EP0041721A2

EP0041721A2 - High pressure sodium lamp having improved efficacy

Info

Publication number: EP0041721A2
Application number: EP81104390A
Authority: EP
Inventors: John F. Waymouth; Elliot F. Wyner
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1980-06-06
Filing date: 1981-06-06
Publication date: 1981-12-16
Also published as: JPS572567U; EP0041721A3; JPH0211717Y2; CA1203559A; DE3169958D1; EP0041721B1

Abstract

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 arctube 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 arctube material having a lowthermal radiant emittance.

Description

This invention is concerned with high efficacy 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 efficacy of such lamps by design changes which reduce the wall loading, and reduce the average arc current density while simultaneously maintaining the wall temperature above about 1100°C.

BACKGROUND OF THE INVENTION

It is well known in the prior art that the useful visible radiation from an arc discharge in a mixture of 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 of HPS lamps, optimized for diameter, wall loading, sodium and mercury pressures by empirical techniques known to the prior art, suffer from a number of intrinsic compromises that have hitherto been unsuspected by the most knowledgeable workers in the field. For instance, we have found that, at constant power input and sodium pressure for a given size tube, efficacy increases with increasing wall temperature by 6 to 10% per 100°K. The reason for this increase is that self-absorption of sodium D line radiation in the self-reversed portion of the line is decreased at constant sodium pressure as wall temperature T_W increases, because the density of neutral sodium atoms in the cooler-gas near the walls decreases as T_W increases, according to P_Na/kT_W, where P_Na is the sodium vapor pressure and k is Boltzmann's constant. In Figure 1 is shown as a shaded area in a spectral power distribution the additional radiation which is emitted (at a constant arc temperature and sodium vapor pressure) at 1500°K wall temperature in comparison to 1300°K. Simultaneously, the loss of energy per unit area from the arc by conduction of heat to the wall decreases as T_W increases, since the temperature gradient between the arc and the wall decreases. Figure 2 illustrates the measured dependence of efficacy as a function of arc tube wall temperature determined from an experiment in which the wall temperature of a lightly-loaded arc tube was varied by operating it inside an independently controllable furnace.
Accordingly, if all other factors were held constant, this factor would cause the efficacy to increase as wall loading (power/ unit area of external wall surface) is increased, because wall temperature increases as wall loading increases. High wall loadings are best achieved by operating at high power input per unit of 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/cm² 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 4000°K, 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 (infrared line emission and infrared continuum emission) are substantially 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 faster than the desired sodium D emission, decreasing the ratio of useful visible to non-useful infrared, and with it the efficacy. Accordingly, at constant wall temperature, constant sodium pressure and constant tube diameter, efficacy 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.
Immediately, 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 efficacy with increasing power per unit length and wall loading. It has never been possible to take advantage of the separate effects of increased efficacy 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 net result in any practical prior art lamp has been an efficacy which slowly increases with power per unit length up to the maximum permitted by the temperature capability of the arc tube material, when 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 from the competition of two effects, to wit: as sodium pressure decreases toward very 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, to which the eye is insensitive. Accordingly, the average lumens per radiated watt of sodium D radiation 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 efficacy then is found at an optimum pressure intermediate to the "low" and "iiigh" pressure domains.
As a consequence of our researches, we have found that the optimum sodium pressure for maximum efficacy depends on tube diameter (d) in the following way. Maximum efficacy is found at a D line separation of 80 to 100 angstroms, independent of diameter, but the sodium pressure P_Na required to yield this D line separation decreases with increasing diameter according to the expression, P_Na is proportional to 1/√d. We further 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 length of arc is proportional to P² _Nad²;
infrared lines per unit length of arc is proportional to P_Nad^3/2;
infrared continuum per unit length of arc is proportional to P² _Nad²; and
heat conduction loss per unit length of arc is approximately independent of P_Na and d.

When P_Ha is restricted to its optimum value, varying as 1/√d, the diameter dependencies of the varying modes 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,

We 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 input power, may 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 total. Since it is a major non-luminous energy loss, when the heat conduction fraction is decreased, luminous efficacy must increase, i.e., luminous efficacy increases with increasing tube diameter (provided sodium pressure is adjusted to the optimum value at each diameter).
Immediately, of course, we again see an intrinsic compromise forced on the lamp designer that has hitherto gone unrecognized by specialists in the field. As tube diameter is increased, the heat input to the wall required to maintain a constant 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 any special measures to improve heat insulation of the wall, the wall temperature will decrease as the tube diameter increases. Because of the already-described large dependence of efficacy 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 infrared lines and continuum than in the visible sodium D line, because of the increase in arc temperature required and the higher temperature coefficients of the former.
As a consequence, the effects of power per unit 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 efficacy increases with increasing wall temperature (all other factors held constant) because of reduced self-absorption 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 about 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 radiation is decreased thereby to a greater degree than the useful sodium D radiation. It is to be noted that this increase in efficacy with decrease in power per unit length does not continue indefinitely to vanishing power per unit length. The continuing increase in efficacy is limited and eventually reversed by the fact that the heat conduction loss itself has a lower coefficient of dependence on arc temperature than 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 increase 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 reduced 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 of efficacy (normalized to that of the prior art 400 watt lamp, 0.7 cm in inside diameter) vs power input per unit length, with tube diameter as a parameter; constant wall temperature and optimum sodium pressure for each diameter is assumed. In this simplified 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 increase 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 unit length is clearly visible in these calculations; the optimum power per unit length appears to be in the vicinity of 20 to 25 watts/cm, substantially below the values of many prior art lamps.
The concepts and principles stated herein are at variance with the prior art understanding of the means of optimizing high pressure sodium lamps for maximum 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 mechanisms 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, a.t optimum sodium pressure and constant wall temperature (about 7500°K), for arc tubes having inside diameters of 2.0, 1.5, 1.1 and 0.7 cm.
FIG. 4 shows an HPS lamp in accordance with this invention.

THE INVENTION

It is an object of this invention to provide means for increasing the operating wall temperature of any HPS lamp which is less than the maximum permitted by the arc tube material (about 1500°K for polycrystalline alumina), thereby permitting an increase in efficacy of about 6 to 10% per 100⁰K 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 radiation 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 optimum sodium vapor pressure.
Another object of this invention is to combine the means for increasing the operating wall temperature with arc tubes of substantially 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 permitted by the material (about 1500⁰K 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 generally 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 invention, 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 constructed in accordance with the teachings of this invention, employing translucent polycrystalline yttrium oxide (yttria) as the arc tube material instead of alumina.
Both translucent ceramics have the property of becoming opaque in the infrared spectral region. Alumina becomes absorbent between about 4 microns and about 7 microns wavelength, whereas yttria becomes absorbent between about 7 microns and about 9 microns; thus yttria will intrinsically thermally radiate less than alumina at temperatures about 1200⁰ _C.
The thermal radiant emittances of translucent polycrystalline 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 alumina are typically 0.20. This permits the yttria arc tube to reach a higher wall temperature for a given power per unit area dissipation or, more importantly for 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 efficacy lamp by means of a larger diameter, lower-wall loaded yttria arc tube maintained at equal or nearly equal temperature as an arc tube designed according to the prior art.

Example 1

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 optimum according to the prior art.
The wall temperatures cited above and elsewhere in this specification are measured by a radiometric method described by deGroot, J.J., "Comparison Between the Calculated and the Measured Radiance at the center of the D-lines in a High Pressure Sodium Vapor Discharge", 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 this invention as compared to a 150 watt 55 volt HPS prior art lamp. The lamp as per this invention 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

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 disclosed 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 our invention employing an yttria arc tube for reduced thermal radiative losses, and two different 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 known prior art, by virtue of its very low wall loading and low arc tube wall temperature. Experimental lamps manufactured according to our invention with yttria arc tubes of identical dimension have substantially increased arc tube wall temperatures and correspondinly 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

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 invention has 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 this invention have been employed yttria arc tubes. However, 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 1100⁰C., preferably near 1200°C, in spite of reduced heat input per unit area to the arc tube walls.
In example 4, below, we describe the use of infrared-reflecting shields to reduce thermal radiative losses.

Example 4

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. However, 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 oxide and tin oxide. Lamp operation is summarized below.
At 400 watts the wall temperature is higher than 1200°C normally associated with the conventional 7.3 mm I.D. design. Thus the quartz sleeve will permit the use of larger 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 art 400 watt lamps. This increase in efficacy has resulted from the reduction in self-absorption of the sodium D radiation brought about by the lower sodium atom density near the wall that 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. by 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 D) without the infrared reflective coating (but with niobium heat shields at the arc tube ends to raise the end temperature, therefore the pressure, of the sodium-mercury amalgam). Performance of the lamps is summarized below.
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 700 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 arc current density, wall loading, and arc loading for a number of high pressure sodium lamps, encompassing all wattages above 70 watts presently commercially available, designed according to the teachings of the prior art, where current
loading = P/_{(Πx 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 diameters respectively.
An important point to notice is the comparison between the 250 and 250S lamps, the latter having been optimized for higher efficacy over the former according to the teachings 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 of 0.587 cm and delivers about 29000 lumens. According to 3,906,272, the optimum diameter for this lamp is approximately 0.55 cm. Thus, the direction of change of 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 teaching is directly opposite the disclosure of this invention.
The lamps in Table I are typically designed for maximum efficacy according to the teachings of the prior art. None of the lamps are designed with a diameter large enough that the current density is as low as 8.0 amp/cm^2. Nor are any of the lamps designed with a wall loading as low as 13 watts/cm². Moreover, the efficacies indicated appear generally to increase with increasing wall temperature, and all wall temperatures appear to be in excess of about 1100°C. Thus, we may conclude that the optimum diameters cited in 3,906,272 for each lamp simply represent the largest possible diameter consistent with a minimum wall temperature of 7100°C for conventionally constructed high pressure sodium lamps.
To repeat once more, the central concept.of 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 7700°C even though the heat energy input per unit area of wall surface may be reduced.
In a preferred embodiment, a lamp in accordance with this invention 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 the 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 of yttria and the results for a 150 watt lamp and a 400 watt lamp made in accordance therewith are shown in Examples 2 and 1 above, respectively.

Claims

1. A high pressure sodium arc discharge lamp having improved efficiency comprising a non-vitreous arc tube having electrodes at its ends and containing sodium, mercury and a starting gas, the wall loading of the lamp during normal operation being less than about 13 watts per square centimeter of arc tube external wall surface, the arc tube wall temperature during normal operation being greater than about 1100°C.

2. A high pressure sodium arc discharge lamp having improved efficiency comprising a non-vitreous arc tube having electrodes at its ends and containing sodium, mercury and a starting gas, the current density of the lamp during normal operation being less than about 8 amperes per square centimeter of arc tube internal cross- sectional area, the arc tube wall temperature during normal operation being greater than about 1100°C.

3. The lamp of Claim 2 wherein the wall loading of the lamp during normal operation is less than about 13 watts per square centimeter of arc tube external wall surface.

4. The lamp of Claim 3 wherein the arc tube is disposed within an outer jacket and means thermally insulating the arc tube are also disposed within, or upon the inner surface of, said outer jacket.

5. The lamp of Claim 3 wherein the arc tube is made of yttria.