DETECTING THE ELECTRODE CONDITION OF ELECTRODES OF A GAS DISCHARGE LAMP

Abstract

Method for detecting the electrode state of at least two electrodes (2, 4) arranged at a predetermined distance apart in a transparent discharge vessel (1) of a gas discharge lamp (6) filled with a gas (7), wherein the at least two electrodes (2, 4) are subjected to an alternating electric current, characterized in that - for a first of two polarities of the alternating current, at least a first electrical voltage is detected between the at least two electrodes (2, 4), - for a second of the two polarities of the alternating current, at least a second electrical voltage is detected between the at least two electrodes (2, 4), - the at least one of the first and the at least one of the second recorded voltages are related to each other in order to determine a reference voltage, and - the electrode state is determined from the reference voltage.

Description

[0001] The invention relates to a method for detecting the electrode state of at least two electrodes arranged at a predetermined distance apart in a transparent, gas-filled discharge vessel of a gas discharge lamp, wherein the at least two electrodes are subjected to an alternating electric current. Furthermore, the invention also relates to a method for detecting the electrode state of at least two electrodes arranged at a predetermined distance apart in a transparent, gas-filled discharge vessel of a gas discharge lamp, wherein the at least two electrodes are subjected to an alternating electric current and the gas discharge lamp is operated in conjunction with a color wheel.The invention further relates to a device for detecting the electrode state of at least two electrodes arranged at a predetermined distance apart in a transparent, gas-filled discharge vessel of a gas discharge lamp, which are subjected to an alternating electric current. The invention further relates to a device for detecting the electrode state of at least two electrodes arranged at a predetermined distance apart in a transparent, gas-filled discharge vessel of a gas discharge lamp, which are subjected to an alternating electric current, wherein the gas discharge lamp is operated in conjunction with a color wheel having at least two color segments.Finally, the invention also relates to a lamp with a gas discharge lamp having at least two electrodes spaced apart at a predetermined distance in a transparent discharge vessel filled with a gas.

[0002] Methods for detecting the state of an electrode, as well as devices for this purpose and lighting devices incorporating such devices, are known in principle, so that a separate written proof is not required. In the prior art, an average electrical voltage between the electrodes is used to detect the electrode state. This average electrical voltage correlates, among other things, approximately with the actual distance between the electrodes. However, this method only allows a statement about the overall state of both electrodes. It does not allow for the determination of the individual electrode states.

[0003] Lighting devices with gas discharge lamps, gas discharge lamps, ballasts for the gas discharge lamps, and methods for their operation are also extensively known in the prior art. A gas discharge lamp of this type, occasionally also called a gas discharge tube, discharge tube, burner, or the like, serves to provide light based on supplied electrical energy. For this purpose, the gas discharge lamp has at least two electrodes arranged at a predetermined distance apart in a transparent discharge vessel. The discharge vessel is generally hermetically sealed and filled with gas. The electrodes are subjected to alternating electric current, so that an electric arc can form between the two electrodes as part of a gas discharge.

[0004] For this purpose, the electrodes can be supplied with an external electrical potential via electrical conductors. The electrodes are usually connected to the ballast, which supplies them with alternating current, thus enabling the desired gas discharge to generate light.

[0005] The discharge vessel is often made of a material such as glass, especially quartz glass, aluminum oxide ceramic, or the like.

[0006] Typically, outside of normal operation, the gas in the discharge vessel is at low pressure at room temperature. The gas can consist of a single gaseous substance or a mixture of several different gaseous substances. Furthermore, it is also possible for the gas to evolve to its desired composition at a later point in operation due to the evaporation of a solid and / or liquid within the discharge vessel. The gas does not need to have a constant composition. Its composition can vary depending on the specific operating conditions of the gas discharge lamp.

[0007] The composition of the gas determines the properties of the gas discharge. The gas discharge, as well as the electrodes, releases heat, which leads to an increase in pressure within the discharge vessel. Measures to reduce such thermal effects are known in the prior art, for example, in US 2002 / 0195953 A1, where the power supply is varied depending on the lamp's orientation or light characteristics to regulate the temperature in the lamp housing. Suitable gases include substances or mixtures such as metal vapors of sodium, rare earth metals, mercury, and / or the like, optionally supplemented with halogens, for example, in the manner of a metal halide lamp or similar.Furthermore, especially to promote the ignition of the gas discharge lamp, a noble gas may be included in the gas, for example argon, xenon, krypton, neon, as well as mixtures of halogens and other metals and / or the like.

[0008] To generate high luminance, gas discharge lamps can be designed as high-pressure or ultra-high-pressure gas discharge lamps. In these lamps, the gas discharge occurs in the area of ​​an arc that forms between the electrodes, thus providing an arc discharge. High-pressure gas discharge lamps are available, for example, as mercury vapor lamps, xenon or krypton arc lamps, or similar designs. Under normal operating conditions, such gas discharge lamps can operate at pressures up to approximately 1 MPa. With ultra-high-pressure gas discharge lamps, such as high-pressure mercury vapor lamps, xenon short-arc lamps, or similar designs, the gas pressure during normal operation can reach up to approximately 10 MPa or even more, for example, around 20 MPa or even 30 MPa.

[0009] Especially in high-pressure and ultra-high-pressure gas discharge lamps, the electrodes are often made of tungsten. The electrode can be, for example, pin- or rod-shaped and may also include a wire winding. In these gas discharge lamps, the current density in the gas is generally so high that a low-pressure discharge can immediately transition into an arc discharge upon startup, causing the internal pressure to increase further due to rising temperature and any evaporating filler material. In addition to the operating electrodes, ignition electrodes may also be provided, for example, in mercury vapor lamps or similar devices.

[0010] Gas discharge lamps intended for use in video projection or entertainment applications are typically designed for AC operation as so-called short-arc lamps. To achieve high optical image quality, these lamps provide an arc length of approximately 1 mm, or sometimes even less, which places particular demands on the stability of the electrodes and their arrangement within the discharge vessel. Video projection applications include, for example, the use of gas discharge lamps in digital video projectors. Entertainment applications include lighting fixtures used for stage lighting, special effects lighting, and similar purposes.Such lighting devices, which can be operated in a highly dynamic manner to create specific lighting effects and backdrops, are also known as moving heads or similar devices. Depending on the manufacturer, the gas discharge lamps used for these devices are called, for example, P-VIP® lamps, Sirius® lamps, UHP® lamps, or similar.

[0011] Due to the high current density in the discharge arc, opposing electrode tips are subjected to high thermal stress during normal operation, which can cause changes in the respective electrode state of the at least two electrodes. These changes in the electrode state can, for example, involve changes in the electrode geometry of one of the at least two electrodes, particularly with regard to electrode backfiring, a change in the contour of the respective tip of the at least two electrodes, a change in the length of the respective electrode or electrode tip, a change in the diameter at the tip of one of the at least two electrodes, tip migration, and / or the like.This can affect the arc, for example, its length, position, and / or the like, potentially impacting both the effective luminous efficacy of the projection application and the general aging behavior of the gas discharge lamp, such as devitrification, blackening, and / or similar effects. Preventing asymmetrical operation over extended periods is achieved, for example, in US Patent 6,683,420 B2, by a method that simultaneously serves to detect the fault during operation. This method calculates a voltage difference from successive voltage signals and compares it to an average voltage. A numerical register is updated based on this comparison, and upon reaching a predefined threshold, the power supply is switched to standby mode.

[0012] In the prior art, it is known to select the most favorable operating mode to positively influence the aging behavior of the gas discharge lamp and to ensure the most consistent possible quality of light generation, particularly with regard to arc stability. This can be achieved, for example, by adjusting the frequency of the alternating current, by using alternating operating intervals with different frequencies, or by providing so-called direct current phases. For example, DE 10 2013 223 138 A1 describes an operating method for improving the service life of a discharge lamp and for flicker-free operation of a discharge lamp, wherein the power supply to the discharge lamp is repeatedly deviated from a predetermined commutation scheme at a predefinable time interval by a direct current phase of a predefinable duration.With suitable timing of the direct current phases, active growth of the electrode tips and stabilization of the tip positions are achieved.

[0013] Such interventions can also be made depending on the measured electrical voltage at the electrodes. For example, US 8,436,545 B2 discloses a light source device that provides for the alternating current operation of the gas discharge lamp in such a way that, depending on a determined orientation of the electrodes in space with respect to the effect of gravity, different amplitudes can be provided for the positive and negative polarity of the alternating current. The orientation is detected by means of sensors that are included in the lighting device. In addition, the lighting device can also include a color wheel. From US 8,436,545 B2 it is further known to additionally provide for the effects of the color wheel by adjusting the positive and negative amplitudes of the alternating current and / or by adjusting the respective frequencies for the positive and negative polarity.Here too, special sensors are required to detect the respective operating states regarding the orientation of the electrodes in space as well as the respective operating state of the color wheel and to provide a suitable control function for the alternating current with the ballast.

[0014] US Patent 2007 / 0164688 A1 describes a method for preventing flicker in high-pressure discharge lamps used in projectors with a dynamic color filter. This involves generating lamp current signals based on lamp voltage signals acquired over a period at least as long as the color sequence cycle of the dynamic color filter.

[0015] Although these principles have proven effective in improving the uncorrected operation of the gas discharge lamp, the use of sensors has proven to be not only complex but also prone to interference. Furthermore, the accuracy of the controls based on these sensors is limited, particularly because they are indirect.

[0016] The invention is therefore based on the objective of improving the state detection of electrodes of gas discharge lamps.

[0017] The invention proposes a solution comprising methods, devices and a lighting device according to the independent claims.

[0018] Advantageous further training opportunities arise from the characteristics of the dependent requirements.

[0019] With regard to a first generic method according to a first aspect, it is particularly proposed that for a first of two polarities of the alternating current at least a first electrical voltage is detected between the at least two electrodes, for a second of the two polarities of the alternating current at least a second electrical voltage is detected between the at least two electrodes, the at least one of the first and the at least one of the second detected voltages are related to each other in order to determine a reference voltage, and the electrode state is determined from the reference voltage.

[0020] In a generic method according to a second aspect in connection with a color wheel, it is particularly proposed that for a first color segment of at least two color segments of the color wheel, at least one first electrical voltage between the at least two electrodes is detected at one of two polarities of the alternating current, for a second color segment of the color wheel, at least one second electrical voltage between the at least two electrodes is detected at the first polarity of the alternating current, the at least one of the first and the at least one of the second detected voltages are related to each other in order to determine a reference voltage, and the electrode state is determined from the reference voltage.

[0021] With regard to a first generic device according to the first aspect, it is particularly proposed that the device is configured to detect a current signal corresponding to the alternating current, to detect at least a first electrical voltage between the at least two electrodes for a first of two polarities of the alternating current, to detect at least a second electrical voltage between the at least two electrodes for a second of the two polarities of the alternating current, to relate the at least one of the first and the at least one of the second detected electrical voltages to each other in order to determine a reference voltage, and to determine the electrode state from the reference voltage.

[0022] With regard to a device according to the second aspect, in which the gas discharge lamp is operated in conjunction with a color wheel having at least two color segments, it is particularly proposed that the device be configured to detect a current signal corresponding to the alternating current, to determine a position of the color wheel, to detect at least a first electrical voltage between the at least two electrodes for a first of the at least two color segments of the color wheel at a first of two polarities of the alternating current, to detect at least a second electrical voltage between the at least two electrodes for a second of the at least two color segments of the color wheel at the first polarity of the alternating current, to relate the at least one of the first and the at least one of the second detected electrical voltages to each other in order to determine a reference voltage, and to determine the electrode state from the reference voltage.

[0023] With regard to a generic lighting device, it is particularly proposed that it includes a device according to the invention.

[0024] The invention is based on the use of asymmetric, i.e., time-dependent and / or polarity-dependent, components of the electrical voltage between the electrodes in order to selectively determine the respective electrode state of each of the at least two electrodes. With the invention, it is therefore possible not only to determine a total state of the electrodes, as is done in the prior art using the average electrical voltage between the electrodes, but also to determine an electrode state for each of the at least two electrodes, in particular an electrode state change, especially a relative electrode state change, and / or especially a state change of one electrode relative to the other electrode.For the sake of clarity, these variants shall each be encompassed by the term “electrode state” or “determining an electrode state” within the scope of this patent application.

[0025] This allows not only a more precise determination of the electrode state, but also enables the counteracting of unfavorable developments in the condition of the respective electrodes by implementing appropriate control measures regarding the way in which the alternating current is applied to the electrodes. The electrode states determined through the use of the invention can thus be used to control the gas discharge lamp.

[0026] For example, compared to the prior art, it is no longer necessary to provide additional sensors in the lighting device to determine the states of the electrodes. Instead, it is sufficient to detect and evaluate the electrical voltage between the electrodes in accordance with the invention in order to determine the individual states of each electrode. In particular, the lighting device does not need to contain position sensors that provide information about the orientation of the gas discharge lamp's electrodes in space. This not only results in a significant simplification compared to the prior art, but also opens up a multitude of further applications, thus improving the reliability of the gas discharge lamp and reducing its aging.

[0027] The invention utilizes the understanding that the electrical voltage between the electrodes essentially consists of three different components. A major component is an ohmic voltage drop across the actual arc. This voltage drop is independent of the direction of the alternating current. However, it can depend on light penetrating the discharge vessel from the outside, for example, due to reflected light radiation or the like.

[0028] Another component arises in the anode region. This component is usually very small in magnitude. Furthermore, due to a diffuse current distribution during the transfer of charge carriers to the respective electrode, it can only depend minimally on the geometric properties of the electrode. This component is generally negligible.

[0029] A third, significant component, however, relates to the cathode region. In the cathode region, the alternating current is primarily carried by electrons supplied by thermoionic emission from the electrode. During a steady-state discharge, an electrical voltage is established such that a sufficient area on the cathode reaches a sufficient temperature (focal spot) to carry the current. The corresponding component of the electrical voltage therefore depends directly on local properties of the electrode, such as its geometry, additional energy input through convection or back reflection, and / or other factors. For example, an elongated, thin electrode tip requires less power to maintain its temperature than a short, thick electrode tip.

[0030] Premium Performance Video Projection (PVIP) lamps are gas discharge lamps for AC operation, meaning that their electrodes periodically switch between anodic and cathodic operation during normal operation. Furthermore, when used in Digital Light Processing (DLP) projectors, the lamp is typically connected to a rotating color wheel, often containing various dichroic filter segments. The operation of the gas discharge lamp can be synchronized with the color wheel.

[0031] A first aspect of the invention is to separately detect the electrical voltage between the electrodes during different phases or polarities of the alternating current and to relate the detected electrical voltages to each other. The electrical voltage can be detected by means of a voltage sensor connected to the electrodes.

[0032] The relationship between the measured voltages can be achieved in various ways. Preferably, a difference calculation is used, whereby a difference is determined between at least one of the first electrical voltages and at least one of the second electrical voltages. Furthermore, a quotient calculation of at least one of the first electrical voltages and at least one of the second electrical voltages can also be used to relate the measured voltages to each other. In addition, other relationships, which are covered by the term "reference voltage" within the scope of this patent application, can be provided to adapt the relationship to specific applications as needed.

[0033] This allows information to be obtained about differing electrode conditions, particularly at their electrode tips, or about a general operating asymmetry, which can occur, for example, when operating a gas discharge lamp with vertically oriented electrodes. This enables the provision of a control signal, through appropriate evaluation, which can be used, for example, to control the ballast in order to at least partially compensate for differing electrode load conditions by adjusting the alternating current. For this purpose, it can be provided, for example, that DC voltage pulses are superimposed on the alternating current to counteract an undesirable change in one of the two electrodes relative to the other, or to compensate for a general operating asymmetry if necessary, as described, for example, in US 8,436,545.An advantage of the invention is that, in principle, the determination of the electrode states does not depend on the aging state of the gas discharge lamp. While the invention allows the electrode states of at least two electrodes, or their changes, to be recorded during simultaneous operation, the result, due to the evaluation of a reference voltage and thus a relative quantity, does not need to depend on the lamp current or voltage at which the lamp is currently operating.

[0034] According to the second aspect of the invention, the effects of operating the gas discharge lamp in conjunction with the color wheel are taken into account. The invention utilizes the effect that, depending on the color of a given color segment, the use of a color wheel reflects different spectral components of the light emitted by the gas discharge lamp towards the color wheel. It has been shown that the gas discharge lamp is affected by this back reflection, which can be detected by measuring the electrical voltage between the electrodes. A difference in measured voltages for different filter segments can thus serve as a measure to determine the strength or intensity of the back reflection. If necessary, a correction for different currents in different color segments can be incorporated.However, it is preferable that the measured voltage is always recorded during the same polarity of the alternating current. Therefore, it is not strictly necessary for this aspect to require the first and second electrical voltages to be recorded during opposite polarities of the alternating current. Nevertheless, a combination with the first aspect is possible.

[0035] The electrode condition refers in particular to a physical, preferably mechanical, property of the electrode, for example, electrode dimensions, electrode tip shape, surface properties, material composition, and / or the like. The electrode condition thus preferably indicates the extent to which an electrode is suitable for its intended operation in the gas discharge lamp. Preferably, differences in the respective electrode condition of the two electrodes can be determined by comparing the first and second electrodes. In particular, by comparing a recorded actual electrode condition with a target electrode condition, it can be determined to what extent the gas discharge lamp is reliably operational. Preferably, relative changes in the respective electrodes can be recorded when comparing them to each other.In particular, relative changes in geometry, such as the shape of the electrodes, can be determined. This may be sufficient to implement corrective measures for the operation of the gas discharge lamp. Therefore, it is not necessary to determine specific values ​​for the physical quantities. The electrode state of one of the at least two electrodes can thus also be a relative quantity, which may, for example, depend on the respective at least one other electrode.

[0036] Regarding the first aspect, it is further proposed in this context that the electrode state be determined taking into account any feedback from a color wheel. This allows additional effects that may occur during operation in the gas discharge lamp in conjunction with the color wheel to be considered and factored into the determination of the electrode state.

[0037] Preferably, the reference voltage is determined at least once for at least one color segment of the color wheel. This allows at least one of the first electrical voltages and at least one of the second electrical voltages to be determined under the same conditions with respect to the color wheel.

[0038] When measuring at least one first electrical voltage and at least one second electrical voltage, the first and second voltages do not need to be measured in immediately successive opposite polarities of the alternating current. Polarities of the alternating current that are temporally separated can also be used. This allows the procedure to be better adapted to the specific characteristics of each application.

[0039] According to a further development, it is proposed that several voltage values ​​are recorded for both the first and second electrical voltages, particularly at predefined times, and that the voltage values ​​recorded for the first and second electrical voltages are averaged to determine the respective first and second electrical voltages. This embodiment is particularly suitable for improving the accuracy of the method according to the invention. For example, this allows the relevance of stochastic deviations to be reduced, resulting in a more accurate determination of the reference voltage overall. It is not necessary for the same number of recorded voltage values ​​to be provided for the first and second electrical voltages. They can differ from each other as required.

[0040] Furthermore, the same or different time periods can be used for acquiring the first and second electrical voltages. These time periods can also differ in their temporal relationship to one another and / or their duration. For each of the first and second electrical voltages, a specific number of time points can be defined within the respective time period at which the respective voltage values ​​are recorded. Overall, the method of the invention can thus be advantageously adapted to a wide variety of applications in virtually any way, enabling the most reliable and accurate acquisition of the first and second electrical voltages with minimal effort.

[0041] Furthermore, it is proposed that the averaging of the voltage values ​​comprises an arithmetic averaging and a subsequent rolling averaging. This further development allows for an improvement in the accuracy and reliability of the invention. Preferably, the arithmetic averaging is performed first. This takes into account, in particular, that the electrical voltage is usually measured at different current levels, for example, within a segment of the color wheel, and that it may not be possible to meaningfully evaluate both polarities of the alternating current in every segment. Only then is a rolling averaging of the two voltages thus generated, namely the first and the second electrical voltage, performed.

[0042] In moving averaging, several averaging operations are performed consecutively over a varying number of recorded voltage values ​​for the first and second electrical voltages. Depending on the number of available voltage values, a subset can be used for each averaging operation. This allows, for example, the determination of a series of arithmetic means of preferably several consecutive voltage values. This method improves the determination of the reference voltage.

[0043] Furthermore, it is proposed that averaging the voltage values ​​includes determining a median value. The median value, preferably determined from the voltage values, is the value that occupies a middle position when the voltage values ​​are sorted sequentially. This allows for averaging the voltage values ​​that is largely free of scatter or noise. This improves the determination of the reference voltage, from which the electrode condition can then be determined with greater reliability. The median also proves advantageous for use in determining the reference voltage because it is less sensitive to outliers in the measured values. Moreover, this also allows for better control of any corrective measures that can be implemented, for example, via the ballast.Corrective measures can include, for example, supplementary phases in which a direct current is superimposed on the alternating current or, alternatively, a direct current is applied to the alternating current.

[0044] It is further proposed that the at least two electrodes be supplied with a direct current, at least temporarily, in addition to or as an alternative to the alternating current, depending on the determined reference voltage. This makes it possible to implement corrective measures to at least partially symmetrize the stress on the electrodes. Using the invention, a control system can be provided that allows for the reduction of unequal stress conditions on the electrodes. The additional or alternative direct current allows the current stress on at least one of the electrodes to be adjusted or varied, with the effect of a direct current component preferably acting primarily on the electrode in the anode phase within the normal power range.This makes it possible to compensate, at least partially, for uneven loads caused, for example, by a vertical orientation of the electrodes to each other in space or by effects due to back radiation or the like, by applying direct current in a suitable manner.

[0045] Preferably, the direct current is set as a function of the median value. This takes into account that the relevant electrode states, which are to be detected by the invention, generally change slowly over time. The median value is therefore particularly suitable for adjusting the frequency or a proportion of the direct current via the reference voltage.

[0046] An advantageous further development proposes that the direct current be supplied for a predetermined period. This design ensures that the uneven current application to the electrodes caused by the direct current is not excessive. Negative effects on the electrodes due to prolonged direct current application can thus be largely avoided. The predetermined period can be, for example, several tens of milliseconds, one or more seconds, one or more minutes, or the like. The predetermined period is preferably adjustable and can also depend on the aging state of the gas discharge lamp, particularly of the at least two electrodes. For this purpose, the aging state can be determined, for example, by measuring the average electrical voltage between the at least two electrodes.For this purpose, the device of the invention may be provided to have a corresponding evaluation unit which makes it possible to determine the average voltage from a continuously recorded electrical voltage between the electrodes.

[0047] According to a further development, it is proposed that the spatial orientation of at least two electrodes be determined from the reference voltage. This can be achieved using a suitable evaluation unit of the device according to the invention. The reference voltage can be compared with predefined reference values, and the orientation of the electrodes in space can be determined based on this comparison. Therefore, the tilt sensors in the lighting device, which are common in the prior art, are not required to determine the orientation of the electrodes in space. This increases reliability and reduces the costs and complexity of the lighting device or ballast.

[0048] The device can preferably detect a current signal corresponding to alternating current. For this purpose, the device itself can include a suitable current sensor by means of which the alternating current can be detected. However, the current sensor can also be a separate component that is merely connected to the device and supplies the device with a corresponding current signal.

[0049] The device preferably further comprises an evaluation unit that can detect the first and second electrical voltages, relate them to each other, and determine the reference voltage. The electrode state can also be determined from the reference voltage using the evaluation unit. The evaluation unit can also be integrally formed with the current sensor. The device can also consist solely of the evaluation unit. The evaluation unit itself can be designed as an electronic hardware circuit comprising electronic components to implement the process according to the invention. Furthermore, the evaluation unit can, of course, include a computer unit that is controlled by a suitable computer program such that it at least partially implements the process according to the invention.Of course, it is also possible for the device to be integrated into a control system that supplies the alternating current for the gas discharge lamp. In this way, the device can, for example, be integrated into the ballast. If the ballast already includes a computer unit, it is possible to supplement the ballast's computer program with a computer program that implements the process according to the invention.

[0050] The advantages and effects specified for the methods according to the invention apply equally to the devices according to the invention and to the lighting device equipped with the devices according to the invention, and vice versa. Consequently, device features can also be formulated for method features, and vice versa.

[0051] Further advantages and features become apparent from the description of exemplary embodiments with reference to the accompanying figures. In the figures of the exemplary embodiments, the same reference numerals denote the same features and functions.

[0052] They show: Fig. 1 in a schematic sectional view a high-pressure gas discharge lamp with two electrodes arranged in a discharge vessel; Fig. 2 in a further schematic sectional view a lighting device with a gas discharge lamp according to Fig. 1 in conjunction with a color wheel; Fig. 3 a schematic sectional view as Fig. 1, wherein the electrodes of the gas discharge lamp are arranged horizontally and a gas flow in the discharge vessel is shown; Fig. 4 a schematic sectional view of the gas discharge lamp according to Fig. 3 along a line IV-IV; Fig. 5 a schematic sectional view as Fig. 3, however, in which the electrodes differ from the Fig. 3 are oriented vertically here; Fig. 6 a schematic diagram representation of a differential voltage as a reference voltage compared to an operating time of the gas discharge lamp according to Fig. 1 without corrective measures regarding the application of an alternating current to the electrodes; Fig. 7 a schematic representation of how Fig. 6, in which the gas discharge lamp is supplied with alternating current taking into account corrective measures; Fig. 8 a schematic diagram representation of a voltage-time diagram regarding a determination of the differential voltage; Fig. 9 a schematic diagram representation of the differential voltage against time, in which the orientation of the electrodes in space is changed several times; Fig. 10 a schematic diagram representation in which a color segment-specific relative reflection component is represented by means of color segments of a color wheel assigned to graphs; Fig. 11 a schematic diagram representation in which, by means of a first graph, the effects of the color wheel on an alternating current with which the gas discharge lamp is supplied are shown, and with a second graph, the electrical voltage between the electrodes is shown; and Fig. 12 a representation like Fig. 11 without color wheel.

[0053] Fig. Figure 1 shows a schematic sectional view of a gas discharge lamp 6, which in this case is a high-pressure mercury lamp, such as the Osram P-VIP®, Osram Sirius®, or the like. The gas discharge lamp 6 comprises a transparent discharge vessel 1 made of quartz glass, in which two electrodes 2, 4 are arranged at a predetermined distance of approximately 1.3 mm. However, the electrode distance can also be approximately 0.8 mm to approximately 1.5 mm. These values ​​for the electrode distance refer specifically to a gas discharge lamp 6 at the beginning of its intended operation. During intended operation, or towards the end of its intended operation, the electrode distance can be greater. Reference numerals 3, 5 denote electrode tips, which in this case have different axial extensions.The different lengths of the electrode tips 3, 5 resulted from the operation of the gas discharge lamp 6 without corrective measures.

[0054] The electrodes 2 and 4 are each formed from a tungsten rod, which additionally comprises a tungsten coil at its end inside the discharge vessel 1. The electrodes 2 and 4 are led out of the discharge vessel 1 via leads 13 and 14 (shown only schematically here) and can be connected via leads (not shown) to a ballast (also not shown), by means of which they can be supplied with a predetermined alternating current during intended operation.

[0055] Fig. Figure 2 shows a further schematic sectional view of a lighting device 12, which in this case is a projection device of the type of a video projector. The lighting device 12 comprises a reflector 8, which has a focal point in which the gas discharge lamp 6 is positioned according to Fig. 1 is positioned approximately as follows. On the aperture side, following the reflector 8, in this order, are an ultraviolet (UV)-infrared (IR) filter 9, a color wheel 10, and an integrator 11. Downstream of the integrator 11, a lens arrangement can be arranged, which represents conventional optical components used to further process light emitted by the gas discharge lamp 6 in the lighting device 12 and finally direct it to a light exit aperture to provide the light emitted by the lighting device 12. The lighting device 12 comprises a housing (not shown) in which the ballast is also located.

[0056] The color wheel 10 ( Fig. 10) is designed as a circular disc, rotatably mounted at its center and rotatably driven. The color wheel 10 comprises six approximately equal color segments arranged adjacent to each other in the circumferential direction. Each color segment is assigned to a single color. The following color sequences result in the direction of rotation: RGBRG B. Here, R represents red, G represents green, and B represents blue. In alternative embodiments, the color wheel can also have additional or fewer segments than the color wheel 10, as is the case, for example, in Fig. The diagram shows 10. Furthermore, the color sequence and the number of colors can, of course, vary. Additionally, segments of different sizes can be provided, depending on the desired application.

[0057] Alternatively or additionally, the lighting device 12 can also be gimbal-mounted, allowing its light output aperture to be oriented in almost any way to provide a variety of lighting effects, lighting backdrops, and light distributions. The lighting device 12 is intended for stage lighting applications. The integrator 11 described above is not strictly necessary for these applications.

[0058] Fig. Figure 3 shows a schematic cross-sectional view of how Fig. 1, in which, however, the electrode tips 3 and 5 are approximately the same shape. In Fig. Figure 3 shows the horizontal arrangement of electrodes 2 and 4. Arrows 15 schematically illustrate a gas flow during the intended operation of the gas discharge lamp 6. It can be seen that the electrodes 2 and 4, and in particular their electrode tips 3 and 5, are subjected to approximately symmetrical stress due to the gas flow. Arrows 15 clarify this. Fig. Figure 4 shows a schematic sectional view of the gas discharge lamp 6 according to Fig. 3 along a line IV-IV.

[0059] Fig. Figure 5 shows a further schematic sectional view of the gas discharge lamp 6 according to Fig. 3, where now - in contrast to the orientation according to Fig. 3 - the electrodes 2, 4 are vertically oriented. According to the vertical orientation, a changed gas flow now occurs, which in Fig. Figure 5 shows the diagram with arrows 16. It can be seen that, due to the gas flow present in the discharge vessel 1, electrode 2 is subjected to a higher thermal stress than electrode 4.

[0060] Arrows 15 and 16 illustrate the formation of convection during horizontal and vertical operation of the gas discharge lamp 6, respectively. In horizontal operation ( Fig. 3, Fig. 4) The convective flow of the gas 7, and thus the convective component of heat transport at both electrode tips 3, 5, is approximately equal. In vertical operation ( Fig. 5) In contrast, a vertical flow forms. The gas 7 heats up in the discharge arc and rises. Accordingly, cooled gas 7 flows downwards along an outer wall of the discharge vessel 1. In this configuration, the upper electrode 2 is subjected to additional thermal stress compared to the lower electrode 4.

[0061] This stress leads, after a sufficiently long operating period, to differently shaped electrode tips 3, 5, as shown by Fig. Figure 1 shows that the uneven stress on the electrodes 2 and 4 can lead to premature aging of the gas discharge lamp 6 if no corrective measures are taken.

[0062] The invention reduces this undesirable behavior. The invention is based on the gas discharge lamp 6, as described in the Fig. This has been explained in sections 1 to 5. The gas discharge lamp 6 is operated under normal conditions with an alternating electric current supplied by the ballast (not shown). This allows an arc to form between the electrodes 2 and 4 in a predictable manner, causing the gas discharge lamp 6 to emit light.

[0063] According to the invention, at least one first electrical voltage is detected between the at least two electrodes 2, 4 for a first of two polarities of the alternating current, and at least one second electrical voltage is detected for a second of the two polarities of the alternating current. Alternating current is generally characterized by the fact that its polarity changes over time. As a rule, this change occurs regularly at a predetermined frequency. However, it is also possible to provide for periods of positive polarity that differ from periods of negative polarity. The first and second electrical voltages are related to each other by calculating a difference to determine a differential voltage as a reference voltage. This reference voltage then serves to determine the electrode state. For this purpose, a suitable evaluation of the reference voltage is performed.As a result, a signal can be provided that corresponds to a specific electrode state.

[0064] This method makes it possible to detect asymmetrical changes in electrodes 2 and 4 at an early stage and to take appropriate countermeasures or corrective measures. This can reduce both aging and undesirable effects caused by periodically alternating and asymmetrical discharges, such as flicker, scintillation, and / or the like. By measuring the first and second electrical voltages at different polarities of the alternating current, it is possible to determine a range of electrode-related information, particularly regarding the electrode condition.

[0065] Fig. Figure 6 shows a schematic voltage-time diagram, in which an abscissa is assigned to time in hours, whereas an ordinate is assigned to voltage in V. Fig. Figure 6 shows, with a graph 17, the time evolution of the differential voltage at the gas discharge lamp 6 according to Fig. 1. During operation without corrective measures regarding electrode tip geometry. It can be seen that even after a short operating time, the differential voltage deviates significantly from the value 0 V. In the Fig. In the embodiment shown in Figure 6, the deviations are particularly large in a range between 100 and 600 operating hours. Fluctuations arise in a horizontal orientation because the electrodes 2 and 4 can develop differently over time due to material redistribution processes, for example with regard to geometric properties such as tip length and tip diameter.

[0066] Fig. 7 now shows a representation like Fig. 6, in which, however, the gas discharge lamp 6 was operated under the influence of corrective measures according to the invention. The corrective measures were provided by the ballast, which is controlled by a signal corresponding to the respective electrode state from a device (not shown) according to the invention. This makes it possible to achieve a symmetry of the stress on the electrodes 2, 4 in the intended operation of the gas discharge lamp 6 by purely electronic means. This is illustrated by a graph 18 in Fig. Figure 7 illustrates this. It is evident that the deviation from the 0 V-line is significantly smaller than in Fig. 6. Consequently, electrodes 2 and 4 also develop much more symmetrically. This reduces the aging of the gas discharge lamp 6.

[0067] For the first and second electrical voltages, several voltage values ​​are recorded at predetermined times. The voltage values ​​recorded for the first electrical voltage and the voltage values ​​recorded for the second electrical voltage are then averaged to determine the first and second electrical voltages, respectively.

[0068] Fig. Figure 8 illustrates this using a schematic voltage-time diagram. The abscissa represents time, now in seconds. The left ordinate represents the electrical voltage in V, while the right ordinate represents the differential voltage in V. Graph 19 shows... Fig. Eight voltage values ​​for the first electrical voltage and 20 voltage values ​​for the second electrical voltage, recorded for different alternating current polarities, are shown in a graph. These values ​​are averaged during an evaluation process, which includes an arithmetic mean followed by a moving average. The dashed lines parallel to the x-axis indicate qualifications of the original measurements by an analog-to-digital converter, in this case based on a 10-bit resolution.

[0069] Based on further analysis, signals can now be provided that reflect the stress on electrodes 2 and 4 during the intended operation of the gas discharge lamp 6. These signals are supplied to the ballast, which then, depending on the determined differential voltage, supplies electrodes 2 and 4 with a direct current in addition to the alternating current, at least temporarily. This makes it possible to at least partially compensate for differing stresses and thus reduce the asymmetrical aging of the electrodes. The direct current is provided for a predetermined period, which is approximately 25 ms. However, this period can also be longer or shorter.After a predetermined period of time, the procedure can be repeated to re-evaluate the operating states of the electrodes and, if necessary, to implement appropriate corrective measures.

[0070] Fig. Figure 9 shows another voltage-time diagram for a further embodiment according to the invention, in which an abscissa is again assigned to time in s and an ordinate to voltage in mV. Fig. Figure 9 shows how the orientation of the gas discharge lamp 6 in space can be determined using the method of the invention without additional tilt sensors. During a period characterized by t < 200 s and whose end is designated T1, the gas discharge lamp 6 is oriented, due to the current operating position of the lighting device 12, such that the electrodes 2, 4 are horizontally aligned. This period ends at time T1. At time T1, the reference voltage, in this example a differential voltage, is approximately U1 = -85 mV.

[0071] At time T1, the gas discharge lamp 6 is pivoted from a horizontal to a vertical orientation. At time T2, electrodes 2 and 4 are now vertically oriented, and a differential voltage U2 (electrical voltage U1) is measured between them. In this configuration, averaging the voltage values ​​includes determining a median value. This configuration is therefore based on a median algorithm. It can be seen that from time T1 to time T2, the differential voltage increases at a relatively rapid rate of change until it reaches a new value U2 of 60 mV at time T2. This orientation is maintained until time T3.

[0072] At time T3, the gas discharge lamp 6 is again moved with respect to its orientation, this time being pivoted into the opposite vertical orientation. From the diagram according to Fig. As can be seen in Figure 9, the symmetry voltage changes again. Here too, a delayed voltage sequence occurs, reaching a value of U3 = -220 mV at time T4. This orientation is maintained until time T5. At time T5, the gas discharge lamp is rotated back to its vertical orientation. Accordingly, the differential voltage changes to a voltage U4 at time T6. The new voltage value U4 at time T6 is higher than at time T2. However, it is evident that the differential voltage slowly decreases until time T7, approaching voltage U2. This is apparently due to thermal equalization processes in the gas discharge lamp 6. At time T7, the gas discharge lamp 6 is rotated back to its initial horizontal orientation. Consequently, the differential voltage returns to its initial value U1 at time T8.

[0073] This embodiment demonstrates how the invention allows the orientation of the gas discharge lamp, and in particular its electrodes 2, 4, in space to be determined without the use of position sensors. For this purpose, only the differential voltage needs to be evaluated according to graph 22.

[0074] Another example based on the Fig. 10 to 12 refers to the use of the gas discharge lamp 6 in conjunction with the color wheel 10. Fig. Figure 10 shows a schematic diagram illustrating the normalized reflection caused by the color wheel 10, depending on each of its color segments R, G, B, with respect to wavelength. The abscissa represents the wavelength in nm, and the ordinate represents reflected radiation.

[0075] From the diagram according to Fig. Figure 10 shows that the color segments R, G, and B exhibit a partially high reflectivity for radiation outside their transparent range. This is particularly evident at short wavelengths, especially below approximately 400 nm, whereas at longer wavelengths the reflectivity is less pronounced, as shown in the diagram. Fig. 10 above approximately 680 nm. The diagram was determined using a light source corresponding to the gas discharge lamp 6, as also used in these embodiments.

[0076] The back reflection caused by the operation of the color wheel 10 in the lighting device 12 affects the detected first and second electrical voltages. If such back reflection is detected, corrective measures, such as power modulation or differently pronounced DC voltage phases, can be implemented in this case as well by appropriate control of the ballast.

[0077] Fig. Figure 11 shows the effects in a schematic voltage / current-time diagram. The abscissa represents time in milliseconds, while the left ordinate represents the electrical voltage between electrodes 2 and 4 of the gas discharge lamp 6, and the right ordinate represents a current in amps flowing through electrodes 2 and 4. This current is of one polarity during one period of the alternating current. The current is represented by graph 23, while the electrical voltage is represented by graph 24. The time frame is chosen to represent one frame, which in this case corresponds to two revolutions of the color wheel 10. Segment transitions of the color wheel 10 are indicated by lines parallel to the ordinate.

[0078] Graph 24 shows that the back reflection from the color wheel 10 has a significant influence on the electrical voltage at electrodes 2 and 4. The electrical voltage is highest in segment G, while on average it is about 130 mV lower in color segment B and about 720 mV lower in segment R. A change in voltage follows a segment change almost immediately.

[0079] A brief change in the alternating current can also be observed at each segment transition, as can be seen in graph 23, which represents the alternating current. In this case, this is observed as a reaction of an output circuit to a change in the electrical voltage at electrodes 2 and 4. This is a characteristic of the ballast, which comprises an output circuit using a buck-filter capacitor and an ignition choke.

[0080] Fig. 12 shows a representation of how Fig. 11, where a current of the gas discharge lamp 6 is represented by a graph 25 and an electrical voltage at the electrodes 2, 4 by a graph 26. In Fig. The color wheel 10 is removed from the beam path at 12. It can be seen that both the alternating current 25 and the electrical voltage 26 remain essentially constant over time.

[0081] This design makes it possible to determine the effects of the color wheel 10 from the electrical voltage at the electrodes 2, 4 of the gas discharge lamp 6 and to implement corresponding corrective measures using the ballast. For this purpose, an evaluation unit (not shown) can be provided within a device according to the invention, which performs the corresponding evaluation and provides a suitable control signal for the ballast. This allows for a corresponding corrective measure to be implemented, by means of which the effects due to backscattering from the color wheel 10 can be reduced.

[0082] Overall, the invention enables a simple improvement in the operation of the gas discharge lamp. By allowing the electrode states of the gas discharge lamp 6 to be detected with minimal effort, appropriate corrective measures can be implemented, for example, using the ballast, to reduce the impact on the reliability and aging of the gas discharge lamp 6. A separate device can be provided for the process according to the invention, comprising corresponding electronic units such as an evaluation unit capable of executing the relevant process steps. Alternatively, the device can include a computer unit that implements the process according to the invention by means of a computer program. Finally, the device can also be integrated into the ballast.The invention does not require any position sensors.

[0083] The exemplary embodiments serve only to illustrate the invention and are not intended to limit it. REFERENCE MARK LIST 1 discharge vessel 2 electrode 3 electrode tips 4 electrode 5 electrode tips 6 Gas discharge lamp 7 Gas 8 Reflector 9 UV-IR filters 10 Color wheel 11 Integrator 12 Lighting equipment 13 electrical line 14 electrical lines 15 Arrow 16 Arrow 17 Graph 18 Graph 19 Graph 20 Graph 22 Graph 23 Graph 24 Graph 25 Graph 26 Graph T1-T8 Time U1-U4 voltage

Claims

[1] Method for detecting the electrode state of at least two electrodes (2, 4) arranged at a predetermined distance apart in a transparent discharge vessel (1) of a gas discharge lamp (6) filled with a gas (7), wherein the at least two electrodes (2, 4) are subjected to an alternating electric current, characterized by , that - for a first of two polarities of the alternating current, at least a first electrical voltage is detected between the at least two electrodes (2, 4), - for a second of the two polarities of the alternating current, at least a second electrical voltage is detected between the at least two electrodes (2, 4), - the at least one of the first and the at least one of the second recorded voltages are related to each other in order to determine a reference voltage, and - the electrode state is determined from the reference voltage. [2] Method according to claim 1, characterized by , that the electrode state is determined taking into account a feedback effect by a color wheel (10). [3] Method according to claim 2, characterized by , that the reference voltage is determined at least once for at least one color segment (R, G, B) of the color wheel (10). [4] Method for detecting the electrode state of at least two electrodes (2, 4) arranged at a predetermined distance apart in a transparent discharge vessel (1) of a gas discharge lamp (6) filled with a gas (7), wherein the at least two electrodes (2, 4) are supplied with an alternating electric current and the gas discharge lamp (6) is operated in conjunction with a color wheel, characterized by , that - for a first color segment (R, G, B) of at least two color segments (R, G, B) of the color wheel (10) at least one first electrical voltage is detected between the at least two electrodes (2, 4) at a first of two polarities of the alternating current, - for a second color segment (R, G, B) of the color wheel (10) at least a second electrical voltage is detected between the at least two electrodes (2, 4) at the first polarity of the alternating current, - the at least one of the first and the at least one of the second recorded voltages are related to each other in order to determine a reference voltage, and - the electrode state is determined from the reference voltage. [5] Method according to any one of the preceding claims, characterized by, that for the first and the second electrical voltage several voltage values ​​are recorded, in particular at predetermined times, wherein the voltage values ​​recorded for the first electrical voltage and the voltage values ​​recorded for the second electrical voltage are averaged to determine the first and second electrical voltages respectively. [6] Method according to claim 5, characterized by , that the averaging of the stress values ​​includes an arithmetic mean and a subsequent moving mean. [7] Method according to claim 5 or 6, characterized by , that averaging the voltage values ​​includes determining a median value. [8] Method according to any one of the preceding claims, characterized by , that the at least two electrodes (2, 4) are supplied with a direct current at least temporarily in addition to or as an alternative to the alternating current, depending on the determined reference voltage. [9] Method according to claim 8, characterized by , that the direct current is adjusted depending on the median value. [10] Method according to claim 8 or 9, characterized by that the direct current is provided for a specified period of time. [11] Method according to claim 10, characterized by that the reference voltage is determined again after the specified period has elapsed. [12] Method according to any one of the preceding claims, characterized by , that a spatial orientation of at least two electrodes (2, 4) is determined from the reference voltage. [13] Device for detecting the electrode state of at least two electrodes (2, 4) arranged at a predetermined distance apart in a transparent discharge vessel (1) of a gas discharge lamp (6) filled with a gas (7) and which are subjected to an alternating electric current, characterized by that the device is designed, - to detect an electrical signal corresponding to alternating current, - to detect at least one first electrical voltage between the at least two electrodes (2, 4) for a first of two polarities of the alternating current, - to detect at least a second electrical voltage between the at least two electrodes (2, 4) for a second of the two polarities of the alternating current, - to relate at least one of the first and at least one of the second recorded voltages to each other in order to determine a reference voltage, and - to determine the electrode state from the reference voltage. [14] Device for detecting the electrode state of at least two electrodes (2, 4) arranged at a predetermined distance apart in a transparent discharge vessel (1) of a gas discharge lamp (6) filled with a gas (7), which are supplied with an alternating electric current, wherein the gas discharge lamp (6) is operated in conjunction with a color wheel (10) having at least two color segments (R, G, B), characterized by that the device is designed, - to detect an electrical signal corresponding to alternating current, - to determine a position of the color wheel (10), - to detect at least one first electrical voltage between the at least two electrodes (2, 4) for a first of the at least two color segments (R, G, B) of the color wheel (10) at a first of two polarities of the alternating current, - to detect at least a second electrical voltage between the at least two electrodes (2, 4) for a second of the at least two color segments (R, G, B) of the color wheel (10) at the first polarity of the alternating current, - to relate at least one of the first and at least one of the second recorded voltages to each other in order to determine a reference voltage, and - to determine the electrode state from the reference voltage. [15] Lighting device (12) with a gas discharge lamp (6) with at least two electrodes (2, 4) spaced apart at a predetermined distance in a transparent discharge vessel (1) filled with a gas (7), characterized by a device according to claim 13 or 14.

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