LED stands for light-emitting diode. LEDs are part of a lighting technology called Solid-State lighting, including OLEDs (Organic Light Emitting Diodes). OLEDs are sheets of carbon-based compounds that glow when a current is applied through transparent electrodes. LEDs become illuminated by the movement of electrons through a semiconductor material. LEDs and OLED technology are advancing fast.

Low-Powered LEDs

LEDs were first used as indicators like a green power button on a computer, or a red blinking light on a video camera.

High-Powered LEDs

LED's can be used in lighting fixtures to illuminate an area. Many tiny LEDs are used in each fixture. LED lighting is more efficient, durable, versatile and longer lasting than incandescent and fluorescents lighting. LEDs emit light in a specific direction, whereas an incandescent or fluorescent bulb emits light and heat in all directions. LED lighting uses both light and energy more efficiently.

An incandescent or compact fluorescent (CFL) bulb inside of a recessed can will waste about half of the light that it produces, while a recessed down light with LEDs only produces light where it’s needed — in the room below.

Incandescent bulbs create light by passing electricity through a metal filament until it becomes so hot that it glows. Incandescent bulbs release 90% of their energy as heat. In a CFL bulb, an electric current is driven through a tube containing gases. This reaction produces ultraviolet light that get s transformed into visible light by the fluorescent coating (called phosphor) on the inside of the tube. A CFL releases about 80% of its energy as heat.LED lighting products use light emitting diodes to produce light very efficiently. The movement of electrons through a semiconductor material illuminates the tiny light sources we call LEDs. A small amount of heat is released backwards, into a heat sink, in a well-designed product; LEDs are basically cool to the touch.

LED lightings are nade if tiny chips comprised of layers of semi-conducting material. LED packages may contain just one chip or multiple chips, mounted on heat-conducting material called a heat sink and usually enclosed in a lens. The resulting device, typically around 7 to 9 mm on a side, can be used separately or in arrays. LED devices are mounted on a circuit board, which can be programmed to include lighting controls such as dimming, light sensing and pre-set timing. The circuit board is mounted on another heat sink to manage the heat from all the LEDs in the array. The system is then encased in a lighting fixture, architectural structure, or even a “light bulb” package.

LED facts

• Brightness is equal to or greater than existing lighting technologies (incandescent or fluorescent) and light is well distributed over the area lighted by the fixture.
• Light output remains constant over time, only decreasing towards the end of the rated lifetime (50,000 hours or 45 years based on use of 3 hours per day).
• Excellent color quality. The shade of white light appears clear and consistent over time.
• Efficiency is as good as or better than fluorescent lighting.
• Light comes on instantly when turned on.
• No flicker when dimmed.
• No off-state power draw. The fixture does not use power when it is turned off, with the exception of external controls, whose power should not exceed 0.5 watts in the off state.

Luminous Efficacy

Light energy efficiency is measured in lumens per watt (lm/W) which is the amount of light produced for each watt of electricity consumed by the light source. At this stage of development, LEDs performs similarly to fluorescent lamps. However, there are other factors to consider, like:

Color Quality

LEDs have high correlated color temperatures (CCTs), often above 5000K, producing a “cold” bluish light. However, LEDs are available in warm white (2600K to 3500K) with color rendering index (CRI) of 80, equivalent to CFLs.

Driver Losses

Fluorescent and high-intensity discharge (HID) lights needs a ballast to operate. LEDs also require electronic devices called drivers. The driver converts line power to the voltage (2 to 4VDC for high-brightness LEDs) and current (200-1000mA)and may also include dimming and/or color correction controls.
LED drivers are typically about 85% efficient. So LED efficacy should be discounted by 15% to account for the driver. To compare, the range of luminous efficacies for traditional and LED sources, including ballast and driver losses are shown below:

Thermal Effects

The luminous flux figures indicated by manufacturers are based on an LED junction temperature (Tj)of 25°C. LEDs are tested under conditions somehow different from actual operation conditions. The luminous flux is measured under instantaneous operation (about 20 millisecond pulse) in open air. Tj is always higher then when operating in regular conditions. LEDs when used in a fixture adequately designed can produce 10%-15% less light than indicated as typical luminous flux.

Comparing LEDs to Traditional Light Sources

Energy efficiency proponents are accustomed to comparing light sources on the basis of luminous efficacy. To compare LED sources to CFLs, for example, the most basic analysis should compare lamp-ballast efficacy to LED+driver efficacy in lumens per watt. Data sheets for white LEDs from the leading manufacturers will generally provide "typical" luminous flux in lumens, test current (mA), forward voltage (V), and junction temperature (Tj), usually 25 degrees Celsius. To calculate lm/W, divide lumens by current times voltage. As an example, assume a device with typical flux of 45 lumens, operated at 350 mA and voltage of 3.42 V. The luminous efficacy of the LED source would be:
45 lumens/(.35 amps x 3.42 volts) = 38 lm/W
To include typical driver losses, multiply this figure by 85%, resulting in 32 lm/W. Because LED light output is sensitive to temperature, some manufacturers recommend de-rating luminous flux by 10% to account for thermal effects. In this example, accounting for this thermal factor would result in a system efficacy of approximately 29 lm/W. However, actual thermal performance depends on heat sink and fixture design, so this is only a very rough approximation. Accurate measurement can only be accomplished at the luminaire level. To view the Comparison Chart click here.

Application Efficiency

Due to the directional nature of LED light emission, LEDs potentially have higher application efficiency than other light sources in certain lighting applications. Fluorescent and standard "bulb" incandescent bulbs emit light in all directions. Much of the light produced by the lamp is lost within the fixture, reabsorbed by the lamp, or escapes from the fixture in a direction that is not useful for the intended application. For many fixture types, including recessed downlights, troffers, and under-cabinet fixtures, it is not uncommon for 40-50% of the total light output of the lamp(s) to be lost before it exits the fixture. LEDs emit light in a specific direction, reducing the need for reflectors and diffusers that can trap light, so well-designed fixtures, like the undercabinet light shown below, can deliver light more efficiently to the intended location.