Lighting System A lighting system consists of : 1.Light sources 2.Luminaires (or fixtures) 3.Ballasts.

Light sources Light is a small section of electromagnetic radiation which produces a sensation of brightness and colour in the human eye. Electromagnetic radiation is a form of energy. The spectrum of such radiation provides information on its energy composition. Prismatic breakdown of light The perception of light is essentially characterised by its prismatic breakdown into its spectral components.

The principle used for the measurement of the human spectral sensitivity is the equivalence of visual effect, the effect in question being the perception of brightness. Radiation consisting of a single wavelength somewhere between 380 and 780 nm will be seen as having both a brightness and a color. Light sources

Light sources consist of two parts : 1.Natural light Natural light is light received on Earth from the Sun, either directly or after reflection from the Moon. 2.Artificial light (electric lamp) The artificial lamp can be divided into two classes; Solid matter lamps and discharge lamps

Artificial light Incandescent lampsDischarge lamps Current is passed through a wire to heat it to high temperature (Produce light by heating a tungsten filament to incandescence) A voltage is applied across two electrodes in a glass enclosure filled with inert gases, metal vapours and rare earths metals to produce an arc discharge. The direct radiation from the gaseous filler substances combines to produce the desired light colour (Produce light by an electric discharge in a gas) Operate directly from the electricity supply Require control gear between the lamp and the electricity supply Example : incandescent lamps and tungsten-halogen lamps. Examples: mercury vapour, metal halide and sodium vapour lamps. Light sources

Artificial light

Light sources Characters of light generating - Thermal radiations Incandescent lamps and tungsten-halogen lamps are thermal radiators. When a tungsten filamente nclosed in a bulb filled with gas is heated by passing electricity through it, these lamps emit a spectrum that is similar to that of the black body radiator. Incandescent lamp Spectrum of an incandescent lamp

Light sources Characters of light generating - Discharge lamps In discharge lamps, light is generated when an electrical current is passed through gas or metal vapour plasma in sealed discharge tubes. There are two types of discharge lamp depending on the pressure of the filler material: high- pressure discharge lamps and low-pressure discharge lamps. High-pressure discharge lamps The most common representatives of this group of lamps are the metal halide lamps. Under high pressure (~10 bar) the filler substances in the gas atmosphere emit directly visible light through electron excitation. The light colours and the colour rendering properties can be influenced by adding carefully controlled doses of what are known as "rare earths". HQI lamp Spectrum of an HQI discharge lamp

Low-pressure discharge lamps A distinction is made here between low-pressure mercury and sodium lamps. The fill pressure is only a few millibars. Like the high-pressure discharge lamps, low-pressure sodium lamps emit directly visible light, but exclusively with a wavelength of 585 nm. Low-pressure sodium lamp Spectrum of an SOX lamp In low-pressure mercury lamps (fluorescent and compact fluorescent lamps) on the other hand,mercury atoms are made to emit UV radiation as a result of electron collisions. This UV radiation is converted into visible light by phosphors applied to the inside of the tube. Principle of a fluorescent lamp Spectrum of a fluorescent lamp

Luminaires (or fixtures) The luminaire is the complete lighting fixture. It consists of a housing socket, the light source (lamps) and the components which distribute the light such as the lens, the diffuser and the reflector. Ballast Light from discharge-type lamps (e.g., fluorescent, mercury vapor,metal halide, high-pressure sodium, and low-pressure sodium) is produced indirectly by a cathode exciting a gas in which an electrical arc forms which then emits light. In a fluorescent lamp, mercury vapor emits ultra-violet radiation which strikes the sides of the lamp wall where phosphors convert it to visible light. A ballast is required to start and operate all discharge lamps.

Photometric value

Luminous intensity is the luminous flux of a light source in a particular direction and not dependent on the size of the recipient. Luminous flux indicates the radiated power emitted by a light source in all directions. This radiation is evaluated according to the sensitivity of the human eye. All other photometric values are derived from this basic value. Photometric value

Luminance indicates the luminous intensity of a light source or an illuminated surface, referred to its observed area. For humans, light is not visible until radiation enters the eye. Luminance is the only variable that can be perceived by humans. Illuminance is the ratio between the luminous flux and the area to be illuminated, irrespective of the reflectance of the surface. An illuminance of 1 lux occurs when a luminous flux of 1 lumen falls evenly on an area of 1 m2. Illuminance reduces with the square of the distance between the light source and the surface. Photometric value