1 LI-FI (LIGHT FIDELITY)-THE FUTURE TECHNOLOGY IN WIRELESS COMMUNICATION INTRODUCTION 1 Most of us are familiar with Wi-Fi (Wireless Fidelity), which uses 2.4-5GHz RF to deliver wireless Internet access around our homes, schools, offices and in public places. We have become quite dependent upon this nearly ubiquitous service. But like most technologies, it has its limitations. While Wi-Fi can cover an entire house, its bandwidth is typically limited to 50-100 megabits per second (Mbps). This is a good match to the speed of most current Internet services, but insufficient for moving large data files like HDTV movies, music libraries and video games. The more we become dependent upon the cloud or our own media servers to store all of our files, including movies, music, pictures and games, the more we will want bandwidth and speed. Therefore RF-based technologies such as todays Wi-Fi are not the optimal way. In addition, Wi-Fi may not be the most efficient way to provide new desired capabilities such as precision indoor positioning and gesture recognition. Optical wireless technologies sometimes called visible light communication (VLC) and more recently referred to as Li-Fi (Light Fidelity) on the other hand, offer an entirely new paradigm in wireless technologies in terms of communication speed, flexibility and usability. 2 Li-Fi is transmission of data through illumination by taking the fiber out of fiber optics by sending data through a LED light bulb that varies in intensity faster than the human eye can follow. Li-Fi is the term some have used to label the fast and cheap wireless-communication system which is the optical version of Wi-Fi. It is possible to encode data in the light by varying the rate at which the LEDs flicker on and off to give different strings of 1s and 0s.The LED intensity is modulated so rapidly that human eye cannot notice, so the output appears constant. More sophisticated techniques could dramatically increase VLC data rate. LiFi is transmission of data through illumination by taking the fiber out of fiber optics by sending data through a LED light bulb that varies in intensity faster than the human eye can follow.Li-Fi is the term some have used to label the fast and cheap wireless-communication system, which is the optical version of Wi-Fi. The term was first used in this context by Harald Haas in his TED Global talk on Visible Light Communication. At the heart of this technology is a new generation of high brightness light-emitting diodes, says Harald Haas from the University of Edinburgh, UK,Very simply, if the LED is on, you transmit a digital 1, if its off you transmit a 0,Haas says, They can be switched on and off very quickly, which gives nice opportunities for transmitted data.It is possible to encode data in the light by varying therate at which the LEDs flicker on and off to give different strings of 1s and 0s.The LED intensity is modulated so rapidly that human eye cannot notice, so the output appears constant.

More sophisticated techniques could dramatically increase VLC data rate. Terms at the University of Oxford and the University of Edingburgh are focusing on parallel data transmission using array of LEDs, where each LED transmits a different data stream. Other group are using mixtures of red, green and blue LEDs to alter the light frequency encoding a different data channel.Li-Fi, as it has been dubbed, has already achieved blisteringly high speed in the lab. Researchers at the Heinrich Hertz Institute in Berlin,Germany,have reached data rates of over 500 megabytes per second using a standard white-light LED. The technology was demonstrated at the 2012 Consumer Electronics Show in Las Vegas using a pair of Casio smart phones to exchange data using light of varying intensity given off from their screens, detectable at a distance of up to ten metres. Fig

Fig.1 Li-Fi enviorment

In October 2011 a number of companies and industry groups formed the Li-Fi Consortium, to promote high-speed optical wireless systems and to overcome the limited amount of radiobased wireless spectrum available by exploiting a completely different part of the electromagnetic spectrum. The consortium believes it is possible to achieve more than 10 Gbps, theoretically allowing a high-definition film to be downloaded in 30 seconds.

WHAT LI-FI STANDS FOR ?

Li-Fi comprises a wide range of frequencies and wavelengths, from the infrared through visible and down to the ultraviolet spectrum. It includes sub-gigabit and gigabit-class communication speeds for short, medium and long ranges and unidirectional and bidirectional data transfer using line-of-sight or diffuse links, reflections and much more. It is not limited to LED or laser technologies or to a particular receiving technique. Li-Fi is a framework for all of these providing new capabilities to current and future services, applications and end users. Technology detailsIt is a 5Gvisible light communication system that uses light from light-emitting diodes (LEDs) as a medium to deliver networked, mobile, high-speed communication in a similar manner as Wi-Fi.[3] Li-Fi could lead to the Internet of Things, which is everything electronic being connected to the internet, with the LED lights on the electronics being used as Li-Fi internet access points.[4] The Li-Fi market is projected to have a compound annual growth rate of 82% from 2013 to 2018 and to be worth over $6 billion per year by 2018.Visible light communications (VLC) works by switching bulbs on and off within nanoseconds, which is too quickly to be noticed by the human eye. Although Li-Fi bulbs would have to be kept on to transmit data, the bulbs could be dimmed to the point that they were not visible to humans and yet still functional. The light waves cannot penetrate walls which makes a much shorter range, though more secure from hacking, relative to Wi-Fi. Direct line of sight isn't necessary for Li-Fi to transmit signal and light reflected off of the walls can achieve 70 Mbit/s. Li-Fi has the advantage of being able to be used in electromagnetic sensitive areas such as in aircraft cabins, hospitals and nuclear power plants without causing electromagnetic interference.[8][9] Both Wi-Fi and Li-Fi transmit data over the electromagnetic spectrum, but whereas Wi-Fi utilises radio waves, Li-Fi uses visible light. While the US Federal Communications Commission has warned of a potential spectrum crisis because Wi-Fi is close to full capacity, Li-Fi has almost no limitations on capacity. The visible light spectrum is 10,000 times larger than the entire radio frequency spectrum. Researchers have reached data rates of over 10 Gbit/s, which is more than 250 times faster than superfast broadband. Li-Fi is expected to be ten times cheaper than Wi-Fi. Short range, low reliability and high installation costs are the potential downsidesHistory

Prof. Harald HaasProfessor Harald Haas, from the University of Edinburgh in the UK, is widely recognised as the original founder of Li-Fi. He coined the term Li-Fi and is Chair of Mobile Communications at the University of Edinburgh and co-founder of pureLiFi.The general term visible light communication (VLC), includes any use of the visible light portion of the electromagnetic spectrum to transmit information. The D-Light project at Edinburgh's Institute for Digital Communications was funded from January 2010 to January 2012. Haas promoted this technology in his 2011 TED Global talk and helped start a company to market it. PureLiFi, formerly pureVLC, is an original equipment manufacturer (OEM) firm set up to commercialize Li-Fi products for integration with existing LED-lighting systems. In October 2011, companies and industry groups formed the Li-Fi Consortium, to promote high-speed optical wireless systems and to overcome the limited amount of radio-based wireless spectrum available by exploiting a completely different part of the electromagnetic spectrum. A number of companies offer uni-directional VLC products which is not the same as Li-Fi.VLC technology was exhibited in 2012 using Li-Fi. By August 2013, data rates of over 1.6 Gbit/s were demonstrated over a single color LED.[22] In September 2013, a press release said that Li-Fi, or VLC systems in general, do not require line-of-sight conditions. In October 2013, it was reported Chinese manufacturers were working on Li-Fi development kits. One part of VLC is modeled after communication protocols established by the IEEE workgroup. However, the IEEE 802.15.7 standard is out-of-date. Specifically, the standard fails to consider the latest technological developments in the field of optical wireless communications, specifically with the introduction of optical orthogonal frequency-division multiplexing (O-OFDM) modulation methods which have been optimized for data rates, multiple-access and energy efficiency have. The introduction of O-OFDM means that a new drive for standardization of optical wireless communications is required.Nonetheless, the IEEE 802.15.7 standard defines the physical layer (PHY) and media access control (MAC) layer. The standard is able to deliver enough data rates to transmit audio, video and multimedia services. It takes into account the optical transmission mobility, its compatibility with artificial lighting present in infrastructures, the devience which may be caused by interference generated by the ambient lighting. The MAC layer allows to use the link with the other layers like the TCP/IP protocol.The standard defines three PHY layers with different rates: The PHY I was established for outdoor application and works from 11.67 kbit/s to 267.6 kbit/s. The PHY II layer allows to reach data rates from 1.25 Mbit/s to 96 Mbit/s. The PHY III is used for many emissions sources with a particular modulation method called color shift keying (CSK). PHY III can deliver rates from 12 Mbit/s to 96 Mbit/s. The modulations formats preconized for PHY I and PHY II are the coding on-off keying (OOK) and variable pulse position modulation (VPPM). The Manchester coding used for the PHY I and PHY II layers include the clock inside the transmitted data by representing a logic 0 with an OOK symbol "01" and a logic 1 with an OOK symbol "10", all with a DC component. The DC component avoids the light extinction in case of an extended line of logic 0.The first VLC smartphone prototype was presented at the Consumer Electronics Show in Las Vegas from January 710 in 2014. The phone uses SunPartner's Wysips CONNECT, a technique that converts light waves into usable energy, making the phone capable of receiving and decoding signals without drawing on its battery. A clear thin layer of crystal glass can be added to small screens like watches and smartphones that make them solar powered. Smartphones could gain 15% more battery life during a typical day. This first smartphones using this technology should arrive in 2015. This screen can also work to receive VLC signals and so can the smartphone camera. The cost of these screens per smartphone is between $2 and $3, much cheaper than most new technology.VLC technology is ready right now; it's being installed in museums and businesses across France, and is being embraced by EDF, one of the nation's largest utilities. Philips lighting company has developed a VLC system for shoppers at stores. They have to download an app on their smartphone and then their smartphone works with the LEDs in the store. The LEDs can pinpoint where they are at in the store and give them corresponding coupons and information based on where aisle they are on and what they are looking at.

The Li-Fi Consortium is an international platform focusing on optical wireless technologies.[2]It was founded by four technology-based organizations in October 2011. The goal of Li-Fi Consortium is to foster the development and distribution of optical wireless technologies such as communication, navigation, natural user interfaces and others. PRINCIPLE OF LI-FI TECHNOLOGY

Heart of Li-Fi technology is high brightness LEDs. Light emitting diodes can be switched on and off faster since operating speed of LEDs is less than 1 s, than the human eye can detect, causing the light source to be appear continuously. This invisible on-off activity enables a kind of data transmission using binary codes. Switching on and LED is a logical 1, switching it off is a logical 0.It is possible to encode data in the light by varying the rate at which LEDs flicker on and off to give different strings of 1s and 0s. Modulation is so fast that human eye doesnt notice .A light sensitive device (photo detector) receives the signal and converts it back into original data. This method of using rapid pulses of light to transmit information wirelessly is technically referred as Visible Light Communication (VLC) though its potential to compete with conventional Wi-Fi has inspired the popular characteristics Li-Fi .

A. Visible Light Communication

Li-Fi is a fast and cheap version of Wi-Fi, which is based on visible light communication (VLC). The Visible light communication is a data communications medium using visible light between 400 THz (780 nm) and 800 THz (375 nm) as optical carrier for data transmission and illumination . Visible light is not injurious to vision. Typical example of visible light communication is given in fig.1 below.

Fig. 1 Typical Example of Visible light communication: Use of LED illumination as a transmitter

Fig. 2 Spectrum of radio and light wave

The importance of using VLC is that Frequency above 3 THz is not regulated by radio regulating laws as shown in fig.2 Visible light LED can be used almost everywhere. Infrared light is already used for communication such as wireless remote control, Infrared wireless LAN and Infrared inter-building communication. Visible light LEDs are beginning to be used in every home and office, which makes visible light LEDs ideal for ubiquitous data transmitter. Growth rate of LED lighting is expected to triple from 2009 to 2012 & market share of LED lighting will be more than 30 % of total lighting market in 2016. Image sensors can be used as receivers. The use of image sensors as receiver makes it possible to detect incoming data and accurate direction of incoming vector from transmitter to receiver. The image sensor technology will allow VLC to various new applications which cant be realized by radio wave technology like indoor navigation, augmented reality, accurate control of robots or vehicles and accurate position measurement.

B. Devices used in visible light communication

Fig 3 shows the example of devices used in visible light communication.

Fig. 3 Devices used in Visible light communication

Devices which are used for transmission purpose in VLC are visible light LED and fluorescent lamp. LED light intensity is modulated by controlling its current. The technology uses fluorescent lamps to transmit signals at 10bit/s, or LEDs for up to 500 Mbit/s. Devices which are used for reception purpose in visible light communication are pin photodiode (high speed reception up to 1Gbps), Avalanche photo diode (very sensitive reception) and Image sensor (simultaneous image acquisition and data reception) as shown in fig 4 below.

Fig. 4 (a) Pin photo diode

Fig. 4 (b) Avalanche photodiode

Fig. 4 (c) Image sensor

Light-emitting diodeA light-emitting diode (LED) is a two-lead semiconductor light source that resembles a basic pn-junction diode, except that an LED also emits light. When an LED's anode lead has a voltage that is more positive than its cathode lead by at least the LED's forward voltage drop, current flows. Electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.An LED is often small in area (less than 1mm2), and integrated optical components may be used to shape its radiation pattern. Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs were also of low intensity, and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.Early LEDs were often used as indicator lamps for electronic devices, replacing small incandescent bulbs. They were soon packaged into numeric readouts in the form of seven-segment displays, and were commonly seen in digital clocks.Recent developments in LEDs permit them to be used in environmental and task lighting. LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are now used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, and camera flashes. However, LEDs powerful enough for room lighting are still relatively expensive, and require more precise current and heat management than compact fluorescent lamp sources of comparable output.LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology.

Parts of an LED. Although not directly labeled, the flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out from mechanical strain or vibration.

A modern retrofit LED lamp with "bulb" shape with aluminium heat sink, a light diffusing dome and E27 screw base, using a built-in power supply working on mains voltage

Technology

The inner workings of an LED, showing circuit (top) and band diagram (bottom)

I-V diagram for a diode. An LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 23 volts.PhysicsThe LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carrierselectrons and holesflow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Thus, light extraction in LEDs is an important aspect of LED production, subject to much research and development.Refractive index

Idealized example of light emission cones in a semiconductor, for a single point-source emission zone. The left illustration is for a fully translucent wafer, while the right illustration shows the half-cones formed when the bottom layer is fully opaque. The light is actually emitted equally in all directions from the point-source, so the areas between the cones shows the large amount of trapped light energy that is wasted as heat.[38]

The light emission cones of a real LED wafer are far more complex than a single point-source light emission. The light emission zone is typically a two-dimensional plane between the wafers. Every atom across this plane has an individual set of emission cones. Drawing the billions of overlapping cones is impossible, so this is a simplified diagram showing the extents of all the emission cones combined. The larger side cones are clipped to show the interior features and reduce image complexity; they would extend to the opposite edges of the two-dimensional emission plane.Bare uncoated semiconductors such as silicon exhibit a very high refractive index relative to open air, which prevents passage of photons at sharp angles relative to the air-contacting surface of the semiconductor. This property affects both the light-emission efficiency of LEDs as well as the light-absorption efficiency of photovoltaic cells. The refractive index of silicon is 3.96 (590nm),[39] while air is 1.0002926.[40]In general, a flat-surface uncoated LED semiconductor chip will emit light only perpendicular to the semiconductor's surface, and a few degrees to the side, in a cone shape referred to as the light cone, cone of light,[41] or the escape cone.[38] The maximum angle of incidence is referred to as the critical angle. When this angle is exceeded, photons no longer escape the semiconductor but are instead reflected internally inside the semiconductor crystal as if it were a mirror.[38]Internal reflections can escape through other crystalline faces, if the incidence angle is low enough and the crystal is sufficiently transparent to not re-absorb the photon emission. But for a simple square LED with 90-degree angled surfaces on all sides, the faces all act as equal angle mirrors. In this case most of the light can not escape and is lost as waste heat in the crystal.[38]A convoluted chip surface with angled facets similar to a jewel or fresnel lens can increase light output by allowing light to be emitted perpendicular to the chip surface while far to the sides of the photon emission point.[42]The ideal shape of a semiconductor with maximum light output would be a microsphere with the photon emission occurring at the exact center, with electrodes penetrating to the center to contact at the emission point. All light rays emanating from the center would be perpendicular to the entire surface of the sphere, resulting in no internal reflections. A hemispherical semiconductor would also work, with the flat back-surface serving as a mirror to back-scattered photons.[43]Transition coatingsAfter the doping of the wafer, it is cut apart into individual dies. Each die is commonly called a chip.Many LED semiconductor chips are encapsulated or potted in clear or colored molded plastic shells. The plastic shell has three purposes:1. Mounting the semiconductor chip in devices is easier to accomplish.2. The tiny fragile electrical wiring is physically supported and protected from damage.3. The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air.[44]The third feature helps to boost the light emission from the semiconductor by acting as a diffusing lens, allowing light to be emitted at a much higher angle of incidence from the light cone than the bare chip is able to emit alone.Efficiency and operational parametersTypical indicator LEDs are designed to operate with no more than 3060 milliwatts (mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die.One of the key advantages of LED-based lighting sources is high luminous efficacy. White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with a luminous efficacy of 1822 lumens per watt (lm/W). For comparison, a conventional incandescent light bulb of 60100 watts emits around 15lm/W, and standard fluorescent lights emit up to 100lm/W.As of 2012, the Lumiled catalog gives the following as the best efficacy for each color.[45] The watt-per-watt value is derived using the luminosity function.ColorWavelength range (nm)Typical efficacy (lm/W)Typical efficiency (W/W)

Red620 < < 645720.39

Red-orange610 < < 620980.29

Green520 < < 550930.15

Cyan490 < < 520750.26

Blue460 < < 490370.35

In September 2003, a new type of blue LED was demonstrated by the company Cree Inc. to provide 24mW at 20 milliamperes (mA). This produced a commercially packaged white light giving 65lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131lm/W at 20mA. Nichia Corporation has developed a white LED with luminous efficacy of 150lm/W at a forward current of 20mA.[46] Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100lm/W at their full power of 10W, and up to 160lm/W at around 2W input power. In 2012, Cree announced a white LED giving 254lm/W.[47]Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.Note that these efficiencies are for the LED chip only, held at low temperature in a lab. Lighting works at higher temperature and with drive circuit losses, so efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17lm/W to 79lm/W).[48]Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208lm/W at room temperature. The correlated color temperature was reported to be 4579K.[49] In December 2012 Cree issued another press release announcing commercial availability of 200lm/W LED at room temperature.[50]Efficiency droopThe term "efficiency droop" refers to a decrease (up to 20%[citation needed]) in luminous efficacy of LEDs as the electrical current increases above tens of milliamps(mA). Instead of increasing current levels, luminance is usually increased by combining multiple LEDs in one bulb. Solving the problem of efficiency droop would mean that household LED light bulbs would need fewer LEDs, which would significantly reduce costs.In addition to being less efficient, operating LEDs at higher electrical currents creates higher heat levels which compromise the lifetime of the LED. Because of this increased heating at higher currents, high-brightness LEDs have an industry standard of operating at only 350 mA. 350 mA is a good compromise between light output, efficiency, and longevity.[51][52][53][54]This effect, first reported in 1999,[citation needed] was initially theorized to be related to elevated temperatures. Scientists proved the opposite to be true that, although the life of an LED would be shortened, the efficiency droop is less severe at elevated temperatures.[55] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.[54] In 2013, a study conclusively identified Auger recombination as the cause of efficiency droop.[56]Possible solutionsResearchers at the U.S. Naval Research Laboratory have found a way to lessen the efficiency droop. They found that non-radiative Auger recombination of the injected carriers is where the droop arises from. They created quantum wells with a soft confinement potential to lessen the non-radiative Auger processes.[57]Researchers at Taiwan National Central University and Epistar Corp are developing a way to lessen the efficiency droop. They are doing this using ceramic aluminium nitride (AlN) substrates, which are more thermally conductive than the commercially used sapphire. The higher thermal conductivity reduces self-heating effects.[58]Lifetime and failureMain article: List of LED failure modesSolid-state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Many of the LEDs made in the 1970s and 1980s are still in service in the early 21st century. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly. [59]The most common symptom of LED (and diode laser) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can occur as well. Early red LEDs were notable for their short service life. With the development of high-power LEDs the devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material and may cause early light-output degradation. To quantitatively classify useful lifetime in a standardized manner it has been suggested to use the terms L70 and L50, which is the time it will take a given LED to reach 70% and 50% light output respectively.[60]LED performance is temperature dependent. Most manufacturers' published ratings of LEDs are for an operating temperature of 25C. LEDs used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in climates where the temperature within the light fixture gets very hot, could result in low signal intensities or even failure.[61]LED light output rises at lower temperatures, leveling off, depending on type, at around 30C.[citation needed] Thus, LED technology may be a good replacement in uses such as supermarket freezer lighting[62][63][64] and will last longer than other technologies. Because LEDs emit less heat than incandescent bulbs, they are an energy-efficient technology for uses such as in freezers and refrigerators. However, because they emit little heat, ice and snow may build up on the LED light fixture in colder climates.[61] Similarly, this lack of waste heat generation has been observed to sometimes cause significant problems with street traffic signals and airport runway lighting in snow-prone areas. In response to this problem, some LED lighting systems have been designed with an added heating circuit at the expense of reduced overall electrical efficiency of the system; additionally, research has been done to develop heat sink technologies that will transfer heat produced within the junction to appropriate areas of the light fixture.[65]Ultraviolet and blue LEDs

Blue LEDsCurrent bright blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light. Modules combining the three colors are used in big video screens and in adjustable-color fixtures.The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at RCA Laboratories.[74] These devices had too little light output to be of practical use and research into gallium nitride devices slowed. In August 1989, Cree Inc. introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide.[75] SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping[76] ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high-brightness blue LEDs were demonstrated.[77] High-brightness blue LEDs invented by Shuji Nakamura of Nichia Corporation using gallium nitride revolutionized LED lighting, making high-power light sources practical.By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber. Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case to form the active quantum well layers, the device will emit near-ultraviolet light with a peak wavelength centred around 365nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375395nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 240nm.[78] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260nm, UV LED emitting at 250270nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365nm) are already effective disinfection and sterilization devices.[79]Deep-UV wavelengths were obtained in laboratories using aluminium nitride (210nm),[70] boron nitride (215nm)[68][69] and diamond (235nm).[67]White lightThere are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that generate high-intensity white light. One is to use individual LEDs that emit three primary colors[80]red, green, and blueand then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.There are three main methods of mixing colors to produce white light from an LED: blue LED + green LED + red LED (color mixing; can be used as backlighting for displays) near-UV or UV LED + RGB phosphor (an LED producing light with a wavelength shorter than blue's is used to excite an RGB phosphor) blue LED + yellow phosphor (two complementary colors combine to form white light; more efficient than first two methods and more commonly used)[81]Because of metamerism, it is possible to have quite different spectra that appear white. However, the appearance of objects illuminated by that light may vary as the spectrum varies.RGB systems

Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is approximately 2427 nm for all three colors.

RGB LED.White light can be formed by mixing differently colored lights; the most common method is to use red, green, and blue (RGB). Hence the method is called multi-color white LEDs (sometimes referred to as RGB LEDs). Because these need electronic circuits to control the blending and diffusion of different colors, and because the individual color LEDs typically have slightly different emission patterns (leading to variation of the color depending on direction) even if they are made as a single unit, these are seldom used to produce white lighting. Nevertheless, this method is particularly interesting in many uses because of the flexibility of mixing different colors,[82] and, in principle, this mechanism also has higher quantum efficiency in producing white light.There are several types of multi-color white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods, include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency will mean lower color rendering, presenting a trade-off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs get closer to their theoretical limits.Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method that we use to produce and control light color. However, before this type of LED can play a role on the market, several technical problems must be solved. These include that this type of LED's emission power decays exponentially with rising temperature,[83] resulting in a substantial change in color stability. Such problems inhibit and may preclude industrial use. Thus, many new package designs aimed at solving this problem have been proposed and their results are now being reproduced by researchers and scientists.Correlated color temperature (CCT) dimming for LED technology is regarded as a difficult task, since binning, age and temperature drift effects of LEDs change the actual color value output. Feedback loop systems are used for example with color sensors, to actively monitor and control the color output of multiple color mixing LEDs.[84]Phosphor-based LEDs

Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband Stokes-shifted light emitted by the Ce3+:YAG phosphor, which emits at roughly 500700 nmThis method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).[85] A fraction of the blue light undergoes the Stokes shift being transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising the color rendering index (CRI) value of a given LED.[86]Phosphor-based LED efficiency losses are due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function). Due to the simplicity of manufacturing the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stoke shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce3+:YAG).White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.Other white LEDsAnother method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.[87]A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly sapphire substrates in relatively small 100- or 150-mm wafer sizes.[88] It is predicted that by 2020, 40% of all GaN LEDs will be made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.[89]Organic light-emitting diodes (OLEDs)Main article: Organic light-emitting diode

Demonstration of a flexible OLED device

Orange light-emitting diodeIn an organic light-emitting diode (OLED), the electroluminescent material comprising the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor.[90] The organic materials can be small organic molecules in a crystalline phase, or polymers.The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut.[91] Polymer LEDs have the added benefit of printable[92][93] and flexible[94] displays. OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players while possible future uses include lighting and televisions.[91]Quantum dot LEDs (experimental)Quantum dots (QD) are semiconductor nanocrystals that possess unique optical properties.[95] Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the CIE diagram. This provides more color options and better color rendering than white LEDs since the emission spectra is much more narrow, characteristic of quantum confined states. Quantum-dot LEDs are available in the same package types as traditional phosphor-based LEDs.[citation needed]There are two types of schemes for QD excitation. One uses photo excitation with a primary light source LED (typically blue or UV LEDs are used). The other is direct electrical excitation first demonstrated by Alivisatos et al.[96]One example of the photo-excitation scheme is a method developed by Michael Bowers, at Vanderbilt University in Nashville, involving coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This method emits a warm, yellowish-white light similar to that made by incandescent bulbs.[97] Quantum dots are also being considered for use in white light-emitting diodes in liquid crystal display (LCD) televisions.[98]The major difficulty in using quantum dots-based LEDs is the insufficient stability of QDs under prolonged irradiation.[citation needed] In February 2011 scientists at PlasmaChem GmbH could synthesize quantum dots for LED applications and build a light converter on their basis, which could efficiently convert light from blue to any other color for many hundred hours.[99] Such QDs can be used to emit visible or near infrared light of any wavelength being excited by light with a shorter wavelength.The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of OLED. A layer of quantum dots is sandwiched between layers of electron-transporting and hole-transporting materials. An applied electric field causes electrons and holes to move into the quantum dot layer and recombine forming an exciton that excites a QD. This scheme is commonly studied for quantum dot display. The tunability of emission wavelengths and narrow bandwidth is also beneficial as excitation sources for fluorescence imaging. Fluorescence near-field scanning optical microscopy (NSOM) utilizing an integrated QD-LED has been demonstrated.[100]MiniatureThese are mostly single-die LEDs used as indicators, and they come in various sizes from 2mm to 8mm, through-hole and surface mount packages. They usually do not use a separate heat sink.[103] Typical current ratings ranges from around 1 mA to above 20 mA. The small size sets a natural upper boundary on power consumption due to heat caused by the high current density and need for a heat sink.Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle.Researchers at the University of Washington have invented the thinnest LED. It is made of two-dimensional (2-D) flexible materials. It is 3 atoms thick, which is 10 to 20 times thinner than three-dimensional (3-D) LEDs and is also 10,000 times smaller than the thickness of a human hair. These 2-D LEDs are going to make it possible to create smaller, more energy-efficient lighting, optical communication and nano lasers.[104]There are three main categories of miniature single die LEDs: Low-current: typically rated for 2 mA at around 2 V (approximately 4mW consumption). Standard: 20 mA LEDs (ranging from approximately 40mW to 90mW) at around:1.9 to 2.1 V for red, orange and yellow,3.0 to 3.4 V for green and blue,2.9 to 4.2 V for violet, pink, purple and white. Ultra-high-output: 20 mA at approximately 2 V or 45 V, designed for viewing in direct sunlight.5 V and 12 V LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V supply.Mid-rangeMedium-power LEDs are often through-hole-mounted and mostly utilized when an output of just tens of lumens are needed. They sometimes have the diode mounted to four leads (two cathode leads, two anode leads) for better heat conduction and carry an integrated lens. An example of this is the Superflux package, from Philips Lumileds. These LEDs are most commonly used in light panels, emergency lighting, and automotive tail-lights. Due to the larger amount of metal in the LED, they are able to handle higher currents (around 100 mA). The higher current allows for the higher light output required for tail-lights and emergency lighting.High-power

High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds)See also: Solid-state lighting, LED lamp and Thermal management of high-power LEDsHigh-power LEDs (HPLEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.[105][106] LED power densities up to 300 W/cm2 have been achieved.[107] Since overheating is destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will fail in seconds. One HPLED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.Some well-known HPLEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HPLEDs manufactured by Cree Inc. now exceed 105 lm/W [108] (e.g. the XLamp XP-G LED chip emitting Cool White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent lights, as LEDs grow more cost competitive.The impact of Haitz's law which describes the exponential rise in light output of LEDs over time can be readily seen in year over year increases in lumen output and efficiency. For example, the CREE XP-G series LED achieved 105 lm/W in 2009,[108] while Nichia released the 19 series with a typical efficacy of 140 lm/W in 2010.[109]LEDs have been developed by Seoul Semiconductor that can operate on AC power without the need for a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HPLED is typically 40 lm/W.[110] A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.[111]Application-specific variationsFlashingUsed as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.Bi-color LEDTwo different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads antiparallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode, so that they can be controlled independently.

A decorative garden light that changes colorTri-colorThree different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color.RGBTri-color LEDs with red, green, and blue emitters, in general using a four-wire connection with one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Others however, have only two leads (positive and negative) and have a built in tiny electronic control unit.Decorative multicolorincorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally simply by varying the supply voltage. (In a cheap 'Melinera' garden lamp supplied by OWIM GmbH & Co KG in 2013 the LEDs are within a clear casting of 5mm diameter, 10mm long which encapsulates 3 LEDs which change between red, green and blue as the DC supply varies between about 2 volts and 3 volts).Alphanumericavailable in seven-segment, starburst and dot-matrix format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5x7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.Digital RGBThese are RGB LEDs that contain their own "smart" control electronics. In addition to power and ground, these provide connections for data in, data out, and sometimes a clock or strobe signal. These are connected in a daisy chain, with the data in of the first LED sourced by a microprocessor, which can control the brightness and color of each LED independently of the others. They are used in strings for Christmas and similar decorations. Two types available as of February 2014 use chips designated WS2811 and WS2812.[112][113]Considerations for usePower sourcesMain article: LED power sourcesThe currentvoltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can cause a large change in current. If the applied voltage exceeds the LED's forward voltage drop by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use constant-current power supplies to keep the current below the LED's maximum current rating. Since most common power sources (batteries, mains) are constant-voltage sources, most LED fixtures must include a power converter, at least a current-limiting resistor. However, the high resistance of 3 V coin cells combined with the high differential resistance of nitride-based LEDs makes it possible to power such an LED from such a coin cell without an external resistor.[114]Electrical polarityMain article: Electrical polarity of LEDsAs with all diodes, current flows easily from p-type to n-type material.[115] However, no current flows and no light is emitted if a small voltage is applied in the reverse direction. If the reverse voltage grows large enough to exceed the breakdown voltage, a large current flows and the LED may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.Safety and healthThe vast majority of devices containing LEDs are "safe under all conditions of normal use", and so are classified as "Class 1 LED product"/"LED Klasse 1". At present, only a few LEDsextremely bright LEDs that also have a tightly focused viewing angle of 8 or lesscould, in theory, cause temporary blindness, and so are classified as "Class 2".[116] The Opinion of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) of 2010, on the health issues concerning LEDs, suggested banning public use of lamps which were in the moderate Risk Group 2, especially those with a high blue component in places frequented by children.[117] In general, laser safety regulationsand the "Class 1", "Class 2", etc. systemalso apply to LEDs.[118]While LEDs have the advantage over fluorescent lamps that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. A study published in 2011 states (concerning toxicity of LEDs when treated as waste): "According to federal standards, LEDs are not hazardous except for low-intensity red LEDs, which leached Pb [lead] at levels exceeding regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103 mg/kg; limit: 1000), nickel (up to 4797 mg/kg; limit: 2000), or silver (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous."[119]Advantages Carbon emissions: LEDs deliver significant reductions in carbon emissions.[120] Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.[121] The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes. Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs. Size: LEDs can be very small (smaller than 2mm2[122]) and are easily attached to printed circuit boards. On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond.[123] LEDs used in communications devices can have even faster response times. Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or HID lamps that require a long time before restarting. Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.[124] This pulse-width modulation is why LED lights viewed on camera, particularly headlights on cars, appear to be flashing or flickering. This is a type of stroboscopic effect. Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED. Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.[59] Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.[125] Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.[126] Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile. Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. However, when large quantities of light are needed many light sources are usually deployed, which are difficult to focus or collimate towards the same target.Disadvantages High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. As of 2012, the cost per thousand lumens (kilolumen) was about $6. The price is expected to reach $2/kilolumen by next year.[127][needs update] The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment or "thermal management" properties. Over-driving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, which require low failure rates. Toshiba has produced LEDs with an operating temperature range of -40 to 100C, which suits the LEDs for both indoor and outdoor use in applications such as lamps, ceiling lighting, street lights, and floodlights.[88] Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.[128] Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460nm and dip at 500nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,[129] red surfaces being rendered particularly badly by typical phosphor-based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs. Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So LEDs are difficult to apply to uses needing a spherical light field, however different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees. In contrast, lasers can emit beams with divergences of 0.2 degrees or less.[130] Electrical polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. To automatically match source polarity to LED devices, rectifiers can be used. Electric shock hazard: There have been LED recalls because of faulty wiring that can cause electric shock, fire or burns.[131] Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.105: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.[132][133] Blue pollution: Because cool-white LEDs with high color temperature emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages using white light sources with correlated color temperature above 3,000 K.[111] Efficiency droop: The luminous efficacy of LEDs decreases as the electrical current increases above tens of milliamps. Heating also increases with higher currents which compromises the lifetime of the LED. Because of this increased heating, the lighting industry does not exceed 350 mA to power a given LED.[51][53][54][134]ApplicationsLED uses fall into four major categories: Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning. Illumination where light is reflected from objects to give visual response of these objects. Measuring and interacting with processes involving no human vision.[135] Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light.[136][137][138][139] See LEDs as light sensors.Data communication and other signalingSee also: Li-FiLight can be used to transmit data and analog signals.Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK cables to the very high bandwidth fiber links that form the internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved.[146]Sustainable lightingEfficient lighting is needed for sustainable architecture. In 2009, a typical 13-watt LED lamp emitted 450 to 650 lumens,[147] which is equivalent to a standard 40-watt incandescent bulb. In 2011, LEDs have become more efficient, so that a 6-watt LED can easily achieve the same results. A standard 40-watt incandescent bulb has an expected lifespan of 1,000 hours, whereas an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.Energy consumptionIn the US, one kilowatt-hour (3.6 MJ) of electricity currently causes an average 1.34 pounds (610g) of CO2 emission.[148] Assuming the average light bulb is on for 10 hours a day, a 40-watt bulb will cause 196 pounds (89kg) of CO2 emission per year. The 6-watt LED equivalent will only cause 30 pounds (14kg) of CO2 over the same time span. A buildings carbon footprint from lighting can therefore be reduced by 85% by exchanging all incandescent bulbs for new LEDs if a building uses only incandescent bulbs.In practice, most buildings that use a lot of lighting use fluorescent lighting, which has 22% luminous efficiency compared with 5% for filaments, so changing to LED lighting would give only 34% reduction in electrical power and carbon emissions.The reduction in carbon emissions depend on the source of electricity. Nuclear power in the United States produced 19.2% of electricity in 2011, so reducing electricity consumption there reduces more carbon emissions than in France (75% nuclear electricity) or Norway (almost entirely hydroelectric).Replacing lights that spend the most time lit make the most savings, so LED lights in infrequently used rooms bring a smaller return on investment.Economically sustainableThis section is outdated. Please update this article to reflect recent events or newly available information. (January 2014)

LED light bulbs could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system is currently hampered by the high cost and low efficiency of available products. 2009 DOE testing results showed an average efficacy of 35 lm/W, below that of typical CFLs, and as low as 9 lm/W, worse than standard incandescents.[147] However, as of 2011, there are LED bulbs available as efficient as 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W. The high initial cost of commercial LED bulbs is due to the expensive sapphire substrate, which is key to the production process. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted.Light sources for machine vision systemsMachine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used for this purpose, and this is likely to remain one of their major uses until price drops low enough to make signaling and illumination uses more widespread. Barcode scanners are the most common example of machine vision, and many low cost ones use red LEDs instead of lasers. Optical computer mice are also another example of LEDs in machine vision, as it is used to provide an even light source on the surface for the miniature camera within the mouse. LEDs constitute a nearly ideal light source for machine vision systems for several reasons:The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually a minor concern. However, it might not be easy to replace a broken light source placed within complex machinery, and here the long service life of LEDs is a benefit.LED elements tend to be small and can be placed with high density over flat or even-shaped substrates (PCBs etc.) so that bright and homogeneous sources that direct light from tightly controlled directions on inspected parts can be designed. This can often be obtained with small, low-cost lenses and diffusers, helping to achieve high light densities with control over lighting levels and homogeneity. LED sources can be shaped in several configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).LEDs can be easily strobed (in the microsecond range and below) and synchronized with imaging. High-power LEDs are available allowing well-lit images even with very short light pulses. This is often used to obtain crisp and sharp still images of quickly moving parts.LEDs come in several different colors and wavelengths, allowing easy use of the best color for each need, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effects of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation. This allows using plastic lenses, filters, and diffusers. Waterproof units can also easily be designed, allowing use in harsh or wet environments (food, beverage, oil industries).

III. CONSTRUCTION AND WORKING OF LI-FI TECHNOLOGY

WORKING TECHNOLOGYThis brilliant idea was first showcased by Harald Haas fromUniversity of Edinburgh, UK, in his TED Global talk on VLC.He explained, Very simple, if the LED is on, you transmit adigital 1, if its off you transmit a 0. The LEDs can beswitched on and off very quickly, which gives niceopportunities for transmitting data. So what you require at allare some LEDs and a controller that code data into thoseLEDs. We have to just vary the rate at which the LEDsflicker depending upon the data we want to encode. Furtherenhancements can be made in this method, like using an arrayof LEDs for parallel data transmission, or using mixtures ofred, green and blue LEDs to alter the lights frequency witheach frequency encoding a different data channel. Suchadvancements promise a theoretical speed of 10 Gbps meaning you can download a full high-definition film in just30 seconds. Simply awesome! But blazingly fast data ratesand depleting bandwidths worldwide are not the only reasonsthat give this technology an upper hand. Since Li-Fi uses justthe light, it can be used safely in aircrafts and hospitals that areprone to interference from radio waves. This can even workunderwater where Wi-Fi fails completely, thereby throwingopen endless opportunities for military operations.Imagine only needing to hover under a street lamp to getpublic internet access, or downloading a movie from the lampon your desk. There's a new technology on the block whichcould, quite literally as well as metaphorically, 'throw light on'how to meet the ever-increasing demand for high-speedwireless connectivity. Radio waves are replaced by lightwaves in a new method of data transmission which is beingcalled Li-Fi.Light-emitting diodes can be switched on and offfaster than the human eye can detect, causing the light sourceto appear to be on continuously. A flickering light can beincredibly annoying, but has turned out to have its upside,being precisely what makes it possible to use light for wirelessdata transmission. Light-emitting diodes (commonly referredto as LEDs and found in traffic and street lights, car brakelights, remote control units and countless other applications)can be switched on and off faster than the human eye candetect, causing the light source to appear to be oncontinuously, even though it is in fact 'flickering'. Thisinvisible on-off activity enables a kind of data transmissionusing binary codes: switching on an LED is a logical '1',switching it off is a logical '0'. Information can therefore beencoded in the light by varying the rate at which the LEDsflicker on and off to give different strings of 1s and 0s. Thismethod of using rapid pulses of light to transmit informationwirelessly is technically referred to as Visible LightCommunication (VLC), though its potential to compete withconventional Wi-Fi has inspired the popular characterisationLi-Fi.2.1 Visible light communication (VLC)-A potential solutionto the global wireless spectrum shortageLiFi (Light Fidelity) is a fast and cheap optical version of Wi-Fi, the technology of which is based on Visible LightCommunication (VLC).VLC is a data communicationmedium, which uses visible light between 400 THz (780 nm)and 800 THz (375 nm) as optical carrier for data transmissionand illumination. It uses fast pulses of light to transmitinformation wirelessly. The main components of thiscommunication system are 1) a high brightness white LED,Which acts as a communication source and 2) a siliconphotodiode which shows good response to visible wavelengthregion serving as the receiving element? LED can be switchedon and off to generate digital strings of 1s and 0s. Data can beencoded in the light to generate a new data stream by varyingthe flickering rate of the LED. To be clearer, by modulatingthe LED light with the data signal, the LED illumination canbe used as a communication source. As the flickering rate is sofast, the LED output appears constant to the human eye. Adata rate of greater than 100 Mbps is possible by using highspeed LEDs with appropriate multiplexing techniques. VLCdata rate can be increased by parallel data transmission usingLED arrays where each LED transmits a different data stream.There are reasons to prefer LED as the light source in VLCwhile a lot of other illumination devices like fluorescent lamp,incandescent bulb etc. are available.

Fig 5.Data transmission using LED

4 Imagine ourselves walking into a complex where GPS signals are unavailable but the complex is equipped with ceiling bulbs that create their own constellation of navigation beacons. As the camera of our cell phone automatically receives these signals, it switches our navigation software to use this information to guide us to the ATM machine were looking for. We conclude our ATM transaction and notice the Giga Spot sign for instant digital movie downloads. We pick out that new data using our phones payment facility and then download within a few seconds the high-definition movie into 2 the Giga Link flash drive plugged into the USB port of our Smartphone. As we walk away, our phone notifies us that the leather jacket featured in the character of movie is on sale nearby. We walk over towards the show window and our image comes up on the screen, wearing that coveted jacket. You turn and pose while the image matches our orientation and body gestures for a digital fitting. When we walk into the store, the clerk handover us the actual jacket in exactly size fitting. Li-Fi is implemented using white LED light bulbs at downlink transmitter. These devices are used for illuminationonly by applying a constant current. By fast and subtle variations of the current, optical output can be made to vary atextremely high speeds. This variation is used to carry high speed data. Working of Li-Fi is shown in fig. 5 below. Anoverhead lamp fitted with an LED with signal processing technology streams data embedded in its beam at ultra highspeeds to the photodiodes. A receiver dongle than converts the tiny changes in amplitude into an electrical signal,which is then converted back into a data stream & transmitted to a computer or mobile device.

Fig. 5 Working of Li-FILIFI CONSTRUCTIONThe LIFI product consists of 4 primary sub-assemblies: Bulb RF power amplifier circuit (PA) Printed circuit board (PCB) EnclosureThe PCB controls the electrical inputs and outputs of the lamp andhouses the microcontroller used to manage different lamp functions.An RF (radio-frequency) signal is generated by the solid-state PAand is guided into an electric field about the bulb. The highconcentration of energy in the electric field vaporizes the contentsof the bulb to a plasma state at the bulbs center; this controlledplasma generates an intense source of light. All of thesesubassemblies are contained in an aluminum enclosure.FUNCTION OF THE BULB SUB-ASSEMBLY At the heart of LIFI is the bulb sub-assembly where a sealed bulb isembedded in a dielectric material. This design is more reliable thanconventional light sources that insert degradable electrodes into thebulb. The dielectric material serves two purposes; first as awaveguide for the RF energy transmitted by the PA and second as anelectric field concentrator that focuses energy in the bulb. Theenergy from the electric field rapidly heats the material in the bulb toa plasma state that emits light of high intensity and full spectrum.

USAGE MODELS 5 Within a local Li-Fi cloud several data based services are supported through a heterogeneous communication system. In an initial approach, the Li-Fi Consortium defined different types of technologies to provide secure, reliable and ultra-high-speed wireless communication interfaces. These technologies included giga-speed technologies, optical mobility technologies and navigation, precision location and gesture recognition technologies. For giga-speed technologies, the Li-Fi Consortium defined Giga Dock, Giga Beam, Giga Shower, Giga Spot and Giga MIMO models to address different user scenarios for wireless indoor and indoor-like data transfers. While Giga Dock is a wireless docking solution including wireless charging for smart phones tablets or notebooks, with speeds up to 10 Gbps, the Giga Beam model is a point-to-point data link for kiosk applications or portable-to-portable data exchanges. Thus a two-hour full HDTV movie (5 GB) can be transferred from one device to another within four seconds. 6 Giga Shower, Giga Spot and Giga-MIMO are the other models for in-house communication. There a transmitter or receiver is mounted into the ceiling connected to, for example, a media server. On the other side are portable or fixed devices on a desk in an office, in an operating room, in a production hall or at an airport. Giga Shower provides unidirectional data services via several channels to multiple users with gigabit-class communication speed over several meters. This is like watching TV channels or listening to different radio stations where no uplink channel is needed. In case Giga Shower is used to sell books, music or movies, the connected media server can be accessed via Wi-Fi to process payment via a mobile device. Giga Spot and Giga MIMO are optical wireless single- and multi-channel Hot Spot solutions offering bidirectional gigabit-class communication in a room, hall or shopping mall for example. COMPARISION BETWEEN Li-Fi & Wi-Fi 7 LI-FI is a term, one used to describe visible light communication technology applied to high speed wireless communication. It acquired this name due to the similarity to WI-FI, only using light instead of radio. WI-FI is great for general wireless coverage within buildings and li-fi is ideal for high density wireless data coverage in confined area and for relieving radio interference issues, so the two technologies can be considered complimentary.

LI-FI is a term of one used to describe visible lightcommunication technology applied to high speed wirelesscommunication. It acquired this name due to the similarity toWI-FI, only using light instead of radio.WI-FI is great forgeneral wireless coverage within buildings, and li-fi is idealfor high density wireless data coverage in confined area andfor relieving radio interference issues, so the two technologiescan be considered complimentary.

Table 1.Comparison between current and future wirelessTechnology

The table also contains the current wireless technologies thatcan be used for transferring data between devices today, i.e.Wi-Fi, Bluetooth and IrDA. Only Wi-Fi currently offers veryhigh data rates. The IEEE 802.11.n in most implementationsprovides up to 150Mbit/s (in theory the standard can go to600Mbit/s) although in practice you receive considerably lessthan this. Note that one out of three of these is an optical technology.

3.1 How it is different?Li-Fi technology is based on LEDs for the transfer of data.The transfer of the data can be with the help of all kinds oflight, no matter the part of the spectrum that they belong. Thatis, the light can belong to the invisible, ultraviolet or thevisible part of the spectrum. Also, the speed of the internet isincredibly high and you can download movies, games, musicetc in just a few minutes with the help of this technology.Also, the technology removes limitations that have been puton the user by the Wi-Fi. You no more need to be in a regionthat is Wi-Fi enabled to have access to the internet. You cansimply stand under any form of light and surf the internet asthe connection is made in case of any light presence. Therecannot be anything better than this technology.Fig 3.Working and advantagesAPPLICATION OF LI-FI4.1 You Might Just Live LongerFor a long time, medical technology has lagged behind the restof the wireless world. Operating rooms do not allow Wi-Fiover radiation concerns, and there is also that whole lack ofdedicated spectrum. While Wi-Fi is in place in many hospitals,interference from cell phones and computers can block signalsfrom monitoring equipment. Li-Fi solves both problems: lightsare not only allowed in operating rooms, but tend to be themost glaring (pun intended) fixtures in the room. And, as Haasmentions in his TED Talk, Li-Fi has 10,000 times thespectrum of Wi-Fi, so maybe we can, I dunno, delegate redlight to priority medical data. Code Red!4.2 AirlinesAirline Wi-Fi. Ugh. Nothing says captive audience like havingto pay for the "service" of dial-up speed Wi-Fi on the plane.And dont get me started on the pricing. The best Ive heard sofar is that passengers will "soon" be offered a "high-speedlike" connection on some airlines. United is planning onspeeds as high as 9.8 Mbps per plane. Uh, I have twice thatcapacity in my living room. And at the same price as checkinga bag, I expect it. Li-Fi could easily introduce that sort ofspeed to each seat's reading light. Ill be the guy WoWing nextto you. Its better than listening to you tell me about yourwildly successful son, maam.4.3 Smarter Power PlantsWi-Fi and many other radiation types are bad for sensitiveareas. Like those surrounding power plants. But power plantsneed fast, inter-connected data systems to monitor things likedemand, grid integrity and (in nuclear plants) coretemperature. The savings from proper monitoring at a singlepower plant can add up to hundreds of thousands of dollars.Li-Fi could offer safe, abundant connectivity for all areas ofthese sensitive locations. Not only would this save moneyrelated to currently implemented solutions, but the draw on apower plants own reserves could be lessened if they haventyet converted to LED lighting4.4 Undersea AwesomenessUnderwater ROVs, those favourite toys of treasure seekersand James Cameron, operate from large cables that supplytheir power and allow them to receive signals from their pilotsabove. ROVs work great, except when the tether isnt longenough to explore an area, or when it gets stuck on something.If their wires were cut and replaced with light say from asubmerged, high-powered lamp then they would be muchfreer to explore. They could also use their headlamps tocommunicate with each other, processing data autonomouslyand referring findings periodically back to the surface, all thewhile obtaining their next batch of orders.4.5. It Could Keep You Informed and Save LivesSay theres an earthquake in New York. Or a hurricane. Takeyour pick its a wacky city. The average New Yorker maynot know what the protocols are for those kinds of disasters.Until they pass under a street light, that is. Remember, withLi-Fi, if theres light, youre online. Subway stations andtunnels, common dead zones for most emergencycommunications, pose no obstruction. Plus, in times lessstressing cities could opt to provide cheap high-speed Webaccess to every street corner.

Photo DetectorsIntroductionPhoto detectors are used primarily as an optical receiver to convert light intoelectricity. The principle that applies to photo detectors is the photoelectric effect, whichis the effect on a circuit due to light. Max Planck In 1900 discovered that energy isradiated in small discrete units called quanta; he also discovered a universal constant ofnature which is known as the Plancks constant. Plancks discoveries lead to a new formof physics known as quantum mechanics and the photoelectric effect E hv which isPlanck constant multiplied by the frequency of radiation. The photo electric effect is theeffect of light on a surface of metal in a vacuum, the result is electrons being ejected fromthe surface this explains the principle theory of light energy that allows photo detectors tooperate. Photo detectors are commonly used as safety devices in homes in the form of asmoke detector, also in conjunction with other optical devices to form security systems.Photo detector:A photo detector operates by converting light signals that hit the junction to avoltage or current. The junction uses an illumination window with an anti-reflect coatingto absorb the light photons. The result of the absorption of photons is the creation ofelectron-hole pairs in the depletion region. Examples of photo detectors are photodiodesand phototransistors. Other optical devices similar to photo detectors are solar cells whichalso absorb light and turn it into energy. A similar but different optical device is the LEDwhich is basically the inverse of a photodiode, instead of converting light to a voltage orcurrent, it converts a voltage or current to light.Photodiodes:A commonly used photo detector is the photodiode. A photodiode is based on ajunction of oppositely doped regions (pn junction) in a sample of semiconductor. Thiscreates a region depleted of charge carriers that results in high impedance. The highimpedance allows the construction of detectors using silicon and germanium to operatewith high sensitivity at low temperatures. The photodiode functions using an illuminationwindow (Figure 1), which allows the use of light as an external input. Since light is usedas an input, the diode is operated under reverse bias conditions. Under the reverse biascondition the current through the junction is zero when no light is present, this allows thediode to be used as a switch or relay when sufficient light is present.

Figure 1 photo diode with illumination window as shown.Photodiodes are mainly made from gallium arsenide instead of silicon because siliconcreates crystal lattice vibrations called phonons when photons are absorbed in order tocreate electron-hole pairs. Gallium arsenide can produce electron-hole pairs without theslowly moving phonons; this allows faster switching between on and off states and GaAsalso is more sensitive to the light intensity. Once charge carriers are produced in thediode material, the carriers reach the junction by diffusion. Important parameters for thephotodiode include quantum efficiency, current and capacitance which will be covered inthe equations section.PIN PhotodiodeAnother type of photodiode is the PIN photodiode; this photodiode includes anintrinsic layer in between the P and N type materials. The PIN must be reverse bias due tothe high resistivity of the intrinsic layer; the PIN has a larger depletion region whichallows more electron-hole pairs to develop at a lower capacitance. The illuminationwindow for a PIN is on the P-side of the diode because the mobility of electrons isgreater than holes which results in better frequency response. The larger breakdownvoltage in comparison to the PN photodiode allows it to be used with a biased voltage ofapproximately 100 which results in a fast response time by the equation below.

Avalanche PhotodiodeAn Avalanche photodiode is operated at reverse bias close to the breakdown,which causes photo excited charge carriers to accelerate in the depletion region andproduce additional carriers by avalanching. The avalanche photodiodes are good for fiberoptic systems that require low light levels with quantum efficiency larger than 100%.PhototransistorPhototransistor is similar to the photodiode except an additional n-type region isadded to the photodiode configuration. The phototransistor includes a photodiode with aninternal gain. A phototransistor can be represented as a bipolar transistor that is enclosedin a transparent case so that photons can reach the base-collector junction. The electronsthat are generated by photons in the base-collector junction are injected into the base, andthe current is then amplified. Since phototransistor detection is on the order of thephotodiode they can not detect light any better than a photodiode. The draw back of ap