Abstract:
An arc lamp with a filter mounted internally has been disclosed. The arc lamp includes an anode, a cathode, a body defining a cavity, wherein the anode and the cathode are inside the cavity, and a filter mounted within the cavity. Other embodiments are claimed and described.

Description:
FIELD OF INVENTION 
     The present invention relates to short arc lamps, and more particularly, to the filter of a short arc lamp. 
     BACKGROUND 
     In optical systems involving the generation and controlled radiation of long or continuous pulses of light, such as spectroscopy, or solar simulation, where high intensity, color correct illumination of sensitive working areas is required, such as in fiber optics illumination devices, it is advantageous to have a light source capable of producing the highest possible light flux density. Products utilized in such applications include short arc inert gas lamps. An existing short arc lamp includes a sealed chamber containing a gas pressurized to several atmospheres, and an opposed anode and cathode defining an arc gap. A window provides for the transmission of the generated light, and a reflector may be positioned surrounding the arc gap. 
       FIG. 1  shows an existing short arc lamp  100 . The short arc lamp  100  includes a window  110 , an external filter  120 , a retainer ring support  125 , a strut  130 , a strut support ring  140 , a cathode  160 , an anode  170 , and a ceramic body  180  having a curved surface  182 , and a base  190 . The window  110  is made of sapphire or quartz. The strut  130 , supported by the strut support ring  140 , braces the cathode  160 . The anode  170  is mounted on the base  190  and is aligned with the cathode  160  along the axis  101 . The ceramic body  180  has a parabolic surface  182  acting as a reflector. The ceramic body  180  is mounted on the base  190  at one end. The external filter  120  is mounted near the window  110  outside of the ceramic body  180  of the lamp  100  to reduce the intensity of light beam with certain wavelengths, such as ultra violet light, infra red light, etc. The retainer ring  125  holds the external filter  120 . 
     One of the problems with the existing short arc lamp is the concentrated beam loading over a small clear aperture area on the filter  120 . Such concentrated beam loading is likely to cause cracking and coating crazing of the filter  120 , which may lead to spectral shifts and light transmission degradation. Moreover, the arc lamp  100  is typically installed within other equipment, such as a projector. Cracking of the filter  120  may damage the equipment within which the lamp  100  is installed. Besides causing property damages, cracking of the filter at high temperature may also cause injuries (e.g., burns, cut, etc.) on the user(s) of the lamp. 
     One of the existing solutions to the above problem is to place the external filter  120  as close as possible to the lamp window  110  in order to reduce beam loading concentration at the center of the external filter  120 . Another existing solution is to put an ultra violet suppression coating on the lamp window  110  in order to reduce the heat and unwanted ozone and ultra-violet light caused by the ultra violet light from the lamp. Furthermore, some existing arc lamps include a hot mirror to reject infra red light, as well as narrow band filters and heat absorbing glass. 
     However, the above techniques do not solve the problem of beam loading concentration satisfactorily because the area of the light beam concentration on the external filter  120  remains about the same. Such concentration in a small area on the external filter  120  still makes the filter susceptible to cracking and coating crazing. Furthermore, the use of various coating on the lamp window  110  also increases the manufacturing cost of the lamp. 
     SUMMARY 
     An arc lamp with a filter mounted internally has been disclosed. The arc lamp includes an anode, a cathode, a body defining a cavity, wherein the anode and the cathode are inside the cavity, and a filter mounted within the cavity. Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the appended claims to the specific embodiments shown, but are for explanation and understanding only. 
         FIG. 1  shows an existing short arc lamp. 
         FIG. 2  shows one embodiment of an arc lamp. 
         FIG. 3  shows one embodiment of an arc lamp. 
         FIG. 4A  shows one embodiment of an internal filter. 
         FIG. 4B  illustrates a cross-sectional view of the internal filter shown in  FIG. 4A . 
         FIG. 5  shows one embodiment of a curved washer. 
         FIG. 6  shows one embodiment of an arc lamp. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known components, structures, and techniques have not been shown in detail in order not to obscure the understanding of this description. 
       FIG. 2  shows one embodiment of an arc lamp. The short arc lamp  200  includes a window  210 , an internal filter  220 , a washer  223 , a strut  230 , a strut support ring  240 , a cathode  260 , an anode  270 , and a body  280  having a curved surface  282 , and a base  290 . The window  210  may be made of sapphire or quartz. The strut  230 , supported by the strut support ring  240 , braces the cathode  260 . The anode  270  is mounted on the base  290  and is aligned with the cathode  260  along the axis  201  to define an arc gap  263  between the cathode  260  and the anode  270 . The body  280  may be made of ceramic. The curved surface  282  of the body  280  may have a reflective coating acting as a light reflector. The curved surface  282  may be parabolic. The ceramic body  280  is mounted on the base  290  at one end. The internal filter  220  is mounted within the cavity defined by the ceramic body  280  of the lamp  200 . In one embodiment, the filter  220  is mounted between the strut  230  and the arc gap  263 . The filter  220  may be mounted closer to the strut  230  than the arc gap  263 . The curved washer  223  preloads the filter  220  into the cavity of the body  280  and holds the filter  220  in place within the body  280 . In one embodiment, an aperture is defined by the filter  220 , through which the cathode  260  is mounted inside the lamp  200 . 
       FIG. 3  shows an alternate embodiment of an arc lamp. The lamp  300  includes a window  310 , an internal filter  320 , a window flange  325 , a strut  330 , a body  380 , abase  390 , a cathode  360 , and an anode  370 . The window  310  is mounted in the window flange  325 , which is coupled to the body  380 . In one embodiment, the body  380  is made of ceramic. The strut  330  braces the cathode  360  at one end and is coupled to the body  380  and the window flange  325  at another end. The body  380  has a reflector surface  382  to reflect light and a flat surface  384 . The anode  370  is mounted in the base  390 , aligned along the axis  301  with the cathode  360 . The base  390  may be integrally attached to the flat surface  384  of the body  380  by brazing. The internal filter  320  is mounted within the cavity defined by the body  380 . In one embodiment, the filter  320  is mounted between the strut  330  and the arc gap  363  between the anode  370  and the cathode  360 . The filter  220  may be mounted closer to the strut  330  than the arc gap  363 . 
     Referring to  FIG. 2 , with the filter  220  mounted within the cavity of the body  280 , power loading by the light beam generated by the anode  270  and the cathode  260  is less concentrated because the light beam covers a larger surface area on the filter  220  than in the existing lamp  100  (referring to  FIG. 1 ). With a larger surface area for power loading, transmission loss and spectral shifts are reduced in the lamp  200 . Because of reduced power loading, the filter  220  is less likely to crack, especially around the aperture  223 . In one embodiment, the internal filter  220  provides four to six times power loading reduction than the existing lamp  100  shown in  FIG. 1 . 
     In one embodiment, the arc lamp  200  is placed near a fiber bundle  299  such that the light beam from the lamp  200  enters the fiber bundle  299  through the entrance  209 . Since the filter  220  is less likely to crack because of less concentrated power loading and the filter  220  is farther away from the fiber bundle  299 , the fiber bundle is less likely to be damaged by cracking of the filter  220 . Moreover, user injury can be avoided by reducing the likelihood of unwanted radiation due to cracking of the filter  220 . 
       FIG. 4A  shows a top view of one embodiment of the internal filter. The internal filter  410  is in the shape of a circular disc. An opening or aperture  420  is defined at substantially the center of the filter  410 . The aperture  420  may also be circular in shape.  FIG. 4B  shows a cross-sectional view of the filter  410 . In one embodiment, the filter has a thickness of approximately 0.043 inches and a diameter of about 1.275 inches. The aperture may have an inside diameter of about 0.125 inches. In one embodiment, the filter  410  is made of quartz. The filter  410  may have an infrared rejection coating on one side and an ultra violet suppression coating on the other side. Alternatively, the filter  410  may be made of narrow bandpass glass or heat absorbing glass. The filter  410  may operate within the temperature range of −40° C. to 500° C. 
       FIG. 5  shows one embodiment of a washer for mounting the internal filter inside the cavity of a lamp body. The washer  500  is curved with an opening  510  defined on the washer  500 . When the filter  220  (referring to  FIG. 2 ) is mounted inside the lamp  200 , the washer  500  is placed between the filter  220  and the strut  230  such that the cathode  260  goes the opening  510 . The washer  500  may be a spring washer. 
       FIG. 6  shows one embodiment of an arc lamp with the strut and the cathode separated from the lamp body and the filter for the purpose of illustration. The components of the lamp illustrated in  FIG. 6  include a body  610 , a filter  620 , a washer  625 , a strut  630 , a strut-holding ring  633 , a cathode  660 , and a window  650 . The filter  620  is mounted inside the body  610  near the top of the body  610 . The filter  620  defines an opening, also referred to as an aperture  623 , substantially centered on the filter  620 . The washer  625  is placed over the aperture  623 . 
     When the lamp  600  is assembled, the cathode held by the strut  630  is inserted into the cavity of the body  610  through the washer  625  and the aperture  623  of the filter  620 . The strut  630  is mounted on or near the end  615  of the body  610  by the strut-holding ring  633 . 
     The foregoing discussion merely describes some exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, the accompanying drawings and the claims that various modifications can be made without departing from the spirit and scope of the invention.