Abstract:
At least one electric discharge lamp capable of generating a broadband output pulse of a range of wavelengths in the visible spectrum, the output pulse having a predetermined time interval and a predetermined total electrical energy input for the pulse, has a drive circuit for delivering energy pulses to the electrical discharge lamp, as well as a sensor for sensing an optical output from the discharge lamp; and a control mechanism for operating the drive circuit in response to variations in optical output detected by the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a Continuation-in-Part Application of U.S. patent application Ser. No. 11/628,417, filed 12 Apr. 2006 now U.S. Pat. No. 7,710,044, being the US national phase of PCT Patent application GB2005/001977 dated 20 May 2005. 

   TECHNICAL FIELD 
   This invention relates generally to controls for discharge lamps suitable for providing broadband incoherent light sources suitable for medical and cosmetic applications. 
   It is known that discharge lamps for providing incoherent light sources for such purposes have several advantages over traditional laser technology, including their low cost, and the facts that they produce multiple wavelengths permitting multiple uses, and are subject to less stringent regulatory control. 
   BACKGROUND OF THE INVENTION 
   A typical discharge lamp for such medical and cosmetic applications comprises a xenon arc flashlamp located within a reflector shaped to direct the optical output from the flashlamp to a treatment site. 
   The flashlamp is typically driven by a capacitor discharge circuit where the electrical energy required is stored in a capacitor until the output optical pulse is required. When the optical output is required, the electrical energy is delivered to the flashlamp, thereby converting the electrical energy to optical output. 
   In such an arrangement, the current flowing through the flashlamp varies during the pulse, proportional to the discharge characteristics of the capacitor. This variation in the current during the pulse produces a varying intensity of optical energy and induces a shift in the output wavelength spectra as the output wavelength is determined by the plasma temperature within the flashlamp, and the plasma temperature is governed by the current flowing. 
   Referring to  FIG. 1A  of the drawings, there is illustrated a simplified version of a conventional flashlamp drive circuit, in which a power supply unit  100  is used to charge a relatively small capacitor  102 , in this case say 500 μF. A switch  104  is provided between the capacitor  102  and the flashlamp  106 . Examples of switches used in the past have included thyristors, which once turned on, generally remain on until the capacitor has fully discharged, and transistors. When the switch  104  is closed, the capacitor  102  is substantially completely discharged to the flashlamp  106 , giving a drive current pulse similar to that illustrated in  FIG. 1B , whereby around (say) 150 J of energy (defined by the area under the curve in  FIG. 1B ) is delivered to the flashlamp in around 5 ms. 
   However, there are applications, particularly medical applications, where the shape of the optical pulses used to drive the flashlamp is important in order to achieve the desired therapeutic effect, and in particular to achieve such effect without damage to areas of the patient&#39;s body not being treated. For example, in optical dermatology, it may be desirable to rapidly heat a target chromophore to a selected temperature, and to then reduce applied energy so as to maintain the chromophore at the desired temperature. It is therefore highly desirable for the shape and duration of the optical pulses delivered to the flashlamp to be controllable. 
   Referring to  FIG. 2A  of the drawings; there is illustrated a simplified form of another known flashlamp drive circuit, in which a power supply unit  100  is used to charge a relatively large capacitor  102  (say, 0.2 F) up to, say 1500 J, and a switch  104  (embodied in this case by a transistor) is used to deliver a small portion of this total energy (say 150 J) at a time. In view of the manner of operation of this type of partial discharge system, an optical pulse can be delivered to the flashlamp  106  with a relatively uniform energy distribution, as illustrated in  FIG. 2B  of the drawings. Effectively, a drive system of the type illustrated in  FIG. 2A  of the drawings, delivers a plurality of small packets  108  of energy. Thus, in the case where 150 J of energy are delivered in a 50 ms-time interval, each packet  108  will consist of 0.03 Jμs. As a result, it is possible, using such a system, to control the shape of the optical pulse delivered to the flashlamp in order to achieve the desired effect. 
   However, a major disadvantage of the partial discharge system described with reference to  FIG. 2A  of the drawings, is the size of the capacitor  102 , whereas it is highly desirable in all flashlamp applications to minimize the size of the capacitor (and therefore the charge it carries) as this has the effect of minimizing the size, weight and cost of the lamp drive circuitry and enhances the safety of such drive circuits by reducing shock risks. 
   A method aimed at producing a constant current during the optical pulse is proposed in U.S. Pat. No. 6,888,319. This approach provides a drive circuit for a pulsed flashlamp which circuit includes a sensor for power through the lamp, and a series regulator which operates an on/off switch between the energy storage capacitor and the flashlamp, the switching frequency being determined by monitoring the current flow or power within the circuit. This approach can provide a relatively constant current output during the overall current pulse and is commonly referred to as a flywheel circuit as described in, for example, U.S. Pat. No. 4,513,360. 
   Whilst providing a constant current pulse does have advantages, this approach does not provide constant optical output, because the output optical power can depend upon many external factors that are not manifested as variation in current. These factors include, but are not limited to, gas fill pressure, gas purity, operating temperature, flashlamp envelope degradation, flashlamp envelope coating (often flashlamps are coated to improve conduction) or flashlamp envelope doping (doping the envelope can selectively filter certain wavelengths). Many of these parameters can vary during usage; for example, it is common for flashlamp output to degrade through usage as contaminants can cause optical fluctuations. Such contaminants can cause optical degradation but may not affect the current flowing in the flashlamp. 
   OBJECT OF THE INVENTION 
   It is an object of the present invention to provide flashlamp drive control, and a corresponding method of driving a flashlamp, whereby the shape and duration of the current pulses delivered to the flashlamp is highly uniform. 
   SUMMARY OF THE INVENTION 
   The present invention therefore provides, in combination, 
   at least one electric discharge lamp capable of generating an output pulse of a range of wavelengths in the visible spectrum, the output pulse having a predetermined time interval and a predetermined total electrical energy input for the pulse, 
   a drive circuit for delivering a plurality of energy pulses to the electrical discharge lamp, 
   a sensor for sensing an optical output from the discharge lamp; and 
   a control mechanism for operating the drive circuit in response to changes in optical output detected by the sensor. 
   In the combination according to the invention, the drive circuit comprises 
   a) a storage capacitor capable of storing electrical energy input, 
   b) charge means for selectively charging the storage capacitor; 
   c) a switch for permitting delivery of electrical energy from said storage capacitor to the discharge lamp; and 
   d) drive means for selectively opening and closing the switch throughout the predetermined time interval so as to deliver a plurality of packets of energy from the storage capacitor to the discharge lamp, each packet being of duration less than said predetermined time interval. 
   Thus, the present invention is intended to provide means for driving and controlling a circuit for a flashlamp, which effectively mimics the operation of the partial discharge system described above with reference to  FIG. 2A  of the drawings, using a relatively small capacitor by providing means for modulating the capacitor output at a high frequency to achieve the desired energy pulse. 
   Also in accordance with the present invention, there is provided a method of driving a pulsed radiation source, using a combination according to the invention, the method comprising providing a storage capacitor so as to be capable of storing electrical energy required to be delivered to the radiation source, and selectively charging the storage capacitor so as to deliver to the radiation source an energy pulse in the form of a plurality of packets of energy within a predetermined time interval, the method further comprising sensing the optical output from the discharge lamp; and controlling delivery of energy from the storage capacitor to the radiation source in response to optical output sensed by the sensor 
   The invention further comprises an electric discharge lamp capable of generating a broadband output pulse of a range of wavelengths in the visible spectrum, the output pulse having a predetermined time interval and a predetermined total electrical energy input for the pulse, in combination with a drive circuit for delivering energy pulses to the electrical discharge lamp, a sensor for sensing an optical output from the discharge lamp; and a control mechanism for operating the drive circuit in response to optical output detected by the sensor. 
   Beneficially, the switch used according to the invention may be an insulated-gate transistor, such as an insulated-gate bipolar transistor (IGBT). 
   In a preferred embodiment, the storage capacitor is connected in parallel with the pulsed radiation source. 
   A combination according to the invention may comprise a plurality of flashlamps, each having associated therewith a respective storage capacitor and respective means for selectively charging and discharging said storage capacitor. Means, such as a digital signal processor and microprocessor, are beneficially provided for controlling the charge/discharge means. 
   The time interval for the optical pulse is typically 1 to 100 milliseconds (such as 10 to 100 milliseconds), whereas the individual packets of energy typically have an order of magnitude lower, such as a duration of 5 to 25 microseconds. 
   Preferably, the control includes a processor unit arranged to compare optical output sensed by the sensor with precalibrated values stored in a memory unit. 
   Preferably the control mechanism includes a high speed analog to digital converter for each sensor output. 
   It is preferred that a plurality of the discharge lamps is used; this enables a more uniform optical output to be achieved. Such a plurality of lamps is typically provided in a single reflector unit with a single light guide for the lamps. 
   When a plurality of discharge lamps is employed, the system preferably includes means for shutting down the discharge lamps when a detector indicates that one of the lamps has failed to generate the output pulse. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which; 
       FIG. 1A  is a simplified circuit diagram of a first flashlamp drive circuit and flashlamp configuration according to the prior art; 
       FIG. 1B  illustrates an energy pulse which can be delivered by the circuit of  FIG. 1A ; 
       FIG. 2A  is a simplified circuit diagram of a second flashlamp drive circuit and flashlamp configuration according to the prior art; 
       FIG. 2B  illustrates an energy pulse which can be delivered by the circuit of  FIG. 2A ; 
       FIG. 3  is a schematic circuit diagram illustrating a flashlamp drive circuit and flashlamp configuration; 
       FIG. 4  illustrates schematically a portion of the circuit of  FIG. 3 ; 
       FIGS. 5A and 5B  illustrate energy pulse forms which can be delivered by the circuit of  FIG. 3 ; 
       FIG. 6  shows an exemplary circuit and optical feedback system for use according to the invention; and 
       FIG. 7  shows an exemplary digital control suitable for use in the system of  FIG. 4 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIGS. 3 and 4  of the drawings, there is illustrated a flashlamp unit including a drive circuit according to an exemplary embodiment of the present invention. The flashlamp  106  may, for example, comprise a delivery head carrying light emitting apparatus in the form of an electric discharge tube containing a high pressure Noble/inert gas such as Xenon or Krypton. The discharge tube operates to produce, in response to the input of a current pulse, a burst of light of a range of wavelengths in the visible spectrum (approximately in the range 400 to 700 nm). However, many different types of flashlamps and other pulsed radiation sources will be well known to a person skilled in the art, and their specific form and structure will not be described in any further detail herein. A bank of, say, six flashlamps or other pulsed radiation sources may be provided in a single unit, as required by the particular application. 
   Associated with the or each flashlamp  106 , there is provided a switch mechanism  110  comprising an insulated-gate bipolar transistor (IGBT)  112  and a corresponding driver  114 . The switch mechanism  110  also incorporates a secondary transistor  116 , having a comparatively very small capacitance of (say) 10 μF. The capacitor  116  and the respective flashlamp  106  are connected in parallel with each other. A controller, comprising a digital signal processor (DSP)  118  and a microprocessor  120 , is provided to control the operation of all of the flashlamps  106  in the bank via the respective switch mechanisms  110 . It will be appreciated that the microprocessor  120  can be programmed so as to cause the digital signal processor  118  to run the bank of flashlamps in accordance with anyone of a number of different programs, depending on the application. 
   A switch mode power supply  122  and a primary capacitor  124  are also provided. In use, each drive pulse delivered to a flashlamp  106  is comprised of a plurality of smaller energy packets resulting from the high frequency, repeated charging and discharging of the respective capacitor  116 , controlled by the DSP  118  via the respective driver  114 . As a result, there is provided flashlamp drive circuitry, and a corresponding method of driving a flashlamp, whereby the shape and duration of the current pulses delivered to the flashlamp is highly controllable, and the size of the storage capacitor required is significantly reduced relative to known arrangements. Examples of the types of energy pulses which can be delivered using the drive circuit described above with reference to  FIGS. 3 and 4  of the drawings; are illustrated in  FIG. 5  of the drawings. 
   Circuits such as those shown in  FIGS. 3 and 4  of the drawings are provided according to the invention with sensors for sensing an optical output from the discharge lamp  106 ; and a controller for operating the driver in response to optical output detected by such sensors. Such sensors and controllers are described in more detail in the following description with reference to  FIGS. 6 and 7  of the accompanying drawings. 
   Referring to  FIG. 6 , a power supply  11  has an AC mains supply  12  (typically at 110V or 240V AC 50/60 Hz) which is converted to a DC voltage. This DC voltage is used to charge energy storage capacitor C 1 , the voltage to which capacitor C 1  is charged being controlled via the SET signal from a digital control system  13  to the power supply  11 . A capacitor voltage Vc is monitored by the digital control system  13 ; when Vc is reached, the control system  13  turns off the power supply  11 . During this charging period, semiconductor switches  14  and  15  are in OFF mode inhibiting current flow through the remainder of the circuit. 
   Flashlamps  16  and  17  (typically Xenon arc discharge lamps) are both in open-circuit mode, that is, there is no conduction path through the flashlamps. Capacitor C 1  maintains its stored charge until required. 
   When optical output from the flashlamps  16  and  17  is required, firstly the flashlamps  16 ,  17  have to be “broken down” or “triggered” to create a conduction path through the gas within the flashlamp  16 ,  17 . To trigger the flashlamps  16  and  17 , a high voltage spike is applied to the external surface of the flashlamp glass envelope via external trigger planes  18  and  19 . When the optical output is required, the control system  13  signals a trigger circuit  20  via a TRIG signal. The trigger circuit  20  applies a voltage pulse to the primary (Pri) winding of each of trigger transformers T 1  and T 2 . The voltage on the primary winding (Pri) is amplified to induce a higher voltage on the trigger transformer secondary (Sec) windings. 
   The resulting trigger spikes or packets of energy V T1  and V T2  are typically 5-10 kV with a duration of 10 microseconds whilst the primary voltage pulse is in the order of 200-400V. This high voltage spike on the exterior of the flashlamp ionizes Xenon gas within the flashlamp leading to the formation of a conduction path from the flashlamp anode to cathode. 
   Simultaneously to the TRIG signal being applied to the trigger circuit  20 , semiconductor switches  14  and  15  are turned on (that is, closed) to provide a conduction path, via control signals SW 1  and SW 2  from the control system  13 . Providing the trigger spikes V T1  and V T2  have induced the necessary ionisation within the flashlamps  16  and  17 , current will flow through inductors L 1  and L 2 , both flashlamps ground producing the optical output from the ionized xenon gas within the flashlamps. Whilst the current is flowing through switches  14  and  15 , and both flashlamps, from capacitor C 1 , inductors L 1  and L 2  store a proportion of the energy delivered from C 1 . 
   When the optical output from either flashlamp  16  or flashlamp  17  reaches a predetermined high level defined within the control circuit  13  and monitored by signal S 1  and S 2  from optical sensors  21  and  22 , the control system  13  opens switch  14  or switch  15  accordingly to prevent further current flow from C 1  through the corresponding flashlamp. For example, if flashlamp  16  reaches a preset optical output value, switch  14  is opened via SW.sub. 1  from the control system  13  thereby preventing further current flow from capacitor C 1 . When switch  14  is opened, the stored energy within the inductor L 1  induces a current which flows through flashlamp  17  via diode D 1  (commonly referred to as a “flywheel” diode”). The optical output is monitored by the control system  13  via S 1  and when this current decays to a predefined low point, switch  14  is closed thereby allowing current flow to resume from C 1  which both maintains output in the flashlamp and stores energy within the inductor. This process operates concurrently and independently for flashlamp  16 . 
   By repeating this process at a frequency in the order of 100-500 kHz, the optical output from the flashlamps can be maintained at a constant level for the duration of the required optical pulse (typically in the order of 1-100 milliseconds). In order to ensure constant output of the flashlamps during the required optical pulse, the duty ratio between the on and off times of both switches  14  and  15  is varied during the pulse to compensate for the voltage drop in capacitor C 1  during the release of its stored energy. 
   Referring to  FIG. 7 , the digital control system comprises a processor unit  31  which contains suitable control software algorithms for operation. The charge voltage of the capacitor C 1  monitored by the V.sub.c signal is fed into an analog to digital converter  32 , the digital output of which is read by the processor unit  31 . Depending upon the required charge voltage V.sub.c, the processor unit  31  controls the power supply via the SET signal, when the desired V.sub.c is reached, the power supply output is terminated. When the stored energy is dissipated after the optical output pulse, capacitor C 1  is recharged by the power supply as commanded by the processor unit  31 . 
   An operator of the apparatus selects the desired output optical parameters such as energy, pulse duration and pulse sequence (single or multiple pulses) through a user Interface  33 . A data table contained within the memory unit  34  is referenced by the processor unit  31  to obtain the predefined sensor readings which correspond to the level of output optical power required. 
   The signals from sensors  21  and  22  are converted to digital format by two independent analog to Digital Converters  34 , 35  to be read by the processor unit and compared to the predefined values as defined in the data table stored in a memory unit  34 .