross-section until the incandescent filament is destroyed at this point. Fig. 4.12. shows the remarkable dependence of light flux and service life on the mains voltage.

Since operation of an incandescent lamp does not call for ballasts and ingnition devices, the lighting fitting is of a simple design.


Fig. 4.12. Luminous flux and service life of an incandescent lamp in dependence on the operating voltage

fV = rated flux;
Un = rated voltage
t1 = service life
tn = rated service life

The colour of the light of this bulb is agreeable in the case of low illumination intensities (» 100 lx). When high illumination intensities are required (> 200 lx), incandescent lamps cannot be used. Because of these properties, the incandescent lamp is today primarily used for the illumination of sitting-rooms, restaurants, theatres and other rooms where frequent switching is necessary (staircases, service-rooms).

Since about I960, halogen lamps have been produced as a special form of incandescent lamps. In contrast to the conventional incandescent lamp, the glass bulb of halogen lamps contains a small amount of a halogen, usually bromine, in addition to the filling gas. The incandescent filament particles evaporating during lamp operation form a cycle with the halogen and are deposited on the incandescent filament again and again. Thus, blackening of the bulb is avoided. The cycle calls for a high temperature at the outer wall; that is why quartz glass bulbs have to be used in relatively small designs.

In many cases, additional cooling is required.. On no account should the glass bulb be touched with bare hands. It should always be gripped at the point of compression or a clean piece of cloth has to be used. If the glass bulb has been touched inadvertently, it must be cleaned by means of a rag soaked in spirit. For some types, a special position of burning has to be observed.

For the dependence of the service life and the light flux on the voltage, the same relations as for the conventional incandescent lamp apply. The mean service life is stated, to be anything between 20 and 2000 hours depending on the type. An advantage is the higher luminous efficiency as compared, with the conventional incandescent lamp and the good spectral composition of the light. For incorporation in optical systems, the small dimensions of incandescent filament and lamp (spot light) are of particular advantage.

Halogen incandescent lamps are used for taking photographs, as projector lamps and special motor-vehicle lamps. For an illumination of sports fields and in other floodlight installations, halogen incandescent lamps of the sizes 1000 and 5000 W are used. Illumination systems in cinematographic film and television studios and in theatres can also be equipped with halogen incandescent lamps.

Already in 1936, the fluorescent lamp was produced for the first time; today it is primarily used for interior lighting in industry and administration. Large quantities of these lamps have been used since 1950, after the incorporation in the lamps of decisive technical improvements.

The fluorescent lamp is a low-pressure gas discharge lamp. Fig. 4.13. shows the basic design. In a tube two electrodes are fused in place which have to enable the electron emission.

As filling in the tube a small amount of mercury and of argon, an inert gas, is provided under the very low pressure of a few hundred pascal (the normal air pressure is about 100,000 pascal). The interior wall of the glass tube is covered with a luminescent material. When voltage is applied and ignition effected, a gas discharge occurs. In this event, the current passage is effected by ions and electrons flowing through the gas. The discharge in the mercury vapour filling produces normally ultraviolet light which is not perceived by our eyes. The portion of argon in the filling produces only a very weak bluish light. The luminescent material in caused to light by the ultraviolet rays. Depending on the type of the luminescent material used, a visible light of different spectral composition is produced.


Fig. 4.15. Design of a fluorescent lamp

1 - Cap in
2 - Cap
3 - Electrode (filament)
4 - Glass tube
5 - Layer of luminescent material
6 - Noble gas filling and mercury vapour

Fig. 4.14. shows the complete circuit of a fluorescent lamp.


Fig. 4.14. Circuit of a fluorescent lamp

1 - Ballast
2 - Fluorescent lamp
3 - Starter with anti-interference capacitor

For operation, a ballast and a starter are required. When connecting the circuit to 220 V a.c., no current will flow through the fluorescent lamp at first. Almost the full mains voltage is applied via the ballast to the starter in which a glow discharge is developing. At the same time, the bimetallic strip in the starter is heated closing the circuit via ballast, heating spiral, starter, heating spiral. The heating spirals start glowing. At the same time, the bimetallic strip is cooling down, opening the circuit. The high voltage peak now generated, by the ballast of more than 400 V leads to the ignition of the discharge in the fluorescent lamp. Due to the voltage drop across the ballast, a burning voltage of 50 V to 110 V remains at the fluorescent lamp depending on the length of the lamp while, with this voltage, a glow discharge cannot be repeated in the starter. The ignition process may occur several times in exceptional cases. For a reliable ignition and a long service life of the fluorescent lamp it is absolutely necessary to use only such ballasts and starters which are appropriate for the type of lamp.

Frequently, a capacitor for power-factor compensation is added according to Fig. 4.15. whose function and mode of operation will be discussed in Chapter 7.


Fig. 4.15. Circuit of a fluorescent lamp with capacitor for power-factor compensation

In fluorescent lamps the luminous flux is largely depending on the operating voltage, as is shown in Fig. 4.16. The dependence of the luminous flux on the ambient temperature must also be taken into consideration; at high outdoor temperatures, a good ventilation of the lighting fittings is required in this event (Fig. 4.17.).


Fig. 4.16. Dependence of the luminour flux on the mains voltage for fluorescent lamps

fn = rated flux
Un = rated voltage


Fig. 4.17. Dependence of the luminous flux on the ambient temperature for fluorescent lamps

fn = rated flux

Fluorescent lamps are made for a power input of 8 to 65 W. For room illumination, especially the power steps 40 and 65 w are used. For sitting-room lighting and direct place illumination, the lower power steps may be used. Primarily fluorescent lamps in rod form are on offer. For decorative and sitting-room lighting, lamps in U-shape and circular shape are included in the offer.

An advantage of the fluorescent lamp in case of rooms of small height is the large light-emitting surface. Even in case of direct sight, no or insignificant dazzling will occur. Different light colours can be achieved by various compositions of luminescent materials. The light colour “daylight white” most closely resembles the daylight but it is felt to be agreeable only in case of very high illumination intensities (> 1000 lx). For working place lighting, the light colour “natural white” is an agreeable colour because of its good colour rendition properties between 200 lx and 1000 lx. In the event of high demands on colour endition, the brilliant light of the light colour “natural white de luxe” should be used; its disadvantage, however, is a lower luminous efficienca. For low values of brightness, the light colour “warm white” has been developed, and in the event of fastidious demands “warm white de luxe” should be used. In Table 4.6. light colours suitable for various tasks of illumination are given.

Table 4.6. Lamp Light Colours Suitable for Various Tasks of Illumination

Task of illumination

Suitable lamp light colour


daylight white

neutral white

neutral white de luxe

warm white

warm white de luxe

administration room


x




conference room


x

x


x

lecture room


x




restaurants, hotels


x

x


x

butcher’s shop



x


x

dwelling




x

x

industry


x




dye-house

x


x



manufacture of ready-made articles of dress

x

x

x



drawing office


x




medical facilities






For special purposes, the light colours “Lumoflor”, “blue”, “green” and “red” are on offer.

For outdoor lighting and for the illumination of high working rooms, small light sources with a high luminous flux (point sources of light) are required. For this purpose, the mercury vapour high-pressure lamps, which have been produced since 1956, the halogen metal-vapour lamps, which have been developed only in recent years, and the sodium-vapour high-pressure lamps are used. These three types of lamps have in common that the light is produced, in a burner in a high-pressure gas discharge; the burner is provided with a further glas bulb. Ballasts are necessary. A running-in period of a few minutes is required until the full luminous efficiency is attained. After a short current interruption, the lamp will go out and can only be ignited again after a cooling time of some minutes.

Today, the mercury-vapour lamp frequently is additionally coated with luminescent material in order to obtain a better spectral composition of the light. Nevertheless, an unnatural colour rendering is invevitable. These lamps are made in power steps from about 50 W to 2000 W. Fig. 4.18. shows the circuit with ballast. The external bulb prevents the high portion of ultraviolet light to get outside. When the external bulb is damaged, the lamp remains serviceable but for man there is the danger of burns due to ultraviolet light. These lamps are primarily used for street lighting.


Fig. 4.18. Circuit of a mercury vapour high-pressure lamp

1 - Ballast
2 - Mercury vapour high-pressure lamp

The sodium-vapour high-pressure lamps, which have been made only recently, offers not only a higher luminous efficiency but also an improved spectral composition of the light and, consequently a more natural colour rendition. For operation, both a ballast and an igniting device are required (see Fig. 4.19.). The lamps are made in power steps from 175 w to 400 W; a further extension of the manufacturing programme is to be expected. Sodim-vapour high-pressure lamps are used to advantage in street lighting, illumination of large open-air facilities (ports, marshalling yards, construction sites) and in high halls without particular demands on colour rendition (foundries, assembling halls, storage halls). Because of their high luminous efficiency, they will replace the mercury-vapour high-pressure lamps in many fields.


Fig. 4.19. Circuit of a sodium vapour high-pressure lamp with different igniting devices

1 - Ballast
2 - Thyristor ignitor
3 - Sodium vapour high-pressure lamp
4 - Starter ignitor

A better colour rendition is provided by the halogen metal-vapour lamp though the luminous efficiency is slightly reduced. In its design it resembles the mercury-vapour high-pressure lamp but contains in the burner an additional substance, i.e. an addition of halogen. By means of various types of additives and the luminescent material used, the light colours of daylight white, neutral white or warm white can be achieved. The lamps are made in power steps from 175 w to 2000 W. For operation, an igniting device is required in addition to the ballast (also see Fig. 4.19.).

The lamp is used in outdoor illumination and in high halls where demands on colour rendering are more fastidious. A few types have been specifically developed for use in colour photography studios where mixing with daylight or with the light emitted by the halogen incandescent lamp is possible.