Discharge Lamps
In electric discharge lamps, light is produced by the passage of an electric
current through a vapor or gas, rather than through a tungsten wire as in
incandescent lamps.
The light production bydischargesources is
more efficient than the electric heating
method used in filament lamps. Discharge
lamps used in architectural lighting are more
efficacious and have a longer life. See table
4 in the Appendix.
Fluorescent Lamps
Afluorescentlamp is a low-pressure mercury
arc discharge source. Its operation relies on
an electrical arc passing between two cath-
odes, one at either end of a glass tube. Fluo-
rescent lamps require aballastto provide
the proper starting voltage and regulate the
lamp operating current.
When the voltage difference between the
two cathodes is sufficient to strike an arc, an
electric current passes through mercury vapor
within the bulb. As the arc current passes
through the vapor, it causes changes in the
energy levels of electrons in the individual
mercury ions. As the electrons change levels,
they release several wavelengths of visible
and ultraviolet energy. This radiation strikes
the tube wall, where it causes phosphor
material to fluoresce (become luminous) and
emit light (figure 7.1).
Because light emanates from the phos-
phor, light from a fluorescent lamp is emit-
ted from the surface of the bulb; the entire
tube is the actual light source. Average lumi-
nance of the lamp is comparatively low
because light is generated from a large area.
See table 5 in the Appendix.
The selection of phosphors and addi-
tives determines the kind of light that is pro-
duced: ultraviolet light, colored light, or the
more commonly numerous variations of
“white” light.
Although operating principles are the
same for all fluorescent lamps, two kinds of
cathode exist: hot-cathode and cold-cath-
ode. (These names are misleading, however,
because the cold-cathode type dissipates
more heat than the hot-cathode kind.)Cold-Cathode
Thecold-cathodelamp is a thimble-shaped
cylinder of soft iron, sometimes coated with
emissive materials (figure 7.2). This large-
area source of electrons has an extremely
long life. Voltage drop at the cathode is higher
than with a hot cathode; therefore, the watt-