Light Emitting Diodes (Leds)
How it works
A Light Emitting Diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device. The effect is a form ofelectroluminescence where incoherent and narrow-spectrumlight is emitted from the p-n junction in a solid state material. LEDs do not require heating of a filament to create light. Instead, electricity is passed through a chemical compound (crystal) that is excited and generates light.
In the largest available light diodes their dimensions are represented by edges of only 1mm. LEDs thus belong to the smallest available, almost point-like, light sources.
An LED often has optics added directly on top of the chip to shape its radiation pattern and assist in reflection. The colourof the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet.
LEDs produce monochromatic radiation and their colour tone is defined by the dominant wavelength. There are LEDs in the colours red, orange, yellow, green and blue.
The following table denotes the different semiconductor material used to achieve the various colours:
Aluminium indium gallium phosphide
red, orange, yellow
Gallium arsenide phosphide
red, orange, yellow
Indium gallium nitride
White light can be produced as a mixture of all wavelengths, for example in LED modules. This arises through an additive mixture of the three RGB colours (Red, Green, Blue).
White light LEDs were developed by placing a phosphor compound on top of a blue LED. Here the light of a blue LED excites the luminescent material which changes a part of the blue light into yellow.
By overlaying the unabsorbed blue light with yellow light emitted by the luminescent material white light is produced. The concentration of luminescent material needs to be guided precisely so that the desired white is realised.
One of the biggest challenges has been to produce a stable ‘white’ light. Earlier and cheaper ‘white’ LEDs have often emit ‘white’ light with a blue tinge or hue.
Constant Colour White LEDs
For all the acknowledged benefits of power LEDs, such as efficiency, sustainability, and durability, the primary hurdle to be overcome is that of providing a supportable, high colour-quality supply of white LEDs. Leading LED maufacturers have recently formulated new phosphor technology which enables specific targeting for correlated colour temperature.
This now puts the manufacturer in control of the colour temperature and tint and allows for production variance to be minimized. Until recently, consistency of LED selection has been via ‘Binning’, where each batch of LEDs is categorised so that the customer can ensure that exactly the same colour LEDs are used.
Red, Green, Blue (RGB) LEDs allow the designer, client or consumer the opportunity to produce a phenomenal array of lighting effects by blending different ratios of Red, Green or Blue.
The CIE have recently concluded that the current CRI method did not describe well those situations where white LED light sources were involved i.e. if white LED light sources were visually ranked together with other light sources.
Low correlation was found between the visual colour differences and the computed colour differences if the current CRI method was applied to calculate those colour differences. The conclusion is that the CIE CRI is generally not applicable to predict the colour rendering rank order of a set of light sources when white LED light sources are involved in this set.
Using the CIE scale, warm white LEDs have a colour reproduction index from Ra ≥70 up to Ra ≥90. For cold white LEDs the Ra value is between 70 and 80.
Correlated Colour Temperature (CCT)
White LEDs are available in a range of Colour Temperatures. White LEDs have above all a cold, neutral white light with a colour temperature > 4,500 K. Further development in the area of convertible luminescent materials is making warmer light colours possible.
Since 2003 there have been warm white (> 2,800 K) and neutral white (3,300 to 3,800 K) LEDs. For the domestic market, White LED lamps are offered in Warm White and Cool White.
Beam Angle (Optics)
As protection against environmental influences the semiconductor crystal is set into a housing. This is constructed so that the light radiates in a semicircle of almost 180 degrees (the current maximum is about 160 degrees).
Guidance of the light is thus easier than in filament or discharge lamps, which generally radiate light in all directions. There are various types of housing for LEDs of low, medium and high performance; they all give good mechanical stability.
There are several procedures for brightness control:
- Analogue dimming – where the LEDs are operated as a variable resistance
This procedure has two disadvantages: at low light intensity the control element has a relatively large performance loss which is converted into heat. Because of this a component for an appropriate diversion of the heat has to be included and this is of relatively large dimensions.
- Digital dimming - control with the aid of digitally controlled switches
Here the LEDs are switched on and off intermittently. This happens so often within one second that the eyes do not notice the flickering of the LED light. Control by pulse width modulation is, for example, such a process.
If the technique of brightness control is combined with the technological option of being able to set various coloured LEDs to individual colours, then colour sequences and plays of colour, as well as mixed colours, are very easily created.
The holy grail of the light source. Offering a white light of up 150 lm/w (in laboratory tests). This is the area where the most focus is being placed. The efficacy of LEDs is very much affected by temperature. LEDs utilise a Heat Sink, a die cast fitting to dissipate heat away from the PCB. The lower the temperature, the higher the efficacy. Average efficacy would be 50-100 lm/w at present but this figure is increasing constantly.
The other key factor in the focus by lamp manufacturers and luminaire designers on LEDs is that they offer unprecedented life of 50,000 hours plus. Some LEDs even promise 100,000 hours. Because LEDs offer such extreme life, one of the biggest challenges is how to achieve actual performance over life measurements i.e. ‘real life’ data.
Testing an LED for 24 hours a day, 365 days per year, would only provide 8,760 hours of data. So, an LED rated at 100,000 hours would naturally require 11.5 years to produce ‘real life’ data. With the pace of technological development and the need to commercialize LEDs, this is obviously not workable.
Some manufacturers have agreed to use a 6:1 ratio to ensure data can be gathered and extrapolated. So, 1,000 hours actual would equate to 6,000 hours and 2,000 hours actual would be 12,000 hours.
The lifespan of an LED depends on its operational and environmental temperature. At room temperature, LEDs (and LED modules) have a very long lifespan of up to 50,000 working hours. In contrast to filament lamps, where a break in the helix (filament) means the end of its life, total failure of an LED is extremely rare. Its light intensity also declines much more slowly: this property is known as degradation.
The period of degradation of the original luminous flux defines the lifespan of LEDs. The degradation of the luminous flux is strongly dependent on the temperature of the light emitting surface in the semiconductor crystal. There must therefore be no build-up of heat in the operation of an LED: the conducting plate or additional heat sink must reliably divert the heat. A too high environmental temperature will equally lead to a decrease in the luminous flux.
Until recently, LED reliability claims were covered under the blanket lumen maintenance statement “70% lumen maintenance at 50K hours,” However, this is not ideal for LEDs.
A conventional lamps’ life is characterized by a mortality curve. This refers to a percentage of the lamps that catastrophically fail. For instance, the most common mortality rating is based on the time at which 50 per cent of lamps will have failed catastrophically, commonly known as a B50 – see graph.
As LEDs experience a gradual reduction in light output during operation and generally do not catastrophically fail, more than 50% of LEDs will still provide a good measure of light at less than the 70% lumen maintenance (known as L70) threshold. The Lighting industry has thus sought to develop alternative methods of defining the mortality of LEDs.
One manufacturer now offers mortality curves based on 2 options. There is the standard 70% lumen maintenance (L70) at 50% survival rate (B50) or an alternative 70% lumen maintenance (L70) at 90% survival rate. The latter measure is known as B10. This is a new and revolutionary way to measure LED lifetime and offers customers the advantage of knowing how long their LEDs will produce optimal light output for.
Control Gear (Drivers)
LED Drivers are current control devices that replace the need for resistors. LED Drivers respond to the changing input voltage while maintaining a constant amount of current (output power) to the LED as its electrical properties change with temperature. The voltage versus current characteristics of an LED is much like any diode. Current is approximately an exponential function of voltage, so a small voltage change results in a large change in current. It is therefore important that the power source gives the right voltage.
If the voltage is below the threshold or on-voltage no current will flow and the result is an unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED. As the LED heats, its voltage drop decreases, further increasing current. Consequently, LEDs should only be connected directly to constant-voltage sources if special care is taken.
The majority of LEDs require Direct Current. Depending on the type of operation, there are two different methods of control for LEDs and LED modules: constant voltage and constant current.
Constant Voltage Driver
The voltage regulated control of LEDs is characterised by the fact that the diodes are operated with a ‘constant voltage’. In this case standard proprietary “direct current” equipment can therefore be used as the power supply.
This method of operation permits easy control of light intensity in LEDs by pulses (switching on and off) of the power supply. With this method it is necessary to limit the current in LEDs, because the forward tension leaks strongly. An incorrectly defined operating current limit can lead to destruction of LEDs and their operational and control equipment.
Constant Current Driver
The current regulated control of LEDs has advantages for constant operation and in the efficacy (lumen/watt). IN this instance, it is important to use a predetermined current. The appropriate wiring, in most cases includes a governor, ensures constant operation. Strongly fluctuating forward tensions play only a small role in this method of operation as the voltage to the LEDs adjusts in proportion to their operational current so that they are not overloaded.
RGB Colour Controllers
Dimming of an LED can be done by either reducing the current level through the diode ( DC-dimming, analogue dimming) or by applying PWM-dimming (short for Pulse Width Modulation) to the LED.
DC-dimming is a straightforward solution to reduce the thermal load (and brightness) of an LED. For example, reducing the LED’s current from 350 mA down to 250 mA, will reduce the thermal load on the LED accordingly. Varying the current of LED may however have side effects on the light output of the LED.
LED can have a noticeable dependency of the output colour on the current that is applied; this is also referred to as a colour-shift of the LED. For white LED reducing (or increasing) the LED current may lead to a change of the white-point. It is important to check whether any colour-shift occurs with DC dimming and whether it is acceptable to the particular application. If the colour-shift is too strong, PWM-dimming can help reduce this effect. In particular for RGB applications it is advisable to use devices with PWM dimming.
PWM-dimming utilizes a different method for reducing the average current through the LED: the current applied to the LED is turned on and off at a high frequency (e.g. 300 Hz) while keeping the current level fixed (e.g. at 350 mA). The average value of the current flowing through the LED is then determined by the length of the on-period as compared to the off-period (the duty-cycle).
The above charts show dimming at 25%, 50% and 100% and the resulting, average current flow through the diode. Since the current through the LED remains unchanged at different dimming levels, there is also no colour-shift introduced due to a change in current. This ensures best performance of the LED in both RGB and white light applications.
Low Power LEDs & High Power LEDs
Low Power LEDs are mostly single-die LEDs used as indicators, and they come in various-sizes from 2 mm to 8 mm,through-hole and surface mount packages. They are usually simple in design, not requiring any separate cooling body. Typical current ratings range from around 1 mA to above 20 Ma, with a luminous flux of approx 1 lm. The small scale set a natural upper boundary on power consumption due to heat caused by the high current density and need for heat sinking.
High power LEDs (HPLED) can be driven at hundreds of mA (vs. tens of mA for other LEDs), some with more than oneampere of current, and give out large amounts of light i.e. in excess of 120 lm. Since overheating is destructive, the HPLEDs must be highly efficient to minimize excess heat; furthermore, they are often mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out in seconds.
LED Junction Temperature is the temperature at the light emission point at the heart of an LED device (called the ‘p-n junction’).
This is a critically important parameter for high reliability LED applications because both LED lifetime and LED light output are directly proportional to junction temperature. By controlling LED junction temperature through a variety of thermal management techniques, optimally efficient LED lighting designs with very long service lives can be realised.
The key to a successful design starts with the transfer of LED heat. Each custom LED lighting design involves the concept of efficiently transferring as much heat as possible away from LED PN junction. The process begins within the LED lamp, where thermal energy released into an integrated slug can potentially exit the light emitting diode. Modern surface mount LED lamps depend on the thermal efficiency of this slug.
In addition, a thermally stressed LED lights will lose efficiency and light output will diminish. If the LED thermal management continues to race out of control, the LED junction may break down causing a state of complete thermal runaway.
The result is typically catastrophic failure. Other affects of overstressed LEDs may include broken wire bonds, delaminating, internal solder joint detachment, damage to die-bond epoxy, and lens yellowing.
LED drivers are designed to convert mains voltage to DC with power factor pre-regulated for LED lighting. Thereby avoiding the problems associated with the higher wattage fluorescent circuits where the phase shift between the supplied voltage and current becomes significant.
Until recently, the use of LEDs has been limited to electronic devices, ambient ligh ting, exterior lighting or for creating effects, because of the versatility of RGB LEDs and Controllers.With their low power consumption, LEDs are ideal for emergency lighting and have been used in this application for a number years.
As the development of White LEDs is progressing at an exponential
rate, the performance of (LEDs), including efficiency, flux level, life, and the variation of colour, is advancing too, meaning we are now beginning to use them as direct light sources.
Example 1. LEDs are ideally suited to street lighting - accidents are scarce and there are few lamp changes, little maintenance, reduced energy use and low costs.
Example 2. LEDs are used as secondary light sources or in smaller rooms i.e. in lifts, railway carriages, corridors etc.
The amazing potential of the LED is that it can offer high efficacy, ultra-long long life, instant start, good colour rendering, no UV, shock-proof, limited environmental impact…the list goes on. Additionally, because LEDs are based on Solid State technology, they can be controlled and programmed, making them the most versatile and flexible of light sources.
White LEDs are still being developed to the point where they can be used as like-for-like replacements for their incandescent, fluorescent or HiD equivalent. Some of the chemicals used in the fluorescent crystals are recognised as hazardous. The main disadvantage would be low operating temperature for optimum life, efficacy and lumen maintenance. In terms of initial expenditure or capital outlay, LEDs are expensive but this argument can be countered over the life of the light source.
An LED module consists of several semiconductor crystals or single LEDs (semiconductor crystals with their housings) which are placed in series next to one another, or combined in some other form, on a conductor plate. The plate is not only a carrier but also makes possible the easy fixing of the LEDs and other optical, electronic or mechanical components.
The electrical layout of the conductor plate can be adapted to a particular application: as well as single operation, coloured LEDs can also be separately fixed using an appropriate layout so that plays of colour and sequences are possible within a module. Colours can be produced with an additive colour mixture because the LED module combines the three RGB colours (red, green, blue). The mixing of basic colours leads to the creation of every favourite tone or to various colour effects.
New LED Technology
There are two competing technologies which could radically change the nature of lighting in the future: OLEDs and PLEDs.
Organic Light Emitting Diodes (OLEDs) are based on multiple layer devices (up to 16 layers) of evaporated low molecular weight molecules.
Polymer Light Emitting Diodes (PLEDs) are solution processible structures based on a single white light emitting polymer, deposited by printing on to charge transport layers, typically PLEDs are 3 or 4 layers thick
Figure 1 depicts a cross-section through both structures. Both devices represent extended emissive surfaces with devices up to 1/5 m2 already being demonstrated from the laboratory.
However in general, laboratory-made devices are much smaller - generally 10’s of square centimetres in area. The main research effort continues within the materials, striving for improved quality of white light, lifetimes and efficacy.
Presently OLEDs outperform PLED technology, on laboratory test samples, but due to the complexity of manufacturing multiple layer structures it is widely believed that solution processible PLED technology will be first to demonstrate high direct yield manufacturing levels.
If the emitting layer material of the LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED). So far, OLEDs are intrinsically well suited for indoor area illumination and could appear as 'glowing wall paper’ or ‘illuminated ceiling tiles’ without the need for 'luminaires'.
OLEDs utilise flat display technology and are made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. OLED’s could tackle in the future the material and production challenges currently encountered with SSL LEDs. OLEDs are already on the market for particular, very flat illuminated displays in portable devices.
Television manufacturers are still weighing up the benefits of OLEDs over LEDs (i.e. for flat panel LED TVs). OLEDs based on organic material are still under development. OLED efficiencies under particular operational conditions have been reported up to 64 lm/W at 1000 Cd/m² but still have to prove their efficacy in actual working conditions (e.g. temperature and life).
PLEDs consist of thin, flexible film made of polymers and capable of emitting the full color spectrum of light. These solution processed light emitting devices are simple 'few layer' devices based on polymeric functional materials. The active polymers serve a dual role in transmitting the charge and also converting it into light. The ability to dissolve the active materials (Hole Injection/Transport Layer, Interlayer/Primer Layer and Light Emitting Polymer) in a solvent to form an "ink" and deposit by a range of printing techniques on a wide variety of substrates at low temperatures provides a number of manufacturing advantages over small molecule OLED technology.
The best devices are currently achieving > 30lm/W with a half life of ~30Khrs at 500 Cd/m². It is expected that by 2011, PLED devices will exhibit > 60lm/W with 70% life being > 20K hrs. These materials emit no UV or IR and the expected thermal uplift, for a 600 sq tile fitting, is <15oC.
A PLED has 4 main layers:
- A glass or plastic substrate - for PLED fabric displays, plastic tends to be a better choice because it's less fragile but more flexible than glass.
- A transparent electrode coating, which is applied to one side of the substrate
- The same side of the substrate is then coated with the light emitting polymer film
- The final layer is an evaporated metal electrode, which is applied to the other side of the polymer film
PLEDs are a good example of 'nanotechnology'.The total thickness of all layers in a PLED display device can be less than 500nm.Human hair 0.1mm thick
Human hair 0.1mm thick
Organic layer thickness 1/2000 a human hair!!
Controlled layer thickness to 10% (5nm)
- BS IEC 60838-2-2 (draft) Special fittings – Part 2: specific Requirements – main section 2: connectors for LED modules.
- BS EN 61347-1 – A1 and A2 (drafts) Equipment for lamps, Part 1: general and safety requirements.
- BS EN 61347-2-13 (draft) Equipment for lamps, Part 2-13: specific requirements for electronic operating equipment for LEDs supplied by direct or alternating current.
- BS IEC 62384 (in preparation) Direct or alternating current equipment for LED modules – requirements for working methods.
- BS EN 60598-1 Luminaires – Part 1: General requirements and tests.
- BS EN 60598-2 Luminaires – Part 2: Special requirements (with corresponding main sections).
- BS EN 60825-1 Safety of laser installations – Part 1: classification of designs, requirements and user guidelines.
- BS EN 55015 Limiting values and measurement procedures for radio interference in electrical lighting installations and similar electrical equipment.
- BS EN 61547 Installations for general lighting purposes – EMC requirements for resistance to interference.
- BS EN 61000-3-2 Electromagnetic compatibility (EMC) – Part 3-2: limiting values – limiting values for harmonically oscillating currents (equipment entry current less than or equal to 16A per conductor).
- BS EN 61000-3-3 Electromagnetic compatibility (EMC) Part 3-3: limiting values: limitation of changes in voltage, voltage fluctuations and flicker in the public low voltage supply network for equipment with a measured current _ 16 A per conductor which is subject to any special connection conditions.
- BS IEC 62031: 09/2008 (LED Modules for General Lighting – Safety Specifications)
- BS IEC XXXXX (LED Module for General Lighting - Performance
- BS IEC 62560:(Lamps - Safety standard for self-ballasted LED lamps)
- BS IEC 62612 (DRAFT) (Lamps – Self Ballasted LED-Lamps for general lighting service > 50V – Performance): 34A/1318/PAS (replaces PRESCO(RTK)075 17th version and also issued as NP)
- BS IEC 61341(Measurement – Intensity & Angle)
- BS IEC 61231 (ILCOS)
- BS IEC 62504 (LED Terms & Definitions)
- BS IEC 60061 (Lamp Caps)
- CIE Div 2 2007 Publication CIE127 (Methods of measurement)
- CIE Div 6 / IEC62471 Photobiological safety / Optical radiation
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