CA1050605A - Electric discharge lamp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Electric discharge lamps of extended life can be produced or cheaper materials used in their manufacture by providing on the internal surface of the envelope and on the exposed surfaces of internal components a protective coating of a metal phosphate or arsenate. External surfaces can also be protected against corrosion in air or in the atmosphere within an outer jacket, where this is employed. The coating may be applied in a liquid medium which, preferably after draining, is warmed to evaporate the medium and thereafter baked to form the vitreous coating.

Description

: 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 . , , : -2-' J
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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~ ;
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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 .

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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.
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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

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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~ ~ , 's ~
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~'~5~5 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~
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~ phosphorus or arsenic from 1:0~1 to 1:209~ Preferred metals , ~:

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} ~ 5 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' } ~7-. - i "

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:~S~ 5 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.

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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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g_ '' ' ,.',`~ ' ~ , ~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 ., -1O-s, ~
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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~
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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 :[~
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2 W Cl6 ~ SiO2 _ ~D 2 W0 C14 ~ iCl4 III
where ~ and y correspond -to several chlorides of tlmgsten.
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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.
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, : ~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~

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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.

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.' As an example, oxygen-containing impurities in lamps containing aluminium trichloride can in some circumstances produce chlorine by the reaction:
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~ 2AlC13 ~ 30 ~ A1203 + 3C12 IV
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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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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 .. . ~
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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~
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. associated with it 7 or i-t mi~ht be used ~ erl required by ;~
~: a p~rticular manufacturinrJ technique.
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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.

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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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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.

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Claims (19)

WHAT WE CLAIM IS:

1. A method of making an electric discharge lamp in which at least those portions of the internal surface of the discharge envelope and the exposed surfaces of internal components which tend to react with the gas fill in the envelope during operation of the lamp are coated with a solution capable of deposit-ing a metal phosphate or arsenate on heating and are heated to form thereon a continuous substantially defect-free glassy coating of the metal phosphate or arsenate.

2. A method according to Claim 1 in which the said metal is at least one of aluminium, iron, chromium, titanium, vanadium and tin.

3. A method according to Claim 1 wherein the coating comprises aluminium phosphate.

4. A method according to Claims 1, 2 or 3 in which the atomic ratio of metal to phosphorus or arsenic in the glass composition is from 1:0.1 to 1:2.9.

5. A method according to Claim 1 in which the said surfaces are coated either separately or after assembly with the said solution and are subsequently heated to evaporate the solvent and to form a glassy coating.

6. A method according to Claim 5 in which, after the solution has been applied to the said surfaces it is allowed to drain and thereafter the surfaces are initially heated to remove the solvent and subsequently baked at a controlled higher temperature to complete the formation of the coating.

7. A method according to Claims 1, 2 or 3 in which the external surface of the discharge envelope and the surfaces of external components thereof are coated before assembly within an outer envelope.

8. A method according to Claims 1, 2 or 3 in which the solution contains a soluble complex phosphate containing coordinated solvent groups.

9. A method according to Claims 1, 2 or 3 in which the solution contains a compound of the metal and an oxyacid of phosphorus or arsenic or a compound capable of forming such an oxyacid in the solution.

10. A method according to Claims 1, 2 or 3 in which the solvent of the solution comprises water, an alcohol, ester, ketone, aldehyde, nitro-compound or ether.

11. An electrical discharge lamp comprising: an arc tube, components including a pair of spaced electrodes in said arc tube and respective conductive leads or support for said electrodes;
a gaseous fill in said tube;
and a transparent, defect-free, solution-deposited protective coating of a vitreous compound selected from metal phosphates and arsenates on at least the internal surface of the arc tube and the exposed surfaces of said leads or supports which tend to react with said fill during operation of the lamp.

12. A lamp according to claim 11 further comprising metallic end caps of said tube, the internal surface of said end caps being also provided with said coating.

13. A lamp according to claim 11 further comprising a coating of a metal phosphate or arsenate glass on the external surfaces of said arc tube and components of said lamp.

14. A lamp according to claim 13 further comprising a light-transmitting outer jacket enclosing said arc tube and components.

15. A lamp according to claim 11 wherein said coating comprises a phosphate or arsenate glass of at least one of the metals aluminium, iron, chromium, titanium, vanadium and tin.

16. A lamp according to claim 11 wherein said coating is composed of an aluminium phosphate glass.

17. A lamp according to claim 15 wherein the atomic ratio of metal to phosphorus or arsenic in the glass composition is from 1:0.1 to 1:2.9.

18. A lamp according to claim 11 wherein said coating is the deposited and baked residue of a solvent-soluble complex phosphate ox arsenate.

19. A lamp according to claim 11 wherein said coating comprises the deposited and baked residue of a compound of said metal and an acid moiety selected from oxyacids of phosphorous or arsenic.