Plasmas under pressure? Eindhoven COST529 March 2006page 1

Plasmas under pressure? Eindhoven COST529 March 2006page 2 Plasmas under pressure? COST 529 Meeting March-April 2006 David Wharmby Technology Consultant

Plasmas under pressure? Eindhoven COST529 March 2006page 3 This Lumileds slide really made me think (Luminance correlates with brightness - nit = candela m -2 = lumen m -2 sr -1 ) Source: Lumileds (Dr R Scott Kern) lm/W

Plasmas under pressure? Eindhoven COST529 March 2006page 4 Source: Lumileds (Dr R Scott Kern) 55 W halogen 1500 >20 Lumileds R&D guess  17W

Plasmas under pressure? Eindhoven COST529 March 2006page 5 Radiance (W m -2 sr -1 ) Radiance L e and its visual weighted counterpart luminance L v, 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) L v = d 2  v / [dAcos  d  lumen m -2 sr -1 or cd m -2 ) We tend to WORSHIP lumens and efficacy: we are not very sensitive to systems needs which almost invariably involve luminance

Plasmas under pressure? Eindhoven COST529 March 2006page 6 Etendue & usable light Emitted luminous flux  v  and luminance L v are related etendue  v  L v units (m 2 sr) Etendue is conserved in perfect optics   A  (small A and  ) 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  source >  gate some emitted light from the source is wasted

Plasmas under pressure? Eindhoven COST529 March 2006page 7 Must match source  to application  Example  In UHP, arc  is large so source area A must be small to comparable with LCD etendue. etendue   A  A  0.1 mm 2    2  True of any display application – we need to think etendue

Plasmas under pressure? Eindhoven COST529 March 2006page 8 LEDs produce little flux & installations rely on high L v etendue  Functional Decorative

Plasmas under pressure? Eindhoven COST529 March 2006page 9 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 T e  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 Cayless M A, “Future developments in lamps”, IEE PROC., vol. 127 part A, , 1980 Planck limits plasmas, but not LEDs, phosphors

Plasmas under pressure? Eindhoven COST529 March 2006page 10 Luminance of thermal & non-thermal radiators CMH Luminance (Gcd/m 2 or Gnit) UHP sun TH non-thermal incoherent sources (LEDs) are catching up lasers CMH

Plasmas under pressure? Eindhoven COST529 March 2006page 11 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 T e ? heat capacity too high UHP lamps bar Hg (Derra et al. 2004) Near the limits – new direction needed

Plasmas under pressure? Eindhoven COST529 March 2006page 12 Summary - low etendue sources Planck (black body) distribution is limit in thermal equilibrium All plasmas are Planck limited, so high T e 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

Plasmas under pressure? Eindhoven COST529 March 2006page 13 But OLEDs are a different matter....

Plasmas under pressure? Eindhoven COST529 March 2006page OLEDs low luminance, high etendue

Plasmas under pressure? Eindhoven COST529 March 2006page Performance Specifications CCT: 4000K CRI: LPW 1200 Lumens Better color quality than fluorescent Still glass substrate (cost, weight) OLEDs low luminance, high etendue but with improved flux Equivalent to 80 W incandescent

Plasmas under pressure? Eindhoven COST529 March 2006page 16 Lighting Wallpaper? Roll to roll line planned 2007 Now - Plastic substrate with very low permeability coatings With thanks to Anil Duggal GE Global Research

Plasmas under pressure? Eindhoven COST529 March 2006page 17 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 SrAl 12 O 19 :Pr, Mg Phosphor “ Sergio M. Loureiro et al., Chem. Mater., 17 (12), , “Visible Quantum Cutting in LiGdF4:Eu3+ Through Downconversion, RT Wegh et al. Science 1999, pp Molecular discharges as light sources R Hilbig et al. LS

Plasmas under pressure? Eindhoven COST529 March 2006page 18 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