Conserving energy is now a mandate, not a choice, and part of that mandate is the need to go Green. When it comes to lighting, we can easily imagine the impact of globally improving the efficiency of lighting sources by 10 percent. But what if it could be improved by 1000 percent? Newly enhanced light emitting diodes (LEDs) have the potential to achieve these efficiency improvements while maintain
Conserving energy is now a mandate, not a choice, and part of that mandate is the need to go Green. When it comes to lighting, we can easily imagine the impact of globally improving the efficiency of lighting sources by 10 percent. But what if it could be improved by 1000 percent? Newly enhanced light emitting diodes (LEDs) have the potential to achieve these efficiency improvements while maintaining high performance and reliability that supersede many currently used sources. In the first part of this four-part series, we look at the LED's physical structure, range of colors, efficiency, LED drivers, and applications.
AnatomyPhysically, LEDs resemble p-n junction diodes. As with p-n junctions, electrons and holes flow towards the junction when a positive differential voltage is applied between the anode (p-side), and cathode (n-side). Once an electron is recombined with a hole, it releases energy. Depending on the physical properties of the p-n junction materials, the released energy can be non-radiative, as for the typical diode applied in discrete circuits, or in the form of emissions in the optical range. For an LED, the wavelength of the emitted light (i.e., its color) depends on the band gap characteristics of its p-n junction material. Performance-wise, LED materials have relatively low reverse breakdown voltages since they have relatively low band gaps.
ColorsRed LEDs were the first to become commercially available in the late 1960s but their light output was very low. Despite this shortcoming, they were commonly used in seven-segment displays. Thanks to advancements in material science, nowadays LEDs are commercially available in a variety of colors with some of them having light outputs that would blind you if you stared directly at them.
Blue LEDs became widely available a few years ago. Mixing blue LEDs with red and green LEDs produces white light. This technique of generating white light provides a large color gamut, dynamic light tuning, and excellent color rendering (CRI), which is well suited for high-end backlighting applications. A simpler and more economical way of producing white light is to use blue LEDs and a phosphor coating that converts some of the blue light to yellow. The yellow light stimulates the red and green receptors of the eye, and therefore mixing blue and yellow gives the appearance of white. This scheme can provide good CRI but the LED's light output may suffer from inconsistent color temperatures due to manufacturing discrepancies and varying thicknesses in the phosphor coating layer.
(Click on Image to Enlarge) Figure 1: LED color chart for the basic colorsEfficiency
