WHAT IS AN HMI LIGHT?
(Edited from Wikipedia 11/7/06)
HMI
or Hydrargyrum Medium-arc Iodide, is a metal-halide HID high
intensity gas discharge medium arc-length lamp. The name implies that
hydrargyrum, an archaic term for Mercury (Hg), is held as a vapor mixed with
other rare halides in a quartz-glass envelope with two tungsten-coated
electrodes of medium arc separation.
Unlike
traditional tungsten lighting units, HMIs use ballasts to regulate and
supply electricity to the lamp head. The lamp operates not by heating a
tungsten-based filament, but rather by creating an electrical arc between
two electrodes within the bulb that excites the pressurized mercury vapor
and provides phenomenal light output with greater efficiency than
tungsten-based lighting units.
The
efficiency advantage is near four-fold, with approximately 85-108 lumens per
watt of electricity. Unlike tungsten bulbs where the gas is inert and solely
for filler and recirculation, HMI bulbs rely heavily on the mercury vapor
for light output, and the other metal halides mixed with the mercury to give
it the light spectrum that bring it to approximately 5600K, or the color
temperature of noon sunlight.
Ballast Operation
In order to
power an HMI bulb, special ballasts act as an igniter to initially start the
arc, and then regulate it by acting as a choke. Two types of ballasts exist
- magnetic and electronic (square-wave or flicker-free). Magnetic
ballasts are generally much heavier and bulkier than electronic ballasts,
yet can usually be obtained at lower cost. Standard magnetic ballasts
exhibit the previously mentioned problems of flicker on film or video.
Within the
last ten years, electronic flicker-free (or Square-Wave) ballasts
have become increasingly popular and affordable as an alternative to
magnetic ballasts by eliminating most of the problems associated with HMI
flicker.
ALZO HMI Ballast
Operation
ALZO HMI ballast is a
patented high frequency modulated flicker free electronic ballast that uses
microcomputer modulated AC power to prevent flicker. The ballast is
specifically designed for video or film recording with regulation circuitry
to maintain consistent color temperature over the life of the bulb.
ALZO HMI HID Bulb
The ALZO HMI Bulb is a
double envelope white frosted Metal Halide bulb with a color temperature of
about 5300K and a life of near 7000 hours. The frosting is not a coating but
is a white glass that will not discolor or wear off the bulb. This white
glass also softens the light emitted from the bulb as uncoated HMI HID bulbs
produce very harsh light.
