LEDs

Lighting Facts Label Manufacturers and distributors of solid-state lighting or LED lighting attaches the Lighting Facts Label to the packaging of their products. This label provides a quick summary of product performance data based on industry-standard testing. Information included are the brightness (in lumen), the luminous efficacy or efficiency of turning electrical energy to light (lumens per watts used), the wattage required to light the product, the correlated color temperature (CCT) or the light's relative color, and the color rendering index (CRI), which is the product's ability to accurately render color of an object.The Lighting Facts Label do not cover indicator applications such as flashlights, nightlights and holiday lighting. It does not guarantee high performance of the product with the label.

The light-emitting diode (LED) is a new light source that differs in important ways from existing light sources. An LED is a semiconductor diode. It consists of a chip of semiconducting material treated to create a structure called a p-n (positive-negative) junction. When connected to a power source, current flows from the p-side or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers (electrons and electron holes) flow into the junction from electrodes. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon (light). The specific wavelength or color emitted by the LED depends on the materials used to make the diode. [1]

LEDs are directional light emitters, compact and low-profile (requires careful design), breakage resistant as it has no glass or filaments, instant on (instant full brightness upon turning on), able to rapid cycle (can be turned and off without effect on performance and useful life), dimmable (not all LEDs are compatible with all dimmers), and infrared and ultraviolet-light free. [2] Modern electronics relies heavily upon LED light bulbs. For instance, LEDs transmit information from remote controls, are used in traffic lights, digital LED clocks, flashlights, and to form images on jumbo television screens.

LED lighting starts with a tiny chip (most commonly about 1 mm2) comprising layers of semi-conducting material. LED packages may contain just one chip or multiple chips, mounted on heat-conducting material and usually enclosed in a lens or encapsulant. The resulting device, typically around 7 to 9 mm on a side, can produce 30 to 150 lumens each, and can be used separately or in arrays. LED devices are mounted or soldered on a circuit board and attached to a lighting fixture, architectural structure, or even a "light bulb" package. [3] To produce illuminance levels equivalent to high output traditional luminaires requires grouping multiple LEDs, each of which increases the heat sinking needed to maintain light output and useful life.[4] LED light bulbs are miniature bulbs that do not use filaments to produce light. Therefore, the life of an LED is much longer than that of a regular incandescent bulb, because there is no filament to burn out. Incandescent bulbs also tend to be much larger in size due to the filament, which must be housed in a vacuum inside the bulb.

LEDs are near-monochromatic light sources. An individual LED chip emits light in a specific wavelength. To be used as a general light source, "white" light is needed. White light can be achieved with LEDs in two main ways: (a) phosphor conversion, in which a blue or near-ultraviolet (UV) chip is coated with phosphor(s) to emit white light; and (b) RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light. The advantages and disadvantages of each approach at the current level of technology development are outlined below.[5]

A sample of LED bulbs. Photos: GE Lighting.

There is another type of solid-state lighting or LED lighting. Organic light-emitting diodes (OLEDs) are based on organic (carbon based) materials. In contrast to LEDs, which are small point sources, OLEDs are made in sheets which provide a diffuse area light source. OLED technology is developing rapidly and is increasingly used in display applications such as cell phones and PDA screens. However, OLEDs are still some years away from becoming a practical general illumination source. Additional advancements are needed in light output, color, efficiency, cost, and lifetime.[6]

[edit] How is the useful life of an LED measured?

Table 1.Lifetime Projections of various light sources.

Studies show that only a decline to 70% from the initial lighting level is noticeable to the general lighting consumers.[7] With LEDs, life of a product is noticeably by the decline of its light output. Switching on and off an LED does not affect its life at all. The electrical and thermal design of an LED system or fixture contributes more on how it will last. Driving to a higher current than that rated as well as operating at a higher temperature than that designed will decrease an LED's useful life.[8] This is because of the unhandled heat generated at the LED chip. Heat is not emitted as infrared radiation by LEDs. It must conduct or convect the heat to underlying circuit board and heat sinks, housings, or frame elements. The industry, however, continues to improve the durability of LEDs at higher operating temperatures.[9]

Table 2. How much heat is generated by LED compared to other white light sources.[10]:

[edit] So how efficient are LEDs?

With efficiency measured in lumens per watt, the most efficacious LEDs are those with high correlated color temperatures (CCTs), often above 5000K, producing a "cold" bluish light. Warm white LEDs (2600K to 3500K) are improving and are now approaching the efficacy of CFLs. Leading warm white LEDs are now available with color rendering index (CRI) of 80, equivalent to CFLs. It should be noted here that luminous efficiency figures cited by manufacturers does not take into account actual operation in a fixture or system or driver losses. LEDs in a well-designed luminaire with adequate heat sinking will produce 10%-15% less light than indicated by the "typical luminous flux" rating. Additionally, LED efficacy should be discounted by 15% to account for the driver losses. Drivers or supplementary electronics coming with LEDs converts line power to the appropriate voltage and current and may also include dimming and/or color correction controls. Currently available LED drivers are typically about 85% efficient.[11]

Luminous efficacy is an important indicator of energy efficiency, but it doesn't tell the whole story, particularly with regard to directional light sources. LEDs emit light in a specific direction, providing more light in a directed area than other light sources and reducing the need for reflectors and diffusers that can trap light. LEDs directionality and intensity result in higher application efficiency even though luminous efficacy is lower relative to other light sources.[12]

[edit] So Where are LEDs used for?

The small size and inherent directionality of white LEDs make them a promising option for a number of general illumination applications.[13]

• More Light. LEDs produce somewhat more light per Watt than do incandescent bulbs; this is useful in battery powered devices.
• Color. LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• Temperature. LED performance inherently increases as operating temperatures drop. This makes LEDs a natural fit for grocery store refrigerated and freezer cases, cold storage facilities, and outdoor applications.
• Rapid cycling capability makes LEDs well-suited to use with occupancy sensors or daylight sensors as well as flashing light displays.
• No ultraviolet light. LEDs are most welcome where ultraviolet light can do extreme damage like on artwork, artifacts, photography and fabrics.
• Vibration resistance. LED's inherent vibration resistance may be beneficial in applications such as transportation (planes, trains, automobiles), lighting on and near industrial equipment, elevators and escalators, and ceiling fan light kits.
• Durability. LEDs are built inside solid cases that protect them, unlike incandescent and discharge sources, making them extremely durable. This feature may provide added value in applications where broken lamps present a hazard to occupants, such as children's rooms, assisted living facilities, or food preparation industries.
• Because LEDs are mounted on a flat surface, they emit light hemispherically, rather than spherically. For task lighting, accent lighting, recessed downlighting, and other directional applications, this reduces wasted light.Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
• Dimmable. An LED driver connected directly to a line-voltage incandescent dimmer may not receive enough power to operate at lower dimming levels or it may be damaged by current spikes. Some LED products can be used with line-voltage incandescent dimmers, but the dimmer and the LED driver electronics must be carefully matched. Because of this, some LED light fixture manufacturers publish lists of specific dimmer products tested and approved for use with their fixtures.[14]
• Long life. LEDs have an extremely long life span: upwards of 100,000 hours, twice as long as the best fluorescent bulbs and twenty times longer than the best incandescent bulbs. (Incandescent bulbs can also be made to last an extremely long time by running at lower than normal voltage, but only at a huge cost in efficiency; LEDs have a long life when operated at their rated power.)
• Quick. LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; LEDs used in communications devices can have even faster response times.
• Further, LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.

[edit] Disadvantages of using LEDs

• LEDs are currently more expensive, in lumens per dollar, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.
• LED performance largely depends on the ambient temperature of the operating environment. "Driving" an LED "hard" in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automative, medical, and military applications where the device must operate over a large range of temperatures, and are required to have a low failure rate.
• LEDs require complex power supply setups to be efficiently driven. In indicator applications a simple series resistor can be used; however, this sacrifices a large amount of energy efficiency.
• LEDs typically cast light in one direction at a narrow angle compared to a incandescent or fluorescent lamp of the same lumen level.
• Most "white" LEDs are actually blue LEDs with a phosphor coating that generates warm or cool white light. Their light does not shift to red when dimmed; some may actually appear bluer with dimming.

[edit] References

1. wikipedia.org -LED
2. United States Dept. of Energy Lighting Facts Program
3. United States Dept. of Energy Energy Efficiency and Renewable Energy Program