International Journal of High Speed Electronics and Systems

We review the history and state-of-the-art of light emitting diode (LED). Unique properties of LEDs– high efficiency, long life time, flexibility in obtaining customized spectral power distribution, fast modulation rate, compact form factor, ability to generate radiation with wavelengths outside the visible range (infrared and ultraviolet, including deep ultraviolet) enable multiple applications for signage, display, general lighting, biology, medicine, agriculture, communications, industrial controls, security, and transportation. Many of emerging LED applications will be in "smart lighting", which are applications that are not possible or difficult to achieve using more traditional lighting sources.

GaN-based light-emitting diodes suffer from high-current loss mechanisms that lead to a significant decrease in internal quantum efficiency at high drive currents. This phenomenon, known as "efficiency droop," is a major problem for solid-state lighting applications, in which light-emitting diodes are driven at high currents to deliver large optical powers. Although substantial effort has been invested to uncover the physical origin and mitigate the effects of efficiency droop, there is still a lack of consensus on the dominant mechanism responsible. In this article, we review several mechanisms that have been proposed as explanations of efficiency droop, including junction heating, carrier delocalization, Auger recombination, and electron leakage from the active region. In addition, device structures intended to mitigate the droop-causing mechanism – (i) thick quantum wellsl, (ii) enhanced hole-injection efficiency structures, and (iii) polarization-matched active region – are discussed.

This paper outlines the various driving and control techniques for Light Emitting Diodes (LEDs). LEDs should be driven from a constant current source. High power LEDs are usually driven from a switching regulator, for reasons of efficiency. The types of drivers described include Buck (step-down), Boost (step-up) and Buck-Boost (step-up or step-down). Isolated drivers and Power Factor Correction (PFC) circuits are also described. This brief paper can only describe the basic outline of these circuits, but this should be sufficient to allow the basic principles to be understood.

Polychromatic lighting sources that are composed of at least four different colored light-emitting diodes (LEDs) offer versatility in color quality of illumination. In this paper, different methods of assessment of color quality of white light are discussed and a general approach to the solution of the color-mixing problem by means of optimization in respect of several color rendition characteristics is considered. Spectral power distributions of model tetrachromatic solid-state sources obtained by maximizing various figures of merit, such as color rendering index, gamut area index, color quality scale, and indices based on the statistical analysis of the just perceivable chromaticity differences for a large number of test color samples, are demonstrated. A concept tetrachromatic lighting source that can be operated within a dynamical trade-off between two opposing color rendition characteristics, the ability to render colors with high fidelity and the ability to render colors with increased chromatic saturation, is introduced. Such "smart" sources with tailored color quality can meet individual needs and preferences of color vision and find numerous applications in lighting.

Light-emitting diodes (LEDs) can and are currently integrated into light microscopes. They have numerous advantages as illumination sources. Most notably, they provide intensity (brightness) and spectral control during bio-imaging. For transmitted light imaging, LEDs can replace the traditional tungsten filament bulb, while offering longer life, little-to-no color temperature shift resulting from an intensity change, reduced emission in the infrared region, (a property important for live cell imaging), and reduced cost of ownership. For fluorescent imaging, in which the typical illumination sources are mercury or xenon lamps, LEDs offer the advantages of a longer lifespan, greater spatial and temporal stability, elimination of the need for mechanical shutters and neutral density filters, significantly lower cost of ownership, and reduction of photon dose at the specimen. Additionally, LEDs permit vibration-free, high-speed spectral and temporal modulation. This modulation allows more information to be obtained for a given photon dose.

It is well-known that the light/dark cycle incident on the retina regulates the timing of the human circadian system. Disruption of a regular, 24-hour pattern of light and dark can significantly affect our health and well-being. A wide range of modern maladies, from sleep disorders to cancer, have been linked to light-induced circadian disruption. Light has been defined, however, only in terms of the human visual system, not the circadian system. Thus, the study of light-induced circadian disruption is in need of a new definition of light (and dark). Here we contrast light as a stimulus for the human visual system with that for the human circadian system to elucidate the significance of developing a new definition of circadian light as it might ultimately be used to improve health and well-being.

Every fifth inhabitant of our planet has no access to electric lighting. Most of them are poor people living in remote areas of developing countries. Recent progress in solid-state lighting technologies offers good opportunities to develop, commercialize and introduce off-grid lighting systems based on application of white light emitting diodes (WLEDs) in combination with photovoltaic solar panels, wind generators or tiny hydro power plants. Though strongly dependent on the mainstream progress in implementation of LEDs for general lighting, application of this technology in developing world has specific challengers, difficulties and even advantages. Lighting technology the developing world is up to leapfrog from splinters and kerosene wick lamps directly to LED lamps leaving incandescent and fluorescence lamps behind. Achievements and problems, history and future of implementation of solid-state lighting in remote villages of developing counties are discussed in this chapter.

The determinants of successful development, commercialization and diffusion of solid state lighting (SSL) are not well understood particularly in a global context. Patent data provide one means to gain insight into corporate and national research and development activities. However, existing SSL patent analyses have focused primarily on United States (US) patents. This study analyzes SSL patents granted worldwide to measure the strengths of US, Japan, Germany, the Netherlands, South Korea, Taiwan, and China, nations all poised to play a key role in SSL's future. It shows a strong and growing role of corporate patent portfolios for firms headquartered in Asian nations. The data cover patents that were applied for and issued from 1937 to March 2009. Our findings suggest a stronger role of non-US organizations and individuals than had been reported in previous studies that focused only on US patents.