Understanding LED Illumination

M. Nisa Khan

• Publisher: CRC Press
• Year: 2013
• ISBN:
  9781000755756
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Understanding LED Illumination elucidates the science of lighting for light emitting diodes. It presents concepts, theory, simulations, and new design techniques that shine the spotlight on illumination, energy efficiency, and reducing electrical power consumption. The text provides an introduction to the fundamentals of LED lamp design, and highli

1 Introduction

1.1 Introduction

Light is a vitally important physical resource for all living beings. Aside from providing us with vision, light is inherently connected to all life, which we describe as photobiological phenomena. While human beings have always had this integral relationship with light, our significant understanding of its properties and behavior only started a few centuries ago, after which the major discoveries unfurled in steps. Starting in the late 1660s, Sir Isaac Newton initiated the corpuscular theory of light, explaining that light was made up of little particles, or “corpuscles” and that each of these particles did not have the unique color of “white,” but rather these white light particles were composed of a spectrum of discrete colors that can be separated with a prism [1]. In practice, many optical phenomena can be handled with Newton’s theory, which forms the basis of geometric optics or ray optics. Around the same time, Newton’s adversary Robert Hooke deduced that light did not have behaviors of a particle, but rather that of a wave, from which Christian Huygens developed his wave theory in 1690 [2]. However, the wave theory was not vindicated until Thomas Young and Augustin-Jean Fresnel did interference experiments that proved that light has a wave-like property that could not be supported by Newton’s corpuscular theory. Subsequently, diffraction theory was established and the study of physical optics, otherwise known as wave optics, was set forth [3].

Wave optics was further embraced when it was unified with electromagnetic theory by James Clerk Maxwell in the 1860s, establishing that light waves were in fact electromagnetic radiation [4]. At the turn of the twentieth century, Max Planck and Albert Einstein formulated the astonishing theory revealing that light has both wave-like and particle-like properties, which they explained using quantum mechanics [5,6]. When viewed as a particle, light was then referred to as “photon,” falling in the category of “boson,” which behaved based on Bose–Einstein statistics, in contrast to electron particles, which are “fermions” following Fermi–Dirac statistics [7]. Based on subsequent developments of quantum mechanics and electrodynamics, the twentieth century became an energetic and exciting field for physics—optics in particular—leading to amazing discoveries in astronomy and various engineering fields including light-emitting diodes (LEDs), lasers, photodetectors, fiber optics, and others.

From the turn of the twentieth century, as the understanding of light at the fundamental level progressively developed in the field of optics as a branch of physics, lighting development for illumination applications took an unprecedented turn at the same time, starting with the invention of the practical incandescent light bulb generally credited to Thomas A. Edison (albeit with much controversy). The significance of the tungsten light bulb was not only its mere invention, but also how quickly it became ubiquitous in households, particularly in the United States, because of its successful deployment via common electrical systems, affordability, and practical benefits. From 1914 to 1945, lamp sales went from 88.5 million to 795 million units, reaching more than five lamps per person per year [8]. Lighting science and engineering became a field in its own right, primarily for illumination and human vision applications, benefits of which have always been imminent and extraordinary.

Although incandescent lamps are widely used in households and commercial buildings because of their practicability, they consume a great deal of electric energy because the incandescence process typically converts only a few percent of electric energy to visible light and over 90% to invisible thermal radiation. As other lighting technologies have become practical and more energy efficient, incandescent lamps have started to be replaced gradually in many applications. These include such electric lights as linear and compact fluorescent lamps, high-intensity discharge lamps, and LEDs.

Enormous improvements in LED lighting technologies in the past decade have led many to wonder if LED lighting will be chosen for nearly all illumination purposes. The idea is becoming increasingly popular as LEDs’ theoretical luminous efficacy, at the small-scale source level, is calculated to be about twice that of the current state-of-the-art fluorescent lamps. However, the question remains whether this higher efficacy can be scaled for practical-sized lamps that can illuminate omnidirectionally. This book investigates such challenges of LED lighting and analyzes some solutions for practical LED lamps. Recognizing these challenges and further developing various existing solutions and perhaps adding new solutions, LED lamps for general illumination applications could take a significant step forward.

1.2 Lighting Fundamentals

1.2.1 A Very Brief History of the Study of Light

According to modern history, investigation of light, vision, and color began when the Greek philosophers Plato, Aristotle, Democritus, and others provided early philosophical and psychological descriptions of light and color. The development of light science or optics leapfrogged in the twentieth century following several hundred years of scientific discoveries since Isaac Newton’s work on optics in the mid-seventeenth century. The subject of “lighting” is duly distinguished within the general field of optics. Lighting is specific to human vision that is entirely dependent on illumination of objects by means of natural and artificial light and thus only deals with light in the visible spectrum. Optics is the branch of physics involving the behavior and properties of light in general, within the entire optical frequency spectrum encompassing visible, ultraviolet, and infrared light.

As the optical sciences have undergone rigorous development and become relevant to many disciplines including astronomy, medicine, photography, and many engineering fields, such as fiber optics and optical communications, the scientific knowledge and quantitative characterization methods of lighting improved as a consequence. Nevertheless, lighting remains primarily practical and experiential; we utilize it every day and many of us have become naturally accustomed to having artificial light with certain illumination quality.

1.2.2 Introduction to Lighting Fundamentals

Daylight reaching us from the sun is the predominant form of natural light from which our familiarity with light and vision began. Vision has descriptions only in the presence of light, some of which include the color, size, and shape of objects we see and how bright they appear. Without light, we do not see our surroundings or any objects present in areas surrounding us unless the objects are some form of light source themselves, such as fire. Lighting fundamentals have been established to describe what we see and how well we see things in the presence of light. Understanding the lighting fundamentals is crucial for all lamp designers and technologists, in particular for those in the LED industry who are generally missing an illumination background. Because artificial lighting has been around for a long time, people already have a great deal of expectations for lighting quality, user friendliness due to ubiquitous standards, extensive product availability, and fairly low up-front costs.

We require vision and hence lighting simply to view our surroundings, perform visual tasks, and enjoy entertainment shows. Using quantitative parameters, lighting fundamentals offer descriptions of how well light sources illuminate for the purpose of our vision in the presence of those light sources. It should be noted that for practical purposes, such described human vision is not absolute but rather derived from what the average person sees based on some reasonable comparative statistics.

1.2.3 Quantitative Parameters of Lighting

1.2.3.1 Color Metrics

For most viewing applications, we use primarily “white” light, which Newton demonstrated to be composed of different colors with his famous prism experiment, basics of which can be easily repeated as shown in Figure 1.1. White light has a very broad spectrum of colors (i.e., it contains a broad range of frequencies or wavelengths). Since these optical frequencies vibrate at very high rates, our eyes or any other currently known detector cannot sense the fast-varying amplitudes of the light wave. Instead, our eyes assess an average optical power flow, which is referred to as luminous flux. Luminous flux is a scalar light power quantity measured in units called lumen (lm).

Figure 1.1. White light from a compact fluorescent lamp is reflected from a shiny object and the light rays are captured by a camera after passing through a prism. The rays passing through the prism in this picture show the range of individual colors within the broad white spectrum of the fluorescent lamp.

Illumination with broad-spectrum white light allows objects of various colors to be seen close to their inherent colors. This property of light is known as color rendering. White light, such as sunlight, contains all visible optical wavelengths or colors and we define sunlight as “pure” white light, as it is able to render all colors perfectly within our visual spectrum based on our definitions. Color rendering is quantified in a relative manner and is specified as a color rendering index (CRI). Although CRI is measured on a scale from 0 to 100, where 100 is considered to be ideal, low values are not meaningful in practice because they would not correspond to white light sources.

Sunlight has different intensity and hue, or tint, during the day from dawn till dusk and daylight varies accordingly, further changing with different atmospheric conditions that affect sky light scattering. This varying tint can be described with a parameter known as color temperature. The color temperature of a visible light source is the temperature correlated to that of an ideal black-body radiator whose emission radiation best matches the tint of that light source. (A black body is an idealized physical matter that absorbs all incident electromagnetic radiation.) It is thus more accurately known as correlated color temperature (CCT) and has the unit of absolute temperature measured in kelvin (K), which is occasionally also denoted as °K.

The CCT of a light source is quantified as the surface temperature of an ideal black body that emits light having the same color tint as the light source. An incandescent light bulb s...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Acknowledgments
• About the Author
• 1. Introduction
• 2. LED Lighting Devices
• 3. LED Module Manufacturing
• 4. Lamp Measurement and Characterization
• 5. LED Lamp Design Considerations
• 6. LED Lighting Design and Simulation
• 7. LED Replacements for Incandescent and Linear-Tubular Fluorescent Lamps
• References