A Brief History of LEDs
Nick Holonyak invented the visible LED while working for General Electric in 1962. Commercial LEDs were introduced by 1968 and commonly used as replacements for incandescent and neon indicator lamps. These red LEDs were primarily used as indicators in TVs, radios, telephones, calculators, and watches, but the light output was not enough to illuminate an area.
Dr. Shuji Nakamura, who invented the first blue LED in 1993, is also commonly credited with inventing the first white LED. From that first invention, LED technology has grown rapidly and the result is seemingly ever-escalating light outputs that maintaining efficiency and reliability at acceptable levels. In fact, high power white light LEDs are now fast replacing incandescent and fluorescent lighting as general illumination sources.
How LEDs Produce Light
LEDs differ from traditional light sources in the way they produce light. In an incandescent lamp, electric current heats a tungsten filament until it glows or emits light. In a fluorescent lamp, an electric arc excites mercury atoms, which emit ultraviolet (UV) radiation. After striking the phosphor coating on the inside of glass tubes, the UV radiation is converted and emitted as visible light.
An LED, in contrast, 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 the 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. White light is created by combining the light from red, green, and blue (RGB) LEDs or by coating a blue LED with yellow phosphor.
Some Basic Advantages of LEDs
- Directional Light Emission: LEDs emit light in a specific direction, so well-designed fixtures can deliver light more efficiently to the intended location. Fluorescent and standard "bulb7#34; shaped incandescent and metal halide lamps emit light in all directions.
- Size Advantage: LEDs can be very compact and low-profile.
- Breakage/Vibration Resistance: LEDs contain no breakable glass or filaments.
- No IR or UV emissions: LEDs intended for lighting do not emit infrared or ultraviolet radiation, and generate no heat in the beam of light.
- Instant On: Unlike traditional sources, like metal halide, LEDs require no "warm-up" time.
- Rapid Cycling Capability: LED lifetimes are not affected by frequent switching.
- Control: LEDs are compatible with electric controls to change light levels and colors.
- Efficacy: Currently five times more efficient than the incandescent light bulb, the cost per lumen of LEDs is decreasing 10-fold with each decade.