Direct Energy, 2018a

7 LAMPS, LEDS, AND LASERS 149
tube can range from 10 −4 Pa to 10 5 Pa for dierent lamps[87, p. 206].
Typical electrode spacing is on the order of centimeters [87]. Some neon
bulbs have an electrode spacing of 1 mm while many uorescent tubes have
an electrode spacing over 1 m. Hundredsto millionsof voltsare applied
across the electrodes [89]. Transformers are used to achieve these high
voltage levels. The voltage between the electrodesionizesthe gasinside
the tube and providesa supply of free electronswhich travel along the
conducting path between the electrodes[89]. The gasmay be ionized, and
electrons supplied, by other methods such as chemical reactions, a static
electric eld, or an optical eld instead [87, Ch. 5]. Electrons may also
be supplied to the gas by thermionic emission, boiling electronso the
cathode.
The optical propertiesof the lamp are determined by the gasinside
the tube. Energy supplied by the electric eld across the electrodes, or
other means, exciteselectronsof the gasatomsto higher energy levels.
Spontaneous emission occurs between distinct allowed energy levels only,
so the emission occurs over relatively narrow wavelength ranges. Gases are
chosen to have allowed energy level transitions in the desired wavelength
range. Typical gases used include helium, neon, sodium, and mercury [87,
p. 514].
Gas discharge lamps are classied as either glow discharge devices or arc
discharge devices. Figure 7.3 shows an example plot of the current between
electrodesasa function of voltage. Asshown in the gure, the currentvoltage
characteristic of a gas discharge tube is quite nonlinear. However,
it can be broken up into three general regions, denoted the dark region, the
glow region, and the arc region. The regions are distinguished by a change
in slope of the current-voltage plot. This gure is used with permission
from [89] which providesmore detailson the physicsof gasdischarges.
The dark region of operation corresponds to low currents and voltages,
and devicesoperating in thisregion are said to have a dark or Townsend
discharge. Optical emission from devices operating in this region are not
self sustaining. While atoms of the gas may ionize and collide with other
atoms, no chain reaction of ionization occurs. The transition between the
dark and glow dischargesiscalled the spark [87, p. 160]. In Fig. 7.3,
V S is the sparking voltage. The second region, corresponding to higher
currents, is called the glow region, and this region is called self sustaining
because ionscollide and ionize additional gasatomsproducing more free
electrons in an avalanche process. Signicant spontaneous emission occurs
in the glow discharge region [87] [89]. The third region, corresponding to
even higher current, iscalled the arc region. Arc discharges are also self
sustaining [87, p. 290], and spontaneous emission is produced. Once the