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 band gap of the semiconductor material used to make the LED. To create white light from LEDs requires 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.
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.
A single LED is a low-voltage solid state device and cannot be directly operated on household AC current without some circuitry to control the voltage applied and the current flow through the lamp. A series diode and resistor could be used to control the voltage polarity and to limit the current, but this is inefficient since most of the applied voltage would be dropped as wasted heat in the resistor. A single series string of LEDs would minimize dropped-voltage losses, but one LED failure could extinguish the whole string. Paralleled strings increase reliability by providing redundancy. In practice, three strings or more are usually used. 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 produce 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 housing with driver electronics, a heat sink and optics.
As of 2010, many LED lamps are available as replacements for the ordinary household incandescent or compact fluorescent light bulbs, ranging from low power bulbs from 5 up to 40 Watts, through conventional replacement bulbs for 60 Watt incandescent bulbs (typically requiring about 7 watts of power), and a few lamps are now becoming available to replace higher wattage bulbs, e.g., giving light equivalent to about 100 Watts of incandescent light using 13 watts in LEDs. (A standard general purpose incandescent bulb produces light at an efficiency of about 14 to 17 lumens/W depending on its size and voltage. According to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60W tungsten bulb must have a minimum light output of 806 lumens.)
Most LED bulbs are not designed to be dimmed (although some models are designed to work with dimmers), and are usually directional. The lamps are declining in cost to between $10 – $20 each (2010). These bulbs are more power-efficient than the compact fluorescent bulbs and offer extraordinary lifespan of 30,000 or more hours (although this lifetime is dependent on operating temperature, and may be lower if the lamp is operated at a higher temperature than specified.) A LED light bulb can be expected to last 25–30 years under normal use. The bulbs maintain output light intensity very well over their life-times. Energy Star specifications require the bulbs to typically drop less than 10% after 6000 or more hours of operation; and, in the worst case, the drop must be less than 15%. They are also mercury free, unlike fluorescent lamps. LED lamps are available with a variety of color characteristics. With the savings in energy and maintenance costs, these bulbs can be attractive.
Watt replacement for a 60-Watt bulb, the “Geobulb”, with an efficiency of 59 lumens/W.. The company also offers wedge-base lamps for replacement in low voltage fixtures. In the Netherlands, a company called Lemnis Lighting offers a dimmable LED lamp called Pharox. The company Eternleds Inc. offers a bulb called HydraLux-4 which uses liquid cooling of the LED chips. Philips makes a number of LED lamps which are commercially available in the United States and come with a six year warranty, and a number of smaller manufacturers can be found that sell LED lights that are screw-in replacements for conventional bulbs, for example, the EvoLux and ZetaLux lamps from EarthLED.
The technology is improving rapidly, and new energy-efficient consumer LED lamps have been announced from three of the lighting industry’s largest manufacturers, Osram Sylvania, Philips, and General Electric, so these listings should be taken as not necessarily representative of what is currently available.
