Discharge lamps generate light using an
electric arc
discharge rather than by an incandescent filament.
HMI
lamps were developed by OSRAM in the 1960s, and were first used in 1969 to light the first color TV
transmission
in Germany. OSRAM was soon followed by other manufacturers who came out with similar discharge lamps under different names such as CSI, CID, HTI and MSR.
Discharge lamps contain a combination of halides of various 'rare earth' metals such as dysprosium, thulium and holmium, and mercury vapor. The name of the specific discharge
lamp
is usually derived from this combination. For instance, the H of
HMI
stands for mercury (Hg) which is necessary primarily to create the
lamp
voltage; M indicates the presence of metals which essentially contribute to the typical daylight
spectrum
of the discharge lamps, and I refers to the addition of halogen compounds - iodides.
Discharge lamps have a quartz glass
envelope
which is able to withstand the heat and tremendous pressure in the lamp. The
envelope
is formed into a cylindrical or ellipsoidal discharge chamber, and electrodes are attached to the base of the
lamp
with special conductors. As the distance between the electrodes is constant, the
light source
is at a fixed point, improving the performance of the optical system.
As the electrodes of discharge lamps are made of tungsten, halogen gas is added to the mixture of gasses, so that a halogen cycle preserves the electrodes by preventing the vaporized
electrode
material from being deposited on the interior wall. Other chemical interactions ensure that the high concentration of vaporous rare
earth
metals needed for generating light is achieved in the hot zones of the arc.
Two different types of bases are available: double ended and single ended. Double ended lamps are bar shaped with a
connector
at each end. Single ended lamps fit into a housing similar to that of incandescent lamps.
Discharge lamps are extremely reliable and have an exceptionally long
lamp
life. They achieve high values of luminous efficacy, typically around 60 lm/W. The largest discharge lamp, a blazing 12,000W
HMI
lamp by OSRAM, has a
luminous flux
of more than 1 million lumens. Each type of discharge
lamp
has its own typical color temperature, a result of the specific gaseous combinations used. Many lamps aim at a
color temperature
simulating daylight (4,500OK to 5,200OK) and are used in the film industry, where compatibility with daylight and with film emulsions are a critical issue. During operation discharge lamps also emit dangerous ultraviolet radiation, necessitating a special filter to screen out the hazardous rays.
There are many types of discharge lamps, manufactured at different factories, each with its own reference code. For instance:
Single ended discharge lamps (top), a double ended discharge
lamp
(bottom).
MSR
(Medium Source Rare earth), single ended, 95 lumens per watt, 56000K Manufactured by Philips, Holland.
CID
(Compact Iodide Daylight), 70-80 lumens per watt, 55000K Manufactured by Thorn UK.
CSI
(Compact Source Iodide), 90 lumens per watt, 40000K Manufactured by Thorn UK.
BriteArc
(Mercury Halide source), 95 lumens per
watt
Manufactured by Sylvania, USA.
XBO
(Xenon source, daylight), 50 lumens per
watt
62000K. Manufactured by OSRAM, Germany.
The compact, bright
light source
of discharge lamps has been traditionally used in follow spots as well as other high power lighting luminaires and
stage
projectors. Many of the new generation of moving lights use discharge lamps.
