Incandescent light bulb: gas, glass, and the filament
Incandescent light bulb construction allows a choice inside the glass: a vacuum or an inert gas. The filament must be protected from oxidation while an electric current heats it until it glows. Evacuating the enclosure is one way to provide that protection; filling it with inert gas is another. A gas-filled bulb is therefore part of incandescent lighting, rather than a different category defined by the presence of gas. The light still comes from the heated filament. The enclosure and its contents make it possible for that filament to operate under conditions suited to producing illumination.
The electrical path runs through terminals or wires embedded in the glass. Contact wires and a base with conductors connect to the filament, while a stem or glass mount allows connections to pass through the enclosure without air or gas leaks. Small wires support the filament and its leads. A socket provides both mechanical support and electrical connections. Filament temperature depends on its type, shape, size, and the current drawn. The heated wire emits a nearly continuous spectrum, including visible light and a much larger contribution of heat.
Incandescent light bulb performance changes when argon or nitrogen surrounds a tungsten filament. Compared with operation in a vacuum, the gas slows the filament's evaporation. That allows higher temperatures and greater luminous efficacy with less reduction in filament life. In 1913, Irving Langmuir found that filling a lamp with inert gas, initially nitrogen and later argon, doubled luminous efficacy and reduced blackening of the bulb. He patented his device on April 18, 1916. The fill was a working part of improving the lamp, with consequences for both the amount of light produced and the condition of the glass.
Gas-filled designs also appeared before the tungsten developments. Alexander Lodygin created an incandescent bulb in 1872 and received a Russian patent in 1874. His glass receiver contained nitrogen and two carbon rods, arranged so current could pass to the second when the first had been consumed. On July 24, 1874, Henry Woodward and Mathew Evans filed a Canadian patent for carbon rods mounted in a nitrogen-filled glass cylinder. They did not commercialize their lamp successfully and sold patent rights to Thomas Edison in 1879. These designs show gas-filled enclosures alongside the carbon materials used in earlier efforts to make electric light.
Incandescent light bulb development also followed changes in filament material. In 1904, Sandor Just and Franjo Hanaman received a Hungarian patent for a tungsten filament lamp that lasted longer and gave brighter light than a carbon filament. Tungsram marketed tungsten filament lamps that year. William D. Coolidge developed a method of making ductile tungsten from sintered tungsten in 1909 while working for General Electric. The resulting material could be made into filaments, and General Electric began selling bulbs with ductile tungsten wire by 1911. Materials and manufacturing methods opened routes to a filament that could serve as a practical lighting component.
Filament shape offered another area of development. Burnie Lee Benbow received a patent for a coiled coil filament in 1917: a filament formed into a coil and then wrapped into another coil with a mandrel. In 1921, Junichi Miura created the first double-coil bulb using a coiled coil tungsten filament at Hakunetsusha, a predecessor of Toshiba. Machinery to manufacture those filaments at scale did not yet exist. Hakunetsusha developed a method for mass production by 1936. The progression links a new physical arrangement to the separate manufacturing problem of making it available in large quantities, rather than only demonstrating the design.
Incandescent light bulb efficiency has limits despite those improvements. Less than 5% of the energy consumed becomes visible light, with the rest released as heat. Luminous efficacy measures visible light output relative to electrical power consumed, in lumens per watt. Incandescent lighting uses more power and emits more heat for a given quantity of light than most other electric lighting. In an air-conditioned building, that heat increases the cooling load. During a heating season, heat from lamps can reduce the need for the heating system, though the building's heating equipment can usually provide the same heat at lower cost.
The purchase price is only one part of lighting cost. An incandescent bulb's initial cost is small compared with the energy it uses during its life, and its lifespan is shorter than that of most other lighting. Replacement can add inconvenience or expense, particularly where changing lamps requires labor. Comparisons therefore need to include required illumination, lamp price, replacement labor, electricity, and effects on heating and cooling. Some lamps also emit less light as they age, which may lead to replacement before complete failure.