WO2007128472A1

WO2007128472A1 - Methods and systems for controlling gas discharge lamps

Info

Publication number: WO2007128472A1
Application number: PCT/EP2007/003858
Authority: WO
Inventors: Koen Geirnaert
Original assignee: Mergeay, Maurice
Priority date: 2006-05-02
Filing date: 2007-05-02
Publication date: 2007-11-15
Also published as: GB2437755A; GB0608568D0; EP2022298A1

Abstract

The present invention relates to a control system (100) for controlling a gas discharge system (10), the lamp control system (100) comprising a detection means (102) for detecting a physical parameter such as e.g. an optical parameter from said gas discharge lamp (10), an evaluation means (104) for spectrally evaluating variations in said optical parameter. The control system may also comprise a controlling means (106) for controlling an operating frequency of said gas discharge system (10) based on said evaluated physical parameter. The system typically is adapted for reducing arc instabilities in a gas discharge lamp, although other gas discharge systems may be adapted as well. The invention also relates to a corresponding optical system, and to a method for controlling a gas discharge system, such as e.g. a gas discharge lamp.

Description

METHODS AND SYSTEMS FOR CONTROLLING GAS DISCHARGE LAMPS

Technical field of the invention

The invention relates to the field of lighting and heating. More particularly, the invention relates to methods and systems for operating gas discharge systems, such as e.g. gas discharge lamps, and corresponding gas discharge systems, such as e.g. gas discharge systems like welding systems.

Background of the invention

The use of high intensity discharge (HID) lamps is widely spread. Due to their high light output, low power consumption and good colour temperature, they are used in a variety of applications such as outdoor lighting, large area lighting, automotive applications such as e.g. as headlights, etc. In a HID lamp typically the light is emitted from an arc discharge between two closely spaced electrodes hermetically sealed inside a small envelope capsule. Typically ballasts are required to supply proper voltage and to control current in order to have good operation of the lamps. A schematic overview of an exemplary HID lamp is shown in Fig. 1. Typically, a HID lamp 10 comprises a lamp bulb 12 containing a small arc tube 14 inside. In the arc tube 14, a top and bottom electrode 16 are positioned closely spaced. Typically the lamp also has a lamp foot 18 for connecting to a power source and providing driving current. The lamp may comprise or may be coupled to ballasts (not shown) present in order to generate arc discharges between the electrodes 16.

A typical problem occurring in HID lamps is arc instabilities, resulting in bad operation and/or disturbing side effects. Several types of instabilities are possible like a deflection of the arc near the electrodes, a deflection to the edge of the gas tube or several deflections in the ionised gas. In Fig. 2 an unstable discharge arc between a pair of discharge electrodes is shown. These arc instabilities can be seen by the human eye through an optical filter by looking in the arc.

The above described arc instabilities can be caused by so called acoustic resonance. Acoustic resonance phenomena are explained in different ways. Initially, standing pressure waves occurring in the gas filling of the lamp were seen as the origin of the problem. In that way the instability can be seen as the resonance when the applied current/voltage frequency is the same as the resonant frequency of the tube-gas system. Another idea was that acoustic resonance is caused by an instability in the ion plasma of the ionised gas.

The resonance phenomena typically is dependent on many external conditions like distance between the electrodes, tube radius, gas pressure, gas temperature, power, etc and thus it is clear that during operation and lifetime the unstable frequency points or band can shift. Not only may several parameters vary during operation and lifetime of a lamp, these and other parameters like gas chemical components and concentrations may also vary when different lamps of a same type are considered. Some acoustic resonance detection methods are known, which are based on observing variations in the electrical parameters of the electric ballast. These parameters can be lamp voltage, lamp current, and lamp impedance. Since these deviations are small under acoustic resonance conditions, known detection methods typically make use of the standard deviation variation of the lamp parameters, rather than an appropriate detection of acoustic resonance conditions. Beside detection methods for detecting acoustic resonance also several control methods for controlling HID lamps are proposed to avoid acoustic resonance, such as frequency hopping or modulation or very high frequency operation (above acoustic resonance occurrence band).

Summary of the invention

It is an object of the present invention to provide alternative or efficient methods for operating gas discharge systems such as e.g. gas discharge lamp systems and to provide efficient gas discharge systems such as e.g. gas discharge lamp systems. It is an advantage of embodiments of the present invention to provide methods for operating gas discharge systems such as e.g. lamp systems and efficient gas discharge systems such as e.g. gas discharge lamp systems resulting in high quality lighting e.g. by reducing or possibly avoiding flickering of the gas discharge system. It is an advantage of embodiments of the present invention that methods for operating and controlling gas discharge lamps, such as e.g. high intensity discharge lamps, are provided that are universally applicable for a broad range of lamp types and ballast topologies. It is an advantage of embodiments of the present invention that operating methods and gas discharge systems are provided that are able to operate at higher frequencies.

It is an advantage of embodiments of the present invention that systems with cheaper and lighter ballast topologies are obtained, systems with a longer lamp lifetime are obtained, systems with a high efficiency are obtained and systems with a high full power control are obtained. The latter is obtained by operating the gas discharge lamp of the system at higher frequencies.

It is an advantage of embodiments of the present invention that gas discharge systems with reduced arc instability can be obtained whereby the gas discharge systems can use cheaper and lighter ballast topologies and ballast topologies having smaller losses.

The above objective is accomplished by a method and device according to the present invention. The present invention relates to a control system for controlling a gas discharge system, the control system comprising a detection means for detecting a physical parameter of the gas discharge system operated at an operating frequency, and an evaluation means for evaluating the temporal behaviour of said detected physical parameter of the gas discharge system, or for spectrally evaluating variations in said detected physical parameter of the gas discharge system.

The present invention relates to a control system for controlling a gas discharge system, the control system comprising a detection means for detecting a physical parameter of the gas discharge system operated at an operating frequency, and an evaluation means for evaluating variations in said detected physical parameter of the gas discharge system as function of temporal frequency.

The physical parameter may vary whereby different frequency components of these variations may be present. The evaluating may be evaluating variations in said detected physical parameter of the gas discharge system as function of temporal frequency of the variations. It is an advantage of embodiments of the present invention that efficient and accurate systems for controlling the gas discharge system are obtained.

The control system may control a gas discharge system being a gas discharge lamp. The gas discharge system also may be e.g. an arc discharge system adapted for heating or welding. It is an advantage of embodiments of the present invention that the control system may be applied to a variety of gas discharge systems, e.g. a variety of gas discharge lamps such as low pressure lamps and high pressure high intensity discharge lamps.

The evaluation means may be an evaluation means for evaluating an intensity of the variations of said physical parameter as function of frequency. As function of frequency thereby may be as function of temporal frequency. The physical parameter thereby may be obtained at one operating frequency. It is an advantage that the control system for reducing or avoiding discharge instabilities, e.g. arc instabilities, can be obtained to control the system during operation, thus being able to compensate for changes of the gas discharge system over time. It thus is an advantage of embodiments of the present invention that they allow both calibration and dynamic control. The evaluation as function of frequency may be adapted for evaluating frequency contributions in a frequency range between 0 Hz and two times the operating frequency. The whole frequency range or predetermined frequencies or predetermined frequency ranges may be evaluated. It is an advantage of embodiments of the present invention that an accurate system for reducing or avoiding discharge instabilities, e.g. arc instabilities, may be obtained.

The frequency range may have a lower limit of 0.1 Hz, 1 Hz or 10Hz and may have a higher limit of 0.8kHz, 1 kHz, 2kHz, the operating frequency of the gas discharge system, e.g. gas discharge lamp, or double the operating frequency of the gas discharge system, e.g. gas discharge lamp. Evaluation means for evaluating frequency contributions may comprise an evaluation means for evaluating frequency contributions at predetermined frequencies or predetermined frequency intervals in said frequency range. As function of frequency thus may mean as function of frequency components of the variations occurring in the physical parameter. It is an advantage of embodiments of the present invention that the number of measurements needed for controlling stability of the gas discharge system, e.g. gas discharge lamp, may be limited.

The control system furthermore may comprise a frequency controlling means for controlling an operating frequency of said gas discharge system based on said evaluated physical parameter of said gas discharge system. It is an advantage of embodiments of the present invention that the control system results in gas discharge systems wherein arc instabilities are reduced automated or automatically, thus resulting in high quality systems which are user-friendly. The physical parameter may be an optical parameter. It is an advantage of embodiments of the present invention that the control system has a high sensitivity for discharge instabilities, e.g. arc instabilities. The optical parameter may be an optical parameter detected from an irradiation of the gas discharge system. The physical parameter alternatively or in addition thereto may comprise an electrical parameter and/or an acoustical parameter. The electrical parameter may be an electrical signal, such as e.g. a gas discharge system voltage and/or gas discharge system current. The variations may be variations below the switching frequency for the gas discharge system.

The evaluation means may be adapted for evaluating an optical power spectral density. It is an advantage of embodiments of the present invention that, by using the optical power spectral density, arc instabilities which are not visible with the human eye may be detected and remedied.

The optical parameter may be obtained at at least one emission colour of a colour spectrum of the gas discharge system, e.g. the gas discharge lamp. The detection means may comprise a photo-chemical sensor. The detection means may comprise a photodiode and an amplifier. Alternatively, the detection means may comprise a pressure sensor or an electrical sensor for sensing electrical signals.

The detection means may comprise a plurality of photodiodes and amplifiers. The plurality of photodiodes and amplifiers may be integrated in one circuit.

The detection means furthermore may comprise an optical filter for selecting an optical parameter. The optical filter may be for example a colour filter, an intensity filter such as e.g. a neutral density filter, or a polarisation filter. The frequency controlling means may furthermore be adapted for altering the operating frequency by scanning the operating frequency throughout a predetermined operating frequency range.

It is an advantage of embodiments of the present invention that the control system may be used for detecting a suitable operating frequency during start up of the gas discharge system or during initial calibration of the gas discharge system.

The frequency controlling means may provide an output signal for drive circuitry of said gas discharge system, e.g. gas discharge lamp, said output signal providing operating frequency information for driving said gas discharge system, e.g. gas discharge lamp. It is also an advantage of lamp embodiments of the present invention that by using the optical power spectral density as parameter, control systems independent of the lamp type or ballast type can be obtained.

The operating frequency information may comprise an operating frequency resulting in optical parameter values below a predetermined reference value.

The detection means may be adapted to obtain the optical parameter at several emission colours or several emission colour ranges of the colour spectrum of the gas discharge lamp. The control system furthermore may be adapted for controlling the output power spectrum by varying the output power at a particular emission colour or in a particular emission colour range. This control method can be applied when the dominant colour is outside the visual spectrum like for HID UV lamps.

The system may be adapted for controlling the emitted intensity in a predetermined part of an emission spectrum of the gas discharge system, taking into account an irradiation intensity measured with said detection means. It is an advantage of embodiments of the present invention that the control system and method can be applied when the dominant emission colour is outside the visual spectrum like for HID UV lamps. The implementation of an optical sensor or sensors can generate an optical feedback loop on top of the electrical feedback loop. This optical feedback loop can detect early lamp failure, end-of life stroboscopic effect and/or end of life colour shift. This optical feedback can be applied for dimming the lamp and pulse start regulation. It can also be applied for measuring the temperature of the lamp for hot re-strike waiting time calculation. The detection means may be adapted to measure an emitted light intensity of the gas discharge system and to measure incumbent light or parasitic light while being shielded from the emitted light.

The control system may control two optical sensors whereby one sensor is adapted to measure the emitted light of the lamp and one is shielded from the emitted light of the lamp and is adapted for measuring incumbent light or parasitic light from other sources. Alternatively or in addition thereto, the system may comprise a moveable shielding means or shield to temporarily shield an optical sensor from the emitted light of the lamp. These optical sensor(s) may be photo- chemical sensor(s) and/or photodiode(s). It is an advantage that in some embodiments according to the present invention parasitic light can be cancelled out of the optical measurement and appropriate compensation algorithms can be implemented.

The system furthermore may comprise an integrated light source for calibrating the detection means. The integrated light source may be an on board light source. The integrated light source may e.g. be a light emitting device. The integrated light source may have a predetermined, known emission characteristic.

It is an advantage of some embodiments according to the present invention that calibration of an intensity or colour measurement can be performed in an efficient way, e.g. without the need for additional components separate from the optical system, and in an automatic and accurate way, without the need for calibrating positions of additional external components.

The system furthermore may comprise an optical guiding means to capture emission output of the gas discharge system and to transport the captured emission output to the detection means. The optical guiding means may be a wave guide. The optical guiding means may be an optical fibre.

The present invention also relates to an optical system comprising a gas discharge system, drive circuitry for driving said gas discharge system and a control system for controlling a gas discharge system as described above. It is an advantage of embodiments of the present invention that they can be applied in a variety of applications and does not put high requirements on the gas discharge system or drive circuitry used.

The present invention also relates to a method for controlling a gas discharge system, the method comprising, for at least one operating frequency, detecting a physical parameter of the gas discharge system and evaluating temporal variations in said physical parameter, or spectrally evaluating variations in said physical parameter, and the method further comprising selecting an operating frequency for said gas discharge system based on said evaluated physical parameter.

The present invention also relates to a method for controlling a gas discharge system, the method comprising, for at least one operating frequency, detecting a physical parameter of the gas discharge system and evaluating variations of said physical parameter as function of temporal frequency, and the method further comprising selecting an operating frequency for said gas discharge system based on said evaluated physical parameter. The physical parameter may vary whereby different frequency components of these variations may be present.

The evaluating may be evaluating variations in said detected physical parameter of the gas discharge system as function of temporal frequency of the variations.

Selecting an operating frequency may comprise altering an operating frequency for said gas discharge system or keeping the operating frequency used for the gas discharge system. Selecting an operating frequency may comprise lowering the operating frequency of the gas discharge system or rising the operating frequency of the gas discharge system. The method may be a method for controlling a gas discharge lamp.

Evaluating may comprise evaluating an intensity of variations of said physical parameter as function of frequency. As function of frequency may be as function of temporal frequency. Evaluating as function of frequency may be adapted for evaluating frequency contributions in a frequency range between OHz and two times the operating frequency. Evaluating frequency contributions may comprise evaluating frequency contributions at predetermined frequencies or predetermined frequency intervals in said frequency range.

The physical parameter may be an optical parameter. The gas discharge system may be adapted for irradiating, wherein said detecting a physical parameter may comprise detecting an optical parameter from irradiation of said gas discharge system.

Detecting an optical parameter from irradiation of said gas discharge system may comprise capturing irradiation of said gas discharge system and guiding said irradiation to a detector.

Detecting an optical parameter of the gas discharge system may comprise detecting the optical parameter at at least one emission colour or in at least one emission colour range of a colour spectrum of the gas discharge system. The method may comprise controlling an output power spectrum by varying an output power at one of said at least one emission colour or emission colour range.

The method may furthermore comprise calibrating the detection means using an integrated irradiation source.

The method may furthermore comprise calibrating the detection means using capturing emitted radiation from the gas discharge system and measure incumbent light or parasitic light while being shielded from the emitted radiation.

The method further may comprise driving said gas discharge system at said selected operating frequency. The at least one operating frequency may be a plurality of operating frequencies and selecting an operating frequency may be selecting an operating frequency of said plurality of operating frequencies.

Detecting a physical parameter of the gas discharge system may comprise detecting a power spectral density of the emitted light. The power spectral density may be the power spectral density for a emission frequency or emission frequency range for characteristic colour emissions of the gas discharge system.

Detecting an optical parameter may comprise filtering irradiation emitted by the gas discharge system. Evaluating may comprise evaluating whether frequency contributions of variations of said physical parameter are larger than a predetermined value.

The predetermined value may be determined as function of a minimum and maximum value of said physical parameter for a given operating frequency or in a given operating frequency range. Selecting an operating frequency may comprise selecting an operating frequency where frequency contributions of variations of said physical parameter have a minimum value for the at least one operating frequency.

The method may comprise varying the operating frequency in steps up or down around an initially selected operating frequency used and performing said detecting and evaluating step until said optical parameter or said evaluated parameter related thereto reaches substantially a minimum value.

The present invention also relates to a method for calibrating a gas discharge system, the method comprising prior to standard operation of the gas discharge system, for a plurality of operating frequencies of an operating frequency range of the gas discharge system, detecting a physical parameter of the gas discharge system and evaluating temporal variations in said physical parameter, or spectrally evaluating variations in said physical parameter, and the system furthermore comprising selecting an operating frequency for said gas discharge system based on said spectrally evaluated variations in said physical parameter.

The present invention also relates to a method for calibrating a gas discharge system, the method comprising prior to standard operation of the gas discharge system, for a plurality of operating frequencies of an operating frequency range of the gas discharge system, detecting a physical parameter of the gas discharge system and evaluating variations in said physical parameter as function of temporal frequency of the physical parameter, and the system furthermore comprising selecting an operating frequency for said gas discharge system based on said evaluated variations in said physical parameter. The physical parameter may vary whereby different frequency components of these variations may be present. The evaluating may be evaluating variations in said detected physical parameter of the gas discharge system as function of temporal frequency of the variations. Evaluating may comprise evaluating an intensity of variations of said physical parameter as function of frequency of the variations. As function of frequency may be as function of temporal frequency.

Evaluating as function of frequency may be adapted for evaluating frequency contributions in a frequency range between OHz and two times the operating frequency.

Evaluating frequency contributions may comprise evaluating frequency contributions at predetermined frequencies or predetermined frequency intervals in said frequency range.

Said physical parameter may be an optical parameter. The gas discharge system may be adapted for irradiating, wherein said detecting a physical parameter may comprise detecting an optical parameter from irradiation of said gas discharge system. Detecting an optical parameter from irradiation of said gas discharge system may comprise capturing irradiation of said gas discharge system and guiding said irradiation to a detector.

Detecting an optical parameter of the gas discharge system may comprise detecting the optical parameter at at least one emission colour or in at least one emission colour range of a colour spectrum of the gas discharge system.

The method may comprise controlling an output power spectrum by varying an output power at one of said at least one emission colour or emission colour range. The method may furthermore comprise calibrating the detection means using an integrated irradiation source.

The method may furthermore comprise calibrating the detection means using capturing emitted radiation from the gas discharge system and measure incumbent light or parasitic light while being shielded from the emitted radiation. The present invention furthermore relates to a method for detecting arc instabilities in a gas discharge system, the method comprising detecting a physical parameter of the gas discharge system and evaluating temporal variations in said physical parameter, or spectrally evaluating variations in said physical parameter.

The present invention furthermore relates to a method for detecting arc instabilities in a gas discharge system, the method comprising detecting a physical parameter of the gas discharge system and evaluating variations in said physical parameter as function of temporal frequency. The physical parameter may vary whereby different frequency components of these variations may be present. The evaluating may be evaluating variations in said detected physical parameter of the gas discharge system as function of temporal frequency of the variations.

Evaluating may comprise evaluating an intensity of variations of said physical parameter as function of frequency of the variations. As function of frequency may be as function of temporal frequency.

The frequency range may have a lower limit of 0.1 Hz, 1 Hz or 10Hz and may have a higher limit of 0.8kHz, 1 kHz, 2kHz, the operating frequency of the gas discharge lamp or double the operating frequency of the gas discharge lamp.

The invention also relates to a computer program product for executing a method for controlling or a method for calibrating or a method for detecting arc instabilities as described above. It also relates to a machine readable data storage device storing such a computer program product and/or the transmission of such a computer program product over a wide area network. Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature. The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

Brief description of the drawings

Fig. 1 is a schematic representation of a high intensity discharge lamp as known in prior art.

Fig. 2 is a schematic representation of arc instabilities that may occur in a gas discharge lamp, as known in prior art.

Fig. 3 is a schematic representation of a test bench as may be used in embodiments according to the present invention.

Fig. 4 is an exemplary illustration of a lamp power spectral density as may occur in a gas discharge lamp and a schematic representation of a photodiode sensitivity curve as may be used in embodiments according to the present invention.

Fig. 5 is an exemplary illustration of a lamp power variation as may occur in a gas discharge lamp known from prior art and as may be controlled using embodiments according to the present invention.

Fig. 6 is an exemplary illustration of a lamp power variation with acoustic resonance, as may occur in a gas discharge lamp known from prior art and as may be controlled using embodiments according to the present invention. Fig. 7 is an exemplary illustration of a power spectral density of a gas discharge lamp in a selected frequency band with and without arc instability, as may be used in embodiments according to the present invention.

Fig. 8 is an exemplary illustration of a filtered spectral density of a lamp power for an arc discharge lamp, as may be used in embodiments according to the present invention.

Fig. 9 is the schematic implementation of the principle in Fig. 10b

Fig. 10a is a flow chart showing different steps of an exemplary method according to an embodiment of the present invention.

Fig. 10b is a schematic illustration of different components of a system according to embodiments of the present invention and the corresponding method steps performed therewith.

Fig. 11 is a graph indicating experimental spectral contributions of variations in detected power for gas discharge lamps operated at different operating frequencies, illustrating advantages of embodiments of the present invention.

In the different figures, the same reference signs refer to the same or analogous elements.

Description of illustrative embodiments The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, bottom and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Similarly, it is to be noticed that the term "coupled", also used in the claims, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression "a device A coupled to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.

In an example, reference will be made to transistors. These are three- terminal devices having a first main electrode such as a drain, a second main electrode such as a source and a control electrode such as a gate for controlling the flow of electrical charges between the first and second main electrodes. It will be clear for a person skilled in the art that the present invention is also applicable to similar devices that can be configured in any transistor technology, including for example, but not limited thereto, CMOS, BICMOS, Bipolar and SiGe BICMOS technology. The following terms are provided solely to aid in the understanding of the invention. These definitions should not be construed to have a scope less than understood by a person of ordinary skill in the art. The term gas discharge system comprises a system wherein glow discharge, low pressure discharge, arc discharge or high pressure discharge occurs. In some embodiments according to the present invention, the gas discharge system may be a low pressure discharge system. Typically the terms "frequency" components, "frequency" controlling means, "spectral" components and "spectrally" evaluating are used to refer to the temporal behaviour of the variations of the physical parameter. The present invention is not limited thereto and the above terms may also refer to the emission wavelength or emission colour of the irradiation of a gas discharge system. However, if reference is made to the emission colour or emission wavelength typically the term "emission" or "colour" will be used as an indicator.

With optical parameter, in the present invention a parameter related to electromagnetic radiation is meant, the invention not being limited to only visible light but also relating to other types of electromagnetic radiation such as e.g. but not limited, to infrared radiation or UV radiation.

The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims.

The present invention relates to gas discharge systems such as for example - but not limited to - gas discharge lamps such as e.g. high intensity discharge (HID) lamps, drive electronics for such lamps, adjusting systems for such lamps and method for controlling and/or driving such lamps. Different types of high intensity discharge lamps are known. HID lamps typically are operated using a magnetic or an electronic ballast. Typically, a magnetic operating frequency is slightly above the normal power line frequencies, i.e. typically 50Hz. High intensity discharge lamps based on electronic ballasts typically may be operated either in DC or in AC regime. In DC regime, typically lamps may be operated at a frequency in a range having a lower limit of 30Hz, or of 50Hz, or of 100Hz and an upper limit of 500Hz, or of 400 Hz, or of 300 Hz, such as e.g. a lamp operated at a frequency of 200Hz. In AC regime, typically lamps are driven in high frequency mode such as e.g. above 1 kHz, preferably above 3 kHz, more preferably above 5kHz. Typical examples of high intensity discharge lamps are high pressure sodium lamps, high pressure mercury lamps and metal halide lamps, although the invention is not limited thereto. Another type of lamp to which the present invention can be applied is low pressure discharge lamps, for example a low pressure sodium lamp.

In embodiments of a first aspect, the present invention relates to a control system for controlling a gas discharge system. Such a gas discharge system typically may be a gas discharge lamp, although the invention also may relate to other systems such as welding systems or heating systems based on arc discharge. The control system thereby is adapted for controlling the operation of the gas discharge system such that arc instabilities are reduced or avoided. Arc instability can be seen as any deflection from a straight arc between the electrodes in the gas tube. A schematic representation of a control system according to embodiments of a first aspect is shown in Fig. 3a. Typically the control system 100 comprises a detection means 102 for detecting a physical parameter of a gas discharge system, e.g. gas discharge lamp 10, operated at an operating frequency. The control system 100 furthermore comprises an evaluating means 104 for evaluating the temporal variations in the physical parameter, or spectrally evaluating variations in the obtained physical parameter or a parameter related thereto. With spectrally evaluating there may be meant that the different frequency components of the variations in the obtained physical parameter may be evaluated. The algorithm using detection of variations below the switching frequency can be implemented on any type of physical parameter. The control system furthermore may comprise a frequency controlling means 106 for controlling the frequency at which the gas discharge system 10 is driven. It will be clear to the person skilled in the art that both the gas discharge system 10 as well as driving circuitry 108 for driving the gas discharge system do not need to be part of the control system 100, but that the control system 100 typically may be adapted to cooperate with these features, i.e. the control system 100, or more particularly the detection means 102, may be adapted for obtaining a physical parameter of the gas discharge system. The physical parameter may be any suitable parameter, such as an electrical parameter from the gas discharge system or the driving circuitry thereof, an acoustical parameter from the gas discharge system or an optical parameter. An electrical parameter may be a voltage or current of the gas discharge system. In the present application, the systems and methods will be illustrated by way of using an optical parameter, the invention thus not being limited thereto. If the physical parameter is an optical parameter, the detection means 102 thus may detect an optical parameter based on receiving radiation from the gas discharge system, e.g. gas discharge lamp 10, in the detection means. The frequency controlling means 106 may be adapted to provide a control signal for the drive circuitry 108 in order to induce the drive circuitry 108 to operate the gas discharge system, e.g. gas discharge lamp 10 at a given frequency. Alternatively the control system 100 also may incorporate drive circuitry for the gas discharge system or part of the drive circuitry. Typically the drive circuitry 108 may comprise the ballasts typically required for operating a gas discharge system, e.g. a gas discharge lamp. The control system 100 typically, in case a gas discharge lamp is considered, may be introduced in an optical system suitable for using a gas discharge lamp, such as e.g. a high intensity discharge (HID) lamp.

A specific example of a control system - the invention not being limited thereto - is indicated in Fig. 3b, indicating a control system 200 with a detection means 102 comprising a photodiode 202 and amplifier 204, an evaluating means 104 comprising a spectrum analyser 206 and a frequency controlling means 106 being a frequency controllable converter 208. A further example will be provided later. In general, the detection means 102 may comprise any suitable type of sensor for detecting a physical parameter of the gas discharge system, e.g. gas discharge lamp. The physical parameter may be any suitable parameter comprising variations induced by arc instabilities, whereby different components of such variations may be present. The physical parameter may be an optical parameter. Alternatively or in addition thereto, the physical parameter also may be an electrical parameter or an acoustical parameter. If an optical parameter is used, the detection means may be any suitable type of sensor for detecting an optical parameter of the gas discharge lamp. If an electrical parameter is used, the detection means may be any suitable electrical equipment for detecting such electrical parameter. If an acoustic parameter is used, the detection means may be any suitable acoustic sensor, such as a highly sensitive pressure sensor, the invention not being limited thereto. By way of example, the detection means will be described in more detail for detection of an optical parameter, the invention not being limited thereto. The detection means may be a sensor allowing to sense and convert photons from the gas discharge lamp into a signal that represent the intensity of the photons for a given frequency band. The optical parameter may be any suitable parameter. Typically such a parameter may be any suitable optical parameter comprising variations induced by arc instabilities, whereby different components of such variations may be present. It may e.g. be or may contribute to the optical power spectral density around one dominant colour of the lamp. The optical parameter possibly also could be the emission wavelength e.g. average emission wavelength, for irradiation of a given gas discharge system. Such optical power spectral density shows variations above the operating frequency of the converter in normal operation. When arc instability or acoustic resonance occurs the optical power spectral density typically may show variations below the operating frequency. In case the frequency of these variations are below the gain bandwidth of the human eye humans see these variations as flickering. Nevertheless, if the frequency of these variations is above the gain bandwidth of the human eye, they still can be detected with an optical sensor with a gain bandwidth or bandwidth higher then the human eye. The sensor of the detection means 102 may e.g. be a photo-detector, such as e.g. but not limited to a photodiode with an amplifier. A radiation guiding means 103 may be provided to capture radiation of the gas discharge system, e.g. HID lamp and guide the radiation to the detection means 102. Such a radiation guiding means may be for example a wave guide or optical fibre. The detection means 102 typically may be adapted to supply input signals for an evaluating means 104. The detection means 102 furthermore may comprise a filtering means or filter 105 for filtering the light signal in front of the sensor, thus increasing the selectivity in order to select a band of power spectral density of emitted light to be measured. Such a filter may be an optical filter filtering e.g. for colour, intensity e.g. using a neutral density filter, or polarisation filter. The filter may be applied before the radiation of the gas discharge system enters the sensor. The detection means 102 may be adapted to have a required level of sensitivity at the peak in the power spectrum of the gas discharge lamp to be studied. Detection means with a wide spectral sensitivity thus may be advantageous as they may be used with a wide variety of gas discharge lamps. If e.g. a sodium gas discharge lamp is to be controlled, typically the spectral sensitivity may be significantly high near a wavelength of 600nm.

By way of example, the invention not limited thereto, the spectral sensitivity of a detection means 102 is shown suitable for a high pressure mercury lamp, in Fig. 4. In graph A, the lamp power spectrum is shown, in graph B, the sensitivity of the detection means 102 is indicated and in graph C, the power spectrum as detected by the detection means 102 is indicated. It can be seen that the detection means 102 selectivity, e.g. photodiode selectivity, overlaps with peaks in the power spectrum of the gas discharge lamp, in the present example high pressure mercury lamp. As indicated in graph A and graph B, the peaks in the power spectrum may show variation over time. Since the ionisation of the gas typically is controlled by the alternating current the variation of the power spectrum may have in normal operation the double frequency as the current. In Fig. 5 the lamp power variation as a function of time in normal operation is depicted. The dotted line shows a halve a period of the alternating control current. The output power of the lamp shows a large DC component with an superposed AC component coming from the operating frequency. This measurement shows that due to the time constant of the gas and tube system the emitted intensity is constant represented by the DC component and slightly varying with the AC component. In Fig. 6 it is shown that under arc instability condition the output power of the lamp shows a different pattern. On top of the DC component not only the operating frequency AC component is added (period T2-T1 ) but also a low frequency component (period T3-T0). In Fig. 6, by way of illustration only one low frequent component is shown but in reality several low frequency components are seen. If some or one of the low frequency components are within the bandwidth of the human eye the phenomena is perceived as flickering. If the frequency components are above the bandwidth of the human eye the phenomena can only be seen in the bending of the arc and still lead to unstable arc but can not be seen by the human eye in flickering light. The evaluating means 104 may be any suitable evaluating means allowing evaluation of temporal variations in said physical parameter, or spectrally evaluation of variations in the detected physical parameter, e.g. optical parameter or acoustical or electrical parameter, or a parameter related thereto outputted by the detecting means 102. Typically the gas discharge system is operated at a given operating frequency. The detected physical parameter may be provided for at least one operating frequency. In case the detected physical parameter is an optical parameter or a parameter related thereto, it may be a parameter that is representative of the emitted light power spectral density in a given spectral band of the light spectrum. This parameter typically has an optical characteristic which is a function of the frequency.

The evaluating means 104 may be adapted for evaluating an intensity of the variations of the detected physical parameter as function of frequency of the variations. In other words, frequency components, also referred to as spectral components, of the variations of the detected physical parameters may be evaluated. The evaluating means may be adapted for evaluating frequency contributions of the variations of the detected physical parameter in a frequency range between 0 Hz and two times the operating frequency. The DC component thus may be excluded and the frequency range may reach up to two times the operating frequency. The evaluation typically may be performed in a smaller frequency range, e.g. up to the operating frequency, the operating frequency not included. The whole frequency range as well as predetermined frequencies or predetermined frequency range contributions may be evaluated. The frequency range may have a lower limit of 0.1 Hz, 1 Hz or 10Hz and may have a higher limit of 0.8kHz, 1kHz, 5kHz, the operating frequency of the gas discharge lamp or double the operating frequency of the gas discharge lamp. Contributions stemming from a single frequency as well as contributions stemming from a frequency interval in the frequency range may be used for the evaluation.

For a given operating frequency of the gas discharge lamp, evaluation may be performed by comparing frequency contributions of variations of the physical parameter with a predetermined reference level. Such a predetermined reference level may for example be a fixed level based on reference systems, may be determined from theoretical models, may be based on previously obtained experimental results or may be determined during calibration of the system. In the latter case, e.g. the operating frequency band may be scanned and the predetermined level for the frequency contributions of the variations of the physical parameter may be put on ( Pmax -Pmin)*X% above the minimum detected level whereby X may be a predetermined number, e.g. between 0.1 and 30, e.g. between 1 and 20, e.g. 10. The symbols Pmax and Pmin thereby may represent the maximum intensity and the minimum intensity. When the maximum intensity for a frequency contribution of the variations of the physical parameter is higher than the predetermined reference level, arc instability is detected. Typically the evaluating means may comprise e.g. - but is not limited to - dedicated computation means such as a programmable logic device, sometimes referred to as PAL, PLA, FPGA, PLD, EPLD, EEPLD, LCA or FPGA. The latter are well- known integrated circuits that provide the advantages of fixed integrated circuits with the flexibility of custom integrated circuits. Such devices allow a user to electrically program standard, off-the shelf logic elements to meet a user's specific needs. In particular, such processing engines may be embedded in dedicated circuitry such as a VLSI. Also a digital signal processor (DSP), a general purpose processor (GPP), an application specific integrated circuit (ASIC), a microprocessor, a micro controller or a microcomputer can be used. The evaluating means may be provided in a number of different ways, e.g. by providing a frequency spectrum analyser or by evaluating a specific filtered frequency contribution of variation of a physical parameter detected. Such filtering may be performed in different ways, such as e.g. using a spectral filter, such as a low pass filter, a band pass filter. Such filters may be higher order filters. A typical example of a circuitry using a band pass filter will be described by way of illustration below. Alternatively, an appropriate signal may be obtained by performing digital signal processing. A further alternative may be applying integration techniques to obtain an appropriate signal.

In Fig. 7 it is by way of example shown that in case there is no unstable arc or acoustic resonance there are no low frequent components up to -8OdB in the frequencies between DC and the double of the operating frequency, indicated by area D in Fig. 7 graph A.. In case arc instability is occurring, the lower frequency components may increase for example with more than 2OdB in the range having a lower limit of 1 Hz and an upper limit of I kHz₁ depending on the severity of the arc instability. By way of example, in Fig. 7 graph B several low frequency components are shown up to -4OdB in the range between DC and the double operating frequency. This means an increase of 4OdB that is clearly detectable. It is an advantage of embodiments of the present invention that the increase in power in the low frequency band, i.e. typically within a range having a lower limit of e.g. 0.1 Hz, e.g. 1 Hz, e.g. 10 Hz and an upper limit of e.g. twice the operating frequency, e.g. the operating frequency, e.g. 5kHz, e.g. 2kHz, e.g. 1 kHz, e.g. 0.8kHz, may be used to determine arc instability. In Fig. 8 an exemplary illustration of a filtered spectral density of a lamp power for an arc discharge lamp.

In order to limit the number of measurements to be performed and or in order to obtain an improved sensitivity, the evaluation means 104 may be adapted to filter the different intensity variations, such that only power variations at frequencies slower than the double of the operating frequency need to be detected. A low pass or alternatively a band pass filter between DC and the double operating frequency can filter the components that are increasing under arc instability. As described above, the latter also may be obtained by integration techniques or by applying digital signal processing. Such solutions avoid the need for an expensive spectrum analyser. The frequency controlling means 106, if present, typically may be frequency controlling means adapted for selecting an operating frequency of the gas discharge lamp. These frequency controlling means 106 may be used both for searching an instability free operating zone and/or operating band in the operation frequency range e.g. for different steps in the evaluation process, and for selecting and/or setting an operation frequency for the gas discharge system.

The control system may be adapted for searching an instability free operating zone and/or operating band by detecting a physical parameter and evaluating temporal variations in the detected physical parameter, or spectrally evaluating variations in the detected physical parameter, e.g. evaluating spectral contributions of the variations in intensity of the detected physical parameter for different operating frequencies of the gas discharge system. In other words, the typical operating frequency range may be checked in order to identify an appropriate operating frequency. The frequency controlling means 106 also may be used for dynamically setting and/or selecting an appropriate operation frequency of a gas discharge system, e.g. gas discharge lamp. The operation frequency of the gas discharge system, e.g. gas discharge lamp, thereby typically may be selected based on input information of the evaluating means, whereby the input provided may be spectral information about variations in a physical parameter, e.g. power spectrum information, for different operating frequencies. In this way, by changing the operation frequency of a gas discharge lamp to an instability free region, further arc instabilities can be avoided.

The control system may be operated at start-up of the gas discharge system for determining a reference intensity level for the parameter to be detected or for calibrating the gas discharge system. Thereby a full scan of the operation frequency band may be performed. The control means furthermore may be used during gas discharge system operation, e.g. gas discharge lamp operation, whereby the control means may be adapted to perform adaptive control of the operating frequency, as the instability frequencies can shift and converge to the operating frequency. It is an advantage of embodiments according to the present invention that shift of the safe operating band resulting from life and operating condition variations can be controlled, reduced and possibly even avoided. The control means thus may provide an output signal for the ballast controller to shift the operating frequency up or down in steps till no arc instability is detected. The latter may be performed automated and/or automatically. Controlling the operation of the gas discharge system for arc instabilities may be performed in predetermined time intervals or may be performed continuously. It may be performed by a processing means using an algorithm, a neural network or using any suitable means.

The control means according to embodiments of the present invention thus may be based on the principle that, in case an optical parameter of a gas discharge system is evaluated, arc instability results in a low frequency flickering resulting, besides power in DC, in more spectral power in the low frequency spectrum while without arc instability most of the power may be detected in the high frequency optical power spectrum around the operating frequency of the converter, also referred to as the operating frequency, and its harmonics.

A more detailed example of a control system - provided by way of illustration only - is indicated in Fig. 9, indicating a detection means 102 and evaluating means 104 comprising a number of blocks. In the present example the detection means 102 and the evaluating means 104 circuitry are made of six different blocks. It will be obvious for the person skilled in the art that the invention is not limited to the present example, but that the latter is provided by way of illustration. The detection means 102 of the present example comprises a photodiode 252 and a two stage amplifier with internal feedback and external feedback 254. The first stage consists, in the present example, of an Nmos input pair and the second stage consists of an operational amplifier with local feedback. In the present example, the evaluation means comprises a bandpass filter 256 allowing signals to pass from 1 Hz to 10kHz. The evaluation means 104 furthermore comprises a buffer and amplifier for the filtered signal, shown in block 258 and a comparator with reference level dependent on the lamp type but generated with a resistive divider, as shown in block 260. The control system furthermore may comprise a controlling means 264 which, when the comparator output is high, the SET - RESET flip flop is set high. The controller furthermore will reset the flip flop when a new detection and evaluation cycle needs to be started and the flip will be set again in case arc instability is still available. In operation, the photodiode detects the emitted light power. The emitted light power typically consists of a large DC component and a large AC component on the double of the operating frequency of the converter in case no arc instability occurs. In this case, in the present example, the bandpass filter 256 rejects the DC component and rejects also the AC component if switching is provided above 5kHz. It is to be noted that the bandpass filter can be adapted to the typical operation frequency used. This results in a low output of the comparator and the flip flop will not be set. When arc instability occurs typically modulation components are superimposed on the DC signal. These superimposed signals pass the bandfilter in case their frequency is between 1 Hz and 10KHz. These signals are amplified by the buffer- amplifier on the filter output and make the comparator toggle. The comparator toggling is setting the flip-flop. When the flip-flop output is high are instability is detected and the frequency of the converter can be switched up or down and a new detection loop can be started. This algorithm can be repeated until the flip flop output stays low after a reset. In a particular embodiment, the optical parameter may be obtained at several emission colours of the colour spectrum of the gas discharge system such as the gas discharge lamp. Measurement of the emission colour spectrum then furthermore may be used to control the output power spectrum by varying the output power in a particular colour range. In other words, a particular range of emission wavelengths may be used to control the output power. The latter can for example advantageously be used when the dominant colour is outside the visual spectrum, such as for example for HID UV lamps. Detection of such emission may be performed using an optical detector, such as e.g. a photo-chemical sensor or photodiode. The light intensity may be detected and used for controlling the emission intensity in a particular part of the spectrum. The latter may e.g. be performed in addition to the gas discharge system control mechanisms as described above. The control method can be applied when the dominant colour is outside the visual spectrum like for HID UV lamps. The implementation of an optical sensor or sensors may allow to generate an optical feedback loop. Such an optical feedback loop may be on top of the electrical feedback loop, used for driving the gas discharge system based on the information obtained by evaluating the variations of a physical parameter as function of a temporal frequency. The physical parameter may vary whereby different frequency components of these variations may be present. The evaluating may be evaluating variations in said detected physical parameter of the gas discharge system as function of temporal frequency of the variations. The optical feedback loop may provide the possibility detect early lamp failure, end-of life stroboscopic effect or end of life colour shift. This optical feedback can be applied for dimming the lamp and pulse start regulation. It can also be applied for measuring the temperature of the lamp for hot re-strike waiting time calculation.

In another particular embodiment, the present invention may be adapted for cancelling out parasitic light in optical measurements or to provide compensation. This may be obtained by measuring emitted radiation of the gas discharge system and by measuring the same parameter but while being shielded from the emitted radiation of the gas discharge system, thus measuring the incumbent light or parasitic light from other sources. In this way the contribution of incumbent light or parasitic light can be derived and it can be cancelled or compensation can be implemented to take it into account. Such measurements may be based on two different optical detectors whereby one is shielded from the emitted radiation by the gas discharge system and another one is not shielded from the emitted radiation. Alternatively or in addition thereto, the system also may comprise a shielding means for shielding an optical detector from the emitted radiation of the gas discharge system. By making the shielding means moveable, different contributions can be measured by varying the position of the shielding means.

In a further particular embodiment, the detection means 102 can be calibrated by using an integrated radiation source for which the emission characteristic is known. The latter may e.g. be an on board light source, such as e.g. a light emitting device (LED) or any other suitable radiation source with known characteristic. The latter results in efficient ways to calibrate the system, as for example improved accuracy in positions of components during the calibration can be obtained. It is to be noticed that the concepts of these particular embodiments may be applied to a control system for a gas discharge system independent of the control system based on evaluation of variations of a physical parameter as function of the temporal frequency, but it is advantageous to combine these and the number of components needed in the control system can be reduced. For example, the detection means may be used for both the temporal frequency analysis as for the analysis of emission colour or parasitic light from other sources.

In embodiments of a second aspect, the present invention relates to an optical system comprising a control system as described in the first aspect, comprising the same features and properties as described above. Such a system comprises a gas discharge system, such as e.g. a gas discharge lamp, such as for example a high intensity discharge lamp as described in more detail above. Such an optical system furthermore comprises drive electronics for driving the gas discharge system using driving conditions as determined using the control system. The optical system may be a general lighting system, an automotive lighting system or any other type of lighting system adapted for using a gas discharge lamp, such as for example a high intensity discharge lamp.

In a third aspect, the present invention relates to a method for controlling operation of a gas discharge system, e.g. a gas discharge lamp, such as for example a high intensity discharge (HID) lamp. The method for controlling operation may both be used for calibrating the gas discharge system, e.g. a gas discharge lamp, thus relating to a method for calibrating a gas discharge system, e.g. a gas discharge lamp, as well as a method for controlling operation of the gas discharge system, e.g. a gas discharge lamp, e.g. dynamic controlling operation of the gas discharge system, e.g. a gas discharge lamp. The method for controlling operation of such a gas discharge system, e.g. gas discharge lamp, typically comprises, for at least one operating frequency of the gas discharge lamp, detecting a physical parameter and evaluating temporal variations in the physical parameter, or spectrally evaluating variations of the physical parameter or a gas discharge system, e.g. gas discharge lamp parameter based thereon and selecting, based on said evaluating, an operation frequency for said gas discharge system, e.g. gas discharge lamp. Selecting an operation frequency for said gas discharge system, e.g. gas discharge lamp may comprise determining an operation frequency for said gas discharge system, e.g. gas discharge lamp and it may comprise using the operation frequency for the gas discharge system, e.g. gas discharge lamp for operating, e.g. driving, the gas discharge system, e.g. gas discharge lamp. Typically, in order to be able to select an operation frequency, at least one operation frequency, e.g. a plurality of different operation frequencies may be tested, whereby for each operation frequency the detecting and evaluating step is performed. A plurality of different operation frequencies may comprise a number of different operation frequencies representative for part of or substantially the full operation frequency range of the gas discharge lamp.

By way of example, the invention not being limited thereto, an overview of different steps are shown in Fig. 10a, showing a flow chart of an exemplary method for controlling operation of a gas discharge system, e.g. a gas discharge lamp.

For a gas discharge system, e.g. gas discharge lamp, at at least one operating frequency, the following first and second step are performed. In a first step 802, a physical parameter of the gas discharge system is sensed. The physical parameter may be any suitable parameter comprising variations induced by arc instabilities, whereby different components of such variations may be present. The physical parameter may be an optical parameter. Alternatively or in addition thereto, the physical parameter also may be an electrical parameter or an acoustical parameter. By way of example, the invention not being limited thereto, the use of an optical parameter will be discussed in more detail. Typically such a parameter may be any suitable optical parameter comprising variations induced by arc instabilities, whereby different components of such variations may be present. The optical parameter may be for example related to the optical power spectral density. The optical parameter possibly also may be an emission wavelength, e.g. average emission wavelength, of irradiance of the gas discharge system. Typically such an optical characteristic provides parameter values for the different spectral contributions in the variation of the optical parameter detected at different frequencies. Frequency components in the variations of the optical parameter may be evaluated in any suitable frequency range, e.g. in a frequency range between 0 Hz and two times the operating frequency. The DC component thus may be excluded and the frequency range may reach up to two times the operating frequency. The evaluation typically may be performed in a smaller frequency range, e.g. up to the operating frequency, the operating frequency not included. The whole frequency range as well as predetermined frequencies or predetermined frequency range contributions may be evaluated. The frequency range may have a lower limit of 0.1 Hz, 1 Hz or 10Hz and may have a higher limit of 0.8kHz, 1 kHz, 5kHz, the operating frequency of the gas discharge lamp or double the operating frequency of the gas discharge lamp. Contributions stemming from a single frequency as well as contributions stemming from a frequency interval in the frequency range may be used for the evaluation. As described above, sensing typically may be performed using any type of sensor suitable for detecting the physical parameter such as for e.g., for optical parameters, e.g. photo-electric or photo-chemical element, that is able to sense and convert photons from light source into a signal that represents the intensity of these photons in a sudden spectral frequency band, i.e. for example for a dominant colour of the lamp. Such a sensor may be for example a photodiode with an amplifier, although the invention is not limited thereto. Sensing an optical parameter of the lamp may comprise converting a light signal into a part of a power spectral density of the lamp, i.e. a part of the amount of power per unit of frequency as function of the frequency. If necessary, sensing an optical parameter thereby also may comprise optically filtering a light signal in front of the sensor e.g. in order to increase selectivity and in order to select a band of the power spectral density of the emitted light. In case electrical parameters or acoustical parameters are used, typically sensitive electric sensors and sensitive pressure sensors may be used. As described above, the optical parameter may be any suitable parameter indicative of the optical output of the gas discharge system. Preferably, the optical parameter may be the optical power spectral density of the lamp. The power spectral density of a light source is a measure of the power carried by each frequency in the spectrum. Frequency thereby refers to the temporal frequency of the output, not to a spectral frequency of the output. As the power spectral density shows variations whereby in normal operation the variations are variations substantially near DC and variations with a double operating frequency or higher and in case of arc instability, the variations at lower frequency in the power spectrum become more dominant. Using power spectral density therefore may allow to use the relationship between high frequency operation and low frequency flickering for detection, e.g. optical detection of the parameter.

In a second step 804, the physical parameter, e.g. optical parameter or electrical or acoustical parameter, or a gas discharge system parameter based thereon is evaluated for temporal variations in the physical parameter, or is spectrally evaluated. This step may comprise evaluating an intensity of the variations of the detected physical parameter as function of frequency of the variations. In other words, frequency components or spectral components of the variations of the detected physical parameters may be evaluated. This step also may comprise evaluating frequency contributions of the variations of the detected physical parameter in a frequency range between 0 Hz and two times the operating frequency. The DC component thus may be excluded and the frequency range may reach up to two times the operating frequency. The evaluating typically may be performed in a smaller frequency range, e.g. up to the operating frequency, the operating frequency not included. The whole frequency range as well as predetermined frequencies or predetermined frequency range contributions may be evaluated. The frequency range may have a lower limit of 0.1 Hz, 1 Hz or 10Hz and may have a higher limit of 0.8kHz, 1 kHz, 5kHz, the operating frequency of the gas discharge system or double the operating frequency of the gas discharge system. Contributions stemming from a single frequency as well as contributions stemming from a frequency interval in the frequency range may be used for the evaluation. Such evaluating may comprise evaluating a parameter that is representative of the emitted light power spectral density in a given spectral band of the light spectrum. The evaluation of the variations of this signal as function of frequency makes it able to detect arc instability. For a given operating frequency of the gas discharge system, evaluating may be performed by comparing frequency contributions of variations of the physical parameter with a predetermined reference level. Such a predetermined reference level may for example be a fixed level based on reference systems, may be determined from theoretical models, may be based on previously obtained experimental results or may be determined during calibration of the system. In the latter case, e.g. the operating frequencies band may be scanned and the predetermined level may be put on (P_MAX-PMIN)*X% above the minimum detected level whereby X may be a predetermined number, e.g. between 0.1 and 30, e.g. between 1 and 20, e.g. 10. The symbols P_MAX and PMIN thereby may represent the maximum intensity and the minimum intensity. When the maximum intensity for a frequency contribution of the variations of the physical parameter is higher than the predetermined reference level, arc instability is detected. In case the optical power spectral density is used, the deviation level may be the total integrated absolute power increase in the selected frequency band. When the intensity of the spectral component of the variation in physical parameter is higher than the predetermined reference level, arc instability may be detected. In order to limit the number of measurements to be performed, only variations slower than the double of the operating frequency may be studied.

In a third step 806, the operating frequency of the gas discharge system is selected, depending on the evaluation of the gas discharge system parameter. Optical parameters obtained by operating the gas discharge system at one frequency may be used, whereby it is decided whether or not the operating frequency of the gas discharge system is appropriate, i.e. does not substantially result in arc instabilities. Alternatively the detection step and evaluating step may be performed for a number of operating frequencies and an appropriate operating frequency may be selected. At start-up, the latter may comprise determining the reference level, whereby a scan of the operation frequency band may be applied. The detection of the resonance free frequencies and selection of the operating frequency may be conducted during a run up stage operation of the lamp, between ignition and steady state. An appropriate instability free operating frequency thereby may be selected. During lamp operation, this third step also may lead to adaptive_. control of the operating frequency, as the instability frequencies can shift and converge to the operating frequency. Typically shift from the instability frequencies to the operating frequency may be detected by a high output obtained in the second step, e.g. a high output by a comparator in a detection system. If the previously used operating frequency thus preferably is not used anymore, selecting the operating frequency of the gas discharge lamp may comprise indicating to the ballast controller to shift the operating frequency up or down in steps till the comparator is low. In this way, in embodiments according to the present invention, operating frequency bands corresponding with the minimum spectral contributions in variations of the physical parameter, e.g. minimum power spectrum density values, for low frequencies can be determined. The latter may be performed automated and/or automatically. It may be performed by a processing means using an algorithm, a neural network or using any suitable means. The method also may be performed by first selecting a suitable operating frequency and further scanning a range around the operating frequency to find a suitable operating frequency, when instability occurs at the first selected operating frequency.

In other words, during operation the acoustic resonance frequency can shift under operating conditions and due to lifetime variations in the operating frequency band. By continuous monitoring of the acoustic resonance parameter adaptive control and flicker free operation can be achieved by setting or shifting the operating frequency in an acoustic resonant free band. Methods according to embodiments of the present aspect typically may be adapted therefor. The method is especially useful in combination with a control system as described in the first aspect of the present invention, as illustrated by way of example in Fig. 10b, indicating possible different steps and different corresponding technical means for performing a control method according to the present invention. It is an advantage of embodiments according to the present invention that operating frequency ranges can be selected wherein no acoustic resonance or other arc instabilities occur.

In a fourth aspect, the present invention relates to a method for detecting arc instabilities in a gas discharge system, e.g. gas discharge lamp. The method for detecting arc instabilities typically comprises the sensing step for sensing a physical parameter of the gas discharge system, e.g. gas discharge lamp, as described in the method according to the third aspect, and the evaluating step for evaluating temporal variations of the physical parameter or for spectrally evaluating variations of the physical parameter or a parameter related thereto as described in the method according to the third aspect. It does not need to be used to alter the operating frequency of the gas discharge system, e.g. gas discharge lamp. The sensing step and the evaluating step typically have the same features and same advantages as described above. The output of the method for detecting arc instabilities may comprise as output specific frequencies at which acoustic or other arc instabilities occur. The method may e.g. be used to evaluate different types and/or different setups of gas discharge systems, such as e.g. gas discharge lamp systems, without, in the present aspect, being necessary to adapt the controlling of the gas discharge system. Evaluating different types and/or different setups of gas discharge systems may result in selection of an optimum gas discharge systems, e.g. in gas discharge lamp systems suffering only little or not from acoustic or arc instabilities.

In a further aspect, the different method steps for controlling a gas discharge system, e.g. gas discharge lamp and/or the different method steps for detecting arc instabilities in a gas discharge system, e.g. gas discharge lamp, may be performed in an automated and/or automatic way on a processing system. The different steps may be implemented in the processing system as hardware or as software. Such a processing system may include at least one programmable processor coupled to a memory subsystem that includes at least one form of memory, e.g., RAM, ROM, and so forth. A storage subsystem may be included that has at least one disk drive and/or CD-ROM drive and/or DVD drive. In some implementations, a display system, a keyboard, and a pointing device may be included as part of a user interface subsystem to provide for a user to manually input information. Ports for inputting and outputting data also may be included. More elements such as network connections, interfaces to various devices, and so forth, may be included. The various elements of the processing system may be coupled in various ways, including via a bus subsystem. The memory of the memory subsystem may at some time hold part or all of a set of instructions that when executed on the processing system implement the step(s) of the method embodiments of the present invention. Thus, while a processing system as such is prior art, a system that includes the instructions to implement aspects of the present invention is not prior art. The present invention also includes a computer program product which provides the functionality of any of the methods according to the present invention when executed on a computing device. Further, the present invention includes a data carrier such as for example a CD-ROM or a diskette which stores the computer product in a machine readable form and which executes at least one of the methods of the invention when executed on a computing device. Nowadays, such software is often offered on the Internet or a company Intranet for download, hence the present invention includes transmitting the computer product according to the present invention over a local or wide area network.

It is an advantage of particular embodiments of the present invention that the control systems, control methods and detection methods are general, independent of lamp type, independent of ballast type and useable over a wide frequency range far above the detection level of the human eye.

It is an advantage of particular embodiments of the present invention that integration of the detection/control method/system in the ballast control loop may result in a universal technique for operating HID lamps of different types, wattage's and manufacturers despite the occurrence of acoustic resonance or arc instabilities among these lamps in a broad frequency range.

The advantages of embodiments of the present invention further are illustrated by way of exemplary experimental results, provided by way of illustration only. By way of example, Fig. 11 illustrates an experimental result indicating spectral contributions of a gas discharge lamp for different operating frequencies. It can be seen that at selected operating frequencies, substantially no spectral contributions are present, at low frequency, e.g. for frequencies under double the operating frequency, whereas for other operating frequencies, substantially large spectral contributions are present at low frequency, e.g. for frequencies under double the operating frequency. It could be seen that for operating frequencies whereby no spectral contributions at low frequency occur no arc instabilities occurred, whereas for operating frequencies whereby spectral contributions at low frequency occur arc instabilities did occur. The latter fully supports the methods and systems as described in the present example.

Other arrangements for accomplishing the objectives of the controlling methods and systems for controlling gas discharge system, e.g. gas discharge lamp systems embodying the invention will be obvious for those skilled in the art. It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention.

Claims

1.- A control system (100) for controlling a gas discharge system (10), the control system (100) comprising

- a detection means (102) for detecting a physical parameter of the gas discharge system (10) operated at an operating frequency

- an evaluation means (104) for evaluating variations in said physical parameter as function of temporal frequency.

2.- A control system (100) according to claim 1 , wherein said gas discharge system (10) is a gas discharge lamp (10).

3.- A control system (100) according to any of claims 1 to 2, wherein said evaluating variations is evaluating an intensity of said variations of said physical parameter as function of temporal frequency.

4.- A control system (100) according to any of claims 1 to 3, wherein said evaluation as function of temporal frequency is adapted for evaluating frequency contributions in a frequency range between 0 Hz and two times the operating frequency.

5.- A control system (100) according to claim 4 wherein evaluating frequency contributions comprises evaluating frequency contributions at predetermined frequencies or predetermined frequency intervals in said frequency range.

6.- A control system (100) according to any of the previous claims, the control system (100) furthermore comprising a frequency controlling means (106) for controlling an operating frequency of said gas discharge system (10) based on said evaluated physical parameter of said gas discharge system (10).

7.- A control system (100) according to claim 6, wherein the frequency controlling means (106) furthermore is adapted for altering the operating frequency by scanning the operating frequency throughout a predetermined operating frequency range.

8.- A control system (100) according to any of claims 6 to 7, the frequency controlling means providing an output signal for drive circuitry (108) of said gas discharge system (10), said output signal providing operating frequency information for driving said gas discharge system (10).

9.- A control system (100) according to the previous claim, wherein the operating frequency information comprises corresponds with an operating frequency resulting in physical parameter values below a predetermined reference value.

10.- A control system (100) according to any of the previous claims, wherein said physical parameter is an optical parameter.

11.- A control system (100) according to claim 10, wherein said evaluation means

(104) is adapted for evaluating an optical power spectral density.

12.- A control system (100) according to any of claims 10 to 11 , wherein said detection means is adapted to obtain the optical parameter at at least one emission colour of a colour spectrum of the gas discharge system.

13.- A control system (100) according to claim 12, wherein the detection means is adapted to obtain the optical parameter at several emission colours or several emission colour ranges of the colour spectrum of the gas discharge lamp and wherein the control system furthermore is adapted for controlling the output power spectrum by varying the output power at a particular emission colour or in a particular emission colour range.

14.- A control system (100) according to any of the previous claims, wherein said detection means (102) comprises a photo-chemical sensor.

15.- A control system (100) according to any of the previous claims, wherein said detection means comprises at least one photodiode and at least one amplifier.

16. A control system (100) according to any of the previous claims, wherein said system is adapted for controlling the emitted intensity in a predetermined part of an emission spectrum of the gas discharge system, taking into account an irradiation intensity measured with said detection means (102).

17.- A control system (100) according to any of the previous claims, wherein the detection means furthermore comprises an optical filter (105) for selecting an optical parameter.

18.- A control system (100) according to any of the previous claims, wherein the detection means is adapted to measure an emitted light intensity of the gas discharge system and to measure incumbent light or parasitic light while being shielded from the emitted light.

19.- A control system (100) according to any of the previous claims, the system furthermore comprising an integrated light source for calibrating the detection means (102).

20.- A control system (100) according to any of the previous claims in as far as dependent on claim 10, wherein the system furthermore comprises an optical guiding means (103) to capture emission output of the gas discharge system and to transport the captured emission output to the detection means (102).

21.- An optical system comprising a gas discharge system (10), drive circuitry (108) for driving said gas discharge system and a control system (100) for controlling a gas discharge system as described in any of claims 1 to 20.

22.- A method (800) for controlling a gas discharge system, the method comprising

- for at least one operating frequency - detecting (802) a physical parameter of the gas discharge system (10)

- evaluating variations of said physical parameter as function of temporal frequency and, - selecting (806) an operating frequency for said gas discharge system based on said evaluated physical parameter.

23.- A method (800) according to claim 22, wherein evaluating variations comprises evaluating intensity or amplitude variations of said physical parameter as function of temporal frequency.

24.- A method (800) according to any of claims 22 to 23, wherein evaluating as function of frequency is adapted for evaluating frequency contributions in a frequency range between OHz and two times the operating frequency.

25.- A method (800) according to claim 24, wherein evaluating frequency contributions comprises evaluating frequency contributions at predetermined frequencies or predetermined frequency intervals in said frequency range.

26.- A method (800) according to any of claims 22 to 25, wherein said physical parameter is an optical parameter.

27.- A method (800) according to claim 26, the gas discharge system adapted for irradiating, wherein said detecting a physical parameter comprises detecting an optical parameter from irradiation of said gas discharge system.

28.- A method (800) according to claim 27, wherein detecting an optical parameter from irradiation of said gas discharge system comprises capturing irradiation of said gas discharge system and guiding said irradiation to a detector.

29.- A method according to any of claims 22 to 28, said method further comprising driving said gas discharge system at said selected operating frequency.

30.- A method according to any of claims 22 to 29, wherein said at least one operating frequency is a plurality of operating frequencies and wherein selecting an operating frequency is selecting an operating frequency of said plurality of operating frequencies.

31.- A method according to any of claims 22 to 30, wherein detecting a physical parameter of the gas discharge system comprises detecting a power spectral density of the emitted light.

32.- A method according to any of claims 22 to 31 in as far as dependent on claim

26, wherein detecting an optical parameter comprises filtering irradiation emitted by the gas discharge system (10).

33.- A method according to any of claims 22 to 32 in as far as dependent on claim

24, wherein evaluating comprises evaluating whether frequency contributions of variations of said physical parameter are larger than a predetermined value.

34.- A method according to claim 33, wherein said predetermined value is determined as function of a minimum and maximum value of said physical parameter for a given operating frequency or in a given operating frequency range.

35.- A method according to any of claims 22 to 34 in as far as dependent on claim

24, wherein selecting an operating frequency comprises selecting an operating frequency where frequency contributions of variations of said physical parameter have a minimum value for the at least one operating frequency.

36.- A method according to any of claims 22 to 35, wherein said method comprises varying of the operating frequency in steps up or down around an initially selected operating frequency used and performing said detecting and evaluating step until said optical parameter or said evaluated parameter related thereto reaches substantially a minimum value.

37.- A method according to any of claims 22 to 36 in as far as dependent on claim 26, wherein detecting an optical parameter of the gas discharge system comprises detecting the optical parameter at at least one emission colour or in at least one emission colour range of a colour spectrum of the gas discharge system.

38.- A method according to any of claims 22 to 37, said method comprising controlling an output power spectrum by varying an output power at one of said at least one emission colour or emission colour range.

39.- A method according to any of claims 22 to 39, wherein the method furthermore comprises calibrating the detection means (102) using an integrated irradiation source.

40.- A method according to any of claims 22 to 40, wherein the method furthermore comprises calibrating the detection means (102) using capturing emitted radiation from the gas discharge system and measure incumbent light or parasitic light while being shielded from the emitted radiation.

41.- A method for calibrating a gas discharge system, the method comprising prior to standard operation of the gas discharge system

- for a plurality of operating frequencies of an operating frequency range of the gas discharge system,

- detecting (802) a physical parameter of the gas discharge system (10) - evaluating variations in said physical parameter as function of temporal frequency,

- selecting (806) an operating frequency for said gas discharge system based on said evaluated variations in said physical parameter.

42.- A method according to claim 41 , wherein said evaluating variations is evaluating an intensity of variations of said physical parameter as function of temporal frequency.

43.- A method according to claim 42, wherein evaluating as function of temporal frequency is adapted for evaluating frequency contributions in a frequency range between OHz and two times the operating frequency.

44.- A method according to claim 43, wherein evaluating frequency contributions comprises evaluating frequency contributions at predetermined frequencies or predetermined frequency intervals in said frequency range.

45.- A method according to any of claims 41 to 44, wherein said physical parameter is an optical parameter.

46.- A method (800) according to claim 45, the gas discharge system adapted for irradiating, wherein said detecting a physical parameter comprises detecting an optical parameter from irradiation of said gas discharge system.

47.- A method for detecting arc instabilities in a gas discharge system, the method , the method comprising

- detecting (802) a physical parameter of the gas discharge system (10) and

- evaluating variations in said physical parameter as function of temporal frequency.

48.- A method according to claim 47, wherein said evaluating variations comprises evaluating an intensity of variations of said physical parameter as function of temporal frequency.

49.- A method according to claim 48, wherein evaluating as function of frequency is adapted for evaluating frequency contributions in a frequency range between OHz and two times the operating frequency.

50.- A computer program product for executing the method as claimed in any of claims 22 to 49.

51.- A machine readable data storage device storing the computer program product of claim 50.

52.- Transmission of a computer program product as described in claim 50 over a wide area network.