Chapter 5

Solid-State Light Sources

O f the three key technologies that all artificial light sources in the
market are based on, two of them (incandescence and plasma-based
sources, otherwise known as discharges) have reached a plateau in efficacies,
but nevertheless there is still ongoing research, and fascinating developments
continue to take place. The third technology, although it started developing
and was marketed much later than the other two, and perhaps for that very
reason, is now showing such fast progress that not only can it compete with
the other technologies, but in some cases it is preferred to them.
This chapter is an introduction to this technology of solid-state light
sources, known as LEDs, which has attracted the interest of many profes-
sionals and covers a range of topics answering the most frequently asked
questions. Some of the topics include the semiconductor and diode tech-
nology on which LEDs are based, ways of creating different colors and
white light, modes of operation, their thermal management, applications,
and comparisons with the other two technologies of light sources. The
chapter addresses a wide range of professionals for whom light and its
sources and, in particular, solid-state lamps, are part of their work.
Solid-state lamps, that is, light-emitting diodes, are considered by many
to be the future of lighting. Indeed, they have evolved to such an extent
that they demonstrate several advantages over the other technologies, and
they have already dominated some applications. Of course, there are still
some issues that are limiting factors for their further development, such as
the issue of thermal management. A large number of scientists are work-
ing intensively on the technology of solid-state lamps and, more particu-
larly, they are focused on understanding how to create light through the

125

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 126    ◾    Light Sources: Technologies and Applications

crystals, the reliability and performance of the materials in order to reduce

production costs, the development of phosphor powders for conversion
of radiation with good quantum efficiency, the geometry and materials
of various parts for better extraction of photons, their sensitivity to tem-
perature and humidity, and, finally, on improving the control electronics
of the large number of units needed to produce high luminous flux and
color reliability.
There are many who parallel the seemingly inevitable future dominance
of LEDs with the case of the transistor, where the hitherto dominant tech-
nology of thermionic valve/tube (glass-metal-vacuum) was replaced by a
solid-state technology.
Finally, let us not forget that a similar but at the same time different
technology of solid-state lighting is based on organic (OLEDs) and polymer
(POLEDs) compounds. This technology may prove even more important in
coming decades if the materials used become cheaper and more flexible.

5.1 Light-Emitting Diode (LED)

The technology of solid-state lighting was the last of the three tech-
nologies to penetrate the market, and it is based on the effect of
electroluminescence.
The term electroluminescent refers to light emission from a solid body
when electric current flows through it or when it is placed in an electric
field, and it is an effect different from incandescence. The first efforts to cre-
ate light in this way focused on the use of phosphorescent powder such as
ZnS (enriched with copper or manganese) in powder or thin-film form for
use as a backlight for liquid crystal displays. These light sources consume
little power but require high voltage (>200 V), while their efficiency is low.
These efforts and the development of semiconductor technology that
gave birth to the solid-state diode led to a new generation of solid-state
light sources.
The controlled addition (doping) of small quantities of certain materials
in a semiconductor’s crystal structure, such as silicon, without damaging
the structure (high-quality crystals must be used, avoiding oxygen and
hydrogen) gives the semiconductor some extra properties and, depending
on the materials used, we have two different cases. In one case, if the addi-
tional material consists of atoms with a number of valence electrons larger
than that of the crystal atoms, we call the semiconductor n-type, and it
will have a surplus of electrons in the crystal structure (for example, by
adding a small amount of phosphorus or arsenic in silicon). In the other

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 Solid-State Light Sources    ◾    127

Si Si

Si Sb Si Si B Si

Si Si

Figure 5.1  Creation of semiconductors with additions of various ele-

ments to crystalline silicon (doping). The addition of antimony creates
free electrons, and the addition of boron creates electron holes.
case, where the added materials consist of atoms with a smaller number
of valence electrons (adding a small amount of boron in silicon or gal-
lium), the semiconductor will have a surplus of positive charges, otherwise
known as electron holes, and we call such a semiconductor p-type (see
Figure 5.1).
The connection of an n-type with a p-type semiconductor creates
between them what is called a p-n junction and functions as a diode, allow-
ing the flow of electricity in one direction only, from anode (p-type) to
cathode (n-type), as shown in Figures 5.2 and 5.3.
During the flow of electricity through such a solid-state diode, electrons
are combined in the semiconductor junction with the positive holes, and
this combination puts the electrons in a lower-energy state. The energy
– +

p-type n-type
– – + +
– +
– – + +

Figure 5.2  Creation of a diode by connecting n- and p-type semiconductors.

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 128    ◾    Light Sources: Technologies and Applications

Light
emission
+

I
R –

Figure 5.3  Radiation emission from the junction of a semiconductor

during flow of electricity.

state difference can be released as electromagnetic radiation (not always,

as it can also be lost as heat in the crystal) with a wavelength that depends
on the materials of the semiconductor. Such a light source is known as
LED (light-emitting diode). LEDs emit radiation of narrow bandwidth
(a range of a few tens of nanometers). The total charge in the crystal must
be distributed as much as possible, and the capsule should be optically
transparent (epoxy). The flux is limited by the heat generated at the junc-
tion. Both the shell (epoxy) and the crystals begin to wear out after 125°C.
Because of the low flux, LEDs have currently limited applications. Due to
the materials used, total internal reflection takes place, easily trapping the
radiation. The extraction techniques of photons are therefore an impor-
tant aspect of this technology.
If the material is an organic compound, then we have an OLED (organic
light-emitting diode), and in case of a polymer compound the acronym
used is POLED.
A brief historical review of the invention and development of this tech-
nology follows:

• H.J. Round—Marconi Labs (1907)—pale blue light from a SiC crystal

• Oleg Vladimirovich (1920s)—first LED
• Rubin Braunstein—Radio Corporation of America (1955)—first infra-
red LED using GaAs, GaSb, InP, SiGe

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 Solid-State Light Sources    ◾    129

• Bob Biard and Gary Pittman—Texas Instruments (1961)—first pat-

ent for an infrared LED using GaAs
• Nick Holonyak Jr.—General Electric Company (1962)—first red LED
• Jacques Pankove—RCA (1972)—first blue LED using GaN
• M. George Craford (1972)—first yellow LED
• T.P. Pearsall (1976)—first use of an LED in telecommunications
• Shuji Nakamura—Nichia Corporation (1993)—first blue LED using
InGaN
• Monsanto Corporation (1968)—first mass production of LEDs,
bringing down the cost dramatically

Figures  5.4 and 5.5 show the anatomy of a modern LED unit, while
Figure 5.6 shows a variety of shapes and forms that LEDs can take.
The following list is representative of compounds that with appropri-
ate additions of materials (doping) and the connection of the p-type and
n-type semiconductors created, result in emission of radiation after flow
of electricity. A general rule is that the energy difference increases (wave-
length of emission decreases) with increasing aluminum (Al) concentra-
tion and decreasing with increasing indium (In) concentration.
AlGaAs—red and infrared:

• AlGaP—green
• AlGaInP—high brightness, orange—red, orange, yellow, and green
• GaAsP—red, orange—red, orange, and yellow
• GaP—red, yellow, and green
• GaN—green and blue
• InGaN—near-ultraviolet, blue—green and blue
• SiC as substrate—blue
• Sapphire (Al2O3) as substrate—blue
• ZnSe—blue
• Diamond (C)—ultraviolet
• AlN, AlGaN—ultraviolet

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 130    ◾    Light Sources: Technologies and Applications

+
-
Figure 5.4  Light-emitting diode diagram (LED).

Figure 5.7 shows the emission spectra of three different LEDs in three dif-
ferent regions of the visible range. LEDs emit radiation of relatively nar-
row bandwidth, and this is depicted in Figure 5.8.
The efficiency of LEDs is defined by several factors such as

• The electric efficiency, which has to do with the number of charges in

the material (>90% achieved)
• The internal quantum efficiency, which is the number of photons per
number of electrons (this depends on the material and construction
of layers; heat and reabsorption are the main problems)

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 Solid-State Light Sources    ◾    131

Epoxy lens/case

Wire bond

Reflective cavity

Semiconductor die

Anvil
Leadframe
Post

Flat spot

+ –
Anode Cathode

Figure 5.5  Solid-state lamp–light-emitting diode (LED).

Figure 5.6  Variety of solid-state light sources (LEDs).

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 132    ◾    Light Sources: Technologies and Applications

400 500 600 700 800

Wavelength (nanometers)

Figure 5.7  Emission spectra of three different LEDs in three different

regions of the visible range.
• The extraction efficiency, which is the number of emitted photons
per total number of photons (the geometry of the material and cap-
sule plays an important role)
• The spectral or optical efficiency, which is related to the eye sensitiv-
ity curve (this factor is not taken into account for an LED emitting at
the limits of the curve)
20° 10° 10° 20°

30°
30°

40° 40°

50° 50°

LED

Figure 5.8  Light from LEDs is emitted in small solid angles.

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 Solid-State Light Sources    ◾    133

5.2 Organic LEDs
If the material used is an organic compound, then it is known as an OLEDs
(organic light-emitting diode). For the organic compound to function as
a semiconductor, it must have a large number of conjugated double bonds
between the carbon atoms. The organic compound may be a molecule
with a relatively small number of atoms, in crystalline form, or a polymer
(PLEDs), which offers the advantage of flexibility. For the time being, the
organic LEDs offer lower luminous efficiencies and average lifetimes than
their inorganic cousins.
OLEDs are steadily making their way into commercial devices such
as cell phones and flat-screen displays. They are fabricated with layers of
organic polymers, which make them flexible, and they use less power and
less expensive materials than liquid crystal displays.
The downside is that because the polymers react easily with oxygen
and water, OLEDs are expensive to produce—they have to be created in
high-vacuum chambers—and they need extra protective packaging layers
to ensure that once they are integrated into display devices, they do not
degrade when exposed to air or moisture.
OLEDs can be made from a wide range of materials (see Figures 5.9 and
5.10 for examples), so achieving good-quality white light is less challenging.
It has not been the quality of light that has let OLEDs down but rather their
efficiencies. Fluorescent lighting typically operates at around 60 to 70 lm/W,
while incandescent bulbs operate at about 10 to 17 lm/W. In contrast, the
best reported power efficiency of an OLED until now has been 44 lm/W.
OLEDs have the potential to grow into a very energy-efficient light
source. In production, levels of between 15 and 20 lumens per watt have

O O
N Al

O
N

Figure 5.9  Chemical structures of organic molecules with double bonds,

used for the development of OLEDs.

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 134    ◾    Light Sources: Technologies and Applications

H5C2 C2H5

H5C2 C2H5

Pd

H5C2 C2H5

H5C2 C2H5

Figure 5.10  Chemical structures of organic molecules with double bonds,

used for the development of OLEDs.

been achieved, but the ultimate potential is for the technology to reach
efficiencies as high as 150 lumens per watt.
A combination of these technologies can also lead to the future light sources.
One idea proposed is a hybrid light-emitting diode, or HLED. The device
would incorporate both organic and inorganic layers, combining the flexibil-
ity of an OLED with the stability of an inorganic light-emitting material.

5.3 LED White Light Emissions

White light can be created with different-colored LEDs (red, green, blue or
yellow and blue or four different colors) or by using a phosphor on a UV
or blue LED (UV LED with a trichromatic powder or a blue LED with a
yellow powder—YAG:Ce).
With three or more primary LEDs (Figures 5.11 and 5.12), all colors can
be created. Red LEDs are the most sensitive to temperature and, therefore,
corrections need to be made as the LEDs heat up. Moreover, the light inten-
sity and angle of incidence of each LED must match and mix appropriately
in order to create the white light correctly. The combination of blue and
yellow light also gives the impression of white light since the yellow light
stimulates the sensors of the eye that are sensitive to red and green, but the
resulting white light will be of low color rendering index.
The other method of creating white light without using more than one
LED, is to convert ultraviolet or blue LED light into different colors by
using a phosphor. The use of phosphor lowers efficiency due to Stokes
losses and other losses on the powder, but it still remains the easiest and
cheapest way of creating white light, while the color rendering is usually
better due to the larger spectral range of the powder. Another disadvantage

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 Solid-State Light Sources    ◾    135

Figure 5.11  (See color insert following page 20.) Combination of dif-
ferent color LEDs for the creation of white or dynamic lighting.

Figure 5.12  (See color insert following page 20.) Combination of dif-
ferent color LEDs for the creation of white or dynamic lighting.

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 136    ◾    Light Sources: Technologies and Applications

4000
Blue LED
3500

3000

2500
Intensity

2000 Yellow phosphor

1500

1000

500

0
300 350 400 450 500 550 600 650 700 750 800
Wavelength (nanometers)

Figure 5.13  Blue LED emission spectrum with phosphor that converts
part of the blue light into other colors/wavelengths (mainly yellow).
of using a powder is the issue of distribution of light. The light emission
angle from the crystal is different from that from the powder; hence, mix-
ing is not very good.
Apart from the UV LEDs that can be used with phosphors, blue LEDs
can also be coated with a powder that converts part of the blue light into
yellow (Figure 5.13). The ratio of blue to yellow can be controlled through
the quantity of the powder used, allowing us to control the color tempera-
ture of the source. However, this method gives us a source with a low color
rendering index, as there is a deficiency in red emissions, something that
can be an issue in some applications such as general lighting but not in
other applications such as signage. This method allows us to create white
light with a color temperature of up to about 5500 K, but with the addition
of another powder that emits in the red part of the spectrum we can also
create a warm white light temperature of 3200 K and better color render-
ing, at the cost of reducing the source efficiency.
The use of powder on a blue LED is the most economical way to cre-
ate white light, and there are even proposals to use a blue LED (InGaN)
with a green powder to replace the low-efficiency green LED or a
blue LED with a red powder to replace the temperature-sensitive red
(AlInGaP) LED.
The use of phosphors on an UV LED can give white light of significantly
higher color rendering index, but at the expense of efficiency (mainly due
to Stokes losses), similar to how a fluorescent mercury lamp operates. The

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 Solid-State Light Sources    ◾    137

different powders must be coated in such a way that there is no absorption

of each other’s emitted light. The powders used in fluorescent lamps are
not appropriate as they are stimulated by the mercury emission lines at
185 and 254 nm, while the UV LEDs emit at 360–460 nm.
With three LEDs, we have better control over color (dynamic light-
ing), while the use of phosphors gives stability and a better mix. There
are, of course, products that use two or three crystals in the same LED
with appropriate wiring in order to create different colors and have better
color mixing and control without the need for phosphors. This technol-
ogy, however, of many crystals in the same LED raises the cost due to the
separate control of each diode that requires more gear.
Whether one uses UV or blue LEDs with appropriate phosphors or
suitable semiconductors, a variety of colors and accents of white can be
produced today according to market demands.
To use three LEDs (each primary color) to create white light means that
they have to be controlled during operation as they wear out differently
and show different sensitivities to heat. The appropriate electronic and
optical components can provide this control. When using phosphors, one
cannot control or make corrections, and the increase in temperature shifts
the emission wavelengths of blue LEDs.
The following table (Table 5.1) lists the advantages and disadvantages
offered by the different ways of creating white light with LEDs.
A third method of producing white light without the use of fluorescent
powders is through combination of radiation simultaneously produced by
the semiconductor and its substrate (blue radiation from ZnSe and yellow

Table 5.1  Comparisons of Different Methods for Creating White Light

Advantages Disadvantages
Mixing different Dynamic lighting Different colors have different
color LEDs Ability to create millions of colors sensitivities to heat/no stability
Better efficacy Complex electronic gear
Control of component colors Not good color mixing
Blue LED with Good efficacy Not good color mixing at certain
phosphor Good color rendering index angles
Wide color temperature range No control or regulation of
Better stability different colors
Ultraviolet LED Good color mix Poor efficacy
with phosphor Wide color temperature range Low power
Good color rendering index Must manage UV light
Better stability No control or regulation of
different colors

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 138    ◾    Light Sources: Technologies and Applications

light from the ZnSe substrate). The absence of fluorescent powder means
higher efficacy.
Finally, a method that is being developed rapidly is the use of quantum
dots: nanocrystalline semiconductor materials with dimensions equal to
a few dozen atoms that emit light (fluoresce) with high efficiency under
electrical or optical stimulation. The wavelength of radiation can be con-
trolled by controlling the size of the nanocrystals, and this method is in
the experimental stage (see Figure 5.14 for related spectra).
This color-tailoring ability solves one of the major problems with using
LEDs for general lighting applications. LEDs are appealing because they last
for years, use perhaps 20% of the electricity of a standard incandescent bulb,
and are highly efficient at converting electricity into visible light instead of
into heat. But to make white light, you either have to mix together LEDs of dif-
ferent colors or use a blue LED coated with a phosphor that emits yellow light

1.4
1.2
Relative Intensity

1.0
Relative Intensity

0.2
0.8
0.6
0.1
0.4
0.2
0.0 0.0
400 500 600 700 400 500 600 700
Wavelength/nm Wavelength/nm
(a) (b)

1.4 1.4
1.2 1.2
Relative Intensity

Relative Intensity

1.0 1.0
0.8 0.8
0.6 0.6
0.4 0.4
0.2 0.2
0.0 0.0
400 500 600 700 400 500 600 700
Wavelength/nm Wavelength/nm
(c) (d)

Figure 5.14  Various emission spectra of white LEDs: (a) blue LED with
phosphor while the other three spectra, (b), (c), and (d), belong to dichro-
matic, trichromatic, and tetrachromatic quantum dot LEDs, respectively.

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 Solid-State Light Sources    ◾    139

Figure 5.15  An LED lamp comprising many LED units.

to produce a whitish mix. The problem with the phosphors is that they do not
emit evenly across the visible spectrum. They tend to have gaps in the green
section and even more so in the red, leading to the harsher, bluish light.
Due to their low levels of emitted light, a lamp must make use of mul-
tiple LEDs in order to be functional as a light source for general lighting.
Many LEDs are needed for most applications (Figure 5.15).
Most LEDs on the market operate at low power, usually less than 1 W,
but some products operate at powers as high as 7 W with an efficacy of
20  lm/W. The highest efficacy ever reported is about 130 lm/W, but for
very low-power LEDs. Generally, LEDs emit from 1 lm to several tens of
lumens, while there is now on the market a 5 W LED 5W with 120 lm of
white light at 350 mA. Depending on their flux, LEDs are categorized as
follows:

• Indicators with <5 lm

• Standard with 5–50 lm
• High Brightness (HB) with 50–250 lm
• Ultra High Brightness (UHB) with >250 lm

If LEDs reach 250 lm/W in the next two decades, they could replace all
fluorescent lamps that are currently limited to 50–120 lm/W.

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 140    ◾    Light Sources: Technologies and Applications

A general rule that has been stated and that so far seems to be hold-
ing is the doubling of light output from each LED every 24 months in
the last 40 years (similar to Moore’s law) and the halving of cost almost
every decade. This pattern was observed and described for the first time
by Roland Haitz. In most cases, LEDs operate with less than 100 mA, and
when the current exceeds that value, they are referred to as power LEDs.
For high-power LEDs, a silicon gel is used instead of a polymer epoxy coat-
ing, and an appropriate heat sink is used for proper heat management.

5.4 LED Operation
The LED is a diode; that is, electric charge flows only in one direction.
Once the LED starts to operate normally, the current is linearly propor-
tional to the voltage; that is, the relationship between voltage and current
for the LEDs is positive, as shown by Figure 5.16.

Vf = Vo + Rs If

Vo is the initial voltage that must be applied before charges start to flow.
The value depends on the material and the energy difference between
states (band gap). In general, the voltage is influenced by the temperature
of the diode. The resistance also depends on the material and is quite low.
During production, there are shifts in Vo and the resistance, so the cate-
gorization of the final products according to their electrical characteristics
(binning) is necessary as the light output is proportional to the intensity of
the current and, therefore, the voltage.

1000
900
Current Intensity (mA)

800
700
600
500
400
300
200
100
0
0.0 1.0 2.0 3.0 4.0 5.0
Voltage (V)

Figure 5.16  Voltage–current relationship in LED starting and operation.

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 Solid-State Light Sources    ◾    141

The two most important factors in limiting light emission are heat and
current density. At large currents, the materials wear out, mainly due to
high temperatures but also due to leakage of charges to the other layers
outside the active crystals (especially when the layer of the active crystal is
thin). The maximum values depend on the material but, in general, more
than 120 Α/cm2 is prohibitive for all cases. High temperature not only
damages the crystal but also causes shifts in the emitted spectrum. For
example, yellow can be converted to red at high temperatures just before
the destruction of the crystal and the diode junction. This change cor-
responds to about 0.1 nm per degree Celsius, something that is a great
disadvantage if color reliability is important.
LEDs generate heat during operation (not in emission), and the materi-
als are very sensitive to it. Thus, the thermal design is an important part of
the design of these light sources.
An LED is a low-power light source, so LED lamps cannot be connected
directly to the mains voltage as they need a current and voltage controller
to keep them at low values and also to be able to run several LEDs at once
(as shown in Figures 5.17 and 5.18). In other words, LED lamps need the
equivalent of ballast, which we call a driver.
The driver can be a constant voltage driver or a constant current driver.
The constant voltage driver is not as reliable because there is a dif-
ferentiation of the required voltage during production; and because the
resistance is small, variations in voltage result in significant variations
in intensity.
The constant current driver is preferable as the intensity is set to the
normal operating intensity of the LEDs, and the voltage is adjusted.
There is also a hybrid driver of constant voltage with a large resistor
parallel to the LED. However, the power consumption by the resistor ren-
ders such drivers inefficient.
A driver can be used for many LEDs, and it is important how they are
connected (series/parallel). If we have a constant voltage driver, then a
resistor should be added so that we have nominal current intensity, while
with constant current drivers, we need to take into account the number
of LEDs. In each case, the current intensity through each LED or series of
LEDs must be carefully estimated.
The position of each LED in a system plays an important role in the
functioning of the whole layout. If an LED fails, it can create an open cir-
cuit (e.g., in a series connection) or a closed circuit with different electrical

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 142    ◾    Light Sources: Technologies and Applications

Figure 5.17  (See color insert following page 20.) LEDs in series mainly
for decorative lighting.

Figure 5.18  (See color insert following page 20.) LEDs in series mainly
for decorative lighting.

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 Solid-State Light Sources    ◾    143

characteristics (e.g., if all parallel) such as higher current in one of the

series leading to asymmetric production of light.
A combination of series and parallel connections offers greater reli-
ability because the current can find alternative paths without overloading
some units.
Protection from large positive (normal flow) and negative (opposite
polarity) voltages is provided by high-voltage diodes in appropriate connec-
tions. LEDs withstand very short (<1 ms) and nonrecurring high-current
pulses (hundreds of milliamperes).
Of course, there are intelligent LED control systems that cannot only
create a wide range of colors by combining multiple light sources and offer
dynamic lighting, but can also take into account variations in temperature
or other electrical characteristics and make the appropriate corrections
and changes. There are many communication protocols to control lighting
systems and the choice depends on the application. It is important for an
intelligent LED driver to be able to receive and analyze the following list
of signals:

• Linear voltage control (0 to 10 V)

• Digital multiplex—DMX512
• Digital addressable lighting interface (DALI)
• Power-line communication (PLC)
• Domotic standards: INSTEON, X10, Universal Power-Line Bus
(UPB), and ZigBee

5.5 Thermal Management of LEDs

All light sources convert electrical energy into heat and radiation
emitted in varying ratios. Incandescent lamps emit mostly infrared
radiation and a low percentage of visible light. Low- and high-pressure
discharge lamps (fluorescent and metal halide, respectively) produce
more visible light but also emit infrared and ultraviolet radiation as
well as heat. LEDs do not emit infrared and undesirable ultraviolet
radiation and apart from visible light produced, the remaining energy
is converted into heat, which must be transferred from the crystal
to the circuit (as the capsule surrounding the LED is not thermally
conductive, the heat flows in the other direction), and from there to

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 144    ◾    Light Sources: Technologies and Applications

Table 5.2  Power/Energy Conversion for Different “White” Light Sources

Incandescent Fluorescent Metal Halide
(%) (%) (%) LEDs (%)
Visible light 10 20 30 15–25
IR 70 40 15 ∼0
UV 0 0 20 0
Emitted energy 80 60 65 15–25
Heat 20 40 35 75-85
Total 100 100 100 100

other parts of the unit, until finally it is transferred to the environment

through the air. See Table 5.2 for a comparison of different technolo-
gies regarding energy conversion.
The removal of heat happens first with its flow from part to part and
then from the surfaces to the environment. This means that many factors
play a role in how efficiently and quickly the heat is removed, and some of
these factors are as follows:

• The materials of which different parts are made (preferred materi-

als are those that are thermally conductive, that is, having low ther-
mal resistance such as metals, but new polymers also exhibit good
conductivity)
• Their connections (there should be a good contact between each
part, and gaps do not help because air is not a good conductor
of heat)
• Their total surface area (large areas with as low a volume as possible)

Heat production (and the temperature increase at the diode junction

which accompanies it) is the major limiting factor and the biggest obstacle
to developing LEDs of higher power and brightness. Therefore, the issue
of thermal management is currently perhaps the most important prob-
lem that scientists and technologists have focused on, and their efforts are
directed toward finding and using material of high thermal conductivity
to reduce as much as possible the thermal resistance R_ of the system so
that heat is removed as quickly and easily as possible.
The thermal resistance Rθ of an LED is defined as the ratio of the tem-
perature difference between the junction and the environment (ambient

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 Solid-State Light Sources    ◾    145

temperature) over the consumed power due to the current that flows
through the LED.

Rθ = (ΔTjunction − ambient/P)

Where
ΔT = Tjunction – Tambient
P = current intensity (I) * voltage (V)

The total thermal resistance must, of course, include the entire system
from the junction to the surfaces that are in contact with the surrounding
air. Each individual part of an LED is characterized by a different thermal
resistance (whose values depend on the geometry, material, and surface
area of each piece), and according to these we can define the thermal resis-
tance of the crystal, which is given by the manufacturers (the smaller the
value, the easier the transfer of heat). Figure 5.19 shows a thermal model
for an LED circuit that is analogous to an electrical circuit.
With the thermal resistance (obtained from the manufacturer) between
the junction and the material on which the crystal rests, and by measur-
ing the temperature differences between the other parts using infrared

TJunction

RJC

RCB RJA

RTIM
PLED

RH

TAmbient

Figure 5.19  Thermal model of an LED “circuit.”

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 146    ◾    Light Sources: Technologies and Applications

detectors or thermocouples, we can calculate the temperature at the junc-

tion for different current intensities and power values.
The thermal resistance, with units °C/W, is deduced from the thermal
conductivity (with unit W/mm), the length of the heat conductor, and its
cross-section. This practical parameter allows us to calculate various tem-
peratures at different parts of the system when the consumed power is
known. The model used is that of an electrical circuit, where the parallel-
isms are

Heat Q (W) ∼ Current intensity

Thermal resistance Rθ (°C / W) ∼ Electrical resistance
Temperature difference ΔT (°C) ∼ Voltage
The equivalent to Ohm’s law is ΔT = Q × Rθ

5.5.1 What Defines the Junction Temperature?

Light and heat are produced at the junction of the diode, which has small
dimensions, so the heat production per unit surface area is very large. The
temperature of the junction cannot be measured directly, but it can be
calculated by measuring the temperature of another part and taking into
account the thermal resistances of all materials.
There are three factors that determine the junction temperature of an
LED: the intensity of the operating current, the thermal resistance of the
system, and the ambient temperature. Generally, the greater the intensity
of the current, the more the heat produced in the crystal. The heat must
be removed in order to maintain the flux, the lifetime, and the color. The
amount of heat that can be removed depends on the temperature and the
thermal resistances of the materials that make up the whole LED.
The products on the market have a maximum temperature at which
they must operate, which is around 120°C. The efficacy and lifetime, how-
ever, begin to decline well before that temperature limit. Very few power
LEDs have the appropriate initial design that allows them to function at
maximum power without using a secondary cooling system. Temperature
increase without proper control and stabilization is certainly the main rea-
son for early destruction of LEDs. Although 120°C is given as the maxi-
mum operating limit, a more realistic limit is 80°C as one must take into
account that fluctuations of the ambient temperature can be of the order of
25°C or higher.

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 Solid-State Light Sources    ◾    147

1.20

Relative Luminous Flux (100%)

1.00

0.80
Blue
0.60
Green
0.40

0.20
Red
0.00
–20 0 20 40 60 80 100 120 140
Junction Temperature in Degrees Celcius

Figure 5.20  Effect of temperatures on lifetime and flux for different-col-

ored LEDs presented for known commercial products.

Manufacturers classify LEDs according to the luminous flux and color

under pulsed current (25 ms pulses), keeping the junction temperature con-
stant at 25°C. But under normal operating conditions, the junction temper-
ature is at least 60°C, so the flux and the color will be different from those
of the manufacturer’s specifications. The worst aspect is the LED thermal
management; the greater are the differences and the efficacy losses.
The rise in junction temperature has various consequences such as reduc-
tions in efficiency, life expectancy, and voltage value as well as shifts of the
radiation wavelengths; the latter, especially, affects the operation of mostly
white LEDs, causing changes in color temperature (see Figure 5.20).
In general, temperature affects each color to a different extent, with red
and yellow LEDs being the most sensitive to heat and blue being the least
sensitive. These different responses of each color to temperature can lead
to changes and instabilities of the white light produced by RGB systems if
during operation the junction temperature Tj is different from that speci-
fied by the manufacturer. But even when phosphors is used, the shifts are
significant because the powders are sensitive to specific wavelengths.

5.5.2 Overheating Avoidance
There are two ways in which we can control the junction temperature to
avoid a malfunction or premature destruction of an LED. One is to decrease
the intensity of the current, so that the LED is operated at a lower power,
and the other is to carefully design so that the overall thermal resistance
of the LED is minimized. The second method, although preferable, is not
so simple and requires us to take into account several parameters such as

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 148    ◾    Light Sources: Technologies and Applications

the total surface area of the materials in contact with the surrounding air.
The greater the contact area, the lower the thermal resistance.
A combination of both methods is common practice, but proper calcula-
tions must be done in each different case. Since the flux decreases with both
increasing temperature and decreasing current, and thus the power, we
must work out whether (taking into account the thermal resistance given
by the manufacturer) a decrease in power, which means operation with
lower brightness, ultimately pays. The voltage also depends on the junction
temperature, and it is reduced as the temperature increases. If we deal with
a single LED or several in a series, then one can control the current; other-
wise, there is a risk of having uneven distribution of the current intensity
due to temperature variations and, ultimately, destruction of the system.
The circuit connected to the LED may include some type of heat sink,
such as the MCPCB LEDs (metal-core printed circuit board), but higher-
power commercial LEDs are using extra heat sinks characterized by their
large size relative to the LED and their large surface area. A typical geom-
etry of such extra large heat sinks includes the use of fins that dissipate
heat more efficiently. Such a heat sink is incorporated in luminaires or can
be itself the luminaire. Finally, some LEDs have the circuit at a distance so
that the heat generated by the circuit does not contribute to the increase
of the junction temperature. See Figures 5.21 and 5.22, which depict LED
lamps with incorporated heat sinks.

Figure 5.21  LED lamps with heat sinks incorporated in the design.

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 Solid-State Light Sources    ◾    149

Figure 5.22  LED lamps with heat sinks incorporated in the design.

In addition to electrical, mechanical, and optical data, a compact model

for Flotherm software is now permanently available at www.osramos.
com/thermal-files for calculating thermal behavior. Customers can find
all the documents and the latest data needed for calculating thermal vari-
ables for different designs without building costly prototypes or carrying
out time-consuming measurements. The data are available for standard
high-power LEDs in the visible range, particularly for the Dragon family,
the Power TopLED range, the Advanced Power TopLED range, and some
Ostar variations.
The compact model available on the Web site is a simplified thermal
geometry model that can be integrated in Flotherm software and used for
customer-specific calculations. It is suitable, for example, in calculating
the temperature distribution in a planned system.

5.6 Dimming/Controlling the Brightness

An LED can be dimmed by controlling the current, and this can be done
in two ways:

• Increasing/decreasing the intensity (DC dimming)

• Pulse control

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 150    ◾    Light Sources: Technologies and Applications

Controlling the DC current intensity has some disadvantages, which are

described in the following text. Binning of LEDs takes place under the
operation current, so by changing the intensity there is no longer reliabil-
ity regarding common features between LEDs of the same category (pro-
posed dimming to ¼ of the intensity only). Low intensity also implies big
changes in parallel LEDs due to changes in voltage and, finally, changes in
intensity can lead to differences in color.
On the other hand, with pulse control, the maximum pulse current
intensity value is set at the normal operating value and, thus, no changes
occur in the characteristics, while the mean or average intensity is defined
by the frequency and duration of pulses. In this way, the LED works prop-
erly, and a linear relationship between current intensity and luminous flux
is ensured. For controlling the duration of the pulses, this is an easy task
because LEDs respond instantly (<μs).
The dimming ratio is defined as the minimum mean intensity value over
the maximum mean intensity of LED current. This percentage is deter-
mined by the shortest possible pulse that the driver can deliver and which
at its maximum reaches the nominal current intensity of LED operation.
The shorter pulse is in turn defined by the rise and fall times of the pulses.
This method makes dynamic lighting possible, and it is imperative that
the minimum frequency chosen should be one at which the source is com-
fortable to the human eye.
Of course, with a combination of pulse control and DC current inten-
sity checking, even smaller dimming ratios are possible.

5.7 General Characteristics of LEDs

A summary of the general characteristics of LEDs follows (see also
Table 5.4):

• Their emission spectra are narrow band (a few tens of nanometers).

• They are characterized by their low brightness (flux), starting from 1
lm for conventional LEDs and reaching 120 lm for high-power LEDs.
• Their efficacy is around 20–30 lm/W, which already exceeds that
of incandescent lamps but lags behind efficacies of discharge lamps
such as low-pressure fluorescent and high-pressure metal halides.
• For high flux, many LED units are necessary.
• Wide range of colors with RGB mixing.

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 Solid-State Light Sources    ◾    151

Table 5.3  As the Temperature Rises, Wavelength

Shifts are Different for Different-Colored LEDs
Color K (nm/°C)
Yellow 0.09
Red 0.03
Blue 0.04
Green 0.04

• They respond instantly without the switching frequency wearing

them out. LEDs with phosphor have a slightly slower response time
due to the powder fluorescence.
• Good brightness control and dimming rations reaching 1/3 with
current intensity decrease and 1/300 with pulse control (a few hun-
dred hertz).
• Their end of life is characterized by a gradual decrease of the lumi-
nous flux and is not sudden.
• They are sensitive (and wear out if exposed) to heat and static elec-
tricity, and, in the case of blue and ultraviolet LEDs, to radiation.
Generally though, they are characterized by their material strength
in contrast to the fragile technology of glass–gas.
• They are characterized by their small size, which means freedom in
luminaire design. For large fluxes, however, the luminaire must also
be large.
• Their average lifetime (70% of initial lm value) is 50,000 h. The end
of life depends on defects in the crystalline structure of the semicon-
ductor or the fluorescent powder.

Table 5.4  General Characteristics of LEDs

Efficacy (lm/W) <130, most around 20–30
Power (W) 0.1–7
Color temperature (Κ) Wide
Color rendering index Up to 90
Lifetime 50,000–100,000
Applications Signage, remote control, fiber-optic
communication, decoration,
advertising,

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 152    ◾    Light Sources: Technologies and Applications

• There is no harmful radiation in the ultraviolet and infrared parts of the

spectrum, which is an advantage compared to other lamps, but sensitiv-
ity and reduced efficacy at elevated temperatures caused by the current
flow are disadvantages. Almost all LEDs have an upper limit of 125°C.
• LEDs are still expensive, not just because of their materials and man-
ufacturing cost but also due to their low luminous flux. Nevertheless,
they offer a low power consumption solution in applications that
require low levels of light.

5.8 Applications
Applications in which LEDs have the advantages are

• Applications that require light of specific color. LEDs produce specific

colors more efficiently than by putting filters in incandescent lamps.
• Applications in which long lamp lifetimes are required due to dif-
ficulty or high cost in replacements.
• Wherever small-sized light sources are required, such as decorative
lighting or small spaces (mobile phones, car interiors, etc.).
• Wherever instant start and dimming are necessary.

Applications where LEDs have disadvantages and must not be widely used
yet are the following:

• In high-temperature environments
• Where high brightness is required
• Where color stability is necessary
• If accurate stability of color temperature and color rendering index
is essential
• If good knowledge of lifetime and lumen depreciation is needed

The goal of LED companies is for this light source technology to dominate
the following applications in the near future:

• TV and computer large screens

• Small projection systems

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 Solid-State Light Sources    ◾    153

• Car headlights
• Interior lighting

Organic/polymer LEDs (OLEDs/POLEDs) will also play an important

role here, but for now they are characterized by short lifetimes.
The different applications can be categorized into those where visual
contact with the source is necessary (signage) and those where the reflected
light is used (general lighting).

5.8.1 Signage—Visual Contact
In applications in which high levels of brightness are not needed but the
creation of optical signals of specific color is the aim, LEDs have already
started dominating the market. Some of those applications are the follow-
ing (LED characteristics that offer the advantages are in brackets):

• Traffic lights (color, sturdiness)

• Car back lights (style, size, electrically compatible) as shown in
Figure 5.23
• Car interior (size, mercury-free)
• Decorative lighting (size, dynamic lighting, dimming) as shown in
Figure 5.24

Figure 5.23  Use of LEDs for automotive brake lights.

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 154    ◾    Light Sources: Technologies and Applications

Figure 5.24  Decorative lighting with strips of red LEDs.

• Monitors and screens (color mixing)

• Road signs (long lifetime, size)
• Mobile phones (size, low voltage)

5.8.2 General Lighting—No Visual Contact with the Source

There are some applications of general lighting where LEDs are preferred
for various reasons:

• Dental treatments/bleaching (blue color, size, long lifetime, replace-

ment of low-efficiency halogen lamps)
• Torches (size, low voltage)
• Architectural lighting

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 Solid-State Light Sources    ◾    155

• Machine sensors (various geometries, durability, sturdiness)

• Telecommunications (instant start)

LEDs are used in a wide range of applications ranging from building light-
ing to large panel displays.
For LEDs to dominate in other markets, the following characteristics
must be improved or changed:

• Crystal materials and layer geometries so that efficacy is increased

(internal and extraction)
• The materials of all parts of the unit so that they withstand higher
temperature; thus, higher power inputs in order to increase the
brightness/luminous flux
• Cost, which can be lowered with the help of the aforementioned
changes
• Better quality of white light with higher color rendering index and
wider range of color temperature
• The production line, so that there is greater reliability and no varia-
tion in efficiency

5.9 Closing Remarks
The future of LEDs and perhaps of lighting in general will depend on
the outcome of various research paths that scientists around the world
are following or must follow. It is essential that a better understanding
of the light-generation mechanism and improvements of internal quan-
tum efficiency be achieved. LED manufacturing techniques must also
improve so that consistency and better quality can lead to better market-
ing. Apart from the actual junction, other parts of an LED are also crucial
and require R&D, such as substrates, packaging, and lenses with proper
thermal management. The ultimate goal is not only to increase the effi-
cacy of each LED but also to achieve long lifetimes and tolerance to high
temperatures. In addition, it is important to develop phosphors capable
of absorbing and converting photons efficiently and, as is the case already
for fluorescent lamps, the development of “quantum-splitting phosphors”
could be a breakthrough for LEDs.

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 156    ◾    Light Sources: Technologies and Applications

The technology of OLEDs also has the potential for rapid growth and
market penetration as the great variety of organic luminescent materials
could give a large number of emitting colors.
As with any other technology, high efficiencies, consistency, long life-
times, good color stability, uniformity over large surfaces, and relatively
low costs are the characteristics desired for a technology to succeed.

References and Useful Links

• Schubert, E.F., Light Emitting Diodes, Second edition, Cambridge
University Press, 2006. United Kingdom.
• A. Zukauskas, M.S. Shur, and R. Gaska, Introduction to Solid State
Lighting (Wiley, New York, 2002).
• www.LightEmittingDiodes.org.
• www.lumileds.com.
• www.lrc.rpi.edu/researchareas/leds.asp.
• www1.eere.energy.gov/buildings/ssl/basics.html.
• scitation.aip.org/journals/doc/PHTOAD-ft/vol_54/iss_12/42_1.
shtml.
• www.netl.doe.gov/redirect.
• www.maxim-ic.com/appnotes.cfm/appnote_number/1883.
• www.ecse.rpi.edu/∼schubert/Light-Emitting-Diodes-dot-org.
• www.intl-lighttech.com/applications/led-lamps.
• ieee.li/pdf/viewgraphs_lighting.pdf.
• http://trappist.elis.ugent.be/ELISgroups/lcd/tutorials/tut_oled.php.
• http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf.

Standards
IESNA LM-79-08
ANSI C82.2 (efficacy)

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 Solid-State Light Sources    ◾    157

IESNA LM80-08 (lumen depreciation)

ANSI C78.377A (CRI)
Τest standards
ANSI C82.77-2002 (PFC)
EN61000-3-2 (harmonics)
EN61000-3-3 (flicker and voltage variations)

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