Sulphur lighting

Sulphur lamps

In 1994, DOE announced that a new, highly efficient lighting system
was illuminating the exterior of the Forrestal Building in Washington,
D.C., and the Space Hall of the Smithsonian's National Air and
Space Museum. The new system is a technological breakthrough that
couples high-power sulfur lamps to a light pipe system that distributes
the light. The lighting of the two buildings is the first working
U.S. example of the high-power version of the sulfur lamp. In
these installations, a hollow pipe distributes focused light from
the sulfur lamp evenly over large areas.

The sulfur lamp bulb consists of a spherical quartz envelope filled
with a few milligrams of sulfur and an inert noble gas, such as
argon, which is weakly ionized using microwaves. The argon heats
the sulfur into a gaseous state, forming diatomic sulfur molecules,
or dimers. The dimers emit a broad continuum of energy as they
drop back to lower energy states--a process called molecular emission.
Molecular sulfur emits almost entirely over the visible portion
of the electromagnetic spectrum, producing a uniform visible spectrum
similar to sunlight but with very little undesirable infrared
or ultraviolet radiation. Conventional mercury lamps and most
other high-intensity discharge (HID) sources are built around
atomic emission and produce an artificial-looking light with many
missing colors.

Unlike conventional sources whose outputs typically diminish 75%
over time, sulfur lamps will maintain their efficiency and light
output over their entire lifetimes. By eliminating the need to
compensate for lamp lumen depreciation, fewer sulfur lamps can
provide a required light level, possibly for long lives of up
to 50,000 hours. In addition, sulfur lamps contain no mercury,
an environmentally toxic substance used in all other conventional
efficient sources.

The sulfur lamp was developed originally by scientists (now at
Fusion Lighting in Rockville, Maryland) who discovered that sulfur
excited by microwave energy could be used in place of mercury
in ultraviolet industrial lamps to produce a high-quality white
light. These lamps operated at power and light output levels (3.5
KW input and 450,000 lumens) too high for most commercial applications.
The high wattage required air-cooling and spinning the lamps to
operate them. Applying their expertise in electrodeless discharge
lamps, LBL researchers developed lower-power lamps using radio
frequencies instead of microwaves. In 1993, they demonstrated
an RF-driven sulfur lamp that produced up to 15,000 lumens with
an RF input of only 100 watts--a luminous efficacy of approximately
150 lumens per RF watt. While the lamps still needed to be rotated,
lower-power operation allowed the air cooling to be eliminated.

Although they are prototypes, the first-generation lamps at the
Forrestal Building and the National Air and Space Museum are nonetheless
energy- efficient. The Forrestal Building's 280-foot light pipe
and two sulfur lamps replaced about 280 mercury HID fixtures,
resulting in a measured energy savings of more than 65% and saving
DOE approximately $8000 annually in energy costs. Because the
sulfur lamp system replaced an old mercury system at the end of
its maintenance cycle, the new light levels were roughly four
times those of the old system. Maintenance costs are also lower,
saving an additional $1500 per year.

DOE is funding Fusion Lighting through LBL to develop a microwave-
operated, high-power sulfur lamp of 1000 watts, producing 125,000
lumens. It is best suited for applications like sports stadiums,
convention centers, aircraft hangars, large maintenance facilities,
highway and street lighting, and shopping mall and industrial
lighting. Another DOE-funded project at Fusion Lighting is aimed
at developing a commercial RF-driven sulfur lamp at lower power
(50-100 watts)--small enough for use in homes and commercial buildings.