Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to high pressure sodium vapor
lamps specially designed ~or operation on sonic frequency
pulses with short duty cyc:Les in order to raise the color
temperature and improve the color rendition, and is con-
cerned with reducing the noise level in such lamps.
High pressure sodium vapor lamps are now well-known
and widely used for street, roadway and area lighting
applications. The basic lamp type is described in U.S.
patent No.. 3,248,590 dated April 26, 1966 - Schmidt,
"High Pressure Sodium Vapor Lamp", and generally comprises
an outer vitreous envelope or jacket of glass within which
is mounted a slender tubular ceramic arc tube. The
ceramic envelope is made of a light-transmissive refractory
oxide material resistant to sodium at high temperatures,
suitably high density polycrystalline alumina or synthetic
sapphire. The filling comprises sodium along with a rare
gas to facilitate starting, and mercury for improved
efficiency. The ends of the alumina tube are sealed by
suitable closure members affording connection to the
electrodes. The outer envelope is generally provided at
one end with a screw base having shell and eyelet terminals
to which the electrodes of the arc tube are connected.
Up to the present time high pressure sodium vapor
lamps have been conventionally operated on 60 cycle alternat-
ing current by means of ballasts which limit the current
to the lamp rating. In such operation, the light generated -
by the discharge is due almost exclusively to the ex- ~ -
citation of the sodium atom through the self-reversal and
broadening of the sodium D-line at 5~9 nanometers. The
lamp e~ficacy is hlgh, up to 130 lumens per watt depending
upon lamp sizej but the color temperature is low, from 1900
to 2100 Kelvin. While object colors in all portions of the -
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spectrum are recognizable, those a-t the "cool" end such as
violets, blues and to some extent greens are muted or grayed
down. As a result, the lamp has not been acceptable for
indoor applications where critical color discrimination is
required.
More recently, the color temperature of high pressure
sodium vapor lamps has been raised and their color re-
ndition has been described in Canadian application Serial
No. 275,374 dated April 1, 1977 of Mitchell M. Osteen,
"Color Improvement of High Pressure Sodium Vapor Lamps by
Pulsed Operation", and assigned like this application. By
utilizing pulse repetition rates in the sonic range from
500 to 2000 hertz and short duty cycles from 10 to 30%, the
color temperature has been increased from the common value
of 2050K to as high as 2700K with substantially no
reduction in lamp efficacy, or even higher than 2700K at
the price of some reduction in efficacy.
Sonic pulse operation of high pressure sodium vapor
lamps of conventional construction produces audible noise.
The pulse repetition rate in the range from 500 to 2000
h2 determines the fundamental frequency and the ear is
sensitive to this range. Also, the noise problem is
aggravated by the short duty cycle which means an abrupt
rise and fall in current at every pulse inducing higher
frequency harmonics which may be even more penetrating.
The object of the :;nvention is -to provide high pressure
sodium vapor lamps which are substantially noisefree on
sonic pulse operat:ion, that is, lamps in which the noise
level is low enough for comfortable use indoors.
~30 I have determ:ined that a major source of noise in ~ -
sonic pulse operat:ion of prior art lamps is the stem press
or seal of the outer glass envelope or jacket wherein the
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welds oE the inner and outer nickel portions to the inter-
mediate portion of the inleads are embedded in glass.
Nickel is magnetostricitive and the climensional changes in
the current carrying nickel wires can couple mechanical
energy to the jacket very effectively. ~y invention replaces
the nickel wires by non-magnetostrictive conductors; pre-
ferably titanium for the inner conductors and copper for the
outer conductors which are connected to the base terminals.
Another source of noise is the frame supporting the
arc tube which has conventionally been made of a nickle-iron
alloy. My invention substitutes a non-magnetostrictive con-
ductor for the nickel-iron of the frame, preferably titanium
which can withstand the temperature of operation and also
makes a good getter for residual hydrogen or oxygen in the
vacuum of the outer envelope. Further noise reduction is
achieved by utilizing non-magnetrostrictive material for the
lamp base r suitably brass for the shell and eyelet, and
phosphor bronze for the internal spring cap.
In a lamp design embodying the invention, assembly
is made in such manner as to avoid loose parts which could
vibrate or rattle during operation, particularly within the
arc tube wherein a pressure wave is generated at each pulse. -
In the drawing:
.
FIG. 1 shows a high pressure sodium vapor lamp in-
tended for sonic pulse operation and having the low noise
features embodying the invention.
FIGS. 2 and 3 are enlarged side and plan details of
the cathode showing the attachment of the anti back-arcing ~-
shield.
Referring to FIG. 1, the illustrated lamp 1 embodying
the invention is a jacketed high pressure sodium vapor lamp
rated for 300 watts input on unidirectional pulse operation.
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The lamp comprises an inner ceramic arc tube 2 enclosed
within an evacuated outer envelope 3 of glass to the neck
of which is attached a standard mogul screw base 4. The
outer envelope or jacket comprises a re-entrant stem press
5 through which extend a pair of relativel~ heavy inlead
conductors 6, 7 whose outer ends are connected to spring cap
8 and to eyelet 9 of the base. The spring cap engages the
neck of the bulb through four springy legs 11 each provided
with an embossment which engages a dimple 12 in the glass.
The base 4 is fastened by screwing it down on the spring
cap and this also makes the connection between inlead 6
and screw shell 13 of the base.
The arc tube 2 centrally located within the outer
envelope comprises a length of alumina ceramic tubing,
either polycrystalline alumina ceramic which is tran-
slucent or single crystal alumina which is clear and trans-
parent. In the drawing, the arc tube is represented as
clear to facilitate illustration. End closures consisting
of metal caps 14, 15 of niobium which matches the expansion
coefficient of alumina ceramic, are sealed to the ends of
the tube by means of a glassy sealing composition. A
metal tube 16, suitably of niobium or tantalum, extends
through cap 14 and serves as an exhaust and fill tubula-
tion during manufacture of the lamp. The illustrated lamp
is intended for base-up operation and the exhaust tube is
sealed off at its outer end and serves as a reservoir in
which excess sodium mercury amalgam condenses during operation.
Electrode 17 within the lamp is attached to the inward
projection of exhaust tube 16, and a dummy exhaust tube 18
extending through metal end cap 15 supports the other electrode
19. Both electrodes may consist of tungsten wire 20 coiled
on a tungsten shank 21, suitably in two superposed layers.
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As example, the arc tube which is 90 mm long by 5.5 mm in
bore contains a fllling of xenon at a pressure of 20 torr
serving as a starting gas, and a chaxge of 25 mg of amalgam
of 25 weight per cent sodium and 75 weight per cen-t mercury.
Exhaust tube 15 is connected by connector 22 and long
frame member or side rod 23 to inlead 6 which provides
circuit continuity to the base shell 13 which is made
positive in unidirectional pulsed operation. Dummy exhaust
tube 18 extends through a ring support 24 fastened to short
L-shaped rod 25; the arrangement provides lateral restraint
while allowing axial expansion of the arc tube. A flexible
metal strap 26 connects dummy tube 18 to side rod 25 which in
turn is welded to inlead 7, thereby providing circuit con-
tinuity to base eyelet 9. The distal end of long side rod
23 is braced to inverted nipple 27 in the dome end of the
envelope by a clip 28 which engages it.
In a lamp intended for sonic pulse short duty cycle
operation, arc stability and overheating of the end
closures, particularly that at the anode and when uni-
directional pulsing is used, can be a problem. In Canadian
application Serial No. ~ 933 dated Sc~~e~ t,t~77
titled, "~Iigh Pressure Sodium Vapor Lamp Stabilized for
Pulse Operation" and assigned to the same assignee as the
present application, a lamp construction having improved
arc stability is described and claimed. Arc stability
and long life are achieved and end closure overheating is
prevented by using electrodes of cross sectional area from
0.3 to 0.4 times the envelope cross section and by in-
creasing insertion depth. In addition, the anti back-arcing
shield 29 is positioned so that the ratio of arc gap to gas ~ - -
column length is less than 0.80, preferably about 0.70.
The lamp illustrated in FIG. 1 has these features and is
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intended for oper~tion on unidirectional pulses in the sonic
range from 500 -to 200 hz at 10 to 30% duty cycle with a
300 watt input. The lamp is operated baseup with the
cathode 19 at the upper end. Since the current is uni-
directional, only cathode 19 is activated with dibarium
calcium tungstate emission material which is contained in
the interstices between the two laye:rs of tungsten coiling.
The physical structure of anode 17 at the lower end is the
same except that no emission material is provided and the
anti backarcing shield is omitted.
In the illustrated lamp the neck of the outer bulb or
jacket is sealed to the flare of a glass stem 5 which also
includes an exhaust tube 30 for evacuating the inter-envelope
space. The lead-in conductors 6, 7 comprise intermediate
portions 6a, 7a which are of wire selected to match the co-
efficient of expansion of the glass, and inner portions 6b,
7b and outer portions 6c, 7c which are butt-welded to the
intermediate portions. The hermetic seal is made at the
intermediate portions 6a, 7a which are completely embedded
in the pressed portion of the stem. In the prior art con-
struction, nickel or nickel-iron alloy was used for the
inner portions and frequentl~ for the outer portions as well.
By tests conducted in a room-size anechoic chamber, I have
determined that major sources of noise are the nickel wires
undergoing magnetostriction in the stem press. Since the
joints or welds of the nickel portions to the intermediate
portions are embedded in glass, their dimensional changes
during pulsing can couple mechanical energy to the jacket ~
very effectively. I have found that subs-tituting non- -
magnetostrictive material for the inleads reduces the noise
level significantly, as much as 10 decibels. For examplein
lamps where the noise level measured 60 db, it dropped to 50
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db when the inleads having inner and outer portions of
nickel were replaced by all tungsten inleads sealed through
a stem of nonex glass which has a coe~ficient of expansion
matching that of tungsten.
In a practical lamp design, tungsten inleads are too
expensive and difficult to work. Other non-magnetostrictive
metals which could be used are molybdenum, titanium, copper
and aluminum. A preferred construction illustrated in the
drawing utilizes an intermediate inlead portion 6a of
tungsten, to which is butt-welded on one side an inner portion
of 6b of titanium and on the other side an outer portion 6c
of copper. The copper wire may be stranded as illustrated,
or solid, and it is preferably nickel-plated as an anti-
oxidation measure. Outer portion 6c is Eastened by welding
or soldering to spring cap 8, and outer portion 7c is
similarly fastened to eyelet 9. The spring cap has in the
past been made of steel which contributes to noise. It is
desirable to replace the steel by non-magnetostrictive
material, suitably phosphor bronze which has the needed -~
springiness. Other metallic parts of the base, namely
eyelet 9 and screw shell 13 are made of brass.
The interior of the jacket is exhausted and sound is
not transmitted through a vacuum. But the frame of mount
which supports the arc tube within the jacket is a source
of noise and in addition couples noise to the exterior.
In the prior art construction long side rod 23 and L-
shaped rod 25 were made of nickel-iron alloy which is -
magnetostrictive. I have found that substituting a non-
magnetostrictive material for these parts reduces the noise
level another 10 db, that is from 50 to about 40 db. I
prefer to use titanium because it is not magnetostrictive
and also makes a good getter for residual hydrogen or oxygen.
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The clip 28 which engages inverted nipple 27 may be made of
titanium sheet or else it may be eliminated and the end of
rod 23 curved into a ring to encircle the nipple. Strap
31 which is attached to side rod 23 and engages insulator
32 to serve as a stiffening brace does not carry current
and does not need to be made of nonmagnetostrictive material.
In the arc tube proper, all the conductive parts are
of tungsten or niobium, neither of which is magnetostrictive.
Nevertheless, it is important to avoid loose parts which
could be set into vibration by the longitudinal pressure
wave which occurs in the arc tube as each pulse. In
particular I have found that the anti back-arcing shield
in the form of a small niobium disc 29 mounted on tungsten
shank 21 behind cathode 19 can be a source of noise. The
prior art practice has been to pierce a clean hole through
the disc slightly larger in size than the tungsten shank.
The disc are loose on the shank and, in pulse operation of
the lamp, they rattle or buzz and sound comes out through
the frame and base to the socket. The problem is cured
by firmly attaching the shield to the shank. For instance,
the disc may be welded to the shank. A preferred simpler
solution is to use a nail punch to make an undersized hole
through the disc and then to ~orce-press the disc on the
shank, causing the formation of sharp points 33 which engage
and bite into the shank, as shown in FIG. 2. In lamps
wherein these precautions have been observed along with the
previously described design features of non-magnetostrictive
inleads, frame and base parts, the noise level on pulse
operation was lowered 30 db (a factor of 1000) below that
of conventional construction. The residual noise level
is low enough for comfortable use of the lamps indoor.
