Note: Descriptions are shown in the official language in which they were submitted.
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USE OF SILVER TO C~ONTRQL LODINE
Fi~ld o~ _th~ Invention
The present invention relates generally to
high intensity metal halide discharge lamps and, more
particularly, to the use of silver in metal halide
discharge lamps for controlling the iodine level
therein and thereby promoting arc stability and
improving lamp performance.
~ack~rQI~ of the I~ven~ion
In operation of a high intensity metal
halide discharge lamp, visible radiation is emitted by
the metal portion of the metal halide fill at
relatively high pressure upon excitation typically
caused by passage of current therethrough. One class
of high intensity metal halide lamps comprises
electrodeless lamps which generate an arc discharge by
e~tablishing a solenoidal electric field in the high-
pressure gaseous lamp fill comprising the combinationo~ one or more metal halides and an inert buffer gas.
In particular, the lamp fill, or discharge plasma, is
excited by radio frequency (RF) current in an
excitation coiL surrounding an arc tube which contains
th~ fill. The arc tube and excitation coil assembly
acts essentially as a transformer which couples RF
energy to the plasma. That is, the excitation coil
acts as a primary coil, and the plasma functions as a
single-turn secondary. RF current in the excitation
coil produces a time-varying magnetic field, in turn
creating an electric field in the plasma which closes
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completely upon itself, i.e., a solenoidal electric
field. Current flows as a result of this electric
field, producing a toroidal arc discharge in the arc
tube.
Typical electrodeless metal halide -
discharge lamps use metal halides for generating white
color lamp emission for general lighting applications.
Disadvantageously, however~ free iodine formation and
devitrification of the arc tube wall occur in
electrodeless high intensity metal halide discharge
lamps after exposure to the plasma arc discharge. The
amount of ~ree iodine in the arc tube increases with
time. This accumulating iodine, beyond a certain
threshold, causes arc instability and eventual arc
extinction.
Accordingly, it is desirable to provide an
iodine getter for controlling the iodine level in
electrodeless high intensity metal halide discharge
lamps and thereby promote arc stability. To be
practicable, such an iodine getter should extend the
useful life of the lamp and hence not enhance
devitrification and etching of the arc tube wall.
$ummaI~ of the Inven~iQn
Silver is added to the fill of an
electrodeless high intensity metal halide discharge
lamp, which includes at least one metal iodide as a
fill ingredient, for controlling the iodine vapor
level ~herein. In operation, silver reacts with free
iodine, forming silver iodide (AgI), which has a
relatively high boiling point and a relatively low
vapor pressure. The iodine level is thus controlled
below an arc instability threshold to promote and
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maintain arc stability. In addition, silver does not
attack the quartz arc tube wall because silica (si2)
is much more stable than silver oxide (Ay2O).
~oreover, the addition of silver to the arc tube does
not accelerate the decomposition of iodides in the
fill, such as sodium iodide (NaI), cerium iodide
(CeI3), lanthanum iodide (LaI3) and neodymium iodide
(NdI3), which would otherwise enhance devitrification
and etching of the quartz wall. Lamp performance and
life are thus substantially improved using silver as
an iodine getter.
The features and advantages of the present
invention will become apparent from the following
detailed description of the invention when read with
the accompanying drawings in which:
Figure 1 is a partially schematic and
partially cross sectional illustration of a typical
electrodeless high intensity metal halide discharge
20 lamp; ~- ;
Figure 2 graphically compares the efficacy
of a group A of electrodeless high intensity metal
halide discharge lamps using silver as an iodine
getter with corresponding control lamps not using the
getter;
Figures 3 and 4 graphically compare the
iodlne absorbance of groups A and B, respectively, of
electrodeless high intensity metal hallde discharge
lamps u~ing silver as an iodine getter with
corresponding control lamps not using the getter;
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Figure 5 graphically compares the color
temperature for silver-gettered and control lamps of
group A; and
Figure 6 graphically compares color
rendition index values for silver-gettered and control
lamps of group A.
Detailed ~escriptlon of_the
Figure 1 illustrates a typical
electrodeless high intensity metal halide discharge
lamp 10. As shown, lamp 10 includes an arc tube 14
formed of a high temperature glass, such as fused
silica. ~y way of example, arc tube 19 is shown as
having a substantially ellipsoid shape. However, arc
tubes of other shapes may be desirable, de~ending upon
the application. For example, arc tube 14 may be
spherical or may have the shape of a short cylinder,
or "pillbox", having rounded edges, if desired.
Arc tube 19 contains a metal halide fill,
including at least one metal iodide, in which a
solenoidal arc discharge is excited during lamp
operation. A suitable fill comprises at least one
rare earth metal halide (e~g., cerium iodide ~CeI3),
lanthanum iodide (LaI3), neodymium iodide (NdI3),
praeqeodymium iodide (PrI3)3 and at least one alkali
metal halide (e.g., sodium iodldei (NaI), cesium iodide
(CCiI) and lithium iodide ~LiI). One exemplary fill
comprises sodium iodide, cerium iodide and xenon
combined in weight proportions to generate visible
radiation exhibiting high efficacy and good color
rendering capability at white color ~emperatures.
j,s;,~ ~, : .,: .:: . ~:: '
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Such a fill is described in commonly assigned U.S.
Pat. No. 4,810,938 of P.D. Johnson, J.T. Dakin and
J.M. Anderson, issued on Mar. 7, 1989 and incorporated
by reference herein. Another exemplary fill comprises
S a combination of lanthanum iodide, sodium iodide,
cerium iodide, and xenon, as described in commonly
assigned U.S. Pat. No. 4,972,120 of H.L. Witting,
issued Nov. 20, 1990 and incorporated by reference
herein.
Electrical power is applied to lamp 10 by
an excitation coil 16 disposed about arc tube 14 which
is driven by an RF signal via a ballast 18. A
suitable excitation coil 16 may comprise, for example,
a two-turn coil having a configuration such as that
described in commonly assigned U.S. Pat. No. 5,039,903
of G.A. Farrall, issued Aug. 13, 1991 and incorporated
by reference herein. Such a coil configuration
results in very high efficiency and causes only
minimal blockage of light from the lamp. The overall
shape of the excitation coil of the Farrall patent is
generally tha~ of a surface formed by rotating a
bilaterally symmetrical trapezoid about a coil center
line situated in the same plane as the ~xapezoid, but
which line does not intexsect the trapezoid. However,
other suitable coil configurations may be used, such
as that described in commonly assigned U.S. Pat. No.
9,812,702 of J.M. Anderson, issued Mar. 14, 1989 and
incorporated by reference herein. In particular, the
Anderson patent describes a coil having six turns
which are arranged to have a substantially V-shaped
cross section on each side of a coil center line.
Still another suitable excitation coil may be of
solenoidal shape, for exampie.
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In operation, RF current in coil 16 results
in a time-varying magnetic field which produces within
arc tube 14 an electric field that completely closes
upon itself. Current flows through the fill within
arc tube 14 as a result of this solenoidal electric
field, producing a toroidal arc discharge 20 in arc
tube 14. The operation of an exemplary electrodeless
high intensity discharge lamp is described in Johnson
et al. U.S. Pat. No. 4,810,938, cited hereinabove.
In accordance with the present invention,
silver is added to the metal iodide fill of an
electrodeless high intensity discharge lamp in order
to control the level o~ iodine vapor therein, thereby
promoting arc stability. In operation, silver reacts
lS with free iodine that has been released due to metal
loss in the arc tube wall, forming silver iodide
(AgI).
Under lamp operating conditions, some of
the silver iodide vaporizes and some remains in the
liquid phase. The vapor pressure of the silver iodide
ls determined by its liquid temperature which, in
turn, is controlled by the power applied to the
syqtem. The iodine that i5 bound to silver in the
liquid phase is not released to the vapor phase
because silver iodide has a relatively high boiling
point tl506 C) and a relatively low vapor pressure.
Hence, the total iodine concentration in the vapor
phase is regulated by the liquid temperature only, and
an excessive iodine buildup i5 avoided. Hence, with
the iodine vapor pressure controlled below an arc
instability threshold, arc stabllity is promoted and
maintained.
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The quantity of silver employed as an
iodine getter according to the present invention in
order to control iodine vapor pressure below an arc
instability threshold is dependent upon such factors
as type and quantity of fill ingredients, size and
shape of the arc tube, excitation power and operating
temperature. An exemplary quantity is in the range,
for example, from approximately 0.4 to 4 milligrams.
Advantageously, silver does not attack (or i
reduce) the quartz arc tube wall because silica (si2)
is much more stable than silver oxide (Ag2O).
Moreover, silver is less stable than the iodides of ~ -
the lamp fill such as, for example, sodium iodide
(NaI), cerium iodide (CeI3), lanthanum iodide (LaI3),
neodymium iodide (NdI3), and praeseodymium iodide
(PrI3), so that the addition of silver to the arc tube
doss not accelerate the decomposition of the iodides
of the fill which would otherwise enhance ~ `
devitrification and etching of the quartz wall. Lamp
performance and life are thus substantially improved
using silver as an iodine getter. `
The performance of two groups A and B of
lamps were compared, each group consisting of lamps
which did employ silver as an iodi.ne getter and
corresponding control lamps which did not employ an
iodine getter. The lamps of groups A and B are
ellipsoid wlth dimensio~s l9mm x 26mm. Each lamp of
group A (and its corresponding control group)
conta~ned 8 mg of a ~ill mixture comprising sodium
iodide (NaI) and neodymium iodide (NdI3) in a 5:1 -
molar ratio. Each lamp of group B (and its
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corresponding control group) contained 10 mg of a fill
mixture comprising sodium iodide (NaI) and neodymium
iodide (NdI3) in a 7:1 molar ratio. The lamps of
group A were dosed with 1 mg of silver, and the lamps
of group B were dosed with 0.49 mg of silver. The
lamps of group A were operated with excitation coils
of 31 mm inner diameter (I.D.), and the lamps of group
B were operated with excitation coils of 34 mm I.D.,
each group being tested at a power level of 300 coil
Watts. Each lamp of groups A and B had a quartz outer
jacket filled with nitrogen gas surrounding the arc
tube, thé group A jackets having an outer diameter
~O.D.) of 30 mm and the group B jackets having an O.D.
of 33 mm.
Photometric data were taken for the lamps
of group A at burn times of 100, 500, 1000 and 2000
hours. The graph of Figure 2 compares the efficacy of
the lamps of group A using silver as an iodine getter
and the corresponding control lamps. Lamp efficacy
was higher for the lamps of group A using silver as an
iodine getter than for the control lamps. In
addition, the lumen loss over the first 2000 hours of
operation was much lower for the lamps of group A
t2.5%) than ~or the control lamps (15%).
Free iodine formed in the arc tubes was
measured by absorption spectroscopy at a wavelength of
515 nm. The results for group A and B and their
corresponding control groups are illustrated
graphically in Figures 3 and 4, respectively. Iodine
accumulated rapidly in the ungettered control lamps.
Advantageously, however, the level of fr~e iodine in
the lamps using silver as an iodine getter ~as very
low. As iLlustrated in Figure ~, arc instability was
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observed in a lamp of control group B at 2642 hours at
an iodine absorbance of 0.36, equivalent to 0.56 mg of
I2 formed in the lamp. At the same burn time, the arc
was stable and the iodine absorbance was near zero in
a corresponding silver-gettered lamp.
Advantageously, the addition of silver as a
fill ingredient also improved the color temperature ;
and color consistency of the lamps of groups A and B.
Figure 5 shows color temperature as a function of lamp
operating time for group A silver-gettered lamps and
group A control lamps. The color temperature
increased by 400 C in the control lamps at 2000 hours,
while a constant color temperature was observed in the
silver-gettered lamps.
Figure 6 compares the color rendition index
(CRI) values measured for the silver-gettered and
control lamps of group A. The CRI values measured at
100 hours were very similar in both the silver- ~
gettered and control lamps. As lamp operation time ~-
increased, the CRI value of the control lamps
increased, while the CRI of the silver-gettered lamps
remained almost constant. Hence, color consistency is
improved with the addition of silver to the fill.
For the lamps of groups A and B, it was
also observed that the additions of silver to the
lamps did not enhance deterioration of the arc tube
walls.
~ hile the preferred embodiments of the
present invention have been ~hown and described
herein, it will be obvious that such ~mbodiments are
provided by way of example only. Numerous variations,
changes and substitutions will occur to ~hose of skill
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in the art without departing from the invention
herein. Accordingly, it is intended that the
invention be limited only by the spirit and scope of
the appended claims.
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