Abstract:
A low-pressure mercury vapor discharge lamp. The lamp includes at least two tube sections substantially parallel to each other for holding a quantity of mercury vapor, sealings for sealing one end of each of two of the tube sections, discharge electrodes for activating the mercury vapor for generating a luminescent discharge, a reduced diameter connecting tube for sealing the remaining ends of the tube sections to one another in successive pairs, and an amalgam for maintaining the mercury vapor pressure at a level which maintains the luminance of the discharge at a substantially constant value.

Description:
This application is a continuation of application Ser. No. 07/055,055, filed May 28, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a low-pressure mercury vapor discharge lamp, and more particularly to a low-pressure mercury vapor discharge lamp made of a single piece glass tube bent into a generally U-shaped configuration. 
     2. Description of the Prior Art 
     The bent type low-pressure mercury vapor discharge lamp such as a fluorescent lamp, basically, has a tubular discharge envelope bent once or several times, which consists of several straight, parallel longitudinal tube sections and at least one transverse connecting tube section. The transverse connecting tube section connects a pair of the longitudinal tube sections so that the longitudinal tube sections are united into a continuous bent tube together with the transverse connecting tube section. 
     Opposite ends of the resultant continuous bent tube are aligned in the same direction and sealed. The sealed ends are provided with stems for supporting discharge electrodes, and covered by a base member which permits the low-pressure mercury vapor discharge lamp to be used in a suitable fixture. The discharge electrodes are electrically connected to terminal pins mounted on the base member. This structure provides a relatively compact low-pressure mercury vapor discharge lamp. 
     Low-pressure mercury vapor discharge lamps are known in which the transverse connecting tube section is curved to a substantially U-shape (e.g., U.S. Pat. No. 4,353,007), or in which the transverse connecting tube section is bent once or several times (e.g., U.S. Pat. No. 4,481,442). 
     Low-pressure mercury vapor discharge lamps are also known in which the transverse connecting tube section is curved and constricted at portions adjacent to the longitudinal tube sections, or in which a straight transverse connecting tube section connects or bridges a pair of longitudinal tube sections at portions near their respective one ends. 
     A problem associated with these known low-pressure mercury vapor discharge lamps is a relatively low temperature region of the mercury vapor in the tubular discharge envelope, within the transverse connecting tube section. 
     For example, one conventional U-shaped type low-pressure mercury vapor discharge lamp (e.g., U.S. Pat. No. 4,353,007) has a curved transverse connecting tube section with a uniform diameter the same as that of the longitudinal tube sections. When this type of low-pressure mercury vapor discharge lamp is turned on, a low temperature region of the mercury vapor tends to occur in a region furthest from the discharge electrodes of the lamp. This occurs because the discharge of the mercury vapor is less activated in this region. 
     The low temperature region of the mercury vapor arises in the curved transverse connecting tube section. This causes the luminance of the low-pressure mercury vapor discharge lamp to be reduced at the low temperature region. Further, the mercury vapor pressure in the low-pressure mercury vapor discharge lamp lowers in accordance with the temperature of the mercury vapor in the low temperature region. As a result, the mercury vapor condenses and results in a so-called black shade, i.e., a film of mercury compound on the glass wall near the low temperature region. 
     Another conventional bent-type low-pressure mercury vapor discharge lamp (e.g., U.S. Pat. No. 4,481,442) has a flat transverse connecting tube section. The resulting discharge envelope has expanded its corner portions between the flat transverse connecting tube section and the longitudinal tube sections. 
     In this type of low-pressure mercury vapor discharge lamp, the discharge of the mercury vapor is less activated in regions near the corners of the expanded corner portions. The temperature of the mercury vapor is lowered at these regions in comparison to the temperature in other regions due to the reduced activation of the discharge. As a result, the luminance of the low-pressure mercury vapor discharge lamp is reduced at the low temperature regions. Morever, the mercury vapor pressure in the low-pressure mercury vapor discharge lamp lowers in accordance with the temperature of the mercury vapor in the low temperature region. The mercury vapor then condenses and results in a black shade film of mercury compound on the corner glass walls near these regions. 
     Referring to FIG. 1, the third conventional curved and constricted type low-pressure mercury vapor discharge lamp 10 has a transverse connecting tube section 11. In the manufacturing process of this type of low-pressure mercury vapor discharge lamp 10, a straight discharge envelope with a diameter DO is heated at its mid portion. Then, the straight discharge envelope is bent at the mid portion by using a mandrel. In the manufacturing process as described above, the discharge envelope tends to be reduced at portions 13a and 13b. On the other hand, the discharge envelope tends to be expanded at a middle portion 12. 
     Thus, the mid portion of the discharge envelope becomes a curved and constricted transverse connecting tube section 11, as shown in FIG. 1. The resulting curved and constricted transverse connecting tube section 11 has an elongated tube portion 12 with a diameter D1 at its mid length portion and a pair of reduced tube portions 13a and 13b, each having the diameter D2, in its end portions adjacent to longitudinal tube sections 14a and 14b, respectively. The diameters have the following relation: D2&lt;DO and D1&gt;DO. 
     In this type of low-pressure mercury vapor discharge lamp 10, the discharge of the mercury vapor is less activated in a region 18 near the elongated tube portion 12. This is because that the mercury vapor pressure lowers in the enlarged diameter portion 12 and the mercury vapor is less activated in a region 15 most far from discharge electrodes. The temperature of the mercury vapor lowers at the region 15 in comparison to the temperature in other regions due to the reduced activation of the discharge. This causes the luminance of the lamp 10 to be reduced at the low temperature region 15. Further, the mercury vapor in the low pressure mercury vapor discharge lamp 10 condenses and results in the so-called black shade, i.e., a film of mercury compound on the glass wall near the region 15, so that the luminance of the lamp 10 is further reduced at the low temperature region 18. 
     Referring to FIG. 2, the fourth bridged type low-pressure mercury vapor discharge lamp 20 has a straight transverse connecting tube section 21. The low-pressure mercury vapor discharge lamp 20 has a pair of longitudinal tube sections 24a and 24b, which are connected together at portions near adjacent ends via the straight transverse connecting tube section 21. Therefore, the longitudinal tube sections 24a and 24b have portions 26a and 26b which extend beyond the straight transverse connecting tube section 21. 
     In this type of low pressure mercury vapor discharge lamp 20, the discharge of the mercury vapor is less activated in regions 25a and 25b in the extended portions 26a and 26b. The temperature of the mercury vapor lowers at the regions 25a and 25b in comparison to the temperature in other regions due to the reduced activation of the discharge. This causes the luminance of the lamp 20 to be reduced at the low temperature regions 25a and 25b. Further, the mercury vapor in the low-pressure mercury vapor discharge lamp 20 condenses and results in the so-called black shade, i.e., a film of mercury compound on the glass wall near the regions 25a and 25b, so that the luminance of the lamp 20 is further reduced at the low temperature regions 25a and 25b. 
     SUMMARY OF THE INVENTION 
     An object of the present invention to provide a bent type low-pressure mercury vapor discharge lamp in which the luminance is not reduced by the conditions described above. 
     Another object of the present invention is to provide a bent type low-pressure mercury vapor discharge lamp in which a low temperature region does not arise. 
     In order to achieve the above objects, a low-pressure mercury vapor discharge lamp according to the present invention is provided with at least two longitudinal tube sections substantially parallel to each other for holding a quantity of mercury vapor, sealings for sealing one end of each of two of the longitudinal tube sections, discharge electrodes for activating the mercury vapor for generating a luminescent discharge, and a reduced diameter transverse connecting tube section for sealing the remaining ends of the longitudinal tube sections to one another in successive pairs, and an amalgam for maintaining the mercury vapor pressure at a level which maintains the luminance of the discharge at a substantially constant value. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will now be described in detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic sectional view of a conventional low-pressure mercury vapor discharge lamp with a discharge envelope bent once; 
     FIG. 2 is a diagrammatic sectional view showing parts of other conventional low-pressure mercury vapor discharge lamps with a discharge envelope bent once; 
     FIG. 3 is a diagrammatic sectional vieW of a low-pressure mercury vapor discharge lamp with a discharge envelope bent once, according to the present invention; and 
     FIG. 4 is a diagrammatic sectional view showing the sealed end of the low-pressure mercury vapor discharge lamp of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the accompanying drawings, i.e., FIGS. 3 and 4. Throughout the drawings, like reference numerals and letters are used to designate like or equivalent elements for the sake of simplicity of explanation. 
     FIG. 3 shows an embodiment of a low-pressure mercury vapor discharge lamp 30 bent once, such as a U-shaped type fluorescent lamp, according to the present invention. The bent type low-pressure mercury vapor discharge lamp 30 has two longitudinal tube sections 31a, 31b and a transverse connecting tube section 32. The two longitudinal tube sections 31a, 31b are aligned in parallel with each other and are connected via the transverse tube section 32 at one end thereof. Thus, the longitudinal tube sections 31a, 31b are united into a continuous tube 33 bent once, by the transverse connecting tube section 32. Opposite ends 34a, 34b of the resulting continuous bent tube 33, i.e., respective other ends of the two longitudinal tube sections 31a, 31b, are sealed. As shown in FIG. 4, the sealed ends 34a, 34b are provided with stems 35a, 35b for supporting discharge electrodes 36a, 36b and covered by a base member 87 which permits the lamp 30 to be used in a suitable fixture. The discharge electrodes 36a, 36b are electrically connected to terminal pins 38a, 38b mounted on the base member 37 for activating luminescent discharge of the lamp 30. 
     The transverse connecting tube section 32 will now be described in detail. The transverse connecting tube section 32 is shaped generally in a U-shape, as shown in FIG. 3. The transverse connecting tube section 32 is so designed that a cross-section of the transverse connecting tube section 32 is at each section less than the cross-sections of the two longitudinal tube sections 31a, 31b. For example, the cross-section of the transverse connecting tube section 32 is gradually reduced from its end portions 39a, 39b adjacent to the longitudinal tube sections 31a, 31b to its mid portion 40. Therefore, the transverse connecting tube section 32 has a cross-section with a minimum diameter D3 at its mid portion 40, and a cross-section with a maximum diameter D5, equal to the diameter of the two longitudinal tube sections 31a, 31b, at its end portions 39a, 39b. Two corner portions 41a, 41b of the transverse connecting tube section 32 have a cross-section with a diameter D4, larger than the diameter D3, but smaller than than the diameter D6. That is, the diameters have the following relation: D3&lt;D4&lt;D5. Two corner portions 41a, 41b  are taken at positions at which the cross-sections have an angle of 60° to the cross section at the mid portion 40. In actual examples, the longitudinal tube sections 31a, 31b are formed to have a cross-section with a diameter of 24 mm. The transverse connecting tube section 32 is formed to have cross-sections with diameters of approximately 20 mm and 22 mm at the mid portion 40 and the corner portions 41a, 41b, respectively. The glass wall of the tube 33 has a uniform thickness, e.g., 1 mm. Thus, the cross-sections of the portions 40, 39a (39b) and 41a (41b) have the inner wall surface diameters 18 mm, 22 mm and 20 mm. 
     The low-pressure mercury vapor discharge lamp 30 further stores a prescribed amount of amalgam 42. One of the stems 35a, 35b formed on the sealed ends 34a, 34b of the longitudinal tube sections 31a, 31b, e.g., the stem 35a, is provided with a thin pipe 43, as shown in FIG. 4. The thin pipe 43 is extended from the stem 35a toward the base member 37 (see FIG. 3). The thin pipe 43 is opened at one end to the interior of the longitudinal tube section 31a, but sealed at its elongated end 44. The thin pipe 43 is further constricted at a predetermined portion 45. The amalgam 42 is stored in the space of the thin pipe 43 between the constricted portion 45 and the sealed end 44. In the actual examples, the amalgam 42 is indium-bismuth amalgam, i.e., an alloy of indium, bismuth and mercury, or indium-bismuth-silver amalgam, i.e., an alloy of indium, bismuth, silver and mercury. The mercury is included in the amalgams at 0.75 to 6 weight %. 
     According to the embodiment of the invention shown in FIGS. 3 and 4, the mercury vapor pressure in the low-pressure mercury vapor discharge lamp 30 is not reduced because the transverse connecting tube section 32 has a progressively reduced cross-section cross-section of positions 39a, 39b, which are equal to the cross-sections of the two respective longitudinal tube sections 31a, 31b toward mid portion 40. Thus, the mercury vapor in the transverse connecting tube section 32 is more activated than the mercury vapor in the longitudinal tube sections 31a, 31b. Therefore, the temperature of the mercury vapor in the transverse connecting tube section 32 is maintained in a level sufficient level. Therefore, the mercury vapor does not condense on the glass wall of the tube 33 and not result in a black shade, i.e., a film of mercury compound. 
     The mercury vapor pressure tends to be increased in the transverse connecting tube section 32 more than in the longitudinal tube sections 31a, 31b. The amalgam 42 in the thin pipe 43 controls the pressure, i.e., a density of the mercury vapor in the tube 33 and prevents an excessive variation or unbalance of the density of the mercury vapor in the low-pressure mercury vapor discharge lamp 30. That is, the amalgam 42 in the thin pipe 43 applies or absorbs the mercury vapor according to the temperature in the low-pressure mercury vapor discharge lamp 30, so that the density of the mercury vapor is maintained at a prescribed value. 
     A Table below shows results of experiments made on samples of the present invention and conventional low-pressure mercury vapor discharge lamps with the rating of 17 W/100 V. The experiments were made on one hundred samples of the present invention and conventional lamps, respectively. In the experiments, the lumen maintenance ratio and the condition of the black shade after 1,000 hours of operation were observed. 
     
                       TABLE______________________________________  Initial Lumen       Black Shade  Luminous          Maintenance (number of samples)  Flux (%)          Ratio (%)   O      Δ                                   X______________________________________Present  100       96          99   1     0InventionConventional    98.6      93          96   3     1Lamp______________________________________ 
    
     In the Table, the initial luminous flux of the present invention was given as 100. The Mark ○ in the Black Shade column shows that no black shade was observed. The Mark Δ shows that a slight black shade was observed. The Mark X shows that a conspicuous black shade was observed. 
     As seen from the Table, the samples of the present invention have the initial luminous flux larger than the samples of the conventional lamp. Further, the luminous flux of the samples of the present invention was higher after 1,000 hours of operation than the samples of the conventional lamp. Also, the samples of the present invention are superior to the the samples of the conventional lamp in the suppression of the black shade. 
     The inventors have also examined samples of low-pressure mercury vapor discharge lamps with different wattage ratings or different sizes. The samples according to the present invention had a more prominent effect than the samples of the conventional lamps. 
     Various modifications and variations may be made in the invention, without departing from the scope or spirit of the invention. For example, the present invention is able to apply to low-pressure mercury vapor discharge lamps consisting of four or more even numbers of longitudinal tube sections, and a necessary number of transverse connecting tube sections for connecting the longitudinal tube sections.