Note: Descriptions are shown in the official language in which they were submitted.
D-20,574 J
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DESCRIPTI~N
HIGH PRESSURE SODIUM LAMP HAVING IMPROVED EFFICACY
This invention is concerned with high e~ficacy high pressure
sodium (HPS) arc discharge lamps. Such lamps have a non-vitreous~
for example, alumina, arc tube containing a Fill including sodium
and mercury plus a starting gas. The invention is particularly
concerned with improving the ef~icacy of such lamps b~ design changes
which reduce the wall loading, and reduce the average arc current
density while simultaneousl~ maintaining the wall temperature above
about 1100C.
BACKGROUND OF THE INVENTION
It is well known in the prior art that the useful visible
radiation ~rom an arc discharge in a mixture o~ sodium and mercury
vapors is only one of several modes of energy dissipation by such
arcs. In order to optimize the efficacy of a high pressure sodium
lamp incorporating such an arc, it is necessary to minimize all the
non-useful modes of energy dissipation as a result of the collective
effects of such variables as arc temperature, sodium and mercury
pressures, power input per unit length, tube diameter and tube wall
temperatures. As a result of such determinations, we have found that
the present designs o~ HPS lamps9 optimized ~or diameter, wall
loading, sodium and mPrcury pressures by empirical techniques known
to the prior art, suf~er from a number of intrinsic compromises
that have hitherto been unsuspected by the most knowledgeable workers
in the Pield. For instance, we have found that, at constant power
input and sodium pressure for a given size tube, e~icacy increases
with increasing wall temperature by 6 to 10% per 100K. The reason
for this increase is that sel~-absorption of sodium D line radiation
in the sel~reversed portion o~ the line is decreased at constant
sodium pressure as wall temperature T~l increases, because the density
of neutral sodium atoms in the cooler.gas near the walls decreases
as TW increases~ according to PNa/kTW, where PNa is the sodium vapor
pressure and k is 801tzmann's constant. In Figure 1 is shown as a
D-20 ~ 57~ i
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shaded area in a spectral power distribution the additional radiation
which is emitted (at a constant arc tempera~ure and sodium vapor
pressure) at 1500;~ wall temperature in comparison to 1300K. Simul-
taneously, the loss o~ energ~ per unit area from the arc b~ conduc-
tion of heat to the ~all decreases as TW increases, since the tempera-
ture gradient between the arc and the wall decreases. Figure 2
illustrates the measured dependence ~,efficacy as a function of
arc tube wall temperature determined from an experiment in which the
wall temperature o~ a lightl~loaded arc tube was varied by operat;ng
it inside an independently controllable furnace.
Accordingly~ if all other factors were held constant, this
factor would cause the efficac~ to increase as wall loading (power/
unit area of external wall surface) is increased~ because wall tem-
perature increases as wall loading increases. High wall loadings
are best achieved by operating at high power input per u~it o~ arc
length in tubes of small wall diameter. This has tended to dictate
empirically developed designs of HPS lamps operating at or above
about 14 watts/cm~ of wall loading, requiring power input per unit
of arc length of about 30 watts/cm or greater and tube inside
diameters typically less than 1 cm.
The arc temperatures which result from such conditions of
operation are typically of the order of 4000K, and increase with
increasing power per unit length. As a result of our researches,
we have determined that the dependencies on arc temperature of two
of the major useless radiative energy-loss mechanisms of the arc
(in~rared line emission and infrared continuum emission) are sub-
stantia11y greater than that of the useful visible emission in the
sodium D lines. Accordingly, as arc temperature increases, these
two useless energy loss mechanisms increase fas~er than the desired
sodium D emission, decreasing the ratio of useful visible to non-
useful infrared, and with it the ef~ficacy. Accordingly, at constant
wall temperature, constant sodium pressure and constant tube diameter,
e-fficacy would decrease with increasing power per unit length, and
therefore wall loading. Correspondingly, from this factor, efficacy
would increase as the power per unit length and the arc temperature
decrease.
D-20 ,574 J
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Inunediately, therefore, we now recognize an intrinsic compromise
inherent in lamps of the prior art. One factor increases efficacy
with increasing power per unit length and wall loading, another
decreases ef~icacy with increasing p~wer per unit length and wall
loading. It has never been possible to take advantage of the separate
effects o-F increased efficac~ at reduced power per unit length, and
increased efficacy at higher wall temperature, since in prior art
lamps power per unit length and wall temperature have been inexorably
tied together. In fact, since the'wall temperature effect is somewhat
larger than the power/unit length effect, the n~t result in any
practical prior art lamp has been an efficacy which slowly increases
with power per unit 1ength up to the'maximum permitted b~ the tempera-
ture capability of the'arc tube material, w~en measurements are made
at optimum sodium pressure.
The empirical dependence of efficacy on sodium pressure at
constant tube diameter and power per unit length is well known to the
prior art~ and results in a maximum efficacy at that sodium pressure
for which the separation between the red wing and blue wing maxima
of the self-reversed sodium D line is 80 to 100 angstroms. This in
turn results ~rom the competition of two effects, to wit: as sodium
pressure decreases toward ver~ low levels, the lumens per radiated
watt of sodium D radiation approaches a constant 525 lumens/watt,
however, the total sodium D radiation decreases with decreasing sodium
pressure, and hence overall efficacy decreases. On the other hand,
at sodium pressures above the optimum, the concomitant broadening
of the sodium D line results in increasing of this radiation in the
far red and near infrared, tc which the eye is insensitive. Accordingly,
the average lumens per radiated watt of sodium D radia~ion decreases
toward 300 lumens/watt. The total fraction of input energy radiated
in the sodium D line tends to approach a saturation value with
increasing sodium pressure, however; consequently the overall lamp
efficacy must decrease with increasing sodium pressure in this domain.
The maximum of lamp e~ficacy then is found at an optimum pressure
intermediate to the '`low" and "lligh'` pressure domains.
D-20 ,574 `_J
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As a consequence of our researches, we have found that the
optimum sodium pressure for maximum ef~icacy depends on tube diameter
(d) in the fol10wing way. Maximum e~ficacy is found at a D line
separation of 80 to 100 angstroms~ independent of diameter, but the
sodiunl pressure PNa required to yield ~his D line separation decreases
with increasing diameter according to the expression~ PNa is propor-
tional to 1/ ~. We fur~her find that the various modes of energy
loss from the arc depend on sodium pressure and tube diameter at
constant arc and wall temperatures in the following way: !
sodium D radiation per unit len~th
of arc is proportional to P2Nad2,
infrared lines per unit length
of arc is proportional to PNad3/2,
infrared continuum per unit length
of arc is proportional to P2Nad2; and
heat conduction loss per unit length of
arc is approximately independent of PN~ and d.
When PNa is restricted to its optimum value, varying as 1/ ~
the diameter dependencies of the varying mod~s of energy dissipation
at constant arc and wall temperature are:
sodium D radiation per unit length
of arc is proportional to d;
infrared lines per unit length of arc
is proportional to d;
infrared continuum per unit length
of arc is proportional to d; and
heat conduction loss per unit length of
arc is approximately independent of d~
~e see, therefore, that the fraction of input energy dissipated
by heat conduction to the arc tube wall, which amounts in a typical
400 watt HPS lamp of the prior art to approximately one-third of the
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inpu~ power, n~ay be effectively reduced by the use of larger
diameter arc tubes; all radiation losses increase with diameter,
while heat conduction loss remains constant, and thereby becomes
a smaller fraction of the ~otal. Since it is a major non~luminous
energy loss, when the heat conduction ~raction is decreased~ luminous
efficacy must increase, i.e~, luminous efficac~ increases with
increasing tube diameter (provided sodium pressure is adjusted to
the optimum value at each diameter).
Immedia~ely, of course, ~e again see an intrinsic compromise
forced on the lamp designer that has hitherto gone unrecognized by
specialists in the field. As tu~e diameter is increased~ the heat
input to the wall required to maintain a constan~ temperature should
increase in proportion to diameter, but as we have seen, the heat
conduction from the arc, a major component of that heat input, remains
constant. Consequently, without an~ special measures to improve
heat insulation of the wall, the wall temperature will decrease
as the tube diameter incre.ases. Because of the already~described
large dependence of ef~icac~ on wall temperature, the decrease in
wall temperature with increasing tube diameter wipes out and reverses
the gain which would have been observed at constant wall temperature.
Moreover, we note that it is of no value to attempt to maintain
the wall temperature constant by simultaneously increasing the power
input/unit length as diameter is increased. This results in a
greater increase in the useless in~rared lines and continuum than
2~ in the visible sodium D line, because of the increase in arc
temperature required and the higher temperature coe~ficients of
the former.
As a consequence, the effects of power per uni~ length and tube
diameter on efficacy uncovered by our researches have in practical
lamps been negated by the inverse effects of wall temperature and
have remained undiscovered by the many specialists throughout the
world attacking the problem of design of HPS lamps by the usual
empirical techniques.
The results of our investigations can be summarized as follows.
1. Luminous efficac~ increases with increasing wall temperature
(all other factors held constant) E:,ecause of reduced selF-absorption
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of radiation in the center of the sodium D line. Each additional
watt of radiation permitted to escape in this region of the spectrum
contributes abouk 500 lumens to the total luminous output.
2. Luminous efficacy increases as power input per unit length
decreases below that of prior art lamps ~all other factors held
constant) because useless infrared radiatîon is decreased thereby
to a greater degree than the use~ul sodium D radiation. It is to
be noted tha~ this increase in efficac~ with decrease in power per
unit length does not continue indefini~ely to vanishing power per
unit length. The continuing increase in e~icacy is limited and
eventually reversed ~y the fact that the heat conduction loss itself
has a lower coe~ficient of dependence on arc temperature ~han any
radiation loss. At some low power per unit length the energy loss
due to heat conduction becomes too large in comparison to the desired
D line radiation, thus limiting and reversing the increas~ in efficacy.
There is therefore an optimum power per unit length which is in the
vicinity of 20 to 25 watts/cm, substantially lower than the operating
values of many prior art high pressure sodium lamps.
3. Luminous efficacy increases as tube diameter increases
(sodium pressure adjusted for optimum, all other factors held constant)
because useless heat conduction loss is r~duced relative to the useful
radiation loss.
The several energy losses, their functional dependencies and
appropriate magnitude coefficients have been incorporated in a simple ~ !
energy balance to yield the result shown in Figure 3, which is a plot `~
o~ efficacy (normalized to that of the prior art 400 watt lamp, 0.7 cm
in inside diameter) vs power input per unit length, wi~h tube diameter
as a parameter; constant wall temperature and optimum sodium pressure
for each diameter is assumed. In this simpli~ied energy balance
picture, the change in the shape of radial temperature profile of the
arc with diameter is neglected; when this factor is included in a more
detailed calculation, the încrease of efficacy with diameter is not
quite as large, but the trend is identical. The existence of a
maximum in efficacy at an optimum power per u nit length is clearly
visible in th~se calculations; the optimum power per unit length
appears to be in the vicinit~ of 20 to 25 watts/cm, substantially
below the values of man~ prior art lamps.
~O~i59
The concepts and principles stated herein are at
variance with the prior art understanding of the means of
optimizing high pressure sodium lamps for maximurn efficacy.
For example, U.S. Patent 3,906,272 discloses, in Figure 1,
an optimum arc tube inside diameter for each wattage lamp
and design center arc drop; the patent does not recognize
that said optimum diameter results from two competing mechan-
isms which we have discovered and disclose herein. We have
discovered that with suitable thermal insulation to maintain
wall temperatures sufficiently high, efficacy continues to
increase with increasing diameter up to at least double the
diameters disclosed in said patent to be optimum.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plot of sodium resonance radiation in
terms of spectral radiant flux versus wavelength, at wall
temperatures of 1300 K and 1500 K.
FIG. 2 shows relative efficacy as a function of
arc tube wall temperature, at optimum sodium vapor pressure.
FIG. 3 is a plot of relative efficacy of HPS lamps
versus input power (watts) per centimeter of arc length, at
optimum sodium pressure and constant wall temperature (about
1500K), for arc tubes having inside diameters of 2.0, 1.5,
l.l and 0.7 cm.
FIG. 4 shows an HPS lamp in accordance with an
embodiment of the invention.
According to one aspect of the invention there is
provided a high-pressure sodium arc discharge lamp having
improved efficiency comprising a non-vitreous arc tube having
electrodes at its ends, and containing sodium, ~lercury and a
starting gas therein, the inside diameter of said arc tube
having a design value ID (in cm.) large enough to satisfy
the inequality
I < 8 amperes/cm2
f _\
~ 2 ~
where I is the design-center operating current of said lamp
in amperes, the arc length of said arc tube (distances between
electrode tips in centimeters) having a design value AL large
enough to satisfy the inequality
~o~
- 7a -
P < 13 watts/cm2
~ x OD x AL
where P is the design-center operating power of said lamp in
watts, and OD is the outside diameter of said non-vitreous
arc tube in centimeters, equal to the said ID-value plus
twice the wall thickness, and wherein means are provided to
reduce the thermal dissipation per unit area of external wall
surface of said non-vitreous arc tube to a sufficient degree
below that of radiatively-cooled polycrystalline alumina in
a vacuum outer jacket that the temperature of the surface
of the wall of said arc tube is greater than 1100C.
According to another aspect of the invention there
is provided a method of manufacturing a high-pressure sodium
arc discharge lamp having improved efficiency including
the steps of providing a non-vitreous arc tube having elec-
trodes at its ends, and containing sodium, mercury and astarting gas therein, forming the inside diameter of said
arc tube to have a design value ID (in cm.) large enough to
satisfy the inequality
I < 8 amperes/cm2
/ID\~
~ 2~
where I is the design-center operating current of said lamp
in amperes, manufacturing the arc length of said arc tube
(distance between electrode tips in centimeters) to have a
design value AL large enough to satisfy the .inequality
P < 13 watts/cm2
~ x OD x AL
where P is the design-center operating power of said lamp in
watts, and OD is the outside diameter of said non-vitreous
arc tube in centimeters, equal to said ID-value above plus
twice the wall thickness, and providing means to reduce the
thermal dissipation per unit area of external wall surface
of said non-vitreous arc tube to a sufficient degree below
that of radiatively-cooled polycrystalline alumina in a
vacuum outer jacket that the temperature of the surface of
the wall of said arc tube is greater than 1100C.
An embodiment of this invention provides means for
increasing the operating wall temperature of any HPS lamp
which is less than the maximum permitted by the arc tube mate-
_.~ ..! .
~o~
rial (about 1500K for polycrystalline alumina), therebypermitting an increase in efficacy of about 6 to 10~ per
100 K increase in wall temperature. The operating wall
temperature may be increased by improved thermal insulation
of the arc tube or by a reduction in primary thermal radi-
ation and/or heat conduction of the arc tube material.
Means should be provided to maintain the sodium-mercury
amalgam reservoir temperature at the value yielding opti-
mum sodium vapor pressure.
It is also possible to combine the means for increas-
ing the operating wall temperature with arc tubes of substan-
tially larger diameter than prior art arc tubes, in order to
achieve the efficacy gain associated with said larger diameter
by keeping the wall temperature at or near the maximum per-
mitted by the material (about 1500 K for polycrystalline
alumina) in spite of the reduced wall loading. Prior art
arc tubes had arc tube outer diameters of about 0.6 to 1.0
cm and operated (when optimally designed) at wall loadings
of about 14 to 20 watts/cm2. Prior art arc tubes also gener-
ally operated at about 25 to 50 watts per cm of arc length;in this invention, the power consumption per cm of arc length
is generally less.
To demonstrate the changes in lamp design which
result from the teachings of this embodiment, consider a
400 watt HPS lamp, such as has been an article of commerce
since the late 1960's and has not changed substantially in
physical dimensions, materials of manufacture or performance
ratings since about 1973. Such lamps are typically rated
at 50,000 lumens, 125 lumens per watt, and do not, on the
average, exceed that rating in performance. Arc tubes used
by all manufacturers are substantially similar in dimensions.
Thus, such lamps can be considered to have been thoroughly
optimized according to the teachings of the prior art.
Example 1, below, illustrates the comparison between
the performance of a prior art lamp and that of a lamp con-
structed in accordance with the teachings of this embodiment,
employing translucent polycrystalline yttrium oxide (yttria)
as the arc tube material instead of alumina.
Both translucent ceramics have the property of be-
o~
- 8a -
coming opaque in the .infrared spectral region. Alumina be-
comes absorbent between about 4 microns and about 7 microns
wavelength, whereas yttria becomes absorbent between about
7 microns and about 9 m.icrons; thus yttria will intrinsically
thermally radiate less than alumina at temperatures about
1200C.
The thermal radiant emittances of translucent poly-
crystalline yttria arc tubes, such as disclosed in U.S.
Patents 4,147,744 and 4,115,134, have been measured to be
about 0.11, while those of polycrystalline~
"
"
"
,/~
,~
,~
,/
/
D-20 ,574
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alumina are ~ypically 0.20. This permits the yttria arc tube to
reach a higher wall temperature for a given power per unit area
dis~ipation or7 more importantly ~or our purposes, to achieve equal
temperature to an alumina arc tube wall at a lower power per unit
area. Thus we can provide a higher effîcacy lamp by means of a
larger diame-ter~ lower~wall~loaded yttria arc tube maintained at
equal or nearly equal temperature as an arc ~ube designed according
to the prior art.
Example 1
This Invention Prior Art
Arc drop, volts 94.2 100.(typical)
Current, amperes 4.84 4.7 (typical)
Diameter ID, cm 1.~09 0.732
Arc length9 cm 10.12 8.4
Current density*, amp~cm2 4.22 11.17
Wall loading, watts/cm2 OD 9.11 17.05
Arc loading, watts/cm arc
length 39.5 47.6
Wall temperature, C 1090. 1200. (typical)
Lumen output 52720. 49000. (typical)
Efficacy, LPW 132. 123.
% Improvement 7.6
* Averaged over the internal cross section.
Note the substantial reduction in both current density and wall
loading of this lamp in comparison to the prior art lamp, and
the substantial increase in efficacy despite a somewhat lower
wall temperature. It is noted that 3,906,272 does not disclose
an optimum diameter for a prior art 400 watt lamp. However, an
extrapolation of the curves therein to the 400 watt level
confirms that 0.732 cm can be considered very nearly nptimum
according to the prior art.
The wall temperatures cited above and elsewhere in this specifi-
cation are measured by a radiometric method described by deGroot, J.~.,
"Comparison Between the Calculated and the Measured Radiance at the
center of the D-lines in a High Pressure Sodium Vapor Discharge"~
~o~
-- 10 --
Proc. 2nd IEE Conference on Gas Discharges, London, p. 124
(1972)~ This method is believed to have an accuracy of plus
or minus 20 to 30.
Example 2 shows the results for a 150 watt 55 volt
HPS lamp made in accordance with an embodiment of this in-
vention as compared to a 150 watt 55 volt HPS prior art lamp.
The lamp as per this embodiment had an 8 mm inside diameter
yttria arc tube while the prior art lamp had a 5.87 mm inside
diameter alumina arc tube, which is very close to the diameter
of 5.75 mm disclosed in 3,906,272 to be optimum for this lamp.
Example 2
This Invention Prior Art
Arc drop, volts 57.255.(typical)
Current, amperes 3.093.2(typical)
15 Diameter ID, sm 0.8 0.587
Arc length, cm 4.98 4.02
Current density*, amp/cm2 6.15 11.83
Wall loadingl watts/cm OD 9.83 16.07
Arc loading, watts/cm arc
length 30.12 37.3
Wall temperature, C 1120.1150. (typical)
Lumen output 16670.15250. (typical)
Efficacy, LPW 111. 102.
~ Improvement 9.3
* Averaged over the internal cross section.
There is a substantial reduction in both current
density and wall loading of this lamp in comparison to the
prior art lamp, and it has higher efficacy as well, even
though the diameter is 39% greater than the diameter dis-
30 closed in 3,906,272 to be optimum. The efficacy gain for
the lamp of Example 2 is greater than that for Example 1
because the wall temperature of the new lamp in Example 2
is closer to that of the prior art lamp.
Example 3 shows the comparison in efficacy between
a 50 watt lamp according to an embodiment employing an yttria
arc tube for reduced thermal radiative losses, and two dif-
ferent versions, A and B of 50 watt prior art lamps. Prior
art lamp A has been manufactured for only about a year and
has been known to not have been optimized according to the
,',lD "': ~
~2~
known prior art, by virtue of its very low wall loading
and low arc tube wall temperature. Experimental lamps
manufactured according to our embodiment with yttria arc
tubes of identical dimension have substantially increased
arc tube wall temperatures and correspondingly increased
efficacy. Recently announced prior art lamp B represents
an attempt to further optimize the 50 watt lamp according
to the known prior art principles, viz., by decreasing the
arc tube diameter, shortening the arc length, increasing
the wall loading.
Example 3
This Prior Art Prior Art
Invention Lamp A Lamp B
_
Arc tube yttria alumina alumina
Arc drop 54. 52. 52.
Current 1.11 1.18 1.18
ID (cm) 0.477 0.477 0.378
Arc length (cm) 3.08 3.08 2.09
Current density*
(a/cm2) 6.21 6.60 10.51
Wall loading
(W/cm OD) 8.14 8.14 16.04
Arc loading
(W/cm) 16.23 16.23 23.92
Wall temperature 1110C 1000C 1085.
Lumen output 3950. 3400. 4000.(nominal)
Efficacy 79. 68. 80.
% Improvement 16. - -
* Averaged over the internal cross-section.
Optimum diameter for this lamp according to 3,906,272 is 0.335
cm. It should be noted that despite a deviation of more than
40% from said optimum diameter, the lamp according to our
embodiments have equivalent efficacy. Moreover, prior art
Lamp A was deliberately designed at less than optimum wall
loading for alumina in order to improve its lumen maintenance
and ease of manufacture, advantages which are retained by our
lamp but are lost in the more recent prior art lamp B.
Thus far, the specific examples used to illustrate
our embodiments have been employed yttria arc tubes. However,
~ ~ ~r;
- 12 -
other means to reduce thermal radiative losses may also be
used to provide the larger diameter, lower wall loading, lower
arc current density arc tubes that are the subject of this
invention, and that have an arc tube surface wall temperature
above about 1100C., preferably near 1200C, in spite of
reduced heat input per unit area to the arc tube walls.
In example 4, below, we describe the use of infra-
red-reflecting shields to reduce thermal radiative losses.
Exam ~
A conventional 400 watt lamp was constructed with
an alumina arc tube, 7.3 mm inner diameter by 8.9 mm outer
diameter, inside the usual type 7720 glass outer jacket. How-
ever, a quartz sleeve, 29 mm inner diameter by 33 mm outer
diameter, surrounded the arc tube within the outer jacket.
On the inner surface of the quartz sleeve was an infrared
reflective coating of indium o~ide and tin oxide. Lamp
operation is summarized below.
Power 400 watts
Arc tube wall temperature 1257C
Separation of D line peaks 52 angstroms
Efficacy 123.~ LPW
~fficacy corrected for 10%
wall reflection loss 136 LPW
At 400 watts the wall temperature is higher than
25 1200 C normally associated with the conventional 7.3 mm I.D.
design. Thus the quartz sleeve will permit the use of largèr
diameter on tubes. However, the use of such a sleeve provides
two additional glass interferences which the light emitted by
the arc tube has to pass through. A large percentage of the
reflected radiation from the glass interferences is then lost
through absorption within the lamp. If the observed efficacy
of about 124 LPW is corrected for this loss, we see that the
efficacy of the arc tube has increased substantially above
that of the same arc tube mounted without heat conserving
means, and is in fact, substantially greater than the
125 LPW obtainable from prior
D-20,574
12~
13 -
art 400 watt lamps. This increase in efficacy has resulted from the
reduction in self absorption o-~F the sodium D radiation brought about
by the lower sodium atom density near the wall ~hat is a consequence
of the higher wall temperature.
In Example 5, below~ we describe the application of the radiant-
reflector principle oF thermal insulation to an arc tube with a
larger diameter.
Example 5
A lamp ~Lamp C) was made comprising a large diameter alumina arc
tube, 11.0 mm I.D. b~ 12.5 mm O.D. within a cylindrical type 7720
glass outer jacket. There was an infrared reFlective coating, similar
to that of Example 4, on the inner surface of the jacket. PerFormance
of Lamp C was compared with that oF a similar lamp (Lamp ~) without
the in~rared reFlective coating (but with niobium heat shields at the
arc tube ends ~o raise the end temperature, therefore the pressure,
of the sodium-mercury amalgam). Performance of the lamps is
summarized below.
Lamp~C Lamp D
Power, wakts 400 650 700 400 700
Arc tube wall temperature, C 1035 1200 ggo 1035
Separation of D line peaks,
angstroms 25 55 64 28 54
Efficacy, LPW 111.0 137.0139.1 106.3103.4
These results show that the infrared reflective coating raises
the arc tube temperature. A comparison of lumens at similar D
lines indicates the advantage gained from the increase in wall
temperature. Conventionally designed lamps operate at 125 LPW at
400 watts and 135 LPW at 1000 watts. Comparison with Lamp C at
700 watts indicates that higher efficacies can be obtained by this
invention than by utilizing conventional methods oF HPS lamp design,
Lamp C having higher efFicacy at ~00 watts than conventional lamps
at 1000 watts.
As a further illustration of the degree to which our invention
differs from the precepts of HPS lamp~design embodied in the prior
art, we oFfer the data in Table I which shows the dimensions~ average
D-20,574 _.
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- 14
arc curren~ density, wall loading7 and arc loading ~or a number of
high pressure sodium lamps~ encompassing all wattages above 70 watts
presently commercially available, designed according to the ~eachings
of the prior art, where current densi~ = 1j77_( ~)2 ~ wall loading =
P/(~i-X OD x AL) and arc loading ~ P/AL~ where I - tamp current, P-
lamp power, AL = distance between electrode tips and ID, OD - inside
and outside d~ameters respectively.
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An important point to notice ;s the comparison be~ween the 250
and 250S lamps, the latter having been optimized for higher ePFicacy
over the ~ormer according to the ~eachings of the prior art. The
250 watt lamp has a wall loading of 14.6 watts/cm2, an ID of 0.732 cm
and delivers about 26500 lumens, while the 250S lamp has a wall loading
of 19.44 watts/cm2, an ID o~ 0.587 cm ~nd delivers about 29000 lumens.
According ~o 3,906,272, the optimum diameter for this lamp is approxi-
ma~ely 0.55 cm. Thus, the d~rection o~ change o~ dimension parameters
for increased efficacy according to the teachings of the prior art
is toward smaller diameter arc tubes, with a resulting increase in
wall loading. That ~eaching is directly opposite the disclosure o~
this invention.
The lamps in Table I are typically designed ~or maximum efficacy
according to the teachings of the prior art None of the lamps are
designed wi~h a diameter large enoug~ that the current density is
as low as 8.0 amp/cm2. Nor are any o, the lamps designed with a
wall loading as low as 13 watts/cm2. Moreover, the efficacies
indicated appear generally to increase with increasing wall tempera-
ture, and all wall temperatures appear to be in excess oF about
1100C. Thus, we may conclude tha~ the optimum diameters ci~ed in
3,906,272 for each lamp simply represent the largest possible diameter
consis~ent with a minimum wall temperature of 1100C for conventionally
constructed high pressure sodium lamps.
To repeat once more, the central concept o~ our invention is
that still higher efficacies can be obtained at still larger
diameters when suitable steps are taken to reduce the thermal
radiative losses from the arc tube surface so that its temperature
can be maintained above 1100C even though the heat energy input
per unit area o~ wall surface may be reduced.
In a preferred embodiment, a lamp in accordance with this inven~
tion comprises a non~vitreous arc tube 1 having electrodes 2 sealed
into the ends. Arc tube 1 contains sodium, mercury and a starting gas,
typically, xenon. A metal framework 3 provides support for ~he arc
tube and an electrical path to the upper electrode. A support wire 4
is embedded in glass press 5 and provides electrical connection to
the lower electrode. The arc tube assembly is contained within an
outer glass jacket 6. Arc tube 1 was made oP yttria and the results
~or a 150 watt lamp and a 400 watt lamp made in accordance therewith
are shown in Examples 2 and 1 above, respectively.
