[0001] This invention relates to a discharge tube arrangement and in particular to such
an arrangement for use as a light source.
[0002] It is known, e.g. as disclosed in EP-A-0225753 (University of California), to generate
and sustain a low pressure discharge in a gas by using electromagnetic surface waves.
Surface waves are created by an energizer (also known as a launcher) which is positioned
around and external of, but not extending the whole length of, a discharge tube containing
the gas. In such an arrangement, it is not necessary to provide electrodes inside
the discharge tube. The power to generate the electromagnetic wave is provided by
a radio frequency (r.f.) power generator.
[0003] It is proposed that such an arrangement be used as a visible light or a UV source.
To provide a visible light source, the discharge tube could be a generic fluorescent
lamp discharge tube containing a mix of inert gases and mercury vapour (e.g. argon
gas and mercury vapour) and having on its inner surface a phosphor which converts
254nm U.V. radiation to visible light. To provide a U.V. source, the discharge tube
could be a generic germicidal or curing lamp discharge tube constructed of quartz
glass and containing a mix of inert gases and mercury vapour, but with no phosphor.
[0004] However, there is a disadvantage in the use of a fluorescent type discharge tube
arrangement to produce visible light. As indicated hereinbefore, a discharge tube
containing a mix of inert gases and mercury vapour radiates primarily in the U.V.
so that a phosphor must be used to convert the U.V. to visible light. In this process
about half the energy of the U.V. quantum is lost.
[0005] It is theoretically possible to use a volatile metal halide to produce a low pressure
discharge which emits visible light. Such metal halides are extremely reactive but
their use in certain types of electrodeless discharge tube arrangements has been investigated.
[0006] In the H-Discharge (also known as the 'inductively coupled discharge') arrangement,
the discharge is produced as a single turn loop forming the secondary of a transformer;
the primary is formed by a coil, which may have a high permeability core. It has been
found that low pressure metal halide discharges operated in this mode exhibit a wide
range of instabilities and so are impractical as light sources.
[0007] In the E-discharge arrangement, the discharge vessel is placed between the plates
of a capacitor excited by a high frequency source. However, the current to sustain
the discharge has to flow as displacement current through the glass or silica wall
of the discharge vessel and so it is difficult to produce a discharge having a significant
amount of power. Though the current increases with the frequency of excitation, so
also does the dielectric loss due to the glass or silica wall, resulting in significant
power losses in the wall of the discharge vessel.
[0008] Another type of electrodeless discharge is known as the 'ultra-high frequency' discharge.
In such a discharge, the wavelength of the exciting source is shorter than or comparable
with the discharge dimension. Such discharges have been investigated over many years
but problems of power generation and geometry mean that they do not offer practical
possibilities as commercially viable light sources.
[0009] It has been found that discharges operated in these three prior art modes using a
low pressure metal halide fill tend, under certain, ill-defined conditions, to form
tentacles which attach themselves to the wall of the discharge vessel. This causes
intense local hot spots and so failure of the light source. The discharges produced
are also unstable and present a fluctuating load to the power generator leading to
difficulties in matching. Furthermore, in the discharge arrangements used, the structure
used to excite and sustain the discharge tends itself to obscure the discharge, reducing
the amount of light that can reach the observer.
[0010] It is an object of the present invention to provide a discharge tube arrangement
for use as a source of light which at least alleviates the problems outlined hereinbefore.
[0011] The inventors have surprisingly found that it is possible to achieve a stable, well-behaved,
low pressure metal halide discharge in a discharge tube without electrodes by exciting
the discharge using surface waves. The metal halide is at least partially dissociated
and light is emitted in the visible region from both atomic and molecular fragments.
It is envisaged that metal oxyhalides will exhibit a similar behaviour to metal halides.
[0012] According to a first aspect of the present invention there is provided a discharge
tube arrangement for generating visible light as exemplified in claim 1.
[0013] A discharge tube arrangement provided in accordance with this aspect of the present
invention can be used to generate visible light.
[0014] Preferably the means for applying an r.f. electric field comprises an r.f. power
generator and a launcher. Accordingly, the applying means can be arranged so as not
to substantially obscure the discharge and the discharge itself can have a length
of the order of centimetres to metres and a diameter of the order of millimetres to
centimetres depending on the power used.
[0015] Embodiments of the invention will now be described, by way of example only, and with
reference to the accompanying drawing which shows a cross-sectional side view of a
discharge tube arrangement provided in accordance with the present invention.
[0016] As shown in Figure 1, a discharge tube arrangement comprises a discharge tube 20
mounted in a launcher 22. The discharge tube 20 is formed of a light-transmissive,
dielectric material, such as glass, and contains a fill 24.
[0017] The launcher 22 is made of an electrically conductive material, such as brass, and
formed as a coaxial structure comprising an inner tube 26 and an outer tube 28. A
first plate 30, at one end of the outer tube, provides a first end wall for the launcher
structure. At the other end of the outer tube 28, a second plate 31, integral with
the outer tube 28, provides a second end wall. The inner tube 26 is shorter than the
outer tube 28 and so positioned within the outer tube 28 as to define a first annular
gap 32 and a second annular gap 33. Each of the first plate 30 and second plate 31
has an aperture for receiving the discharge tube 20. The outer tube 28, the first
plate 30 and the second plate 31 form an unbroken electrically conductive path around,
but not in electrical contact with, the inner tube 26 to provide an r.f. screening
structure therearound.
[0018] Suitable dimensions for the launcher of Figure 1 are as follows:
[0019] The thickness of the electrically conductive material is of the order of millimetres,
or less, depending on the construction method used.
[0020] An r.f. power generator 34 (shown schematically) is electrically connected to the
inner tube 26 of the launcher 22 via a coaxial cable 35 and an impedance matching
network 36 (shown schematically as comprising capacitor 37 and inductor 38). The r.f.
power generator 34, the impedance matching network 36, the coaxial cable 35 and the
launcher 22 constitute an r.f. powered excitation device to energise the fill to produce
a discharge.
[0021] A body 40 of dielectric material inside the launcher 22 is provided as a structural
element, to keep the size of the gaps 32, 33 constant and to hold the inner tube 26
in position. The body 40 also helps in shaping the electric field in the gaps 32,
33 for ease of starting or other purposes. Suitable dielectric materials which exhibit
low loss at r.f. frequencies include glass, quartz and PTFE. Alternatively, the launcher
may be partially or completely air filled, provided that means to support the inner
tube are provided.
[0022] When the r.f. power supply 34 is switched on, an oscillating electric field, having
a frequency typically in the range of from 1MHz to 1GHz, is set up inside the launcher
22. At the first and second gaps 32, 33, this electric field is parallel to the longitudinal
axis of the discharge tube 20. If sufficient power is applied, the consequent electric
field produced in the fill 24 is sufficient to create a discharge through which an
electromagnetic surface wave may be propagated in a similar manner to the arrangement
of EP-A-0225753. Accordingly, the launcher 22 powered by the r.f. power generator
34 creates and sustains a discharge in the fill - the length and brightness of the
discharge depending, inter alia, on the size of the discharge tube 20 and the power
applied by the r.f. power generator 34. Such a discharge tube arrangement may therefore
be used as a light source.
[0023] In order to produce a discharge which emits visible light, the fill 24 may comprise
a noble gas, such as argon, together with a compound selected from the group consisting
of metal halides and metal oxyhalides. Mercury may also be added.
[0024] The inventors have tried a fill which contained the noble gas, argon (Ar), together
with aluminium chloride (AlCl₃). This was found to produce a stable discharge, emitting
visible light, when excited by a surface wave.
[0025] Halides of metals from the transition series of the periodic table, such as titanium,
iron and niobium, can advantageously be used. These halides are sufficiently volatile
to produce a vapour pressure at which a discharge can be generated at the wall operating
temperatures of the discharge tube. They can be dissociated by electron impact. The
resulting excited atoms, ions and molecules emit radiation; the metal atoms have large
numbers of relatively low-lying energy levels giving rise to radiation throughout
the visible region.
[0026] Halides of neodymium (Nd) and other rare earth metals on excitation also give rise
to radiation throughout the visible region. They are relatively involatile but can
form complexes with other metal halides (known as complexing agents). The vapour pressure
of the complex so formed can be factor of 10⁵ greater than that of the rare earth
metal halide. The complex should have a total vapour pressure exceeding about 10⁻³
torr at the operating temperature of the lamp, e.g. up to 250°C. Examples of complexing
agents include the halides (ie chlorides, bromides or iodides - X is Cl, Br or I)
of aluminium (AlX₃), indium (InX₃), gallium (GaX₃), tin (SnX₄), titanium (TiX₄) as
well as the compound di-iron (III) chloride (Fe₂Cl₆). Examples of complexes include
NdAlCl₆ (a complex of NdCl₃ and AlCl₃) and NaAlCl₄ (a complex of NaCl and AlCl₃).
[0027] The inventors envisage that stable discharges can be generated by surface waves from
fills containing the following mixtures:
Tin (II) iodide (SnI₂) + sodium iodide (NaI) + AlCl₃ + Ar;
AlBr₃ + SnCl₂ + niobium (V) Chloride (NbCl₅) + Ar;
Indium (I) bromide (InBr) + AlCl₃ + Ar;
Thallium iodide (TlI) + AlCl₃ + Ar;
SnCl₂ + AlBr₃ + Ar;
Iron (II) iodide (FeI₂) + AlBr₃ + Ar;
TlI + NaI + FeI₂ + AlCl₃ + Ar;
NaI + AlBr₃ + Ar;
TlI + NaI + FeI₂ + AlBr₃ + Ar
InI + AlBr₃
[0028] The argon is used to increase overall vapour pressure and may be replaced by other
noble gases, such as neon, helium or krypton.
[0029] It is further envisaged that the oxy-halides (i.e. oxychlorides, oxybromides or oxyiodides
- X is Cl, Br or I) of certain metals, such as chromium (CrO₂X₂) and vanadium (VOX₂
and VOX₃), molybdenum (MoO₂X₂ and MoOX₄), and tungsten (WO₂X₂ and WOX₄) can be used
in fills to produce visible light on excitation. Such oxy-halides are volatile liquids
at room temperature.
1. A discharge tube arrangement for generating visible light, the arrangement comprising
a low pressure discharge tube (23) having walls made of a light-transmissive dielectric
material, and having means for applying a radio frequency (r.f.) electric field over
a part of a wall of the discharge tube at a power sufficient to excite a surface wave
in the fill, whereby, in use, the fill is excited to generate visible light, characterised
in that the discharge tube contains a fill (24) comprising aluminium trichloride or
aluminium tribromide together with an inert gas and at least one compound selected
from the group consisting of metal halides and metal oxyhalides.
2. An arrangement as claimed in claim 1, wherein said means for applying an r.f. electric
field comprises an r.f. power generator and a launcher.
3. An arrangement as claimed in claim 1 or 2, wherein said at least one compound comprises
a halide of a transition metal.
4. An arrangement as claimed in claim 1 or 2, wherein said at least one compound comprises
a metal oxyhalide.
1. Entladungsröhrenanordnung zum Erzeugen von sichtbarem Licht, wobei die Anordnung eine
Niederdruck-Entladungsröhre (23) mit Wänden, die aus einem lichtdurchlässigen dielektrischen
Material hergestellt sind, und eine Einrichtung aufweist zum Anlegen eines elektrischen
Hochfrequenzfeldes über einen Teil von einer Wand der Entladungsröhre mit einer ausreichenden
Leistung, um eine Oberflächenwelle in einer Füllung anzuregen, wodurch, bei Verwendung,
die Füllung zur Erzeugung von sichtbarem Licht angeregt ist, dadurch gekennzeichnet,
daß die Füllung (24) der Entladungsröhre ein Aluminium-Trichlorid oder Aluminium-Tribromid
zusammen mit einem Inertgas und wenigstens einer Verbindung enthält, die aus der aus
Metallhalogeniden und Metalloxyhalogeniden bestehenden Gruppe ausgewählt ist.
2. Anordnung nach Anspruch 1, wobei die Einrichtung zum Anlegen eines elektrischen Hochfrequenzfeldes
einen Hochfrequenz-Leistungsgenerator und einen Starter aufweist.
3. Anordnung nach Anspruch 1 oder 2, wobei die wenigstens eine Verbindung ein Halogenid
von einem Übergangsmetall aufweist.
4. Anordnung nach Anspruch 1 oder 2, wobei die wenigstens eine Verbindung ein Metalloxyhalogenid
aufweist.
1. Dispositif de tube à décharge pour produire de la lumière visible, le dispositif comprenant
un tube à décharge (23) basse pression avec des parois faites d'un matériau diélectrique
transmettant la lumière et comprenant un moyen pour appliquer un champ électrique
haute fréquence (HF) sur une partie d'une paroi du tube à décharge à une puissance
suffisante pour produire une onde de surface dans le gaz de remplissage, ce qui fait
que lors d'une utilisation le gaz de remplissage est excité pour produire de la lumière
visible,
caractérisé en ce que le tube à décharge contient un gaz de remplissage (24) contenant
du trichlorure d'aluminium ou du tribromure d'aluminium associé à un gaz inerte et
au moins un composé choisi dans le groupe formé par les halogénures de métaux et les
oxyhalogénures de métaux.
2. Dispositif selon la revendication 1, dans lequel ledit moyen d'application d'un champ
électrique HF comprend un générateur de puissance HF et un amorceur.
3. Dispositif selon la revendication 1 ou 2, dans lequel ledit composé au nombre d'au
moins un est un halogénure d'un métal de transition.
4. Dispositif selon la revendication 1 ou 2, dans lequel ledit composé au nombre d'au
moins un est un oxyhalogénure métallique.