Light Bulb

Overview of the light bulb

The light produced by conventional light bulbs (more properly termed “incandescent light bulbs”) is actually a form of thermal radiation: when a metal is heated, a point is reached when its rising temperature causes it to glow. This form of thermal radiation reaches the point where it radiates within the visible light range of the electromagnetic radiation spectrum (the term thermal radiation is conventionally used to cover the non-visible parts of the spectrum, such as infra-red). Glowing through heating is termed “incandescence”, which derives from the Latin word “incandescere”, which means “to glow white.”

Within the sealed glass chamber of a light bulb, a fine filament of wire is subjected to a temperature high enough to induce incandescence by passing electrical current through it. The sealed glass bulb is either filled with inert gases or has air gases extracted from it to form a vacuum in order to stop the filament from reacting with atmospheric oxygen and oxidising. Current can be sent through the inner filament via wires which are fed through the base of the bulb and sealed during manufacture. The fragile base of the bulb is usually encased in a cup-shaped metal sleeve, fashioned into a bayonet-style shape or screw base, and includes the bulb’s external electrical terminals.

Incandescent light bulbs are cheap to manufacture, can operate with alternating, as well as direct, current and are available in an enormous variety of sizes, shapes, light outputs and colours. Their voltage ratings range from 1.5V and 300V and they are now used so ubiquitously that it is difficult to imagine a time before their invention. And yet for much of human history, far less effective means of lighting the dark, such as candle and flame lights, were the only available sources of man-made illumination, stretching over millennia (see below for a fuller account of the light bulb’s history).

Despite their widespread use across the world, they are considerably less efficient than newer generation lighting technology: compact fluorescent bulbs, for example, are capable of a luminous efficiency of 60 lumens per watt (lm/W), whereas the average incandescent bulb can yield a mere 16 lm/W. Of the energy incandescent bulbs consume, below 5 percent is converted into visible light. The rest is simply lost in the form of heat, although this infra-red radiation is harnessed to useful effect in applications such as poultry brooding boxes, incubators, and some industrial heating and drying systems.

At approximately 1,000 hours, their lifespans are also considerably shorter than far newer, energy-saving forms of lighting such as LEDs, CCFLs (cold cathode fluorescent lamps) and CFLs (compact florescent lamps) which last for as much as 100,000 hours. The latter convert four to six times as much of the energy they consume into light by comparison with incandescent bulbs.

Gradually and slowly, this newer, much more efficient lighting technology is replacing incandescent light bulbs across the field; the United States and the European Union along with several other jurisdictions are progressively phasing out the use of incandescent light bulbs in favour of energy saving alternatives.

 

History of the light bulb

While numerous inventors and scientists in the nineteenth century experimented with various forms of incandescent electric light bulb, the names most closely associated with its invention are the British scientist Joseph Swan, whose home became the first in the world to be illuminated by his invention in 1880, and Thomas Edison, the entrepreneurial American inventor who produced the first commercially viable light bulb using a carbonised bamboo filament in 1879.

Prior to Swann and Edison, light bulbs emitted only very dim light and were very short-lived, shortcomings which rendered them commercially nonviable. Both Swann and Edison extended the incandescent filament’s lifespan considerably by evacuating the bulb with a vacuum pump and sealing it, thereby ensuring that during high-heat incandescence, the filament would not “rust” and disintegrate through oxidation. Whereas in 1899, Edison’s bulb lasted just 13.5 hours, by the following year he had improved the design to such an extent that his new invention lasted for 1200 hours. The light bulb was soon taken up commercially on an unprecedented scale, as more and more homes and business premises desired electric illumination.

In 1880, another American inventor, Hiram Maxim (who later moved to England), extended the light bulb’s lifespan by coating its carbon filaments with hydrocarbons.

The first metal filament lamp to become commercially viable was invented in 1889, when the Austrian scientist Carl Auer von Welsbach discovered that osmium functioned as exceptionally well as the incandescent component. The aim was to overcome the blackening that occurred inside the bulb with carbon filaments although in 1903, an inventor named Willis Witnew found that the effect could be prevented by coating carbon filaments with metal.

The tungsten filament lamp was launched in 1904 by the Hungarian inventors Sándor Just and Croatian Franjo Hanaman. Its lifespan was substantially longer than its predecessors and the luminescence it produced when the bulb chamber was filled with inert gases proved to be twice as brilliant as that produced in a vacuum.

Whereas approximately 300,000 incandescent light bulbs were sold in 1885, by the time tungsten filament lamps became commercially available in the early twentieth century, the number of light sockets in existence in the United States alone had skyrocketed to 50 million. By 1945, lamp sales had multiplied to 795 million per year.

 

Technical aspects of the light bulb

While early light bulbs were evacuated to prevent filament oxidation during heating, today most are filled with inert gases which have proven more effective. Typically, a mix of 93 percent argon and 7 percent nitrogen at a pressure of 70kPa will prevent oxidation and minimise evaporation to negligible levels (krypton is also sometimes used)

The filament is usually made of tungsten and is heated to between 2,000 and 3,000 degrees Kelvin to produce incandescence, safely below the material’s 3,696 K melting point.

While the glass envelope of the bulb is often left clear, many bulbs are first coated internally with a fine, chalk like white powder known as kaolin. This diffuses the incandescence to produce a gentler light, and pigments may be added to it to produce coloured decorative lighting for Christmas trees and parties.

The tungsten filament is manufactured in the form of several coils of already coiled tungsten wire to improve the bulb’s efficiency. Inside a typical 240V 60W bulb, the length of the tungsten “coiled coil” is on average 22.8 inches (the filament itself is very fine, being a mere 0.046mm or 0.00028 inches in diameter).

 

Where light bulbs are used in manufacturing

The short entry under this heading is, quite simply, “everywhere”. Not only are incandescent light bulbs used to illuminate domestic and industrial/commercial/work spaces of all sizes, they are also used in car headlamps, flashlights, decorative lights for Christmas trees and illuminated advertising displays.

Increasingly, they are being incorporated into smart lighting systems, as new automation and control technology allow lights to only come on in response to human movement. This can be useful for security purposes to detect human intruders in prohibited or restricted areas or to automatically illuminate normally darkened areas as humans walk into them. By activating only when required to do so, these systems save energy, reduce costs and improve efficiency.

 

How the light bulb differs from other light sources

Unlike energy-saving compact fluorescent lamps, which rely on the fluorescence of mercury vapour (i.e., it emits light upon absorbing electromagnetic radiation), or light-emitting diodes, which rely on the fact that electrons release energy as light photons when they fall to a lower-energy state, incandescent light bulbs produce thermal radiation, most of it in the non-visible near-infrared region of the electromagnetic spectrum. At a high enough temperature, however, they exhibit incandescence, releasing some of the thermal energy in the visible light range.

 

Revolutionary aspects behind the light bulb

Human productivity was massively enhanced by the invention and mass uptake of the incandescent light bulb. Toward the close of the Victorian era, it became a vastly safer alternative to open flames or gaslight, especially in factories and in underground mining operations. Production effectively became a 24-hour activity, with factories and industrial plants well lit at night as well as by day – developments which would scarcely have been imaginable without the advent of the humble light bulb.