lumens per watt—but their extremely narrow
spectral range makes these lamps unsuit-
able for use in interiors (color plate 26).
The lamp, consisting of two tubes, one
inside the other, has a mixture of neon and
argon gas, plus sodium metal in the inner
tube and an evacuated outer bulb. Initially,
the arc discharge is through the neon and
argon gas and, as the sodium metal heats
up and vaporizes, the yellow-amber color of
sodium is produced (figure 7.15).
The light produced by the LPS arc, con-
sisting only of radiation in the yellow region
of the visible spectrum, is monochromatic.
No mercury is present in the discharge; UV
radiation emission is not a concern if the
outer bulb breaks. The LPS lamps require
specific ballasts; no retrofit lamps exist.
In contrast to HID sources, LPS systems
maintain constant lumen output over life
and light output is unaffected by changes in
ambient temperature. Normal end of life
occurs when the lamp fails to start or warm
up to full light output.
INTERIOR LIGHTING FOR DESIGNERS
Figure 7.15Low-pressure sodium lamp construction.