LEDs- The new revolution in lighting : Foreword

International audienceThe award of the 2014 Nobel Prize for Physics to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources" illustrates the worldwide societal impact of light-emitting diodes (LEDs). It also emphasizes the importance of lighting throughout the history of human societies, relying on ever-improved lighting technologies and acting as a lead user when new energy sources emerged, leading to fast-increasing light production with improvement of service and cost decrease. Nitride-LED-based lighting, with a 100% theoretical power conversion efficiency limit and novel light quality capabilities, is in line with this historic evolution of lighting technologies enabling new implementations and higher lighting use along history. Due to the many high-impact effects of the revolution brought by LED lighting compared to former lighting solutions, it is therefore timely to devote an issue of the Comptes rendus Physique to the exploration of the various aspects of the LED lighting revolution. A first paper by Claude Weisbuch [1] explores the long history of lighting, a major human need throughout the ages, and the serial revolutions brought along by technological advances: the oil lamp, the candle, the gas and kerosene lamps, electrical light bulbs, fluorescent tubes, and finally LEDs. A remarkable feature of this history is that lighting has continuously been at the leading edge of many energy revolutions, the most recent ones being the successive appearance of gas, petrol, and electricity. At the origin of the LED lighting revolution was the invention of the high-efficiency blue LEDs based on nitride semiconductor materials. The invention, development, and status of the blue LEDs is reviewed by Daniel Feezell and Shuji Nakamura [2]. They detail the long road from the first production of device-quality GaN to the present high-efficiency LEDs, which incorporate many advances such as heterostructures and quantum wells, alloy active materials, low-resistivity contacts, light-extracting structuration of LED chips, etc. They also discuss the remaining scientific and technological roadblocks to reach the physical limits of nitride devices. They end up with a discussion of a possible next technology: laser-based lighting. The transition to high-efficiency solid-state lighting (SSL) made possible by LEDs will enable significant reduction in electricity consumption. The state of the art is reviewed by Paul Morgan Pattison, Monica Hansen, and Jeffrey Y. Tsao [3], and they analyze the various sources of losses in lighting systems and their mitigations. They also discuss applications of SSL beyond the simple gain in efficacy made possible by the properties of LED light. The association of high-efficiency LEDs and small low-cost solar panels will have a large impact in both developed and developing economies-in the latter by bringing light to those many parts of the world where there is no electrical grid today. This is part of a worldwide trend towards individual or local pico-power off-grid systems that, associated with other super-efficient appliances, will bring major changes in the quality of life for more than a billion people. The design and impact of these systems is discussed by Peter Alstone and Arne Jacobson [4]. Visible light is produced by LEDs with highly-controllable color rendering. Therefore, a new era in light quality opens up, with a large variety of applications depending on the environment and their implementation-be it in museums, commercial displays, architecture, hospitals, plant factories, etc. At issue is the rendering of natural light by LED luminaires. Marc Fontoynont [5] discusses the present performance as well as the opportunity to generate natural light through LED luminaires. While classic light sources have only a few choices of stable and reproducible colors, LEDs with their versatile spectra open the question of color rendering, and then first of color characterization. Aurélien David and Lorne A. Whitehead [6] describe the recent progress in color science and color rendering norms triggered by the appearance of LEDs