336 Electrical Power Systems Technology
Another type of vapor lamp is called a metal halide type. This light
source is a high intensity mercury-vapor lamp in which metallic substanc-
es, called metal halides, are added to the bulb. The addition of these sub-
stances improves the efficacy of the lamps. The efficacy of a metal halide
lamp is typically 75 lumens/watt, compared to approximately 50 lumens/
watt for mercury-vapor lamps.
Another type of vapor lamp is the high-pressure sodium (HPS)
lamp. Sodium is the primary element used to fill the lamp’s tube when
it is manufactured. These lamps have very high efficacies-approximately
110 lumens/watt.
Street Lighting
The lighting systems of today are highly reliable, compared to the
systems that were installed many years ago. Earlier systems were turned
on and off either manually, or by timing devices that were regulated by
the time of day, rather than by the natural light intensity. Some were con-
trolled by electrical impulses that were transmitted on the power lines.
Now, most systems are controlled by automatic photoelectric circuits. The
lights that are now used to illuminate streets and highways have photo-
electric controls. They operate during periods of darkness and are auto-
matically turned off when natural light is present. These street lights are
usually connected to existing 120/240-volt power distribution systems.
There are many types of street lights in use. The earliest types of elec-
tric street lights used were 200- to 1000-watt incandescent lamps. Now, mer-
cury-vapor and sodium-vapor lamps are the primary types used. (Mercury
lamps produce a white light, and sodium lamps have a yellowish color.)
Several different lamp designs and mounting fixture designs are used.
Several years ago, street lights were converted from incandescent
lamps to mercury-vapor lamps. Now the trend in street lighting seems to
be toward the use of sodium-vapor lights, and several areas of the country
have converted to the use of sodium-vapor lamps. Sodium-vapor lights
produce more illumination than a similar mercury-vapor light. They also
require less electrical power to produce a specific amount of illumination.
Thus, the ability of sodium-vapor lights to deliver more light with less
power consumption makes them more economically attractive than mer-
cury-vapor lights.
Comparison of Light Sources
The purpose of a light source is to convert electrical energy into light