CA1165375A - Metal vapor lamp having internal means promoting condensate film formation - Google Patents

Abstract

METAL VAPOR LAMP HAVING INTERNAL MEANS
PROMOTING CONDENSATE FILM FORMATION
ABSTRACT
In a high intensity metal halide discharge lamp, means are provided associated with the interior surface of the envelope to promote thy formation and spreading of a liquid film of condensate thereon. Such a film can lower the color temperature as a result of pressure broadening and self-reversal of the sodium line, and also by acting as a color correcting filter. The film-promoting means may be a coating which imparts an ir-regularity to the surface such that the reduction in exposed surface area by coverage with a uniform liquid film is energetically favored. Alternatively, a chemi-cally different surface better wetted by the metal halide dose may be used.

Description

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MæTAL VAPOR LAMP ~ G I~TE~N~L ME~NS
PROMOTIN~ CON~NSATE FILM FORMATION

The i.nvention relates to high intensity metal vapor discharge lamps which operate with an unvaporized excess of metal and more particularly to metal halide lamps con-taining an excess of metal halide in liquid orm.
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BACKGROU~D OF THE INVENTION
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Metal halide lamps began with the addition of the halides of various light-emitti~g metals-to thP high pressure mercury lamp in order to modi~y its color and raise its operating ef~icacy as propose~ by U.5. :~.Patent 3,234,421 - Reiling, issued in 1966. Since then metal halide la~p~ have become commercially useful for general :~ illumination; their ¢onstxuction and mode of operation are described i~ IES Lighting Handbook, 5th Edition, 1~72, published by the Illuminating Engineering Society, pages 8-34.
The metal halide lamp generally operates with a sub-stantially fully vaporized charge o mercury and an un-~aporized excess c~n~isting mostly of metal iodides inuid form. The favored filling comprises the iodides of sodium, scandium and thorium. The operating conditions : together with the ~eometrical design of the lamp enve-lope must pr~vide su~ficiently high temperatures, parti~
cularly in the ends, to vaporize a substantial quantity of the i~dides, especially o~ the NaI. In general, this requixes minimum temperatures under operating conditions i3'~
~ D 7993 of the oxder of 700C.
; The quantity of NaI which may be accommodated in the vapor state within a given volume at a given temperature, ~or instance at 750C, can be readily calculated. However, the charge of NaI that is put into most lamps o~ commercial manu~acture is many time~ greater, for instance 100 or more timesj than such calculated quantityA Although most o the added NaI remains as condPn~ate within the arc tube, the quantity participating in the arc discharge in-creases at a diminishing rate with the total quantity putinto the tube. In Electric Discharge Lamps, MIT Press 1971, Chapter 8, Section 8.4, Effects of Arc Tube &eometry, John F. Waymouth ~pecula~es on this phenomenon and pro-poses, as explanation based on the non-isothermal natuxe of the bulb~ ~ha~ a film of condensate spreading beyond the point of minimum temperature towards higher tempera~
tur~s would result in increasing the NaI pressure. ~e also offers an alterna~ive explanation wherein the NaI
pressure in the arc is viewed as being determinea by a dy~amic ra~her than an equilihra~ory process; convection currents bring gases that are much hotter than the wall past the surface of the condensate film, evaporating excess NaI to be carxied through the arc before condensing elsewhere.
Irrespective of the explanation adopted, Waymouth concludes that-it is desirable to have a condensate film as extensive as possible to get the maximum pressure o~
~aI in- the gas for a given quantity of NaI added. In par~icular, he desires that the condensate ~e distri-buted over the barrel of the arc tube, spread as thinly as possible over as larg~ an area as possible, and not condensed in the end~ where there might be crevices or pockets that could store relati~ely large ~uantities with low surface areaA To achieve this result, Waymouth wants the arc tube designed in such a way that the end temperatures are higher than those in the middle, so that excess iodide will conaense in the middle of the arc tube.
We have observed that the condensate does not form 5 a true film in the sense of a continuous layer on the inside of the quartz arc tube, but tends to remain as discrete droplets. We have found tha-t the extent to which the area of the condensa~e can be incre~sed by ob-serving the ~aymouth recommendations is ~uite limited.
10 Also we ha~e encountered other problems when much con-densate coats the envelope walls about the middle of the arc tube, for instance as an equatorial band spaced away from both electrodes. The relatively large droplet~ o condensate in the band reduce transmission and m~y cause 15 flickeri~y as they form and move about. Another problem is the occurrence of flashes of reddish light, particu-larly during warm up. These flashes appear to be due to rapid vapsrization of drops of metal halide dose which form in the equatorial band a~d run down i~to the hotter 20 end zone~ Such problems -are particularly acute in mini-ature metal halide arc tubes of one cubic centi~eter or less such as disclosed in patent 4,161,672 - Cap et al, July 1979.

SUMMARY OF THE INVE~TIOW
The object of our in~ention is to achieve within a metal halide dis~harye lamp a ~ilm of condensate which is more extensive and con~inuous than possible up to now, and which pre~erably extends over substantially the entire 30 inside surface o~ the discharge envelope. Such a film is useul to increase efficacy and improve color rendition by getting the maximum effective quantity of metal halide and particularly NaI in vapor form into the discharge.
Another benefit from such a film is a filter effect whi~h 3Scan be used to lower the color temperature of the emitted light.

In ac~ordance with our invention, we provide means associated with the interior surface of the discharge envelope which promote the foLmation and spreading of a liquid film of condensa~e thereon. The formation of a continuous ilm is favored when its presence reduces the total surface energy. A film will tend to form when the surface energy in the wall to liquid dose interface, plus the surface energy in the liquid dose to Yapor interface, is less than-the surface energy in the wall to vapor interface. Letting:
c = roughness factor of wall surface, eW_d = energy per unit area in wall to dose interface, ed v = energy per unit area in dose to vapor intexface, eW_v = energy per UIlit area in wall to vapor interface, the required relationship may be expressed algebrically by:
c eW_d ~ ed-v < C ew_v The film~promoting means associated with the interior surf~ce may be a finish or a coating which imparts a rough-ness or irregularity to the surface such that the reductionin exposed surface area by coverage with a uniform liquid film is energetically favored. Alternatively the means may be a coating which provides a chemically different surface on the inside which is better wetted by the metal iodide aose. Or again a roughening coating which is also chemically favorable to film formation may be provided.
The d~iving force desirably should be at least sufficient to cause the condensate liquid to flow at a rate which replenishes the 10s5 from e~aporation at any point on the surface, thus avoiding the occurrence of bare spots.
While the inside surface could be roughened by mechan-ical means such as sand blasting, we prefer to coat the inside surface o~ the en~elope with a smoke o a re~ractory oxide, for instance a silica smoke. The smoked envelope is then torched fr the outside to partially sinter the film into the wall and compact the smoke into a more rugged 53'~

structure. The resulting surface causes the condensate to spread out by wick effect. We ha~e found that the evenly dispersed dose or condensate lowers the color temperature as a result of pressure broadening and self-reversal of the sodium line, and also by acting as a color-correcting filter.

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DESCRIP~IO~I OF DR~INGS
In the drawings:
FIG. 1 shbws a miniature metal halide arc lamp em-bodying the invention.
~ IG. 2 is a diagram of the process of smoking theinterior of the arc tube.
FIGS. 3 and 5 are enlarged photographs of miniature metal halide arc lamps under operating conditions, the former without and the latter with a film-promoting coat-ing according to the invention, FIG. 4 is a sketch outli~ing the principal features in the photograph of FIG. 3.
FIG. 6 is a chromaticity diagram comparing lamp performance with and without film-promoting coating.

DETAILED DESCRIPTION
A miniature arc tube 1 having an internal finish in the central bulb portion 2 causing the li~uid condensate to spread into a film by capillary action in accordance with the invention is shown in FIG. 1. The bulb portion

2 may be formed by the expansion of quartz tubing in known manner and the inside is then coated with a smoke o a suitable metal oxide, for instance Si02. Silica smoke is con~eniently produced by the com~ustion of chlorotrimethylsilane in a gas-oxygen flame. Referring to FIG. 2 illustra~ing a suitable setup for laboratory use, illuminating gas is supplied to tubing 3 and bubbled through liquid chlorotrimethylsilane 5 in stoppered beaker .

-- 6 -- .
6. The gas caxries the chlorotximethylsilane vapor out through tubing 7 extending through T junction 8 and outer tube 9. Oxygen is supplied through branch tubing 11 to the T-junction and thence flows in the annular channel be-tween outer tube 9 and internal duct 7. Outer tube 9penetrates through neck 1~ into the bulb portion 2 of the arc tube. The illuminating gas ana chlorotrimethylsilane are ignited and burn in the oxygen producing a small flame 13. For the illustrated bulb, the gas flow is adjusted to have a flame about 2 to 3 millimeters in size. The products of combustion are water vapor and a white smoke of SiO2 which coats the inside of the bulb; the bulb is rotated an~ mo~ed to ana fro axially during smoking to . o~tain an even coating.
15 ~fter smoking, the bulb is immediately heated to drive out any water vapor which could damage the fragile smoke film. The smoked bulb is then torched from the outside to partially ~inter the film into t~e wall of the bulb and eompact the smoke into a more rugged structure.
The bulb temperature during torchiny must be closely watched since too hot a flame will completely meit the smoke into the fused silica wall, while too cool a flame will not produce the required densification and attach- -ment. The coated bulb is then processed into an arc tube 2 5 in known mannex .
An alumi~a coating has the additional advantage over a silica coating that it is moxe readily wetted }~y the metal halide dose. We ha~e applied such a coating by al-ternate generation of silica and alumina smokes within the quartz bulb. The silica smoke was produced by focus-ing a las~x beam on the tip of a silica rod inserted into the bulb through one neck. The laser beam was convenient-ly aimed through the other neck. Then the silica rod was replaced by an alumina xod and alumina smoke gener-ated in the same way. ~y so doing, the alumina becomesfirmly attached to ~he wall when subse~uently sintered, ;3~6~3~

the silica apparently serving as a binder. The alumina smoke all by itself when sintered ~orms a crusty layer that lacks adherence. The alumina coating changes the chemical na~ure of the surface in a direction to favor S wetting and film formation when contacted ~y a metal halide such as sodium iodide.
Xn the completed lamp shown in FIG. 1, the seals are made by collapsing through heat-softening, assisted by vacuum if desired, the quartz o~ the necks 12,12' upon the molybdenum foil portions 14,14' of the electrode inlead assemblies~ Leads 15,15' welded to the foils project externally of the necks while electrode shanks 16,16' welded to the opposite sides of the foils extend through th~ necks into the bulb portion. The lamp is in-tended fo~ unidirectional current operation and theshank 16' texminated by a balled end 17 suffices for an anode. -The cathbde comprises a hollow tungsten helix 18 spudded on the end of shank 16 and terminating at its distal end in a mass or cap 19 which may be formed by melting back a few turns of the helix.
A typical metal halide arc tube inte~ded ~or a lamp of 35 watt size may have a bulb outer diametex o~ about O.7 cm and a discharge volume of 0.1 to 0.15 cubic centi-meter. A suitable filling for the envelope comprises argon or other inert gas at a pressure o~ several tens of torr to serve as a starting gas, and a c~a~ge com-prising mercury and the metal halides NaI, ~e3 and ThI4.
The charge may be intraduced into the arc chamber through one of the nec3cs before sealing in the second electrode.
The illustrated arc tube is usually mounted within an outer protecti~e envelope or ~ac]cet ~not shown) having a base to whose contact terminals the inleads 15,15' are connected.
- The sintered smoke layer in bulb 2 causes a wick effect which succeeds in spreading out the liquid dose or condensate into a substankially continuous film.

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The effectiveness of the layer is readily apparent when FTG. 3 in which the bulb does not have a sintered smoke layer is compared with FIGo S in which a layer is present. Both figures are photographs of the images pro-5 duced on a screen by focusing the light from operatinglamps thereon through a converging lens.
In FIG. 3 where no layer is present, the conden-sate does not form a continuous film but discrete drop-lets which tend to persist. Some droplets become larger as more condensa~e joins the mass, for instance droplets 21 and 22 indicat~d in FIG. 4. Eventually the weight o~
a large dxoplet may cause it to roll down the wall into the ~nd. Sudden vaporization of the droplek should it touch the hot sha~k of the electrode may produce a red-dish ~lash, and movement of the droplets causes someflickering.
In FIG. 5 whexein a silica smoke layer embodying the invention has been provided, a continuous film of con-densate is present covering substantially the entire in--terior surface o~ the bulb. The large droplets have beendisprsed in the film. The film produces improved vapor-ization of the dose which results in the desired lower color temperature.
In ~IG. 6, the magnitude and direction of the shift in spectral output is indicated by means o ICI chroma-ticity coordinates. The two cross-hatched circles rep-resent the mean or range o variation in spectral response at two power levels, 18 watts and 35 watts, ~or lamps o~
the kind shown in FIG~ 1 but without any means promoting the ormatio~ of a liquid condensate ilm. The spectral response of a lamp provided with a sintered silica smoke coating is shown by the two dots-within-triangle, and the corresponding lumen outputs are also indicated. The solid curving line conventionally represents the black body locus, and the sloping lighter lines r the correlated loci o color temperatures o 4000X, 3500K and 3000R.

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The shift in spectral response to the right along the black body locus to a lower color temperature caused by the coating is particular.ly evident at low power. The color temperature tends to be too high at low power due to inadequate sodium iodide ~apor p.ressure. The indi-cated shift to a lower color temperature is more pro-nounced a~ the lower leveI where it is most needed. The shift results from pressure broadening ana self-reversal of the sodium.emission, and also ~he ~ilm o~ evenly dis-persed halide dose acts as a color correcting filter.
In lamps having the continuous film of liquid doseon the inside which our invenkion makes possible, flicker during start up is completely eIiminated. When power is turned o~f, the dispersed halide dose condenses and crystallizes over the entire interior surface of the bulb. When the lamp is subse~uently turned on, melt-ing and vaporization occur smoothly and evenly and the liquid condensate film is pr~mptly re~ormed.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A high intensity metal vapor discharge lamp comprising:
a sealed envelope having light-transmissive walls, a filling in said envelope producing a vapor in which an electric discharge generates light, said filling including a dose of metal salt substantially in excess of the quantity vaporized in operation of said lamp, said dose being liquid at the wall temperature in said envelope during operation, electrode means for supporting a discharge within said envelope, and film-formation promoting means associated with the interior surface of the walls of said envelope which imparts a roughness or irregularity to the surface for promoting the formation and spreading of a liquid condensate film thereon.

2. A lamp as in claim 1 wherein the film-formation promoting means enables the surface energy in the wall to liquid dose interface plus the surface energy in the liquid dose to vapor interface to be less than the surface energy in the wall to vapor interface.

3. A lamp as in claim 1 wherein the film-formation promoting means is a surface finish which substantially increases the interior surface area of said envelope.

4. A lamp as in claim 1 wherein the film-formation promoting means is a coating on the interior of said envelope which exposes a surface area which is substantially reduced when a liquid film forms over it.

5. A lamp as in claim 1 wherein the film-formation promoting means is a coating of fine refractory oxide particles adherent to the interior surface of the walls of said envelope.

6. A lamp as in claim 1 wherein the film-formation promoting means is a coating produced by causing a smoke of refractory oxide particles to con-tact the interior surface of said envelope and form a film thereon, and by thereafter partially sintering the film to compact it into a more rugged structure and improve its adherence to the envelope.

7. A high intensity metal vapor discharge lamp comprising:
a sealed vitreous envelope, a discharge-supporting filling in said envelope comprising mercury and metal halide, the quantity of metal halide being substantially in excess of that vaporized in operation of said lamp, said metal halide being liquid at the temperature of the interior of said envelope during operation, electrode means for supporting a discharge within said envelope, and film-formation promoting means associated with the interior surface of said envelope which imparts a roughness or irregularity to the surface for promoting the formation and spreading of a liquid condensate film thereon.

8. A lamp as in claim 7 wherein the film-formation promoting means is a surface finish which substantially increases the interior surface area of said envelope.

9. A lamp as in claim 7 wherein the film-formation promoting means is a coating on the interior of said envelope which exposes a surface area which is sub-stantially reduced when a liquid film forms over it.

10. A lamp as in claim 7 wherein the film-formation promoting means is a coating of fine refractory oxide particles adherent to the interior surface of said envelope.

11. A lamp as in claim 10 wherein the envelope is fused silica and the coating comprises silica particles.

12. A lamp as in claim 10 wherein the envelope is fused silica and the coating comprises alumina particles.

13. A lamp as in claim 10 wherein the envelope is fused silica and the coating is produced by causing a smoke of refractory oxide particles to contact the interior surface of said envelope and form a film thereon, and by thereafter partially sintering the film to compact it into a more rugged structure and improve its adherence to the envelope.