(1) mercury vapor, (2) metal halide, and (3)
high-pressure sodium lamps.
HID lamps consist of an arc tube enclos-
ing two electrodes and one or more metals
that are vaporized and ionized to conduct
current in an electric arc from one electrode
to the other. When a lamp is energized, an
electric field is established between the
starting electrode and the main electrode,
causing individual particles of the starting
gas to become electrically charged (figure
7.13). With most HID lamps, the arc tube is
enclosed in an outer glass bulb.
The electrons that comprise the current
stream, or arc discharge, are accelerated to
tremendous speeds. When they collide with
the atoms of the gas or vapor, they tempo-
rarily alter the atomic structure, and light
results from the energy given off in the form
of radiation as the atoms return to their
normal state. The lamp warm-up process
takes three to seven minutes, depending on
ambient temperature conditions.
Each kind of HID lamp is unique. With
mercury vaporlamps, light is produced by an
electric discharge through mercury vapor.
The electrodes are made of tungsten, in
which an emission material is embedded
within the turns of an inner tungsten coil pro-
tected by an outer tungsten coil. The elec-
trodes are heated to the proper electron-
emissive temperature by bombardment
energy received from the arc. After the arc
strikes, its heat begins to vaporize the mer-
cury, which results in poor color quality of a
greenish hue: 15 to 20 CRI (color plate 27).
The addition of phosphor coatings on the
inside of the bulb improves color rendering:
45 to 50 CRI (color plate 28).
Inmetal halidelamps, the electric dis-
charge is through the combined vapors of
mercury and metal halides, which are intro-
duced into the arc tube as compound
iodides. When the lamp attains its full oper-
ating temperature, the metal halides in the
arc tube are partially vaporized and the
metals radiate their spectrum. The metal
halides radiate a wider spectrum than the
mercury lamp, yielding greater efficacies and
better color rendering: 65 CRI (color plate
29). The addition of phosphor coatings on
the inside of the bulb provides diffusion and
some additional color improvement: 70 CRI
(color plate 30).
Pulse-startmetal halide lamps use an
arc tube with substantially higher fill pres-
sure than standard metal halide lamps. This
reduces tungsten evaporation from the elec-DISCHARGE LAMPSFigure 7.13Typical high-intensity discharge lamp. With
all HID lamps, the light-producing element is the arc
tube; it contains metallic vapors, gases, and the elec-
trodes at the ends of the arc tube, where the arc origi-
nates and terminates. The base connects the lamp
mechanically and electrically to the luminaire.