Method for applying a protective coating to a high-intensity metal halide discharge lamp

A method for applying a protective coating to the inner surface of the arc tube of a high-intensity metal halide discharge lamp involves dosing the arc tube with an inert gas that is doped with a metal hydride gas. Preferably, the metal hydride gas comprises silane. The arc tube is heated to a sufficiently high temperature to decompose the silane gas. As a result, silicon is deposited as a protective coating on the inner surface of the arc tube wall. The hydrogen gas that is generated by the silane decomposition is removed from the system either by pumping it out before dosing the arc tube with the final arc tube fill, or by diffusion through the arc tube wall during operation of the lamp.

FIELD OF THE INVENTION 
The present invention relates generally to high-intensity metal halide 
discharge lamps. More particularly, the present invention relates to a 
method for applying a protective coating to the inner surface of the arc 
tube of such a lamp. 
BACKGROUND OF THE INVENTION 
In operation of a high-intensity metal halide discharge lamp, visible 
radiation is emitted by the metallic 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 
establishing a solenoidal electric field in the high-pressure gaseous lamp 
fill comprising the combination of 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 the fill. The arc tube and excitation coil assembly 
acts essentially is 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 is the excitation coil 
produces a time-varying magnetic field, in turn creating an electric field 
in the plasma which closes completely upon itself, i.e., a solenoidal 
electric field. Current flows as a result of this electric field, thus 
producing a toroidal arc discharge in the arc tube. 
High-intensity, metal halide discharge lamps, such as the aforementioned 
electrodeless lamps, generally provide good color rendition and high 
efficacy in accordance with the principles of general purpose 
illumination. However, the lifetime of such lamps can be limited by the 
loss of the metallic portion of the metal halide fill during lamp 
operation and the corresponding buildup of free halogen. In particular, 
the loss of the metal atoms shortens the useful life of the lamp by 
reducing the visible light output. Moreover, the loss of the metal atoms 
leads to the release of free halogen into the arc tube, which may cause 
arc instability and eventual arc extinction, especially in electrodeless 
high-intensity metal halide discharge lamps. 
The loss of the metallic portion of the metal halide fill may be 
attributable to the electric field of the arc discharge which moves metal 
ions to the arc tube wall. For example, as explained in Electric Discharge 
Lamps by John F. Waymouth, M.I.T. Press, 1971, pp. 266-277, in a 
high-intensity discharge lamp containing a sodium iodide fill, sodium 
iodide is dissociated by the arc discharge into positive sodium ions and 
negative iodine ions. The positive sodium ions are driven towards the arc 
tube wall by the electric field of the arc discharge. Sodium ions which do 
not recombine with iodine ions before reaching the wall may react 
chemically at the wall, or they may pass through the wall and then react 
outside the arc tube. (Normally, there is an outer light-transmissive 
envelope disposed about the arc tube.) These sodium ions may react to form 
sodium silicate or sodium oxide by reacting with a silica arc tube or with 
oxygen impurities. As more and more sodium atoms are lost, light output 
decreases, and there is also a buildup of free iodine within the arc tube 
that may lead to arc instability and eventual arc extinction. Furthermore, 
the arc tube surface may degrade as a result of the ion bombardment. 
As described in commonly assigned, copending U.S. patent application of 
Witting et al., entitled "Protective Coating for High-Intensity Metal 
Halide Discharge Lamps", Ser. No. 553,304, filed July 16, 1990, now 
allowed a suitable protective coating comprises, for example, a silicon 
layer which is sufficiently thick to prevent a substantial loss of the 
metallic component of the metal halide fill, but which is also 
sufficiently thin so as to allow only minimal blockage of visible light 
output from the arc tube. According to the cited patent application, which 
is incorporated by reference herein, one method of applying the protective 
coating involves a chemical vapor deposition process wherein the coating 
is initially applied to both the inner and outer surfaces of the arc tube, 
the outer coating being subsequently removed by immersing the arc tube in 
a suitable etchant. 
Although the method of the hereinabove cited Witting et al. patent 
application, Ser. No. 553,304, is effective in applying a protective 
coating to arc tubes of high-intensity metal halide discharge lamps, it 
may be desirable to provide a simpler method of applying such a coating, 
thereby simplifying the lamp manufacturing process. 
Accordingly, an object of the present invention is to provide a new and 
improved method for applying a protective coating to the inner surface of 
an arc tube of a high-intensity metal halide discharge lamp. 
SUMMARY OF THE INVENTION 
The foregoing and other objects of the present invention are achieved in a 
method for applying a protective coating to the inner surface of the arc 
tube of a high-intensity metal halide discharge lamp which involves dosing 
the arc tube with an inert gas that is doped with a metal hydride gas. 
Preferably, the metal hydride gas comprises silicon hydride, or silane. 
The silane gas is decomposed into silicon and hydrogen by exposing the arc 
tube to a temperature of approximately 550.degree. C., either by heating 
in an oven or by driving a discharge in the arc tube. As a result, silicon 
is deposited as a protective coating on the inner surface of the arc tube 
wall. The hydrogen gas generated by the silane decomposition is removed 
from the arc tube either by pumping it out before dosing the arc tube with 
its final fill, or by gradual diffusion through the arc tube wall during 
lamp operation.

DETAILED DESCRIPTION OF THE INVENTION 
The sole drawing FIGURE illustrates a high-intensity, metal halide 
discharge lamp 10 employing a protective coating 12 in accordance with the 
present invention. For purposes of illustration, lamp 10 is shown as an 
electrodeless high-intensity metal halide discharge lamp. However, it is 
to be understood that the principles of the present invention apply 
equally well to high-intensity metal halide discharge lamps having 
electrodes. As shown, electrodeless metal halide discharge lamp 10 
includes an arc tube 14 formed of a high temperature glass, such as fused 
silica, or an optically transparent ceramic, such as polycrystalline 
alumina. By way of example, arc tube 14 is shown as having a substantially 
ellipsoid shape. However, arc tubes of other shapes may be desirable, 
depending 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 14 contains a metal halide fill in which a solenoidal arc 
discharge is excited during lamp operation. A suitable fill, 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, comprises a sodium halide, a 
cerium halide and xenon combined in weight proportions to generate visible 
radiation exhibiting high efficacy and good color rendering capability at 
white color temperatures. For example, such a fill according to the 
Johnson et al. patent may comprise sodium iodide and cerium chloride, in 
equal weight proportions, in combination with xenon at a partial pressure 
of about 500 torr. The Johnson et al. patent is incorporated by reference 
herein. Another suitable fill is described in commonly assigned U.S. Pat. 
No. 4,972,120, issued Nov. 20, 1990 to H. L. Witting, which patent is 
incorporated by reference herein. The fill of Witting U.S. Pat No. 
4,972,120 comprises a combination of a lanthanum halide, a sodium halide, 
a cerium halide and xenon or krypton as a buffer gas. For example, a fill 
according to the Witting patent may comprise a combination of lanthanum 
iodide, sodium iodide, cerium iodide, and 250 torr partial pressure of 
xenon. 
Electrical power is applied to the HID lamp 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, 
copending U.S patent application of G. A. Farrall, Ser. No. 493,266, filed 
Mar. 14, 1990, now allowed which patent application is 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 application is 
generally that of a surface formed by rotating a bilaterally symmetrical 
trapezoid about a coil center line situated in the same plane as the 
trapezoid, but which line does not intersect the trapezoid. However, other 
suitable coil configurations may be used, such as that described in 
commonly assigned U.S. Pat. No. 4,812,702 of J. M. Anderson, issued Mar. 
14, 1989, which patent is 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 example. 
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 
HID lamp is described in Johnson et al. U.S. Pat. No. 4,810,938, cited 
hereinabove. 
The protective coating 12 on the inner surface of arc tube 14 is of 
sufficient thickness to prevent a substantial loss of the metallic portion 
of the metal halide fill and hence a corresponding substantial buildup of 
free halogen. In addition, the protective coating is sufficiently thin to 
allow only minimal blockage of visible light output from the arc tube. 
Advantageously, since the metallic portion of the fill generates the 
visible radiation during lamp operation, the useful life of the lamp is 
extended by preventing a substantial loss thereof. Furthermore, since a 
buildup of free halogen typically causes arc instability and eventual arc 
extinction, preventing such a buildup likewise extends the useful life of 
the lamp. 
In a preferred embodiment of lamp 10, as described in Witting et al. U.S. 
patent application, Ser. No. 553,304, cited hereinabove, arc tube 14 is 
comprised of fused silica, and protective coating 12 comprises a layer of 
silicon. A preferred thickness of silicon coating 12 is between 3 and 40 
nanometers, with a more preferred range being from 10 to 20 nanometers. 
Silicon is a preferred protective coating because it has a relatively low 
thermal expansion coefficient and a high melting point. In addition, 
silicon may be advantageously employed as a coating on fused silica arc 
tubes because it is chemically compatible with silica and because it 
reacts with oxygen impurities to form silica. Moreover, for metal halide 
lamps having sodium as one of the fill ingredients, silicon is a preferred 
coating because it is a poor solvent for sodium and does not form 
compounds therewith. 
In accordance with a preferred embodiment of the present invention, silicon 
coating 12 is applied to the inner surface of arc tube 14 by filling the 
arc tube with an inert gas that is doped with silicon hydride, or silane, 
and then heating the arc tube for a suitable time period, e.g. 1 to 90 
minutes, in the range from approximately 500.degree. C. to 900.degree. C. 
Heating of the arc tube can be accomplished either by heating in an oven 
or by driving a discharge in the arc tube, or a combination thereof. In 
particular, heating of the arc tube in an oven causes the silane gas to 
decompose thermally. On the other hand, driving a discharge in the arc 
tube results in both thermal and plasma decomposition of the silane gas. 
In either case, however, silane decomposition causes nucleation and 
deposition of silicon coating 12 on the inner surface of arc tube 14. The 
total silicon content inside the arc tube and the resulting average 
silicon coating thickness are determined by the partial pressure of the 
silane gas. Those skilled in the art of chemical vapor deposition will 
recognize that the coating time can be reduced by increasing the coating 
temperature and that the coating temperature can be reduced if the coating 
time is increased. Moreover, if heating is accomplished by driving a 
discharge, the plasma decomposition further reduces the coating time. 
After coating 12 has been applied to the inner surface of arc tube 14, the 
arc tube is evacuated in order to remove the hydrogen that was generated 
by the dissociation of the silane. The arc tube is then filled with a 
typical dose of at least one metal halide and at least one inert gas, and 
finally sealed. 
EXAMPLE 
A fused silica arc tube of spherical shape having an inside volume of 3 
cubic centimeters is filled with a 5% silane-doped inert gas to a total 
pressure of 250 torr and is heated for 5 minutes in an oven at 
approximately 550.degree. C. As a result, approximately 0.05 milligram of 
silicon is deposited as a coating having a thickness of approximately 20 
nanometers on the inner surface of the arc tube. The arc tube is then 
evacuated in order to remove the hydrogen that was generated by the 
dissociation of the silane. The arc tube is then filled with a solid dose 
of 4.75 milligrams of sodium iodide and 2.25 milligrams of cerium iodide, 
and also with a gaseous dose of krypton, and finally sealed. 
An alternative method of the present invention involves adding the silane 
gas directly to the arc tube fill which typically includes at least one 
metal halide and at least one inert gas. The arc tube is sealed and then 
heated either in an oven or by driving a discharge in the arc tube, or by 
a combination thereof. As a result, silicon coating 12 is deposited on the 
inner surface of the arc tube wall. The hydrogen gas that is generated by 
the decomposition of silane is removed by diffusion through the hot arc 
tube wall. Hydrogen diffuses through hot silica at a relatively fast rate 
due to its small atomic diameter. 
EXAMPLE 
A fused silica arc tube of spherical shape having an inside volume of 3 
cubic centimeters is filled with a solid dose of 4.75 milligrams of sodium 
iodide and 2.25 milligrams of cerium iodide, and with a gas dose of 95% 
krypton and 5% silane at a total pressure of 250 torr. The arc tube is 
sealed and then heated for 30 minutes at approximately 550.degree. C. As a 
result, silicon in the quantity of approximately 0.05 milligram is 
deposited on the inner surface of the arc tube as a coating having a 
thickness of approximately 20 nanometers. A discharge is then driven in 
the arc tube for an additional 60 minutes. The discharge heats the arc 
tube to a temperature of approximately 800.degree. C. and allows free 
hydrogen to diffuse through the arc tube wall. 
Although the method of the present invention has been described in detail 
with reference to a silicon coating, it is to be understood that the 
method of the present invention may be used to apply to high-intensity 
metal halide discharge lamps other suitable protective coatings 
comprising, for example, other metals or metal silicates. 
While the preferred embodiments of the present invention have been shown 
and described herein, it will be obvious that such embodiments are 
provided by way of example only. Numerous variations, changes and 
substitutions will occur to those of skill 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.