F6 Generation Of Light
F6 GENERATION OF LIGHT
a. Light Sources
Introduction
Many different processes convert energy into visible radiation (light). Some basic processes are described below.
Generation of Light
Incandescence
- 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
- Luminescence is the emission of light not ascribed directly to incandescence.
- Two important types of luminescence are electric or gas discharge, and fluorescence.
Electroluminescence
- 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.
Fluorescence
- 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
Definition
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
page revision: 9, last edited: 19 Nov 2010 16:51