The Fundamentals and Applications of Light-Emitting Diodes

The Revolution in the Lighting Industry

Govind B. Nair,Sanjay J. Dhoble

• Publisher: Woodhead Publishing
• Year: 2020
• ISBN:
  9780128231616
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The Fundamentals and Applications of Light-Emitting Diodes: The Revolution in the Lighting Industry examines the evolution of LEDs, including a review of the luminescence process and background on solid state lighting. The book emphasizes phosphor-converted LEDs that are based on inorganic phosphors but explores different types of LEDs based on inorganic, organic, quantum dots, perovskite-structured materials, and biomaterials. A detailed description is included about the diverse applications of LEDs in fields such as lighting, displays, horticulture, biomedicine, and digital communication, as well as challenges that must be solved before using LEDs in commercial applications.

Traditional light sources are fast being replaced by light-emitting diodes (LEDs). The fourth generation of lighting is completely dominated by LED luminaires. Apart from lighting, LEDs have extended their hold on other fields, such as digital communications, horticulture, medicine, space research, art and culture, display devices, and entertainment. The technological promises offered by LEDs have elevated them as front-runners in the lighting industry.

• Presents a concise overview of different types of light-emitting diodes (LEDs) based on inorganic phosphors, organic materials, quantum dots, perovskite-structured materials, and biomaterials
• Includes a discussion of current and emerging applications in lighting, communications, horticulture, and medical fields
• Addresses fundamentals, luminescence mechanisms, and key optical materials, including synthesis methods

Part One

Fundamentals

Outline

1. Introduction to luminescence

2. Fundamentals of LEDs

1: Introduction to luminescence

Abstract

Light-emitting diodes (LEDs) have taken the world by surprise with its flair and application in almost every field that can come from human imagination. To understand the imperative presence of LEDs in today’s world, it is essential to know the complete journey from the prehistoric period that slowly and gradually led to the development of this outstanding technology. The history of lighting is broadly classified into four generations and LEDs have dominated the fourth and the latest generation of lighting. This chapter takes through the different phases of lighting that started with the burning of firewood and oil lamps, followed by the lamps working on the principles of incandescence, fluorescence, and, finally, the LEDs. Here, the concepts associated with the luminescence phenomenon and its mechanisms are also highlighted.

Keywords

History of lighting; Luminescence; Photometry; Radiometry; Rare-earth ions

1.1. Characteristics and quality of light

The quality and quantity of the lumen output originating from a light source justifies the imperial amount of research dedicated to improve the efficiency of the luminaire. Time has witnessed many drastic improvements in the lighting industry. From the dawn of life on the earth, the role of the ultimate source of light has been redeemed by the sun. Unconditionally, the sun provides the best quality light that is still unmatched with the present-day luminaires. The eminence of light can only be determined by the eye that perceives it. Apparently, the features of a luminaire are adjusted to give a pleasant perception to the human eye. To optimize the lighting systems from the perception of a human eye, several quantitative metrics were developed.

The interpretation of a light source by a human eye differs from individual to individual due to the differences in their physiology and, consequently, the quantified perception is likely to change from person to person. To standardize the perceptual responses to light stimuli, a statistical analysis was prepared involving several human subjects. Based on those results, the photometric standards for different sources of light were put forward. The part of the eye that is most sensitive to light is the human retina, which consists of two types of cells, viz., the rod cells and the cone cells. Rod cells and cone cells are together known as the photoreceptors. It is estimated that 120 million rod cells and 60 million cone cells exist in the retina. The rod cells are more sensitive than the cone cells and are sensitive to the entire range of the visible light spectrum. On the other hand, the cone cells are sensitive to only a small range in the visible light spectrum and they are characterized into three types depending on their sensitivity to red, green, and blue light spectrum. The cone cells are functional at high light-levels while the rod cells show their functioning at low light-levels. At intermediate light-levels, both of them function together. Based on the light-levels, the vision regimes are categorized into three types, namely photopic vision, scotopic vision, and mesopic vision. The cone cells mediate the photopic vision, and the rod cells mediate the scotopic vision while the mesopic vision relates to light-levels between the photopic and scotopic vision regimes [1].

Radiometric and photometric units are, in general, used to characterize the physical properties of electromagnetic radiation that is significant to the optical quantities for light-emitting diode (LED) metrology. Light is characterized by radiometric units in terms of physical quantities, for instance, the radiant flux, photon energy, and the number of photons. Ideally, the radiometric quantities are four in number, and those are listed as radiant flux, radiant intensity, irradiance, and radiance. Radiant flux is the net power emitted per unit time by a light source and is expressed in the units of watt (W). Another quantity is the radiant intensity, which is expressed in units of watt per steradian (W/sr) and it is defined as the power emitted per unit solid angle. The radiant power is measured using a detector with an active area, which is positioned at some distance from the point source of light. The ratio of the radiant power and the area of the detector is the irradiance, which is measured in the units of watt per square meter (W/m²). The radiant power emitted from an area per unit solid angle is called radiance, and it is generally measured for extended light sources. Radiance is measured in the units of watts per steradian per square centimeter (W/sr cm²). Nevertheless, human eyes fail to have any perception of radiations other than the visible light, and hence, these radiometric units have less significance while considering the light perception by a human eye. In this regard, the photometric units came to the picture and played a great role in the standardization of the perception of light by human eyes [2]. The photometric quantities such as luminous intensity, luminous flux, illuminance, and luminance define the quality of light originating from a source. The luminous intensity represents the light intensity of an optical source as perceived by the human eye and is expressed in the units of candela (cd). A unit candela is defined as the luminous intensity obtained when a monochromatic light source emitting an optical power of (1/683) watt at 555 nm into the solid angle of 1 steradian (sr). The luminous flux is another photometric quantity, which is expressed in the units of the lumen (lm). It represents the light power of a source as perceived by the human eye. Technically, the lumen can be defined as the luminous flux achieved after a monochromatic light source emits an optical power of (1/683) W at 555 nm. On associating the definitions of lumen and candela, it is found that 1 lumen per steradian is equal to 1 cd. When the luminous flux is measured per unit area, the quantity is expressed in terms of illuminance. Lux is the unit used to measure illuminance and it is equal to lm/m². Yet another photometric quantity is the luminance, which is expressed in the units of cd/m². Luminance is defined as the ratio of the emitted luminous intensity to the projected surface area in a certain direction. All the earlier mentioned metrics are useful while defining only the quality of light from a source and do not necessarily describe the actual merit of a luminaire.

1.2. Conceptual background of luminescence

Luminescence is a cold-body radiation or the spontaneous emission of light from a source that does not accompany heat with it. Technically, luminescence is defined as “the emission of light by bodies, which is in excess of that attributable to black body radiation; and persists considerably longer than the periods of electromagnetic radiations in the visible range after the excitation stops” [3]. The materials exhibiting this phenomenon are called as luminescent materials, or simply phosphors. When a phosphor is stimulated by an excitation source, it absorbs the energy from that source and remits it in the form of light. This light can be either in the ultraviolet (UV) region, visible region, or infrared region. On providing proper stimulus to the phosphor, the electronic system of the material gets excited from the ground state to the higher energy state and, finally, decays from this higher energy state to a lower energy state with the emission of photons/light. The wavelength of the emitted light is the characteristic of the material and not of the excitati...