Summary of doctoral thesis in Material science: Research and development of micro-nano optical materials and free form optics using in solid state lighting

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The research objectives of the thesis: Improve the usability and quality of LED-based lighting products; develop a new generation of high-efficiency LED lights, evenly distributed, eliminating light pollution, and replacing existing LEDs.

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Nội dung Text: Summary of doctoral thesis in Material science: Research and development of micro-nano optical materials and free form optics using in solid state lighting

MINISTRY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY ----------------------------- GRADUATED UNIVERSITY SCIENCE AND TECHNOLOGY DUONG THI GIANG RESEARCH AND DEVELOPMENT OF MICRO-NANO OPTICAL MATERIALS AND FREE-FORM OPTICS USING IN SOLID STATE LIGHTING Major: Optical, optoelectronic and photonic materials Code: 9440127 SUMMARY OF DOCTORAL THESIS IN MATERIAL SCIENCE Ha Noi – 2020
The thesis has been completed at: Graduated University of Science and Technology – Vietnam Academy of Science and Technology Science supervisors: 1. Ass. Prof. Tran Quoc Tien 2: Ass. Prof. Pham Hong Duong Reviewer 1: … Reviewer 2: … Reviewer 3: …. This thesis was defended at Evaluation Council held at Graduated University of Science and Technology – Vietnam Academy of Science and Technology on , 2020 The thesis can be further referred at: - The library of Graduated University of Science and Techonology - National library of Viet Nam
1 INTRODUCTION 1. The necessity of the thesis The process of urbanization is fast, people live and work mainly indoors under artificial lighting. The artificial lighting environment is different from natural light in terms of length, spectral structure, day and night cycle, and the circadian rhythm of residents will be reversed and cause a lot of physical health consequences, physiology and psychology such as insomnia, obesity, poor physiology, infertility, fatigue, even cancer. Therefore, creating a lighting environment for humans to protect eyevision and health is essential. In this thesis, I will systematically present problems to be solved in the field of interior lighting, derived from the knowledge about the role of light for people. Next is the design, manufacture and application of solid lighting products on the basis of LED combined with free-form optical components in interior lighting. 2. The research objectives of the thesis Improve the usability and quality of LED-based lighting products. Develop a new generation of high-efficiency LED lights, evenly distributed, eliminating light pollution, and replacing existing LEDs. 3. The main research content Design and fabricate white LEDs with Remote-phosphor configuration. Calculate, design, simulate and fabricate free-form optical components (FO) for used in solid lighting. Design and fabricate SkyLED luminaires with integrated FO lenses and build some realistic lighting models.
2 CHAPTER I. INTRODUCTION 1.1. Solid state lighting and LED light sources Solid state lighting with a focus on LED light sources has opened up a revolution in lighting where key contributors were awarded the 2014 Nobel Prize in Physics, and in 2015 declared by the International Union is the light year. With outstanding advantages such as energy saving, high efficiency, ability to change the structure of the emission spectrum, safety for the environment and users, LED light source has become the main alternative light sources replacing all other traditional light sources. 1.1.1. Working principle of LED LED - Light-Emitting Diode. To create luminescent devices, straight band gap semiconductors are used with two impurity types donor (n type) and acceptor (p type), creating a zone called p-n junction, while the device is called the diode. In order to increase the quantum efficiency of components, instead of using homogeneous semiconductors, people use semiconductor materials with different band gap Eg to create a p-n junction called a heterogeneous junction. When the p-n junction uses a semiconductor material with a lower Eg sandwiched between two materials with a higher Eg, the free carriers will be locked inside a region with low Eg, called a quantum well. The quantum well structure increases the recombination probability of the carrier, which increases the luminescent efficiency of the component [3]. 1.1.2. Phosphor materials Phosphor-converted materials are very important components that directly affect the quality and efficiency of the LED light source.
3 Commercial luminescent materials currently used for white LEDs include: Garnet, Alluminat, Silicate and Nitride. The challenge for material scientists and manufacturers is to reduce the emission spectral width of these phosphors or to find new materials to replace. 1.1.3. White LED packages (WLED) There are 3 way to fabricate WLED: 1) Using 4 types of LED chips Blue - Amber - Green - Red combined together to form a White LED Module ( RGBA-WLED); 2) UV LED chip combined with Blue Green Red 3-color phosphor; 3) BLED chip combined with 560nm yellow phosphor. The phosphor-converted White LED (Pc-WLED) is the configuration that plays a key role in today's LED packaging market with the advantages: simplicity, high temperature resistance and stable color quality. 1.1.4. LED luminaire An LED luminaire (LED luminaire) used in general lighting is composed of four components, which are the LED module, the driver source, the heat sink, and the optics. 1.1.5. The characteristics of LED light suorces Characteristics that evaluate the quality and effectiveness of light sources are described by the following parameters: optical parameters (luminous flux, power, optical efficiency), color parameters (emission spectrum, color temperature correlation, color rendering coefficient, color cleanliness), luminous intensity and luminous intensity distribution. 1.2. Free-form optical Devices 1.2.1. Free-form optical devices
4 FO optics is the next generation of modern optics, offering outstanding differentiation features and high system integration. FO is defined as the optics of asymmetrical surfaces or of any shape, designed with non-traditional technologies, including: rotating symmetrical spherical or aspherical components (off-axis section), non-standard rotating symmetrical profiles such as cones, arcs or any other shape, and the FO component conforms to the geometry of the system. FO creates new opportunities for optical designers, while also presenting challenges to fabrication technology and measurement methods. FO is widely used in the fields of green technology, solid lighting, aerospace, agriculture and biomedical. FO components have the potential to revolutionize the optical industry, so now this is an active research field in terms of both fundamental and applied research as well as the development of processing tools, measurement, shown through the number of more than 230 important publications, hundreds of patents and industrial products in recent times. 1.2.2. FO applications in solid state lighting The application of FO components in LED-based solid state lighting technology is one of the most important in recent years due to the benefits of solid state lighting technology in combination with FO. However, the design of a non-imaging optical system requires a new approach, especially because of the variety of optical characteristics of different types of LEDs. In the field of interior lighting, there are many studies focusing on the design and application of smart lighting systems using LEDs
5 [49-51, 78-92], however the non-uniformity of luminance distribution is still the weakness of solid lighting systems. Integrating FO lenses with LED light sources for the purpose of evenly illuminating the ceiling is a trend that attracts a lot of attention from designers. 1.3. Lighting 1.3.1. Human vision Recent research results show that the human eye is not only for seeing, but also a means of connecting the human internal biological clock with the Earth's rotation and the solar system. In urban life, when artificial lighting environment is different from natural light in length, spectral structure, there is no signal at the beginning and the end of day, circadian rhythm will be reversed and has many consequences for physical, physiological and mental health. 1.3.2. Lighting pollution In modern times and in big cities, people spend more and more time in artificial lighting environments. The difference between artificial light environment and natural light environment can be considered as light pollution. As a result of these differences is one of the causes of nearsightedness (myopia) and other human physiological ailments such as poor performance, depression, insomnia, heart disease, weight gain and even cancer [45]. Light pollution factors derive from the limited functionality of the light source types, as well as due to weaknesses in lighting design. 1.3.3. Human centric lighting
6 HCL-Human Centric Lighting- is the most important topic and most actively discussed today, as the industrial revolution 4.0 comes to life. Recent studies also show that the huge difference between the natural light environment and the artificial light environment is one of the causes of various types of distortion such as myopia, amblyopia, and sleep, amnesia, depression, infertility ... [46, 59-64]. Scientists, R&D centers of lighting companies have many researches, looking for new human centric lighting solutions, that is, creating artificial lighting environments that are close to the natural light environment that consistent with human circadian rhythms [106-109]. In this thesis, I will focus on the research, design and fabrication of new light sources based on LED light sources integrated with FO components to solve pressing problems in lighting such as glare, inconvenience, loss of rhythm, protect eyesight and health for users. CHAPTER II. METHOD, TECHNIQUES AND TECHNOLOGY 2.1. Calculate, design and simulate using assistant software We used specialized software such as Excel, Origin to calculate and analyze data, especially in the process of processing measurement data of fluorescence spectra, reflection spectrum, transmission spectrum... To design optical or mechanical components, we have chosen Solidworks software to design lens profiles, light trays and auxiliary details such as headlamps, suspension bars... After designing the lens profile, we conducted optical simulation to evaluate the light redistribution efficiency of the optical
7 lens system using optical simulation software such as Optgeo, Tracepro, Zeemax... 2.2. Techniques and technology 2.2.1. Fabricating FO prototypes After designing the structure for FO components, we fabricate FO prototypes using one of two technologies: CNC cutting from PMMA materials and 3D printing using transparent optical material. The prototype after creating has many defects, which needs to be completed by 3 methods: grinding and polishing; processing temperature; optical coating. Experimental results for the optical coating method gave the best results. 2.2.2. FO manufacture using plastic injection molding technology We use only thermoplastic injection technology in the manufacture of FO lenses and other optical components in our LED luminaire production chain. The chosen materials for use depend on the needs and specific use of the product. Specifically, the asymmetrical and narrow lens sample is manufactured from PS material, while the lens for fish lamps uses PC material. 2.2.3. Aluminum extrusion technology For large-scale production, aluminum casting technology and aluminum extrusion technology are used instead of CNC machine tool cutting technology. We choose to design linear lamps with the aim of saving costs, creating a competitive advantage when wanting to commercialize the product.
8 2.3. Measurement and evaluated method 2.3.1. Methods and equipment for measuring characteristics of materials We have used some methods and equipment of the Key Laboratory of the Institute of Materials Science to measure the characteristics of materials and components. Fluorescence measurement system investigates the emission spectrum of phosphor materials for LEDs. To determine the gloss of FO components after thermoplastic injection molding, the FSEM scanning electron microscope is a suitable device. Usually the gloss of optical components is about λ/5 to λ/10 equivalent to 100nm to 50nm. 2.3.2. Equipment for measuring characteristics of LED light suorces Intergrating sphere measuring system is a combination of measuring devices including: intergrating sphere, spectrometer, luminous flux probe, computer with display software support, used to measure optical - electrical parameters of the light source. Goniophotometer: is a device for measuring the distribution of light intensity with angle. Measurements are standardized and stored in IES format, where the luminance (unit of measure cd) is a function of the projection angle. 2.4. Lighting model installation Simulating the lighting environment using Dialux Evo software We used Dialux Evo simulation software to calculate the lighting options for buildings to make the best choice. The actual
9 construction model gives the same results as the simulation when choosing the simulation values close to the actual state of the model. CHAPTER III. DESIGN AND FABRICATION OF WHITE LED LIGHT SUORCE USING REMOTE-PHOSPHOR CONFIGURATION 3.1. Introduction Today's white LEDs are mainly produced by coating phosphor directly onto blue LED chips. With this configuration, most of the light comes out from the LED chip, fluorescent light is reflected back and causes loss. The configuration to take the phopshor away from the LED chip has been proposed by many authors, effectively increasing lighting efficiency and lamp life [95, 96, 105]. In this thesis, we have proposed and fabricated white LEDs with RP (remote-phosphor) configuration that have superior features than previously used configurations. 3.2. Design and fabrication of white LED light suorce with RP configuration 3.2.1. Design white LED light suorce with RP configuration White LED remote - phosphor configuration uses a flat LED plate designed by us includes the blue LED array as the source, the radiator tube for the LED, a light guide plate covered with phosphor + scattering film, diffuser plate, reflector plate. 3.2.2. Design and fabrication of light guide plate The novelty of the solution proposed by us in this solution is the light redistribution and conversion structure that allows the creation of a new, high-performance, non-glare light source. The selected
10 light guide plate is made by PMMA material with a rectangular shape. Light beam will propagate along the plate due to the total reflection effect at the interface between air (n=1) and optical medium (n=1.5) with the critical angle 42o. On the upper interface of guide plate, a matrix of extraction dots was printed using silk screening technique. The shape, composition and size of the extraction dots will determine the proportion of the composition of the light coming out from surface of the light guide sheet. The matrix of extractions dots is made by silk screen printing method of a mixture YAG:Ce3+ and EPI glue (a solvent used in screen printing technology). 3.2.3. Fabrication of white LED light suorce with RP configuration A light guide plate made of PMMA material with size 160x270x5 mm3 above is printed with extraction dots inserted into the slot of the heatsink, below is stuck with LED printed circuit. The LED we choose to fabricate is Osram's OSLON with efficiency about 56%. Different color RP-configured white LEDs have been successfully fabricated (Figure 3.5). Fig. 3.5. White LED lamps with RP configuration
11 3.2. Measuring and investigating optical parameters of RP- configured white LEDs The fluorescence imaging of the extraction dots showed that the phosphor particles were unevenly arranged and clustered together, forming a multilayer structure in some locations. The measurement of the optical parameters of the luminaire show that, when the phosphor ratio in the glue/phopshor mixture increases, the CCT of the luminaire decreases, but the color rendering coefficient (CRI) of the luminaire is almost unchanged. . The CRI enhancement solution for luminaires is to add the red phopshor component to the fabricated glue / phosphor mixture. CHAPTER IV. COMPUTERING, DESIGN, SIMULATION AND FABRICATION OF FREE-FORM OPTICS 4.1. Design and simalution of free-form optics By the ray drawing method, we simulated and selected asymmetric lens (AL) profile and the optimal installation conditions of the ceiling beacon to produce a secondary light source (ceiling house) has the most uniform intensity of light in a fixed size room. 4.1.1. Design free-form optics profile We started with 04 cylindrical AL lens profiles with cross section as shown in Figure 4.1. The inner surface of the AL lens has an asymmetric profile, with the refractive power increasing gradually from the left side to the right side. 4.1.2. Simulating illuminance on the ceiling and on the floor We simulated the intensity distribution on the ceiling and the illumination distribution on the floor with the following conditions:
12 room size: 4x4m2, height 3m; distance from LED to ceiling: 0,4m; initial selected projection angle from the horizontal: 60o; optical detector placement: on the ceiling or on the floor. Fig. 4.1. Designed lenses by modifying inner and outer curvatures and thickness The results of simulating the light intensity distribution on the ceiling and the illuminance distribution on the floor show that for the selected asymmetrical profile of 4 AL lens samples, the lens sample with B4 profile gives the best result. This lens model has been selected for further calculations of the distance and angle of the lamp suspension. 4.1.3. Multiparameters optimization After selecting the B4 profile, we continued to simulate the dependence of the uniformity on the distance and projection angle to optimize the lighting solution. Distance factor optimization: We evaluated the lighting results for a 4x4x3m3 room by varying the distance from the lamp to the
13 ceiling. Simulation results show that uniformity increases with increasing distance from lamp to ceiling. Optimized projection angle: We evaluated the lighting results for a room with the size of 4x4x3m3, the distance from the lamp to the ceiling was 0,4m and the projection angle changed. The results show that the angle of 60o is optimal. Conclusion: In [CT 1], we have proposed a ray drawing simulation method to optimize the AL lens profile suitable for a typical lighting model. The limitation of simulation method in the project [CT 1] is that the criteria for selecting AL lenses with profiles B1 to B4 are just comparing the illumination uniformity with each other, not giving the total criteria. 4.2. Theoretical calculation to creat a criterion for lens design [CT 2] project proposes a different approach to indirect lighting solution using LEDs to create a high uniform intensity distribution. This is a theoretical analysis method to create a criterion for the design of LED luminaires incorporating FO lenses with an ideal light intensity distribution curve. 4.2.1. Modeling and theoretical analysis Our model to be illuminated is a room with length many times larger than width, whereby the ceiling is lighting by two LED light sources with the length equal to the length of the room. The illuminance on the ceiling will be calculated by the formula:
14 The width of the room is L and dividing into n parts of nx∆L. The angles of θi and αi form between Li and the light source and between the right-edge Li and the side-wall, respectively. We have investigated the effect of LED installation location and room size on the illuminance distribution by varying parameters h and L. The results show that when the h/L ratio increases, the uniformity increases. 4.2.2. Alternative approach to uniform lighting system We found it impossible to achieve an evenly distributed indirect lighting system using two long conventional LED arrays. A completely new design approach and theoretical approach is required to create a room with a uniformly illuminated ceiling, becoming a user-friendly secondary light source. In order to obtain a perfectly uniform luminance distribution on the ceiling when illuminated by two linear light sources from opposite sides, all light sources that produce illuminance distributions on the ceiling with a Logistic function satisfy this condition. The Logistic function is represented as: where L is the curve’s maximum value; k is the Logistic growth rate; x is the distance; xo is the value of x at the midpoint of ceiling. Figure 4.14 (a) illustrates the luminance distribution line in the form of a function f (x) with speed k = 3 on a ceiling projected from two opposite light sources. When both light sources work together, the illuminance on the ceiling will be completely uniform due to the
15 superposition of the symmetrical beam across the center of the ceiling. Figures 4.14 (c) and (d) illustrate the schematic of the angular illuminance distribution ω between the light ray and the horizontal direction, plotted in the perpendicular and polar coordinates of an LED light source integrated with the FO lens system with k values ranging from 1 to k = 5. It is found that the opening angle of the beam at half height is quite narrow (~ 12o) and is asymmetrical, extending to the right with a large angle ω. Fig. 4.14. (a) The light distribution for both left and right LEDs with k = 3; (b) the light distribution of the left LED source following Logistic function for several k; and (c) and (d) luminous intensities as the function of beam angle of the left LED source for several k Conclusion: By the mathematical analysis method, we have given the general formula for LED luminaires to achieve absolute uniformity, based on Logistic function form with different k growth rate. Calculation results in some cases from k = 1 to k = 5 show that the larger k, the faster the variable speed, so to facilitate the design of FO lenses, the small k value should be chosen.
16 Another result of the mathematical analysis method in [CT 2] is the angular luminous intensity distribution diagrams of the lens- integrated LED light source with different k growth rates. These obtained theoretical luminance distribution plots allow direct comparison with the measured luminance distribution plots of actual luminaires. 4.3. Fabrication of free-form optical components 4.3.1. AL-Asymmetric lens Asymmetric profile lenses are empirically designed based on an idea that has been protected in Utility solution [GPHI- 11]. The AL lens profile is divided into three parts: the converging part is a half-convex cylindrical lens, the non- refractive transmission part is 1/4 cylindrical, and the flat base is used to attach into heatsink, drawn on Figure 4.19a. AL lens version V1 is made by thermoplastic injection molding method, with starting material is GP-PS (Figure 4.19 b). Figure 4.19 a / (left) AL lens profile V1 version; b / (right) Lens photo V1 version made of GP-PS 4.3.2. NAL - narrow angle lens The V1 first version of the NAL narrow angle lens is designed based on the concept protected by the Utility Solution [GPHI 6], following the technological process employed to
17 make asymmetric lenses AL. The purpose of this Solution is to propose an LED structure with a limited illumination angle of less than 80o, while at the same time creating a uniform luminance distribution on the table top, in order to save energy and prevent glare for the student when they are looking at the board, it contributes to reducing the rate of refractive errors. Fig 4.27. Light intensity distribution diagram simulated for LEDs with integrated NAL lenses (left) and the photo of NAL lenses made of GP-PS (right) Figure 4.27 shows some NAL lenses fabricated by thermoplastic injection molding, along with a 3D image of the IES intensity distribution chart of lens integrated LED luminaires. 4.3.3. FO lens for fishing lamps FO lenses are the core difference of the fishing lamp that we designed and manufactured, in order to redistribute light on the sea surface. The cylindrical lens surface is divided into three parts including the converging part, the non-refractive transmission part and the base part used to attach the heatsink. Asymmetrical cylindrical lenses made by thermal injection have the dimensions 165mm long, 120mm wide and the cross- section as shown in Figure 4.29.
18 Fig. 4.29. Sample FO lens for fishing lamps CHAPTER V. DESIGN, FABRICATION OF SKYLED LUMUNAIRES INTERGRATED FO LENSES AND INSTALLATION LIGHTING MODEL 5.1. LED luminaire incorporates asymmetric profile lenses 5.1.1. Wall mounted SkyLED luminaires Wall mounted SkyLED luminaire designed by us (Figure 5.1) and manufactured (Figure 5.3) [SC7] uses an LED light source integrated with AL asymmetric lens to redistribute the light, etch to deal with the glare and inconvenience of existing lamps. The optical - electrical parameters of the luminaire were measured on an intergrating sphere measuring system at the Institute of Materials Science. The results showed that the parameters of the luminaire met the specified standards: color temperature CCT 5054K, CRI ~ 85, optical efficiency E = 97lm/W, total power of the luminaire is 18W.