In essence, an LED consists of a P–N junction, that is, a junction made of
P- and N-type semiconductor materials. A P-type material is one which has a defciency of electrons resulting from molecular bonding when forming a crystal. Tis electron defciency is described as electron vacancy or hole so that the P-type material has excess holes which can carry current and contribute to electrical conduction. Similarly, an N-type material has a surplus of electrons arising from its molecular bonding. Tese electrons move freely in the crystal serving as charge carriers. When P- and N-type materials are close together, electrons from N-side fll the holes in P-side, creating anelectrically neutral zone called the depletion region between the two sides. Tis electrical barrier is enlarged or reduced by applying a “reverse” or a “forward” external bias, respectively. Light is emitted in a forward biased diode when injected minority carriers (electrons in the P-region and holes in the N-region) recombine with each other. Light is generated in a narrow wavelength band due to current ﬂowing under forward bias; it is of one color or monochromatic. Te wavelength of the light generated depends on the bandgap energy of the material in which the P–N junction is made.
LEDs are available in a wide variety of sizes, colors and power ratings and development is proceeding at a rapid rate. Whilst LEDs come in a variety of styles, Figure illustrates two common forms
The main components of a LED are as follows. The chip of semiconductor material in the center of the lamp may be made of a wide variety of materials. Differing materials result in a different color of light being produced. The chip is mounted onto one of the lead in wires. In high power LEDs the mounting is designed in such a way as to conduct heat away from the chip. The other lead wire is bonded to the chip generally connecting to a very small area close to the actual semiconductor junction. The whole device is then potted in a plastic resin, usually epoxy.
The Light of the Future
Light-emitting diodes are the shooting stars of lighting. Tiny and extremely efficient, they are revolutionizing the world of light – delivering a whole new quality of lighting, addressing an ever growing number of applications and saving a great deal of energy. LEDs are the light of the future and are conquering the realm of general lighting.
Whether indoors or out, decorative or functional – LEDs (light-emitting diodes) permit solutions today that would have been in conceivable even a few years ago. Starting out as a colored signal indicator, the energy-efficient semiconductors advanced rapidly to become one of the principal light sources for accent and orientation lighting. With white light and intelligent manage mint, LEDs now ensure a high quality of lighting right across the range of outdoor and indoor applications.
LED technology is regarded as the most important invention in the history of lighting since Edison’s development of the “lightbulb” over a hundred years ago. Never before has so much light come from such a small fitting; never before have light sources worked so reliably for so many years and consumed so little electricity. Even recently, attention still focused on the richness of colour achieved by LEDs; today, high-performance LEDs are transfiguring general lighting.
The many positive characteristics of the light-emitting diode include:
> extremely long life and virtual freedom from maintenance
> high efficiency
> white and colored light with good color rendering properties
> insensitivity to vibration
> light with almost no heat generation, no IR or UV radiation, no interference with nocturnal insects
> instant, flicker-free lighting that is infinitely
> very compact design
> no mercury content and no end-of-life
What is the UGR value? When is it required and used?
The abbreviation UGR stands for »unified glare rating«. The UGR value is a dimensionless parameter which provides information about the degree of psychological glare of a lighting installation in an indoor space. UGR values are defined in steps within a scale of 10 to 30. In DIN EN 12464-1:2011-08 the steps within this scale are 13, 16, 19, 22, 25 and 28. In the final instance these steps express the statistical perception of glare experienced by a large number of observers. So UGR 19, for example, means that 65% of observers »did not really feel disturbed« by the glare. Conversely, of course, this also means that the remaining 35% felt disturbed by the glare. The lower the UGR value, the less direct glare is experienced by the observers.
The UGR value can only be calculated; it cannot however be directly determined photometrically. Where there are lighting installations with luminaires from which 65% of the light is emitted indirectly and where narrow beam spots or asymmetrically radiating luminaires are installed, then, by definition, it is not possible to indicate a UGR value.
Contrary to widespread opinion the UGR value is not really a property of a luminaire. Here we are dealing with much more than the interaction of the »brightness level« of the luminous surfaces of a luminaire in relation to the »brightness level« of the surroundings and the position and viewing angle of the observer. The average »brightness« of the light emitting surface of a luminaire is defined in this context as the average luminance of the luminaire and the »brightness« of the background or the surroundings as background luminance.
The following example taken from a real-life situation demonstrates clearly the influence which the ratio of these brightness levels to each other can have on the glare effect: Imagine that you are driving along a road at night with no street lighting. A car now comes towards you with headlights on full beam. You are blinded by the strong light and are hardly able to keep your eyes on the road. Imagine the same situation on a sunny summer's day. The same vehicle approaches again with the headlights on full beam. Now you are far less likely to be blinded by the headlights. Yet the properties of the headlights have not changed at all. The degree of direct glare results here mainly from the contrast to the surroundings (i.e. the background luminance).
The position and the viewing angle of the observer also have to be borne in mind. For, if the luminaire is not in the field of vision of a person, then this same person cannot be affected by glare. In certain norms, depending on the field of activity, adherence to UGR thresholds is required. These can be found in the current DIN EN 12464-1:2011-08 under »5 Index of Lighting Requirements«. Since the issue here is maximum UGR thresholds, the term UGRL (Unified Glare Rating Limit) is used. In accordance with DIN EN 12464-1:2011-08 the lighting designer must provide evidence of the direct glare categorization with the aid of the tables of the CIE Unified Glare Rating method (acc. to CIE 117-1995). The purpose of the tabular method is to make it easier for the lighting designer to apply the very complex formula behind the UGR value.
Limitations of the tabular method when determining the UGR value
The tabular method is a procedure which is followed in order to determine the UGR value of a lighting installation in a standard room. However, the designer must bear in mind that the »standard room« usually has very little to do with real situations. According to the tabular method the floor has a maximum degree of reflectance of 20%, walls of 30% to 50% and the ceiling of 50% to 70%. White walls or ceilings with a degree of reflectance of 75 to 90%, such as frequently occur in architecture, are not taken into consideration in the tabular method. In the tabular method the observer can be positioned either across or along the luminaire axis. The tabular method does not recognize an angle of vision diagonal to the luminaire axis and is based exclusively on rectangular room geometries. This method must be applied for each individual type of luminaire if different luminaires are present in one room, since each type of luminaire has its own UGR table.
Why is the UGR value shown as a figure in the luminaire data sheet?
though the UGR value is not in itself a property of a product, nevertheless, in the data sheets of many manufacturers details such as »UGR < 19« can often be found. It is, however, not correct to deduce that this is a luminaire property. Unless the manufacturer provides further details, this figure refers to the UGR value which the luminaire would have in a reference situation with room dimensions of 4H/8H and degrees of reflectance of 20% for the floor, 50% for the walls and 70% for the ceiling. In real situations this value could be lower or even higher.
Despite the restrictions of the tabular method it is of benefit in evaluating products when comparisons are desired. A luminaire with a UGR value of 16 is, in practice, less likely to cause psychological glare than a luminaire which has the same parameters but a UGR value of 25. Therefore, it is important for the designer to be aware of the limitations of the tabular method. If he wishes to determine the UGR value for a specific position of the observer with greater accuracy, we recommend calculating this with the aid of a software which will then take into account the degrees of reflectance and room geometries as they are in reality. All the luminaires present in the room will also be included in the calculation.
The lifetime of a lamp is usually specified in
hours. For LEDs, high-pressure discharge lamps as well as fluorescent and compact fluorescent lamps with plug-in base it is given as the rated lifetime. All these light sources degrade, i.e. their brightness diminishes with operation. The rated lifetime (given as L) therefore describes the time in which the luminous flux of the light source falls to the specified value. For general lighting, typical values are L80 or L70. Thus the average rated lifetime of an LED is reached when the luminous flux reaches 70 percent of its value at installation.
The degradation and failure of LEDs is determined essentially by the let-through current and the temperature inside the LED; in
the case of modules, the electrical wiring of
the LED, the ambient and operating temperature and further module characteristics also play a role.
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields. In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e., those on a line from reddish/orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of, e.g., a green or a purple light. Color temperature is conventionally expressed in kelvin, using the symbol K, a unit of measure for absolute temperature.
Color temperatures over 5000 K are called "cool colors" (bluish white), while lower color temperatures (2700–3000 K) are called "warm colors" (yellowish white through red). "Warm" in this context is an analogy to radiated heat flux of traditional incandescent lighting rather than temperature. The spectral peak of warm-colored light is closer to infrared, and most natural warm-colored light sources emit significant infrared radiation. The fact that "warm" lighting in this sense actually has a "cooler" color temperature often leads to confusion.
Luminaires in this class are electrically insulated and provided
with a connection to earth. Earthing protects exposed metal
parts that could become live in the event of basic insulation
Luminaires in this class are designed and constructed so
that protection against electric shock does not rely on basic
insulation only. This can be achieved by means of reinforced or
double insulation. No provision for earthing is provided.
Here protection against electric shock relies on supply at Safety
Extra - Low Voltage (SELV) and in which voltages higher than
those of SELV are not generated (max. 50V ac rms).