Patent Publication Number: US-7915825-B2

Title: Starting aid for discharge lamp

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Provisional Patent Application No. 60/857,443, filed Nov. 7, 2006. 
    
    
     TECHNICAL FIELD 
     This invention relates to arc discharge lamps and more particularly to starting aids for such lamps. 
     BACKGROUND ART 
     Arc discharge lamps, such as metal halide lamps, are very efficient light sources. Because of the difficulty in beginning the arc discharge various forms of starting aids have been employed. For example, in mercury lamps it has been the practice to use a probe that penetrates the arc chamber. When a voltage is supplied to the lamp a low current discharge occurs between the probe and an adjacent electrode. This low current discharge initiates electrode-to-electrode current conduction, thus starting the lamp. This technique has been used also in metal halide lamps, particularly those having fill pressures below 50 torr of a rare gas. The relatively low pressure leads to arc tube blackening during starting and, additionally, the probe must be electrically neutralized after starting to avoid pinch seal failure by electrolysis. Typically, this is accomplished by means of a bi-metal switch. 
     In recent years the industry has changed to metal halide lamps that use elevated fill pressures that do not use probes. The ballast for this type of lamp produces high voltage starting pulses for ignition. However, these higher-pressure lamps have slow and erratic starting absent some form of starting aid. 
     One form of starting aid employs radioactive Kr85, which is injected into the arc tube as a low percentage of the total rare gas fill, and which act to initiate breakdown. However, use of radioactive materials requires specialized equipment in manufacturing as well as significant expense to document compliance with regulatory agency control in manufacturing and transportation. 
     The starting aid usually employed is a sealed glass capsule containing conditions that generate a burst of UV energy. This technique is described in U.S. Pat. No. 4,721,888 to Proud, et al. In practice, electroded UV capsules such as that disclosed in U.S. Pat. No. 4,818,915 and electrodeless capsules such as that disclosed in U.S. Pat. No. 4,812,714 have been used. While these work very well, they are expensive to manufacture and difficult to automate. Another UV source is disclosed in U.S. Pat. No. 5,323,091 in which the UV source is incorporated into the seal area of the arc tube. 
     DISCLOSURE OF INVENTION 
     It is, therefore, an object of the invention to obviate the disadvantages of the prior art. 
     It is another object of the invention to provide a simple UV source for starting arc discharge lamps. 
     Yet another object of the invention is the provision of a starting aid for arc discharge lamps that is relatively easy to automate. 
     These objects are accomplished, in one aspect of the invention, by an arc discharge lamp having: an arc tube containing an arc generating and sustaining medium and first and second spaced apart electrodes: an envelope surrounding the arc tube and an atmosphere within the envelope; first and second electrical lead-ins sealed within the envelope, the first lead-in being electrically connected to the first electrode and the second lead-in being connected to the second electrode; the improvement comprising: means contained within the envelope and exposed to the atmosphere for generating a UV-producing spark within the envelope atmosphere. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic elevational view of an embodiment of the invention; 
         FIG. 2  is a diagrammatic view of an alternate embodiment of the invention with parts eliminated for clarity; 
         FIG. 3  is a diagrammatic view of another embodiment of the invention; and 
         FIG. 4  is a table comparing various lamp starting times. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings. 
     Referring now to the drawings with greater particularity, there is shown in  FIG. 1  an arc discharge lamp  10  having an arc tube  12  containing an arc generating and sustaining medium  13  and first and second spaced apart electrodes  14 ,  16 , respectively. An envelope  18  surrounds the arc tube  12  and contains an atmosphere  19  within it. The atmosphere is of a composition and pressure that will provide a burst of UV radiation in response to a spark generated within the envelope  18  and, in a preferred embodiment of the invention, is selected from argon or nitrogen (with nitrogen being preferred) at a pressure of from 150 to 400 torr. 
     First and second electrical lead-ins  20 ,  22  are sealed within the envelope  18 , with the first lead-in  20  being electrically connected to the first electrode  14  and the second lead-in  22  being connected to the second electrode  16 , for example, by connector wire  22   a . Means  30  is contained within the envelope  18  and exposed to the atmosphere  19  for generating a UV-producing spark within the atmosphere  19 . As shown in the embodiment of  FIG. 1  the means  30  comprises an isolated pin  24  mounted in the seal area  26  of the arc tube  12  adjacent the first electrode  14 , a resistor  28  electrically connected between the second lead-in  22  and the isolated pin  24 . A spark gap G is formed between the first electrode  14  and the isolated pin  24 . 
     When the lamp is energized with high voltage starting pulses, a spark occurs between the isolated pin  24  and the first electrode  14 . As used herein the term “isolated pin” refers to a metal body sealed into the press seal of the arc tube that does not enter the arc tube chamber. In a preferred embodiment of the invention the isolated pin  24  can be the remnant of the hairpin used to construct the electrode feedthrough of the first electrode  14 . An exemplary embodiment of a hairpin is shown at  16   a  where it forms the second electrode  16 . 
     A space S exists between the first electrode  14  and the isolated pin  24  and the spark gap G is less than the space S. 
     A more particular embodiment is shown in  FIG. 2  wherein the arc tube  12  is supported by a frame member  32  having a first end  34  electrically and mechanically secured to the first lead-in  20  and a second end  36  extending away from the first lead-in  20  and including means  38 , which can be in the form of a pair of straps as is known in the art, engaged with and supporting the arc tube  12 . As in the prior embodiment an isolated pin  24  is sealed in the end  26  of the arc tube  12  adjacent the first electrode  14  and a resistor  28  is electrically connected between the second lead-in  22  and the isolated pin  24 . An electrical connector  40 , which preferably is ribbon shaped, has a proximal end  42  fixed to the frame member  32 , an intermediate portion  44  fixed to the first electrode  14 ; and a distal portion  46  that terminates in the spark gap G in conjunction with the isolated pin  24  thus forming the means  30 . In a preferred embodiment the ribbon is nickel; however, other materials can be used. 
     The invention in its broadest form is shown diagrammatically in  FIG. 3  wherein the means  30  comprises the resistor  28  having a first end  28   a  electrically connected to the second lead-in  22  and a second end  28   b  formed in a spark gap G relation with the first lead-in  20 . 
     This latter embodiment can easily be employed where the arc tube construction does not provide an isolated pin. 
     As mentioned, during starting the igniter of the ballast creates a high voltage pulse that is transmitted across the stem of the leads and then across the spark gap G, causing a small spark to occur. The series resistor  28  limits the current to low values, on the order of 10 milliamperes. While spectral measurements have not been taken, it is believed that the nitrogen spark emits UV from nitrogen molecular bands, which in turn causes liberation of electrons within the arc tube, thus initiating the breakdown process. 
     Test results, shown below in  FIG. 4 , illustrate the efficacy of the starting aid with various spark gaps, no UV enhancer, and a control utilizing a prior art UV enhancer. 
     As can be seen from  FIG. 4 , the lamps with the starting aid employing the spark gap yielded shorter starting times than either prior art construction, or the construction without a UV starting aid, especially in cold environments (last column on the right in  FIG. 4 ) where there is no possible breakdown within the arc tube from the interaction of the argon fill gas and the mercury vapor additive. 
     Lamps constructed as above were evaluated for physical damage when left pulsing for ˜31,000 seconds. To put this in perspective, if the average time to start is 5 seconds, then the lamp will have lasted the equivalent of 6,000 starts. With a normal cycle of 10 hours per start and 20,000 hours rated life, there would be 2,000 starts. At the end of the evaluation period no visible damage to the starter was observed. 
     While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.