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Light bulbs

Light bulbs product information

The common light bulb is one of the main inventions responsible for the growth of our industrial society over the past two hundred years. First invented in 1802 by Sir Humphry Davy, light bulbs were not practical until 1880 when Thomas Edison invented a long lasting filament. The most common types are the incandescent, halogen, gas-discharge, and fluorescent. Solid-state light bulbs are a recent development that are very energy efficient, and may even replace the other types soon.

An incandescent light bulb uses a thin coiled metal filament strung between two terminals, and supported in the middle by several wires. The filament is usually made of tungsten, and is placed inside a sealed glass bulb containing a low pressure inert gas. One terminal is connected to the base of the cap, while the other is connected to the side, and the base and side are separated by an insulator. When mains voltage is passed through the filament, it heats to several thousand degrees and releases a small fraction of this energy as light. Incandescent light bulbs are found in almost all domestic light fittings, but there is interest in replacing them with fluorescent bulbs that have better efficiency and use much less power.

The halogen light bulb is a special type of incandescent that uses a mixture of inert and halogen gas. The amount of halogen gas is tiny but it is enough to substantially increases the lifetime of the bulb, by combining with the tungsten released from the filament to form a halide gas. In the high temperature area around the filament, the tungsten is released from the halide gas and recombines with the filament. Halogen light bulbs are used in spot lights, navigation lights, desk lamps, and other devices that require high-intensity light and small size. Care must be taken when handling them, as even the oil from fingerprints can cause the fragile quartz bulb to break when operating at high temperatures.

Gas-discharge light bulbs are often used to illuminate stadiums, factories, warehouses, freeways, and other large areas. Electrical current ionizes a gas mixture inside a bulb containing two electrodes, and light is produced from the electrical arc between the electrodes. Mercury vapor lamps were once commonly used but have largely been replaced by sodium vapor lamps. These lamps require a ballast to limit the current flow through them, since the ionized gas acts like a short circuit. As sodium lamps get older, they develop a condition known as cycling, where they suddenly stop because the ballast cannot provide enough current. The lamps only start up again once they have cooled down and their internal pressure decreases.

Fluorescent lighting is a special type of gas-discharge lighting. Fluorescent light bulbs are essentially tubes formed into a compact shape. They work by passing electrical current through mercury vapor to make it release ultraviolet light, which is absorbed by the phosphorous coating and released as visible light. The light can be varied by changing the mixture and thickness of the phosphorous coating, and the bulbs are often classed by their color, such as soft white or cool white. Fluorescent bulbs are environmentally friendly because they use less power but there are concerns about the level of toxic mercury in them. The bulbs are a recent innovation and not yet as common as the tubes, which are wildly used in factories, schools, office buildings, and streetlights.

Solid-state light bulbs are an exciting new product because they are much more efficient than other types. They use many light emitting diodes (LEDs) mounted on a circuit board, which also holds their terminals and other components. The LED is a semiconductors mounted inside a small plastic globe. Unlike other light bulbs which release light by thermal radiation, the light from an LED is produced without heating. Consequently, they use far less energy and convert more of it into light, and are cool to touch when operating.

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