A PORTABLE RGB LED LIGHT SOURCE WITH COLOR CONTROL MECHANISM FOR
BACKLIGHTING APPLICATION
SAIFUL AZRUL BIN SAARIF
UNIVERSITI SAINS MALAYSIA
2016
A PORTABLE RGB LED LIGHT SOURCE WITH COLOR CONTROL MECHANISM FOR BACKLIGHTING APPLICATION
by
SAIFUL AZRUL BIN SAARIF
Thesis submitted in fulfillment of the requirements for the degree of
Master of Science
September 2016
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ACKNOWLEDGEMENT
I would like to express my sincere and heartfelt gratitude to my supervisor Dr. Arjuna bin Marzuki for guiding me through with his expertise till the successful completion of this project. This research would not have been successful without his valuable guidance and constructive criticisms.
I also would like to express my sincere thanks to Mr. Suardi bin Kaharuddin and the Final Year Project (FYP) students, Nur Adibah binti Rozali, for their helps through the project development. Not to forget to Mr. Abdul Latip bin Hamid, Mrs.
Sharina binti Mustapa Albakari, Mrs. Roziana binti Mat Said, Mrs. Zammira binti Khairuddin and to all the lab technicians that I’ve met, thank you for the support throughout my research.
Special thanks to my wife, Siti Rohaya binti Abdul Aziz for her advice, encouragement and understanding, also thanks to my parents and my parent’s in-law for their continuous support in going through this research. Thanks to all those who had been supporting me in one-way or another during my research. They helped me out far more than they ever realized, their supports are very much appreciated.
Last but not least my obligations and an appreciation go to the School of Electrical and Electronic Engineering and Institute of Postgraduate Study (IPS) Universiti Sains Malaysia (USM) for allowing me to use their facilities throughout my research. Special thanks to Mr. Mohd Rahmat bin Ariffin for his assistant in student matters.
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TABLES OF CONTENTS
Page
ACKNOWLEDGEMENT ii
TABLE OF CONTENTS iii
LIST OF TABLES vii
LIST OF FIGURES ix
LIST OF ABBREVIATIONS xii
LIST OF SYMBOLS xiv
ABSTRAK xv
ABSTRACT xvi
CHAPTER ONE – INTRODUCTION
1.1 Overview 1
1.2 Problem Statement 2
1.3 Objectives 3
1.4 Scope of Work 3
1.5 Thesis Organization 4
CHAPTER TWO – LITERATURE REVIEW
2.1 Introduction 5
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2.2 RGB LED Application 5
2.3 Color 6
2.3.1 Metrics to Describe Color 9
2.3.2 Additive Color Mixing by RGB LED 11
2.4 Driving and Dimming the LED 12
2.5 Color Control Techniques 15
2.5.1 LED Temperature Detection 16
2.5.2 Optical Sensor 20
2.5.3 Combine Method 22
2.6 Elimination of Ambient Light Disturbance 25
2.7 Summary 26
CHAPTER THREE – METHODOLOGY AND IMPLEMENTATION
3.1 Introduction 27
3.2 Color Control Mechanism Design Flowchart 27
3.3 Hardware Development 29
3.3.1 Specifications 29
3.3.2 Hardware Block Diagram 30
3.3.3 LED Characterization 32
3.3.4 Photosensor Characterization 32
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3.3.5 Microcontroller Characterization 33
3.4 Software Development 35
3.4.1 System Parameter 35
3.4.2 Closed-Loop System Flowchart 38
3.4.3 PWM Frequency and ADC Acquisition Time 41
3.4.4 Source Code Implementation 43
3.5 Control Mechanism Development 43
3.5.1 LED Junction Temperature 43
3.5.2 LED Radiant Intensity 45
3.5.3 Ambient Light Disturbance Elimination 47
3.5.4 RGB LED Input Circuit 47
3.5.5 Sensor Output Circuit 50
3.5.6 Resistor-Capacitor Filter 53
3.5.7 LED Forward Voltage with Resistor-Capacitor Filter 55 3.5.8 Transimpedance Amplifier with Resistor-Capacitor Filter 56
3.6 System Development 57
3.7 Summary 58
CHAPTER FOUR – RESULTS AND DISCUSSION
4.1 Introduction 59
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4.2 Maintaining Color Point 59
4.2.1 RGB LED Characterization 59
4.2.2 PWM Direct Connection to the RGB LED 63
4.2.3 PWM through Resistor to the RGB LED 64
4.2.4 Additive Color Mixing 65
4.2.5 Photodiode Amplifier Feedback Resistance 66
4.2.6 LED Forward Voltage 73
4.2.7 Open-Loop System 80
4.2.8 Closed-Loop System 81
4.3 Ambient Light Disturbance Elimination Results 90
4.4 Summary 91
CHAPTER FIVE – CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 92
5.2 Recommendations and Future Work 93
REFERENCES 94
APPENDICES
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LIST OF TABLES
Page
Table 2.1: Wavelength of distinct colors 6
Table 2.2: Radiometric and photometric unit of measurement 8
Table 3.1: Specifications of microcontroller 29
Table 3.2: Specification of RGB LED 30
Table 3.3 Hardware Components 31
Table 3.4: Reference and random reading parameters description and abbreviation
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Table 3.5: Obtained calculated resistance 50
Table 3.6: Parameters values for initial gain amplification 51
Table 4.1: Temperature Influenced on LED Forward Voltage 60
Table 4.2: Temperature Influenced on LED Radiant Intensity 61 Table 4.3: Microcontroller port test result without connection to LED 63
Table 4.4: Microcontroller port connection to LED 64
Table 4.5: Microcontroller port connection to potentiometer and LED 64
Table 4.6: PWM values for RGB LED additive color mixing 65
Table 4.7: RGB LED radiant output for 250 kΩ gain amplification results 67
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Table 4.8: ADC results of the RGB LED mixed-color radiant output 69
Table 4.9: Red LED radiant output ADC results 70
Table 4.10: Green LED radiant output ADC results 71
Table 4.11: Blue LED radiant output ADC results 71
Table 4.12 Cree RGB LED forward voltage ADC results 74 Table 4.13: Forward voltage ADC results with RC Filter 75
Table 4.14: Open-loop test of LED forward voltage 80
Table 4.15: Open-loop test of LED radiant output 81
Table 4.16: Cree LED forward voltage ADC results in closed-loop system 81
Table 4.17: Seoul LED forward voltage ADC results in closed-loop system 82 Table 4.18: Cree LED radiant intensity ADC results in closed-loop system 84
Table 4.19: Seoul LED radiant intensity ADC results in closed-loop system 85
Table 4.20 Control mechanism performance for Cree RGB LED 87 Table 4.21 Control mechanism performance for Seoul RGB LED 87
Table 4.22: LED radiant output with ambient light 90
Table 4.23: LED radiant output with ambient light elimination 91
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LIST OF FIGURES
Page Figure 2.1: Illustration of electromagnetic radiation solid angle 7
Figure 2.2: Distinct color areas in CIE 1931 chromaticity diagram 9
Figure 2.3: Δuʹvʹ shifts in CIE 1976 chromaticity diagram 10 Figure 2.4: Color gamut location creates by RGB LED 12
Figure 2.5: Illustration of PWM duty cycle 14
Figure 2.6: Junction temperature measurement for RGB LED 17
Figure 2.7: Junction temperature measurement in system 18
Figure 2.8: Thermal resistance model 19
Figure 2.9: Optical feedback system for backlight application 20
Figure 2.10: Control structure of the RGB LED lighting system 22 Figure 2.11: Improved control structure of the RGB LED lighting system 24
Figure 2.12: Light measurement in a PWM single period 26 Figure 3.1: Flowchart of designing color control mechanism 28
Figure 3.2: Block diagram of the proposed design system 30
Figure 3.3: LED placement position 32
Figure 3.4: TIA 33
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Figure 3.5: Color error in different PWM resolution 34 Figure 3.6: Control mechanism closed-loop block diagram 36
Figure 3.7: Closed-loop system flowchart 39
Figure 3.8: Radiant output intensity reading for RGB LED 46
Figure 3.9: Ambient light disturbance reading 47
Figure 3.10: PWM output to R, G and B LED 48
Figure 3.11: PWM output to resistor in series with R, G and B LED 50
Figure 3.12: Photodiode and TIA connection 51
Figure 3.13: Simple RC filter 53
Figure 3.14: RC filter with 0.47 ms time constant 54
Figure 3.15: RC filter with 1 ms time constant 55
Figure 3.16: RC filter to LED input 56
Figure 3.17: RC filter to TIA output 57
Figure 3.18: PCB integration for the system 58
Figure 4.1: RGB LED Forward Voltage and Temperature 60 Figure 4.2: RGB LED Radiant Intensity and Temperature 62
Figure 4.3: Additive Color Mixing by RGB LED 66
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Figure 4.4: Mixed-color Radiant Output of RGB LED 68
Figure 4.5: Oscilloscope Test Results for Implemented RC Filter 69
Figure 4.6: RGB LED radiant output result in ADC values 73
Figure 4.7: Forward voltage of RGB LED Cree LED 75
Figure 4.8: RGB LED forward voltage ADC result with RC filter 76 Figure 4.9: Forward voltage changes versus PWM duty cycle 78
Figure 4.10: Forward voltage changes versus Temperature 79
Figure 4.11: Cree RGB LED forward voltage compensation 83 Figure 4.12: Seoul RGB LED forward voltage compensation 83
Figure 4.13: Regulated Cree RGB LED radiant intensity 88 Figure 4.14: Regulated Seoul RGB LED radiant intensity 89
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LIST OF ABBREVIATIONS
ADC Analog-to-Digital Converter CCFB Color Coordinate Feedback CCFL Cold Cathode Fluorescent Lamp CCR Continuous Current Reduction CCT Correlated Color Temperature
CIE International Commission on Illumination CRI Color Rendering Index
DC Direct Current
DIP Dual Inline Package
EEPROM Electrically Erasable Programmable Read Only Memory FFB Flux Feedback
I/O Input and Output
LCD Liquid Crystal Display LED Light Emitting Diode LSB Least Significant Bit
PC Phosphor-Coated
PCB Printed Circuit Board
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PIC Peripheral Interface Controller PID Proportional Integral Derivative
PWM Pulse Width Modulation
RAM Random Access Memory ROM Read Only Memory
RC Resistor-Capacitor
RGB Red Green Blue SSL Solid State Lighting TFF Temperature Feed Forward TIA Transimpedance Amplifier
UART Universal Asynchronous Receiver/Transmitter
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LIST OF SYMBOLS
cd Candela
μ Micro
f Frequency
F Farad
Hz Hertz
° Degree
Ω Ohm
∆ Delta
λ Lambda
τ Time Constant
K Kelvin
C Celsius
ω Omega
sr Steradian
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PUNCA CAHAYA DIOD PEMANCAR CAHAYA MERAH HIJAU BIRU MUDAH ALIH DENGAN MEKANISME KAWALAN WARNA UNTUK
APLIKASI LAMPU BELAKANG ABSTRAK
Tesis ini mengemukakan pembangunan mekanisme kawalan warna untuk sumber cahaya oleh Diod Pemancar Cahaya (LED) Merah Hijau dan Biru (RGB).
Pencampuran secara penambahan daripada LED asas tersebut memerlukan mekanisme kawalan kerana warna yang dihasilkan tertakluk kepada variasi. Ini adalah disebabkan peningkatan suhu ambien dan pemanasan sendiri oleh LED tersebut. Titik warna akan berubah kerana penurunan keluaran keamatan sinaran tiap-tiap LED adalah tidak sama. Pengawal mikro dicadangkan sebagai unit pusat untuk mendapatkan set titik permulaan pencampuran warna seterusnya mengawal setiap LED apabila sisihan titik warna baru berubah melebihi had. Kaedah ini bergantung pada jangkaan voltan ke depan terhadap setiap LED dan sisihan ukuran sinaran oleh penderia optik. Mekanisme kawalan warna yang telah dibangunkan boleh mengekalkan gabungan titik warna dengan berkesan apabila tertakluk kepada ujian suhu ambien terpecut menerusi penenggelam haba yang dilekatkan pada LED.
Sistem ini diuji pada dua jenis LED RGB. Sistem ini berupaya untuk mengurangkan 83 % ralat relatif untuk LED jenis pertama dan 78.6 % ralat relatif untuk LED jenis kedua. Ralat relatif diperolehi semasa proses mengukur dengan mekanisme kawalan tertutup mengurangkan ralat sistem kawalan terbuka daripada set titik permulaannya.
Gangguan cahaya ambien juga berjaya dihapuskan semasa penderia beroperasi.
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A PORTABLE RGB LED LIGHT SOURCE WITH COLOR CONTROL MECHANISM FOR BACKLIGHTING APPLICATION
ABSTRACT
This thesis presents the development of color control mechanism for Red, Green and Blue (RGB) Light Emitting Diode (LED) light source. The additive mixing by the primary LED requires a control mechanism since a color produced is subjected to variation. This is due to the influenced of elevated ambient temperature and self- heating of the LEDs. The color point will vary because the individual LEDs output radiant intensity degradation is not the same. A microcontroller is proposed as a central unit to obtain an initial set point of the color mixing thus controlling each LED when the difference of new color point exceeded tolerance. The method relies on the forward voltage prediction to the individual LEDs and the radiant intensity measured by an optical sensor. The developed color control mechanism can effectively maintain the mixed color point when it is subjected to an accelerated ambient temperature test through heating the heat-sink attached to the LED. The system is tested on two types of RGB LED. The system is able to reduced 83% of relative error for the first type LED and 78.6 % of relative error for the second type LED. The relative error is obtained during the measuring process with the closed- loop control mechanism reducing the open-loop control system error from its initial set point. The ambient light disturbance is also successfully eliminated during the sensor in operation.
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CHAPTER ONE
INTRODUCTION
1.1 Overview
Solid-State Lighting (SSL) devices such as a light emitting diode (LED) and an organic LED (OLED) have shown a great potential to enhance the performance of artificial lighting. LED light source has shown its significant economic benefit due to their lower operating condition and longer lifetime’s service (Azevado et al., 2009).
Thus, the LEDs are better in energy consumption compared to the other conventional light sources such as incandescent light bulb, halogen light bulb and most compact fluorescent light (CFL). Although the LED luminaires lag slightly behind fluorescent light luminaires, continuous innovation has shown competitive improvement in its efficacy (Bardsley et al., 2014).
Despite that the CFL shows a good light efficacy, it is not environmental friendly since mercury is added in the fabrication process and poses a danger to our environmental health. Hence, LED is a greener choice for lighting purposes (Kim and Schubert, 2008). Moreover, LED shows inherent controllability that their luminance can be controlled or dimmable compared to other conventional light sources. This SSL device also offers saturated colors, robustness and small size device (Jacobs, Jie and Hente, 2008). Other advantages of LEDs are they are anti- vibration and shock resistant devices (Bergh et al., 2001).
There are two methods to generate white light for general illumination for example by the phosphor coated (PC) white LED or by mixing the light output of red, green, and blue (RGB) LED. The PC white LED suffers from shorten lifetime
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because of phosphors deterioration and efficiency dropped caused by the Stoke-shift characteristic of the down conversion which a phosphor absorbs shorter wavelength energy and emits it at a longer wavelength (Zukauskas et al., 2002).
Theoretically, color mixing from RGB LED could provide better efficiency and enable color adjustability. At present, this method is most admirable feature for future LED lamps or other applications such as backlighting because of it individually controllable. In turns, various shade of white or color can be created by tuning the ratios of each RGB LED strings (Ying, Tang and Huang, 2006), (LED Color Characteristic, 2012). In order for the RGB LED light source to achieve reliable luminous output and color stability for its application, various color control feedback are applied (Deurenberg et al., 2005). The control system in return could stabilize the luminous and color output that are influenced by ambient temperature and also self-heating of the LEDs.
1.2 Problem Statement
In general, luminous output and wavelength of LEDs are affected by temperature changes, driving current and ageing. The changes in LEDs junction temperature will result in a degradation of lifetime, efficiency and its forward voltage (Manninen and Orrevetelainen, 2007). As for the RGB LED, it is subjected to different luminous output degradation rate of individual LED due to the self-heating and ambient temperature changes. Thus, it will impact on its color point (Chhajed et al., 2005). Therefore, it requires color control mechanism system that able to tune the electrical power of each RGB LED to regulate the luminous output and its color balance.
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Since the optical sensor operates based on the input optical signal, the environment illuminant might affect the accuracy of the detected output. Thus, it is required to prevent the input optical signal from being affected by external environment so that be reliable and usable to mobile outdoor (Muthu and Gaines, 2003).
Most of the explored control techniques are dependent on the International Commission on Illumination (CIE) standard to express a color. However, every human interprets a color differently with their own perception. Because of that, there is not entirely necessary for a system to produce an accurate color. Therefore, the aim of this research is to develop an embedded control mechanism on an arbitrary color point input according to the user desire with using a simple microcontroller.
1.3 Objectives
This project is carried out for the following objectives:
i. To develop an embedded color control mechanism system to maintain color point within minimum accuracy of at least 20 LSB employing 10-bit ADC converter for portable application.
ii. To eliminate the effect of ambient light disturbance when sensing in operation.
1.4 Scope of Work
The scope of the project is to design and implement the control mechanism to regulate the radiant intensity output losses of RGB LED and maintain its color point.
4 1.5 Thesis Organization
Chapter 1 comprises of the overview of the project which introduce the feature and future of RGB LED lighting, problem statement, objectives, scope of the project and thesis organization.
Chapter 2 summarizes the literature review for the RGB LED light source and it’s luminous and color output control system design. Several LED driving techniques will be explained in this chapter. Various type of color control feedback or mechanism used to stabilize the RGB LED color light output will be discussed.
Some background on the RGB LED and its application is studied as well.
Chapter 3 reveals the methodology used for this project. The road map and flow chart of the design will be discussed within this chapter. This chapter will describe the proposed block diagram of the color control circuit, parameters involved in the design, closed-loop flowchart design for the source code implementation and hardware design. The hardware integration is described and implementation using the optical sensor and forward voltage prediction is explained. In addition, the mechanism source code implemented in C programming language is described.
Chapter 4 indicate the results and discussions. The ambient temperature test on the RGB LED light source will be analyzed. This result implies the design of the color control mechanism. All other practical results and analysis from each approach of the methodology are discussed.
Chapter 5 is the conclusion for the achievement of the objectives.
Furthermore, this chapter includes a brief statement of successfulness and future work.
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CHAPTER TWO
LITERATURE REVIEW
2.1 Introduction
In this chapter, some background on the RGB LED and its application is studied. The perception of color and additive color mixing by using the RGB LED is discussed. In addition, a review is done on the methods of driving and dimming the LED. The color control feedback techniques are studied and elaborated to gather the information in order to implement the control mechanism system.
2.2 RGB LED Application
RGB LED can provide white light sources with variable colour point in consideration of tunable white point, wider color gamut, faster response, longer lifetime, high dimming ratio and higher luminance efficacy make RGB LED suitable as a backlight for high end Liquid Crystal Displays (LCD) applications (Muthu et al., 2002), (Hsieh et al., 2009). The advantages of LCD are light weight, high resolution and good color performance. LCD used for a wide range of display applications in consumer products such as notebooks, PC monitors, televisions, cellular phones and multimedia products (Cho and Kwon, 2009), (Nam et al., 2009). As display size and resolution are increases, it is also fundamental to optimize display backlighting.
Backlighting module is important to drive light source in LCD technologies in consideration of LCD display quality since LCDs are not self-luminance display devices (Liu et al., 2010).
For general lighting illumination purposes, RGB LED has shown an advantage over Phosphor-Coated (PC) white LED as its luminous output can be
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tuneable. It could provide different white color temperature such as cool white, warm white and can be set to other white light according to the user mood (Lim et al., 2006).
2.3 Color
Color is defined as perceptual results of light in the visible region of spectrum. The human eye is sensitive to a small frequency band, known as visible light, ranging from approximately 390 nm to 720 nm. Human eye perceives certain color corresponding to certain wavelength within the electromagnetic spectrum. For instances, human will interpret visible light of the largest wavelength as red while that of smallest wavelength as blue, other colors will be interpreted according to their corresponding wavelength as shown in Table 2.1.
Table 2.1: Wavelength of distinct colors (Schubert, 2006) Color Wavelength
(nm) Color Wavelength (nm)
Ultraviolet < 390 Yellow 570 – 600
Violet 390 – 455 Amber 590 – 600
Blue 455 – 490 Orange 600 – 625
Cyan 490 – 515 Red 625 – 720
Green 515 – 570 Infrared > 720
The radiometry is the detection and measurement of electromagnetic radiation across the total spectrum. Some terms used in the radiometric measurement such as the radiant power, radiant intensity, irradiance and radiance. The radiant power is radiant energy emitted over a surface in unit time interval. For the radiant intensity, it is defined as a result of the radiant power emitted by a source in an extremely small solid angle. The irradiance is a result of radiant power incident on a
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surface, whereas the radiance is a measure of radiant power at the receiver (Wyszecki and Stiles, 1982).
Figure 2.1 depicts the illustration of solid angle of the electromagnetic radiation. As depicted, the solid angle cuts off an area on the surface of a sphere, S centered at C and radius of r. The solid angle size is obtained by
(2.1) The measurement unit of solid angle is in steradian unit denoted as sr.
Figure 2.1: Illustration of electromagnetic radiation solid angle (Wyszecki and Stiles, 1982)
On the other hand, photometry is a radiometric power that is scaled by the spectral response of the human eye. Photometry is most likely equivalent to radiometry except the measurement is within the visible wavelength range. Table 2.2 lists some units of measurement of the radiometric and photometric in comparison.
