The design and implementation of a spectrally tuneable LED-based light source: towards a new era of intelligent illumination Wim Hertog A thesis presented for the degree of Doctor of Philosophy in Optical Engineering at the Department of Optics and Optometry of the Universitat Politècnica de Catalunya Thesis Supervisor Internal Examiner Dr. Josep Maria Carreras Molins Dr. José Arasa Martí Barcelona, December 2016 i Acknowledgements I still remember my first torch: a yellow Playskool flashlight with a built-in red-green colour filter. It was my absolute favourite toy as a toddler. The love for lights did not let go. From scrounging around markets in far-away countries to setting up a lighting company, illumination has always been a fervent hobby. At the end of my studies, this hobby turned into a full time job that, later on, led to the start of this PhD. Thanks, mom, dad and Joachim for the years and years of encouragement that fuelled my passion. Without you I would not be where I am now. My sincere gratitude goes towards Dr. Carreras, whom I met for the first time during a CIE conference in Gent, Belgium. Thank you, Josep, for introducing me in IREC, accepting me as your PhD student and giving me the chance to meet countless like-minded people all over the world. Charles, Francisco, Jordi, Jorge, Oscar, Mariano and especially Jesús: thanks for being such helpful colleagues and more importantly, good friends. Didem, keeping the most important one for the end, thank you for sharing all my happy and sad moments. Thank you for all those far-away adventures. Our time together is my number one energy resource. Wim Hertog Barcelona, 2015 ii Abstract Rising population numbers place ever increasing demands on energy resources. A large percentage of the worldwide energy production is reserved for the generation of electricity and a significant portion of the electrical energy generation is used for illumination purposes. At the same time, people demand brighter light sources that provide better light quality. The luminaire of today is not just a simple lamp, but a complex, intelligent piece of technology designed for a specific purpose. This doctoral dissertation aims to provide a link between the theoretical universe surrounding the physics of electromagnetic radiation and the practical illumination world. A theoretical framework presents the physical properties of light and connects, through the human visual system, to a number of perceptual models. Based on these perceptual models, the colourimetric qualities of an illumination spectrum are analysed and a theoretical framework that aims to optimise the balance between colour fidelity and energy efficiency is proposed. The result of this optimisation is a spectrum that needs to be implemented into a light source. Recent advancements in semiconductor technology led to the development of highly efficient light emitting diodes. The monochromatic nature of these light sources offers the possibility of creating a spectrally tuneable luminaire that is able to reproduce these optimised illumination spectra. A fully integrated spectrally tuneable light engine combines knowledge on characterising and driving LEDs, optical design and thermal management. Each group of monochromatic LEDs needs a variable current source that ensures predictable behaviour regardless of their dimming level or temperature. An advanced optical solution enables efficient light extraction from the LEDs, provides excellent luminance and chroma homogenisation and, finally, delivers a suitable beam pattern for the intended illumination application. Proper thermal management establishes sufficient heat extraction to guarantee low semiconductor temperatures. Finally, the LED light engines created during this doctoral research are incorporated into three spectrally tuneable illumination devices, each designed for a specific purpose. A set of spectrally tuneable downlights installed in the laboratories of the lighting group of the Catalonia Institute for Energy Research (IREC) in Barcelona, Spain and the department of neuroscience of the University of Newcastle in the United Kingdom serves as a valuable tool for experiments that evaluate both physical and psychovisual properties of selected illumination spectra. A compact spotlight, used during the “Making Colour” exhibition of the National Gallery in London, is optimised to provide a set of specific illumination spectra to illuminate art reproductions. Lastly, a high power luminaire was designed to generate specialised spectra to irradiate greenhouse plants, steering their morphology and the production of plant-specific compounds. iii List of abbreviations and symbols AC Alternating Current irradiance in watt per square metre radiant intensity in watt per solid angle radiance in watt per solid angle . surface area radiant exitance in watt per square metre sensitivity function of the rods of the human eye Boltzmann´s constant radiant flux in watt colour difference / energy difference temperature difference A amplitude a* a* coordinate of the CIELAB colour space AIC Association Internationale de Couleur AM Amplitude Modulation B magnetic field in teslas b* b* coordinate of the CIELAB colour space BC Before Christ BDI Beck Depression Inventory C capacitance in farads / celsius, a unit of measurement for temperature c speed of light in metres per second / impact of surrounding in CIECAM02 CAD Computer Aided Design CAM Colour Appearance Model CAT Chromatic Adaptation Transform CBE Chemical Beam Epitaxy CCD Charge-Coupled Device CCFL Cold-Cathode Fluorescent Lamp CCR Constant Current Reduction CCT Correlated Colour Temperature cd candela, SI unit of luminous intensity CDI Colour Discrimination Index CFL Compact Fluorescent Lamp CMF Colour Matching Function CGL Charge Generation Layer CIE Commission Internationale de l’Eclairage CMS Colour Matching System CMYK Cyan, Magenta, Yellow, Black COB Chip On Board CQS Colour Quality Scale CRC Colour Rendering Capacity CRI Colour Rendering Index CRM Colour Rendering Map CT Colour Temperature D diode / degree of chromatic adaptation to the reference white in CIECAM02 D65 CIE standard illuminant D65 iv DALI Digital Addressable Lighting Interface DC Direct Current / chromaticity distance DE Germany DH Double Heterojunction E electric field in newtons per coulomb / flux in watt EBL Electron Blocking Layer EL Electroluminescence EMI ElectroMagnetic Interference EML Emission Layer ErP Energy related Products ES España Esp electric field generated by the space charge ETL Electron Transport Layer eV electronvolt, a unit of energy Ev illuminance in lux F factor determining the degree of adaptation in CIECAM02 f frequency in hertz FCI Feeling of Contrast Index FET Field-Effect Transistor FM Frequency Modulation FP7 Seventh Framework Programme for Research and Technological Development FSCI Full Spectrum Colour Index FWHM Full Width at Half Maximum GSM Global System for Mobile communications hamiltonian operator HBL Hole Blocking Layer HID High Intensity Discharge HIL Hole Injection Layer HI-LED Human – centric Intelligent LED engines for the take up of SSL in Europe HPS High Pressure Sodium HTL Hole Transport Layer I current in amperes i(t) induced current in amperes IC Integrated Circuit IES Illuminating Engineering Society If forward current of an LED in amperes ipRGC Intrinsically Photosensitive Ganglion Cell IR Infrared IRC Infrared Coated IREC Institut de Recerca en Energia de Catalunya IRTA Institut de Recerca i Tecnologia Agroalimentàries Is source current / reverse bias saturation current in amperes ISBN International Standard Book Number ITO Indium Tin Oxide Iv luminous intensity in candela J current density in amperes per square metre JND Just Noticeable Difference K kelvin, SI unit of temperature v KSS Karolinska Sleepiness Scale L inductance in henries L* L* coordinate of the CIELUV and CIELAB colour space LA absolute luminance of the adapting field in CIECAM02 LCD Liquid Crystal Display LED Light Emitting Diode LER Luminous Efficacy of Radiation Lmes Luminance under mesopic conditions in candela per square metre LPS Low Pressure Sodium LTE Long Term Evolution Lv luminance in candela per square metre lx lux, SI unit of illuminance MBE Molecular Beam Epitaxy MCPCB Metal Core Printed Circuit Board MCRI Memory Colour Rendering Index MEQ Morningness – Eveningness Questionnaire MH Metal Halide MOVCD Metalorganic Vapour Deposition MQW Multiple Quantum Well MV Mercury Vapour n index of refraction Nc chromatic induction factor in CIECAM02 NHC N-Heterocyclic Carbenes º symbol for degree OLED Organic Light Emitting Diode OSA The Optical Society p purity of a spectrum PANAS Positive and Negative Affect Scale PAR Photosynthetically Active Radiation PC Phosphor Converted / Polycarbonate PCB Printed Circuit Board PD power dissipation in watt PET Polyethylene Terephthalate PLED Polymer Organic Light Emitting Diode PMMA Poly(Methyl Methacrylate) POF Plastic Fibre Optic Popc flux of a phosphor-converted LED in watt Popp flux of a phosphor pump in watt PSQI Pittsburgh Sleep Quality Index PSU Power Supply Unit PVN Paraventricular Nucleus PWM Pulse Width Modulation Q thermal flux in watt Qe radiant energy in joule Qv luminous energy in lumen . seconds qVa electrostatic potential energy of a charge q in point a R resistance in ohms Ra the CIE general colour rendering index vi RAM Random Access Memory RCRI Rank-order Colour Rendering Index Rf feedback resistance in ohms RGB(W) Red, Green, Blue(, White) RHT Retinalhypothalamic Tract Ri colour rendering