Direct Energy, 2018a

148 7.3 Devices Involving Spontaneous Emission
eciency. Much of the electromagnetic radiation emitted by a blackbody
radiator falls outside the visible range.
The main advantage of incandescent lamps over other technologies is
their simplicity. For this reason, incandescent lamps were some of the
earliest lamps developed. Humphry Davy demonstrated that blackbody
radiation could be used to produce visible light in 1802, and practical incandescent
lamps date to the 1850s [88]. In order to develop these practical
incandescent lamps, vacuum pumping technology had to be developed, and
technology to purify the metal used to make lamp laments was required
[88].
In some ways, an incandescent lamp is similar to an antenna. In both
cases, the input takes the form of electricity, and this electrical energy is
converted to electromagnetic energy by passing through a conducting wire.
In an antenna, the input is time varying to encode information, and the
output is at radio or microwave frequencies. However, in an incandescent
lamp, the input is typically AC and does not contain information. The
desired output of an incandescent lamp is visible light, but it also produces
heat and electromagnetic radiation at infrared frequencies and at other
non-visible frequencies. Additionally, antennas are typically designed to
operate at a wavelength close to the length of the antenna, and such antennas
can produce waves with specic electromagnetic polarization and
radiation patterns. Spontaneous emission in incandescent lamps, however,
is necessarily unpolarized and incoherent.
7.3.2 Gas Discharge Lamps
A gas discharge occurs when a conducting path forms through a plasma,
an ionized gas [89]. Gas discharge devices convert electricity to light by
spontaneous emission when this type of conducting path forms. In 1802
in addition to demonstrating blackbody radiation and proposing the idea
of a fuel cell, Humphry Davy demonstrated a gas discharge device [3, p.
222] [88]. W. Petrov demonstrated a gas discharge around the same time
[88]. One of the rst practical gas discharge lamps, a carbon arc lamp, was
built by Leon Foucoult in 1850, and it was used for theater lighting [88].
Development of gas discharge lamps required the ability to purify gases in
addition to the development of vacuum pumping technology [88]. Examples
of gas discharge devices in use today include include sodium vapor lamps,
mercury arc lamps, uorescent lamps, and neon advertising signs [89].
A gas discharge lamp is made from a sealed tube containing two electrodes
and lled by a gas. The glass tube contains the gas, maintains the
gas pressure, and keeps away impurities. The pressure of the gas inside the