Note: Descriptions are shown in the official language in which they were submitted.
80~
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 - `~
8~
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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80~
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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B
80~
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
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047-L
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~
~.
8047-L
~44~
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
~, ,?~
8047-L 1~9~
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.
~ '7 ~ 9~
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.
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