Note: Descriptions are shown in the official language in which they were submitted.
: rThc~ present invention relates to el~ctric dischar~e
; devices and more especially to discharge lamps -the envelopes
of ~;hi.ch contain a fill vf' a reactive gas or ~apour~ '~he
invention is concerned ~ith the provision of protective
coatings on i.nternal surf'aces of such lampsO
Various types of discharge lamp have gas fills
contairing ~ases or vapours which are reaGtive or potentially
reac-tive wlth materials commonly used for lamp envelop~s ~nd
the internal components of the 18~ps~ ATnong these t~es may
be particularly mentioned metal halide discharge lar~lps~ and
metal vapour discharge lamps containin~r the vapours of re-
' , active metals, such as sodium,
,. ,
In metal halida discharge lamps the radiation isproduced by all electric arc be-tween two primary electrodes
extending into an en~elope~ or arc tube, containin~ mercury
; and one or more metallic halides which axe at least partially
vaporised and dissociated by the heat of the arc during
, 1 operation of the lamp. '~he envelope is commonly constructed
of vitreous fused silica, although sometimes another trans--
. 20 parent or translucent glass, ceramic ox crystalline material
,~ is used~
One problem which limits the perform~nce of such
lamps is reaction between the material of the en~elope, or of
the electrodes and supports, and the metallic halide conterts.
Such reactions c~n cause any of the following: darkening ox
.. ..
i obscuration o~ the envelope wall, with consequent loss of
,...
' light; erosion of th~ electrodes ~nd supports; loss of the
. , ,
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:
constituent vapours i~ the discharge ca~sin~ changes of colo~
or electrical characteristics; or mechanical failure of the
envelope and destruction of the lamp~
Other discharge lamps have arc tubes formed of a trans-
lucent ceramic material or translucent crystalllne material and
frequently contain highly react:ive metallic elements, such a~
sodium or other alkali metal. These elements cannot be used in
arc tubes constructed of vitreous silica or most glasses,
because of the tendency of the metal vapour to react with these
..... .
materials.
In accordance with this invention, attack by reactive
fill substance on portions of the structure of a discharge lamp
or device is reduced or prevented by providing, on at least those
- portions of the internal surface of the discharge envelope or
the exposed surfaces of internal components which tend to react
~ with the fill, a coating of a metal phosphate or arsenate. The
;; ~ surfaces to be covered will usually include the internal surfaces
of the envelope and the electrode supports~ ;
:
In accordance with a furth~r aspect of this invention,
, 20 in discharge lamps where the arc tube is enclosed in an outer
; envelope which itself contains a ~as or gases, external surfaces
of the arc tube or components thereof may also be protected by a
coating of metal phosphate or arsenate and thereby preserved
from attack by gases or vapours in the outer envelope.
Thusg in accordance with the ~resent teachings,
an improvement is provided in the method of manufacturing
an electric discharge lamp which comprises an arc tube,
components including spaced electrodes and leads or supports
s~ therefor as well as a gaseous fill in the tube. The improvement
in ~he manufacture comprises the steps of coating the internal
; surfaces of ~he arc tube or the exposed surfaces of the
components, which tend to react with the fill components
during lamp operation, with a liquid composition capable of
.
-3-
, ~ '
~al5~6~
generating a glassy metal phosphate or arsenate on heatiny and
including a ]iquid medium. Heating the coated surfaces to remove
the liquid medium and to form on the surface a defect~free
protective coating of the metal phosphate or arsenate.
The protective coating provided in accordance with
~` this invention may be applied to conventional materials used
for the fabrication of lamp components, for example to protect
;, ~ them from highly reactive fill substances, or they may enable
cheaper and more readily available materials to be substituted
for conventionally used materials without unacceptable loss
in performance or life.
The coating is preferably derived ~rom an aluminum
phosphate complex as described in German Offenlegungschrift
, ~
(DOS) No. 2,028,839 of Imperial Chemical Industries, open ~or
inspection March 4th, 1971, one or more of the metal phosphate
, - or arsenate compositions prepared in accordance with DOS No.
2,235,651 (I.C.I. open to public June 7, 1973) or from a
composition comprising an aluminum phosphate and containing a
- titanium compound. Combinations of these compositions can also
~ 20 be used.
. .
` ~ For the purposes of this invention, preferred metal
.
phosphates and arsenat~s are those o~ atomic number 12 to 14,
20 to 32, 39 ~o 50, 56 to 80, 90 or 92. rrhe term 'phosphate'
is here mean~ to include ortho-, meta- and pyro- phosphates
together with phosphinates and phosphonates.
Especially preferred sources of metal phosphate
coatings arP solvent-soluble complex phosphates containing
coordinated solvent groups, such as water or polar organic
solvents~ as described in DOS Nos. 2,028,839 and 2,235,651. Not
only are the isolated complex phosphates themselves ~uitable,
but the compositions which are therein described containing
phosphate precursors may also be usedO
" ` ~
., ~ ' .
...
,~ .
1~5~5
'~ Liqui.d coating comp~sitic3ns m-ly b~ used whichcomprise a soluti.on, of (a) a met~l compound and (b) .~n o~y~
~ acid of pho~phorus or arsenic, or a compound capable of
:~ form:Lngr such an oxyacid in the 50 l.utlonO At least part of
the solvent may be organic. These compo6itions are capable of
decomposing to a metal phosph~lte or arsenate on bein~ heated.
The solvent is selected from water or the wide range
of organic solvents which d.i.ssolve the components of th.e
compositionO ~he organic solvent, when used 9 is prefe~ably
selected from alcohols, esters, ketones, aldehydes, nitro-
compounds and ethers, especially monohydric alcohols of the
structure XOH, esters of the structure R1COOR2 9 ethers of the
structure R10R2, ketones of the structure R1COR2, nitro-
compounds of the structure R1N02 and ethers of the structure
oR3, where R, R~ and R2 are alkyl groups or substituted al~yl
. grollps containing from 1 to 10 carbon a-toms each, and R3 is
a divalent alkyl group having from 4 to 7 carbon atoms one of
which may be replaced by an oxygen atom~ Mixtures of Gne or
more solvents may be used. Diluents may also be present,
provided they do not bring about precipitation of the
:,~
:-- ~ components of the composition~
.. Aliphatic alcohols containing 1 to 10 carbon atoms
are particularly convenient, especially lower molecular weight
alcohols conta~ ing 1 to 4 carbon atoms, for examp].e
methanol, ethanol~ n- or iso-propanol and substitu-ted
alcohols especially metho~y- or ethoxy-ethanol. Suitable
esters a:re ethyl acetate or carbonate~ ~cetyl acetone may
.~ --5
s~ ~ ,
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., : . ~ .
,. :
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be used. q`etrahydrofuran i i.~ the moC;t preferred ether to
use, though dioxan may also be used. Aromatlc hydro~.y
compounds can be used, but; solubility is lo~.~ in such
materials.
~he composition may be formed by dissolving an
~ ,:
isolated compiex of the t~pe described in the specifications
referred to ahove in a solvent. 'l7he metal compound may
itself be a phosphate and so provide the oxyacid of phosphorus
;~ or ar~enic, in which case an additional acid May be required1G to form a homogeneous solution, e.g~ hydrochloric or nitric
acid.
A wide range of metal compc~,unds may be used Simple
inorganic compounds including oxides and hydroxides are
suitable, as are sa]ts such as halides, carbona-tes, ni-trates,
phosphates, perchlorates and cyanates. Sulphates may be used
;~ - in some cases but they can be disadvantageous owing to the
difficult~ with which they are thermally decomposed.
Also s-uitable are salts of organic acids such as
acetates, benzoates~ oxalates, propionates or formates.
Alkoxides are also useful.
~- Alternatively co-ordination complexes of the met-almay be used, for example complexes having ligands derived
from acetylacetone, ethylenedithiol, ethanolamine, carbon
monoxide or phosphines.
Preferred compositions are those in which the metal
,-~ and oxyacid are present with atomic ratios of metal t~
.: ~
~ phosphorus or arsenic from 1:0~1 to 1:209~ Preferred metals
, ~:
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a:re alumini~ , iroII~ chxomium, titanium~ va~adium and tin~
A solv~nt-solub].e alu~i.nium phosphate Ma~ be used,
fox example the acid ort~lophosphatec; Al2(~0~!3 an.d
Al(~I2P0~ , and mixtu:res cont~ining them.
Normal aluminium o:rthophosphate is insoluble in
water but soluble in dilute mi.neral acids (for exa~nple hyclro~
chloric and nitric acids) and in some carboxylic acids (for
example citric acid) and such solutions may be used for the
purpose of this invention. Moreover, solid complex al~nini.um
~,~ 10 phosphates containing the anion of the acid and chemically
bound water or alcohol (or a mixture thereof) may also be
' used.
Where the complex contains an alcohol group, it is
preferred that it be an aliphatic alcohol containing from one
to four carbon atoms, for example methyl alcohol, ethyl
alcohol, n~-propyl alcohol or isopropyl alcohol, although
:i . complexes with higher alcohols are known and may be used if
,- desired.
.,,
,: The complex phosphates most commonly con-tain from
three to five molecules of the hydroxy compound per phosphorus
atomg for example water~containing complexes may have an
. empirical formula corresponding to AlPO~HCl.(E20)x where x i~
in the range 3 to 5,
~he complex aluminium phosphates containing alcohol
and their solutions may be prepared by reacting aluminium
j compound, preferably halide, with an alcohol and phosphoric
.. acid. One such compound has the empirical foxmula
Al P Cl ~I25G88'
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The complex phosphate containiny water can be made as
above or by hydrolysing the alcohol-containing complex phosphates
or, for example, by contacting ~luminium phosphate hydrate with
gaseous hydrogen chloride.
Iron, chromium, vanadi~m, ~itanium and tin phosphate-
,; containing coatings may be prepared by dissolving a salt,
preferably a halide, in an alcohol and adding phosphoric acid or
a source thereof.
The layer should be free from pin-holes or othex
defect or imper~ection which might cause it to break down
during operation of the lamp. In one preferred method of making
lamps according to this invention, the desired portions of the
:,
internal surface of the envelope and the surfaces of internal
components which are exposed in the finished lamp are coated
either separately or after assembly with a liquid composition
capable of generating the desired metal phosphate or arsenate,
and subsequently heated to evaporate the solvent and cure the
composition to form a defect-free metal phosphate or arsenate
coating. It has been found valuable in the production of
defect-free coatings to al~ow the applied liquid coaking
composition to drain thoroughly and thereafter to bake initially
at a relatively low temperature to remove ~he solvent and sub-
sequently at a controlled higher temperature to comple~e the
formation of the protective coating. The preferred baking
temperatures vary with the particular composition of coating
material employed, but can be determined by expeximent.
,............. .
.. . .
~ 8
~ . .
:;
s~
Various aspects of this inven~ion will be described
with reference to the accompanying drawings in which:
Figures 1, 2 and 3 are respective diagrammatic views
of three forms o discharge device to which the invention may be
~ applied; and
.. Figure 4 is a diagrammatic view of a fourth form of
l discharge device to which the invention may al50 be applied.
In accordance with one aspect of ~his invention, surfaces
of electric discharge lamps and devices or components thereof
tending to react with the lamp contents are provided with
i .
coatings of the phospha~e or arsenate described above.
In Figures 1 to 3 are shown three examples of
. discharge devices or lamps. In each case the arc tube or envelope
.. : 10 is constructed of vitreous fused silica, into which are
sealed electrodes 11 on tungsten shanks 12, electrically
connected to external leads or connectors 13 through molybdenum
.~. foil pinch seals 14~ An exhaust tube 15 is pxovided in the
:
- wall of the tube 10 and is sealed off in the finished lamp, as
.. . .
shown in the drawing~
~ 20 The form of lamp shown in Figure 1 commonly contains
; a mixture of the iodides of scandium, sodium, thorium and
mercury, in addition to metallic mercury and a quantity of argon
gas. An additional ~uxiliary electrode 16 may be sealed through
one end of the tube 10 for starting purposes. An arc tube of
~ this kind is normally sealed in an outer glass iacket (not shown)
`'~ which is either evacuated or filled
t
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¢~ 30
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g_
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~s~ s
with an inacti~e gas, erld ~hich ~a~ be corlted with a
phosphor. ~allps of this type are commoniy desi~ncl~ed MBI
or MBIF lamps. Other lamps of th;s .ons~ructjc)n contain
-the ha]ides of aluminium or tin, ~-~(l are commonl~J re~crred
t~ 5 to as molecular arc lamps.
~he form of lamp shown in Figure 2 rnay contain the
halides of sodium, gallium~ -thallium and mercury~ together
with metallic mercury and a rare gas such as xenon~ '~hese
lamps are very compact and operate at a higher pressure
than those of ~igure 1. ~1hey are not normally operated ln
an outer envelope (although they ~ay sometimes be) and are
com~only designated Compact Source lamps, or C~SI lamps.
The form of la~p shown in Figure 3 has a lon~ex and
narrower arc tube than those of Figure 1, and is desiglled to
operate without ar. hermetically sealed outer jacket, but
usually in a special closed Xitting. ~hese lamps may contain
similar halide mixtures to those used in the type of la~p
shown in ~igure 1, but other versions intended for photo-
chemical and other spec-ial purposes contain other halides~
such as the iodides of gallium, indiwn or bismuth. I,amps of
~- this type are commonly designated MB~ lamps.
In ~igure 4 is shown a further type of arc tube 10
which is made of a translucent cer~mic material, such as
alumina, or a transparent crystalline material, such as
crystalline alumina or sapphire, and in which electrocles 11
are sealed, either through metallic caps 17 or ceramic
~ plugs sea]ed to the ends~ ~he metalliG Gaps are often of
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~L~S~i;f~35
nio~ ol1 o.r a !~i ob~ rn ~lloy, \.~hi.c~l is par-ticulaxly re-
active wi.th n~etn.Ll:ic hal~le corl.poun(ls~ l,ar!lps of t~liS typ~
frequentlv contai~ ni.ghly reactive metallic elelnents, s;uc,h
as sodium or tlle othe~ cl:lkcl1i meta:Ls, l~hich cannot ~e llsed
in arc tubes construc-led of. vi-treous s,j.llca or most gl,lssesb
~he arc tubes of these l~1lps are commorily fi~ted ~lithin
outer jackets.
~ y possi~le chemical reactions rnay :I.imit the
.:?
per~ormance of such lampsO ~lost of these are not,- fully
understood, but cer-tain likely reactions are believed to b~
. ,.
responsible for the observe(l effectsA
- As an e~ample, when a lamp o~ t,he t~>e show.rl in
: ~ ~igure 1 containing al~n:inium t.richloIide was opera-ted the
.. ~ tungsten electrode supports ~Jere rapidly eroded, the
i 15 surrounding areas of the silica envelope were darkened and
a more general attack and dev.itrification of the envelope
was apparentO Although these effects have no-t yet been
positively identified as caused by specific chemical re-
` actions~ the fol.lowing types of reaction are known to be
... ~ 20 possible:
.. ~ . hlC13 ~ ~ AlCl ~ 2 Cl
xW ~ 7Cl - -~ WXCly J
.,:~, ,
.~ 4AlC13 ~ SiO2 ~---~ 3Si.Cl~ 2A1203 :[~
::. .
2 W Cl6 ~ SiO2 _ ~D 2 W0 C14 ~ iCl4 III
where ~ and y correspond -to several chlorides of tlmgsten.
. ~,. . .
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;,,'. . .` :
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(rl"tle i :i .r~,t i).rl:rt; of react:i~n I ~jroba'bly occllrs in the
, hi~-,hel te!.,p~ratu~e par-t.s of the di~charge~ while t;he se~corld
pclrt oecurs ~t the coole:c metal sl~.rf'ace as a resu:Lt of
i.ncornplete reco~binatio:rl o:f the ch:l.orine wit,h the mono-
chlori~l~j. 'l'hese could readil~ accou.x~t ~or the main
featu~es of the observed ~e~haviour.
Such interactions with the lamp coml~onents are not
limi.ted to normal chemical :reactic,ns ~Jith t~.e halides i.n the;.r
,.,`- - solid~ liquid or gaseous for~c,. The presence of the discharge
and the associated electric fields permits a much wider rs~lge
... of interaction, ero~ion or att;ack, involving the products o.i'
:,, d.issociation of the compounds, and c~xcited or ioni~ed speci,es
,=.' derived from them, ~lectrolytic processes ~ay prod.uce fur.ther
.~ ~ reactions, and m~y increase the rate a-t which some of the
interactions occur~
~'~ - As an example, the presence of an electric field with ::
'``" a definlte mean polarity across the tube ~all, associa-ted with
.,~`'~` the current leads to the dîschar~e, is known to greatly
~s ~
enhance the migratio~ of sodium~ de.rived f.rom the dissociaticn
~:: 20 of sodium iodi,de :in such la~ps, through tha silica walls.
-:
.; .
, : ~his gives rise to a loss of sodium from the axc 7 with conse-
quent deleterious chan~es in colour and e]..~-ctrical character-
,:~ -
,;? istics. Near the electrodes, electrolytic actlon of thls k~.nd
;,' : is a frequent cause OI' mechanical failure of the lalllp~
' 25 . Also, impurity gases or vapours commonly present in
'~ lc~p envelopes such as water vapour or o~"T~en may gi.ve rise
'.~, to still further reactio~s~ and enharlce the rate of others~
12-
.~, .j,,
,~; . ~ . . .
, .: .: -: . .. . . -
~. .. - :: . :
,.... ~ . :. . .
.. .. ..
.. . .
3~5~5
Traces of such impurities are commonly left in the gaseous
fil.ling during processing, or are in~roduced with the halides,
or may be released from the var:ious components by the action
of the discharge.
:.:
.' As an example, oxygen-containing impurities in lamps
containing aluminium trichloride can in some circumstances
produce chlorine by the reaction:
,: .
~ 2AlC13 ~ 30 ~ A1203 + 3C12 IV
:;;
leading to a greatly increased rate of erosion of the electrodes
. 10 by reactions of the type I.
The present invention can be utilized to prevent,
control or reduce the incid~nce of, deleterious interactions
- of these kinds between the metallic or halide contents and
the components of the lamp in contac~ with them, by coa~ing the
surfaces in contact with the metals or halides with a
:; .
;5 protective layer of metal phosphate or arsenate composition which
~ is resistant to interactions of these kindsO
:~. The surfaces to be protected usually include the
~ internal surface of ~he envelope and the exposed surfaces of the
~ 20 electrode leads or supports, and me~al end caps when present~
;
~ together with, when possible, ~he regions where different
.~ components join. ~he active surface of the electrodes will not
*j normally be covered.
: ~ In some cases there are advantages also in coating the
external surfaces of the arc tube~ leads or adjoining components,
. as a protection against the surrounding atmosphere. For example,
the metalLic caps 17 of lamps of the
,s,
~ 30
'~`.
. .
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.
, . . .
~ s~
type S~lOWIl iJl ~ Ie /~ are 5u~;ceptib.1e to reaction with
i.mpurity c~;ca.se.j or va~ourC; such as ~-ater vapour OI` o~.ygen i21
the surroundi11g outeT jclcket. ~irnilarly the lead-; ad~acent
to the axc tube in l~mps of t;he t~)e shown ln ~ gure ~ are
.~iabl.e to reac-ti.on ~ th the surroundin(~ ai.r in the ~itting~.
Bot}1 t11~se may be protecl.ed by extern.al coatin~C., of. l;h.ls
.: type~
he protective layer shoulcl ~e effectively free from
pinholes or other defects or imperfectlons which will cause it
to brea~ down durir1g operati.on of the lamp, although a
substantial degree of protection, and consequent improv~ment
in ].an1p csuality may be obtained i.n some cases when such
perfection is not fully achieved~
In a preferred method of providi.ng the coating, it is
appli.ed to the whole of the interi.or of` the arc tube af:ter
.... .
-~: the components have been substantially assembled~ ~his has
:~ - -the advantage that the regions where the compo~ents joi.n, s
.~ which are often particularly susceptible to attack, are fully
coated. The active areas of the electrodes may also be
.. . ~
:
20 coated in this process, but the coating on these wi.ll norIaally
be removed when the arc is first struck, or the lamps first
ope.rated.
~ he protective coating may al-terna-tively be applied
'.,J' . by coatin~ the components individually before a~sembly. ~`his
i~. 25 method mig~t be used, for example, where it is essential
. that the acti~re surface of the electrode is not brought into
cortact with the coating material or any products of reacti.on
~: 14~
,Y~
:~ .
. ~ .. ... . .
. - ,
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.
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.. . . .
l: ~
. associated with it 7 or i-t mi~ht be used ~ erl required by
;~
~: a p~rticular manufacturinrJ technique.
., .
he coating can be applied by f~ g, inj~cti.ng or
spra~.ing the :insi.de of t~ie arc tubc with the complex ox a
solution of the complex cmd subsequently removing or draini.ng
any sur~lus, or it can be appli.ed to the -individual components
by any Or the methods desc:ribed in DOS NoO 2,235~651. The
~' complex is then decomposed by heating to yield a phosphate
(or arsenate) layer by heating at a temperature below 1000C
as described ln the same specifi.cation. '~he construction .~d
.~; processing of the lamp i8 then completed in the normal way.
.~ The following is one example o~ the applica~ion of the
r,~' inVentiOI!o
~ xample
:. ~
A 400~ aluminium chloride discharge lamp with an arc
~ ~ tube of the form shown in ~igure 1 is protected by coa-ting
*, . the insida of the arc tube 10 in the fol.lowing mcmner. A
~- solution is prepared by adding slowly and with stirri.ng,
4.646 g anhydrous aluminium chloride to 91.45~ g meth~nol.
0 3.866 G orthophosphoric acid (~8%) is then addedO '~he
- resultant solu-tion is dispensed from a hypodermic syringe~
~.~ through the lamp exhaust tube 15, before this is sealed off~
.~ It is distributed around the inside of the arc tube, which is
then inverted c~nd left to drain, leaving only a thin layer
~: 25 adhering to the inside surfaces. ~he resultant coating is
baked. at 100C in a ~vacuum for one hour and finally formed
. ~ by bc~king at 400C for -three minutes.
15~
~ .
, , "
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, . . .
. . . . , ~ ..
. ~
! . ~, .
~S~
The lamp is then processed in the normal manner for
discharge lamps of this type -to give an arc tube of volume 2.2
cm which contains 7.4 mg AlC13 and 44 mg Hy together with a
pressure of 20 Torr of argon at room temperature. During
processing the arc is struck and the layer on the active surface
of the electrode i~s removed, ieavLng the protective layer over
the internal surfaces of the arc tube and the cooler parts of
the electrode structures.
The layer substantially increases the resistance of
the cooler parts of the ~lectrode leads from erosion by the
chloride vapour, preventing blackening of the envelope surface
by deposited tungsten and other reaction products. The silica
is alsQ protected from reaction with the aluminium chloride.
Instead of the particular aluminium phosphate
. . ~
composition described in the above preferred example, aluminium
phosphate or arsenate coatings may be used, prepared from
solutions of halogen-containing complex phosphates or arsenates
of aluminium, coating the internal lamp surfaces, and heating
to cure the coating.
Instead of one of the above compositions, coatings may
be used prepared from liquid compos~itions of other metal
compounds and oxyacids of phosphorus or arsenic as disclosed in
., .
~ DOS.No. 2,235,651, coating the internal lamp surfaces and
., .
:
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... .
:,
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s~
heating under the conditions substantially as disclosed in the
same Application, the remainder of the processing following
.
the same general lines as in the above preferred example.
s It should be noted that it is not an essential part
of the process of this inventio,n to coat the envelope and
internal components after assembly, as described in the above
-i preferred example, and individual components may be coated before
lamp assembly. The essential feature of the invention is the
provision of a continuous glassy layer consis~ing essentially
of a metal phosphate or arsenate covering the intexior surface
of the envelope or any internal components that could react
with the fill or contents of the lamp at the operating
temperatures.
Similarly, any external surfaces to be protected may
be coated either before or after assembly.
.X
.~
~. .
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:, .
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~ 30
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