Plasmas%20under%20pressure?

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(No Transcript)
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Plasmas under pressure? COST 529 Meeting
March-April 2006
David Wharmby Technology Consultant david_at_wharmby.
demon.co.uk
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This Lumileds slide really made me think
Source Lumileds (Dr R Scott Kern)
lm/W
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85
21
30
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(Luminance correlates with brightness - nit
candela m-2 lumen m-2 sr-1)
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Lumileds RD
guess 17W
55 W halogen 1500 gt20
Source Lumileds (Dr R Scott Kern)
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Radiance (W m-2 sr-1)

• Radiance Le and its visual weighted counterpart
  luminance Lv, are fundamental
• determine how effectively the light can be used
• no-one would use a fluorescent lamp in a
  projector or projector lamp to light a room
• this is why so many LED applications are
  decoration not illumination
• Luminance is conserved along a ray (or decreases
  in a lossy system)
• Lv d2Fv/ dAcosq.dW (lumen m-2 sr-1or cd
  m-2)
• We tend to WORSHIP lumens and efficacy we are
  not very sensitive to systems needs which almost
  invariably involve luminance

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Etendue usable light

• Emitted luminous flux Fv and luminance Lv are
  related etendue e Fv/Lv
• units (m2 sr)
• Etendue is conserved in perfect optics
• e AW (small A and W ) so a large object/small
  solid angle becomes small image/large solid
  angle
• e.g. if a projector system the etendue is
  defined by the LCD gate dimensions If esource gt
  egate some emitted light from the source is
  wasted

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Must match source e to application e

• Example
• In UHP, arc W is large so source area A must be
  small to comparable with LCD etendue.
• etendue e AWA 0.1 mm2W 2p

True of any display application we need to
think etendue
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LEDs produce little flux installations rely on
high Lv etendue e
Functional
Decorative
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Fundamental limitations to light production

• A lot of light is produced by accelerating
  electrons
• For plasmas in gases (and wires) electrons are
  thermalised by collisions at Te
• radiance (luminance) limited by Planck function
• In semiconductor sources electron energy directly
  excites luminescent level
• not limited by Planck function
• But there are many competing processes in
  semiconductors
• In phosphors, radiation directly excites
  luminescent level
• radiation is not limited by Planck (or Stokes)
• In lasers are far from equilibrium, population
  inversion is required

Planck limits plasmas, but not LEDs, phosphors
Cayless M A, Future developments in lamps, IEE
PROC., vol. 127 part A, 211-218, 1980
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Luminance of thermal non-thermal radiators
CMH
CMH
lasers
non-thermal incoherent sources (LEDs) are
catching up
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Example - UHP projection limits?

• Increase arc temperature - But how?
• Increasing power only increases peak T slowly
• More current? - electrodes on the edge
• Cannot increase arc gap - etendue decreases
• Make spectrum more Planck-like
• Peak temperature may decrease
• Molecular absorption increases
• Increase ionization losses, but how?
• Use plasma hot spots caused by electrode cooling
• Use other emitters, but which?
• ???
• Transient discharges for high Te?
• heat capacity too high

UHP lamps 100-200 bar Hg (Derra et al. 2004)
Near the limits new direction needed
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Summary - low etendue sources

• Planck (black body) distribution is limit in
  thermal equilibrium
• All plasmas are Planck limited, so high Te
  essential
• LEDs are not Planck limited have ever
  increasing luminance
• Many applications for high luminance/ low
  etendue sources
• automotive forward lighting
• projection
• decorative/accent lighting
• display lighting
• BUT . . .I find it difficult to believe LEDS will
  ever produce adequate lumens/ for general
  illumination

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But OLEDs are a different matter . . . .
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OLEDs low luminance, high etendue
2002
2003
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OLEDs low luminance, high etendue but with
improved flux
2003 Performance Specifications CCT 4000K CRI
88 15 LPW 1200 Lumens Better color quality than
fluorescent Still glass substrate (cost, weight)
Equivalent to 80 W incandescent
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Lighting Wallpaper?
Roll to roll line planned 2007
Now - Plastic substrate with very low
permeability coatings
With thanks to Anil Duggal GE Global Research
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General illumination

• Fluorescent lamps are near their limit
• Possible approaches to improvement of low
  radiance sources are
• quantum splitting phosphors (aimed at Hg free but
  apply to Hg-RG)
• needs fundamental work on very complex crystal
  field and phonon interactions in crystal lattices
  1,2
• direct production of white light in the visible
  region
• molecular discharges seem the only option but
  fundamental work is needed to understand what is
  possible 3
• OLEDS are not Planck limited
• a long way short, but improving rapidly
  potentially low cost

1. First Observation of Quantum Splitting
Behavior in Nanocrystalline SrAl12O19Pr, Mg
Phosphor Sergio M. Loureiro et al., Chem.
Mater., 17 (12), 3108 -3113, 2005 2. Visible
Quantum Cutting in LiGdF4Eu3 Through
Downconversion, RT Wegh et al. Science 1999,
pp663-666 3. Molecular discharges as light
sources R Hilbig et al. LS10 2004
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Conclusions

• Emission from plasmas (solid and gaseous) limited
  by Planck
• Existing sources seem to show no room for major
  improvements
• Possible major improvements in phosphors and
  visible emission from LP discharges
• Massive scientific effort needed in both areas
• Emission from solid state light sources not
  limited by Planck function
• but there are many complex competing processes
  reduce conversion efficiency
• these are being sorted out with great vigour
• LEDs show greatest potential in low etendue
  applications (display, projection, decorative)
• OLEDs show greatest potential in general lighting
  applications (high etendue)

We need new ideas for Planck limited sources