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Increasing the Brightness of Light Sources Ling Fu Increasing the Brightness of Light Sources Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften (Dr. rer. nat.) dem Fachbereich Physik der Philipps-UniversitÄatMarburg vorgelegt von Ling Fu aus der V.R. China Marburg 2006 Vom Fachbereich Physik der Philipps-UniversitÄatMarburg als Dissertation angenommen am: . Erstgutachter: Prof. Dr. H. Ries Zweitgutachter: Prof. Dr. H. JÄansch Tag der mÄundlichen PrÄufung: 16.11.2006 Zusammenfassung Das Ziel dieser Arbeit ist es, die ErhÄohung der spektralen Strahldichte von Licht- quellen mittels Licht-Recycling zu untersuchen und eine AbschÄatzung des Poten- tials dieser Methode zu erarbeiten. Dazu wurde ein physikalisches Modell der Licht- erzeugung in thermischen und lumineszenten Quellen erstellt und an verschiedenen Typen von Lichtquellen sowie einem Prototyp der Carambola, einer von uns ent- wickelten und hier vorgestellten Optik furÄ deterministisches Licht-Recycling veri¯- ziert. FurÄ moderne Beleuchtungssysteme sind kompakte GrÄo¼e und hohe Helligkeit (Strahldichte) wichtige Eigenschaften. Licht-Recycling kann die Strahldichte des von einer Lichtquelle ausgesendeten Lichts um den Preis des Verringerns der insgesamt in den Phasenraum abgestrahlten Energie erhÄohen. Licht-Recycling bedeutet die Reflexion eines Teils des ausgestrahlten Lichts zur Quelle. Ein Teil dieses reflektier- ten Lichts entgeht der Absorption in der Quelle und steht weiter zur Verfugung.Ä Infolgedessen kann die Strahldichte eines verkleinerten Phasenraums im Vergleich zur Helligkeit der ursprunglichenÄ Quelle erhÄoht werden. In dieser Arbeit wird diese Grundregel des Licht-Recycling auf verschiedene kunstlicheÄ Lichtquellen angewen- det, um eine ErhÄohung der Helligkeit zu erreichen. ZumÄachst werden die MÄoglichkeiten zur ErhÄohung der Helligkeit von Lichtquellen mittels Licht-Recycling theoretisch uberprÄ uft,Ä auf der Grundlage der Gesetze der Thermodynamik, insbesondere des Kirchho®schen Strahlungsgesetzes, des Planck- schen Gesetzes, des Lambert-Beerschen Gesetzes, der Etendueerhaltung und des Helligkeitstheorems. Aus experimenteller Sicht werden die Strahlungseigenschaften der drei unterschied- lichen Lichtquellen Kurzbogenlampen, GluhlampenÄ und Leuchtdioden (LEDs) im Hinblick auf ihre Eignung furÄ das Licht-Recycling untersucht. Exemplare dieser Lichtquellen werden in Experimenten zum Licht-Recycling eingesetzt, um ² E®ekte intrinsischen Licht-Recyclings zu erkennen, beispielsweise das durch die Wicklung des Drahtes entstehende Licht-Recycling in Gluhlampen.Ä ² die zur Erstellung von physikalischen Modellen benÄotigten Parameter, wie EmissivitÄat und AbsorptivitÄat der Kurzbogenlampen oder die ReflektivitÄat und den Quantenwirkungsgrad von LEDs zu messen. ² die Grundlage furÄ die Entwicklung von Optiken furÄ das Licht-Recycling auf der Basis der gemessenen Parameter zu scha®en. Die Carambola ist eine von uns furÄ deterministisches Licht-Recycling entwickelte Optik. Zwei physikalische Modelle zur Simulation der Strahlungsverteilung von Licht- quellen, eins furÄ Gluhlampen,Ä das andere furÄ lumineszente Quellen (LEDs) werden erstellt. Beide sind mit Messungen hoher AuflÄosung validiert. Die physikalischen Mo- delle sind in der Lage, die Strahlungsverteilung mit einigen erforderlichen Parame- tern (Geometrie, Materialeigenschaften und Betriebsbedingungen der Lichtquelle) analytisch zu modellieren. Sie sind auf andere Quellen mit Äahnlichen Eigenschaf- ten anwendbar. Die Vorteile der PrÄagnanz, der hohen Genauigkeit und der breiten Anwendbarkeit kombinierend, kÄonnen die physikalischen Modelle in Strahlverfol- gungssoftware integriert werden. Zur experimentellen Untersuchung des E®ekts des Licht-Recycling ist eine opti- sche Vorrichtung, die Carambola, mit deterministischem (im Gegensatz zu stocha- stischem) und mehrfachem Licht-Recycling bestimmt. Um die mÄogliche maximale ErhÄohung der Strahldichte, die mit der Carambola erreichbar ist, zu simulieren, werden einige Kombinationen von Quellen und Carambola mit Strahlverfolgung modelliert. Dieses wird auf Quellen mit unterschiedlicher EmissivitÄat und unter- schiedlichen Strahlungseigenschaften (optische Dicke) und auf Carambola mit unter- schiedlichen Geometrien und optischen Eigenschaften angewendet. Daraus kann man schlie¼en, dass Xenon Hochdrucklampen aufgrund ihrer geringen optischen Dicke, ihrer gleichmÄassigen spektralen AbsorptivitÄat furÄ das Licht-Recycling besonders ge- eignet sind. In der Strahlverfolgung wird diese Eignung der Xenonlampe und der furÄ das Licht-Recycling bestÄatigt. Im Laufe dieser Arbeit wurde ein Prototyp einer funf-rippigenÄ reflektierenden Carambola aus Aluminium hergestellt. Die experimentellen Resultate mit der Carambola liegen unterhalb der theoreti- schen Erwartungen, die Diskrepanzen kÄonnen erklÄart werden. Eine Carambola mu¼ ein hohes ReflexionsvermÄogen und eine genaue Form haben, wenn ein deutlicher Ef- fekt des Licht-Recycling gezeigt werden soll. Die Carambola kann verbessert werden durch das Polieren und Versilbern der Reflektoren, durch Verwendung von Quel- len kleiner optischer Dicke und kompakter GrÄo¼e, hohem tatsÄachlichem Reflexions- vermÄogen und externem Quantenwirkungsgrad (LED). Theoretische Untersuchung und experimentelle Messungen des Licht-Recycling zeigen, da¼ dieser E®ekt entweder eine tatsÄachliche Eigenschaft der Quelle ist, oder durch eine besonders entworfene Optik verursacht werden kann. Durch das Wie- derverwerten des Lichtes kann die spektrale Strahldichte eines Phasenraums erhÄoht werden. Sowohl die theoretische Untersuchungen als auch die experimentellen Messungen des Licht-Recyclings demonstrierten eine signi¯kante ErhÄohung der Strahldichte op- tisch dunnerÄ Quellen. Contents Zusammenfassung i List of Figures ii List of Tables iv 1. Introduction 1 1.1. Motivation . 1 1.2. Brightness enhancement of light sources: brief history . 3 2. Theory of light recycling 9 2.1. Concepts . 9 2.2. Kirchho®'s law on radiation and light recycling . 13 2.3. Light recycling with short-arc lamps . 18 2.3.1. Experimental method and setup . 18 2.3.2. Results and discussion . 21 2.4. Summary . 30 3. Thermal light sources 31 3.1. Physical modelling of ¯lament lamps . 33 3.1.1. Geometrical model of the ¯lament . 33 3.1.2. Thermal model . 35 3.1.3. Radiation model . 39 3.2. Measurements and model veri¯cation . 43 3.2.1. Experimental setup . 43 3.2.2. Parameter identi¯cation . 43 3.2.3. Model veri¯cation . 47 3.3. Summary . 51 4. Luminescent light sources 53 4.1. Electroluminescence of LEDs . 53 4.2. Physical modelling of LEDs . 55 4.2.1. Geometrical model of the light-emitting surface . 56 4.2.2. Electrical model . 58 i 4.2.3. Material and radiation model . 60 4.3. Light recycling with an LED . 71 4.3.1. Experimental method and setup . 71 4.3.2. Results and discussion . 74 4.4. Summary . 76 5. Optical device for light recycling¡Carambola 79 5.1. Optical principle . 79 5.2. Prototyping . 80 5.2.1. Geometry . 80 5.2.2. Reflective version . 83 5.2.3. Refractive version . 85 5.3. Light recycling in the Carambola . 86 5.3.1. Source . 86 5.3.2. Brightness enhancement . 90 5.3.3. Results and discussion . 93 5.4. Summary . 95 6. Summary and prospects 97 A. Symbols and Abbreviations I B. Author's Publications V Bibliography VII Acknowledgements XII Academic Career XIII ii List of Figures 1.1. Magic lantern of G. Sibbald . 4 1.2. UHP arc lamp without and with dichroic coating . 5 1.3. A dual-paraboloid reflector system for projection display . 6 2.1. De¯nition of the ¶etendue. 10 2.2. Etendue conservation and brightness conservation . 11 2.3. Radiation of a black body, a grey body and a selective radiator . 15 2.4. Principle of light recycling . 16 2.5. Absorption and emission in a volume source . 17 2.6. Experimental setup for light recycling . 19 2.7. Photographs of high-pressure short-arc lamps . 20 2.8. Light recycling with a short-arc lamp . 21 2.9. Spectral radiation of the HBO lamp . 22 2.10. Energy levels of mercury atoms and Boltzmann probability distribu- tion vs. energy . 23 2.11. Spectral absorptivities in the HBO lamp . 24 2.12. Light recycling e®ect in the HBO lamp . 25 2.13. Total absorptivity of the plasma in the HBO lamp . 26 2.14. Spectral distribution of the HBO lamp and blackbody radiation . 27 2.15. Light recycling e®ect in the XBO lamp . 30 3.1. Photograph of the 20 W halogen lamp . 32 3.2. Flow chart of the physical modelling of ¯lament lamps . 32 3.3. Simulation model for deriving geometrical recycling factors of the ¯lament . 35 3.4. Local recycling factor vs. revolving angle around the coil . 36 3.5. Global integrated recycling factor of the ¯lament lamp . 37 3.6. Thermal model of the ¯lament lamp for DC operation . 39 3.7. Temperature vs. position of the ¯lament lamp . 40 3.8. Thermal model of the ¯lament lamp for AC operation . 40 3.9. Material emissivity and e®ective emissivity of a ¯lament lamp . 42 3.10. Brightness enhancement vs. geometrical recycling factor and material emissivity . 42 3.11. Experimental setup for radiation measurements of a ¯lament lamp . 44 iii 3.12. Flow chart of the parameter identi¯cation . 46 3.13. Temperature distribution vs. wavelength via two-wavelength ther- mometry . 48 3.14. Temperature distribution vs. position via two-wavelength thermometry 49 3.15. Modelled radiance and a photograph of the ¯lament . 49 3.16. Comparison between modelled radiance and experimental spectro- scopic measurement . 50 4.1. Flow chart of the physical modelling of LEDs . 56 4.2. Schematic of a double heterostructure LED . 57
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  • Etendue and Optical Throughput Calculations Authors: H

    Etendue and Optical Throughput Calculations Authors: H

    TECHNICAL NOTE #002-2-14-2011 Etendue and Optical Throughput Calculations Authors: H. Zhu and P. Blackborow Introduction This note is intended to help decide whether a Laser-Driven Light Source (LDLS™) is the appropriate source for an application. In this note we will consider only the optical performance and not other benefits of the LDLS, such as stability or ultra-long life. One of the main factors to consider is whether or not the étendue of the LDLS matches the étendue of the optical system. To have an optimized throughput in an optical system, the étendue of the light source, the collecting optics, and the étendue of the light receiving optics, optical fibers or monochromators, need to be closely matched. (In this Application Note, a monochromator can also mean a spectrometer or a spectrograph.) In addition to the term “étendue,” other terms often used are “f-number” (F/#) and “numerical aperture” (NA). Etendue and its relationship to F/# and NA of an optical system will be discussed, in addition to discussions on how to use étendue and throughput calculations to select appropriate LDLS applications. For simplicity, diffraction and coherence effects are excluded in these discussions. What is Etendue and Why Etendue Matching is Important for Effective Light Coupling The term étendue comes from the French word for extent or spread. Other terms used for this property are Geometrical Extent (G), Optical Extent, and Optical Invariant. We will use G to represent étendue. For a LDLS source with relatively small cone of radiation (so paraxial optical approximation is valid), the étendue (G) of a light source is equal to the source emitting area (S) times the solid angle (Ω) from which the light is collected for a specific application, G ≈ SΩ [mm2-sr].