Everything about LEDs


LED: Light emitting diode

LED emits incoherent narrow-spectrum light when electric current is applied on p-n junction. This effect is the electroluminescence. Practically LED is a little electronic device which consists of a chip and an optical component. The emitted light's color can be from infrared to ultraviolet depending on basic material and quality of applied components.


Discovery and history of LED
  • Henry Round of Marconi Labs discovered Electroluminescence in the early 1900s.
  • Russian Oleg Vladimirovich Losev, independent researcher created the first LED in the middle of 1920s. His research results were published in German, Russian and British scientific journals, but his work was no recognized by international scientific world of the time because the emitted light's luminous intensity was very small.
  • In 1955 Rubin Braunstein of the Radio Corporation of America developed the first experimental LED diode emitting infrared light. Gallium arsenide (Ga As) was the base of this diode.
  • In 1961 Bob Biard and Garry Pittman of American Texas Instruments carried on researching with diodes using gallium arsenide (GaAs) emitting infrared radiation in invisible-spectrum. Biard and Pittman recognized their own work importance and had infrared LED patented.
  • In 1962 Nick Holonyak Jr. expert of the General Electric Company created the first visible-spectrum LED.
  • In 1971 a former student of Holonyak, Dr. M. George Craford developed the first yellow LED which could be used in practice too. Shortly after red and red-orange LEDs were launched, their brightness was improved by a factor af ten comparing with yellow LEDs.
  • In 1972 blue LED was invented which supposed a breakthrough, its first version is linked with the name of Jacques Pankove, experimenter of RCA Laboratories.
  • In 1993 blue LEDs reached shocking high improvement in brightness thanks to Shuji Nakamura's work, experimenter of Nichia Corporation. From that time onward to our days many companies produce LED chips using perfect technology, but basic principles have not changed from 1993.
  • In 1999 Philips Lumileds produced its first 1 W LED of continuous use. This LED had much bigger size than other traditional LEDs. These LEDs can only be used set up to radiators and from that time started use of LEDs for lighting purposes.

How does LED work?

Like any common diode, the LED includes a chip of semiconducting material in which a p-n junction is created. Current can flow only in one direction between electrons and holes. Direct band gap's width determine the volume of voltage necessary to maintain the process. When an electron jumps across the band gap, it's arrival makes another electron rise into a higher energy level, this involves energy income that the atom radiates by releasing a photon. Wavelength, and thus the emitted light's color depends on the band gap's energy, and it's width.


From Lumen to Watt

People's light demand have been increasing from the appearance of light sources which use electric current. Requirements that artificial light sources have to meet to light working and living places lacking natural light keep growing more and more. Like early lamps which burnt different materials, in place of lighting based on incandescent bulbs, new light sources were demanded which offered more uniform and nicer lighting for available space. By the end of 20th century because of growing electric energy costs a new demand appeared: economical lighting in which case emitting luminous flux needs decreasing voltage. Latest light sources consist of LEDs (light-emitting diode). The candela is the unit of luminous intensity. A useful property of LED light sources is that they emit directed light from a point so it is not necesssary to reflect light by mirrors or projector from the opposite side of living space into the direction where they are used, like in case of incandescent bulbs and fluorescent tubes. LEDs are manufactured in versions to 120oC view angle degree. LEDs not only are available as light sources which resemble bulbs and they attract us to replace existing light sources because of their simplicity - but in the form of rigid bars or flexible strips by which we can create uniformly distributed lighting in space. Light sources manufactured with LEDs are sensitive to two factors: they require stabilized current and working temperature below 80oC. Fortunately LED prices are approaching to earlier type prices, but they are still considered relatively expensive, although their exceptional high luminous efficiency (1-5W demand of output) accompanied by their long lifetime make possible an economical lighting.


Luminous efficacy:

Luminous efficacy is the ratio of luminous flux to radiant flux.

Unit: lm/W


Demand of light:

It is the measure of light necessary for perform a visual task without visual exhaustion.


Radiant point:

A light source which dimensions in space are negligible.


LED light source:

LED light source is a radiant point, but basically is a light source which has a directed light emmittance into a limited view angle degree of space (without any projector).


LED's light emitting viewing angle degree:

The angle on whose limits radiant flux intensity is 50% of nominal luminous intensity.


Candela:

Candela is the unit of luminous intensity. The ratio of luminous flux emitted by the light source into a viewing angle of space to the viewing angle.

Symbol: Iv
Unit: cd (kandela)

The candela is the luminous intensity of a light source which emits monochromatic radiation of frequency 540x1012 Hz (555,016 nm wavelength) with a radiant intensity of 1/683 W/sr (Watt per steradian).


Lumen:

Lumen is the unit of luminous flux.

Symbol: lm

1 lumen is the luminous flux if we consider a light source which emits 1 candela of luminous intensity in every direction from a radiant point into 1 sr solid angle.


Lux:

Lux is the unit of illuminance.

Symbol: lx

Illuminance is of 1 lux made by luminous flux of 1 lumen which is distributed uniformly on a surface of 1 m2.


Supply demand:

The power demand (W) used to produce nominal luminous flux of a given light source can be calculated as the product of voltage (U) and electric current (I).


Stabilized voltage supply:

Stabilized voltage supply is provided by a power supply unit which will not change output voltage not even in case of input voltage fluctuations produced between given limits (for example: 230 V of main voltage) nor in case of change of load connected to output of power supply unit.