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At the latest since the first smithies appeared, man has become aware that substances glow and produce visible light if they are heated up enough. In the case of ferrous materials, this light emission can have colours from dark red through yellow tones to a light yellow-white, depending on the temperature. A smith can make a good assessment of the temperature of a glowing workpiece simply by looking at it. The black body theory of Planck describes this phenomenon in physical terms. Here, a preferably black body is slowly heated. As the temperature rises, it first begins to emit long-wave light (infrared) and the radiation gradually shifts into the visible reddish area. As the temperature rises further, the colour becomes more and more yellowish and “whiter”, until it finally reaches a hue comparable with daylight. The incandescent bulb and the halogen lamp make use of this principle. The light emission caused by such thermal excitation is conspicuous because of its continuous spectrum. The light of incandescent and halogen lamps thus comes closest to sunlight.
In an incandescent lamp, there is a tungsten wire in a glass bulb filled with an inert gas. Tungsten has the advantage of very high temperature stability and is protected from premature oxidation by the filler gas.The disadvantages of an incandescent lamp are its short service life, its low level of efficiency as well as continuous deterioration in efficiency caused by the blackening of the bulb. However, its advantages cannot simply be dismissed: For example, as a temperature radiator it generates white light with a continuous spectrum, which in turn is reflected in very good colour rendering.
Furthermore, its light generation process is relatively slow. For this reason, it is relatively easy to regulate the output of an incandescent lamp (dimming). Here, it is possible to regulate the output directly with the mains frequency, for example by means of leading-edge or trailing-edge dimming. Flickering does not occur with these lamps thanks to their relatively high inertia.
A halogen lamp represents the further development of the incandescent lamp and tries to reduce some of the substantial disadvantages of the incandescent lamp. On account of the very high temperatures of the filament of an incandescent lamp (3000 °C), vaporisation leads to a continuous deterioration of the tungsten wire. As a consequence of the enormous temperatures, tungsten atoms can relatively easily leave the fixed metallic compound of the wire and diffuse in the filler gas. The tungsten atoms released in this way are deposited on the relatively cold bulb of the lamp and give it a dark colour. This causes the luminous efficiency to fall continuously. In time, this weakening of the wire leads to a failure of the filament and the lamp “blows”.
In a halogen lamp, small quantities of halogens (chlorine, bromine, iodine) are added to the inert filler gas. Here too, tungsten atoms from the filament are transmitted into the filler gas through vaporisation. Here, however, they are “caught” by the halogens; in a chemical reaction, for example, tungsten iodide is formed from tungsten and iodine. This compound cannot be deposited on the jacket of the lamp, and thus the bulb cannot turn black. However, the tungsten iodide is deposited again on colder areas of the filament, where it decomposes into tungsten and iodine again. The tungsten is deposited on the metal of the wire, while the iodine remains in the inner atmosphere of the lamp and is available for another cycle. This process is also described as the halogen cycle and has led to an improvement in service life and luminous efficiency.
A fluorescent lamp is a low-pressure gas discharge lamp, and consists essentially of a glass tube filled with mercury vapour. At the ends of this tube there are electrodes, between which voltage is applied. If this voltage is greater than the ignition voltage required, the gas column within the glass tube is ionised, creating low-pressure plasma. Modern fluorescent lamps have electrodes formed as heating coils. Thanks to the fact that these coil electrodes are preheated, the required ignition voltage can be drastically reduced. After ignition, a current flows between the electrodes through the mercury plasma, causing its atoms to emit light. This light emission is mainly in the ultraviolet range. The inside of the glass tube is therefore coated with luminous material, which fluoresces through the ultraviolet light of the plasma in the visible spectral range. The coating thus transforms the emitted UV light into visible light. Through the careful selection and mixture of the luminous materials used, white light is generated. Most of the UV radiation which is not transformed is absorbed by the glass of the tube.
The diagram above shows the typical spectrum of a normal fluorescent lamp. Compared to the spectrum of sunlight, its highly discontinuous structure is striking; the light from such a lamp consists of many peaks. These are caused by the different luminous materials within the lamp. Our eyes regard the emitted light of such a lamp as white, since they cannot adequately evaluate the spectral composition of light. For this reason, such a lamp frequently has poorer colour rendering than an incandescent or halogen lamp.
Without some form of limitation, the current flow in the lamp would increase rapidly and lead to the destruction of the lamp. As with all other discharge lamps, therefore, the fluorescent lamp must also be operated with a ballast.
Fluorescent lamps are characterised by acceptable luminous efficiency (up to 100 lm/W), a relatively long service life (up to 45,000 hours) and moderate to good colour rendering. As with many light sources, the properties of efficiency and light quality compete with each other. If high light quality is desired, this is usually accompanied by low efficiency.
Sodium vapour lamps use the light emission of a sodium plasma to generate light. They consist of a discharge tube, in which sodium and at least one assist gas are enclosed. At both ends of the discharge tube, there are electrodes through which ignition voltage can be applied.
Since sodium is in its solid phase at room temperature, the atmosphere of the discharge tube usually contains neon as an assist gas. This can be ionised quite simply, and begins to conduct the current through the lamp as plasma. This causes the lamp to heat up, and the sodium begins to vaporise. As the sodium content increases, the light, which is initially reddish, becomes more and more yellow. Unlike fluorescent lamps, the gas discharge already emits light in the visible range, and conversion by a luminous material is not necessary. In spectral terms, sodium vapour lamps emit very uneven light. As a rule, low-pressure lamps have monochromatic light, while high-pressure lamps (discharge lamps) generate not only the dominating yellow sodium peak but also emissions in other spectral ranges. Both types of lamps have very poor colour rendering, but are characterised by high efficiency and a moderate service life.
In technical terms, metal halide lamps are based on mercury vapour lamps and contain mixtures of halogen compounds, rare earths and an assist gas (usually argon, xenon or neon) in their discharge tubes. As with the sodium vapour lamp, some of the substances used are initially in the solid phase, and are vaporised during the start-up phase by the sharp rise in temperature in the arc tube. Here, the lamp is ignited by the operating device with the help of high voltage impulses (5 - 80 KV), and initially only current flows through the ionised assist gas. This heats up all other components, and finally leads to the vaporisation and ionisation of all filler materials. Only after this start-up phase does the lamp reach its full brightness. As a rule, this process lasts 40-60 seconds. The composition of the filling leads to light generation with a high degree of efficiency (up to 100 lm/W), and the emitted light is characterised by high quality. At up to 15,000 hours, the service life of these metal halide lamps can be regarded as positive.
In an LED, light is generated by means of an inorganic semiconductor. To put it simply, a semiconductor consists of two differently composed (doped) areas that are immediately adjacent to each other. Electrons can move in both areas, however, they move at different quantum-mechanical energy levels. In the so-called n area there is a surplus of electrons, while in the p area there is a surplus of defect electrons (so-called holes). In the contact area between the two layers, diffusion processes lead to a balance between the two types of charge. On account of the electrostatic conditions created, this process is self-inhibiting and separates the two areas electrically from each other; it is also said that the diode blocks the current.
If a voltage source is then applied to the diode in such a way that its negative pole is connected to the n area (cathode), the external voltage works with the internal voltage, so to say, and the blocking layer can be overcome or suspended, and the diode becomes conductive. If the polarity of the voltage source is reversed, the external voltage reinforces the blocking effect, and the diode remains blocked for the flow of current. A diode can thus be compared with a non-return valve in hydraulics.At the transition point between the n and p areas, the electrons switch from a high energy level to a low quantum-mechanical energy level. Since no energy can be lost in nature, the “surplus” energy must be dissipated. This can happen in the form of dissipated heat, for example.
With light emitting diodes, the materials of the diodes are such (doped) that the surplus energy is emitted as light. As the differential energy at the transition point of the electrons from n to p is approximately constant for all pairs of materials, and the energy is always emitted in the form of a light quantum, the spectrum of the emitted light is fairly narrow. In other words, a normal LED always emits a fixed colour of light. White light can be generated with single-colour LED by means of additive colour synthesis. Here, for example, the emissions of red, green and blue LED are mixed with each other. If the intensities of the individual colours are matched to each other, the human eye perceives such mixed light as white.
However, the light from such a light source is not of optimum quality. Its spectrum is very discontinuous on account of the individual peaks of the LED. In particular the colour rendering of such a system is thus relatively poor. For this reason, such systems are usually only used today for creating dynamic coloured light.
Significantly better white LED light can be achieved with the help of conversion phosphors. Here, the relatively high-energy (short-wave) light of a blue LED is used to induce the phosphor to emit light. Usually phosphors with yttrium, aluminium and gallium (YAG phosphors) are used. They emit light in the yellow-green range, and the mixture of their emissions together with unconverted blue components of the LED result in white light. Depending on the quality of the phosphor and the LED, good to very good white spectra can be created; on account of the wide spectral emission of the phosphor, its spectra are relatively homogeneous. As a rule, however, a distinct blue peak can still be detected.
Current developments in the field of phosphors therefore aim to improve the spectral composition and thus try to raise the quality of the generated light.
OLED (Organic Light Emitting Diode) are millimetre-thin glass wafers with organic materials enclosed in them. These form layers which are around 400 nanometres thin and which current can flow through. The organic layers are enclosed by an anode and a cathode layer, which function as the electrical contacts from both sides. The organic layer contains molecules which begin to glow when an electric current passes through them. The particular molecular structure determines the colour of the light. The organic layers are coated to protect them from external effects.
An organic light emitting diode consists of several organic semiconducting layers between two electrodes, at least one of which is transparent. In the manufacture of an OLED, successive organic layers are applied to a conductive substrate, followed by another conductive electrode. In general, two different classes of material are used in the manufacture of organic, light-emitting components: polymer substances and what are known as small molecule materials, which have no orientation properties and thus form amorphous layers.