Osram brilliant mix concept
Osram brilliant mix concept

SEPTEMBER 2011: Big things come in small packages. This statement holds 100 per cent true for light-emitting diodes (LEDs), which are now celebrated for many salient features including small form factor, long life and low power consumption.

That said, you might be surprised to learn that the first LEDs were far from being useful. They could only operate at bizarre temperatures of -77ºC and below, would burn out in only a few hours, guzzle power like demons, and produce very little light in the bargain. As we can see, all of these cited weaknesses have over time transformed into strengths—mainly due to process and material improvements. In short, the technological and business developments in the LED industry have been, and continue to be, evolutionary and revolutionary at the same time.

In the past few years, improvements in efficacy, packaging and price of LEDs have resulted in a greater realisation of its benefits, more applications and a larger market. “The range of applications for LEDs continues to grow significantly. One of the fastest growing and most promising uses is in illumination applications. LEDs are penetrating this market rapidly due to the combination of excellent colour saturation and energy savings that satisfy today’s demand for reduced energy ‘green’ products, and long life,” explains Hari Kiran Chereddi, managing director, Sujana Energy.

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“LEDs are one of the most energy efficient and green lighting solutions available today, and this has been the primary reason for their rapid popularity across the world, amid rising awareness about the environment. These are one of the most sustainable and long-lasting lighting sources and facilitate reduction in carbon dioxide emissions, in addition to helping save energy and maintenance costs,” adds Indranil Goswami, head-lighting application services, Philips Lighting India.

In this article, we take a quick look at the recent improvements in LED technology and its exciting applications, as also the challenges that continue to daunt the industry. However, before we foray into the details, it might be wise to begin with Chereddi’s simple summary of recent developments: “As LED materials technology became more advanced, the light output has increased, and LEDs have become bright enough to be used for illumination. But, with higher power, it became increasingly necessary to get rid of the heat, so the packages have become more complex and adapted for heat dissipation. The efficacy of white LEDs has risen from about 15 lumens per watt (lm/W) to over 100 lm/W. In the past two years, there has been a significant fall in the cost of LEDs; the reasons behind the price fall are mass manufacturing and improved technology.”

Understanding LEDs

In order to understand the improvements in LED technology, it is very important to know the basics of how an LED works and how it is made.

Concept. Diodes are electrical valves made of very thin layers of semiconductor material. One layer will have an excess of electrons, while the next will have a deficit of electrons. This difference causes electrons to move from one layer to another, thereby generating light. More the number of electrons crossing from one layer to another, brighter the light generated.

Construction. A semiconductor is a crystalline material. It conducts electricity only when there is a high density of impurities in it. The semiconductor wafer is a single uniform crystal—for the purpose of LEDs, the semiconductors are usually made from gallium arsenide (GaAs), gallium phosphide (GaP) or gallium arsenide phosphide (GaAsP). Once the slice or wafer of semiconductor is ready, various impurities like zinc, nitrogen, silicon, germanium or tellurium are suspended in it through a process called doping; there are various techniques for this. The impurities create a layer with excess or deficit electrons. Once these layers are ready, wires are stuck to the semiconductor to introduce electricity. This completes the LED device. Depending on the semiconductor materials (called substrates) and the impurities used, the LED produces different colours of light.

Challenges. The key to successfully designing and manufacturing LEDs is to understand that it is both an electronic and optic device, affected by electrical, electronic and optic principles. Properties such as colour, brightness and efficiency are affected by the size of the diode, the semiconductor materials used to make it, the thickness of the diode layers, the type and amount of impurities used to dope the semiconductor, etc. The goal of an LED designer is to optimise these parameters.

All key parameters seem to be on the rise
Most of the key parameters used to evaluate LEDs such as light output (flux), efficacy (lm/W), colour quality and stability have improved greatly in the recent past, thanks mainly to material sciences and semiconductor fabrication technologies. “LED technology has been evolving rapidly since the last one or two years. The diode technology has improved significantly and as a result high-power LEDs with higher lumen output are making it possible to replace other lamps with LED lamps. This has resulted in wider application of LEDs. The past few years have also seen a remarkable improvement in the efficacy of LED chips, packages and lamps, measured in terms of the number of lumens emitted per watt of electrical power consumed,” says Goswami.

There are several products in the market exemplary of these properties. Quite recently, Cree reported a record efficacy of 231 lm/W for a white, power LED. As per company reports, the LED efficacy was measured using a single-die component at a correlated colour temperature of 4500 Kelvin, standard room temperature and 350mA testing.

Sharp Corporation has also introduced high-output, high-colour-rendering 25W-class LED lighting devices that feature a high luminous efficacy of 91 lm/W, a luminous flux of 2370 lm and a high colour rendering index (CRI) of 83. These devices have also achieved incredibly low energy consumption through the adoption of LED chips and phosphor, both of which have excellent high-temperature properties. They are well suited for use in product display spotlights in stores.

OSRAM Opto Semiconductors, a leading technology-driver in the LED market, has launched its high-current UX: 3 chip technology. The next-generation thin-film indium gallium nitride (ThinGaN) chip technology used here makes LEDs capable of handling high currents. “Normally, the efficiency of high-power LEDs reduces as the operating current increases. The greater the volume of the active chip area, the weaker the effect. This is where UX: 3 chip technology comes in, reducing the current density in the multi-quantum well structure. The result is a high-power LED with high current capability for high-power applications—mobile flash, projection and, of course, general lighting applications,” explains Dr Michael Fiebig, director-marketing and business development, OSRAM Opto Semiconductors.

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‘Brilliant-Mix’, another key concept from OSRAM, stands for a 30 per cent greater luminous efficacy with a high CRI too. The intelligent colour mix, based on Oslon solid-state lighting (SSL) LEDs in EQ-White and Amber, covers a broad white spectrum from 2700 to 4000 K. It also offers warm white light with a high luminous efficacy (110 lm/W) and a colour rendering index of more than 90.

Philips living colour
Philips living colour

Looking ahead, Chereddi comments, “Significant technological challenges remain to be solved in order to achieve the cost and performance target set by the industry to enable adoption of LEDs for a large number of applications. Solving the high current efficiency drop remains a priority. Thermal management remains the main challenge, impacting device lifetime, stability and limiting driving currents. The development of new phosphors is also critical in achieving high efficiency and bringing colour consistency, stability and rendering at the levels requested for general lighting applications.”

Size is reducing
Today’s LEDs are much smaller in size as compared to a year or two ago. Single-die miniature LEDs, which are nothing but a single chip with a connecting wire, are commonly available. This reduction in size can be attributed to improvements in fabrication technology.

“A few years ago, the size of an LED was two to five millimetre, that too in lead mounting. These were used mainly as signal indicators. Now, LEDs are available in surface-mount device (SMD) packaging, and we have multiple LEDs in one small package. The size of a package consisting of three LEDs is no more than five millimetre. These three LEDs may be of the same colour or of different colours. By changing the colour intensity of each LED in one package, we can generate the required true colours. So, such a tiny package can replace a pixel on an LED screen,” remarks Dr Anita Agrawal, assistant professor in the department of electrical and electronics engineering, BITS, Pilani-K.K. Birla Goa Campus.

Solid State Supplies’ small footprint 40W RGBW LED emitter is an example of the reducing size of LED components. The LZC-00MD40 RGBW LED emitter has a high light output, high flux density and superior colour mixing. It offers twelve red, green, blue and white LED dies in a single, compact emitter. With 40W power capability and a 9.0×9.0×5.4mm³ ultra-small footprint, this package provides exceptional luminous flux density, producing 2000 lm in daylight white or 1500 lm in warm white.

One step ahead, UK-based E-wave, a maker of tailor-made LEDs, claims that with the latest fabrication and packaging technologies as small as 50×50µm² LED chips can be made with even smaller emitting areas. These can be provided as chips or packaged LEDs. The chips can be mounted on small ceramic (or silicon) sub-mounts, and then encapsulated in plastic for applications that require package foot-prints smaller than two millimetre.

To understand the reason behind the reducing size of LEDs we should remember that an LED is both an electronic and an optic device. It is almost like a little chip. So, as semiconductor fabrication technology improves, the size of LEDs also reduces. In fact, semiconductor fabrication affects many aspects of LEDs—right from size to the price (volume manufacturing at low costs). 

Some of the key parameters used for evaluating LEDs

Light output. The light output of LEDs is normally the luminous flux, or energy per unit time that is radiated from a source over visible wavelengths. It refers mainly to energy radiated over wavelengths sensitive to the human eye, from about 330nm to 780nm. Flux is normally measured in lumen.

Performance. The performance of an LED is the light produced per diode and it depends on many factors ranging from material and size to temperature. Like the chip world has Moore’s Law, the LED world has Haitz Law: “The amount of light that can be produced per diode (performance) would increase 20-fold every decade, while the cost of that light would decrease 10-fold.”

While many generally use luminaire efficacy as a measure of performance, others believe that it should be seen as a standalone metric, but should be assessed in the light of other parameters like temperature, etc, relevant to the application. In general, high-performance LEDs offer a greater light output in a smaller footprint, and high luminous intensity for low currents.

Efficiency and efficacy. Similarly, both the efficacy and efficiency of LED bulbs have also been improving significantly. In lighting, efficacy refers to the amount of light (in lumens) produced by a certain amount of electricity (in watts). On the other hand, lighting fixture efficiency is the ratio of the total lumens exiting the fixture to the total lumens initially produced by the light source.

Colour rendering index (CRI). CRI is a measure of colour quality; that is, how well light sources render the colours of objects, materials, etc. The test procedure to arrive at the CRI number involves comparing the appearance of eight colour samples under the light in question and a reference light source. The average differences measured are subtracted from 100 to get the CRI. So, small average differences will result in a higher score, while larger differences give a lower number. However, in recent times, the International Commission on Illumination (CIE) has raised some doubts as to the relevance of the older general colour rendering index (Ra) for measuring the quality of white LEDs.

To do away with any discrepancies, a long-term research and development process is underway to develop a revised colour quality metric that would be applicable to all white light sources. In the meantime, the CIE recommends that the CRI can be considered as one data point in evaluating white LED products and systems, but it should not be used to make product selections in the absence of in-person and on-site evaluations. The recommendations also add that if colour appearance is more important than colour fidelity, one should not exclude white light LEDs solely on the basis of relatively low CRI values. Some LED products with CRIs as low as 25 still produce visually pleasing white light.

Lumen maintenance. Lumen maintenance simply compares the amount of light produced from a light source or from a luminaire when it is brand new to the amount of light output at a specific time in the future.

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In addition, the footprint, thermal management and optical control capabilities of LEDs are also considered, depending on the applications they are to be put to.

Cost is going down too
According to some recent reports, improvements in device efficacy have been driving big improvements in lumens per dollar, but that is getting close to the theoretical maximum, and it is not easy to double it again. But, costs still need to come down further. So, the industry is turning to manufacturing efficiency for the solution to this problem.

Here, one of the main techniques to reduce cost is to use larger wafer substrates. Other areas of improvement include designing easy-to-manufacture devices, improving cost-of-ownership of equipment, better wafer-level testing and traceability for tracking defects back to root causes, better measurement of process conditions, improved ways to prepare the substrate before epitaxial film/wafer growth, improvement in the handling of the delicate wafers after laser lift-off, lower-cost ways to make patterned sapphire substrates, and so on.

“With latest manufacturing technologies, manufacturers are able to produce LEDs that are not only super bright but also consume less power while producing light of higher intensity. Volume manufacturing on 15.24cm (6-inch) wafers started at the end of 2010, and massive transition from 5.08-10.16cm (2-inch to 4-inch) is set to begin in 2011. However, the adoption of 20.32cm (8-inch) wafers remains uncertain and will require that sapphire wafer manufacture achieves significant cost reductions to make mass production more affordable,” explains Chereddi.

Uses aplenty
LEDs have innumerable uses, including…
1. Indicators on electronic devices, stereos, automobile dashboards, microwave ovens, etc.
2. Numeric displays on clocks, digital watches and calculators
3. Simple light bulbs for domestic and public lighting
4. Smart lighting systems for offices, homes
5. Short range optical signal transmission, such as in TV remote controls
6. Fashion accessories, such as blinking coloured lights on helmets
7. Back-lighting for liquid crystal display (LCD) screens
8. Automotive lighting
9. Power-smart displays for mobiles

Other technological developments like inorganic opto-semiconductors and organic light-emitting diodes with improved lumen efficiency and manufacturing processes might also reduce cost and enable mass manufacture in the near future.

And of course, there is the unbeatable force of the market. In India, the Bureau of Energy Efficiency (BEE) is coming up with a roadmap for enhancing the usage of priced LEDs in India. This would include standardising LEDs and developing testing, monitoring and verification protocols of LED lightings. The increasing applications and use by architects for aesthetic and energy-management applications will also increase demand and help reduce prices. Central government’s 2011 budget has proposed to reduce excise duty and countervailing duty on imports of LED lights, which would further reduce the price of LED lightings in India.

Some ongoing efforts across the world...
1. Design of surface-mounted LED package using a ceramic sub-mount as the carrier and InGaN LED flip chips attached to a ceramic sub-mount
2. Improving wafer-level packaging to reduce cost; silicon-based wafer-level packaging offers high fabrication reliability, high yield, superior thermal management, integration of optical elements, and the direct integration of the driver IC in the package. The form factor can be further reduced using through-silicon vias, increasing the maximum amount of chips per wafer
3. Futuristic innovations in semiconductor, phosphorescent material and control
4. Optimisation of LED component platforms to help make LED lighting systems more cost-effective, efficient, robust, quicker to develop, higher in quality and with longer lifetimes
5. Use of reflectors instead of lenses to control optical properties in LED lighting systems, for various applications such as street lighting, back-lighting and adjustable office lighting
6. Overcoming the limitations in the design of plastic optical elements, which are cheaper, lighter and more versatile than glass optics
7. Reducing wastage due to over-illumination. Normally, the optics for LEDs produce circular or elliptical beams, which results in wastage when illuminating non-circular areas. New advances in optical design and meld tool manufacturing open the possibility of avoiding over-illumination by using optical components that make use of free-form optical surfaces
8. Combination of optical free-form surfaces with solid-state light sources for strongly asymmetric light distribution such as that required by many aesthetic applications and even street lighting, in a cost-effective way using methods like rapid prototyping
9. Design considerations for constant voltage, constant current and mixed-mode switching power supplies, including switching topologies and power factor correction configurations, in driving LED lighting systems
10. Sophisticated heat management, as required by the increase of luminous flux, usage of high-power LEDs, and thermal stress caused by extensive thermo-cycles
11. Use of multidimensional all-in-one boards to support flexible optical design and simple integration of intelligent light management electronics
12. Reducing fluctuations in the photometric characteristics within LEDs of one batch
13. Advance light measurement instrumentation required to detect the above-mentioned fluctuations (spectroradiometric light measurement techniques are being explored)
14. Modern spectrometric measurement systems and sophisticated algorithms for quick and precise calibration of systems consisting of a number of multi-chip LEDs—by measuring parameters like correlated colour temperature, colour coordinates or colour rendition indices, and automatically adjusting them in a reliable and predictive way.
15. Advanced 3D-pick-and-place machines that can be fully integrated in an automated assembly line; to explore new design opportunities for LEDs
16. New standards, norms and test procedures for the LED systems and technologies in terms of materials, drivers, system architecture, controls and photometric properties

Expanding applications
“LEDs in general are much more tailored to the specific requirements of the general lighting market than a couple of years before. Technical developments as well as a broad understanding of the customers’ requirements together with industry adaptations can be seen in the latest SSL products. The new LEDs are dedicated to specific applications to gain the improvements of this technology,” says Dr Fiebig. This, in many ways has impacted the type of applications that LEDs are being put to nowadays.

Solid-state LED lighting is already well-established in some market segments like architectural lighting, decoration/beautification, signage, automotive lighting, LCD backlights, wall washers, video walls, etc, and beginning to gain ground in other segments like outdoor facade lighting, professional shop lighting and residential lighting. The use of red, green and blue (RGB) combinations is expected to become increasingly popular in these segments due to the resulting flexibility of colour selection and the simplification of the optical aspects of lighting designs. Independent market research institutes such as Strategies Unlimited have shown that the main market is in architectural lighting, which will be overtaken by retrofit lighting solutions. The residential sector, retail displays, shop lighting and outdoor lighting will support this growth.

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If the improvements in performance and cost continue in the same trend, LED lamps might ram into the do-mestic market too and one day replace CFLs and even the ubiquitous 60W incandescent bulb. Dr Agrawal explains the possibility, “Ultra-high brightness (UHB) LED is becoming popular because of its high efficiency, small size, directional property (incandescent, halogen or compact fluorescent bulbs give out omni-directional light) and long life. The operational life of current white LED lamps is 100,000 hours. This is eleven years of continuous operation, or 22 years of 50 per cent operation. The long operational life of an LED lamp is a big contrast to the average life of an incandescent bulb, which is approximately 5000 hours.”

Using their intelligence intelligently
Due to their digital nature and ability to be controlled and managed, the SSL industry will be characterised above all by the development of intelligent solutions that adjust the amount of light according to demand and the particular situation. That is, ‘tailor-made’ light with optimum energy efficiency. Such intelligent solutions using presence or motion sensors will, in fact, bring out the true potential of LEDs.

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“Take for example, street-lighting, where conventional solutions always show the same brightness, no matter how dark the night is. The Arquiled LED streetlight from Arquicity is a combination of intelligent lighting management and LED lighting. The brightness of this street light is automatically adjusted according to the traffic. Intelligent lighting control can be provided by an ambient light sensor. In fact, we believe that light will be used more and more to make people feel better. Light will also be used as a design element, in other words not just to make rooms bright but to give them a different look, a different feel, a different atmosphere. With SSL, light can be much more than just illumination,” explains Dr Fiebig.

Philips is leveraging many of the exciting features of LEDs and launching such products in the Indian market too. “LEDs have digital control programmability. Unlike other lighting solutions that are analogue, LEDs are digital, highly versatile and provide many new avenues and options in terms of creativity and design across applications. LEDs provide the option of intrinsic coloured lighting and directional lighting. These provide greater control in any lighting application. In addition, through the innovative combination of various-coloured LEDs, dramatic colour-changing effects can be produced from a single fixture through dynamic activation of various sets of LEDs,” explains Goswami.

Philips’ LivingColors range enables users to express themselves by creating striking light effects to personalise their own ambience with the colour of their choice, with just the press of a button! Philips has also launched the first indigenously-developed consumer LED bulb in India, which is now available in five and seven watt variants. Recently, the company also launched an LED-based solar street lighting range for Indian roads. The solution will be applicable for urban areas as well as off-grid locations in rural areas.

Truly, there are many exciting applications in store for LEDs. LEDs, for example, have grown significantly in the medical field with new and unique equipment that have LED lights embedded in them. Ultraviolet light is used in innovative printing and curing applications. Space requirement and weight are important features in compact plants; and ultraviolet LEDs are a space-saving and efficient option to equip printing systems. LEDs can be switched on and off without delay to follow the process cycles and consume energy only when they are operated.

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An essential advantage becomes obvious: the service life of the LEDs is equal to the time they are actually used. This makes ultraviolet LED light sources very efficient and economic in cycled operation. The advantages are longer servicing intervals and reduced cost of maintenance in addition to a reduction in production downtimes. LED technology is also beginning to be used in horticulture. LEDs can be used to emit specific lights that help plants grow.

Still a growing market
With so many advantages and potential applications, LED is sure to succeed in a large market such as India—provided the price factor is addressed. As per recent Frost & Sullivan and The Economic Times reports, the LED segment currently accounts for less than three per cent of the country’s organised lighting market, which is estimated at 70,000 million and is growing at 25-30 per cent a year. The LED market alone is estimated at 2000 million.

LEDs are fast emerging as the lighting technology of the future in the Indian lighting market. The overall Indian LED lighting market is expected to reach $399.2 million by 2015 from $49.6 million in 2009 at an estimated compounded annual growth rate of 41.5 per cent till 2014.

From an overall global perspective, Dr Fiebig concludes: “On one hand, the technology is still developing rapidly, the industry is on a growth curve in terms of brightness, efficacies and colour control, and on the other hand, there is still room for further advancements in terms of productivity, thereby reducing cost per lumen. With this ongoing trend toward higher performance and lower-cost LEDs, many new applications open up (e.g., within general lighting) and others are expanded (e.g., within automotive). In this sense, the industry is still in the growth phase and we expect that the creativity that we unleash in the market based on the advances in LEDs will keep us there for years to come. Another important topic is to adapt the lighting systems to the LED technology. That means in order to use the complete potential of LED, new aspects of lighting concepts have to be taken into account. This will help improve the system and application efficiency and finally cost competitiveness vis-à-vis conventional technologies.”


The author is a technically-qualified freelance writer, editor and hands-on mom based in Chennai

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