Commonly used types of light sources, including incandescent, flourescent, and high intensity discharge lamps:
Incandescent means glowing due to heat. development of the incandescent lamp marked the beginning of efficient electric lighting. Tungsten wire is generally used as the filament. when an electric current flows through the tungsten wire, the wire becomes warm due to I^2R heating effect. If the current through the wire is high enough, the heating causes the wire to glow. This is the principle of an incandescent lamp.Most incandescent lamps are either evacuated or have an inert gas instead of a vacuum inside the bulb. Most bulbs are coated or frosted to diffuse the light, making it more appealing and reducing glare. Only about 6-12 percent of the energy emitted from an incandescent bulb will be in the visible range. Most of the radiation is in the infrared region.
In many application fluorescent lamps have several advantages over incandescent lamps including:
- an effeciency of two to three times as many lumens per watt as an incandescent. This means they can operate at 50% or less of the ennergy cost of equivalent tungstent lightning system
- Less heat is given off per lument of lighting
- Light produce less glare since the source is distributed over a larger area
- under ordinary operating conditions, five to ten times the life expectancy of incandescents.
Fluorescent lamps are available in three main types, classified by their type of starting and operating circuit: Hot cathode, preheated-starting;hot cathode, instant starting;and cold cathode. The lamp consists of a glass tube fitted with special sockets for each type. The inside of the tube is coated with chemicals called phosphors which give off visible light when exposed to ultraviolet light.
- High intensity discharge lamps
High intensity discharge (HID) lamps include the groups of lamps commonly known as mercury, metal halide and high pressure sodium. The light producing element of these lamp types is an arc discharge within an envelope (arc tube)
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