Title: Low-pressure mercury vapor discharge lamp
Abstract: Low-pressure mercury vapor discharge lamp comprising a discharge vessel (10) having a first and a second end portion (12a, 12b), the discharge vessel (10) containing mercury and a rare gas, wherein the end portions (12a, 12b) each support an electrode (20a,20b) arranged in the discharge vessel (10) for initiating and maintaining a discharge in the discharge vessel (10), wherein an electrode shield (22a,22b) substantially encompasses at least one of the electrodes (20a,20b), and wherein said electrode shield (22a,22b) comprises an inner wall (23a) and an outer wall (24a), said walls (23a,24a) being spaced apart.
Patent Number: 6,977,469 Issued on 12/20/2005 to Seinen, et al.
| Inventors:
|
Seinen; Peter Arend (Turnhout, BE);
Van Der Eyden; Josephus Theodorus (Eindhoven, NL)
|
| Assignee:
|
Koninklijke Philips Electronics N.V. (Eindhoven, NL)
|
| Appl. No.:
|
476808 |
| Filed:
|
May 8, 2002 |
| PCT Filed:
|
May 8, 2002
|
| PCT NO:
|
PCT/IB02/01635
|
| 371 Date:
|
November 4, 2003
|
| 102(e) Date:
|
November 4, 2003
|
| PCT PUB.NO.:
|
WO02/091423 |
| PCT PUB. Date:
|
November 14, 2002 |
Foreign Application Priority Data
| Current U.S. Class: |
313/613; 313/616; 313/492 |
| Intern'l Class: |
H01J 017/02 |
| Field of Search: |
313/238,239,352,492,493,609,613,616
|
References Cited [Referenced By]
U.S. Patent Documents
| 6646365 | Nov., 2003 | Denissen et al.
| |
| Foreign Patent Documents |
| 2147735 | May., 1985 | GB.
| |
Primary Examiner: Patel; Vip
Claims
1. A low-pressure mercury vapor discharge lamp comprising a discharge vessel
(
10) having a first and a second end portion (
12a,
12b),
the discharge vessel (
10) containing mercury and an inert gas, wherein the
end portions (
12a,
12b) each support an electrode (
20a,
20b)
arranged in the discharge vessel (
10) for initiating and maintaining a discharge
in the discharge vessel, and wherein a double walled electrode shield (
22a,
22b)
substantially encompasses at least one of the electrodes (
20a,
20b),
said double walled electrode shield (
22a,
22b) comprising
an inner wall (
23a) and an outer wall (
24a), which
walls are spaced apart, a space between the inner wall (
23a) and
the outer wall (
24a) being between 0.2 mm and 2 mm.
2. The low-pressure mercury vapor discharge lamp as claimed in claim 1, wherein
the electrode shield (
22a,
22b) is substantially manufactured
from a single piece of sheet material.
3. The low-pressure mercury vapor discharge lamp an of claim 2, wherein the single
piece of sheet material is manufactured from stainless steel.
4. The low-pressure mercury vapor discharge lamp of claim 2, wherein the single
piece of sheet material is manufactured from chromium nickel steel having a composition
comprising in percent by weight of a maximum of 0.08% C, a maximum of 2% Mn, a
maximum of 2-3% Mo and a remainder Fe.
5. The low-pressure mercury vapor discharge lamp of claim 2, wherein the single
piece of sheet material is manufactured from a CoNiCrMo alloy having a composition
of: 41.5% CO, 12% Cr, 4% Mo, 8.7% Fe, 3.9% W, 2% Ti, 0.7% Al and a remaining % Ni.
6. The low-pressure mercury vapor discharge lamp as claimed in claim 1, wherein
the electrode shield (
22a,
22b) is provided on an outer
wall (
24a) with a low-emissivity coating layer (
28a)
to reduce radiation losses of the electrode shield (
22a,
22b).
7. The low-pressure mercury vapor discharge lamp as claimed in claim 6, wherein
the low-emissivity coating layer comprises a material selected from the group consisting
of a precious metal, titanium nitride, chromium carbide, aluminum nitride and silicon carbide.
8. The low-pressure mercury vapor discharge lamp of claim 6, wherein the low-emissivity
coating layer is polished which reduces the radiation of heat through the electrode
shield (
22a,
22b).
9. The low-pressure mercury vapor discharge lamp of claim 6, wherein the low-emissivity
coating layer comprises a precious metal.
10. The low-pressure mercury vapor discharge lamp of claim 9, wherein the chromium
nickel-steel is (AlSi 316).
11. The low-pressure mercury vapor discharge lamp as claimed in claim 1, wherein
the electrode shield (
22a,
22b) is provided, on an
inner side wall with an absorbent coating layer to absorb radiation.
12. The low-pressure mercury vapor discharge lamp as claimed in claim 11, wherein
the absorbent coating layer contains carbon.
13. The low-pressure mercury vapor discharge lamp of claim 1, wherein the double-walled
electrode shield
22a,
22b has a nominal operating temperature
higher than 450 C.
14. The low-pressure mercury vapor discharge lamp of claim 13, wherein the nominal
operating temperature is such that the radiation output of the lamp is at least
80% of that during which the mercury vapor pressure is at its optimum.
15. The low-pressure mercury vapor discharge lamp of claim 13, wherein the operational
temperature causes the lamp to be dimensionally stable, corrosion-resistant and
have a relatively low heat emissivity.
16. The low-pressure mercury vapor discharge lamp of claim 15, wherein the precious
metal is a gold film.
17. The low-pressure mercury vapor discharge lamp of claim 1, wherein the cross-section
of the electrode shield is selected from the group consisting of quadrangular,
cylindrical, triangular and poly-angular.
18. A low-pressure mercury vapor discharge lamp comprising a discharge vessel
(
10) having a first and a second end portion (
12a,
12b),
the discharge vessel (
10) containing mercury and an inert gas, wherein the
end portions (
12a,
12b) each support an electrode (
20a,
20b)
arranged in the discharge vessel (
10) for initiating and maintaining a discharge
in the discharge vessel, and wherein a double walled electrode shield (
22a,
22b)
substantially encompasses at least one of the electrodes (
20a,
20b),
wherein said double walled electrode shield (
22a,
22b)
comprises an inner wall (
23a) and an outer wall (
24a),
which walls are spaced apart and wherein the inner wall (
23a) is
substantially encompassed by the outer wall (
24a) and is connected
to the outer wall (
24a) by a connecting portion (
25a),
the inner wall (
23a) and outer wall (
24a) being comprised
of three or more sub-wall regions, wherein respective sub-wall regions are proximally
aligned and are spaced apart, each sub-wall region of the respective inner wall
(
23) and outer wall (
24) forming a substantially right angle with
an adjoining sub-wall region.
19. A low-pressure mercury vapor discharge lamp of claim 18, wherein the spacing
is between 0.2 mm and 2 mm.
20. A low-pressure mercury vapor discharge lamp of claim 18, comprised of four
or more sub-wall regions.
21. A low-pressure mercury vapor discharge lamp of claim 18, wherein a central
portion of the connecting portion (
25a) is removed to enhance an
insulating effect between the inner wall (
23a) and the outer wall (
24a).
22. A low-pressure mercury vapor discharge lamp of claim 18, wherein the electrode
shield (
22) is constructed from a single piece of sheet material.
23. A low-pressure mercury vapor discharge lamp comprising a discharge vessel
(
10) having a first and a second end portion (
12a,
12b),
the discharge vessel (
10) containing mercury and an inert gas, wherein the
end portions (
12a,
12b) each support an electrode (
20a,
20b)
arranged in the discharge vessel (
10) for initiating and maintaining a discharge
in the discharge vessel, and wherein a double walled electrode shield (
22a,
22b)
substantially encompasses at least one of the electrodes (
20a,
20b),
said double walled electrode shield (
22a,
22b) comprising
an inner wall (
23a) and an outer wall (
24a), which
walls are spaced apart; wherein respective sub-wall regions are proximally aligned
and are spaced apart, and wherein the inner wall (
23a) is substantially
encompassed by the outer wall (
24a) and is connected to the outer
wall (
24a) by a connecting portion (
25a).
Description
The invention concerns a low-pressure mercury vapor discharge lamp comprising
a discharge vessel having a first and a second end portion, the discharge vessel
containing mercury and a rare gas, wherein the end portions each support an electrode
arranged in the discharge vessel for initiating and maintaining a discharge in
the discharge vessel, and wherein and electrode shield substantially encompasses
at least one of the electrodes.
Such a low-pressure mercury vapor discharge lamp is described in the non-prepublished
European patent application No. EP 0011119 (PHD 99.160). With this lamp the electrode
shield is manufactured from stainless steel sheet material that is formed into
a tube.
In mercury vapor discharge lamps mercury forms the primary component for the
(efficient)
generation of ultraviolet (UV) light. On an inner wall of the discharge vessel
a luminescent layer comprising a luminescent material (such as fluorescent powder)
is present to convert UV into other wavelengths, such as UV-B and UV-A for tanning
purposes (sun beds), or to visible radiation. Such discharge lamps are for this
reason also referred to as fluorescent lamps.
In the description and claims of the present invention the expression "nominal
operation" is used in order to refer to operating conditions in which the mercury
vapor pressure is such that the radiation output of the lamp is at least 80% of
that during optimum operation, meaning under operating conditions in which the
mercury vapor pressure is at its optimum.
For correct operation of low-pressure mercury vapor discharge lamps the electrodes
of such discharge lamps comprise an (emitter) material with a low so-called work
function (lowering of the output potential) for the delivery of electrons to the
discharge (cathode function) and the receipt of electrons from the discharge (anode
function). Known materials with a low work function are, for example, barium (Ba),
strontium (Sr) and Calcium (Ca). It is noted that during ignition and during operation
of low-pressure mercury vapor discharge lamps material (barium and/or strontium)
evaporates and sputters from the electrode(s). In general the emitter material
is deposited on the inner wall of the discharge vessel and on the electrode shield,
if the low-pressure discharge lamp includes such an electrode shield. It also appears
that the above-mentioned Ba and Sr that is deposited elsewhere in the discharge
vessel no longer takes part in the light generating process. The deposited (emitter)
material also forms mercury-containing amalgams on the inner wall, as a result
of which the quantity of mercury available for the discharge (gradually) falls,
which can adversely affect the lifetime of the lamp. In order to compensate for
such a loss of mercury during the life of the lamp, in the lamp a relatively high
dose of mercury is necessary which is undesirable from the environmental point
of view.
By providing an electrode shield that encompasses the electrode(s) and that during
nominal operation has a temperature that is higher than 250° C., there is
a fall in the reactivity of materials in and on the electrode shield for reaction
with the mercury present in the discharge vessel to prevent the formation of amalgams
(Hg—Ba, Hg—Sr).
Experiments have also shown that the emitter material, that evaporates
from the electrode, forms oxides (BaO or SrO). During (nominal) operation of the
discharge lamp mercury forms a bond with such oxides of evaporated emitter material.
If reactive oxygen is present in the vicinity of the electrode, BaO, SrO and/or
HgO are formed, and possibly also SrHgO
2 and BaHgO
2. If tungsten
(from the electrode) is also deposited (during cold starts sputtering of tungsten
takes place), WO
x and HgWO
x are also formed. Without it being
necessary to give a theoretical explanation, it seems that, although BaO and SrO
under normal thermal conditions do not react with mercury, the presence of the
discharge in the discharge area plays a role in the formation of these compounds
of mercury and the oxides of evaporated emitter material. At temperatures higher
than 450° C. the mercury is released again, due to dissociation of the said
compounds of mercury and the oxides of evaporated emitter material, and the released
mercury is again available for discharge. HgO, BaO and SrO in particular dissociate
from 450° C. upwards. The compounds SrHgO
2 and BaHgO
2 are
somewhat more stable, the dissociation of these requiring a higher temperature
of at least 500° C.
The aim of the invention is an efficient low-pressure mercury vapor discharge
lamp of the kind described in the opening that uses less mercury.
To that end the electrode shield comprises an inner wall and an outer wall that
are spaced apart. In this way an electrode shield is obtained with good insulating
characteristics, so that the temperature of the inner wall is higher than for a
single wall so that, as described above, less mercury is bonded. For a good insulating
effect the spacing between the inner wall and the outer wall is preferably between
0.2 mm and 2 mm.
Preferably the electrode shield is manufactured predominantly from a single
piece of sheet material, and preferably it is manufactured from stainless steel.
Stainless steel is a material that is resistant to high temperatures. The material
has, compared with iron for example, a high corrosion resistance, a relatively
low thermal conduction coefficient and a relatively poor thermal emissivity. By
manufacturing the shield from a single piece of sheet material it can be produced
in a low-cost manner.
Preferably the electrode shield is provided on a side facing away from
the electrode with a low emissivity coating layer to reduce radiation losses of
the electrode shield, which coating layer preferably contains a precious metal
or chrome. By applying such a layer to the outer surface of the electrode shield
it is simpler to reach the desired relatively high temperatures of the electrode
shield. Other suitable materials for a low-emissivity coating layer on the outer
surface of the electrode shield are titanium nitride, chromium carbide, aluminum
nitride and silicon carbide. In an alternative embodiment of the low-pressure mercury
vapor lamp the outer surface is polished. The polishing treatment of the outer
surface of the electrode shield also reduces the radiation of heat through the
electrode shield.
The electrode shield is preferably provided on a side directed towards to the
electrode with an absorbent coating layer for absorption of radiation, which coating
layer preferably contains carbon. By using a layer with a relatively high emissivity
in the infra-red radiation range, the heat absorbing power of the electrode shield
is increased. In this way it is simpler to reach the desired relatively high temperatures
of the electrode shield.
The invention will now be explained in more detail using an example and the figures,
in which:
FIG. 1 is a schematic and longitudinal cross-sectional representation of an
embodiment of the low-pressure mercury vapor discharge lamp in accordance with
the invention;
FIG. 2 is a perspective view of a detail of FIG. 1;
FIG. 3 is a perspective view of a detail of FIG. 2; and
FIG. 4 is a representation of the average wall temperature of an electrode shield
of a low-pressure mercury vapor discharge lamp in accordance with the invention
as a function of the spacing between the walls.
FIG. 1 shows a low-pressure mercury vapor discharge lamp provided with a glass
discharge vessel
10 with a tubular portion
11 around a longitudinal
axis
2, which discharge vessel allows the radiation generated in the discharge
vessel
10 to pass through and is provided with a first and a second end
portion
12a,
12b. In this example the tubular portion
11 has a length of 120 cm and an internal diameter of 24 mm. The discharge
vessel
10 encompasses in a gas-tight manner a discharge area
13 provided
with a filling of 1 mg of mercury and an inert gas, for example argon. The wall
of the tubular portion is customarily coated with a luminescent layer (not shown
in FIG. 1), comprising a luminescent material (for example fluorescent powder),
that converts the ultraviolet (UV) light generated by the mercury excited as it
is incident into (predominantly) visible light. End portions
12a,
12b each support an electrode
20a,
20b arranged
in the discharge area
13. The electrode
20a,
20b
is a winding of tungsten that is covered with an electron-emitting substance,
in this case a mixture of barium, calcium and strontium oxide. From the electrodes
20a,
20b current supply conductors
30a,
30a′,
30b,
30b′ extend
through the end portions
12a,
12b to the outside of
the discharge vessel
10. The current supply conductors
30a,
30a′,
30b,
30b′, are connected
with contact pins
31a,
31a′,
31b,
31b′ that are secured to a lamp base
32a,
32b.
Generally around each electrode
20a,
20b an electrode
ring is arranged (not shown in FIG. 1), on which a glass capsule is clamped, through
which mercury is dosed. In an alternative embodiment, an amalgam—comprising
mercury and an alloy of PbBiSn is provided in an exhaust tube (not shown in FIG.
1) that is connected with the discharge vessel
10.
In the embodiment of FIG. 1 the electrode
20a,
20b is
encompassed by a double-walled electrode shield
22a,
22b,
that in nominal operation has a temperature that is higher than 450° C. At
the said temperatures, dissociation causes mercury that is bonded to BaO or SrO
on the electrode shield
22a,
22b to be released and
become available again for discharge in the discharge area. A particularly suitable
temperature of the electrode shield is at least 550° C. In the example of
FIG. 1 the electrode shield
22a is manufactured from stainless steel.
Such an electrode shield is, at the said high temperatures, dimensionally stable,
corrosion-resistant and has a relatively low heat emissivity. A suitable material
for the manufacture of the electrode shield is chromium nickel steel (AlSi
316)
having the following composition (in % by weight): a maximum of 0.08% C, a maximum
of 2% Mn, a maximum of 2-3% Mo and the remainder Fe. A further particularly suitable
material for the manufacture of the electrode shield is Duratherm 600, a CoNiCrMo
alloy with an increased corrosion resistance and having the following composition:
41.5% CO, 12% Cr, 4% Mo, 8.7% Fe, 3.9% W, 2% Ti, 0.7% Al and the remaining % Ni.
FIG. 2 is a perspective view of a detail of FIG. 1, wherein the end portion
12a supports the electrode
20a via the current supply
conductors
30a,
30a′. The double-walled electrode
shield
22a is supported by a support wire
26a that
in this example is positioned in the end portion
12a. In an alternative
embodiment the support wire
26a is connected with one of the current
supply conductors
30a,
30a′. In the example
of FIG. 2 the support wire
26a is made from stainless steel. Stainless
steel has a relatively very low thermal conduction coefficient relative to the
known materials (iron, for example) that are used as the support wire. The electrode
shield
22a can maintain a relatively high temperature, inter alia
because the support wire
26a effectively reduces heat discharge from
the electrode shield
22a. In a further alternative embodiment the
electrode shield is mounted directly on the current supply conductors, for example
through the electrode shield being provided with constrictions that are a press
fit on the current supply conductors.
FIG. 3 shows a perspective view of an embodiment of the essentially quadrangular
electrode shield
22a as shown in FIG. 2, comprising an inner wall
23a, and an outer wall
24a that at least substantially
encompasses the outer wall
24a, and a connecting portion
25a.
The electrode shield does not necessarily have to be quadrangular in shape, but
can for example also be cylindrical, triangular or polyangular in cross-section.
The electrode shield in this example is manufactured from a single piece of sheet
material, and in the connecting portion
25a the central piece is
removed, for example by punching, so that only two connecting limbs remain on the
side edges, which enhances the insulating effect between the inner wall
23a
and the outer wall
24a. In order to be able to achieve temperatures
of the inner wall
23a of the electrode shield
22a in
excess of 450° in operation, preferably of at least 550° C., an outer
surface of the outer wall
24a of the electrode shield
22a
is provided with a low-emissivity coating layer
28a to reduce
radiation losses of the electrode shield
22a. The low-emissivity
coating layer
28a preferably comprises a chromium film. In an alternative
embodiment the low-emissivity coating layer
28a comprises a precious
metal, for example a gold film. Also in FIG. 3, the inner wall
23a of
the electrode shield
22a is provided on an inner surface with an
absorbent coating layer
29a for absorption of (heat) radiation. The
absorbent coating layer
29a preferably comprises carbon.
The spacing between the two wall portions
23a,
24a is
preferably between 0.2 and 2 mm. FIG. 4 shows, for an embodiment, the relation
between the wall spacing on the one hand and the average wall temperature (a) of
the inner wall and the average wall temperature (b) of the outer wall on the other
hand. "(c)" gives the temperature that is reached with a single wall, or with a
wall spacing of 0 mm. The graph shows clearly that a double wall results in a higher
temperature of the inner wall
23a than a single wall, and that a
greater spacing between the two wall portions
23a,
24a
likewise contributes to a higher temperature, but that the effect of this drops
as the wall spacing increases. It is conceivable that the wall spacing should not
be too great, since otherwise the "double-wall" effect is lost.
*
