Lighting Reference Guide – Generation of Light

6 Generation of Light

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

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.

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.

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
– LED

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

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

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

A lighting unit consists of:

A typical lighting system consists of:

A lighting system environment consists of: