EP2174533A1

EP2174533A1 - Control for discharge lamp

Info

Publication number: EP2174533A1
Application number: EP08776221A
Authority: EP
Inventors: Michael Noel Kiernan; Robert Marc Clement
Original assignee: Cyden Ltd
Current assignee: Cyden Ltd
Priority date: 2007-07-20
Filing date: 2008-07-17
Publication date: 2010-04-14
Also published as: JP2010534387A; WO2009013524A1; GB0714126D0; CN101828431A

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 optical parameters detected by the sensor.

Description

Control for discharge lamp

This invention relates generally to controls for discharge lamps suitable for providing broadband incoherent light sources suitable for medical and cosmetic applications.

Background 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.

Prior Art

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. Figure 1 of the accompanying drawings shows such a lamp which includes a discharge tube 1 mounted in a reflector housing 2 having an aperture 3 over which is located a suitable transparent light guide 4, which is used to collect both the reflected and direct illumination light from the flashlamp and deliver it to a treatment site.

Such a flashlamp is typically driven by a capacitor discharge circuit where the electrical energy required is stored in a capacitor until an output optical pulse is required; Fig 2 of the accompanying drawings shows a simple circuit for such a flashlamp, in which discharge tube 1 is supplied by a trigger transfomer 5 and a sttorage capacitor 6f .

When the optical output is required, the electrical energy is delivered to the flashlamp, thereby converting the electrical energy to optical output. In this type of discharge circuit, 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.

It has previously been proposed to attempt to produce a constant current during the output pulse, in order to beneficially ensure that the output wavelength remains constant; one such . A method aimed at producing a constant current during the optical pulse is proposed in US Patent 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, US Patent 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 enevelope 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.

We have therefore developed a system that monitors the optical output and ensures substantially constant output during the pulse.

The present invention therefore provides a system comprising, in combination, at least one electric discharge lamp capable of generating an incoherent output optical pulse of a range of wavelengths in the visible spectrum, the optical pulse having a predetermined time interval and a predetermined total electrical energy input for the pulse, and a drive circuit for delivering a plurality of energy pulses to said electrical discharge lamp, the drive circuit comprising a) a storage capacitor capable of storing electrical energy input to the circuit, b) a charger for repeatedly charging the storage capacitor; c) a switch for delivering the electrical energy input from the storage capacitor to the discharge lamp; and d) drive means for selectively opening and closing the switch throughout the predetermined time interval to permit delivery of a plurality of packets of energy to the discharge lamp, each packet being of duration less than the predetermined time interval; the combination further comprising an optical sensor for sensing an optical output from the discharge lamp; and a control for operating the drive means in response to changes in optical output detected by the sensor.

The present invention further provides 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 parameters detected by the sensor.

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.

The present invention further provides a method of driving a pulsed radiation source such as a discharge lamp, 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 the energy pulse to the radiation source in the form of a plurality of packets of energy within a predetermined time interval, in which the optical output from the radiation source is sensed and delivery of energy from the storage capacitor to the radiation source is controlled in response to optical parameters sensed by the sensor.

Still further, the invention provides an optical cosmetic method of treatment, which comprises providing an electrical energy input to a discharge lamp so as to produce an optical output pulse directed towards animal tissue, the discharge lamp being driven by a method as just described.

Preferred features of the present invention will now be described with reference to the accompanying drawings, in which:

Fig 1 schematically illustrates a typical reflector and light guide combinatiuon for use in a system according to the invention,

Fig 2 is a graph showing optical output versus time for a conventional flashlamp system; Fig 3 shows an exemplary circuit and optiical feedback system for use according to the invention; and

Fig 4 shows an exemplary digital control suitable for use in the system of Fig 3.

Description of Operation

Referring to Figure 3, a power supply 11 has an AC mains supply 12 (typically at 110V or 240V AC 50/60Hz) which is converted to a DC voltage. This DC voltage is used to charge energy storage capacitor C1 , the voltage to which capacitor C1 is charged being controlled via the SET signal from a digital control system 13 to the power supply 1 1. 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 1 1. 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 C1 maintains its stored charge until required.

When optical output from the flashlamps 16 and 17 is required, firstly the flashlamps have to be "broken down" or "triggered" to create a conduction path through the gas within the flashlamp. 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 T1 and T2. 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 Vp₁ and V_T2 are typically 5 to 1OkV with a duration of 10 microseconds whilst the primary voltage pulse is in the order of 200 to 400V. This high voltage spike on the exterior of the flashlamp ionises 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 SW₁ and SW₂ from the control system 13.

Providing the trigger spikes V_Ti and V_T2 have induced the necessary ionisation within the flashlamps 16 and 17, current will flow through inductors L1 and L2, both flashlamps ground producing the optical output from the ionised xenon gas within the flashlamps. Whilst the current is flowing through switches 14 and 15, and both flashlamps, from capacitor C1 , inductors L1 and L2 store a proportion of the energy delivered from C1.

When the optical output from either flashlamp 16 or flashlamp 17 reaches a pre- determined high level defined within the control circuit 13 and monitored by signal S1 and S2 from optical sensors 21 and 22, the control system 13 opens switch 14 or switch 15 accordingly to prevent further current flow from C1 through the corresponding flashlamp. For example, if flashlamp 16 reaches a preset optical output value, switch 14 is opened via SW₁ from the control system 13 thereby preventing further current flow from capacitor C1. When switch 15 is closed, the stored energy within the inductor L1 induces a current which flows through flashlamp 16 via diode D1 (commonly referred to as a "flywheel" diode"). The optical output is monitored by the control system 13 via S1 and when this current decays to a predefined low point, switch 15 is closed thereby allowing current flow to resume from C1 which both maintains output in the flashlamp and stores energy within the inductor. This process operates concurrently and independently for flashlamp 17.

By repeating this process at a frequency in the order of 100-50OkHz, the optical output from the flashlamps can be maintained at a constant level for the duration of the required optical pulse (typically having a duration in the order of 1 to 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 C1 during the release of its stored energy.

Referring to Figure 4, the digital control system comprises a processor unit 31 which contains suitable control software algorithms for operation. The charge voltage of the capacitor C1 monitored by the V_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_c, the processor unit controls the power supply via the SET signal, when the desired V_c is reached, the power supply output is terminated. When the stored energy is dissipated after the optical output pulse, capacitor C1 is recharged by the power supply as commanded by the processor unit 31.

An operator of the apparatus selects the desired output optical parameters of 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 optical 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 31 and compared to the predefined values as defined in the data table stored in the memory unit 34.

Claims

1 In combination, at least one electric discharge lamp capable of generating an output pulse of a range of wavelengths in the visible spectrum, said output pulse having a predetermined time interval and a predetermined total electrical energy input for said pulse, and a drive circuit for delivering a plurality of energy pulses to said electrical discharge lamp, the drive circuit comprising a) storage capacitor means capable of storing electrical energy input to said circuit, b) charge means for repeatedly charging said storage capacitor means; c) a switch for permitting delivery of electrical energy from said storage capacitor means to said discharge lamp; and d) drive means for selectively opening and closing said switch throughout said predetermined time interval so as to deliver a plurality of packets of energy from said storage capacitor means to said discharge lamp, each said packet being of duration less than said predetermined time interval; the combination further comprising optical sensor means for sensing an optical output from the discharge lamp; and control means for operating said drive means in response to changes in optical output detected by said sensor means.

2. A combination according to claim 1 , wherein said control means includes a processor unit arranged to compare optical output sensed by the sensor means with precalibrated values stored in a memory unit.

3. A combination according to claim 1 or 2, which includes a plurality of said discharge tubes.

4. A combination according to claim 3, which further comprises means for shutting down said discharge tubes when a detector indicates that one of said tubes has failed to generate said output pulse.

5. 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 parameters detected by the sensor.

6. A lamp according to claim 5, which includes a plurality of electric discharge tubes.

7. A lamp according to claim 5 or 6, wherein the control mechanism includes a processor unit arranged to compare optical output sensed by the sensor means with precalibrated values stored in a memory unit.

8. A method of driving a pulsed radiation source such as a discharge lamp, the method comprising providing a storage capacitor so as to be capable of storing electrical energy required to be delivered to said radiation source, and selectively charging said storage capacitor so as to deliver to said radiation source said 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 radiation source; and controlling delivery of energy from the storage capacitor to the radiation source in response to parameters sensed by the sensor.

9. An optical cosmetic method of treatment, which comprises providing an electrical energy input to a discharge lamp so as to produce an optical output pulse directed towards animal tissue, wherein said discharge lamp is driven by a method according to claim 8.