A light-emitting-diode lamp is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. Since the light output of individual light-emitting diodes is small compared to incandescent and compact fluorescent lamps, multiple diodes are often used together. LED lamps can be made interchangeable or retrofitted with other types. LED components use DC so LED lamps must also include internal circuits to operate from standard AC voltage. LEDs are damaged by being run at elevated temperatures, therefore LED lamps typically include heat management elements such as heat sinks and cooling fins. LED lamps offer long service life and high energy efficiency, but initial costs are higher than those of fluorescent lamps.
General purpose lighting requires white light. LEDs emit light in a very small band of wavelengths, producing strongly colored light. The color is characteristic of the energy bandgap of the semiconductor material used to make the LED. To create white light from LEDs requires either mixing light from red, green, and blue LEDs, or using a phosphor to convert some of the light to other colors.
The first method (RGB-LEDs) uses multiple LED chips each emitting a different wavelength in close proximity to create the broad white light spectrum. The advantage of this method is that one can adjust the intensities of each LED to “tune” the character of the light emitted. The major disadvantage is the high manufacturing cost, which is problematic for commercial success.
The second method, phosphor converted LEDs (pcLEDs) uses a single short wavelength LED (usually blue or ultraviolet) in combination with a phosphor, which absorbs a portion of the blue light and emits a broader spectrum of white light. (The mechanism is similar to the way a fluorescent lamp produces white light from a UV-illuminated phosphor.) The major advantage here is the low production cost, and high CRI (color rendering index), while the disadvantage is the inability to dynamically change the character of the light and the fact that phosphor conversion reduces the efficiency of the device. The low cost and adequate performance makes it the most widely used technology for general lighting today.
To be useful for illumination for home- or work spaces, a number of LEDs must be placed close together in a lamp to combine their illuminating effects. This is because individual LEDs produces only a fraction of the light of traditional light sources. When using the color-mixing method, a uniform color distribution can be difficult to achieve while the arrangement of white LEDs is not critical for color balance. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. LED lamps usually consist of clusters of LEDs in a housing with both driver electronics, a heat sink and optics.
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