LEDs (light-emitting diodes) are a widely recognized and important electronic device in this high technology era, and are used in many applications nowadays. Traffic lights, cars, screens, projectors, smart phones and many more devices use LEDs to emit light.
This report looks briefly at some applications of LEDs, and how it has developed so far in the 21st century to become the most significant light emitting device. After giving a good basis and understanding of LEDs, this report will then look specifically at the use of LEDs in the Light-Fidelity (Li-Fi) technology, and how its use could be developed over the coming years.
LEDs have become the most favoured light source for many situations, due to their high efficiency and small size. Although the technology of Li-Fi is still new and not widely recognized yet, a lot of research is being carried out to develop it. With the increase in handheld technology, wireless connectivity has formed a very important part of our lives, and could even be considered a commodity just like electricity, gas and water. Li-Fi has the potential to become the leading source for data connections in the world, and it is expected that data transfer through wireless networks will rise by a factor of 10 in next five years, with the US Federal Communication Commission warning of a “spectrum crisis” due to spectrum saturation.  There is therefore a huge market waiting to be exploited.
Introduction to LEDs
LED stands for “light-emitting diode”. The feature that differs LEDs from any other light bulbs is that light bulbs typically emit light by heat generation, but LEDs emit light by electronic excitation. The working principle of LEDs is almost the same as any other p-n junction diode. One of the principal uses in p-n junction diode is to control the direction of the flow of electricity.  LEDs consist of a chip of semiconducting material. The p-n junction diode is created by doping impurities in the semiconducting material. Impurities within the semiconductor are introduced to make the LED function correctly and are used to create the required electron density.
Diodes, in general, are made of very thin layers of semiconductor materials: one layer will have an excess of electrons, while the next will have a deficit in electrons.  Electrons move from a region of high electronic density to a region of low electronic density. When an electron drops from the higher energy level to lower energy level, energy is released in the form of light (a photon). The frequency of the light (photon) produced depends on the amount of energy released. In other words, the more electrons that get passed across the boundary between layers, known as junction, the brighter the light. This phenomenon is known as electroluminescence.  The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light. The colour of the light is determined by the energy band gap of the semiconductor. 
One advantage of LEDs is that they can emit coloured light. Different coloured light or even multi-colour light can be produced by controlling the amount of electricity flowing through the diode and thus controlling the drop of energy in the electron. Other advantages of LED compared to other light-emitting devices are that LEDs use less power, have longer lifetimes, produce a lot less heat, are smaller in size, and they are shock resistant.
On the other hand, LED also have some disadvantages and one is that LEDs performance is temperature dependent. Operating in an extreme temperature will affect the LED performance and can even cause failure.  Other disadvantages are that LEDs have higher initial prices.
Oleg Losev reportedly made the first LED in 1927. He shared his research with the world which allowed further development.  In 1962 there were major stepping stones for LEDs. The first patent for LEDs emitting infrared light diode was placed. The same year the first LEDs producing visible light were created.
When the first LED was created; diodes with the semiconductor GaAs were being used. Through the late 60’s and early 70’s, developments in different colors of LED’s was made. In 1976 the first high brightness LEDs were made. This is also the year where high-efficiency LEDs were being tested for optical fiber communication. Multi-colour LEDs have also developed over time with systems such as RGB, di-colour and tri-colour lighting. 
There has also been a lot of commercial development observed. In the early stages it was not really a competitor for fluorescent lighting as its cost around US$200 per unit. However they quickly began to replace fluorescent lighting as the prices dropped, because they were more efficient and reliable. Since the 1960s LED technology has doubled its efficiency and light output every 36 months.
LED uses fall into five 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 , Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light .
Firstly, the LEDs are most commonly used as indicators, signs and lightings. The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays and as dynamic decorative displays. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.
Weather and all-hazards radio receivers also use LEDs. Because of the LEDs long life, fast on-off switching times, and their ability to be seen in broad daylight due to their high output and focus, LEDs have been used for car’s high-mounted brake lights, trucks, and buses, and in turn signals for some time. However, many vehicles now use LEDs for their rear light clusters. The advantages of using LEDs for this application include it improves safety, it forms thinner lights than incandescent lamps, it has high-efficiency and high-power LEDs. However, LEDs are more expensive compared to the most conventional lighting technologies, LEDs are also sensitive to voltages; voltage supplied must be above the threshold while the current below the rating and LEDs depend heavily on the ambient temperature of the operating environment.
It has also become possible to use LEDs in lighting and illumination. For examples, LED street lights are also employed on poles and in parking garages. In 2007, the Italian village Torraca was the first place to convert its entire illumination system to LED .
LEDs are also used in aviation lighting. Airbus has used LED lighting in their Airbus A320 Enhanced since 2007, and Boeing plans its use in the 787. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.
LEDs are also suitable for backlighting for LCD televisions and lightweight laptop displays and light source for DLP projectors. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting .
LEDs are increasingly finding uses in medical and educational applications. The main objective of using LEDs in medical application is growing plants in space. NASA has even sponsored research for the use of LEDs to promote health of the astronauts. Moreover, Ronald W. Ignatius, founder and the chairman of the board at the Quantum Devices Inc. (QDI), of Barneveld, Wisconsin proposed the use of LEDs as the photon source in growing plants in space.
Other examples are LEDs as mood enhancement, and new technologies such as AmBX, exploiting LEDs versatility. The advantages of LEDs in medical and educational applications include it has less IR or heat radiation, less heating effect and air conditioning (cooling) systems have less heat to dispose of, reducing carbon dioxide emissions .
LEDs can also be used as Illumination such as digital microscopes, and those which can be directly attached to the USB port of a computer. An example of this is USB microscopes and endoscopes which enable images to be recorded and stored directly on the computer without the use of a separate camera.
Light can be used to transmit data and analog signals due to its high data bandwidth. LEDs use various types of optic fibre cables to send data from digital audios to other places. Some computers are equipped with IrDA interferences which allows them to send and receive data to nearby machines via infrared. Examples of applications for this means of process are optical fiber and free space optics communications.
Lastly, LED as light sensors. In addition to emission, an LED can be used as a photodiode in light detection. This capability may be used in a variety of applications including ambient light detection and bidirectional communications. As a photodiode, an LED is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. This implementation of LEDs may be added to designs with only minor modifications in circuitry. An LED can be multiplexed in such a circuit, such that it can be used for both light emission and sensing at different times. For examples, remote controls, movement sensors in optical computer mice and touch sensing; touch-sensing screen that registers reflected light from a finger or stylus .
Future applications : Researchers have come up with a way to place a light-emitting diode on a contact lens. Recently, researchers have invented nanomaterials which can be used to equip soft contact lenses with display capabilities, thereby flashing information to the user in real-time and also help in continuous monitoring . The research team tested these high tech lenses on rabbits with positive results. In the future, computer embedded contact lenses could be developed to work in a similar manner as Google Glass.
Introduction to Li-Fi
Li-Fi is a wireless data communication technology, so can be directly compared to other wireless technologies, the most prominent of which are Wi-Fi, Bluetooth and 3rd and 4th generation mobile networks such as HSPA and HSPA+. Some of the features for which Li-Fi will be judged on include download/upload speed, capacity, availability, security and efficiency.
Li-Fi works on the same principle as Wi-Fi, however it utilises another range of the electromagnetic spectrum, using the visible light spectrum rather than radio waves to send data.
It is therefore a form of Visible Light Communication, however offers far better speeds than fluorescent lamps. Visible light Communication can be used for ubiquitous computing, whereby computing is made to appear everywhere and anywhere, as lighting is used everywhere in our daily lives. 
LEDs can be used to send signals due to their ability to be switched on and off so rapidly. A receiver can then pick up the light, with the light being ‘on’ representing a digital ‘1’, and the light being ‘off’ representing a digital ‘0’. This to upload and download information if you have a pair of LEDs working together, thereby allowing access to the internet.
In order to encode the data desired all that needs to be done is to vary the rate at which the LEDs’ flicker, which simply requires a small modulator within it, that controls when the light is on and off, and a receiver to sense the signals.  This is not only cheap but also very simple. Furthermore, as humans it is impossible for us to notice these changes in signal as they occur so rapidly, so we would experience no difference except the ability to access the internet through a simple light!
For further improvements, arrays of LEDs for parallel data transfer can be used, or mixture of blue, green and red LEDs may be used to overcome the light’s frequency with each frequency encoding a different data channel. With these developments a theoretical speed of 10Gbps can be achieved which means a full high-definition film can be downloaded in only 30 seconds.
The first functioning Li-Fi transceiver has recently been shown at the 2014 Mobile World Congress, named Li-1st and produced by ‘pureLiFi’, a company set up by Harald Haas, the pioneer of this technology. This product has a downlink and uplink speed of 5Mbps, which is relatively slow in comparison to current technologies. However, this technology is designed to “provide a platform for pilot projects with pureLiFi partners, and to establish engagement on pureLiFi’s high-speed technology path” , so considering it is the first product to demonstrate this technology, the possibilities are huge.
In 2012 the Li-Fi industry was worth around $96m, however this is expected to increase to around $6b in 2018 , almost doubling every year. This is a huge potential market that could be exploited.
Advantages and disadvantages of Li-Fi
There are several advantages of Li-Fi over Wi-Fi. The primary benefit is the speed at which data can be transmitted, both uploaded and downloaded. Due to the speed at which LED’s can be switched on and off, the metaphorical ‘on’ and ‘off’ pulses can be transmitted at extremely high speeds. Therefore data is transmitted far faster, allowing speeds of more than 10Gbps, meaning a high definition film could theoretically be downloaded in just 30 seconds.  This is a speed which is almost unimaginable with current technology.
On top of this, the visible light spectrum is 10,000x larger than the radio wave spectrum that can be used for data transmission. This would immediately alleviate the strain on capacity that could potentially affect data transmission in the years to come. 
Li-Fi also has security benefits, as the data signal can only be intercepted if it is in the path of the light. Therefore it is very easy to safeguard against any unwanted receivers picking up the signal, as you can simply face the light in another direction, or put an opaque object in front of the transmitter.
Another benefit is that Li-Fi can be used anywhere that light can be. Therefore, in situations where radio signals may interfere with other signals such as on aircraft and hospitals, there would be no issue with Li-Fi. This increases the number of places that people could have access to the internet, yet another benefit of this technology.
However, possibly the greatest benefit is that this technology could be implemented anywhere that LED lighting is currently or could possibly be used, with minimal extra cost. LEDs would still be able to perform their primary function of lighting alongside the transmission of data, without it being noticeable to humans.  The light is modulated at such high speeds that the human eye does not see any difference to other lighting. This allow Li-Fi, and therefore the internet, to be implemented anywhere light can be, making endless applications such as Li-Fi underwater, in aircrafts, and in street lights.
However, unfortunately there are also some disadvantages to this technology.
One of the major demerits of this technology is that the artificial light cannot penetrate into walls and other opaque materials which radio waves can do.  Therefore, signal may be lost due to instant or permanent block of the receiver. Furthermore, any devices that do not receive the direct light will not receive any transmission since it works only with a direct line of sight.
This contributes to the short range of Li-Fi in comparison to Wi-Fi, which could mean that more sources are needed. However, it is likely that in places where a long range is necessary, such as in a workplace, there would be a number of light sources, meaning there could be a number of sources of Li-Fi, therefore eliminating this problem.
On the whole the advantages of Li-Fi greatly outweigh the disadvantages, and there is simply more research needed to increase the download and upload speeds. A method of adding a signal modulator to an already installed LED light would also be very beneficial, rather than having to install the whole package brand new, as there are plenty of LEDs already installed that could be taken advantage of.
Current research and developments
The father of the Li-Fi, Harold Haas, said that it is possible to achieve more than a gigabit per second and that it is a conservative estimate and specified an expectation of obtaining 10 gigabits per second in the future.  The University of Edinburgh, where Professor Harold Haas is the chair of Mobile Communications, is opening a new centre which will be aiming to further develop Li-Fi technology. They also specify that this new technology is seen as the successor to the latest 4G wireless internet systems that came on stream in the UK in 2012. The Centre is hoping that they will form an environment where world leading experts in the University, major international electronics companies and other key research institutes around the world will be encouraged to collaborate with a target establishment of a major new $6billion(£3.57 billion) Li-Fi industry. 
With this progress in visual light communication it appears that the UK is becoming one of the leading countries in visual light communication. Now, for the next three years Engineering and Physical Sciences Research Council (ESPSRC) is targeting to improve micron-sized LEDs that will be produced from a material called gallium nitride which is excellent for high-power, high-frequency operations. These micron-sized LEDs are capable to turn on and off very quickly, around 1000 times quicker than the bigger diodes which means micron-sized LEDs can transfer data much more rapidly. Instead of current big size LEDs which occupies an area of 1mm2 micron-sized LEDs would occupy the same area with 1000 of them in which each of these LEDs will be able to operate as an independent communication channel and also transfer a million times information compare to one 1mm2 LED.  This offers huge potential, making ubiquitous computing a true possibility in the near future.
Professor Martin Dawson is a current staff of the University of Strathclyde, a centre of research for materials and devices underpinning solid state lighting and also leading the EPSRC funded initiative. “Imagine an LED array beside a motorway helping to light the road, displaying the latest traffic updates and transmitting internet information wirelessly to passengers’ laptops, netbooks and smart phones. This is the kind of extraordinary, energy-saving parallelism that we believe our technology could deliver.” 
There are many areas that Li-Fi for which Li-Fi would offer a great advancement in data transmission over radio signals. For example, under the ocean where radio signals are fairly ineffective, hospitals where radio signals interfere with vital equipment, streetlights, underground public transport, and aircraft cabins are just a few of the situations where Li-Fi would completely revolutionise their access to data.
On 18th October 2011 in a press release the new Li-Fi Consortium was launched with the aim of promoting Optical Wireless Communications. In the press release it was underlined that the mission of the consortium to promote new high-speed optical usage models both indoors and outdoors. The Li-Fi consortium is not only encouraging developers to design interesting and new products but also providing resources to them as well.
Optical wireless communication covers a wider spectrum of waves including infra-red and visible light. So, OWC has a bigger population of industry in which the Li-Fi consortium is open for commercial development of OWC technology. Founding members of the consortium are: Fraunhofer IPMS, Germany, IBSENtelecom, Norway, Supreme Architecture, Israel/USA and TriLumina, USA.
In conclusion, Li-Fi would appear to have a huge amount to offer, especially considering the issues that will begin to affect Wi-Fi in the coming years, such as lack of capacity and slow download/upload speeds. However, Li-Fi is currently not in a position to take over just yet, with it still being in a mainly development stage. As shown in Harald Haas’ TED talk though, a working prototype has been shown, in which a high definition video was streamed. To have those sorts of data transmission speeds at such an early stage of development is hugely promising.
Ultimately, some issues with Li-Fi will never be resolved, such as the short range and the ability to interrupt the signal simply by blocking the light. However, the advantages in transmission speeds, capacity, security and ability to use it almost anywhere outweigh these immeasurably.
Ultimately, it is very likely that Li-Fi will become a technology that we see more and more of in the coming years, until it eventually outpaces Wi-Fi. It appears that Li-Fi will lighten up our future effectively and efficiently very soon.
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This report is written by Robert Mills, Mert Chavushoglu, Jitpal Heer, Fadzlina Harun, and Ahmad Nizar