HIGH PRESSURE SODIUM LAMP

Abstract: A high pressure sodium lamp includes an arc tube (37) having an inner diameter greater than 12 mm. An amalgam including a prescribed quantity of sodium and mercury is sealed in the arc tube. To achieve the maximum lamp efficiency in different values of the inner diameter of the arc tube, the lamp should satisfy the following equations: where ⊘ (mm) is the inner diameter of the arc tube, X (wt%) is the ratio of the quantity of sodium to the quantity of amalgam sealed in the arc tube, Y (mg/cc) is the quantity of sodium sealed in the arc tube, and ΔV (v/cm) is the electric field strength when the lamp is operated.

Description: The present invention relates to high pressure sodium lamps. In particular, the invention relates to a saturated vapour pressure type sodium lamp having an arc tube wherein an excess amount of mercury and sodium is provided in an amalgam state. Almost all the amalgam is accumulated at the coolest portion of a metal tube attached to one of the ends of the arc tube. A proper amount of rare gas to facilitate starting is also sealed in the arc tube. High pressure sodium lamps have, in general, a high efficiency. The sodium lamps are widely used as light sources for lighting wide areas, e.g., parking places, open spaces, etc. Recently, there has been a greater demand for high efficiency and large luminous flux lamps. Such a high efficiency high pressure sodium lamp may be obtained by increasing the diameter of the arc tube of the lamp (hereinafter referred to as a conventional design technique). In general, the efficiency of the high pressure sodium lamp depends on the inner diameter of the arc tube, the amalgam sodium ratio in the tube and the electric field strength of the tube. The amalgam sodium ratio is the ratio of the sodium in the amalgam to the amalgam which is sealed in the arc tube of the high pressure sodium lamp. The electric field strength is obtained by dividing the voltage applied between a pair of electrodes of the lamp by the length of the arc tube. In conventional high pressure sodium lamps, metal vapour having a proper amount of sodium and a prescribed amalgam sodium ratio which correspond to the inner diameter of the arc tube, as shown in Figure 1 of the accompanying drawings, is sealed in each arc tube. Furthermore, each lamp having the above described arc tube is operated under the prescribed electric field strength corresponding to the inner diameter of the arc tube, as shown in Figure 2. However, if the diameter of the arc tube is increased to achieve a large luminous flux, the lamp efficiency initially increases to a maximum value when the inner diameter of the arc tube is about 12 mm, and then sharply increases as the inner diameter is increased further, as indicated by the dotted curved line in Figure 3. This is because, with the increased of the inner diameter of the arc tube beyond 12 mm, the quantity of sodium vapour, located near the wall of the arc tube, which absorbs D-­ resonance rays from the arc increases while the heat loss from the end portions of the arc tube decreases. As described above, if the conventional design technique is applied to an arc tube having a large inner diameter, the lamp efficiency sharply decreases when the inner diameter is greater than 12 mm. Therefore, the efficiency of the high pressure sodium lamp is adversely affected, and it is difficult to put a high efficiency sodium lamp to practical use. It is an object of the present invention to maintain a substantially stable lamp efficiency when the inner diameter of the arc tube of a high pressure sodium lamp is increased beyond about 12 mm. Accordingly a high pressure sodium lamp is characterised in that the inner diameter of the arc tube is at least 12 mm; and the amalgam substantially satisfies the following equations: where ⊘ (mm) is the inner diameter of the arc tube, X (wt%) is the ratio of the quantity of sodium to the quantity of amalgam sealed in the arc tube, Y (mg/cc) is the quantity of the sodium sealed in the arc tube, and ΔV (v/cm) is the electric field strength when the lamp is operated. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompany drawings, in which:- Figure 1 is a graph showing the relationship between the amalgam sodium ratio and the quantity of sodium in an amalgam sealed in an arc tube for difference values of the inner diameter of the arc tube in a conventional high pressure sodium lamp; Figure 2 is a graph showing the relationship between the inner diameter of the arc tube and the electric field strength of the conventional high pressure sodium lamp shown in Figure 1; Figure 3 is a graph showing the maximum lamp efficiency of one embodiment of the present invention and of the conventional sodium lamp for different values of inner diameter of the arc tube; Figure 4 is a sectional side view illustrating one embodiment of the present invention; Figure 5 is a graph showing the relationship between the lamp efficiency and the electric field strength when the inner diameter of the arc tube is 36 mm; Figure 6 is a graph showing the maximum lamp efficiency versus the electric field strength for the tube of Figure 5 when the quantity of sealed sodium is 0.3 mg/cc, and the amalgam sodium ratio is varied between 3 wt.% and 25 wt%; Figure 7 is a graph showing the maximum lamp efficiency versus the electric field strength for the tube of Figure 5 when the quantity of sealed sodium is 0,75 mg/cc, and the amalgam sodium ratio is varied between 15 wt% and 25 wt%; Figure 8 is a graph showing the lamp efficiency versus the electric field strength for the tube of Figure 5 when the quantity of sealed sodium and the amalgam sodium ratio are varied between predetermined limits; Figure 9 is a graph similar to Figure 8 showing the lamp efficiency compared with the electric field strength when the inner diameter of the arc tube is 12 mm; Figure 10 is a graph similar to Figure 8 showing the lamp efficiency compared with the electric field strength when the inner diameter of the arc tube is 24 mm; Figure 11 is a graph similar to Figure 8 showing the lamp efficiency compared with the electric field strength when the inner diameter of the arc tube is 48 mm; Figure 12 is a graph showing the optimum electric field strength compared with the inner diameter of the arc tube; Figure 13 is a graph showing the relationship between the amalgam sodium ratio and the electric field strength when the inner diameter is 12 mm, and the quantity of the sealed sodium is varied between 0.3 mg/cc and 1.5 mg/cc. Figure 14 is a graph showing the relationship between the amalgam sodium ratio and the electric field strength when the inner diameter of the arc tube is 24 mm, and the quantity of the sealed sodium is varied between 0.3 mg/cc and 1.5 mg/cc; Figure 15 is a graph showing the relationship between the amalgam sodium ratio and the electric field strength when the inner diameter of the arc tube is 36 mm, and the quantity of the sealed sodium is varied between 0.3 mg/cc and 1.5 mg/cc; Figure 16 is a graph showing the relationship between the amalgam sodium ratio and the electric field strength when the inner diameter of the arc tube is 48 mm and the quantity of the sealed sodium is varied between 0.3 mg/cc and 1.5 mg/cc; Figure 17 is a graph showing the optimum relationship between the quantity of the sealed sodium and the amalgam sodium ratio when the inner diameter of the arc tube is 12 mm, 24 mm, 36 mm and 48 mm; and Figure 18 is a graph showing the upper and lower limit values of the electric field strength plotted against the inner diameter of the arc tube when a lamp efficiency greater than that of the conventional lamp is achieved. In general, the following factors are considered to have influence on the efficiency of a high pressure sodium lamp: 1. the inner diameter of the arc tube; 2. the length of the arc tube; 3. the transmittance of the arc tube; 4. the pressure of the rare gas sealed in the arc tube; 5. the quantity of sodium sealed in the arc tube; 6. the amalgam sodium ratio; 7. the electric field strength; and 8. the wall loading of the arc tube during lighting. In the conventional design technique for a high pressure sodium lamp, the pressure of a rare gas sealed in the arc tube and the wall loading are increased within a practical range to enhance the lamp efficiency. However, it has been found that the excessive degradation of the lamp efficiency resulting from the increase of the inner diameter of the arc tube can be avoided by carefully controlling the quantity of the sealed sodium, the amalgam sodium ratio and the electric field strength without increasing the pressure of the rare gas and the wall loading of the arc tube. The inventors of the present invention conducted an extensive series of tests in which the amalgam sodium ratio, the quantity of the sodium and the electric field strength during lighting were varied for different values of inner diameter of the arc tube. As a result, an optimum condition for each variable was discovered. Referring to Figure 4, an outer tube 31 made of glass is provided with a stem 33 at one end. A base 35 is fitted to the one end of the outer tube 31 to seal it. An arc tube 37 is housed in the outer tube 31. Arc tube 37 is made of a light permeable ceramic having a 96 % transmission factor. Both ends of arc tube 37 are individually sealed by niobium sealing caps 39 and 41. One of a pair of electrode support tubes 43, 45 is inserted into each of the individual sealing caps 39, 41, respectively. One of the electrode support tubes 43, 45 also serves as an exhaust tube. Electrodes 47, 49 are individually supported by respective electrode support tubes 43, 45. Thus, the electrodes are positioned at opposite end portions of the arc tube 37. A prescribed quantity of mercury and sodium are sealed in the arc tube 37. Xenon is also provided in the arc tube 37 as a starting rare gas at a prescribed sealed pressure, e.g. 2.67 x 10³ Pa. The outer end of each electrode support tube 43, 45 is supported by a support element 51 through a holder 53. Furthermore, each support element 51 is electrically and mechanically connected to base 35 through a lead wire 55 supported by stem 33. The following experiment was carried out with respect to an individual high pressure sodium lamp. A 6 KW rated sodium lamp having an arc tube inner diameter of 36 mm, a separation distance between the pair of electrodes of 256 mm, a sealed amalgam sodium ratio of 15 wt% and 0.175 mg/cc of sealed sodium was tested. The relationship between the electric field strength and the lamp efficiency was obtained, as shown in Figure 5, when the lamp was lit under different values of electric field strength, which varied in response to the temperature change of the coolest portion of the arc tube of the lamp. As can be understood from Figure 5, the optimum electric field strength which gave the maximum lamp efficiency was 4.5 v/cm in the lamp constituted as described above. The optimum electric field strength in comparison to an amalgam sodium ratio which varied between 3 wt% with 25 wt% was observed in a lamp with an arc tube inner diameter of 36 mm, a separation distance between the pair of electrodes of 256 mm, and 0.3 mg/cc of sealed sodium. The results are shown in Figure 6. Similarly, a third experiment was carried out wherein 0.75 mg/cc of sealed sodium was contained in the lamp, and the amalgam sodium ratio was varied between 3 wt% and 25 wt%. The results are shown in Figure 7. In figures 6 and 7, the points of the maximum efficiency for each amalgam sodium ratio are connected to one another by a dotted line. The dotted curved line in Figure 6 coincides with the dotted curved line in Figure 7. As can be understood from the above description, the relationship between the electric field strength and the maximum lamp efficiency follows a similar curved line, even though the quantity of the sealed sodium has changed. A similar experiment was carried out with regard to various different quantities of sealed sodium between 0.5 mg/cc and 1.5 mg/cc, as shown in Figure 8. In Figure 8, all maximum lamp efficiency values obtained in the previous experiments are plotted. From Figure 8, the specific relation between the lamp efficiency and the electric field strength may be confirmed. That is, the lamp efficiency varies in accordance with changes of the amalgam sodium ratio and the quantity of sealed sodium. However, in each case, the curved line connecting the maximum lamp efficiency values has a peak value when the electric field strength is 4.5 v/cm, as shown in Figure 8. Figures 9, 10 and 11 show how the lamp efficiency changes compared with the electric field strength for each experiment when the inner diameter of the arc tube of the high pressure sodium lamp is varied between 12 mm and 48 mm. In each experiment, a plurality of lamps were used to observe the maximum efficiency of each lamp. The amalgam sodium ratio was varied between 5 wt% and 80 wt%, and the quantity of the sealed sodium was also varied between the 0.3 mg/cc and 1.5 mg/cc. In Figure 9, the points of maximum lamp efficiency for each of a plurality of lamps are connected to one another by a solid line. Each lamp had an arc tube inner diameter of 12 mm. Figures 10 and 11 show graphs similar to Figure 9 with regard to lamps having arc tube inner diameters of 24 mm and 48 mm, respectively. According to the experiment described above, it is necessary to optimise the amalgam sodium ratio, the quantity of the sealed sodium and the electric field strength to achieve the maximum lamp efficiency. As shown in Table 1, the relationship between the maximum lamp efficiency and the optimum electric field strength for arc tubes of different inner diameter are obtained from Figures 8, 9, 10 and 11. Figure 12 shows a graph illustrating the relationship between the inner diameter of the arc tube and the optimum electric field strength shown in Table I. The graph shown in Figure 12 may be expressed by the following equation: logΔV = -0.304 log ⊘ + 1.13 ...(1) wherein V is the electric field strength, and ⊘ is the inner diameter of an arc tube. Next, a plurality of high pressure sodium lamps wherein the amalgam sodium ratio was varied between 5 wt% and 80 wt%, and the quantity of the sealed sodium was also changed between 0.3 mg/cc and 1.5 mg/cc were manufactured. Each of the previous experiments was carried out on the above described lamps to observe the optimum amalgam sodium ratio and the optimum quantity of the sealed sodium corresponding to the maximum lamp efficiency. Figures 13, 14, 15 and 16 show the result of these experiments. As stated before, since the value of the optimum electric field strength compared with the inner diameter of the arc tube is approximated by the above-­ described equation (1), the intersection of each solid line and the perpendicular line from the value of the optimum electric field strength indicated by a dotted line, as shown in Figures 13, 14, 15 and 16, indicated the optimum amalgam sodium ratio and the optimum quantity of the sealed sodium for achieving the maximum lamp efficiency for each inner diameter of the arc tube. The following Table II shows the amalgam sodium ratio for achieving the maximum lamp efficiency when the quantity of the sealed sodium is changed between 0.30 mg/cc and 1.50 mg/cc for each inner diameter of the arc tube tested. Table II corresponds to Figures 13, 14, 15 and 16. As can be understood from the above description, for example, when the inner diameter of the arc tube is 36 mm, the quantity of sealed sodium is 1.5 mg/cc, and the amalgam sodium ratio is 26.9 wt%, the maximum lamp efficiency is achieved when the lamp is lit under approximately 5 v/cm of electric field strength, as shown in Figure 15. Figure 17 shows the relationship between the quantity of sealed sodium and the amalgam sodium ratio when the maximum lamp efficiency is achieved for each of four different inner diameter arc tubes. As can be seen from Figure 17, the above described relationship is approximated by the following equation: log Y = B log X + C ...(2) where Y is the quantity of the sealed sodium, X is the amalgam sodium ratio, and B and C are constants. B and C for each inner diameter of the arc tube are obtained from the above described experiments, as shown in Table III. The following equations are derived from TABLE III. log B = (-0.717) log ⊘ + 0.848 ...... (3) log (-C) = (-0.200) log ⊘ + 1.033 ...... (4) Therefore, the following equation is obtained by substituting equations (3), (4) into equation (2). log Y = 10⁰·⁸⁴⁸ ⊘ (-0.171) log X -10¹·³³ ⊘ (-0.200) ........... (2)′ As can be understood from the above description, the maximum lamp efficiency is achieved for each inner diameter of the arc tube when the following equations are satisfied: logΔ V = -0.305 log ⊘ + 1.13................... (1) log Y = 10⁰·⁸⁴⁸ ⊘ (-0.171) log x -10¹·³³ ⊘ (-0.200) .......... (2)′ The lamp efficiency of the high pressure sodium lamp which satisfies the above described equations (1) and (2)′ is indicated by the solid line in Figure 3. On the other hand, the lamp efficiency of a conventional high pressure sodium lamp is indicated by dotted line in Figure 3. As will be clearly understood from Figure 3, enhancement of the lamp efficiency is achieved in the high pressure sodium lamp of this embodiment. A lamp efficiency higher than that of the conventional lamp may be achieved from Figure 3. As can be understood from Figure 3, when the inner diameter of the arc tube exceeds 48 mm, the lamp efficiency thereof has to achieve at least 59 1 m/w in order to exceed the lamp efficiency of the conventional lamp. Likewise, when the inner diameter exceeds 36 mm, the lamp efficiency has to achieve at least 91 1m/w. When the inner diameter exceeds 24 mm, the lamp efficiency has to achieve at least 124 1m/w. When the inner diameter exceeds 12 mm, the lamp efficiency has to achieve at least 157 1m/w. Each electric field strength range in which a lamp efficiency greater than the above described lower limit values is achieved is shown in Table IV which is derived from Figures 8, 9, 10 and 11. The upper limit value and the lower limit value of the electric field strength for each inner diameter of the arc tube shown in Table IV is approximated with the following equations: logΔVl = -7.97 log ⊘ + 14.8 (lower limit value) logΔVu = 84.7 log ⊘ - 84.5 (upper limit value) As can be understood from the above-equations, a suitable electric field strength range greater than that of the conventional lamp should satisfy the following equation: -7.97 log ⊘ + 14.8 < logΔV < 84.7 log ⊘ - 84.5 .... (A) Table V shows the range of the amalgum sodium ratio (X) and the range of the quantity of the sealed sodium (Y) corresponding to the above described upper and lower limit value of the electric field strength for each inner diameter of the arc tube. Each range shown in Table V is derived from Figures 13, 14, 15 and 16. The relationship between the amalgam sodium ratio and the quantity of the sealed amalgam against each inner diameter of the arc tube shown in TABLE V is approximated with the following equations: 4.55 log X - 6.64 < log Y < 4.55 log X - 6.23 when the inner diameter of the arc tube is 12 mm; 4.14 log X · 6.00 < log Y < 4.14 log X = 4.50 when the inner diameter is 24 mm; 3.84 log X - 7.01 < log Y < 3.84 log X - 3.52 when the inner diameter is 36 mm; and 3.63 log X - 6.98 < log Y < 3.63 log X - 2.91 when the inner diameter is 48 mm. Furthermore, the above-described equations also may be expressed by the following equation with regard to the inner diameter of the arc tube: As stated above, when the inner diameter of the arc tube of a high pressure arc lamp is greater than 12 mm, the lamp efficiency thereof may exceed that of the conventional lamp if the contents of the high pressure arc lamp, e.g., the amalgam sodium ratio and the quantity of the sodium satisfy the above-described equations (A) and (B). In the above-described embodiment, each experiment was carried out with regard to inner diameters of 12 mm, 24 mm , 36 mm , and 48 mm. However, the present invention may be applied to any sodium lamps which have inner diameters greater than 12 mm. The present invention has been described with respect to a specific embodiment. However, other embodiments based on the principles of the present invention will be obvious to those of ordinary skill in the art. Such embodiments are intended to be covered by the claims.

Claims: 1. A high pressure sodium lamp for operation under a prescribed electric field strength comprising an arc tube having electrode means for generating an arc therein; and an amalgam including a prescribed quantity of sodium and mercury sealed in the arc tube; characterised in that the inner diameter of the arc tube is at least 12 mm; and the amalgam substantially satisfies the following equations: where ⊘ (mm) is the inner diameter of the arc tube, X (wt%) is the ratio of the quantity of sodium to the quantity of amalgam sealed in the arc tube, Y (mg/cc) is the quantity of the sodium sealed in the arc tube, and V (v/cm) is the electric field strength when the lamp is operated. 2. A high pressure sodium lamp as claimed in claim 1, characterised in that the arc tube is of a light permeable ceramic material.

Inventors: HIROSE, YOSHIO, IKE, YUUZIROU, KAMIYA, AKIHIRO, OUSIMA, SINICHI, YONEZAWA, AKIHIRO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: H01J61
IPC codes: H01J61