Subject of the investigation

The case study looked at the use of light sources for illumination. For this purpose, three different types were compared: The two traditional light sources, the incandescent lamp and the energy-saving lamp, and the white LED based on nanoscale layers.

Variant 1: the incandescent lamp

The incandescent lamp, with a luminous efficiency of ca. 15 lm/W (WKO 2003), is the most widely used electric light source. It can be arbitrarily switched on and off and is used in all fields of interior and exterior lighting. The average service life is roughly 1,000-1,500 hours. The incandescent filament, a double-coiled wire between two lead-in electrodes inside a glass bulb, begins to glow when an electrical current is applied. A gas mixture inside the glass bulb prevents rapid vaporization of the filament. Tungsten, with its high melting point of roughly 3,400°C, is used for the filament, as it provides a longer service life than would other metals (Wuelfert 2000).

When the current passes through the bulb, the filament is rapidly heated up to a temperature of about 2,600°C. This causes bright light to be radiated. The electrical energy is converted into light very inefficiently - 9095% of the energy is converted into undesired heat. The incandescent lamp has a very poor energy efficiency. Only a small percentage of the energy is converted into visible light. The energy efficiency can be enhanced by increasing the filament temperature; this requires filling the glass bulb with a halogen mixture to maintain the service life of the lamp. This further development is called the tungsten-halogen lamp. In halogen-filled lamps, the tungsten wire may achieve a temperature of roughly 3,000°C.

Variant 2: the energy-saving lamp (compact fluorescent)

Saving energy and resources is a guiding principle of our time. Using discharge-type lamps allows us to apply energy much more efficiently for lighting than in the case of conventional incandescent lamps. They include the common fluorescent tube lamp, often found in the home and workplace. In particular, energy-saving lamps (compact fluorescents) - a miniature, enhanced form of the fluorescent lamp - make four to five times better use of electrical energy than do incandescent lamps. They are more expensive than normal incandescents, but convert up to 25% of the energy applied to light and have a luminous efficiency of about 60 lm/W (WKO 2003). Moreover, compact fluorescent lamps have a service life of 8,000 to 14,000 hours (KEVAG 2003).

A gas mixture, e.g. mercury, is a major component in discharge-type lamps. When, as a consequence of an applied voltage, a current begins to flow, electrons move from one electrode to the other. The electrons collide with the mercury atoms in the gas mixture inside the tube. These collisions cause energy in the form of ultraviolet light to be released. The ultraviolet light is absorbed by the phosphor coating (e.g. metallic salts) on the inner surface of the glass tube. The phosphors are thus stimulated and emit visible light. A discharge-type lamp requires a ballast, which provides the necessary high-voltage for ignition and also the normal operating voltage.

Variant 3: the (white) light-emitting diode

Recently the use of white light-emitting diodes (LEDs) has come into discussion as an alternative to conventional light sources such as incandescents and fluorescent tubes; this is because it is generally assumed that LEDs can more efficiently produce light. Light generation by means of light-emitting diodes is based on semiconductor lighting technology. This makes it possible, for the first time, to generate a cold light without a significant heat component. The technical problem is that although the internal quantum efficiency of an LED is very high, only a small portion of the light can be decoupled from the component. Internally, up to 90% of the energy can be transformed into visible light, but only about 20-30% (this number is increasing) is externally available (CJ-Light GmbH 2003).

Light-emitting diodes have a very long service life. The service life of the LED is characterized by the number of broken components during a given period and by the decrease of light flux as compared to the original value (50%). A complete failure of all components during service life is practically impossible when operating norms are observed. Service life, efficiency, and light color of the LED depend very much on temperature, with the consequence that the thermal balance must be carefully observed when using LEDs. Additionally, the light-emitting diode can be destroyed by application of a too-high voltage. Additional electronic components in the form of a ballast are necessary for LED systems and modules.

The maximum service life of 100,000 hours can be achieved in red, yellow, and orange LEDs under normal ambient temperatures and at 50% of the maximal allowed current recommended by the manufacturer. The service life of white light-emitting diodes is greatest at an ambient temperature 30 K below the specified value and at 50% of the original current rating (FGL 2003). White LEDs have an average service life of about 15,000 hours.

At the core of light-emitting diodes are semiconductor crystals that generate light when a current is sent through them. Inside the crystals is an n-conducting region having a surplus of electrons (negative charge) and a p-

conducting region with a deficit of electrons (positive charge). Between those is a transition area (called the p-n junction or depletion zone). By applying a voltage, the electrons in the n-region gain enough energy to overcome the depletion zone. As soon as these electrons arrive at the p-region, they unite with the positive charges. Energy is released, which is discharged in the form of electromagnetic radiation. The semiconductor material determines the color of the light emitted. The colors red, green, yellow, and blue can generated. The materials are produced on the basis of aluminum indium-gallium phosphide or aluminum gallium arsenide (AlnGaP or AlGaAs) for red and yellow LEDs, and (indium) gallium nitride (InGaN or GaN) for green and blue diodes (FGL 2003).

Fig. 42. Structure of an LED88

There are two ways to produce white LEDs: By the additive mixture of colors or by luminescence conversion. In the first case, a white LED is produced by combining chips emitting light of different colors into one LED. The various emitters (e.g. red, blue, and green) are placed together so tightly in the device that from a sufficient distance they cannot be distinguished by the human eye and appear as one color; thus producing the impression of white light. This method is also suitable for producing many other colors of light. LEDs producing colored light in this manner (including white) are also called multi-LEDs. The white light produced by multiLEDs, however, is not as convincing as that produced by means of luminescence conversion. The rendition of the color quality using the multi-LED is poorer; the differing brightness and operating conditions of the various LED chips make this a complicated and therefore expensive solution. Generation of white light by luminescence conversion is achieved us ing a combination of a blue or ultraviolet LED and a luminescence dye. When current is flowing, part of the short-wave light is absorbed by the dye, exciting it to emit light. Yellow-orange (long-wave, low energy) light is emitted. The superimposition of various spectral colors is perceived as white light.

Only with the development of white light LEDs, did the devices first become interesting for illumination purposes. The first white light LED was developed by the Japanese firm Nichia in 1995 and was based on the blue LED, also developed by them; it has been commercially manufactured since 1997. At the same time as Nichia, the Fraunhofer Institute for Applied Solid-State Physics (IAF) developed a white light LED as well as - in close cooperation with Osram OS - a manufacturing process. The know-how was transferred there and production began in summer 1998. Agilent (Lumileds) likewise began mass production of white LEDs in summer 1998. General Electric (GELcore) and Toyoda Gosei have been in production since 1999. White LEDs are now offered by almost all major manufacturers. Today LEDs are increasingly used for the most varied lighting purposes. Table 34 lists possible applications of LEDs according to their properties.

Table 34. Properties and applications of light-emitting diodes89

Properties

Application

UV- and IR-free illumination

Low-voltage

Small-size

Low-temperature

Energy saving and long lifestationary or mobile

Durability

Museum and showcase lighting illumination of fine art refrigerated display cases medical lighting illumination of light-sensitive materials damp and subaqueous locations portable applications furniture lighting directional lighting displays surface-mount applications area lighting, cabinet lighting furniture lighting refrigerated display casesexplosion-hazard environments service transit shelter lighting emergency and exit lighting garden lighting mobile applications mobile devices with task illumination public spaces signal and emergency lights maintenance (repair) lighting

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