CA1203559A - High pressure sodium lamp having improved efficacy - Google Patents
High pressure sodium lamp having improved efficacyInfo
- Publication number
- CA1203559A CA1203559A CA000377021A CA377021A CA1203559A CA 1203559 A CA1203559 A CA 1203559A CA 000377021 A CA000377021 A CA 000377021A CA 377021 A CA377021 A CA 377021A CA 1203559 A CA1203559 A CA 1203559A
- Authority
- CA
- Canada
- Prior art keywords
- arc tube
- arc
- lamp
- wall
- sodium
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J61/00—Gas-discharge or vapour-discharge lamps
- H01J61/02—Details
- H01J61/30—Vessels; Containers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J61/00—Gas-discharge or vapour-discharge lamps
- H01J61/82—Lamps with high-pressure unconstricted discharge having a cold pressure > 400 Torr
- H01J61/825—High-pressure sodium lamps
Abstract
ABSTRACT OF THE DISCLOSURE
The efficacy of a high pressure sodium arc discharge lamp is increased by reducing the wall loading and/or current density of the arc tube. Adequate arc tube wall temperature of at least about 1100°C is maintained by the use of radiant or thermal insulation or infrared reflection films or by the use of an arc tube material having a low thermal radiant emittance. The lamp is constructed from a non-vitreous arc tube having electrodes sealed into its ends. A metal framework provides support for the arc tube and an electrical path to the upper electrode.
The efficacy of a high pressure sodium arc discharge lamp is increased by reducing the wall loading and/or current density of the arc tube. Adequate arc tube wall temperature of at least about 1100°C is maintained by the use of radiant or thermal insulation or infrared reflection films or by the use of an arc tube material having a low thermal radiant emittance. The lamp is constructed from a non-vitreous arc tube having electrodes sealed into its ends. A metal framework provides support for the arc tube and an electrical path to the upper electrode.
Description
D-20,574 J
~Z~3SS~
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 J
S~
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
~_ .
~L~3~9 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
~0355~
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 D-20,574 J
-~2Q3 ~ 5 --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
~Z~3SS~
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 J
S~
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
~_ .
~L~3~9 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
~0355~
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 D-20,574 J
-~2Q3 ~ 5 --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 D-20,574 _J
~3~9 ~ 6 -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.
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 D-20,574 _J
~3~9 ~ 6 -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 _ g _ 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~
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;
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~
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 _.
J
5~
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.
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 _ g _ 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~
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;
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~
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 _.
J
5~
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.
Claims (6)
1. A high-pressure sodium arc discharge lamp having improved efficiency comprising:
(a) a non-vitreous arc tube having electrodes at its ends, and containing sodium, mercury and a starting gas therein, (b) the inside diameter of said arc tube having a design value ID (in cm.) large enough to satisfy the inequal-ity where I is the design-center operating current of said lamp in amperes, (c) the arc length of said arc tube (distances between electrode tips in centimeters) having a design value AL large enough to satisfy the inequality 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 (d) 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.
(a) a non-vitreous arc tube having electrodes at its ends, and containing sodium, mercury and a starting gas therein, (b) the inside diameter of said arc tube having a design value ID (in cm.) large enough to satisfy the inequal-ity where I is the design-center operating current of said lamp in amperes, (c) the arc length of said arc tube (distances between electrode tips in centimeters) having a design value AL large enough to satisfy the inequality 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 (d) 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.
2. The lamp of Claim 1 wherein the arc tube is disposed within an outer jacket and means thermally insulat-ing the arc tube are also disposed within, or upon the inner surface of, said outer jacket.
3. The lamp of Claim 2 wherein the arc tube is made of yttria.
4. A method of manufacturing a high-pressure sodium arc discharge lamp having improved efficiency includ-ing the steps of:
(a) providing a non-vitreous arc tube having electrodes at its ends, and containing sodium, mercury and a starting gas therein, (b) forming the inside diameter of said arc tube to have a design value ID (in cm.) large enough to satisfy the inequality where I is the design-center operating current of said lamp in amperes, (c) 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 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 (d) providing means to reduce the thermal dissi-pation 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.
(a) providing a non-vitreous arc tube having electrodes at its ends, and containing sodium, mercury and a starting gas therein, (b) forming the inside diameter of said arc tube to have a design value ID (in cm.) large enough to satisfy the inequality where I is the design-center operating current of said lamp in amperes, (c) 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 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 (d) providing means to reduce the thermal dissi-pation 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.
5. A method according to claim 4 further including the steps of disposing the arc tube within an outer jacket and thermally insulating the arc tube by means within said outer jacket or upon the inner surface thereof.
6. A method according to claim 4 or 5 further in-cluding the step of making the arc tube of yttria.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US15699680A | 1980-06-06 | 1980-06-06 | |
| US156,996 | 1980-06-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1203559A true CA1203559A (en) | 1986-04-22 |
Family
ID=22561939
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000377021A Expired CA1203559A (en) | 1980-06-06 | 1981-05-07 | High pressure sodium lamp having improved efficacy |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0041721B1 (en) |
| JP (1) | JPH0211717Y2 (en) |
| CA (1) | CA1203559A (en) |
| DE (1) | DE3169958D1 (en) |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1266181A (en) * | 1960-08-26 | 1961-07-07 | Lampes Sa | Clear alumina shell discharge lamps |
| JPS5710543B2 (en) * | 1972-03-16 | 1982-02-26 | ||
| US3906272A (en) * | 1974-04-01 | 1975-09-16 | Gen Electric | Low wattage high pressure sodium vapor lamps |
| GB1597162A (en) * | 1977-03-10 | 1981-09-03 | Ngk Insulators Ltd | Transparent polycrystalline alumina and high pressure vapour discharge lamp |
-
1981
- 1981-05-07 CA CA000377021A patent/CA1203559A/en not_active Expired
- 1981-06-05 JP JP1981083699U patent/JPH0211717Y2/ja not_active Expired
- 1981-06-06 DE DE8181104390T patent/DE3169958D1/en not_active Expired
- 1981-06-06 EP EP19810104390 patent/EP0041721B1/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| EP0041721B1 (en) | 1985-04-17 |
| JPS572567U (en) | 1982-01-07 |
| JPH0211717Y2 (en) | 1990-03-28 |
| EP0041721A2 (en) | 1981-12-16 |
| EP0041721A3 (en) | 1982-09-15 |
| DE3169958D1 (en) | 1985-05-23 |
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| MKEX | Expiry |