Patent Publication Number: US-6909250-B2

Title: Apparatus and method for operating a portable xenon arc searchlight

Description:
RELATED APPLICATION 
     The present application is a division of U.S. patent application Ser. No. 09/440,105 filed on Nov. 15, 1999, now U.S. Pat. No. 6,702,452, to which priority is claim pursuant to 35 USC 120. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to xenon arc lamps and in particular to compact or handheld xenon short arc searchlights or illumination systems. 
     2. Description of Prior Art 
     Handheld lighting devices with focused beams or spotlights or searchlights, whether battery-powered or line-powered, are commonly used by military, law enforcement, fire and rescue personnel, security personnel, hunters and recreational boaters among others for nighttime surveillance in any application where a high intensity spotlight is required. The conditions of use are highly varied, but generally require the light to deliver a desired field of view at long distances, be reliable, durable and field maintainable in order for it to be practically used in the designed applications. Typically the light is hand carried and must be completely operable using simple and easily access manual controls which do not require the use of two hands. 
     In prior art xenon short-arc searchlights or illumination systems, whether handheld, portable or fixed mounted, the luminance distribution of the arc has been positioned facing in the direction of the beam (cathode to the rear), to provide a uniform beam pattern when the arc is at the focal point of the parabolic reflector. When the luminance distribution of the arc is positioned in this manner, a majority of the light output is collected in the low magnification section of the reflector and in a slightly divergent manner in the far-field. When the beam is diffused into a flood pattern, a large un-illuminated area or “black hole” is projected. Reversing the lamp position so that the full luminance distribution of the arc is in the high magnification section of the parabolic reflector produces a more concentrated beam in the near- and far-field and hence greater range can be achieved. Additionally, when the beam is diffused into a flood pattern no characteristic “black hole” of prior art configurations is produced. When the arc is moved slightly beyond (or slightly rearward of) the reflector&#39;s focal point, the combination of a placing all available light in the high magnification section of the reflector and collecting it in a slightly convergent manner produces roughly twice the operating range as a conventional anode-forward device. 
     The operation of the xenon arc lamp requires a power supply capable of supplying a regulated current to insure ignition of the lamp and maintenance of its operation. Typically three voltage are required to ignite an arc lamp, bring it into operation and maintain its operation, namely: (1) a high voltage RF pulse applied across the lamp electrodes to ignite or break down the non-ionized xenon gas between the lamp electrodes; (2) a second voltage higher than the operating voltage of the lamp to be applied across the lamp electrodes at the time the high voltage radio frequency (RF) pulse is applied in order to establish a glowing plasma between the electrodes; and (3) a lower voltage to sustain the flow of plasma current at a level sufficient to create a bright glow after the lamp has been ignited. 
     In prior art battery powered searchlights, large high voltage transformers and large storage capacitors have been required to generate a high voltage current of sufficient magnitude to power the lamp&#39;s ignition. A separate voltage boosting circuit for generating the second voltage to establish the plasma adds to the size, weight and component count of the lamp circuitry. The resulting circuitry in prior art has traditionally been less than optimum, with excessive energy lost to heat, and relegating battery running times to less than desirable. 
     Therefore, what is needed is an optical assembly to increase light collection efficiently and dissipate associated heat to produce a significantly more concentrated beam and a circuit topology by which the arc lamp regulated current can be supplied, but with a reduction in the size, weight and component count of the lamp circuitry and at high circuit efficiency to maximize battery life and minimize heatload. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a searchlight for generating a beam of light comprising an arc lamp, high-efficiency electronic ballast circuitry coupled to the arc lamp, a wide range power supply plus an internal battery and battery charger coupled to the ballasting circuit for powering the ballasting circuit and the arc lamp. A single converter circuit is used both for battery charging from an external power source and ballasting an arc lamp. In the illustrated embodiment the arc lamp is a xenon arc lamp, but it expressly is intended to include other kinds of plasma lamps, including without limitation metal halide and halogen lamps. In addition, although the invention is described in terms of a portable battery powered light, nonbattery-powered or line-powered lights in fixed configurations are within the express scope of the invention. For example, the use of the claimed light in aircraft and vehicular systems is included as is simple security lighting in a fixed site. 
     The invention is characterized as a searchlight comprising a lamp, a reflector disposed about the lamp to reflect light generated by the lamp, a lamp holder to position the lamp precisely along the reflector&#39;s axis of optical symmetry, a reflector positioner so that the reflector is selectively moved by user with respect to the searchlight while the lamp remains fixed relative to the searchlight, and a lamp circuit coupled to the lamp for powering and controlling illumination produced by the lamp. 
     The lamp is a xenon arc lamp having an anode and cathode. The xenon arc lamp is mounted within the searchlight so that the anode of the xenon arc lamp is in the rearward position relative to the direction of a beam projected by the searchlight so that field illumination of the beam is slightly convergent and more concentrated and therefore delivers much longer range of operation. This orientation is unique in searchlight and illumination systems employing xenon short arc lamps. 
     The lamp is affixed in a lamp holder that allows precision alignment, and is designed to be quickly replaceable. The lamp module locks into a fluted heat sink to conductively dissipate lamp heat from the anode, as opposed to radiating heat in conventional anode-forward searchlights. 
     The reflector has an optical axis of symmetry. The lamp is positioned on the optical axis of symmetry. The reflector positioner moves the reflector in two opposing directions along the optical axis of symmetry. The lamp is radially adjustable relative to the reflector to be disposed on the optical axis of symmetry. The radial adjustment of the lamp on the optical axis is field adjustable. The reflector positioner retains the relative position of the reflector with respect to the lamp at a last relative position between the lamp and reflector which was selected when last using the searchlight. Thus, the design has a last use memory for the beam focus or adjustment. 
     The lamp, reflector, and reflector positioner are removable from the lamp housing as a unit to allow different reflector materials (for example nickel rhodium, aluminum, gold) to be easily substituted for maximum reflectivity depending on specific applications. The searchlight comprises a housing for containing the lamp, lamp circuit, reflector and reflector positioner. 
     The invention is still further characterized as a searchlight comprising a housing; a lamp disposed within the housing, a lamp circuit disposed within the housing, and a reflector disposed within the housing. The housing is characterized by a mounting fixture adapted to permit quick field coupling to a second device so that movement of the housing to direct the beam from the lamp is integrally manipulated with the second device. 
     The searchlight further comprises a searchlight housing in which the battery is included with the battery charging circuit, the ballasting circuit and the arc lamp as a single unit. 
     The electronic ballast circuitry is comprised of a converter and igniter. The converter has an output coupled across the arc lamp for providing a converted direct current (dc) current and voltage to the arc lamp. The igniter is coupled across the arc lamp to provide a high voltage RF ignition current to the arc lamp. The converter is controlled by a smooth variation of current and voltage to the arc lamp to correspondingly smoothly vary light output from the arc lamp between high and low intensities. By “smooth variation” it is meant that the changes in intensity of the lamp can be made very small so that they are not or are almost not visually perceptible by an ordinary human observer. The converter is controlled to provide the smooth variations between high and low intensities by a multiplicity of small digital current steps. Alternatively, the converter is controlled to provide the smooth variations between high and low intensities by an approximate or digitally simulated analog variation in current intensity provided to the arc lamp. The ballasting circuit is controlled by a control circuit to turn the arc lamp on after ignition at minimum intensity level of operation. 
     The searchlight further comprises a handle with a mounting formed as part of the housing to allow portability for the searchlight and for mounting to the second device. The mounting is a tripod mount so that the portable searchlight may be fixed in the field to a tripod with the second device. The mounting on the handle is a thumb screw mount to permit mounting of an optical detection device onto the searchlight and rigidly fixed to the housing 
     The searchlight further comprises a field changeable filter disposed on the searchlight to select frequency ranges transmitted in the beam to a selected frequency range depending on application. The filter is selected to permit transmission of light in the beam through the filter for illumination in one of the environments comprised of illumination in a smoky environment, for infrared illumination, for underwater illumination, for ultraviolet or any specific color in the visible range. The filter can also be selected for reduction of intensity of the beam from the searchlight to present a minimum intensity output in the beam below which the arc lamp could not operate but for the filter. 
     The invention and its various embodiments may now be visualized by turning to the following drawings where in like elements are referenced by like numerals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the assembled light. 
         FIG. 1   a  is a bottom elevational view of the assembled light of FIG.  1 . 
         FIG. 1   b  is a rear elevational view of the assembled light of  FIGS. 1 and 1   a.    
         FIG. 2  is a side cross-sectional view of the light of  FIG. 1  showing the interior components in an assembled configuration. 
         FIGS. 3   a - 3   d  are depictions of the anode-rear positioning and the consequent benefit as compared to prior art anode-forward positioning. 
         FIG. 3   a  is a depiction of the luminance distribution of an arc from a xenon short arc lamp in a horizontal position. 
         FIG. 3   b  is simplified diagram of a parabolic reflector depicting the focal point and high magnification area of the reflector. 
         FIG. 3   c  illustrates how anode-rear positioning of a short-arc lamp places the luminance distribution in the high magnification area of the reflector. 
         FIG. 3   d  is a graphical comparison of the illuminance of a 75 W xenon short arc lamp in an anode-rear vs. anode-forward position. 
         FIG. 4  is a partially cutaway bottom view of the light of  FIG. 1  showing the relationship of the battery, the circuit board, the lamp and the reflector in an assembled configuration. 
         FIG. 5  is a simplified exploded view of selected components of the searchlight of the invention. 
         FIG. 6  is a perpendicular cross-sectional view of the searchlight of the invention as seen through section lines  5 — 5  of FIG.  2 . 
         FIG. 7  is a perpendicular cross-sectional view of the searchlight of the invention as seen through section lines  6 — 6  of FIG.  2 . 
         FIG. 8  is a simplified graph of the current as a function of time in a xenon arc lamp. 
         FIG. 9  is a simplified graph of the voltage as a function of time in a xenon arc lamp. 
         FIG. 10  is a simplified schematic diagram of the pulse width modulator, converter and ignition circuit of the arc lamp of the invention. 
         FIG. 11  is a simplified schematic diagram of the power supply circuit of the invention. 
         FIG. 12  is a simplified schematic diagram of a lamp current sensing circuit of the arc lamp of the invention. 
         FIG. 13  is a simplified schematic diagram of a reference voltage circuit of the invention. 
         FIG. 14  is a simplified schematic diagram of a programmed logic device in the circuit of the arc lamp of the invention. 
         FIG. 15  is a simplified schematic diagram of a battery charging circuit of the arc lamp of the invention. 
         FIG. 16  is a side cross-sectional view of a printed circuit board showing multiple conductive paths for high current circuit segments. 
     
    
    
     The invention now having been illustrated in the foregoing drawings, turn now to the following detailed description of the preferred embodiments 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A xenon arc searchlight or illumination device incorporates a circuit that both provides for lamp ballasting and charging of the system battery from an external power source. The tolerance to variations in the system supply voltage as well as external voltage are increased by providing logic control of the converter circuit through a programmed logic device (PLD). The intensity of the arc lamp is smoothly decreased or increased in a continuous manner from a maximum intensity to a minimum intensity beam. Ignition of the lamp at its minimum illumination levels is thereby permitted. The lamp beam is narrowed or spread by relative movement of a reflector with respect to the lamp by advancing or retracting the reflector along its optical axis of symmetry on which the lamp is also aligned. The reflector has short focal length of the order of magnitude of approximately 0.3-0.4 inch which maximizes collection efficiency and beam collimation. The lamp is designed so that the lamp, reflector and battery assemblies are easily field replaceable without tools. The lamp, ballast, battery and charger are provided in a single rugged package which is sealed for field use. The searchlight is combined by an appropriate mounting adaptable with other optical detector devices such as cameras, binoculars and night vision telescopes. The beam output is similarly usable with a combination of filters to allow the most varied intensity and wavelengths for a particular application, such as smoke filled environments, surveillance employing near-infrared or infrared illumination, underwater, ultraviolet or any color in the visible range illumination. The xenon arc lamp is oriented within the searchlight with respect to the reflector to provide the most concentrated and convergent field of illumination on which the lamp is capable, namely with the anode of the lamp turned away from the forward beam direction in the reflector. 
       FIG. 1  is a perspective view of searchlight  1   1  which shows a body  232 , an integral handle  306  in which a mounting hole  304  is defined, a heat sink  278  and a rotatable bezel  298  in which a faceplate  299  is fixed. Pushbutton switch  88  is disposed into body  232  just forward of handle  306  where a user&#39;s thumb would normally be positioned when holding searchlight  11  by handle  306 . Pushbutton switch  88  is a sealed momentary contact switch which may be provided with an internal LED which is lit when searchlight  11  is operating and may indicate different modes of operation (on; flashing for charging, solid for full charge, intermittent flash for float charge, etc.). Searchlight  11  is a compact, rugged, and portable battery powered light about the size of a large flashlight or lantern that can produce an adjustably collimated, and adjustable high intensity beam of light for more than a mile in clear atmospheric conditions. 
     Turn now to the exploded assembly drawing of the mechanic elements of the searchlight  11  as depicted in FIG.  5 . Elements of the searchlight  11  have been omitted from the drawings for the sake of simplicity of the illustration. The searchlight  11  includes a housing  232  shown in cut-away perspective view in  FIGS. 2 and 4 . A base plate  234  is provided behind which is a space  236  which carries the battery  237  for searchlight  11  as shown in  FIGS. 2 and 4 . Base plate  234  is mounted to housing  232  through molded end standoffs  238  one of which is shown in FIG.  4 . The molded battery wall  240  integrally extends through standoffs  242  through holes  244  and U-shaped indentation  246  defined through circuit board  234  shown in FIG.  5 . 
     Battery  237  is accessible through the rear of housing  232  as shown in  FIG. 1   b . Three screws  308  fasten a circular rear plate  310  to housing  232 . A recessed electrical connector  312  is provided in rear plate  310  through which an external power supply may be connected either to operate searchlight  11 , to recharge battery  237  or both. Electrical connector  312  is recessed to provide a rugged configuration so that the connector will not be damaged by rough handling. 
     Housing  232  incorporates a housing mounting hole  302  as shown in  FIG. 1   a  on its bottom surface, an integral handle  306  and a hole  304  defined in handle  306  for receiving a handle mount with a thumb screw (not shown) with which to mount or stack another device such as a camera, binoculars, night vision scope and the like on top of searchlight  11 . In this manner two units may be used in combination, namely the searchlight of the invention moved or manipulated as a single unit with an optical detection device of some sort. The entire assembly may also be place on a support tripod or mount using the housing mounting hole  302  shown in  FIG. 1   a.    
     Transformer  68  mounts onto base plate  234 . Circuit board  248  is carried on a plurality of standoffs  250 , which is shown in  FIGS. 2 and 5  for the mounting of a resilient spring assisted connector  252  which engages anode nut  254  disposed onto the anode terminal  256  of xenon lamp  66 . The opposing pin  258  of the resilient spring assisted connector  252  shown in  FIG. 2  is disposed through circuit board  248  and secured thereto by means of a push nut  260 . Pin  258  of the resilient spring assisted connector  252  is then connected by a wire or means not shown to transformer  68 . A banana plug receptacle  262  is similarly connected by a wire or means not shown to lamp ground  62  of FIG.  10 . Banana plug  263  as shown in  FIG. 5  is connected by a wire not shown to the cathode of  264  of lamp  66  shown in FIG.  2  and is plugged into banana plug receptacle  262 . 
     Lamp  66  is disposed in a ceramic sleeve  266  which in turn is affixed into an aluminum jacket  268  as shown in FIG.  5 . The aluminum jacket  268  is disposed in a cylindrical cavity  270  defined in lamp base  272 . There is sufficient clearance between aluminum sleeve  268  and cylindrical cavity  270  defined in lamp base  272  to allow a limited amount of radial displacement of sleeve  268  about the longitudinal axis of lamp housing  232  which is parallel to the longitudinal axis of symmetry of reflector  274 . A pair of access holes  273  through finned heat sink  278  and lamp base  272 , which holes  273  are shown in  FIG. 6  in lamp base  272 , allow access by means of an Allen wrench to two orthogonally positioned socket-head set screws  275  on one side of sleeve  268  and which are each opposed by a spring  277  on the opposite side of sleeve  268  to adjustably center sleeve  268  in lamp base  272 . In this manner, the placement of the arc or plasma in lamp  66  can be accurately and easily adjusted in the field if need be in a plane perpendicular to the beam axis to lie precisely on axis. Because lamp base  272  is centered on the optical axis of symmetry of reflector  274  best shown in  FIG. 5 , lamp  66  can thus be adjusted in the field to be optically aligned onto the axis of symmetry of reflector  274 . Hence, the beam of light from lamp  66  can be focused for maximum collimation. 
     Lamp base  272  is disposed in a cylindrical bore  276  defined in fluted heat sink  278  thus as best visualized in cross-sectional view of FIG.  4 . Fluted heat sink  278  also includes bosses  284  which mate with molded standoffs  242  of housing  232  and are connected thereto by screws  286  disposed in threaded bore  287  defined in bosses  284  and standoffs  242  as shown in FIG.  2 . Lamp base  272  is disposed into cylindrical bore  276  until radial flange  280  of lamp base  272  makes contact with shoulder  282  of fluted heat sink  278 . It will be appreciated from the description below that reflector housing  284  shown in  FIG. 5  can be easily detached from the front of searchlight  11  by unscrewing reflector housing  284  from the front of lamp base  272  as best seen in FIG.  4 . This then allows lamp base  272  to be withdrawn from cylindrical bore  276 , unplugging banana plug  263  from banana socket  262 . Lamp  66 , ceramic sleeve  266  and aluminum jacket  268  are thus handled as a unit with lamp base  272 . If lamp  66  burns out, then it can readily be removed in the field as a unit without special tools or procedures in the manner just described above with the old lamp base  272  and a new lamp base  272  with a new lamp  66 , ceramic sleeve  266  and aluminum jacket  268  inserted. This has the advantage that new lamp  66  is already electrically assembled in an operative unit and is optically aligned with the optical axis of reflector  274 . Such easy field replaceability has a high value in search and rescue equipment. 
     With lamp anode  256  uniquely oriented toward the rear or light housing  232  away from reflector  274 , it is been determined that the field of illumination from lamp  66  is slightly convergent in the far-field and much more concentrated with conventional xenon arc lamps than would occur if the direction or orientation of the lamp were reversed, i.e. with the cathode in the rearward condition. This is due to positioning the full luminance distribution of the arc ( FIG. 3   a ) in the high magnification (behind the focal point,  FIG. 3   b ) section of the parabolic reflector ( FIG. 3   c ), instead of in the low magnification for prior art anode-forward configurations. The resulting illuminance is significantly greater than in anode-forward, as shown in  FIG. 3   d . Hence with the lamp anode  256  in the rear position as shown in  FIG. 5 , a hole in illumination or lessening of variation of intensity in the central part of the spot or beam is reduced. 
     The anode-to-the-rear orientation also means that more heat is projected back into the searchlight toward circuit board  248 . Finned heat sink  278  is provided and thermally connected to lamp housing  272  to ameliorate this condition. A metal heat sink block  235  shown in  FIG. 5  is coupled to circuit board  234  to make thermal contact with fluted heat sink  274  by means of a pair of fingers  273 . Fingers  273  clasp a mating internal heat sink flange (not shown) of heat sink  278 . 
     Reflector housing  284  has an internal collar  287  provided with threading  288 . Threading  288  engages threading  290  defined in the outer cylindrical extension of lamp base  272 . Thus, when assembled into housing  232 , reflector housing  284  screws onto lamp base  272  to further control the accuracy of rotation, as shown in  FIG. 4. A  tight tolerance sleeve and ring are used to stabilize the rotation. Reflector  274 , which is described below, is attached to reflector housing  284 , and thus may be longitudinally advanced or retracted along this longitudinal axis by rotation of reflector housing  284 . The longitudinal axis of reflector housing  284  is coincident with the longitudinal axis or optical axis of  274 . This allows for variable collimation of the beam of light. 
     Reflector  274  is disposed in reflector housing  284  so that forward flange  290  of reflector  274  abuts a shoulder  292  of reflector housing  284  as shown in FIG.  2 . Reflector  274  is attached to reflector housing  284  by means of an adhesive sealant. Screws  294  connect reflector housing  284  to a bezel  298 . Thus, bezel  298  thereby clamps a front transparent (or special ultraviolet, colored or infrared filter) faceplate  299  against a gasket  300 , reflector  274  and shoulder  292  of reflector housing  284 . A bezel ring  297  is threaded into an interior thread defined in bezel  298 . Reflector housing  284  is completely sealed for water resistance and tempered glass window  299  is designed to be usable in hazardous environments. Reflector housing  284  and reflector  274  thereby rotate as a unit and are threaded onto lamp housing  272 . An  0 -ring and groove combination  303  is defined the exterior surface of reflector housing  284  to provide for water sealing. Reflector housing  284  as described above is threaded to lamp housing  272  which allows lamp  66  to be longitudinally moved and focused inside of reflector  274  as stated. Lamp housing  272  is fixed with respect to heat sink  278  and hence body  232  by means of two cupped set screws  310  shown in  FIG. 6  threaded into heat sink  278  and bearing against lamp housing  272  which slip fits into heat sink  278 . Thus, by loosening set screws  310 , which have exterior access holes  312 , the entire head assembly of searchlight  11  can be removed including lamp housing  272 . Lamp housing  272  can then be unscrewed from reflector housing  284  and then replaced. 
     The rotation of reflector housing  284  about lamp housing  272  and hence heat sink  278  is better depicted in the perpendicular cross-sectional view of FIG.  7 . Heat sink  278  has a finger which extends from one of the fins forwardly or to the right in  FIG. 2  so that it is in interfering position with stops  316  screwed to and carried on reflector housing  284 . Therefore, as bezel  298  is rotated by hand, thereby rotating reflector housing  284  with it, its rotation is limited to one revolution or slightly less by the interference between fixed finger  314  and rotating stops  316 . In this manner the head assembly cannot be inadvertently unscrewed from lamp housing  272 , and further the focus range of lamp  66  as it is longitudinally moved on the optical axis of reflector  274  is retained within a desired or optimal range. 
     Reflector  274  may be moved by hand as described by rotating reflector housing  284  or maybe adjusted by means of an electric motor or lever adjustment (not shown). The lamp is focused by positioning the arc gap in lamp  66  at the focal point of reflector  274 . 
     Also included within bezel  298  may be a filter body carrying a filter (not shown) disposed on or adjacent to faceplate  299 . The filter body screws into an interior thread defined in the inner diameter of bezel  298  or may be clamped between bezel ring  297  and bezel  298 . Filters may be chosen according to the purpose desired for providing a effective spotlight in smoky conditions, for ultra violet radiation, infrared radiation or for selecting a frequency band of illumination effective for underwater illumination. Filters may also be employed for attenuation of light intensity in lower illumination applications, such as often occur in infrared applications. 
     The present invention provides a unique circuit topology for providing the current and voltage necessary to ignite, sustain and to adjust the operation of an arc lamp and in particular a xenon lamp in a portable, hand-held battery operated light. The challenge is to provide the current and voltage requirements necessary to ignite and sustain an arc lamp from a wide range of the supply input voltage. Therefore, before considering the circuitry of the invention consider the typical current and voltage requirement xenon arc lamp graphically depicted in  FIGS. 8 and 9  as a function of time. 
       FIG. 8  is a graph of the current supplied to a xenon lamp as a function of time, while  FIG. 9  shows the graph of the voltage as a function of time.  FIGS. 8 and 9  are aligned with respect to each other so that equal times appear at equal positions on the x-axis of each graph. Curve  10  of  FIG. 8  illustrates the current of a xenon lamp while curve  12  in  FIG. 9  illustrates the voltage. The lamp is turned on at time t=0. The power supply, described below turns on and rises quickly, i.e. within about 2 milliseconds, to provide a 90 volt dc open circuit voltage across the lamp at time  14  in FIG.  9 . In the illustrated embodiment a 20 kilovolt RF pulse is generated at time  18  shown in  FIG. 9  to start ignition of the lamp. The power rises rapidly to 100-125 watts. In the illustrated embodiment the RF pulse is about 400 kHz although many other frequencies and range of frequencies can be utilized without departing from the scope of the present invention. Typically the lamp is ignited within a short time, about one millisecond or less during which the current quickly falls as shown by falling edge  20  in FIG.  8 . During this time a current is delivered from a storage capacitor at time  22  to deliver additional energy to heat the plasma and lamp electrodes in order to sustain its operation. 
     As will be described below, a converter circuit holds the heating power at time  24  in  FIG. 9  to deliver the additional current. Once the lamp is started the converter may deliver a constant or regulated current to the lamp at any power level, although typically most lamps are only stable within the range of plus or minus 15 percent of the rated lamp current beginning at time  28  in FIG.  9 . According to the invention, the lamp is started at an optimal power level for the lamp in question. From this point forward the current supply to the lamp and the intensity of its light output can be smoothly transitioned to any level within an operational range without visually perceptible stepped transitions or altered in a step change manner. For example, in the illustrated embodiments the user may manually manipulate the controls as described below to increase the current to a maximum power and brightness at time  30  in  FIG. 9 , thereafter at a later time smoothly decreasing the current and brightness of the lamp to a minimum power level at time  32  in FIG.  8 . 
     The general time profile of the current and voltage of the xenon lamp through its phases of operation now having been illustrated in connection with  FIGS. 8 and 9 , turn to the schematic diagram of  FIG. 10  wherein the pulse width modulator (PWM), converter, lamp circuit and igniter are illustrated.  FIG. 10  is a simplified circuit schematic which illustrates the essential operation of the invention. It must be understood that many conventional circuit modifications for electromagnetic interference (EMI), circuit spike protection, temperature compensation and other conventional circuit modifications could be made in the circuit of  FIG. 10  without departing from the spirit and scope of the invention. 
     The converter, generally noted by reference numeral  34 , is controlled by a signal, PWM, on input  36 . Input  36  is coupled to the gates of a pair of parallel FET&#39;S  38  and  40  through an appropriate biasing resistor network, collectively denoted by reference numeral  42 . The parallel FETs  38  and  40  contribute to the high efficiency of the circuit which results in a high conversion of the battery power to useful illumination. A light made according to the invention produces a beam twice the distance as conventional lights or xenon searchlights running at the same power. 
     The source node of transistors  38  and  40  are coupled to node  44  which is coupled to the input of diode  46  and to one side of inductor  48 . The opposing side of inductor  48  is coupled to the supply voltage, +VIN  50 . Also coupled between supply voltage  50  and the output of diode  46  is a storage capacitor  52 . Energy is stored in capacitor  52  from converter  34  and is delivered as additional energy to heat the plasma and lamp electrodes to sustain its operation as was described in connection with  FIGS. 8 and 9  in connection with time  26 . 
     Node  54 , also coupled to the output of diode  46  and one end of capacitor  52  is the voltage of the lamp power supply, VSENSE+. The current of the lamp power supply is measured by measuring the voltage drop across resistor  56  and is designated in  FIG. 10  as the signals I SENSE+ and I SENSE−. The converter or power supply output is thus formed across nodes  54  and  58  and is delivered to a bank of filtering capacitors, collectively denoted by reference numeral  60 . The lamp DC ground is thus provided at node  62  while the filtered converted lamp power is provided at node  64 . 
     Xenon arc lamp  66  is coupled between lamp ground  62  and a lamp high voltage node  67 . The lamp current supply from node  64  is coupled across the secondary coil of transformer  68 . The primary of transformer  68  is coupled to the igniter, generally denoted by reference  70 . The igniter takes its input from a signal, TRIGGER DRIVE  72 , which is a 40 kHz signal which is ultimately communicated to the gate node of igniter transistor  74  in a manner described below. Igniter transistor  74  is coupled in series with the primary of transformer  76 . The secondary of transformer  76  is coupled to diode  78  and then to an RC filter  80  for deliverance of a high voltage RF signal to a spark gap  82 . When the voltage has reached a pre-determined minimum, the current will jump the spark gap  82 , and current will then be supplied to the primary of transformer  68 . In this manner, the 40 kHz RF pulse which is generated to start the ignition of lamp  66  is delivered to lamp high voltage node  67 . 
     Before considering further the circuit used for the high voltage RF trigger communicated to the gate of transistor  74 , consider first how the current to lamp  66  is controlled through PWM  136 , which in the illustrated embodiment is a Unitrode model UC3823 pulse width modulator. Understanding how this is achieved will then facilitate an understanding of the control of the ignition trigger. One of the main problems to light a xenon lamp has been the initial ignition phase. In the past a high voltage is applied across the lamp (approx. 100 volts), the gas is ionized with a high voltage RF pulse (&gt;10,000 volts) and a large capacitor is used to supply the energy to heat the plasma before reaching the normal running voltage which is about 14 volts for a 75 Watt lamp. 
     When using a switching power supply to run lamp  66  the conventional configuration is to use a “Boost Converter”, that is to boost the 12 volts from the battery supply to the running voltage of the lamp. The problem with this type of power converter is that the input voltage must be lower then the output voltage. This causes problems with the operation in many conventional automobiles for example, as the normal battery voltage can be over 14 volts. In the system of the invention an “Inverted Buck-Boost Converter” is used. This allows the converter to supply the proper lamp voltage while the input voltage can be anywhere from 10 to 28 volts. 
     In a conventional system, the starting high voltage is generated by running the converter in open loop and fixing the voltage to about 100 volts by setting the converter to a fixed duty cycle. This voltage also charges the capacitor that supplies the heating energy. The problem with this is that the converter must also supply power during the heating phase. During this heating phase the converter must supply more power than the running power for a short time. Because the duty cycle is fixed, changes in the input voltage will cause large changes in the power being supplied during this phase. A 10% increase in input voltage could cause, for example, the converter to try to supply more power than it is capable of producing. This will cause it to shutdown due to excessive current demand. The reverse, namely a 10% lower voltage in the input supply voltage, causes the converter not to supply enough power thereby causing the lamp not to light. The other problem is the converter must change from open-loop to closed-loop control to regulate the power being supplied to the lamp. 
     In the system of the invention, the heating power is semi-regulated by sensing the input voltage being supplied and adjusting the open-loop duty cycle. This relationship from voltage to duty cycle is not a one-to-one relationship. By using a percentage of the input voltage to adjust the RC time constant the resultant power delivered to the load will remain constant. 
     Turn again to  FIG. 10  for a concrete illustration of this principle. The input voltage, +VIN, on one side of resistor  157  together with the fixed voltage supplied on resistor  163  (here shown as +10 volts) is summed at the junction  161  of resistors  157 ,  163 , and  159 . This summed voltage is the slope and offset adjusted voltage and is used to set the minimum duty cycle. Capacitor  145  filters this signal and provides a low pass filter. Resistors  159  and variable resistor  163  with capacitor  143  provide the RC time constant for the circuit, which is presented at node  147 . Node  147  is coupled to current shutdown pin (ILIM/SD) on PWM  136 . When the PWM output drive  36  coupled into FETs  38  and  40  is high, the RC circuit just described charges. When a predetermined threshold voltage is reached the PWM signal is turned off. This will keep the power constant across lamp  66  during the heating phase over the total operating input range of the supply from 10 to 32 volts. 
     When PWM drive  36  is low, capacitor  143  is reset through voltage discriminator  149  coupled to the gate node of transistor  151 . When transistor  151  is turned on by discriminator  149 , capacitor  143  is discharged to ground. Discriminator  149  is active high whenever PWM  36  drops below the reference voltage provided at the other input to discriminator  149 , which in the illustrated embodiment is +5.1 volts. When PWM  36  goes high, the RC node  147  begins to charge and voltage on node  147  rises until it reaches a fixed threshold. At this point PWM  136  turns off PWM drive  36  and the cycle repeats. A percentage of the input supply voltage, +VIN, is coupled through resistors  157 ,  159 , and  163  and is used to adjust the RC time constant at node  147  so that the resultant power delivered to lamp  66  remains constant even when there is a wide variation in the supply voltage. Variations in the DC power supply between 11 to 32 volts is easily accommodated by the claimed invention. 
     Consider now the circuitry used to provide the trigger to ignition transistor  74 . Analogous circuitry is used to control the ignition trigger as was just described for the control of PWM drive  36 . Resistors  157   a , and  163   a  coupled to capacitor  145   a  perform the same function and form the same circuit combination as resistors  157 , and  163  coupled to capacitor  145 . Node  161   a  where resistors  157   a , and  163   a  and capacitor  145   a  are coupled together is in turn coupled to resistor  159   a  and capacitor  143   a  which perform the same function and form the same circuit combination as resistor  159  and capacitor  143 . The ignition signal, TRIGGER, is coupled to the gate of transistor  151   a  which in turn discharges RC node  147   a  in a manner as previously described in connection with PWM drive  36 . TRIGGER is generated by programmable logic device (PLD)  164  described below. 
     RC node  147   a  is coupled to one input of voltage discriminator  200 , whose other input is coupled to a reference voltage, i.e. +2.5 V. In this way a threshold value is set for TRIGGER. When TRIGGER is not active, RC node  147   a  charges up and when the threshold is exceeded will be output from discriminator  200 , filtered by filter  202 , signal conditioned by inverters  204  and provided to the gate of transistor  74 , the driver to the primary of the ignition transformer  76 . When TRIGGER goes active, RC node  147   a  is discharged and the output of discriminator  200  is pulled to ground through pull-down transistor  206 . Again, a percentage of the input supply voltage, +VIN, is coupled through resistors  157   a ,  159   a , and  163   a  and is used to adjust the RC time constant at node  147   a  so that the resultant power delivered to lamp  66  during ignition remains constant even when there is a wide variation in the supply voltage. 
     Consider now the power supply for converter  34 . The searchlight may be powered either by an external 12 volt power supply provided line  84  shown in  FIG. 11  or by the current from an internal battery, +BATT, line  86  of FIG.  11 . The manual operation of the lamp is provided by means of a closure of a push button switch  88  shown in  FIG. 14  which is used to provide a grounded signal, RELAY DRIVE from PLD  164 . When RELAY DRIVE goes active, relay  116  is energized and the supply voltage, +VIN, on line  99  is switched to the internal battery, +BATT. When RELAY DRIVE goes inactive, relay  116  is de-energized and the supply voltage, +VIN, is switched to an external terminal  97 . Either an externally provided power supply signal or the battery power supply is provided by means of control of a double pole-double throw relay  116  powered by the signal, RELAY DRIVE, on line  94 . Contacts  120  of relay  116  thus either provide an exterior power supply voltage  122  or the battery voltage, +BATT, as the circuit power supply  50 , +VIN. 
       FIG. 15  illustrates the circuit for a battery charger controller  104  provided within the searchlight to charge the battery. A signal, CHG DRIVE, is provided from PLD  164  on input  96  to the gate to controller  104 . The signal, SENSE+, from node  54  is also coupled as an input to controller  104  from converter  34 . Battery charger controller  104  is a conventional integrated module. 
     The converter and igniter circuitry and battery supply current now having been described, turn to the control circuitry of FIG.  10 . The current sensing nodes  58  and  59 , I SENSE− and I SENSE+ respectively, are provided as inputs to a transconductance amplifier  124  which is characterized by high impedance and provides an amplified voltage output to the input of diode  126 . In the illustrated embodiment a Maxim high-side, current-sense amplifier model  472  is used. The output of diode  126  is fed back on line  127  to node  132 . The voltage at node  132  is provided through resistor  134  to the inverted input pin, INV, of pulse width modular  136 . Pulse width modulator  136  produces from its various inputs a PWM drive  36  which was described above as being coupled to the input of converter  34 . The other inputs and outputs of pulse width modular  136  are conventional and will thus not be further described unless relevant. 
     The signal provided on node  132  is affected by several adjustments. Node  132  is resistively coupled to transistor  142  whose base is controlled by control signal, CURRENT OFF, also output from PLD  164 . Thus, when transistor  142  are turned on, node  132  is pulled low. This causes PWM drive  36  to go low. 
     Node  132  is also resistively coupled to ground through transistor  144  whose base is resistively coupled to a control signal, HI LO POWER as provided by PLD  164 . The emitter of transistor  144  is coupled to node  132  through a conventional binary coded decimal (BCD) resistive ladder  146  so that the maximum current on node  132  is continuously and smoothly digitally controlled as it is adjusted from high to low power and visa versa. Binary coded decimal (BCD) resistive ladder  146  is controlled by the BCD output  165  from PLD  164  so that the amount of resistance provided by ladder  146  is digitally controlled and varied in amounts which are visually imperceptible when hi/lo power is active. 
     The control signal to input NOT INVERTED (NI) of pulse width modulator  136  is controlled through an adjustable resistive network, collectively denoted by reference numeral  150 . The control signal E/A OUT of pulse width modulator  136  is similarly provided from a filter network  152  for the purpose of rejecting unwanted frequencies. The control signal  153 , (ILM REF) is similarly provided from a biasing network  154  with the purpose of setting the threshold voltage at which RC node  147  will cut off PWM drive  36 . A CLOCK signal is provided from pulse width modulator  136  to PLD  164  for the purposes of clocking programmable logic device  164  shown in FIG.  14 . 
     The lamp high voltage set point is produced in part by the circuitry of FIG.  12 . High voltage from node  54 , V SENSE+, is resistively provided to the input of differential amplifier  214 . The opposing input of amplifier  214  is resistively coupled to the supply voltage +VIN, and the output of feedback amplifier  214  is then provided to one input of differential amplifier  216  whose other output is coupled to the +2.5 volt reference. The output of feedback amplifier  216  is the command signal +LAMP SENSE, which is provided as one of the inputs to PLD  164  and which provides a feedback signal of what the voltage on lamp  66  is. 
     The control of light intensity and many other lamp control functions are provided by PLD  164  which is a conventional programmable logic device such as model XC9572 manufactured by Xilinx. The programming of PLD  164  is conventional. The input signals to PLD  164  include CLOCK, +VIN, +LAMP SENSE and PWM, , while the output signals are CURRENT OFF, RELAY, TRIGGER, Hi LO POWER whose functions are described above. Push button  88  is programmed in PLD  164  so that a single momentary depression of push button  88  turns on the light. A second single momentary depression of push button  88  turns off the light. However, when push button  88  is turned on and held on for more than a few seconds, HI/LO POWER goes active and BCD signals  165  begin to count up causing resistance ladder  146  to be driven to gradually increase the power. As long as button  88  is held down, BCD signals  165  count up and light intensity increases. As soon as button  88  is no longer depressed, counting stops and the light intensity remains fixed. If the light is turned off and then turned on again, it will light at the light intensity that was last chosen. The BCD signals  165  count cyclically, i.e. after reaching the maximum count, BCD signals  165  return to the minimum count and hence minimum light intensity. The cycle is then repeated. If desired, PLD  164  could also be programmed to count down or in the opposite direction of light intensity variation. Push button  88  can be programmed in PLD  164  in many different ways from that described without departing from the spirit and scope of the invention. 
       FIG. 13  is a schematic which shows a conventional manner in which the 5.0 and 2.5 volt reference signals are respectively generated using resistor divider  155 . 
     The circuitry now having been described in detail, several observations can be made. The circuit, as previously stated is markedly more efficient in producing light from lamp  66  than prior circuits. This is due to several factors. First, the use of parallel switching FETs  38  and  40  described above contributes to increased power conversion efficiency into light output. Second, the use of a high voltage battery may contribute. Typically, battery voltages of 12 volts are employed. In the present invention batteries with outputs in the range of 16-22 volts are used. Third, converter  34  is run at a higher switching frequency. Whereas prior circuits are operated at about 20 kHz, the present invention is configured to drive converter  34  at a much higher frequency, such as 100 kHz. 
     Finally, the circuit boards are laid out and fabricated to minimize power losses in the lines. A four layer printed circuit board is used. In high current lines such as the circuit path from +VIN to node  50 , inductor  48  and FETs  38  and  40 , and in the power lines in  FIG. 11 , lines  97 ,  84 ,  120 , and  86 , multiple printed circuit board lines are fabricated in parallel for the same line on the schematic. For example, in each of the lines just mentioned four parallel printed circuit board lines are fabricated and coupled in parallel with each other as shown in FIG.  16 . For example, pads  320  and  322  diagrammatically represent nodes in the circuit between which a high current occurs. The circuit board, generally denoted by reference numeral  336 , is comprised of four layers  334 . A vertical riser or via  324  is defined from pads  320  and  322  through all four layers  334 . Vias  324  are coupled with wide and thick conductive printed circuit lines  326 ,  328 ,  330  and  332  disposed on the bottom of each of layers  334 . Circuit lines  326 ,  328 ,  330  and  332  are in parallel circuit with each other and therefore provide a very low resistance, low loss line for high current loads. 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus, if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. 
     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.