david wharmby

8/7/2019 David Wharmby

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Toulouse COST 2002 page 1

Light Source Technology

Science and technology of light sources

State of the art

Future Challenges

David Wharmby

Technology Consultant Ilkley, UK

[email protected]


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Outline

Basic quantities related to light

Types of light sources

Incandescent sources

Discharges

HID

LP

Semiconductor sources

Lighting systems

Key areas for work

Developments depended on

advances in materialscomputer modelling to aid research & design

integrated design of complete light system

Materials advances System Design - Models

Further reading

Lamps and Lighting, 4th edition, edited by J R Coaton and A M Marsden,(London:Arnold), 1997

Up-to-date description of lamps types, major processes involved in producing light,

photometric and colour issues and circuits for lamp operation.

Electric Discharge Lamps J F Waymouth, (Cambridge:MIT Press), 1971.

The classic book. Despite its age, many of the explanations of lamp operation and

electrodes have never been bettered.

International Symposium on the Science and Technology of Light Sources

LS-8

8th International Symposium on Science Technology of Light Sources, Greifswald,

Germany, August 1998

LS-9 9th International Symposium on Science Technology of Light Source, Ithaca, NY

USA, August 2001

These volumes contain many papers of interest to researchers on light sources.


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Light

- What are we selling?

illumination OR illuminance - both related to luminous flux

colour and constancy of colour

minimal flicker effects

stable operation with agreed electrical characteristics

- Eye sensitivity and luminous flux

Photopic eye sensitivity is standardised V()

Used to weight the spectrum P() in (W/nm)

Subscript vimplies eye-sensitivity weighting

- Colour vision

Colour appearance has related weighting

functions

Good colour rendering is critical for most

lighting applications

can be achieved with narrow bands

Good colour uniformity and stability are

essential

0

0.5

1

400 500 600 700

wavelength (nm)

relativeoutput

)()( VP

)(V

)(P

=760

380

)()(683)( dVPv

Vision

The photopic eye-sensitivity curve applies to vision at normal light levels. It is the

relative sensitivity for cone vision. The scotopic sensitivity curve applies to low light

level vision, which uses the rod cells in the retina. These are examples ofAction

Curves. Every process - sun tanning, photosynthesis, curing of printing ink etc. - has

its own specific action curve.

The process is always the same - measure the spectrum of the source P() in W/nmas a function of the wavelength . Then weight the spectrum with the action curve

and integrate numerically over a sensible wavelength range to give the effect of the

source for that particular action curve. The result is in action curve weighted watts.

For historical reasons the photopic-eye-sensitivity weighted-watts are multiplied by a

683 and called lumens.

Colour

Colour vision is also expressed by weighting functions and we like to see lamps that

have white colours in the colour temperature range 2500 to 6500K. The spectrum

needs also to have good colour rendering (preferably the CRI > 80). The initial colour

uniformity between lamps must be largely imperceptible, as must the colour shift

through life.

See Lamps and Lighting chapters 1-3.


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Illumination

In most lighting we need to perform a task on an illuminated area (reading, walking about etc.)llluminance Ev is the key quantity

Ev = luminous flux per unit area (lm/sq.m or lux)

Analogous to irradiance Ee (radiated power per unit area)

For an area dA illuminated with flux d

Ev= d/dA

For an extended source luminance of all elements of source contribute

A

A

point source extended source

System design

Illumination optics

For many lighting applications sources are used to produce illumination on a floor, wall

or work surface. Illuminance is the key quantity. For example, lighting designers will

generally specify the maximum and minimum levels of illuminance in an installation

such as an office. This can then be calculated from the luminous flux of the lamps, the

optical properties of the fittings, and the reflectance and shape of the room. In turn this

allows the designer to decide how many lamps and fittings are needed.

For definitions of radiometric and photometric quantities see, for example

Optical radiation measurements, volume 1, Radiometry Grum F and Becherer R J,

Academic Press, 1979 (see chapter 2).

For every radiometric quantity there is an analogous photometric quantity in which

power (W) is replaced by flux (lumen). The same symbols are used for each, butsubscript e means radiometric and subscript v means photometric.


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Projection applications

Key quantity is radiance Le -related to brightness

- Radiance is conserved along a ray (or decreases in a lossy system)

Le= d2/dAcosd (W m-2 sr-1)

- Lv is visible analogue (lm m-2 sr-1 or cd m-2)

- Flux in the beam is =GL(W or lumen)

- G is called called geometrical extentoretendue

geometric property of the beam that is conserved (or increases) along ray

etendue is conversion factor from radiance to flux

- GAfor small area and solid angles

source area A

dA

optics

In applications, etendue may fixed by LCD gate (or fibre-optic) requirements

If source etendue is greater than this, some light will be lost.

System design

Projection optics

In projection applications the requirement is usually to maximise the utilization of the

light from the lamp. We know intuitively that we need a small source with very high

brightness. This applies to automobile headlamps, LCD projectors and projection TV.

Similar requirements apply to optical fibre illuminators.

This intuitive feel is actually based on the second law of thermodynamics which

ensures that the brightness (more strictly the radiance) of an image cannot exceed the

brightness of the object.

Radiance Le (and its photometric analogue Lv) is the fundamental quantity. It is

conserved along a ray (or decreases if the optical system has losses).

Le= d2/cosdAd (W m-2 sr-1)

for a source of projected area dAcospassing light into a solid angle d.

The geometric quantity G is known as the etendue orgeometric extent.

G = cosdAd (m2 sr)G also conserved along a ray (or increases as a result of aberrations, scatter & other

optical imperfections).

In a lossless system e= G Le - so G is the geometricquantity that converts radiance

to flux.

Etendue can be calculated for the beam in a projector. A low etendue (small area

and/or small solid angle) means that that the beam can be coupled into a small area

light valve. Since sources generally emit into large solid angles, this means source

emitting area must be very small. To achieve the required projected flux the luminance

must also be very high.

See Stupp E H and Brennesholtz M S, Projection Displays, Wiley, 1999, chapter 11

For careful definition of geometric extent see Grum and Becherer (see previous slide).


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Essential properties of sources

Luminous flux (illumination)

Radiance / luminance (projection applications)

Uniformity (back lighting)

Efficacy (lumen/W)

Colour appearance

Colour rendering

Colour uniformity

Colour stability

Lack of flicker

Instant light

Small size and weight

Life

But there must be an acceptable cost/benefit ratio


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Light generationMost artificial light is the result electronic transitions between the energy levels in materials

In incandescent and discharge lamps this is the result of accelerating electrons so they havea distribution function in which velocities are nearly random

- fundamental radiation limit is Planck distribution at electron temperature Te

High electron temperature needed for efficient generation in visible region

- incandescent < melting point (3650K for W)

- HID 4000 7000K (LTE discharges)

- LP 10000 20000K (non-LTE discharges)

Planck radiance (W m-2 sr-1 nm-1)

0

20000

40000

60000

80000

200 600 1000 1400 1800

wavelength (nm)

radiance 3000 K

5000K

7000K

visible

Radiating processes

Whatever the radiating process the radiance may not exceed the Planck radiance at

the electron temperature. In order to achieve high radiance high temperatures are

essential.

Incandescence

In incandescent lamps the electrons are accelerated by the applied field. Some

electrons gain enough energy to excite atoms, which then radiate. Because of the

extremely high density in the solid the atomic lines are highly broadened (forming

bands). Electrons make collisions with the lattice and excite phonons that cause IR

emission from the vibrational energy levels. The very great optical thickness means

that light is emitted from the surface layers. Therefore the band structure of the layers

near the surface is important in determining emittance. Predictions of emittance are

only just becoming possible (see LS9 paper (not included in Abstracts) by Novikov D et

al. (Arthur D Little Inc.), Modelling of emissive properties of materials in search of

improved incandescent light bulb filaments).Discharges

In gas discharges electrons form a near-Maxwellian energy distribution. Atoms, ions

and molecules are excited by the fast electrons and radiate. The higher the electron

temperature, the greater the populations (see Lamps and Lighting chapter 5). At high

pressures the electron temperature and gas temperature converge, populations are

determined by Boltzmann factors, and other LTE conditions apply.

At high pressure line broadening becomes increasingly important in the light emission.

Molecular emission and radiation from colliding atoms (forming temporary molecules)

may also be very important. These processes dominate in tin halide, very high

pressure sodium and extremely high pressure mercury arcs.


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Light generation devices

Incandescent lamps

- metal ions fixed, electrons mobile, neutral overall

- electrons excite lattice and ions, radiation emitted

- density is so high that spectrum is continuous - mostly IR Planck limited

Discharge lamps

- light is generated in a plasma (equal numbers of + and -)

- electrons form near Maxwellian distribution

- fast electrons excite atoms & molecules, which radiate

- radiation is from atomic lines and molecular bands - Planck limited

LEDs

- drive electrons and holes into p-n junction

- e and h recombine and MAY emit light

- light is extracted through absorbing medium with high index

- carriers not randomised so not Planck limited

Phosphors

- conversion of UV to visible in ionic site in lattice; lattice is heated, 50% loss

- not Planck limited

- quantum splitting phosphors are possible


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Incandescent lamps

Tungsten is most used filament material- melting point MP 3650K

- selective emittance increases efficacy

Other filament materials

- attempts to find replacement

- higher temperature operation

- more selective emission

- predictive calculations now possible (LS9)

- high emittance fibres are brittle and coatings evaporate

- look for high selectivity but operate at low temperature

Tungsten-Halogen cycle

- removes evaporated W from walls

- permits 100K increase in filament temperature for similar lifeand wattage

- chemical cycles to return W to hot filament have not been

successful

Emittance of tungsten at 2800K

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

100 1000 10000

wavelength (nm)

emittance

Tungsten-halogen concept has spawned many innovations


Tungsten and other filament materials

Tungsten appears to be the supreme material for filaments. There is a huge base of

R&D that helps to keep it pre-eminent.

It can be operated at temperatures close to the melting point. Like all metals the

emittance falls in the IR and this gives an important efficacy advantage.

Many attempts have been made to find materials that can survive at higher

temperatures than the melting point of tungsten. They also need to be reasonably

selective in their emittance. No one has yet found a material that is (a) sufficiently

conducting (b) sufficiently robust and (c) does not degrade or evaporate (if a coating).

Other approaches have been to pattern the surface of tungsten in a matter that

enhances selectivity (Waymouth J F, J. Light Vis. Env., volume 13(2) pp 51-68, 1989.

Another approach that has not been tried was suggested to me Dr J R Coaton: if the

emittance is highly selective, then low temperature operation might still result in a

higher lamp efficacy. A coating on tungsten may be sufficiently stable if operated at

low enough temperature (see Bigio LS8 paper B08).Tungsten-halogen lamps

Additions of halogens to tungsten lamps keep the walls clean. The bulb can be smaller

and stronger and this allows the inert gas pressure to be higher so that tungsten

evaporation is suppressed. This allows temperature to be increased by about 100K

for similar wattage and life, with an increase of about 20% in efficacy. Perhaps more

important is that a vast range of products has been generated because of the reduction

in envelope size. The design often involves integrated optics and electronics.

The option of returning the tungsten to the hot parts of filament using a halogen cycle

offers the possibility of a highly efficient lamp with long life. Only fluorine is capable of

doing this and no one has been able to make tungsten-fluorine cycles work properly.


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Reducing IR losses

2000 30002500 3500

0.2

0

0.6

0.4

tungsten temperature (K)

fraction of radiated power

IR 750-2000nm

vis


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Discharge lamps

-

Planon

dielectric

barrier

metal halide

sulphur

Genura

QL, Endura

electrodeless

inductive

microwave

metal halide

metal-

high pressure

low Te, high nehigh brightness

fluorescent

all types

fluorescent

backlights

low pressure

high Te, low nelow brightness

hot cathodecold cathodedischarge type


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Te ~Tg

~ 5000K max.

-R R0

Structure of a HP discharges

All particles at the same temperature - LTE appliesSome lines reach Planck limit and are self-reversed

Lamp voltage depends on partial pressure of most

easily ionised vapour

Hg added to reduce thermal conduction losses and

adjust lamp voltage

A small number of elements are sufficiently volatile,

but many elements have volatile halides: examples

are the iodides of Tl, In, Sc, Dy, Al, Sn.

NaI

Na

Na+

liquid NaI

Hg at highpressure

Na*

Temperature profile sets itself so that arc centre is hot

enough to provide electrons so that current satisfies

circuit equations

Fundamental limitationInevitable temperature gradient ensures conduction loss

Radiation efficiencyrad = Prad/Pin 40% to 60%

Look for ways to increase Te involve circuit?

System design

Principles of HP discharges

The surface of the sun is a hot gas and this is why it glows. High pressure discharges

generate volumes of hot gas and they also glow, with the characteristic spectrum of the

elements and molecules in the gas. The temperature in the centre is in the region of

4000-6000K and at the tube wall ~ 1000K. There is a substantial conduction loss

down this temperature gradient limiting the radiation efficiency of HP discharges less

than about 60%. Electrons gain energy from field and transfer it to other particles so

all particles have the same temperature. This condition, called Local Thermal

Equilibrium (LTE) greatly simplifies the treatment of these very complicated devices.

Very few metals have the vapour pressure, spectrum and chemical properties needed

to produce efficient light. Sodium (Na) at a pressure of 50-300 torr is the pre-eminent

example. The pressure depends on thecool spottemperature (CST), the temperature

at the coolest place in the lamp where the sodium condenses. CST and Na pressure

increase with power. As sodium is easily ionised, higher powers lead to higher plasma

conductivity. The lamp, fixture and ballast design must accommodate this

phenomenon. Under extreme conditions (White Light SON lamps) the ballast must

control the power within tight limits.

Metal halide

Metal halides can produce the same result. For example when sodium iodide (NaI)

evaporates into a Hg arc, the molecule dissociates Na and I. The Na* radiates its

characteristic orange radiation similar to that from a metal arc.

For most other metals the halides are usually much more volatile than the metal (Na

halides are exceptions). Such halide can be evaporated to produce white light of the

required CCT and Ra in a huge number of ways. Examples are indium, scandium,

dysprosium and many others.

See Lamps and Lighting chapter 5.


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Recent types of HID lamps for illumination

CMH white-light metal halide

- precision arc tube made from alumina

- Na, Tl and rare earth iodides + Hg

- offers high efficacy and good colour properties as a

result of high vapour pressures, which give a veryfull spectrum

- latest 20W, 1700 lumen 3000K version providesstrong competition for tungsten halogen lamps insome markets

- designed to be operated from electronic control gear

photo courtesy GE Lighting


0.00E+00

5.00E+05

1.00E+06

1.50E+06

2.00E+06

2.50E+06

3.00E+06

400 450 500 550 600 650 700 750

wavelength (nm)

intensity(W/cm)

58.8W

70.0W

82.8W

97.5W

predicted spectra

measured spectra

PH7B Philips CDM70/T

0

50

100

150

200

250

300

400 450 500 550 600 650 700 750

wavelength (nm)

intensity

(mW/nm)

60 W

70 W

80 W

90 W

70W CMH


20W CMH

Lamps for illumination

Ceramic metal halide (CMH) lamps provide improved performance over conventional

metal halide lamps. This is the result of

(a) Higher operating temperatures than in silica lamps, giving higher vapour

pressures. The result is improved colour rendering index >80 with efficacy >80 lm/W.

(b) Precision arc tube volume giving very tight initial colour spread.

(c) Low reactivity of envelope means that colour appearance is closely controlled

through life.

(d) Operation on electronic control gear ensuring clean starting under all conditions,

rapid run up to full light output, no 50Hz flicker and good control of steady state

operation. The range of lamps is now extended to powers as low as 20W, whilst giving

4 times the efficiency of tungsten halogen lamps of comparable light output.

CMH lamps typically contain Na, Tl, rare earth iodides with a high pressure of Hg

which acts as a thermal insulator, controls the voltage drop and assists in rapid run-up.The spectrum has many RE lines that appear to be a continuum when measured at

low resolution. This accounts for the good colour rendering.

These major advances in performance are the result of improved materials and a

system design approach.


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secondary aperturematches etendue of

downstream opticsprimary

aperture

spherical discharge

bulb containing

metal halide

non-imaging

optic

1cm

highly

reflectivealumina

thermally

conductiveceramic outer

carrier

Inductively coupled using single turn coil at 712MHz

Fusion Lighting LCD projector unitHID projection lamps pioneered by Philips

Short arc mercury (200 bar) arc tube lightproduced by atomic lines and molecular bands.

Reflector unit below



courtesy Fusion Lighting

24 mm

Recent types of HID lamps for projection


Projector lamps

High intensity arcs

Philips pioneered the use of super high pressure mercury lamps for projectionpurposes LCD projectors and projection TV.

The arc tube, containing 200 bar Hg when hot, has an arc gap of about 1mm. It is

mounted in a reflector to give a source of very low etendue, compatible with the LCD

light valves used for projection. The radiance the mercury discharge is very high

(maximum arc temperature in the region 6-7000K). At these temperatures and

pressures the deep UV lines of Hg are completely self-absorbed. Radiation comes

from the visible atomic lines and very strong continuum from atomic bremsstrahlung

and molecular emission from Hg2.

Development of this lamp has relied heavily on computer modeling, especially of

thermal properties, and also on innovative systems design.

Another approach

This Fusion Lighting projector lamp tackles the problem of etendue by using a very

high radiance electrodeless high-pressure metal halide discharge at 712MHz. Most of

the light escapes from an aperture in the reflecting surface around the 1cm diameter

bulb. This light is then concentrated in the forward direction using a non-imaging optic.

The lamp is shown built into an LCD projector.

For both these sources a major issue is what fraction of the input power enters the

downstream optics as this is what determines screen illumination.


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Electrodeless microwave

Sulphur discharge Fusion Lighting100 000 lumen source

record is 170 lumen/microwave W

microwave efficiency is a critical issue

reflector &microwavecavity

2.54 GHzmagnetron

S2 band radiationpressure is a few bar

self-reversalminimum

High efficiency molecular visible radiation is possible

System design

S2 discharge in

36 mm diameterbulb

Microwave lamp

The Fusion Lighting lamp is so far the only commercial microwave lamp. Used wherelarge lumen packages (105 lumen) are needed.

The radiating silica capsule is excited in a mesh cavity by a 2.54GHz microwave

magnetron.

Light Generation

The light is generated by S2 molecules at pressures of10 bar. S and S2 at low

pressures radiate mainly in the UV. As pressure is increased the molecular emission

increases until it becomes Planck limited and starts to self-reverse in the UV. By

analogy with sodium D lines, the peak we see in the visible is the peak of the self-

reversed red wing of the molecular band. In the dark region around 300nm there is

very high self-absorption.

Efficacy

Lamp efficacies of up to 170 lumen/microwave watt are the highest ever observed for awhite light source. This shows that direct generation of light from molecules is possible

without excessive vibrational losses.

The efficiency of light generation is offset by poor magnetron efficiency. Microwave

generation using magnetrons also limits the lamps to very high lumen packages that

are quite costly.

See Lamps and Lighting chapter 11. For explanation of operation see Johnston et al.

J. Phys. D: Appl. Phys. 35, 32-351,2002. Picture from Lighting Equipment News May

1998.


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Structure of a LP rare-gas discharge

VT (volt)

any length

10m 0.3 mm

anodefall ~ 5 V

cathodefall ~10 - 15 V

Te ~ 12000K

ne ~ 1018m-3

Te ~ 9000K

distance

E~1V/cm

E~10 kV/cm

~ 2 V

On ac this reverses each half cycle

anode +positive column

negative glow

Faraday dark spacecathode sheath

cathode

anode sheath

rare gas Argon 2 torr

Hg 6 mtorr

ne~1016 m-3

phosphor converts UV

to visible 50% loss

Life limited by electrodes

Operation

A fluorescent lamp discharge is a typical low pressure (LP) plasma. Most of the

radiation is produced in the positive column (PC) as UV at wavelengths 185 and 254

nm. This is converted, using a phosphor, to white light of the required CCT and CRI.

The conversion efficiency power into UV in the positive column can be as high as 80%.

In the PC there are equal numbers of electrons and positive ions; typically 1016 m3.

Electrons have Maxwellian distribution with temperature of about 12000K (1.1 eV),

whilst the gas temperature is about 40 celsius - this is a hot electron plasma. The

electrons gain their energy from the uniform field of about 1V/cm.

The anode fall is space charge region of about 0.3 mm width which adjusts

automatically to ensure that the correct number of electrons lands on the anode to

satisfy the requirements of current continuity.

At the cathode surface there is a narrow region of high field - the cathode fall (CF) -

which accelerates electrons toward the anode and accelerates the ions towards the

cathode. The cathode is designed to be heated by the ions so that it emitsthermionically. Under these conditions the CF is ~10-15V and the cathode emits the

required current with little loss. This process is aided by a low work function coating on

the surface. When the cathode is operating thermionically the lamp is in the arc

mode.

If the cathode is cold (for example, at start) the CF increases to ~100V to extract the

required electron current from the cathode; this is called a glow discharge. The high

energy ions sputter material from the cathode and cause damage and blackening. A

rapid transition from glow to arc is an essential requirement for long life.

See Lamps and Lighting chapter 5. Waymouth, chapters 2-4, de Groot and van Vliet,

chapter 6


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LP rare-gas discharges

Main type are fluorescent lampsLP Hg discharge radiating with efficiency of75% at 254 +185 nm

Some mileage left here for innovation

Main challengesHow to avoid 50% phosphor conversion loss?

- quantum splitting phosphors 2 visible photons for

each visible photon some progress

- generate white light directly in visible region by

using high volatility molecules

Avoiding use of Hg- Xe discharges are and option; not as efficient as

LP Hg, but give instant light

Heliax CFL (GE)

Self-ballasted MicroLynx (Sylvania)

Materials advances System Design

Innovative developments of fluorescent technology

GE Heliax is very high efficiency compact source. The helical shape aids the escape

of visible radiation and leads to a very compact source. Unfortunately it proved too

expensive to make in mass production (photo GE literature)

Sylvania MicroLynx integrated miniature lamp and fixture (Lighting Equipment News

page 6, April 2001). This small self-ballasted lamp plugs into a standard twist and lock

type socket. The small depth makes it ideal for use in display lighting (photo and

drawing from Lighting Equipment News)

Avoiding the phosphor losses

A quantum splitting phosphors of quantum efficiency ~ 1.9 has recently been

discovered (see Physics World, pp 17-18, April 1999 and Wegh R T et al. Science

volume 283 pp 663-666, 29 Jan 1999). LiGdF4:Eu3+ produces ~ 2 red photons for

each 172nm UV photon from a LP Xe discharge. Fundamental theoretical and

materials research is needed to make establish that this is not a one-off accident. Will

be highly beneficial for LP Xe discharges.

Generating light in the visible region

Can a non-LTE source with high electron temperature be found that generates white

light in the visible region? There are no atoms that combine high volatility with multi-

line emission in the visible region. There are many molecules that have these

properties e.g. transition metal chlorides or oxychlorides. Electrodeless operation is

probably necessary with these highly reactive doses. What about the vibrational and

rotational losses? Work on S2 discharges shows that this is not a fundamental

limitation.


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Electrodeless inductively coupled

Transformer with single turn secondary

Circuit must drive Rll +jLlat specified power.

In this lamp a half bridge circuit is used at~2.65 MHz

phosphor coating

re-entrant withcoil inside oncoil support

circuit

exhaust tube

housing

bulb with Kr

and amalgamcontains discharge

La

Ra

Rl+jL

l

plasma

System Design - Models

Electrodeless lamps

Electrode failure is the usual reason for end of life. Electrodeless lamps eliminate it as

a cause.

Various types of electrodeless discharge are possible. The type shown here is an

induction lamp. The low pressure mercury plasma forms a single turn secondary

around the primary coil. The electrons are accelerated by the azimuthal electric field

which results from the changing magnetic field. The discharge produces UV which is

converted to visible light in the normal way. Life usually depends on the robustness of

the circuit.

The circuit must be able to drive an unloaded transformer, producing enough voltage

across the coil to cause breakdown of the gas. After starting the transformer primary is

loaded by the impedance of the plasma Ra +jLa in the plasma. The circuit must control

the power into the primary to the required level for a loaded impedance Rl+jLl.

At the high frequencies needed for efficient operation, very careful attention is needed

to keep electromagnetic emission within the internationally agreed limits.

The design of such a lamp requires close integration of the system comprising circuit,

coil, discharge, optics and electromagnetic emission control.

Lamps and Lighting chapter 11


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Dielectric barrier discharge (DBD)

Osram Planon DBD and control gear

Planon lamps form display

Osram Planon lamp

main applications are high uniformity back lighting forcopiers, fax machines and computers

Xe discharge with UV radiation from Xe 2* excimer


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Light emitting diodes (LED)

LED is a p-n junction forward biased to 2 V

- inject minority carriers into junction

- e and h mayrecombine to produce light

- light has to be extracted from high index material

- LEDs are operated in series or parallel with

approximately constant current

Major advances have been made in the last 5 years

- breakthrough was Nichias efficient blue LED

- improved light extraction

- white LEDs now possible

Claims for LED performance are confusing

They generate no heat.

No mention of circuit lossesWild extrapolations

New LEDS promise 330 lumens a watt

absorbing

substrate

1991

transparent

substrate

1994

large

junction

1994

truncated

pyramid

2000

3 x 5 x 1.5 xflux improvement

Nevertheless progress is astonishing

Materials advances System Design

Semiconductor LEDs

These now come in colours throughout the visible range and also UV. By combining

colours or by using UV + phosphor, white light LEDs of impressive brightness can now

be made. LED manufacturers seem to be taking little notice of the requirements for

good colour rendering at yet. The pace of development is truly astonishing and there

seems little doubt that the takeover from conventional lamps will occur in certain

markets. LED life can be very long, but white light LEDs still suffer from serious

degradation of light output with life.

The possible efficiencies of LEDs can be very high in some case nearly 100%

conversion of injected carriers is converted to light. However the light has to escape

from a very high index material. The multiple reflections involved ensure that even

small amounts of absorption causes substantial losses. The last of the 4 pictures

structure that allows the light to escape with fewer reflections.

It is very difficult to get a clear idea of the real efficiency of LED systems from

commercial literature, and in some cases figures are quoted that seem fanciful in theextreme. Standardisation is urgently needed to bring some order. Extrapolations of

future performance LED efficiency also often seem over-optimistic. Whilst LEDs are

not subjected to the same thermodynamic limitations as discharges because the

electron motion is not randomised, they still have losses such as traps for carriers,

optical absorption and circuit losses. It remains to be seen whether high-flux, high

colour rendering index LEDs can outperform CMH discharges.

Organic LEDs

and light emitting polymers (OLEDs) are some way behind in development for light

sources, although the display market seems assured. They potentially offer low cost

route to solid state light generation. At the moment it difficult to see where the lumens

might come from. Watch this space.

Photo Lighting Equipment News


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LED applications

LEDs generate light of colour requiredfor signals

Traffic and vehicle lighting

- already a large market for efficient,

robust, long lived LEDs

photo courtesy Lumileds

But what about this?

Lumileds-Philips street lighting demo.

(LEDs on the left)

- Meets CIE illumination requirement

- LEDs allow better light control than LPand HP sodium

- Energy consumed per km comparable to

sodium

System design

photo courtesy Lumileds

Applications

The main challenge for LED systems if they are to displace conventional lamps is

achieve comparable lumen packages with comparable optical properties to the

sources they are displacing. In lighting terms LEDs are still very low flux devices.

A second challenge is cost. Currently the cost per white light lumen is currently about

100 times higher than from conventional lamps. Whether this will reduce to levels

where the cost/benefit ratio is acceptable remains to be seen.

Auto and signal applications

Traffic lights are the killer application for LEDs. Energy savings over filtered

incandescent are massive, so even without the reduced maintenance costs they look

good. It also seems that rear cluster and internal display lighting in automobiles will

inevitably go the LED route, since reliability and robustness are superior to

incandescent lamps.

It looks as if the whole of the miniature lamp business will be changed to solid state

lighting within very few years.

Illumination applications

As to real illumination look a the road scene. Although this is a only demonstration, the

very fact that it could be mounted at all says a great deal about the potential of LEDs

for illumination.

I have no details except that the sodium lighting on the left and the LED lights on the

right both meet the CIE road illumination requirements. One benefit of LEDs can be

seen a greater fraction of the light produced illuminates the road. The lamp-fixture

system thus benefits from the greater optical control of the LED light. Thus the LED

flux does not have to be as high as the discharge lamp flux.


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22


Light Source TechnologyA typical lighting system every stage counts

AC/DCpf 1

lamp driver

control

lampP Pcir Prad PvisPdc

Peye

=760

380)( dPradviscirradrad PdP

high

low

/)(=

=760

380)()( dVPviseye

optics with

effectivetransmittance

Peff

viseff P=

dccircirPP/=PPdcdc /=

illuminated

surface

effeyevisradcirdcsys =


Lighting systems

This schematic diagram applies to anylight source run from an electronic supply.

The integration of electronics with lighting is a rapidly growing an inevitable trendbecause of the benefits it brings for the customer and manufacturer. The customer

benefits from better performance in many different ways, and the ability to control the

lighting to suit needs. For the manufacturer, electronic control can overcome some of

the intrinsic problems associated with some lamps, whilst the use ballasts that can

adapt to the lamps reduces inventories.

Each step in this long train must be examined in future development because

accumulation of even small inefficiencies at each stage rapidly drags the overall

efficiency down.

For consistency I have not converted to lumens by multiplying the eye-sensitivity

weighted watts to lumens with the factor 683.

Some of the issues with each stage are examined in the next slides.


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23



AC/DC lamp driver

control

lamp Prad Pvis

System Issues - 1light or person sensitivityIR switches

bus control (DALI)mains signalling

lamp type sensefailed lamp check

lamp typeexcitation mechanism

operating frequency

waveformstart-up

shut down

temperature

EM compatibility

overall efficiency, size, cost, weight

spectral dist.temperature

emissivity

energyconservation

lamp typeincandescent

LED

HIDfluorescent

electrical

size

thermalsafety

radiationmechanisms

to enhance

visible

energy storagepower factor

stability

rippleuniversality

Some of the factors affecting the system efficiency

AC/DC conversion energy storage is often controlling influence on cost & size,

greater stability could benefit some sources. Output independent of input voltage.

Lamp driver depends on lamp type and excitation. e.g. Incandescent may require soft

start. Discharge requires clean ignition & maybe controlled shut down for good life,

and waveforms control for stability. Discharge excitation can be via electrodes or

electrodeless or microwave.

Electronic control (internal or external) is becoming a major way to add value.

Lamp types determine many other factors such as electrical characteristics which

impact on size materials and safety issues

Prad is may be determined by the characteristic temperature with a black body limit.

Also by spectral emissivity of solid (incandescence) or vapour (discharge). Reflectors

can reduce IR losses in some cases. In may discharge lamps the radiation reaches

the black body limit and shows self-reversal as it escapes from the lamp.

Pvis determined by radiator type and density. High densities lead to continuous spectra

with large IR losses.


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24



Peye

optics with

effective

transmittance Peff

efficacycolour rendering

3, 4 line spectra

high - avoid

obstructionfilament supports

sealscomplex shapes

scatteringlight leakage

reduce absorption

fitting design to optimise

illuminationprojected beam

illuminated

surface

System Issues - 2

Some of the factors affecting the system efficiency

The radiator type determines how well the light is use by the eye. The constraint here

is on the simultaneous requirement on colour rendering. Excellent colour rendering

numbers can be achieved using light at 3 narrow bands at wavelengths ???

The next stage is to utilise the light from the lamp is the best way.

In compact fluorescent lamps the construction determine how much of the emitted

visible light escapes without being absorbed in the lamp structure.

In the electrodeless reflector lamp the optical design is critical in ensuring that light is

reflected forward and escapes from the front of the lamp.

In a tungsten-halogen lamp this may mean building into a suitable dichroic reflector.

For many lamps used for illumination this is the province of the fixture manufacturer.

The LED street light example shows that power can be saved whilst maintaining

illuminance if source and optics are carefully matched. Small sources make this

simpler.For projector applications careful optical design with regard to etendue and reducing

reflection at optical surfaces is required. In projectors using incandescent lamps the

structure of the filament supports is very important. For projector applications optical

design needs to be based on actual data rather than crude lamp models as a lot of light

is comes from wires and seals and in the case of discharge lamps, the electrodes.


25/25



Key areas for workGeneral

Future developments will rely heavily on materials advances, system design and models

Long term, high gain and speculative

- quantum splitting phosphors

improved theoretical basis

- LP high Te white light sources (molecular discharges)

fundamental understanding

- Incandescence highly selective emitters (not necessarily high temperature)

use theoretical knowledge as guide

Shorter term

- attack every conversion stage in system to gain small % gains

- better understanding of lamps/control gear interactions

- improve understanding of lamp/power supply interactions

- life prediction to shorten development cyclesIt will happen anyway

- increase flux from LEDs & reduce cost by a factor100

This will need interaction between circuit designers, lamp scientists,

materials scientists, optical designers and others

david wharmby

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