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563

Coherent

Part C

Part C Coherent and Incoherent Light Sources

10 Incoherent Light Sources

Dietrich Bertram, Aachen, Germany

Matthias Born, Aachen, Germany

Thomas Jüstel, Steinfurt, Germany

11 Lasers and Coherent Light Sources

Orazio Svelto, Milan, Italy

Stefano Longhi, Milano, Italy

Giuseppe Della Valle, Milan, Italy

Stefan Kück, Braunschweig, Germany

Günter Huber, Hamburg, Germany

Markus Pollnau, Enschede, The Netherlands

Hartmut Hillmer, Kassel, Germany

Stefan Hansmann, Darmstadt, Germany

Rainer Engelbrecht, Erlangen, Germany

Hans Brand, Erlangen, Germany

Jeffrey Kaiser, Mountain View, USA

Alan B. Peterson, Mountain View, USA

Ralf Malz, Jena, Germany

Steffen Steinberg, Jena, Germany

Gerd Marowsky, Göttingen, Germany

Uwe Brinkmann, Bovenden, Germany

Dennis Lo

†

, Hong Kong, P. R. China

Annette Borsutzky, Kaiserslautern, Germany

Helen Wächter, Zurich, Switzerland

Markus W. Sigrist, Zurich, Switzerland

Evgeny Saldin, Hamburg, Germany

Evgeny Schneidmiller, Hamburg, Germany

Mikhail Yurkov, Hamburg, Germany

Katsumi Midorikawa, Saitama, Japan

Joachim Hein, Jena, Germany

Roland Sauerbrey, Dresden, Germany

Jürgen Helmcke, Braunschweig, Germany

12 Femtosecond Laser Pulses: Linear Properties,

Manipulation, Generation and Measurement

Matthias Wollenhaupt, Kassel, Germany

Andreas Assion, Vienna, Austria

Thomas Baumert, Kassel, Germany

565

Incoherent Li

10. Incoherent Light Sources

Since the invention and industrialization of

incandescent lamps at the end of the 19th

century electrical lighting has become a com-

modity in our daily life. Today, incoherent

light sources are used for numerous appli-

cation areas. Major improvements have been

achieved over the past decades with respect

to lamp efﬁciency Fig. 10.1, lifetime and color

properties.

In the following chapters an overview of var-

ious lamp types and their properties is given.

They are subdivided by light generation mech-

anism: thermal emission of radiation close

to thermal equilibrium (incandescent lamps),

atomic and molecular emission in gas dis-

charge lamps, and emission from solid-state

light sources (LEDs).

10.1 Incandescent Lamps ............................. 565

10.1.1 Normal Incandescent Lamps .......... 565

10.1.2 Tungsten Halogen Lamps .............. 566

10.2 Gas Discharge Lamps ............................ 566

10.2.1 General Aspects ........................... 566

10.2.2 Overview of Discharge Lamps......... 567

10.2.3 Low-Pressure Discharge Lamps ...... 567

10.2.4 High-Pressure Discharge Lamps ..... 570

10.3 Solid-State Light Sources....................... 574

10.3.1 Principle of Electroluminescence .... 574

10.3.2 Direct Versus Indirect

Electroluminescence ..................... 575

10.3.3 Inorganic Light-Emitting Diodes

(LEDs) ......................................... 575

10.3.4 Organic LEDs ................................ 578

10.4 General Light-Source Survey.................. 581

References .................................................. 581

10.1 Incandescent Lamps

The incandescent lamp is the oldest electrical light

source still in widespread use. It can be found in almost

any application, especially where comparatively small

lumen packages are required and where simplicity and

compactness are favored.

10.1.1 Normal Incandescent Lamps

Incandescent lamps produce light by the electrical heat-

ing of a metal wire to such a high temperature that

radiation in the visible part of the spectrum is emit-

ted [10.1]. The metal wire is mounted in a glass bulb

ﬁlled with an inert gas (Fig.10.2).

According to Planck’s law the ﬁlament must be

heated up to at least 2400 K for a white emission color.

In view of the efﬁciency for converting electrical en-

ergy into visible light, even higher ﬁlament temperatures

would be favorable. Unfortunately, increasing the tem-

perature reduces lamp life due to enhanced evaporation

rates of the metal. Since tungsten has a high melting

point and a low vapor pressure it permits high operating

temperature and consequently higher efﬁciencies can be

attained than by any other metal. A possible measure to

improve lifetime is the reduction of the tungsten evapo-

rationrate by theaddition of raregases (Kr, Xe).Alterna-

tively, halogens are used in a so-called regenerativecycle

Efficacy (lm/W)

Year

1870

2000

200

175

150

125

100

1880

1890

1900

1910

1920

1930

1940 1960

1970

1980

19901950

LP sodium

HP sodium

HP metal halide

LP fluorescent

HP mercury

White LED

Incandescent/Halogen

LP compact fluorescent

Fig. 10.1 Temporal development of the luminous efﬁcacy

of electrical light sources (LP = low pressure, HP = high

pressure)

Part C 10

566 Part C Coherent and Incoherent Light Sources

Fill gas

Filament

Bulb

Support wires

Lead-in wires

Stem

Fuse

Lamp cap

Fig. 10.2 Schematic drawing of an incandescent lamp

effectively to transport tungsten back to the ﬁlament. An

increasein luminous efﬁciencyis achievedby coiling the

tungsten wire. A coiled ﬁlament allows higher operation

temperatures for a speciﬁed lifetime (typically 1000 h).

Most common operation parameters yield luminous ef-

ﬁciencies between 8lm/W and 17 lm/Watﬁlament

temperatures of 2400K and 3100 K, respectively. These

values correspond to an energy efﬁciency of a few per-

cent. Incandescent lamp wattages range up to 2000W.

Fig. 10.3 Principle of the chemical transport cycle in tung-

sten halogen lamps

10.1.2 Tungsten Halogen Lamps

In a normal incandescent lamp, tungsten evaporates off

the ﬁlament and condenses on the bulb wall, result-

ing in so-called blackening. Halogen lamps comprise

a halogen, i. e. iodine, bromine, chlorine, added to the

normal gas ﬁlling. These form volatile tungsten com-

pounds at the glass wall, which are transported back

to the hot ﬁlament. Here, the tungsten halides are de-

composed enabling a so-called chemical transport cycle

(Fig. 10.3).

By reduction of the net tungsten evaporation rate,

the ﬁlament of halogen lamps can be operated at higher

temperature compared to standard incandescent lamps.

Thus, luminous efﬁciencies can be increased for reduced

lamp size. This allows their application in compact re-

ﬂectors. Halogen incandescent lamps are available up to

2000 W with luminous efﬁcacies up to 25 lm/W. This

value has recently been improved to 35 lm/Wbycoat-

ing the glass bulb with infrared reﬂective multilayers,

and lifetimes up to 2000h were obtained.

W + 2Br WBr

Filament

Bulb wall

W+ 2Br

WBr

2Br

1500 K

10.2 Gas Discharge Lamps

10.2.1 General Aspects

A gaseous discharge is obtained by driving an

electric current through a gas, typically present

between two electrodes. Alternatively, electrodeless

microwave-exciteddischargesand pulsed dielectric-bar-

rier discharges (e.g. used for plasma displays) are known

as incoherent light sources.

The actual carriers of the electric current in the

gas are electrically charged particles, positive ions and

negative electrons. In a neutral nonconductive gas the

number of charge carrying particles is extremely small.

These particles can be released from the ﬁll gas or the

cathode surface by energetic collisions. Many physical

factors inﬂuence the properties of a gas discharge, the

most important ones being the type and pressure of

the gas, the electrode material, the operating tempera-

ture of the electrodes, the shape and surface structure

of the electrodes, the distance between the electrodes,

the geometry of the discharge vessel, and the cur-

Part C 10.2

Incoherent Light Sources 10.2 Gas Discharge Lamps 567

rent density. For the purpose of light generation, two

main types are distinguished: low-pressure and high-

pressure discharge lamps. For lighting applications,

both are operated in the arc discharge mode, which is

characterized by high current densities



> 1A/cm



To limit the discharge currents, electronic ballasts are

used [10.2].

In a low-pressure discharge lamp (gas pressure typ-

ically less than 100 Pa) the electrons have a mean

free path length larger or of the order of the ves-

sel diameter (e.g. a few cm). Due to low collision

rates with the neutral gas atoms they gain high en-

ergies (> 1 eV) from the applied electrical ﬁeld and

effectively excite the cold atoms by inelastic collisions.

The electrons and atoms are not in thermal equilibrium

(Fig. 10.4).

In high-pressure discharge lamps the operating pres-

sureis typicallyin therange between 10kPaand 10 MPa.

Here, collisions between electrons and atoms or ions are

much more frequent, resulting in a thermal equilibrium

that is characterized by equal particle temperatures.

In low-pressure discharges atomic line radiation

is emitted preferably from resonance transitions of

the element with the lowest excitation potential (e.g.

Hg: 185 nm and 254 nm, Na: 589 nm). In high-pressure

lamps various contributions to the spectrum are ob-

tained: broadened atomic lines (resonance-, van der

Waals- and Stark-broadening), molecular radiation

bands and quasi-continuous emission due to free–

free (Bremsstrahlung) and free–bound (recombination

of electrons with ions and atoms) transitions. As

a result of these quasi-continuous spectra, the color

Sulphur

High pressure

microwave

Rare gas

Low pressure

580 – 720 nm

74 nm

(Phosphors)

p ≈ 0.5 bar

DBD, PDP

Xe/Ne

147 + 172 nm

Phosphors

High pressure

Sodium

Low pressure

Na/Ar/Ne

Na 589 nm

High pressure

Na/Hg/Xe

High pressure

p > 1bar

Hg/Ar

•p ≈ 20 bar

•p ≈ 200 bar (short arc)

Metal halide lamps

• 3line radiators

NaX/TIX/InX, X = I, Br

• Multi-line/Molecular

NaX/TIX/REX

RE = Dy, Ho, Tm, Sc

SnX

Mercury

Low pressure

p < 1 mbar

Hg/Ar

Hg/Ne

185 + 254 nm

(Compact)

Fluorescent

lamps

Phosphors

Fig. 10.5 Overview of gas discharge lamps

Temperature (K)

Pressure (Pa)

–1

10 10

Fig. 10.4 Relationship between electron temperature, gas

temperature and gas pressure

rendering properties of high-pressure discharge lamps

are fair to excellent, depending on the type of ﬁll-

ing [10.3].

10.2.2 Overview of Discharge Lamps

In Fig.10.5 the most relevant types of discharge lamps

are displayed. The light sources are distinguished with

respect to their emission spectra and application ﬁelds.

In the case of low-pressure mercury and xenon excimer

lamps luminescent materials are applied for conver-

sion of ultraviolet (UV) radiation into visible light.

The temporal improvement in efﬁciencies is depicted

in Fig. 10.1.

Part C 10.2

568 Part C Coherent and Incoherent Light Sources

10.2.3 Low-Pressure Discharge Lamps

The most widely applied radiators in low-pressure dis-

charge lamps are Hg and Na. These elements are the

best choice with respect to efﬁciency for the conversion

of electrical input power into radiation.

Low-Pressure Mercury Lamps

The working principle of a low-pressureHg (also known

as a ﬂuorescent lamp) is given in Fig. 10.6. Fluorescent

lamps are generally designed in the form of a linear or

bent tubular bulb with an electrode sealed in at each end

(electrodeless versions are available where the electrical

energy is coupled inductively into the discharge ves-

sel via metal coils). The discharge vessel is ﬁlled with

an inert gas (typically Ar) and a few mgHg. Since the

major part of the emission of Hg atoms (97%) at low

pressures (e.g. 5 Pa) is in the ultraviolet, the inner sur-

face of the bulb is coated with a ﬂuorescent powder or

phosphor, which converts the UV radiation into visible

light. The composition of the phosphor determines the

spectral power distribution and the color of the emitted

light.

The generation of UV photons is due to transitions of

Hg atoms between the excited state levels

and

the ground state level

. About 64% of the electrical

input power is converted into photons at a wavelength

of 185 nm and 254 nm. According to the transitions in-

dicated in Fig. 10.7 only 3% are emitted in the visible

part of the spectrum. As a result of the Stokes shift, the

overall efﬁciency is only 28%. This value corresponds

to a luminous efﬁciency of 100lm/W.

Typical lamp parameters for ﬂuorescent lamps are:

electrical input power up to 140 W, luminous efﬁciency

up to 100 lm/W, color temperature between 2700 K

and 8000 K. Lamps are available in a large variety

Ultraviolet

radiation

Visible radiationElectrode

Electrons Mercury atom

Fluorescent powder

Fig. 10.6 Sketch of a low-pressure mercury discharge (ﬂuorescent) lamp

Energy level above stable level

Ionization

level

Metastable

level

313 nm

Various

excited

levels

Stable level

366 nm

577 nm

1208 nm

1014

408 nm

436 nm

546 nm

185 nm

254 nm

Fig. 10.7 Simpliﬁed energy-level scheme and radiative

transitions of the Hg atom

of geometries, e.g. cylindrical or circular or U-shaped

tubes. The latter are also known as energy-saving or

compact ﬂuorescent lamps. Depending on the con-

struction of ﬂuorescent lamps, the lifetime ranges

from 5000 to 25 000 h.

Fluorescent Coatings

The most important component of a ﬂuorescent lamp is

the ﬂuorescent powder. It is coated onto the inner side

of the glass tube. The powder consists of one or several

luminescent materials (phosphors), which are in general

inorganic compounds doped by transition metals



e.g.

Part C 10.2

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