Basic principles of LED (Light Emitting Diodes)
•    An LED consists of several layers of semiconducting material
•    When an LED is operated with DC voltage light is generated in the active layer
•    The generated light is radiated directly or by reflections
•    In contrast to lamps, which emit a continuous spectrum, an LED emits light in a certain color
•    The color of the light depends on the used material
•    Two systems of material (AllnGaP and InGaN) are used in order to produce LED with a high luminance in all colors from blue to red and also in white (luminescence conversion). Therefore different voltages are necessary in order to operate the diode in conducting direction