1. HIGH-INTENSITY DISCHARGE LAMPS
High-Intensity Discharge (HID). This term denotes a general group of lamps consisting of
mercury,
metal halide, and high-pressure sodium lamps. A mercury lamp is an electric discharge lamp
in which the major portion of the radiation is produced by the excitation of mercury atoms. A
metal
halide lamps is an electric discharge lamp in which the light is produced by the radiation from
an
excited mixture of a metallic vapor (mercury) and the products of the dissociation of halides
(for example, halides of thallium, indium, sodium). A high-pressure sodium lamp is an electric
discharge
lamp in which the radiation is produced by the excitation of sodium vapor in which the partial
pressure of the vapor during operation is of the order of 104 N/m2.
Lamp Construction and Designation. HID lamps consist of a cylindrical transparent or
translucent
arc tube which confines the electric discharge and the associated gases. That tube is further
enclosed in a glass bulb or outer jacket to exclude air to prevent oxidation of the metal parts
and to
stabilize operating temperatures and significantly reduce ultraviolet radiation emitted by the
excitation
of the vapors. The mount structure of many HID lamps is anchored to the “dimple top” of the
outer glass bulb, assuring greater structural integrity and more accurate alignment of the arc
tube.
The construction of a typical mercury lamp is shown in Fig. 26-19. The basic elements are the
arc
tube, fabricated from fused silica and filled with a drop of mercury and a rare gas at low
pressure;
the electrodes; and the outer envelope, which may or may not have a phosphor coating on the
Lamp Characteristics. Light output from each of the three types of HID lamps has its own color
appearance (chromaticity), and the spectral power distributions vary as shown as in Fig. 26-22.
Table 26-12 lists the radiated energy of typical 400-W HID lamps.
Performance Characteristics. Table 26-13 lists some typical metal halide and high-pressure
sodium
HID lamps for general lighting along with their light output (reference initial and mean lumens).
The
basis for published data may vary with manufacturer. For a qualitative comparison of HID lamps
with incandescent and fluorescent lamps, see Table 26-14.
Many mercury and high-pressure sodium lamps, and some metal halide lamps, can be operated
in any position. Other metal halide lamps have restricted narrow ranges of acceptable burning
positions,
outside of which light output and rated life may be adversely affected. Some metal halide

2. lamps must be used only in enclosed luminaires, or may operate at higher temperatures which
would
exceed the temperature rating of explosion-proof or other hazardous-area luminaries. Certain
metal
halide lamps have compact outer bulbs and bases at each end, for smaller display-, sports- and
floodlighting
luminaires. Some HID lamps have sufficient ultraviolet output to produce skin burn and/or
eye injury, thus requiring specialized luminaires equipped with safety interlocks.
A new family of pulse-start metal halide lamps resulted from studies showing that improved
lumen maintenance and color stability of metal halide lamps could result form (1) reducing the
HID Lamp Operation. The practical limit of an HID lamp’s current-carrying capacity is how
high a temperature its enclosing tube can withstand without rupturing. By connecting an
impedance
in series with the lamp, the current is controlled. In most lamps about one-half the supply
voltage is absorbed by a series ballasting device. A variety of ballasts are available for operating
lamps, singly or in pairs. Single lamp ballasts may have low (0.50 minimum) or high (0.90
minimum)
power factor, 2 lamp ballasts have inherently high power factor. The simplest lamp ballast
is the reactor-type used in series with the lamp when line voltage is sufficient for reliable
starting. This is not recommended where line-voltage fluctuations exceed 5%. A reactor-type
ballast
can be used when the line voltage is approximately twice the rated lamp voltage. The
autotransformer-type ballast is used on circuits where the line voltage must be changed to suit
the lamp requirements. Constant-wattage types of the autotransformer or isolated-secondary
design are widely used because of better regulation, low line starting currents, and lower
dropout
voltage. They are also called stabilized or regulated. Heavier wiring, oversized circuit breakers,
and time-delay relays that may be required by the relatively high starting currents of non-CW
and
non-CWA ballasts are eliminated with stabilizing ballasts, as the starting current is less than the
rated operating current.
One of the limitations of the HID lamp is the effect of power-supply interruptions. In the event
of a power interruption or voltage dip lasting for more than 1 cycle, HID lamps extinguish and
do
not restart for several minutes. The exact magnitude of the voltage drop to cause this condition
depends on the ballast design. Regulator ballasts withstand a greater drop than other types.
The
delay in lamp restarting is caused by the high pressure that develops in the arc tube during
operation.
The open-circuit voltage of standard ballasts is not sufficient to restart the lamp until the lamp

3. cools and the pressure decreases. In installations where this characteristic might be a safety
hazard,
the use of a few incandescent or fluorescent luminaires along with the HID units assures
emergency
illumination until the HID lamps restart. Tungsten-halogen auxiliaries are available for HID
industrial
luminaires to provide standby illumination in the event of momentary power failure. For indoor
and outdoor sports lighting, and other applications where instant restrike is preferable, special
ignitor
are available, as are special instant-restrike high-pressure sodium lamps. Metal halide lamps
with a wire lead (at the end opposite the base) are used with auxiliary ignitors to achieve
instant
restrike. For aisle lighting in warehouses and other interior and exterior situations where
illumination
levels for accurate seeing are not needed all of the time, high/low electrical components,
combined
with occupancy detectors, transmitters, and luminaire-mounted receiver scan provide major
energy saving by reducing input wattage 50% to 70% during intervals when the spaces are
unoccupied.
ILLUMINATION 26-29
TABLE 26-12 Energy Output for Some HID Lamps
400-W 400-W high
400-W metal Pressure
Type of energy Mercury halide sodium
Light 14.6% 20.6% 25.5%
Infrared 46.4 31.9 37.2
Ultraviolet 1.9 2.7 0.2
Conduction-convection 27.0 31.1 22.2
Ballast 10.1 13.7 14.9