What are LEDs and How Do They Work?

An LED is a two-lead semiconductor light source. Like organic light–emitting diodes (OLEDs) and light–emitting polymers (LEPs),LEDs are a type of Solid State Lighting (SSL). They generate light through a process called electroluminescence: a direct conversion of electrical energy that occurs when electric current passes through a semiconductor material and light is emitted in response.

The simplest semiconductor device is a diode, which is an electronic component that allows electricity to flow in only one direction. The semiconductor in an LED is specially constructed to release a large number of photons outward instead of absorbing them, resulting in a very efficient light source. Unlike incandescent, fluorescent, and high intensity discharge (metal halide, high pressure sodium, etc.) lamps that emit light in all directions, individual LEDs are directional, emitting light in one direction.

LEDs are commonly found in amber, red, green, and blue. The white light required in most general lighting applications is achieved by mixing various colors of LED together or by using a phosphor material that converts the color of the light. Phosphors are commonly yellow and visible on some LED lamps.

LED-Related Terminology

Here are some terms to help you understand the performance and light output of LED products.

  • Lumen — Unit of measurement for the amount of light produced from a source.
  • Color Rendering Index (CRI) — A scale from 0 to 100 (worst to best) that indicates how well a light source represents an object’s color.
  • Correlated Color Temperature (CCT) — A temperature scale that represents the shade of white light in units called Kelvin (K). Products with a CCT of 2,700 – 3,000K have an orange cast and are generally described as “warm” or “warm white.” Products with a CCT of 4,000 – 6,500K have a white or even slightly blue cast and are often described as “cool white” or “daylight.”
  • Efficacy — The efficiency/performance of the light source, or luminaire, expressed in number of lumens produced per Watt of energy drawn (lm/W).
  • Luminaire — The technical term for a light fixture. It encompasses the entire lighting system including all light source/lamps, reflectors, diffusers, wiring, ballast/drivers, and sockets.
  • LM-79 — The testing standard for LED lamps and luminaires. Test results indicate performance metrics (lumen output, CCT,CRI, and efficacy) at the beginning of a product’s life.
  • LM-80 — The testing standard for LED packages. Test results indicate lumen output depreciation of a diode after a minimum of 6,000 hours of operation.
  • TM-21 Calculator — The standard methodology for calculating the timeframe that the lumen output from an LED is expected to reach 70% of initial output based on LM-80 results.

LED Benefits

LEDs can offer a range of benefits when compared with more mature lighting technologies, like incandescent, halogen, fluorescent, or high intensity discharge:

  • More efficient — They provide more lumens per watt than most other technologies.
  • Longer lifetime — There is no filament or arc tube to burn out.
  • Inherent directionality — This makes it easier to direct light at desired angles.
  • Instant on — There’s no lag time to reach full lumen output and product lifetime is not negatively impacted by frequent switching.
  • Reduced heat output — LEDs can reduce the lighting system’s contribution to cooling loads of HVAC and refrigeration systems.
  • Dimmable — Many LED products are dimmable to 10% of total output or less without the loss of efficacy or color rendition.

In addition, LEDs produce less heat and many products are dimmable to 10% total output or less without diminishing efficacy.