Gas-discharge lamps are a family of artificial light sources that generate
light by sending an electrical discharge through an ionized gas, a plasma.
Typically, such lamps use a noble gas; or a mixture of these gases. Some
include additional substances, like mercury, sodium, and metal halides,
which are vaporized during startup to become part of the gas mixture. In
operation the gas is ionized, and free electrons, accelerated by the electric
field in the tube, collide with gas atoms. Some electrons in the atomic
orbitals of these atoms are excited by these collisions to a higher energy
state. When the excited electron falls back to a lower energy state, it emits a
photon of light of a characteristic frequency. The color of the light
produced depends on the emission spectra of the atoms making up the gas, as well
as the pressure of the gas and other variables. Gas discharge lamps can
produce a wide range of colors. Some lamps produce ultraviolet radiation
which is converted to visible light by a fluorescent coating on the inside of the
lamp's glass surface. The fluorescent lamp is perhaps the best known
gas-discharge lamp. Compared to incandescent lamps,
gas-discharge lamps offer higher efficiency, but are more complicated to
manufacture and require auxiliary electronic equipment such as ballasts to
control current flow through the gas. Some gas-discharge lamps also have a
perceivable start-up time to achieve their full light output. Still, due to
their greater efficiency, gas-discharge lamps are replacing incandescent lights
in many lighting applications. History
The history of gas-discharge lamps began in 1675 when French astronomer
Jean-Felix Picard observed that the empty space in his mercury barometer
glowed as the mercury jiggled while he was carrying the barometer.
Investigators, including Francis Hauksbee, tried to determine the cause
of the phenomenon. Hauksbee first demonstrated a gas-discharge lamp in
1705. He showed that an evacuated or partially evacuated glass globe, in
which he placed a small amount of mercury, while charged by static
electricity could produce a light bright enough to read by. The phenomenon of
electric arc was first described by Vasily V. Petrov, a Russian scientist,
in 1802; Sir Humphry Davy demonstrated in the same year the electric arc at the
Royal Institution of Great Britain. Since then, discharge light sources have
been researched because they create light from electricity considerably more
efficiently than incandescent light bulbs.
The father of the low-pressure gas discharge tube was German glassblower
Heinrich Geissler, who beginning in 1857 constructed colorful artistic cold
cathode tubes with different gases in them which glowed with many different
colors, called Geissler tubes. It was found that inert gases like the noble
gases neon, argon, krypton or xenon, as well as carbon dioxide worked well in
tubes. This technology was commercialized by French engineer
Georges Claude in 1910 and became neon lighting, used in neon signs.
The introduction of the metal vapor lamp, including various metals within
the discharge tube, was a later advance. The heat of the gas discharge vaporized
some of the metal and the discharge is then produced almost exclusively by the
metal vapor. The usual metals are sodium and mercury owing to their visible
spectrum emission. One hundred years of research later led
to lamps without electrodes which are instead energized by microwave or radio
frequency sources. In addition, light sources of much lower output have been
created, extending the applications of discharge lighting to home or indoor
use. Color
Each gas, depending on its atomic structure emits certain wavelengths, its
emission spectrum, which determines the color of the light from the lamp. As a
way of evaluating the ability of a light source to reproduce the colors of
various objects being lit by the source, the International Commission on
Illumination introduced the color rendering index. Some gas-discharge
lamps have a relatively low CRI, which means colors they illuminate appear
substantially different from how they do under sunlight or other high-CRI
illumination. Types
Lamps are divided into families based on the pressure of gas in the bulb, below.
A second distinction used is whether the cathode is heated:
Hot-cathode lamps have electrodes which operate at a high temperature, which
during operation are heated by the arc current in the lamp. The heat knocks
electrons out of the electrodes by thermionic emission, which helps
maintain the arc. In many types the electrodes consist of electrical
filaments made of fine wire, which are heated by a separate current at startup,
to get the arc started. Cold-cathode lamps have electrodes which
operate at room temperature. To start conduction in the lamp a high enough
voltage must be applied to ionize the gas, so these lamps require higher
voltage to start. = Low pressure discharge lamps=
Low-pressure lamps have working pressure much less than atmospheric pressure. For
example, common fluorescent lamps operate at a pressure of about 0.3% of
atmospheric pressure. Fluorescent lamps, a heated-cathode
lamp, the most common lamp in office lighting and many other applications,
produces up to 100 lumens per watt Neon lighting, a widely used form of
cold-cathode specialty lighting consisting of long tubes filled with
various gases at low pressure excited by high voltages, used as advertising in
neon signs. Low pressure sodium lamps, the most
efficient gas-discharge lamp type, producing up to 200 lumens per watt, but
at the expense of very poor color rendering. The almost monochromatic
yellow light is only acceptable for street lighting and similar
applications. A small discharge lamp containing a
bi-metallic switch is used to start a fluorescent lamp. In this case the heat
of the discharge is used to actuate the switch; the starter is contained in an
opaque enclosure and the small light output is not used.
Continuous glow lamps are produced for special applications where the
electrodes may be cut in the shape of alphanumeric characters and figural
shapes. A flicker light bulb, flicker flame
light bulb or flicker glow lamp is a gas-discharge lamp which produces light
by ionizing a gas, usually neon mixed with helium and a small amount of
nitrogen gas, by an electric current passing through two flame shaped
electrode screens coated with partially decomposed barium azide. The ionized gas
moves randomly between the two electrodes which produces a flickering
effect, often marketed as suggestive of a candle flame. US patent 3238408,
Kayatt Philip J., "Flicker glow lamps", issued 1966-03-1
= High pressure discharge lamps= High-pressure lamps have a discharge
that takes place in gas under slightly less to greater than atmospheric
pressure. For example, a high pressure sodium lamp has an arc tube under 100 to
200 torr pressure, about 14% to 28% of atmospheric pressure; some automotive
HID headlamps have up to 50 bar or fifty times atmospheric pressure.
Metal halide lamps. These lamps produce almost white light, and attain 100 lumen
per watt light output. Applications include indoor lighting of high
buildings, parking lots, shops, sport terrains.
High pressure sodium lamps, producing up to 150 lumens per watt. These lamps
produce a broader light spectrum than the low pressure sodium lamps. Also used
for street lighting, and for artificial photoassimilation for growing plants
High pressure mercury-vapor lamps. This lamp type is the oldest high pressure
lamp type, being replaced in most applications by the metal halide lamp
and the high pressure sodium lamp. It requires a shorter arc length.
= High-intensity discharge lamps= A high-intensity discharge lamp is a
type of electrical lamp which produces light by means of an electric arc
between tungsten electrodes housed inside a translucent or transparent
fused quartz or fused alumina arc tube. Compared to other lamp types, relatively
high arc power exists for the arc length. Examples of HID lamps include:
Mercury-vapor lamps Metal halide lamps
Ceramic discharge metal halide lamps Sodium vapor lamps
Xenon arc lamps HID lamps are typically used when high
levels of light and energy efficiency are desired.
Other examples The Xenon flash lamp produces a single
flash of light in the millisecond-microsecond range and is
commonly used in film, photography and theatrical lighting. Particularly robust
versions of this lamp, known as strobe lights, can produce long sequences of
flashes, allowing for the stroboscopic examination of motion. This has found
use in the study of mechanical motion, in medicine and in the lighting of dance
halls. See also
References Further reading
Waymouth, John. Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press.
ISBN 0-262-23048-8. National Highway Traffic Safety
Administration. "Glare from headlamps and other front mounted lamps". Federal
Motor Vehicle Safety Standard No. 108. US Department of Transportation.
Retrieved 2006-01-23. External links
Lamps and Indicators at DMOZ