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Lighting Reference Guide – Generation of Light

6 Generation of Light

a. Light Sources


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

Generation of Light

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

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:

Lamp Type (Lumens per Watt) Rated Average Life (hours)
Incandescent 10 to 35 1,000 to 4,000
Mercury Vapour 20 to 60 24,000+
Light Emitting Diode 20 to 40 see below
Fluorescent 40 to 100 6,000 to 24,000
Metal Halide 50 to 110 6,000 to 20,000
High Pressure Sodium 50 to 140 24,000 to 40,000
Low Pressure Sodium 100 to 180 16,000

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

b. 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

Lighting System Illustration

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