F6 Generation Of Light


a. Light Sources


Many different processes convert energy into visible radiation (light). Some basic processes are described below.

Generation of Light



  • Solids and liquids emit visible radiation when they are heated to temperatures above 1,000 K.
  • The intensity increases and the appearance becomes whiter as the temperature increases.
  • This phenomenon is known as incandescence or temperature radiation.
  • Application: incandescent lamps.


  • Luminescence is the emission of light not ascribed directly to incandescence.
  • Two important types of luminescence are electric or gas discharge, and fluorescence.


  • Electroluminescence is the emission of light when low voltage direct current is applied to a semi-conductor device containing a crystal and a p-n junction.
  • The most common electroluminescent device is the LED.

Electric or Gas Discharge

  • When an electric current passes through a gas, the atoms and molecules emit radiation, whose spectrum is characteristic of the elements present.
  • In low pressure discharge, the gas pressure is approximately 1/100 atm or 0.147 PSI.
  • In high pressure discharge, the gas pressure is approximately 1 to 2 atm or 14.7 to 29.4 PSI.
  • Application: gas discharge lamps.


  • Radiation at one wavelength is absorbed, usually by a solid, and is re-emitted at a different wavelength.
  • When the re-emitted radiation is visible and the emission happens only during the absorption time, the phenomenon is called fluorescence.
  • If the emission continues after the excitation, the phenomenon is called phosphorescence.
  • In the fluorescent lamp, the ultraviolet radiation resulting from the gas discharge is converted into visible radiation by a phosphor coating on the inside of the tube.
  • Application: fluorescent, phosphor-coated HID lamps.

b. Lamp Types


An electric lamp is a device converting electric energy into light.

Lamp Types by Light Generation Method

  • Incandescent lamps
  • Gas discharge lamps
    • Low pressure discharge
    • Fluorescent lamps
    • Low pressure sodium (LPS) lamps
  • High pressure or HID
    • Mercury vapour (MV) lamps
    • MH lamps
    • High pressure sodium (HPS) lamps
  • Electroluminescent lamps
    • LEDs

Lamp Types by Standard Classification

  • Incandescent lamps
  • Fluorescent lamps
  • HID lamps
    • Mercury vapour (MV) lamps
    • Metal halide (MH) lamps
    • High pressure sodium (HPS) lamps
    • Low pressure sodium (LPS) lamps
  • LED sources

Lamp Efficacy or Efficiency

The efficacy of the various types of lamps is shown below:


Rated Average Life

  • Rated average life is the total operated hours when 50% of a large group of lamps still survive; it allows for individual lamps to vary considerably from the average.
  • Incandescent lamp life can be extended by use of dimming to reduce maximum power.
  • Compact fluorescent lamps have relatively long lives of about 10,000 hours.
  • Gas discharge lamps have long lives of about 20,000 hours or more.
  • LED sources have life based on different criteria. When the LED has lost 50% of its original output, it is considered failed. This is a range from 50,000 to 100,000 hours. This methodology is used by most manufacturers.

c. Lighting Systems

Lighting Unit or Luminaire

A lighting unit consists of:

  • A lamp or lamps,
  • A ballast (for gas discharge lamps),
  • A fixture or housing,
  • An internal wiring and sockets,
  • A diffuser (louver or lens).

Lighting System

A typical lighting system consists of:

  • Luminaires,
  • Lighting control system(s).

Lighting System Environment

A lighting system environment consists of:

  • Room (ceiling, wall, floor),
  • Room objects.

Lighting System Illustration