CA1044909A - Photoflash lamp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A photoflash lamp having a beadless ignition structure with short sloping inleads and containing an excess a-mount of oxygen for sufficiently burning back the inleads to prev-ent post-ignition shorting. The combustible fill material in the lamp comprises fine shreds of metal foil, and preferably, the in-leads are of a much larger cross-section than the shreds so that burning back of the inleads will occur near the end of the flash cycle.

Description

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BACKGROUND OF THE INVENTION

This invention relates to the manufacture of photoflash lamps and,more particularly, to flashlamps containing filament inlead wires and means for preventing post-ignition short circuits therebetween.
Photoflash lamps generate an actinic light out-put by the burning of energetic fuel, such as finely shredded zirconium, hafnium or aluminum metal foil, in a combustion support-ing atmosphere, such as oxygen. In some of the tubular electric-ally ignitable photoflash lamps presently manufactured, the ig-nition means comprises a pair of inlead wires sealed through a press at one end of the tubular glass envelope and supported in a mutually parallel spaced apart relationship by a glass bead fused about the wires. Such an arrangement is shown in U.S. Patent 3,739,166. A tungsten filament is mounted across the inner ends of the two inlead wires with the ends of the wires at their junct-ions with the filament being coated with a primer material such as a powaered zirconium mixture.
Another form of ignition structure which has been employed is a beadless construction, such as illustrated in Fig. l of U.S. Patent 3, 770,362. In this arrangement, the inleads within the lamp envelope emerge from the press at opposite sides of the lamp, slope toward one another, and then proceed for a substantial portion of their length within the envelope in a generally parallel spaced apart relationship to the inner ends at which the filament is attached. In both arrangements, the ignition structure appears to extend inwardly into the lamp for about ~0~ of the internal length of the envelope.
In operation, when battery power is applied to the external projecting portions of the two inlead wires, the filament glows to incandescen~e,causing the primer ma-terial to ignite, which in turn ignites the finely shredded metallic comb-us-tible in the lamp produce a predetermined quantity of light out-,~
put. - l - `~

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The oxygen within the lamp is initially present at an elevated pressure, e.g., 8 atmospheres. During lamp flash-ing, the oxygen is heated and the internal pressure rises to a peak value approximately 60% higher than the initial value. At the same time, molten globules of metals and oxides from the act-inic combustion impinge upon the inner glass surface of the en-velope. The resulting localized thermal shock and stressing can readily cause the glass to spall, crack or disintegrate. Accord-ingly, in order to reinforce the glass envelope and improve its containment capability, it has been common practice to coat the outer surface of the lamp envelope with a protective lacquer, such as cellulose acetate.
As stated above, the internal pressure in the flashed lamp reaches a high peak value. This occurs early in the flash cycle (e.g., 13-20 milliseconds). From that time on, as the reaction consumes oxygen, the internal pressure within the lamp gradually declines so that by the time of 160 milliseconds, the internal pressure is approximately one atmosphere. It is seen, therefore, that the potential severity of lamp rupture is a time dependent function as it is determined by the difference between lamp internal pressure and atmospheric pressure. Both contamination and the additive effect of manufacturing tolerances, however, can give rise to occasional lamps with higher internal pressure excursions than normal. Accordingly, it has been standard practice in the flashlamp industry to load lamps with an excess of shredded combustible foil above the stoichiometric ratio (e.g., 5% to 10% excess) to more completely consume the oxygen in the lamp envelope and thus serve as a flash quencher to reduce the internal pressure to a near atmospheric condition upon cooling.
In some of the more recent lamp types, however, improved and thicker lamp coatings and/or the use of stronger glasses have allowed lamp fabrication with equal or near stoichiometric balance to thereby provide improved light output efficiencies.

8047 In addition to the use of a glass bead for firmly supporting the filament inleads in a spaced apart relationship, certain other lamp types, such as described by U.S.Patent 3,816,054, further employ a glass sleeve disposed about a portion of one of the inlead wires as an insulating shield extending from the glass bead toward the ~ilament for preventing post ignition short circuits across the inlead wires. Such a feature is required for the proper operation of certain flash sequencing circuitry for controlling linear arrays of flashlamps. For example, in one presently marketed photoflash array application, if a short circuit occurs between the melted inlead wires in the first (or subsequent) lamp of the array to be flashed by the sequencing circuitry, the entire array of lamps is rendered useless.
Although reliably providing the desired inlead isolating function, the use of a glass insulating sleeve signiEic-antly increases both the manufacturing and materials cost of the lamp unit. For example, according to a present manufacturing method for processing the ignition mount structure, the lead wires are initially provided in the form of a hairpin with the closed end facing downward. The glass bead is then melted and fused about the lead wires to retain the spacing therebetween, this is a conventional beading operation. Next, the closed end o~ the hairpin is trimmed off in the normal manner. The mount structure is then rotated 180, and a glass tube is placed over one of the lead wires so that it rests on the beàd. The end of tfi~ tu~is:i~contact with the bead is then heated just enough to fuse it to the bead but without distorting its upper portion.
The mount structure must then again be rotated by 180 to make it ready for further processing.
It will be noted that this method requires turning the mount structure head 180 at two separate locations. It also requires tube feeder and loader, a device to locate one of the wires so that the glass tube can be fed over it, and a great deal of skill on the machine attendant's part.

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A further disadvantage of this relatively massive bead-sleeve ignition structure is that it substantially decreases the internal volume of the lamp, a factor which is of considerable importance in the currently popular subminiature lamp sizes having internal volumes much less than one cubic centimeter. The presence of the bead sleeve structure results in a higher initial pressure and also causes difficulty in obtaining good fill distribution within the envelope.
In addition to the above noted disadvantages, it has been determined through many tests that the relatively massive beaded and bead-sleeve inlead structures in subminiature flashlamps cause a decrease in flash illumination e~ficiency due to their heat absorbing effects. In fact, it has been found that this èfect can reduce the light output efficiency by as much as 10 to 15~ when compared to lamps employing ignition structures which do not contain a glass bead or bead and sleeve.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide an electrically ignitable photoflash lamp having improved light output efficiency.
A principle object of the invention is to provide an electrically ignitable photoflash lamp having improved means for providing an open circuit condition after flashing of the lamp.
It is a particular object of the invention to provide at substantially reduced manufacturing cost an electrically ignitable photoflash lamp construction which avoids postignition short circuits across the inleads while providing improved output efficiency.
These and other objects, advantages and features are attained, in accordance with the principals of this invention, by employing a beadless ignition structure and a selected amount of combustion supporting gas in excess of the quantity required for stoichiometric chemical reaction with the shredded combustible ~ 8047~ 449~9 foil in the lamp. More specifically, I have discovered that if a selected additional amount of oxygen is provided in the lamp~
the inleads will be oxidized, or burned bac~, almost completely during flashing of the lamp so as to create an open circuit condition. It has been further noted, that if the cross-section-al area of each of the inleads is several times greater than the average cross-sectional araa of each of t:he strands of filamen-tary combustible material, consumption of the inleads will occur, as preferred, near the end of the flash cycle so as to have little or no deleterious effect on the usable light output.
The amount of oxygen in excess of that required for stoichiometry with respect to the combustible material is at least 40%, and preferably from about 50% to 100%, of the stoichiom-etric quantity required for complete chemical reaction with the portions of the inleads inside the envelope. To aid inlead burn-back in a manner assuring an open circuit, the inleads inside the envelope emerge from the seal at the end of the envelope on ~ppoqite sides thereof and slope toward each other for substan-tially the entire distance to the inner ends thereof where the filament is attached.
The improvement in light output efficiency resulting from this lamp construction permits a 10~/o to 15% reduction in the quantity of shreddedaombustible foil required while still maintaining the necessary design light output characteristics.
If about the same quantity of oxygen based ~on the former com-bustible stoichiometry is maintained, then sufficient excess is available to oxidize, or consume, the inlead wires so as to create an open condition after flash. This new`stoichiometry , has an effective lower internal pressure in the vessel due to the larger internal volume resulting from the beadless structure.
The relative simplicity of the lamp permits improved production fabrication at a much lower cost.

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BRIEF DESCRI~TION O~ TH~ DRAWINGS

This invention will be more fully described here-inafter in conjunction with the accompanying enlarged scale drawing, the single FIGURE of which is an elevational view, partly in section of a photoflash lamp in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIM:~:NT
Referring to the drawing, an electrically ignited photoflash lamp is shown comprising an hermetically sealed, light transmitting envelope 10 of glass tubing having a press 12 defin-ing one end thereof and an exhaus~ tip 14 defining the other endthereof. A quantity of filamentary combustible material 16, such as shreded zirconium, or hafnium foil, is located within the lamp envelope. The envelope is also provided with a filling of a comb-ustion-supporting gas, such as oxygen, in an amount in excess of the quantity required for stoichiometric chemical reaction with the combustible material I6~ The selected quantity and purpose of this excessive oxygen stoichiometry is a particular aspect of the invention and, thus, will be discussed in detail hereinafter.
Further, in accordance with the invention, the ignition structure comprises a pair of inlead wires 18 and ~0 sealed through the press 12 and extending inside one end of the envelope. A filament 22 is attached across the inleads near the inner ends thereof, and beads of primer 24 and 26 are located on the inner ends of the inleads 18 and 20, respectively, at their junctions with filament. Tt will be noted that the end of the envelope, namely, the press seal 12 is the sole means for support-ing the inleads in a spaced apart relationship within the envelope.
The inleads are formed of a metallic wire having a coefficient of thermal expansion substantially matching that of the glass envelope 10, whereby a match seal is provided. For example, if the lamp envelope is formed of a borosilicate hard glass having a coeffic-ient of linear thermal expansion between 0 and 300C about in the range of 40 to 50 x 10-7 per C, the inleads may comprise an alloy of iron, nickel, and cobalt, such as Kovar, which has a mean co-efficient of thermal expansion of about 50 x 10~7per C, between 25c and 300C. (Kovar is a U.S. registered trademark of Westinghouse Electric Corp.) The portions of the inleads inside the lamp emerge from the press seal on opposite sides of the envelope proximate the tubular sidewalls thereof, and slope toward each other for substantially the entire distance of the inner ends thereof. To further assure a minimum intrusion o~ the ignition structure upon the internal volume of the lamp envelope, the internal length of the inleads is limited to extend inwardly from the press a distance of from about 20% to 25% of the internal length of the envelope.
In accordance with the invention, the amount of oxygen contained in the lamp in excess of that required for stoichiometry with respect to the combustible material 16 is sufficient so that, upon flashing of the lamp, the combustible material 16 is completely consumed and the inleads 1~ and 20 within the lamp are sufficiently burned back to provide an open circuit. In determining the additional stoichiometry necessary for complete combustion of the internal inlead structure, the theore~ical calculation indicates that approximately 0.25 atmos-pheres of oxygen ~re required for each mill gram of inlead material per cubic centimeter of envelope volume. Eowever, in actual practice~ it has been discovered that if at least 40D/o, and prefer-ably from about 50~0to 100%, of the additional oxygen require-ment is provided, ik will satisfy the necessary afterflash open circuit condition. The total stoichiometric consideration for the system can now be determined quite easily by combining the weight of the combustible material 16 plus the weight of the inlead material within the lamp envelope to determine the nec~sary oxygen requirement in the envelope.
As is well known in the art, 100 % stoichiometry refers to the idealized chemical reaction between the combustihle metal and the oxygenin the flashlamp in which thère are enough atoms of oxygen to react with every atom of metal, e~g., if the metal is hafnium, the reaction may be expressed Hf ~ 2 = HfO2~

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The standard formula for relating the moles of oxygen to the moles of metal to be burned is W +' P~7 K
where W equals the fill weight in milligrams, P equals the pressure in centimeters of mercury, V equals the lamp volume in cubic centi-meters, and K is a derived proportionality constant which permits relating the milligrams of metal to the oxygen pressure in centi-meters of mercury and the cubic centimeters of lamp volume. Hence, K is a function of the metal used, i.e., the atomic weight of the metal, the gas constant from the ideal gas law, and the mole ratio of the metal and oxygen in the final oxide product. For example, it has been determined that K is equal to approximately 20.4 for the alloy Kova~ 20.4 for zirconium and 10.4 for hafnium.
Accordingly, the above equation can be solved for fill weight (W) to determine the metal required to completely consume the oxygen in the lamp, or it can be solved for P to determine the oxygen pressure needed to burn all the metal present in the lamp, such a relation being termed 100% stoichiometry.
For example, a typical lamp in accordance with the invention may comprise a tubular borosilicate glass envelope hav-in a press seal with Kovar or Rodar inleads as shown i;nthe draw-ing. (Rodar is a U.S. registered trademark of Wilbur B. Driver Co.
and refers to an alloy similar to Kovar). The outside surface of the envelope is coated with four layers of cellulose acetate.
Dimensionally, the coated envelope has an outside diameter of about 0.280 inch, an inside diameter of 0.200 inch an an internal length of approximately 11/16 of an inch. The inleads extend inwardly inside the envelope to about 1/8 inch from the end of the envelope at the press seal. The spacing between the inleads in the press is about ~/16 inch, and the spacing between the inner ends of the inleads inside the envelope is about 1/16 inch. The diameter of each inlead is about 14 mils, although it may typically vary from 10 to 16 mils for different lamp designs. As illustrated in the drawing the Kovar or Rodar inleads 18 and 20 support a fine tungsten filament 22 within the lamp, with beads of primer material 24 and 26 about the inner ends ~, ,?~

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of the leads.
The lamp envelope has an internal volume of about 0.35 cubic centimeters, and the fill of combustible material 16 comprises about 25 milligrams of shredded hafnium foil, with the cross sectional area of each strand of ha~nium foil being approximately one square mil, although it may vary to two square mils for different lamp designs. To determine the amount of oxygen fill in atmospheres of pressure, the following formula, derived from that above, may be used:
p = WK
V . 76cm In accordance with~ the invention the lamp is to include sufficient oxygen to provide the required 100% stoichio-metric chemical reaction with the combustible material hafnium plus an adequate excess of oxygen to burn bac~ the inleads suf-ficiently to provide an open circuit. As mentioned above, I
have determined that the amount of oxygen in excess of that required for stoichiometry with respect to the combustible material should be at least 40% of the stoich~ometric quantity required for chemical reaction with the portions of the inleads inside the envelope. An excess stoichiometry of well over 100%~
however, reduces the containment capability. Accordingly, I
prefer a range between 50% to 100% as the excess stoichiometry requirement for burning back the inleads. In the present specific example, I use an excess 50% of the stoichiometric quantity of oxygen required for chemical reaction with the portions of the inleads inside the envelope, the weight of which has been deter-mined to be about 6 milligrams. The K for the inlead material ~Kovar or Rodar) is approximately double the K for hafnium, so using onl~ 50% of the excess oxygen required for stoichiometry with respect to the inlead material has the effect of approximate-ly equating the two constants. That is the total equation for solving for the oxygen requirement would appear as p = WlK1 + 0.5 W2K2 V 76cm. V 76cm.

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where Wl is the weight of hafnium in the lamp, W2 is the inlead weight in the lamp, Kl is the constant for hafnium (ie., 10.4) and K2 is the constant for the Kovar inlead material (ie 20.4).
As 0.5 K2 equals about 10~2, which is near:Ly equal to Kl, the formular may be simplified to P - (Wl + W~)K
V . 76~cm then ~bstituting values we obtain, P = (25 + 6) (10.4) = 12 atmospheres (0.35) (76) Accordingly, the lamp of our example is filled with approximately 12 atmospheres of oxygen to assure an efficient light output, and sufficient burning back o~ the inleads to provide the desired post-ignition prevention of short ciruits.
Further, as the cross-sectional area of the inleads is several times greater than the average cross-sectional area of each of the strands of shredded hafnium foil, the finer shreds will burn first to provide efficient light output, while the inleads will burn toward the end of the flash cycle.
It is noted that a U.S. Patent 3,817,683 discloses a stoichiometric range which includes the use of excess oxygen in a lamp which employs a beaded ignition structure, how-ever, there is no suggestion in that patent for using a selected excess of oxygen for the purpose of preventing post-ignition short circuits. Further, U.S. Patents 2,272,059 and 3,263,457 illustrate beadless flashlamp constructions, but again, neither of these patents discuss the elimination of post-ignition shorts.
In summary, what I have discovered is a stoi-chiometric consideration that provides additional oxygen to perf-orm a specific function within the flashlamp envelope, i.e., to burn back the internal inleads sufficiently to provide an open circuit condition after flash. Further, the excess oxygen and beadless construction provides a higher light output efficiency, which permits a 10 to 15~ reduction in the quantity of shredded combustible foil. If about the same quantity of oxygen based on -- 10 ~

~JgL9L9~19 the former combustible stoichiometry is maintained, then sufficient excess is available to consume the inlead wires so as to create an open circuit condition after flash. This new stoichiometry has an effective lower internal pressure in the vessel due to the larger internal volume resulting from the substantial reduction in size of the ignition structure. And the simplicity of the ignition structure permits improved production fabrication at a much lower cost.
Although the invention has been described with respect to specific embodiments it will be appreciated that mod~
ifications and changes may be made by those skilled in the art without departin~ from the true spirit and scope of the invention.

Claims (9)

1. A photoflash lamp comprising:
an hermetically sealed, light-transmitting envelope;
a quantity of combustible material located in said envelope;
a quantity of oxygen in said envelope in an amount in excess of the quantity required for stoichio-metric chemical reaction with said combustible material;
and ignition means disposed in said envelope in operative relationship with respect to said combustible material, said ignition means including a pair of inleads sealed through and extending inside one end of said envelope, said end of the envelope being the sole means for supporting said inleads in a spaced apart relationship within said envelope;
the amount of said oxygen in excess of that required for stoichiometry with respect to said combustible material being at least 40% of the stoichiometric quantity required for chemical reaction with the portions of said inleads inside the envelope whereby, upon flashing of the lamp, said combustible material is consumed and said inleads are sufficiently burned back to provide an open circuit.

2. The lamp of claim 1 wherein the amount of said oxygen in excess of that required for stiochiometry with respect to said combustible material is from about 50% to 100% of the stiochiometric quantity required for chemical reaction with the portions of said inleads inside the envelope.

3. The lamp of claim 1 wherein said ignition means further includes a filament disposed within said envelope and attached across said inleads near the inner ends thereof, and the portion of said pair of inleads inside the envelope emerge from the end of the envelope on opposite sides thereof and slope toward each other for sub-stantially the entire distance to the inner ends thereof.

4. The lamp of claim 1 wherein said combustible material comprises a plurality of strands of filamentary material, and the cross-sectional area of each of said inleads is several times greater than the average cross-sectional area of each of said strands of filamentary combustible material.

5. The lamp of claim 4 wherein said average cross-sectional area of each of said strands of filamentary combustible material is from approximately one to two square mils, and the wire diameter of each of said inleads is from approximately 10 to 16 mils.

6. The lamp of claim 1 wherein said envelope is formed of glass tubing with a press defining one end thereof and an exhaust tip defining the other end thereof, and said inleads extend through and are sealed into said press, the inleads being formed of a metallic wire having a coefficient of thermal expansion substantially matching that of the glass.

7. The lamp of claim 6 wherein said inleads extend inwardly from said press a distance of from about 20% to 25 % of the internal length of said envelope.

8. The lamp of claim 7 wherein said ignition means further includes a filament disposed within said envelope and attached across said inleads near the inner ends thereof, the portions of said inleads inside the envelope emerge from said press on opposite sides of said tubular envelope proximate the tubular sidewalls thereof and slope toward each other for substantially the entire distance to the inner ends thereof, and the amount of said oxygen in excess of that required for stoichiometry with respect to said combustible material is from about 50% to 100% of the stiochiometric quantity required for chemical reaction with the portions of said inleads inside the envelope.

9. The lamp of claim 8 wherein the inside diameter of said tubular envelope is about 0.200 inch, the spacing between said inleads in said press is about 3/16 inch, and the spacing between the inner ends of said inleads inside the envelope is about 1/16 inch.