LED Lamp Technology overview
General purpose lighting requires white light. LEDs by nature 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 the fact 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 probably most important in 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 cost, while the disadvantage is the inability to fine tune the character of the light without completely changing the phosphor layer. So while this will not yield high CRI (color rendering index) values without sacrificing some other performance property, the low cost and adequate performance makes it the most suitable technology for general lighting today.
To be useful as a light source for a room, a number of LEDs must be placed close together in a lamp to add their illuminating effects. This is because an individual LED produces only a small amount of light, thereby limiting its effectiveness as a replacement light source. If white LEDs are used, their arrangement is not critical for color balance. When using the color-mixing method, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. 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.
In 2008, SSL technology advanced to the point that Sentry Equipment Corporation in Oconomowoc, Wisconsin, USA, was able to light its new factory almost entirely with LEDs, both interior and exterior. Although the initial cost was three times more than a traditional mixture of incandescent and fluorescent bulbs, the extra cost will be repaid within two years from electricity savings, and the bulbs should not need replacement for 20 years.