# Metal halide lamp

Metal halide lamp with a double cap.

Metal vapor lamps are gas discharge lamps in which metal atoms are excited by collision processes with electrons and emit energy in the form of light. Choke coils are mostly used to limit the current.

On the right and left you can see the electrodes that reach into the round bulb here. The distance is only a few centimeters.

## construction

A metal halide lamp consists of a glass bulb into which a small amount of metal and an inert gas are filled. The noble gas is used to build up a spark between the two electrodes reaching into the piston. The eponymous metal evaporates, making the lamp ready for use.

When operating a metal halide lamp, a ballast consisting of an ignitor and a choke must be used. The ignition device provides a high voltage to generate the first spark, after which the lamp can be operated with alternating current in the range of 80 V to 100 V (depending on the lamp type and the filling). The choke limits the current through the lamp to protect it from self-destruction.

## functionality

Low-pressure metal vapor lamps ( Cd , Hg , Na ) with the associated emission spectra

When the lamp is switched on, the ignition device generates a high voltage ( ignition voltage ) that causes a spark to jump between the electrodes. The deposits of the filler metal are evaporated by the resulting heat and help - together with the contained noble gas - to maintain the flow of current between the electrodes at the lower operating voltage.

The flow of current in the tube causes electrons to move from the cathode to the anode. Since the electrons are accelerated between the electrodes, they can absorb enough energy to carry out inelastic collisions with the metal atoms present in the lamp: The accelerated electrons can move an electron in the shell of a metal atom to a higher shell (the atom is excited ). From an energetic point of view, the electron moves from the ground state to an excited state . The space that has become free is occupied again; an electron in the atom thus moves from one state to the free state , with the energy difference in the form of a photon with the energy ${\ displaystyle E_ {1}}$${\ displaystyle E_ {n}}$${\ displaystyle E_ {n}}$${\ displaystyle E_ {1}}$

${\ displaystyle E _ {\ text {Photon}} = E_ {n} -E_ {1}}$

is released. The characteristic emission spectra (for example the typical yellow light from sodium vapor lamps) with the wavelengths result from this energy

${\ displaystyle \ lambda = {\ frac {h \ cdot c} {E _ {\ text {Photon}}}}}$

and therefore the frequencies

${\ displaystyle \ nu = {\ frac {E _ {\ text {Photon}}} {h}}}$

with equal to Planck's quantum of action and the speed of light .${\ displaystyle h}$${\ displaystyle c}$

However, this only applies to low-pressure metal halide lamps, since the electron and gas temperatures are hardly coupled.

In lamps with higher pressures, the coupling of the electrons to the gas temperature leads to a broadening of the spectral lines .

## application

So-called spectral lamps contain a characteristic element and are used to calibrate spectrometers and as sources for monochromatic light. Alkali metals are well suited for use in spectral lamps. Due to their electron configuration, they only have one free electron and therefore only have a few, sharp spectral lines. Furthermore, hydrogen, helium, mercury and cadmium etc. are used as fillers. The spectral lines are designated with the letters of the Latin alphabet based on the lowest energy (i.e. the greatest wavelength). So has z. B. the i-line of mercury has a wavelength of 365  nm or a frequency of 821.35 THz.

These are also often found in terrariums for desert animals.

Sodium lamps and high pressure mercury lamps are used for. B. used for street lighting. Sodium vapor lamps can be recognized by their characteristic yellow light color.

Bulb with a socket on one side, used for example in pursuiters

In event technology , metal halide lamps are used for lighting systems with a very high level of brightness. These are, in particular, pursuit lights for arenas and effect lights , the light output of which is further reduced by effect wheels .

Metal halide lamps are also used as nocturnal outdoor lighting for buildings, as the extremely high light output with lower power consumption than conventional incandescent lamps in combination with the almost bluish light is ideal for such scenarios.

Mercury vapor is also used as the primary light source in fluorescent lamps . However, the glass bulb is coated with a luminescent material that converts the UV lines of the mercury into visible light. The so-called energy - saving lamps also belong to the category of fluorescent lamps. The choice of fluorescent material affects the hue of fluorescent lamps. Lamps with the designation WW (warm white) set a reddish-warm accent, TW (daylight white) a bluish-cold one.

In addition to metals, metal halide lamps also contain halogens in the filling and, compared to high-pressure mercury lamps, have a particularly color-accurate (white) light spectrum. You will u. a. Used to illuminate shop windows and in daylight film spotlights (referred to in English as hydrargyrum medium-arc iodide , HMI).

Metal halide lamps usually cannot be operated directly with mains voltage, but require a ballast that is adapted to the output of the lamp.

## literature

• Winfrid Hauke, Rolf Thaele, Günter Reck: RWE Energie Bau-Handbuch. 12th edition, Energie-Verlag GmbH, Heidelberg 1998, ISBN 3-87200-700-9 .
• Hans R. Ris: Lighting technology for practitioners. 2nd edition, VDE-Verlag GmbH, Berlin-Offenbach 1997, ISBN 3-8007-2163-5 .
• Wilhelm Gerster: Modern lighting systems for indoors and outdoors. 1st edition, Compact Verlag, Munich 1997, ISBN 3-8174-2395-0 .
• Michael Ebner: lighting technology for stage and disco; A handbook for practitioners. 1st edition, Elektor-Verlag, Aachen 2001, ISBN 3-89576-108-7 .