mercury lamps, metal halide lamps, and high-pressure
sodium lamps. All high-intensity-discharge lamps
produce light from an arc tube that is usually contained
in an outer glass bulb.
Figure 6-14 shows the basic configuration of a HID
lamp. In these lamps, a material, such as sodium,
mercury, or metal halide, is added to the arc tube. In
design, the lamp has three electrodesone acting as a
cathode and the other as an anode with the other
electrode being used for starting. The arc tube contains
small amounts of pure argon gas, halide salts, sodium,
and vapor to aid in starting. Free electrons are
accelerated by the starting voltage. In this state of
acceleration, these electrons strike atoms and displace
other electrons from their normal atomic positions.
Once the discharge begins, the enclosed arc becomes
the light source.
Commercial companies that produce these light
bulbs claim a 100-percent increase in lamp life over
tungsten filament bulbs that produce the same amount
of light. The power in watts required to operate these
lamps is less than one half of that required for filament
lamps. The initial cost of the components for lights is
Figure 6-14.HID lamp configuration.
substantially greater as these lights will require ballasts;
however, this cost can be made up later by the savings of
energy costs. The selection of lighting fixtures will
depend on budgeted dollars for new installation projects
versus maintenance dollars.
Most discharge lighting fixtures are supplied with
the required ballast installed in the fixture. In some
cases ballasts, usually called transformers, are
High Pressure Mercury Lamps
This lamp consists of a quartz arc tube sealed within
an outer glass jacket or bulb. The inner arc tube is made
of quartz to withstand the high temperatures, resulting
when the lamp builds up to normal wattage. Two main
electronemissive electrodes are located at opposite
ends of the tube; these are made of coiled tungsten wire.
Near the upper main electrode is a third, or starting,
electrode in series with a ballasting resistor and
connected to the lower main-electrode lead wire.
The arc tube in the mercury lamp contains a small
amount of pure argon gas that is vaporized. When
voltage is applied, an electric field is set up between the
starting electrode and the adjacent main electrode. This
ionizing potential causes current to flow, and, as the
main arc strikes, the heat generated gradually vaporizes
the mercury. When the arc tube is filled with mercury
vapor, it creates a low-resistance path for current to flow
between the main electrodes. When this takes place, the
starting electrode and its high-resistance path become
Once the discharge begins, the enclosed arc
becomes a light source with one electrode acting as a
cathode and the other as an anode. The electrodes will
exchange functions as the ac supply changes polarity.
The quantity of mercury in the arc tube is carefully
measured to maintain quite an exact vapor pressure
under design conditions of operation. This pressure
differs with wattage sizes, depending on arc-tube
dimensions, voltage-current relationships, and various
other design factors.
Efficient operation requires the maintenance of a
high temperature of the arc tube. For this reason, the arc
tube is enclosed in an outer bulb made of heat-resistant
glass that makes the arc tube less subject to surrounding
temperature or cooling by air circulation. About half an
atmosphere of nitrogen is introduced into the space
between the arc tube and the outer bulb. The operating
pressure for most mercury lamps is in the range of two