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LED lighting overview and market survey
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EE4401 – OPTOELECTRONICS
SOLID STATE LIGHTING: THE WHITE REVOLUTION
LIM FANG JENG
U076372R
NATIONAL UNIVERSITY OF SINGAPORE
2010/2011
Table of Contents
1 Introduction ........................................................................................................................ 1
1.1 Light and Life .............................................................................................................. 1
1.2 Definition of Quality of Light ..................................................................................... 1
1.3 Evolution of Lighting Devices: The Emergence of Solid-State Lighting ................... 2
2 Technological Overview of White LED ............................................................................. 3
2.1 Working Principles and Type of Semiconductor White LED ..................................... 3
2.2 Phosphor-Converted White LED ................................................................................ 4
2.3 RGB Color-Mixed White LED ................................................................................... 6
3 Market Overview of White LED ........................................................................................ 6
3.1 Current LED market .................................................................................................... 6
3.2 Market Survey on LED Production ............................................................................. 7
4 Conclusion .......................................................................................................................... 9
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
Lim Fang Jeng
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1 Introduction
1.1 Light and Life
Lighting has far become a development critical for our daily lives. Light is not only
important, but an essential element to enable us to navigate, search and to discover different
things. The best natural light sensor of the universe – our eyes are responsible to receive and
process the information to produce an image or a vision. The structure of eyes have enabled
human to receive light within an illumination range and different spectral qualities (colors).
The most important part in our eyes which give us the ability to receive lights at different
ranges and spectral qualities is to be attributed to the retina. The retina is made of different
light-sensitive photoreceptive cells used for different type of vision – rod cells for scotopic
(dark) vision and cone cells for photopic (bright) vision. The scotopic vision has the highest
luminous efficiency at 507 nm where photopic vision achieves maximum efficiency at 555
nm). The density of rod cells in an eye is much higher compared to that of cone cells. Thus,
the eye muscles will be weary more easily when one tries to see under low light condition;
this suggests the importance of lighting applications to human lives, human’s vision is at its
most comfortable state when the surroundings is bright and illuminated which cone cells are
used as the primary light receptor.
1.2 Definition of Quality of Light
To meet the need for better lighting, various light sources have been developed aims
to increasing the efficiency of light vision to the highest, such development ranges from the
use of fire, invention of the famous Edison incandescent light and fluorescent light as well as
the applications of solid-state lighting. In current state, the light-emitting diodes (LED) which
uses the solid-state lighting technology holds the highest potential to achieve ever growing
quality. The quality of light can be quantified by two different quantities: luminous efficiency
and luminous efficacy, as further defined in the table below.
Table 1 - Definition of luminouse efficiency and luminous efficacy
Luminous efficiency Luminous efficacy
Both luminous efficiency and luminous efficacy share the same unit (i.e. lm/W) and
provide a mean to measure the efficiency of a light source. Luminous efficiency measures
efficiency based on the radiant flux while luminous efficacy measures based on the electrical
power input. The spectrum of the sunlight at wavelength 555 nm gives the maximum possible
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
Lim Fang Jeng
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luminous efficiency available of 683 lm/W, corresponds to 100% efficiency. In all lighting
applications, improvements will be made progressing towards this theoretical limit.
1.3 Evolution of Lighting Devices: The Emergence of Solid-State Lighting
For the past century, the development of lighting was mainly limited by
incandescence, fluorescence and high-intensity discharge (HID). However, none of these
technologies were able to exceed the maximum efficiency of 25%. It was not until the mid-
20th
century when the first commercialized light-emitting diodes (LED) was brought to the
market, the development of the efficiency of solid-state lighting (SSL) far overrides the
incandescent light bulbs and fluorescent tubes. SSL is forecasted in the next few decades to
achieve a white light efficiency of 50%. Figure 1 shows the development of lighting
technologies in the past 200 years [2].
SSL is a technology in which it makes use of the electroluminescent properties of
semiconductors to produce light-emitting devices such as LED. The underlying reason
explaining the potential of SSL LEDs of achieving high efficiency is their energy-efficient
nature. Its electroluminescence principle and semiconductor properties have made the light
produced having low energy consumption, longer lifetime, faster response and small in size.
LED is very versatile in its applications ranging from smallest mobile devices, traffic lights,
computer monitor screens to general lighting [1] (see in Figure 2). If all the lighting
applications are to be replaced with LEDs, the global consumption of electricity would have
to decrease by 10% [3]. Furthermore, its gradual decrease of its costs have also made it more
economical competitive than the various existing lighting technologies. The LED
technologies have already reached a stage at which governments of different nations
Figure 1 - Historical Development and Forecast of the luminous efficacy of lighting technologies in 200 years (1800 to
2050) (Compiled by JY Tsao (Sandia Labs) using various datas) [2]
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
Lim Fang Jeng
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including the European Union, Japan, United States, Canada, Australia and the Republic of
China are aggressively pursuing the elimination of incandescent bulbs.
In Singapore, the LED market is also growing rapidly and application of white LEDs
as general lighting is slowly emerging. Many global SSL companies such as Philips, Seoul
Semiconductor, Epistar, Toyoda Gosei from various nations are also starting to invest into
Singapore’s market. JTC Corporation and Foxsemicon Integrated Technology from Taiwan
has started to install new LED streetlights at parts of street roads including CleanTech Park,
One-North and few more proposed stretch of roads as test-beds. It is estimated that a large-
scale LED streetlight replacement project will take place in 2 to 3 years in the nation [4]. This
technological shift has been made more and more significant since the past few years. As a
result, an understanding on white LED is crucial and important to keep track with the
technological and economic advancement.
In this paper, technological and market overviews will be presented. Firstly, the
principles of LEDs will be studied, followed by overview of different types of white LED.
Secondly, current LED market and market survey on white light LEDs production by 3 main
LED companies will be presented and evaluated.
2 Technological Overview of White LED
2.1 Working Principles and Type of Semiconductor White LED
The working principles of LED is simple: when a forward biased connection is made
across a p-n junction, electrons and holes will recombine and giving photon with certain
energy, if the photon is extracted out from the chip, an LED is made. The energy of the
Figure 2 - Applications of LED in different scales and range
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
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photon is limited by the bandgap of the semiconductor and different energy of photons will
give rise to different color of light emission and by making use of these colors, white light
LEDs can be made.
In practice, there are 2 methods to produce white light LEDs: phosphor-down
conversion and color-mixing method. Phosphor-down conversion involves conversion of
LED of UV/purple color to excite phosphor or other down-conversion materials to give a
wavelength-down appearance of white light; color mixing method involves the mixing of
multiple colored LEDs to create a RGB white light. Figure 3(a) and (b) show the basic lamp
structure of phosphor-converted white LED and color-mixed white LED respectively. Both
phosphor-down and color-mixed LEDs have their own pros and cons in producing white light
and they will be discussed accordingly as the paper proceeds.
2.2 Phosphor-Converted White LED
As mentioned in the above section, the phosphor-converted white LED (pc-LED)
makes use of technology of down conversion. This method of producing white light has been
commonly used in fluorescence, only with the replacement of a higher efficiency and light
source with better quality. Under phosphor conversion, the blue light experiences a Stokes
shift from shorter wavelengths to longer. Different types of material can be used as the
phosphor layer depending on the original LED color. If several phosphor layers of distinct
colors are applied, the emitted spectrum can be broadened and effectively raising the color
rendering index (CRI) of a given LED [5]. A typical spectral characteristics produced by a
pc-LED has two peaks, the first higher peak is attributed to the LED light source (blue or UV)
while the second peak attributes to the Stokes shift of the LED light, which gives rise to white
light.
(b)
Figure 3 - (a) Lamp Structure for Phosphor-down White LED; (b) Lamp Structure for Color-Mixed White LED
(a)
White Light White Light
RGB Phosphor
UV/purple
LED
Mixing Optics
RGB LEDs
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
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To manufacture a white pc-LED, a InGaN blue LEDs can be encapsulated inside a
common yellow phosphor coated epoxy such as cerium-doped yttrium aluminium garnet
(Ce3+
:YAG). On the other hand, a near ultraviolet (NUV) LED source can also be used as the
lamp source, coated with a mixture of high efficiency europium-based (red and blue emitting)
phosphors and copper and aluminium doped zinc sulfide (ZnS:Cu, Al, green emitting)
phosphor. Using a blue LED with Ce3+:YAG phosphor coated is not as efficient as using the
UV LED with the three-color mixed phosphors because the Stokes shift of the former is
larger, causing more heat losses, but produces light with higher CRI due to its broad spectral
broadening. However, using UV LED is still not preferred due to safety concerns; UV light
may leak out and causes harm to human eyes or skin.
Since the nature of this design is to convert the original UV/Purple LED to a broader
spectrum by additional coated layers, certain loss is to be expected. Depending on the design
of the lamp, certain reflective losses and absorption losses can affect the CRI of the white
light produced. This essentially means that there will be a loss in power when light is
delivered. Besides, heat loss due to the Stokes shift, phosphor degradation of the device such
as oxidation, formation of deep electron taps, thermal quenching, etc and lack of tunability to
produce whole array of efficient white light are also the factors affecting the low efficiencies
and low versatility of while light produced by pc-LEDs.
However, due to the simpler and cheaper design and production of a pc-LED
compared to a complex RGB system, the phosphor method is still the most popular method
for making high intensity white LEDs. In fact, majority of high intensity white LEDs on the
market are still manufactured by using phosphor light conversion [6-8]. Different ways can
be employed to increase the efficiency of the light, for instance, a better package design or by
Stokes Conversion
(By Phosphor Layer)
Blue/ UV LED
Figure 4 - Typical Spectral Characteristics of pc-LED
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
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using a more suitable type of phosphor having lower Stokes energy loss as well as lower
degradation, the efficiency can be increased. A more homogeneous white light is claimed to
be produced by Philips Lumileds by employing a conformal coating process, which can make
the phosphor thickness uniform [9]. With development ongoing, the efficiency of phosphor
based LEDs generally rises with each new product announcement.
In overall, due to the low system complexity of pc-LED and the existence of
technology of using phosphor with UV/purple LEDs, albeit with improvements to be desired,
is the clear current design choice for various lighting applications.
2.3 RGB Color-Mixed White LED
The design of RGB color-mixed LED is, however, different from the pc-LED. It
involves the theory of color-mixing in forming white light, hence no down conversion
processes is present as in the pc-LED. The fundamental color to form white light with
optimal CRI is essentially red, green and blue (RGB). Hence, making use of this principle, a
high efficiency and high intensity white light can be produced. To make the color mixing
works, an
Theoretically, color-mixed LEDs should give a higher efficiency compared to pc-
LED because there is no down-conversion loss; this eventually will save energy and also
lower the cost of LED per lumen, as will be discussed in the later section. However, color
mixed LED is still unable to be produced in general lighting. Based on the current technology,
the ‘Green Gap’ problem is still the hurdle and bottleneck for producing a high efficiency
white color-mixed LEDs [10]. Furthermore, due to its principle of additive color mixing in
producing white light, they could not provide richly saturated and natural looking white lights
[11]. Nevertheless, in a long run, this design is still more favorable than pc-LED due to the
absence of down-conversion losses and a lower capital and production costs.
3 Market Overview of White LED
3.1 Current LED market
According to the most recent report by Strategies Unlimited, the global market for
high brightness LED in 2010 is $10.8 billion and an expected increase to $18.9 billion by
2015, with dominant region in Japan and Korea (61% of the market, shown in Figure 5) [12].
LED is expected to have a dominant market by the year 2014 due to the ever rising demand
for energy efficiency and phasing out of incandescent bulbs.
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
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Figure 5 - Regional breakout of world wide high-brightness LED supply by Revenue
Major high brightness LED manufacturers such as Nichia, Cree, Lumiled, Osram
Opto Semiconductors, Seoul Semiconductor, Epistar and Toyoda Gosei have developed the
LED technologies in different applications such as backlight illumination, general lighting,
vehicle lighting, etc. All these development are expected to meet the Solid State Lighting
LED (SSL-LED) roadmap from U.S Department of Energy (DoE) in reaching a power
conversion efficiency of 50% by 2020 (Table 2). Apart from the improvement of power
conversion efficiency, improvement of lifetime and production cost is also vital. By 2020,
SSL-LEDs are expected to have a lifetime of 100,000 hours (~ 10 years), 100 times and 10
times longer than that of incandescence and florescence respectively. Though the production
cost of SSL-LEDs are more costly than that of the conventional incandescence and
fluorescence, their longevity, power usage and efficiency clearly outweigh the conventional
lighting technologies. The roadmap seems aggressive, but with rapid development of LED
technologies, it is not impossible for LED technologies to reach a power conversion
efficiency of 50% by the year 2020.
Table 2 - SSL-LED Roadmap with comparisons to traditional lighting technologies [2]
SSL-LED 2012 SSL-LED 2020 Incandescent Fluorescent
Luminous Efficacy (lm/W) 150 200 16 85
Power Conversion efficiency (%) 37.5 50 4 21.25
Flux (lm) 1,000 1,500 1,200 3,400
Input Power (W) 6.7 7.5 75.0 40.0
Lifetime (hr) 100,000 100,000 1,000 10,000
Lamp Cost ($/klm) 5.0 2.0 0.4 1.5
Lamp Cost ($/lamp) 5.0 3.0 0.5 5.0
Color Rendering Index (CRI) 80 80 100 75
3.2 Market Survey on LED Production
A market survey was done by comparing the most recent highest performance LED
by three major LED production companies, i.e. Nichia Corporation, Cree and Seoul
33%
28%
14%
2%
23%Japan
Korea
Taiwan
China
US/Europe
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
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Semiconductor. Narukawa et.al. from Nichia Corporation from Japan have claimed to
successfully develop a white LED with very high luminous efficacy of 183 lm/W with
luminous flux of 203 lm at 350 mA, achieving a power conversion efficiency of 45.8% [13].
Cree Research Center from the U.S has also successfully developed a high efficacy warm
white LED using their own unique phosphor deposition technology that reaches as high as
138 lm/W at 350 mA, with power conversion efficiency of 34.5% [14]. Acriche, Seoul
Semiconductor’s main product has announced the launch of 4-chip RGB LED (Z-power Z6
and Z7) which could produce 440 lm with 1 W of power for each LED chip and they claim
the LED could produce a diverse temperature of white light if all the 4 chips are used [15].
The performances of LEDs in all these companies are tabulated in Table 3 below.
From Table 3, we can see that the Nichia Corporation apparently sits at the highest
position in luminous efficacy, with the highest power conversion efficiency. However, the
capability of such ultra high brightness LED to be commercialized is still not revealed, so is
the cost of such LED. Though Seoul Semiconductor seems to have the lowest LED efficacy
among the 3 manufacturers, their low-cost technology might earn them higher revenues. High
luminous efficacy should not be the sole evaluation for a best LED, apart from power usage
and efficiency, the cost of LED should also be considered. A good LED should be a balance
between performances and cost in order for it to be commercially available to consumers. It
can also be seen that except for Seoul Semiconductor, most LEDs are that of a phosphor-
converted (pc-LED). This shows the pre-maturity of color mixed LED in current market and
more challenges are to come in the future. Nevertheless, in terms of efficacy, most companies
have moved closer or exceeded the scenario of SSL-LED for 2012 (see Table 2); it is indeed
possible for LED technology to reach a state as projected by SSL-LED 2020.
Table 3 - Comparison of highest performance LED of three majore LED companies
Nichia Corporation [13] Cree [14] Seoul Semiconductor [15, 16]
Luminous Efficacy (lm/W) 183.00 138.00 110.00
Forward Current (mA) 350 350 350
Wall-plug power usage (W) 1 0.97 4
Power Conversion efficiency (%) 45.8 34.5 27.5
LED type pc-LED pc-LED 4-chip color-mixed RGB White
LED
LED Package Surface Mount Surface Mount Ceramic Circuit
Applications General Lighting
General Indoor and
Outdoor lighting,
directional lighting
General Lighting
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
Lim Fang Jeng
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4 Conclusion
By the motivation of have comfortable lighting to enhance the photopic vision of
human, various lighting technologies ranging from candles to fluorescence have been
developed since the last 200 years. The emergence of solid state lighting has made a giant
leap towards a low-cost, energy efficient lighting source. The energy saving, high efficient
and easily manipulated nature has made solid state white LED to have versatile applications
ranging from mobile phone backlight to general lighting. In this paper, overview of two main
types of white LED, pc-LED and color-mixed LED, were evaluated and presented. The
exploration to future projection of solid state lighting LED to 2020 and market survey on
current ultra brightness LED market by Nichia, Cree and Seoul Semiconductor were also
documented. It is believed that with the current state of rapid development, the market will
not merely aiming to reach a high performance LED lighting, but the needs for human in
pleasing the vision could also be met. With these technological advancements, surely a
‘bright’ future will be one more step closer, when the ‘sun’ never sets.
Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R
Lim Fang Jeng
References
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Lim Fang Jeng
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