Types of Electric Lamp

Electric lamps fall into a few broad families that produce light in very different ways. Incandescent bulbs heat a tungsten filament until it glows; they are inexpensive but inefficient. Gas-discharge lamps, by contrast, create light by passing an electric current through a gas or vapour, exciting its atoms so that they emit light. Two prominent members of the discharge family are the neon sign and the fluorescent lamp, both of which rely on a sealed tube containing a low-pressure gas and electrodes at each end.

Neon signs are made from glass tubes bent into letters and shapes and filled with a noble gas at low pressure. When a high voltage is applied across the electrodes, the gas ionises and glows. Pure neon gives the familiar orange-red light, while other gases and gas mixtures, often combined with coloured or phosphor-coated glass, produce blues, greens and many other colours. A neon-sign transformer supplies the high voltage needed to strike and sustain the discharge. Skilled glass bending and gas filling make neon a craft as much as a technology.

Fluorescent lamps use a similar discharge but optimise it for efficient general lighting. The tube contains a low-pressure mixture of an inert gas and a little mercury vapour. The discharge produces mostly ultraviolet light, which strikes a phosphor coating on the inside of the glass and is re-emitted as visible white light. Different phosphor blends set the colour temperature, from warm to cool white. A ballast limits the current and provides the starting voltage. Fluorescent lamps are far more efficient than incandescent bulbs, which is why they were long used in offices, schools and shops before LED lamps became widespread.

Frequently asked questions

How do neon signs make different colours?

Different noble gases and gas mixtures emit different colours, and coloured or phosphor-coated glass tubing further alters the light, so a range of hues is possible beyond neon's natural orange-red.

Why does a fluorescent lamp have a white coating inside?

The discharge produces mainly ultraviolet light. The phosphor coating absorbs the ultraviolet and re-emits it as visible white light, and the phosphor mix sets the colour temperature.

What is a ballast for?

A ballast limits the current through a discharge lamp and supplies the voltage needed to start it, because the lamp itself cannot safely be connected directly across the supply.