Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 118

internal ignition strip getter

lamp cap electrode discharge tube

outer bulb

Fig. 10.7 Main parts of a tubular high-pressure sodium lamp

10.2.3 High-Pressure Sodium Lamps

High-pressure sodium (HPS) gas-discharge lamps belong to the group of high inten-
sity discharge HID lamps. HPS lamps, in common with all high-pressure discharge
lamps, are relatively compact. By increasing the vapour pressure in a sodium lamp,
the spectrum around the typical yellow sodium line broadens. The result is that colour
rendering improves and the colour appearance changes from yellow to yellow-white,
albeit at the cost of a decrease in efficacy. However, the resulting efficacy is more
than double that of a high-pressure mercury lamp. At its introduction in the late
1960s, a very efficient alternative was thus obtained for the many high-pressure mer-
cury lamps employed at that time, and which are still sometimes employed today,
in road lighting. Since then, road-lighting installations all over the world very often
use high-pressure sodium lamps. LED solutions have now become an alternative.

10.2.3.1 Construction

Figure 10.7 shows the construction of the high-pressure sodium lamp. The discharge
tube is made of sintered ceramic material because it is resistant to sodium at very
high operating temperatures. In many versions, the sodium is brought into the gas-
discharge tube as a sodium-mercury amalgam composition, which partially vaporises
when the lamp reaches its operating temperature. At 10 kPa the operating vapour
pressure is ca. 10,000 times higher than that of a low-pressure sodium lamp, but
still lower than 1 atmosphere (0.1 atm). To thermally insulate the gas discharge tube
and to protect its components from oxidation, an outer bulb is employed. Xenon is
usually added as a starting gas, which in mercury-free versions, also acts as a buffer
gas that regulates the voltage and reduces thermal losses. Increase of the pressure
of the xenon gas increases the efficacy of the lamp. In versions making use of this
phenomenon, an auxiliary ignition wire or strip, also called an antenna, is required
and added very close to the discharge tube to ensure proper ignition. Like most gas
discharge lamps, this lamp's outer bulb also contains a getter to maintain the vacuum
during the life of the lamp.