Nowadays LED lighting is developing fastly, however, it still remains at an early stage in comparison with other technologies, for example, fluorescent or high-pressure discharge lamps that were the best ones over the decades. Since several years, renowned manufacturers of lighting successfully introduce LED products on the market. Due to the improvement of technology and progress in solving technical failures, manufacturers offer products with higher and higher technical parameters and broad scope of usage. At the same time, a growing number of market overrated low quality LED products is dramatically increasing. Such products do not meet the basic requirements of safety.
The lack of certain legal regulations, standards and market surveillance in the area of declared technical parameters results in dangerous increase of low quality LED products segment. It represents the threat of the potential inhibition of the development of LED lighting technology, as well as loss, valuable benefits for consumers, competitiveness of the economy and the environment. This article has been prepared by a team of experts of the Polish Committee on Illumination and The Union of Lighting Equipment Manufacturers “Pol-lighting” in order to approximate the problem of shining diodes for everyone, who works in the field of lighting (designers, electrical installers) or for whose who make decisions related to the purchase of lighting products (wholesalers, retailers). Authors are willing that the content of this article will be useful while discovering LED technology and making rational decisions. Elaboration reflects the most significant issues of diodes lighting according to the current state of knowledge and practice. The rapid development of LED technology and improvement in the field of standards, as well as, research methods will call for an update of the information covered in this article.
The name LED is an abbreviation for the English name for Light Emitting Diode. It is a semiconductor light source. Is available across the visible, ultraviolet, and infrared light. The LED diode consists of a semiconductor connector resulting from the combination of two types of semiconductor (type n and p). The release of energy in the form of photons or light, occurs in the flow of electrical current through the LED in a forward direction (or from layer p to layer n).
Fig. 1.1. The symbol of a light-emitting diode and an example electronic circuit with its use
LEDs can emit light of different wavelengths (different colors) depending on the used dopant for material of the basic semiconductor. Notwithstanding, it is always a limited spectral division resulting in rather one color
Fig. 1.2 U-I light-emitting diode characteristics - Up - threshold voltage, Uz - barrier voltage, from 2 to 3V and from 2 to 5V, respectively.
Fig. 1.3 Spectral characteristics of radiation of examplary light emitting diodes
In the case of LED there are three possibilities to obtain white light:
1. Mixturing colors of three onefold LED (red, green and blue) – RGB system.
2. Supplementing the blue color LED with yellow/red phosphor – complementray colors system.
3. Supplementing LED with its UV radiation with phosphor – UV conversation system.
LED radiation phenomenon is associated with radiative recombination mechanism in a semiconductor. During the current flow through the p-n junction charge carriers are injected thereto (holes from the p and the electrons from the n) and recombine there radially. The electroluminescent process in diodes can reach efficiency up to 50%. In the case of band-band recombination, radiation energy is set to the width of bandgap.
Fig. 2.1. Radial recombination process in the light-emitting diode
There are two types of semiconductors:
Intrinsic semiconductor — is a pure semiconductor without any significant dopants. In such semiconductor there is only one possibility of transport of electric charge — energy must be supplied to electron(eg. by heating), greater than the bandgap energy, as it could find itself in the conduction band. In places abandoned by the electrons in the valence band, are being formed the so-called holes, which are, easily explaining– positive charge carriers (electrons are negative charge carriers).
Extrinsic semiconductor – involving intentional introduction of other atoms into the crystal lattice, modifying its properties. There are two types of impurity atoms: donor and acceptor.
Acceptor dopants have three valence electrons that is one less than the crystal, in which those atoms are applied. In order to fill the fourth bond, electron is gathered out of the neighbouring atom, resulting in the excess hole. This type of semiconductor is named an acceptor (p type).
The donor dopant atoms have five valence electrons that is one more than the crystal in which those atoms are used. Four electrons create the conduction band with the crystal lattice, whereas the fifth one becames extra valence electron. This type of semiconductor is named a donor (type n). In consequence of doping to a new band model, new energy levels and differentiation of light color are introduced.
Radiative recombination may occur in several ways:
- band to band recombination – electron from the conduction band directly combines with a hole in the valence band and releases a photon with an energy similar to the band gap
- recombination by a shallow acceptor levels – electron from the conduction band recombines with the hole from the acceptor level – produced photon with a lower energy is released
- recombination through the shallow donor levels – electron from the donor level combines with a hole from the valence band – photon is released
- donor-acceptor recombination – electron from the donor level combines with a hole from the acceptor level
- recombination through deep levels – in such case, photon energy is much more lower than of energy gap.
Fig. 2.2. Diode listing depending on the dopants used.
Apart from lighting aims, one of the benefits of the LED is the ability to directly produce a connector of light of a selected color. It means no need to use color filters, which significantly affect the energy efficiency of such lighting. The color of produced light depends on the chemical content (doping way) of material which does semiconductor include. Underneath the basic types of materials used in LED are presented together with the radiation produced by it:
- Aluminium gallium arsenide (AlGaAs) – LED emits red color light and infrared radiation
- Aluminium gallium phosphide (AlGaP) – LED emits green color light
- Aluminium gallium indium phosphide (AlGaInP) – LED emits orange-red, orange, yellow and green color light
- Gallium(III) phosphide (GaP) – LED emits red, yellow and green color light
- Gallium(III) nitride (GaN) – LED emits pure green color and blue color light
- Indium gallium nitride (InGaN) – LED emits radiation in the near ultraviolet, blue-green and blue color light
- Zinc selenide (ZnSe) – LED emits blue color light
- Diamont © – LED emits ultraviolet radiatione
- Aluminium nitride (AlN), Aluminium gallium nitride (AlGaN) – LED emits radiation in the near ultraviolet
What are the available types of lighting equipment using LED??
Single-diode (LED) – intended to be powered by AC at a stabilized value of current, mainly 350mA, 700mA or 1A. LED diodes with a power of 1W and above are classified as high power LED that corresponds approximately to the current of 350mA.
Fig. 4.1. Examples of individual LEDs
Moduły LED składające się z kilku lub kilkunastu pojedynczych LED zamontowanych na wspólnym podłożu np. płytce drukowanej. Moduł LEDczęsto zawiera inne elementy np. optyczne ( np. soczewkę skupiającą światło), mechaniczne lub elektryczne i elektroniczne. Moduły LEDzawierające odpowiedni układ stabilizujący punkt pracy LED mogą być zasilane zasilaczem napięciowym a nawet napięciem sieciowym. Głównie stosowane są moduły LED przewidziane do zasilania napięciem o wartości 10V i 24V. Przykłady modułów LED przedstawione są na rys. 4.2.
Fig. 4.2. Examples of LED modules
LED modules combined from few single-diode LED mounted on the common base, for example, PCB. LED module often contains other elements, for instance, optical (light focusing lens), mechanical, or electrical and electronic components. LED modules containing the suitable layout stabilizing the operating poinf of LED can be powered by voltage power supply or even by network voltage. Mostly LED modules which are adapted to be powered by 10V or 24V voltage are being used.
Fig. 4.3. Examples of LED lamps, replacements for traditional bulbsand halogen bulbs
LED lamp – lamp, in which the light source is LED or LED modul(es)..
Fig. 4.4. Examples of LED luminaires
LED durability, as well as the durability of other light sources, is calculated in hours as the time of lamp lighting (under particular conditions) in which it preserves the suitable usage properties. In case of LEDs, similarly as with fluorescent or other discharge lamps, which do not expire suddenly as classic bulbs, the basic criterion for determining the durability is the time during which the suitable value of luminous flux is maintained. “The suitable value” is usually defined in % of the initial value. For fluorescent and other discharge lamps, the value of luminous flux is accepted on the level from 70% to 90%. The durability of LED is similarly determined. Predominatigly, the durability of LED is defined as the lighting time during which the luminous flux will not decrease below 70% of the initial value. Such durability is indicated with L70 symbol. L50 symbol can be also found that is related to the durability for the period during which the luminous flux will not decrease below 50% of the initial value. The ruthless value of the durability declared in the second case will be higher, that still does not mean, that the product is better, however, more gentle criterions are adopted to this kind of product. The optimal solution for the user (designer) would be the presentation by the manufacturer of the values of preservation of luminous flux as a function of lighting time.
The above solutions on the topic of defined durability through the coefficient of luminous flux preservation, is only the part of characteristics of light source, including LED, indicating its durability. This feature relates to the particular product. From the user’s point of view, the declared durability of individual light source is not the only important parameter, but also an information about what percentage of the light sources reaches the declared durability L, either what percentage of the ligh sources can be damaged before the time L. In this assessment, those lamps with the highest decrease of luminous flux than it was declared, may be classified as damaged ones. Such a way of considering the durability may be defined according to the coefficient of durability (used mainly for fluorescent and other discharge lamps), indicating the percentage of lamps, which after time L still preserve its characteristics or through the coefficient of damages (mainly used for LED) indicating the percentage of LED lamps being damaged during the particular exploitation time, for instance, declared durability L. Such coefficient of damages may be defined by Fy symbol where y can be accounted as 10 or 50, and should be read as 10% or 50% of possible LED damages during the declared durability time Lx.
Comparing the durability of LEDs with different construction or made by different manufacturers, it is worth to take into consideration criterions and conditions indicated by the manufacturers. Durability may vary for different types of LED depending on the operating conditions (humidity, temperature), current value.
Data related to the durability L70 is presented below, in division of types, which may be found in the literature and in the offers of LED manufacturers.
- LED emitting red color light – approximately 150 thousand hours.
- LED emitting yellow color light – approximately 125 thousand hours.
- LED emitting orange color light – approximately 250 thousand hours.
- LED emitting green, blue and white color light – approximately 30 – 70 thousand hours.
These durability periods occur in case of optimal values obtained in laboratory conditions, which differ from the actual ones found in lighting equipment.
The durability of LED does not depend on the operating cycle and the number of on/off inclusions. Carrying study on LED durability is problematic because of its long lighting time. Even when working 24 hours a day to test the LED during 50 thousand hours would take 5 – 7 years, results obtained after the completion of the study would be less useful because of changes in LED construction, which would occur during this time. Because of the facts mentioed above, nowadays testing the durability of LED or LED modules take place in 2 stages:
- Stage 1 – LED or LED module is being tested at a current and/or the maximum voltage during 1000 hours (heating and stabilization period). It is crucial as for the majority of LEDs, luminous flux increases during the first 1000 hours of operation.
- Stage 2 – turned LED works for the following 5000 hours. Value of luminous flux measured after the heating period (1000 operating hours) is accepted as the initial value (referential). These measurements made between 1000 and 6000 hours of operation are comparable with the initial level (1000 hours). During those 6000 hours the luminous flux will not fall below 70% value of the initial lumious flux (applicable for the general lighting), an exploitation is made on the basis of obtained data. Far exploitation may be performed only for the period of time which is 6 times testing time, or 36000 hours.
Due to this method, verification of declared long durability is possible. Another option to indicate the durability is currently being discussed within the IEC1. It introduces an evaluation of LED durability on the basis of luminous flux measurements during the operation at three temperatures: 550C, 850C and temperature specified by the manufacturer. The temperature is measured at the measuring point on the LED module, which is adviseable by the manufacturer. Luminous flux measurements are made for 20 LED items for at least 6000 hours. The expected LED durability is extrapolated on the basis of executed measurements of luminous flux drop, under the condition that the luminous flux is lower than 30% of the initial value. The basis for this method is strict dependence of LED durability (luminous flux drop) on the temperature obtained in the connector.
The main cause of the drop of luminous flux and reduction of LED durability is the heat generated at the semiconductor junction (junction “p-n”). In order to guarantee proper operation of LED, the heat from “p-n” junction must be transferred by conduction or convection, for example, by means of elements which conducts heat well and ventilators. For heat conduction from connector, radiators and ventilators are used. In case of the lack of usage of appropriately designed heat sink or ventilator, the temperature of diode increases, as constant operation at high temperature will cause durable decrease of luminous flux (luminous efficiency) and LED durability. Shortening LED durability may also take place because of inadequate power supply. For instance, in case when LED power supply current value is higher than the maximum value, it results in increase in the connector temperature and reduction of durability. The use of well-fitting power systems and radiators (recommended by LED manufacturers) guarantees an obtainment of the maximum durability.
The basic and most frequent reason of damages of the electroluminescent diodes is overheated connector. Excessive heat sources can be energizing too high power supply current value, too high environment temperature or combination of these factors. Slight or short-term overheat of the structure causes partial damage of material and light weakness. Following cycles, when LED diode is being treated in very high temperature, are summing up resulting in the absolute destruction of the crystal. LED diode also can be charged by many times the peak current limit value, however the average value of current should be located within the limits approved by manufacturers. Factor which causes an increase of LED diode temperature may be too poor heat transfer to the radiator consequential from inappropriate montage of diode based on inaccurate adhesion of diode’s housing heating contact to the ground or ground to radiator. Moreover, the heat sink may not indicate sufficiently high efficiency of heat transfer to the environment because of the air flow limit either too high temperature of the reduced air flow in environment. Overheat of radiator may be caused by its impurity or closure in the housing. Wrong pins soldering can also be a threat to the LED diodes.
Irreversible damage to the diode can be caused by applying a voltage in the reverse direction with a value greater than the voltage barrier. In electroluminescent diodes voltage which is comparable to the threshold voltage for conduction. That is why, connection to the power supply with opposite direction is very dangerous for LED diodes. Some copies of the diodes are factory-equipped with protection against overload and reverse connection, although such solution is not popular. Damage of LED may be also caused by the presence of surge.
Diodes are optoelectronic elements, which can be easily damaged mechanically. In spite of the fact that diodes are monolithic objects and do not possess mobile elements nor subject to vibration, its structure is sensitive to the effects of high mechanical force. Lead wires to the crystal are made from wire with a cross-section of the order of tenths of a millimeter square, that can be easily broken. Material of lens covering the diode is chosen due to the high efficiency of light emission not because of durability which may lead to damages. Under the influence of the force, shining crystal can separate from the ground, lose thermal contact and overheat.
The declared luminous efficiency of white LED light is systematically growing from year to year. Under laboratory conditions the highest light efficiency values for white color diodes exceeds 170lm/W. It is worth mentioning that the laboratory data for LEDs does not have any reflection to the real working conditions, so we cannot go by it. Nowadays diodes generating white light with luminous efficiency of 100lm/W are available on the market(catalogue data for LED is given for the junction p-n temperature of 25 degrees C). The lighting efficiency of LED depends on work conditions, mainly temperature. Some LED and LED modules manufacturers declare data for optimal working conditions, which that can be hardly achieved in practical usage, so given data can be unrealistic. The resulting efficiency of LED may be also affected by some other factors as: power supply efficiency, optical system efficiency, loses resulting from additional elements, etc.. What is more, light conversion in phosphor causes a loss of energy (known as Stokes shift), when phosphor conducts the conversion of wavelength from the shorter to longer one that reduces the overall LED efficiency.
Diode should be chosen in such a way that the color temperature or Correlated Color Temperature-CCT of the emitted through the diode radiation would be located in the tolerance area for the light source of 2700 K, 3000 K, 3500 K or 4000 K color temperature. It is important to take a look also on the quality of color rendering by iluminated with white color objects. If in order to determine the quality of color rendering Ra indicator is used, it would be best that it amounted for more than 80. The color temperature of the emitted light radiation or the quality of color rendering of illuminated objects depend on the quality and technology in which LED was made. The basic information on this topic is presented below.
The white light is a mixture of radiations in a range of 380nm to 780nm. For numerical and graphical display depending on the mixing radiations, International Commission on Illumination CIE in 1931 created a triangle of x,y colors shown in Figure 11.1 In this triangle each color has an assigned coordinations of x,y position of its chromaticity. While choosing two colors with different positions of chromaticity points and connecting these points with each other, we can visualize each color of light due to the mixture of suitable proportions of these 2 monochrome radiations. Analogically, 3 colors may be mixtured so the resulting color would lay in a triagle. Skilfully choosen, more than 3 mixtured radiations allows the increase in the quality of obtained white light. In comparison with the other white light sources, LED can present different tones defined by means of the color temperature (similar to fluorescent lamps). That is why, in order to obtain the similar color it is important to ensure that all LED have an identical color temperature.
Fig. 10.1 Chromaticity card x, y. Illustration of adding colors.
From the principle of LED operation it is obvious that it emits radiation in a narrow wavelenght range, so in the visible range the human eye picks it as monochrome.
Using principles valid for colors mixing in order to obtain white LED light, 2 groups of methods were developed.
The first group of methods allows to obtain white light by mixing an appropriate proportions radiation of monochromatic LED. Most frequently, few LED chips producing different color light are being used in one housing. Generally, LED housing includes at least two LED chips (producing blue and yellow light), sometimes three (producing red, blue and green light) or four (producing red, blue, green and yellow light). LED produce white color on the principle of colors mixing have potentially the highest luminous efficiency, however the quality of obtained light (the value of the general color rendering indicator Ra) is low.
The second group of methods using luminescence phenomenon taking place in phosphor that is the conversion of wavelengths (radiation falling on luminofor with some wavelength is being conversed by it to the radiation with longer wavelength in comparison with the one which was falling on the luminofor). There are several technological solutions::
- LED chip emits blue color light. The part of this radiation falls on the luminofor, which converts it to yellow color. This yellow light mixes up with the remaining part of the blue light and as a result we obtain an impression of white light.
- LED chip emits blue color light, which falls on several luminofors, each of which converts it to the other color light. These different colors mix up with the remaining directly obtaining from the chip blue color light. As a result, we obtain white color light with the higest overall color rendering indicator Ra in comparison with the method of one luminofor.
- LED chip produces ultraviolet radiation (UV) falling on three-lane luminofor (producing three-lane light: red, green and blue). Different colors of light mix up with each other, giving the white color light of the highest quality (the highest overall color rendering index Ra).
The general color rendering index or CRI, as a measurement of the quality of white color light, from the viewpoint of the reliability of color rendering of the things which are in its new environment started to be used before LED has appeared. Overall color rendering index Ra is perfect, widely used measurement describing the quality of white light. General color rendering index Ra for the solar light, as well as bulbs amounts for 100, however for fluorescent lamps lighting from 50 to 99. The light source with the best color rendering index may amount for 100. Nowadays LED produce white light with the overall color rendering index of 70 to 90. However this index does not characterize all parameters of color rendering of LED, because in case of several LEDs which have the overall color rendering index Ra accounting for 80 detailed index R9(red color rendering) for this diode accepts negative values. That is why it is important that the light emitted by one LED would be as for such index R9 is bigger than 0. Technical data prepared by the manufacturer may include data related to Ra value and delaited indicator Ri=9. Willing to change the drawback of LED determination except Ra and R9 in NIST (USA) new scale of colors projection CQS or Color Quality Scale, reflects the parameters of color projection better than Ra.
Yes, relatively easy. LED controllers ordinary use one of two methods for luminous flux dimming: decreasing the value of the active current or voltage (phase adjustment), either pulse width modulation. PWM method is more common in case of LED luminous flux dimming because of the wide range of regulation (dimming) and linear dependence between the modulation level and the value of luminous flux. LED luminous flux can be dimmed using numerous devices regulating controlled by analog signal 1-10V or digital type DMX and DALI.
LEDs light color changes during the operation because it depends on such factors as:
- 1. diode operating temperature
- 2. power supply method– charging with direct current or pulse power
- 3. luminofor degradation-in case of white color with luminofor LED, the color depends on the characteristics of luminofor
Pace of aging depends on the quality of LED module and the correctness of its application in the particular solution (correctness of power supply, cooling correctness).
LED can emit light of different color temperature. It depends on numerous factors, mainly on the method of doping a semiconductor. Typical color temperatures of white light produced by LED are the following: 2700K, 3000K, 3300K, 4000K, 5400K, 6500K. It is not excluded that some manufacturers offer LED emitting light with other color temperatures.
LED also vary in terms of luminous efficiency. Nowadays LED of luminous efficiency from 30lm/W to 150lm/ W are available. Lower values of luminous efficiency are presented by diodes with a lower power. In the laboratory conditions even higher luminous efficiency are obtained.
LED ligting system efficiency is smaller because it is calculated taking into account the “lost” power supply system values. LED which are offered nowadays are direct replacements of traditional bulbs and represent luminous efficiency in average from 50lm/W to 70lm/W. Such high luminous efficiency allow to obtain approximately 80% energy savings in comparison with the traditional bulbs, as well as, approximately 30% in relation to compact fluorescent lamps. It concerns only such LEDs, which represent A energy efficiency class. Energy efficiency class is indicated on the products or its packages. Energy efficiency class is not indicated for reflector light sources. It is important to pay attention on the fact that diodes emitting light of higher color temperature usually have higher luminous efficiency and give white light, even up to blue tint. Lower color temperature LEDs represent lower efficiency, but emmit warm color lighting.
The color of white light can be very diverse, from warm white (for example, color temperature Tc=2700K or Tc=3000K) to cold (for example, Tc=6500K).
During LED production, the production dispersion of its usage parameters occur. Dispersion of light color is crucial. Due to the present LED manufacturing technology the execution of diodes producing the same ideal light color is not possible. In case of numerous LED usage at the same time, for example, in case of LED modules or LED housings type “wall washer” or LED matrix, the difference of light colors may be visible. Wherefore, manufacturers produce LED due to the dispersion of coordinates tristimulus within the same nomical color of the light, creating so called bins. In such case, the law developed by Mac Adam in 1937 is used. Mac Adam has defined fields in the chart of chromaticity (colors triangle) named ellipses of Mac Adam. Lighting colors, which are situated in the field of this ellipse, depending on its measurements are more or less distinguished by the human eye. Manufacturers use it while producing shining diodes in such a way that light colors are situated in the indicated fields (ellipses) in the chromaticity chart. Diodes are produced to the so called bins.
Bin is a definition related to LED division due to the color of produced light source. Depending on the selection process as well as tolerance range choosen by the manufacturer, LED diodes are situated in separate categories (bins).
Fig.1.5.1 Mac Adam Ellipses on the chromaticity diagram
Each bin includes diodes producing the light of the same color and is indicated with the specific code. LED producing white color light are being sorted on the basis of the color temperature of the light. LED producing colorful light are being sorted on the basis of the dominated wavelength.
Fig. 15.2. Tolerances for white light
The biggest manufacturers increase their criterions for division of bins, mainly in case of LED producing white light. Exacerbation is based on the decrease of tolerance range of color temperatures or the wavelength dominating highly lower than of ranges obtained from Mac Adam’s law. Exacerbated sorting criterions and tolerance range have allowed to improve LED quality.
Thanks to better parameters of LED diodes, mainly bigger luminous efficiency of white diodes currently finding application in all fields of light application for the purposes of lighting. Likewise, the richness of its housings range (simultaneous diodes, multi-source modules, LED panels, LEDs, LED housing) will increase the range of application possibilities. LED can be used for the static lighting installation (for example, functional indoor lighting with workplaces), for dynamic installations (for example, decorative lighting, advertisements) with the change of intensity and colors, for furniture, showcases, etc.. Due to the long durability, LED diodes are suited mainly for the emergency and safety lighting.
In general, LED diodes are used in the following fields:
- decorative lighting
- architectural lighting (illumination of objects)
- general lighting
- accentuating lighting
- advertisements lighting
- signaling lighting
- outdoor lighting
- road lighting
- vehicle lighting
LED represent the range of crucial benefits compared to other sources of lighting. Such benefits are:
- high luminous efficiency, defining it as energy-saving light sources
- high durability, independent from the frequency of switching LED on and off
- immediate light up while obtaining the full luminous flux value
- high shake resistance.
LED benefits can bring measurable advantages from its usage mostly in all fields of lighting usage, mainly in lighting and signaling in places which are hardly available, in places exposed to shake and vibrations (for example, bridges, vehicles), lighting signaling, etc.. It is important to mention benefits resulting from the usage of the particular LED features, which can include:
- the possibility of direct light obtainment with different colors as well as its smooth change, without the usage of additional filters can be used in alarm systems and decorative lighting
- the lack of infrared radiation and negligible share of UV radiation, important in the light of works of art and other sensitive objects
- the possibility of forming a diverse individual LED light distribution, consequently an obtainment of any distribution in light housings without the necessity to use additional reflectors.
LED with its undisputed advantages indicate some shortcomings, which limitate its usage or require specific actions in order to reduce the negative impact from its application. These include:
- strong dependence of LED parameters (luminous flux efficiency, durability) on the connector temperature, as so on the environment temperature. It requires the necessity to use heavy radiator and avoidance of LED usage in places with higher temperature, for example, strongly sunny.
- Resultant performance of LED housing depending on the quality of LED module application (power supply, optics, cooling, etc.).
- interaction of LED circuits with the paremeters of supply chain due to the generation of troubles (higher harmonic). For the reduction of this unefficient phenomenon it is necessary to use particular power supply.
- Small measurements of shining lamp with narrow angle of radiation may cause high luminances resulting in dazzle. Several LEDs can emit too strong radiation in blue band (“cold” light LED), exceptionally harmful fot the human eye. These negative LED characteristics force LED manufacturers to indicate appropriate, fair information and guidance for the users.
Diodes are manufactured in different housings with different initial optical systems. Each of it, differently shapes the distribution of luminous flux. Generally, typical distributions can be divided into 4 basic groups:
- narrow distribution
- lambertowski distribution
- wide distribution or batwing
- lateral distribution or side emitting.
The basis while choosing an appropriate diode for the particular usage is the indicated photometric data included in the catalogue card, however for the purpose of rapid commercial classification the simplified breakdown of diodes was implemented due to the nature of the distribution of luminous flux.
Diodes have photometric bodies with rotational symmetry represented through the average light curves. The parameter describing the nature of distribution which is indicated by manufacturers is the distribution angle (useful divergence V1/2).
Narrow angle diodes are characterized by the distribution angle not bigger than 90º. Often obtained diodes are the ones with the distribution angle from 15º to 30º.
The perfect Lambert distribution is characterized by the angle of 120º. Diodes which refer to the group of Lamber diodes have distribution which is only similar to the ideal one. The biggest differences occur in case of wide lighting angles. Diodes of such group would be described by the distribution of angles from 110° to 130°. It is the most frequently obtained distribution angles among the SMD diodes or high power diodes.
Wide-angle distribution named also as butterfly is characterized by the angle wider than 120° as well as the fact that the maximum brightness has different direction than the optical axis, tilted from it at 30° to 60°. It frequently occurs among the high power diodes.
Side distribution is characterized by the angle wider than 180° as well as is tilted in the direction of maximum brightness at more than 60° from the optical diodes axis. Due to the complicated structure of the initial optical arrangement realizing such distribution, it is not so frequently obtained and occurs only among high power diodes.
Fig. 18.1. Average typical light curves of different LED groups.
In practical use, in lighting equipment not single LED diodes are being used but prepared LED modules. In such cases the manufacturer of the final equipment indicates the resulting lighting distribution similarly as in case of traditional lighting housings. Having in mind ways of LED lighting, it is worth to remember about the illumination problem. The whole luminous flux of diode is emitted from the ground of a few square millimeters. The illumination of this ground accounts for in dependence on diode’s construction and the power of its working from several up to more than 50 millions cd/m2. It is important to carefully conduct the analysis of possible dazzles for LED lighting and installation.
Considering potential photobiologic damages deriving from LED it is important to demarcate white color LED provided for the general lighting aims from LED with clearly identified lighting color or strict spectral distribution of optical radiation. In the last case, can be obtained LED emitting in purposeful way UV radiation, IR or also blue, which in case of unappropriate usage can result in significant threats for the human eyesight or skin. It is the same problem for the usage of other types of radiators on the basis of discharge lamps or incandescent lamps, when it is crucial to go by the manufacturer’s instructions.
Evaluating potential threats obtained from white color LED, for the general lighting goals, it is necessary to look at it as in case of other light sources, either incandescent or discharge. Criterions for photobiologic safety evaluation of all light sources types are indicated in the European Standard EN 62471 based on the IEC standard with the same number and CIE Publication S009.
The basic types of photobiologic threats obtained from the radiation of light sources may be grouped in such way:
- UV radiation threat (actinic and close) harmful for cornea and lens of the eye;
- Blue light threat resulting in photochemical damage to the retina of the eye;
- Thermal radiation threat resulting in thermal damage to the retina of the eye;
- IR radiation threat harmful for cornea and lens of the eye.
In practice, the level of risk for each group depends on radiation power in the indicated spectrum range and time of its influence on the human eye. Additionally, in case of damage to the retina not only the radiation power but also the angle in which this radiation is being sent is important. The slighter the solid angle is the higher is the risk level. In the Standard named above, the maximum values are indicated, as well as, the intensity of radiation or energetic illumination, which are the basis for light sources classification to three fundamental groups of damages: without risk, low level or medium level. It is crucial to notice that set fixed requirements are based on the assumption of appropriate usage of light sources what means the avoidance by the user of the long-time contact with light source in conditions of clear risk for the human eye, for example, looking at filament or other shining element or excessive impending of the light source to the human eye.
Evaluating white LED light according to the foregoing criterions, it is worth to verify that the majority of literature data classify it similarly as the other light sources for the general lighting aims, to the group with no threats to the human eye. Some LEDs can be exceptions, mainly those of cold-white color, which refer to the group of potential threat by blue light as to the group two, what means with significant risk. Comparing LED parameters with other light sources, for example, halogen bulb or metal halide lamps, it may be obvious to claim that those last groups significantly more often exceed the levels of threats considered as safe.
It is worth mentioning that radiation resulting from the light sources, is ranked as threats included in the basic safety requirements indicated in Voltage Directive, compliance with which is the condition for legal implementation of product to the market. It means, that manufacturers of lighting equipments on the basis of LED indicating their products with CE symbol, ensuring the completion of basic safety requirements, also have to take into consideration threats connected with optical LED radiation. Their evaluations may be based on the information coming from LED manufacturer. For this reason, LED manufacturers have to evaluate and classify their products to indicated group of photobiologic threats, and valuable information may be included on the label of the product. In case of threat occurrence, recommended precautions or required actions in order to eliminate or limit threats should be proposed. Tips for manufacturers working in this field are mentioned in Technical Report IEC/TR 62471 – 2.
Summing up contemplations, it is worth to mention that the way of light projection while using LED differs from other light sources, well analized technologies, admitted as traditional. Further actions aiming at detailed examination of photobiologic threat resulting from LED and possible establishment of safety usage conditions for this type of products.
In the foregoing analysis, some issues were consciously skipped: the ones considering typical for light sources-dazzle effect related to the high illumination of shining sources or light pulsation related to charging with low voltage considering that experiences of traditional bulbs usage are enough for appropriate application and usafe of LED.
Because of its current-voltage characteristics, LEDs require an application of appropriate regulatory-stabilizing equipment, power supply (AC), which aim at transformation of alternate voltage into the fixed voltage with indicated value, for example, 12V, 24V or 48V. Some solutions of power supplies (AC) are equipped with controlling systems allowing dimming effect or color change of matrix. Often controlling equipment constitutes the integral part of LED modules.
From the point of view of conditions for LED powering, two basic types of equipment are distinguished: with current stabilisation named “constant-current” and with voltage stabilisation named “constant-voltage”. It is recommended to use these first positively affecting on the value and stability of luminous flux. Modern power supplies (AC) have to be equipped with compensation system of environment temperature flow as well as the system preventing from AC overheating made on the basis of, for example, thermistors. In case of common powering aims of LED, ACs are used.
Fig. 20.1 An example block diagram of an impulse power supply
Chain voltage with the frequency 50Hz is rectified with the help of bridge rectifier. Later on, rectified voltage is being impulsed by semiconductor switch controlled by Pulse Width Modulation(PWM) generator with the row frequency of several dozen Hz. High- frequency transformer is used in order to adjust the values of voltage to the required value, which is later rectified and smoothed. Isolation (Opt) served for separating of galvanic controlling system and AC output circuit. Input filtr is aiming at limitation of damages generated to the chain through the power supply, mainly higher current harmonics. Fuse protect the power supply frin the damages of short-circuti and overload. Modern high quality constructions of impulse power supplies have eve practically sinusoidal shape of input current curve.
The integral part of each LED housing powered from the chain with 230V voltage is the power supply, which is aimed at the change of alternate voltage of electronical chain into the fixed voltage amounted for 12, 24 or 48V. Present-day power supply solutions used in LED housing are based on the typical solutions for impulse power supplu. The basic advantages of such solution are low expenses, lower measurements as well as weight, and higher efficiency. Power supply gathers from the chain not the constant current but impulses of current with big content of highest harmonics, which stands for the crucial drawback. Every waveform sygnal, which is not sinusoidal may be presented as the sum of n sygnals with frequencies being a multiple of the basic frequency of this sygnal. In the powering chain, the basic frequency is equal 50Hz. The frequency of harmonic row 3 amounts for 150Hz, but row 5 amounts for 250Hz. Therefore, row harmonic n has it frequency nx50Hz.
Rys.21.1. An exemplary course of a distorted signal along with its distribution into individual harmonics.
The most important problem is that currents of higher harmonics flowing in the powering chain cause voltage spoles on the chain elements that, as a result, negatively affects the powering voltage, which may be used for the powering of other collectors. In order to realize an objective evaluation of an impact of particular collector on the chain powering, it is required to know the the spectrum of current harmonics, which allows to calculate the value of Total Harmonic Distortion coefficiend, that stands for the overall deformation coefficient. The value of THD current coefficient of LED housing amounts for 26%.
Rys.21.2. Momentary flow of supply voltage and current of an exemplary LED fixture
Rys.21.3. The harmonic spectrum of current of sample LED luminaire
The lower the value of THD current coefficient, the lower is the level of negative influence of housing to the powering chaing. Negative affection of higher harmonics on the powering chain is:
- Overload of electroenergetic cables related to the increase of current value,
- overload of neutral wire resulted from the sum of 3 row harmonics, which sources are one phase receivers,
- deformation of powering voltage, which are causes of inappropriate work of sensitive receivers,
- overload of element of electroenergetic chain, for example, transformers, capacitor banks, etc..
Taking into account the above, it is required that LED housings meet requirements for receivers indicated as C class according to the standard PN-EN 61000−3−2: 2007.Electromagnetic compatibility (EMC), Part 3 – 2: Allowed levels – Allowed leveld of current harmonics emission (phasic current powering the receiver ≤ 16A). Zgodnie z zaleceniami podanej powyżej normy wartości dopuszczalne harmonicznych prądu wejściowego oprawy nie mogą być większe niż wartości podane w tabeli (rys. 21.4.).
Fig. 21.4. Table: Acceptable levels for Class C equipment according to PN-EN 61000-3-2
What is color rendering index (CRI)??
Why CRI is so important issue of LED lighting?
The colors are a result of interaction of objects with lighting.
Which of the presented photographs accurately reflects the colors of items, in your opinion?
To determine how far the light spectrum differs from the spectrum of natural light, there was created a parameter called color rendering index. Commonly abbreviated as CRI, Color rendering index is not always properly interpreted issue. To get good color rendition and contrast, you should use a light source with a high color rendering index. Its maximum value is 100 and it gives us information about the extent to which the light source enables observation of color.
Good color rendering index, combined with a pleasant color temperature of light, for most people provides the best conditions for rest. Just compare streets and parkings lit by sodium lamps with metal halide or LED luminaires, whose color rendering is much higher.