Method for operating a discharge lamp and projection arrangement

Abstract

A method for operating a two-electrode discharge lamp for a projection arrangement, wherein the discharge lamp is supplied with an alternating current (I) with a mean frequency and a predetermined waveform having a predetermined commutation scheme (16a; 16b), wherein the predetermined commutation scheme (16a; 16b) is defined by a predetermined temporal sequence of commutations (K) of the lamp current (I), wherein the commutation scheme (16a; 16b) has a current segment (18a, 18b, 18c, 18d and 18e combined, 18f) between each pair of commutations (K), wherein the absolute values ​​of the current in at least two of the current segments (18a, 18b, 18c, 18d and 18e combined, 18f) are different, and wherein the predetermined commutation scheme (16a; 16b) is repeatedly deviated from the predetermined sequence at at least one predetermined time interval. Commutation scheme (16a;16b) is deviated from by at least one direct current phase (20) with a predetermined time duration, characterized in that the at least one predetermined time interval is a maximum of 50 seconds.;

Description

Technical area

[0001] The invention is based on a method for operating a discharge lamp according to the preamble of patent claim 1 and a projection arrangement according to the preamble of patent claim 12. State of the art

[0002] State-of-the-art projection systems, such as DLP projectors, comprise a color wheel and a discharge lamp for illuminating the color wheel. The lamps are operated with alternating current and controlled by a ballast. Commutation of the lamp current reverses the polarity of the two electrodes of the discharge lamp. By adapting the operating frequency of the discharge lamp to the color wheel, the ballast provides a suitable lamp current with a specific waveform according to a commutation scheme.

[0003] Gas discharge lamps for video projection applications consist of a pair of tungsten electrodes on which, under appropriate operating conditions, small tips grow. These serve as the starting point for the discharge arc and are essential for good lamp performance, particularly in terms of high luminance, low flicker, and low burnback.

[0004] For stable lamp performance, both the geometry and the position of the tips on the electrode head must be maintained as constant as possible over the lamp's lifetime. During operation, the electrode tips reach temperatures close to the melting point of tungsten at their front end, causing tungsten to continuously evaporate. Accordingly, material must be continuously replenished from the electrode head. This can be achieved by cyclically varying the extent of the molten tungsten zone in the electrode tip through temperature modulation. The resulting melting and solidification processes, in conjunction with the high surface tension of tungsten, cause material to be transported forward from the electrode head into the electrode tip.

[0005] Furthermore, the lamp operation must be closely tailored to the customer's application. Especially in DLP projectors, precise synchronization with the color wheel commonly used in these projectors is essential.

[0006] It is known in the prior art that cyclical tip melting and the associated growth of the tips can be achieved with the aid of a so-called maintenance pulse at the end of each current half-cycle immediately before commutation, as described, for example, in EP 766906 B1. Furthermore, it is known from US 7,994,734 B2 and DE 10 2009 006 338 A1 to repeatedly use direct current phases to prevent excessive tip growth and to reshape the tips. These direct current phases are used depending on the lamp voltage. Since the lamp voltage increases proportionally to the distance between the electrode tips, the distance between the tips can be determined from the lamp voltage.

[0007] US 7,994,734 B2 deals with the regression of electrodes, as excessive electrode growth leads to flickering and excessive lamp current. Since, as mentioned, the lamp voltage provides information about the distance between the electrode tips, according to US 7,994,734 B2, the operation of the lamp is also regulated depending on the measured lamp voltage. If the lamp voltage falls below a limit, commutations are suppressed in the commutation scheme of the lamp current with which the lamp is operated, so that direct current phases also arise here. This causes the electrode tips to melt and thus to regress. Direct current phases are inserted at intervals that are typically greater than 150 seconds. However, this measure only prevents excessive tip growth; it does not stabilize the tip position.

[0008] According to DE 10 2009 006 338 A1, a test is carried out during lamp operation to determine whether the lamp voltage is lower than a lower limit, higher than an upper limit, or between these two limit values. Depending on the lamp voltage range, DC voltage phases are repeatedly applied at intervals of time, the duration of which depends on the measured lamp voltage. The time interval is between 180s and 900s to avoid excessive strain on the lamp electrodes. Very long DC voltage phases melt the entire end of the electrode for a short time; the surface tension causes the electrode ends to become spherical, thus causing the electrode tips to reform. Short DC voltage phases merely cause the electrode tips to melt over, which can influence the shape of the electrode tips.To promote tip growth, a maintenance pulse as mentioned above is applied after a long direct current phase. By applying these measures, depending on the lamp voltage, the distance between the electrode tips can be influenced and cracking of the electrode tips can be avoided. However, this method also cannot achieve sufficient stabilization of the tip position, since even non-cracked electrode tips can migrate away from the center over the lamp's lifetime, thus shortening the lamp's lifespan.

[0009] Thus, these methods can only sufficiently stabilize the size of the tip, not its position. However, with the progressive reduction of the effective aperture in today's projectors, tip shifting is no longer tolerable, as a change in tip position leads to a massive reduction in the coupling efficiency of the light into the projector optics and thus to a premature end of its service life.

[0010] One approach to solving this problem is to temporally modulate the frequency of the lamp current (=waveform) with which the lamp is operated, as described, for example, in WO 2013092750 A1, or to interrupt the operating waveform with an alternating current of a different frequency, as described, for example, in US 2012 / 0313546 A1. The advantage of this is the well-dosed melting of the electrode tips, which enables sufficient growth on the one hand, but also stabilizes the tip position on the other. This effect is usually achieved most effectively with waveforms whose center frequency is in the range of 90 Hz.

[0011] The disadvantage of this solution, however, is flickering, known as scintillation, which is clearly visible on the projection screen. Currently, this flickering can only be counteracted by using either waveforms with a symmetrical 60 Hz frequency or frequency-modulated, asymmetrical waveforms with significantly higher frequencies. Both variants allow for flicker-free lamp operation, but at the cost of significantly reduced lamp life.

[0012] A further disadvantage of this solution has been shown to be that, due to the rigidly specified color wheel, it is often difficult to find a suitable waveform with an advantageous commutation scheme for a specific customer application. In addition, predictions about the behavior of the lamp and its electrode tips for a specific commutation scheme are difficult or even impossible to make. In order to check whether a specific commutation scheme is suitable, i.e. whether it meets certain criteria with regard to the formation of the electrode tips and thus the service life of the lamp, as described, for example, in EP 2 852 261 A1, it is necessary to operate a lamp with a lamp current according to this commutation scheme for at least a large part of its service life or even for its entire service life. This is extremely time-consuming and thus complicates the search for suitable commutation schemes and operating modes for a discharge lamp.However, a method of operating a discharge lamp that satisfactorily reconciles the two requirements of a long service life of the discharge lamp and flicker-free operation of the discharge lamp has not yet been found. Description of the invention

[0013] The object of the present invention is therefore to provide a method for operating a discharge lamp and a projection arrangement by means of which improvements can be achieved with regard to the service life of the discharge lamp and flicker-free operation of the discharge lamp.

[0014] This object is achieved by a method for operating a discharge lamp and a projection arrangement according to the independent claims. Advantageous embodiments can be found in the dependent claims.

[0015] In the method according to the invention for operating a discharge lamp with two electrodes for a projection arrangement, a lamp current in the form of an alternating current is supplied to the discharge lamp during operation, with a medium frequency and a predetermined waveform having a predetermined commutation pattern. The predetermined commutation pattern is predetermined by a predetermined temporal sequence of commutations of the lamp current. Furthermore, at least one predetermined time interval repeatedly deviates from the predetermined commutation pattern by at least one direct current phase with a predetermined duration, wherein the at least one predetermined time interval is a maximum of 50 s.

[0016] At least one predeterminable time interval means in particular that the time intervals between the direct current phases do not necessarily have to be the same, so that several time intervals of different lengths can be provided between each two direct current phases, but the time intervals are each a maximum of 50s.

[0017] In contrast to the assumptions and procedures made in the prior art, in which DC phases are used to regenerate the electrode tips and it is assumed that excessively frequent DC phases place excessive strain on the electrodes and thus negatively impact their service life, surprisingly, DC phases can also be used, with appropriate timing, to actively grow the electrode tips and, moreover, stabilize the tip position. This effect can be achieved if the time interval between the DC phases is chosen to be suitably short, namely a maximum of 50 seconds. Such short DC phases not only do not reduce the service life of the discharge lamp, but actually significantly increase it.This is because the resulting stabilization of the tip position prevents the electrode tips from wandering over the course of their service life, thus significantly extending the service life of the discharge lamp. A further very significant advantage is that, in contrast to solutions in which frequency modulation can be used to stabilize the tip position at 90 Hz, a frequency that subsequently determines the flicker behavior of the lamp, the method according to the invention is not tied to a specific operating frequency of the discharge lamp. This means that it has been shown that providing DC phases at very short intervals in time increases the service life of lamps across a broad spectrum of operating frequencies. This makes it particularly advantageous to select medium frequencies of the lamp current at which the flicker behavior of the discharge lamp is improved.The method according to the invention thus makes it possible to extend the service life of the discharge lamp and at the same time to improve the flicker behavior.

[0018] Furthermore, it has been shown that particularly good stabilization of the peak position can be achieved if the time intervals between two direct current phases are even shorter. Therefore, a particularly advantageous embodiment of the invention provides that the at least one predeterminable time interval is a maximum of 25 seconds, preferably a maximum of a single-digit second range, and particularly preferably a maximum of a millisecond range. Furthermore, the predeterminable time interval is, in particular, at least 5 milliseconds. It has been shown that particularly significant advantages can be achieved with regard to extending the service life of the discharge lamp, particularly with predeterminable time intervals in the single-digit second range and shorter.At time intervals in the millisecond range, the DC phases can also be used to cause asymmetries in the given commutation scheme, which, as has been shown, also has a positive effect on the service life of the discharge lamp, thus further enhancing the service life-extending effect of the very short DC phases.

[0019] Likewise, it has proven particularly advantageous if the at least one direct current phase lasts between 5 milliseconds and 100 milliseconds. This is because, here too, it has been shown that this configuration of the phase durations has a positive effect on stabilizing the electrode tips and thus on the service life of the discharge lamp. Particularly preferred in this case is the configuration in which the phase duration of the direct current phases is predetermined, i.e., the discharge lamp is operated with a lamp current whose commutation pattern is predetermined and from which the direct current phases deviate in a predetermined manner with a predetermined time interval(s) and with a predetermined phase duration(s).The invention makes it possible in a particularly advantageous manner to specify a commutation scheme and the above-described design of the direct current phases and thereby ensure a positive behavior of the electrode tips over the entire service life and thus an extension of the service life of the discharge lamp.

[0020] In an advantageous embodiment of the invention, at least one parameter of the at least one direct current phase is predetermined by a measured current intensity of the lamp current. The at least one parameter preferably represents a duration of the at least one direct current phase and / or the at least one predeterminable time interval. It is particularly advantageous to regulate the parameters of the direct current phases such that, as the measured current intensity decreases, the frequency of the direct current phases increases, i.e., shorter time intervals are predetermined, and particularly preferably, as the measured current intensity decreases, the duration of the direct current phases increases.In particular, for a first measured current, which is smaller in magnitude than a second measured current, the at least one parameter is specified such that the duration of the at least one direct current phase is longer and / or that the at least one predeterminable time interval is shorter than for the second measured current. A decrease in the current or a lower current can be caused, for example, by an increasing service life of the discharge lamp and / or also by the fact that the discharge lamp is operated in a dimmed mode in which the operating power of the discharge lamp is reduced compared to the nominal power.Since the melting processes of the electrodes responsible for extending the service life may no longer occur at lower lamp currents due to the direct current phases, service life optimization can advantageously be achieved even in such situations by appropriately adjusting the parameters of the direct current phases depending on the measured lamp current, as described above. For this purpose, for example, certain current intensity intervals of the lamp current can be specified, with each current intensity interval being assigned a set of parameters, for example, relating to the duration and / or the at least one time interval of the direct current phases.

[0021] At least one direct current phase also means that several different direct current phases can be provided, which differ, for example, in their polarity and / or their duration, wherein in particular the phase durations of each direct current phase are advantageously in the range between 5 ms and 100 ms.

[0022] Furthermore, a direct current phase is preferably a period of time during which the polarity of the lamp current is at least effectively not reversed, and the two electrodes of the discharge lamp maintain their polarity at least effectively for the duration of the direct current phase. Effective is understood here to mean that the electrodes remain in the respective anodic or cathodic phase during the direct current phase, i.e., they do not switch from the anodic to the cathodic phase or vice versa.

[0023] A deviation from the specified commutation pattern due to the DC phase is understood to mean that in the specified temporal sequence of commutations, at least at a point in time at which, according to the specified commutation pattern, a commutation of the lamp current would take place and a respective electrode would change from the anodic phase to the cathodic phase and vice versa, this change does not take place and the electrodes remain in their respective anodic or cathodic phase. This can be achieved by omitting one or more consecutive commutations according to the commutation pattern, e.g., by maintaining the polarity of the lamp current instead of changing the polarity of the lamp current. Due to a simpler implementation in terms of circuitry, this "omission" of commutations is preferably implemented by double commutations, i.e.The lamp current changes polarity twice in immediate succession at a given point in time. The at least one DC phase is thus provided by at least one commutation being performed as a double commutation according to the specified commutation scheme. This double commutation is designed such that the two commutations performed follow one another so closely that the electrodes of the discharge lamp do not effectively change from the cathodic to the anodic phase and vice versa, but remain in their respective phases. These two commutations of the double commutation take place within a time interval of approximately 30 microseconds at most.

[0024] Whether an electrode is in the anodic or cathodic phase influences the temperature of the electrode. An electrode heats up in the anodic phase and cools down in the cathodic phase. A direct current phase therefore causes a specific temperature profile for each electrode. The temporally spaced direct current phases according to the invention can therefore create a temperature profile of the electrode temperatures that has a particularly beneficial effect on stabilizing the tip position. Double commutations during the direct current phases hardly affect the temperature profile of each electrode due to the very brief polarity change, which is why it effectively makes no difference whether a direct current phase is implemented by omitting commutations or by corresponding double commutations.

[0025] In a further advantageous embodiment of the invention, the predetermined commutation scheme is deviated from by several consecutive DC phases, which have at least one predetermined time interval between them, wherein the two consecutive DC phases each have opposite polarity. Furthermore, it is advantageous if the DC phases each have the same phase duration.

[0026] The DC phases are therefore equal in their phase duration and alternate in their polarity, so that two consecutive DC phases at at least a predetermined time interval have the same phase duration, and in particular, the lamp current in each of two consecutive, temporally spaced DC phases has different polarity. The uniform phase duration of the DC phases makes the process particularly simple to implement, and especially in combination with the alternating polarity of the DC phases, it ensures that both electrodes are stressed equally.As an alternative to alternating polarity, this could also be achieved by having a first plurality of consecutive DC phases have a first polarity and the same plurality of subsequent DC phases have the opposite polarity, particularly preferably assuming the same average current intensity during these respective DC phases. There are many other possible configurations regarding the phase duration and polarity sequence of the DC phases, but the configurations mentioned above are the simplest and most effective.

[0027] Advantageously, the same stress on the electrodes should be ensured not only by a corresponding design of the direct current phases, but generally over the entire operating life of the discharge lamp.

[0028] Therefore, in a particularly advantageous embodiment of the invention, the predetermined commutation scheme is designed such that the current intensity curve according to this commutation scheme is configured such that, on average, no DC component remains for a predetermined time interval. This fundamental condition ensures uniform loading of the electrodes, in particular because each electrode spends the same amount of time in the anodic phase and the same amount of time in the cathodic phase over this predetermined time interval.The specified time interval can represent the entire operating time of the discharge lamp or, for example, only a periodicity interval in the case of a periodically repeating specified time sequence of the commutations and the DC phases, because by fulfilling these conditions for a periodicity interval, these conditions are also guaranteed for the entire operating time of the discharge lamp.

[0029] Therefore, in an advantageous embodiment of the invention, the predetermined commutation pattern is periodic. In particular, the temporal profile of the current intensity according to this periodic commutation pattern can be repeated periodically several times during the at least one predetermined time interval. This profile does not necessarily have to be repeated in integer numbers. Furthermore, it can also be provided that this temporal periodicity affects only the predetermined commutation pattern, i.e., the deviations from the predetermined commutation pattern due to the DC phases can be configured such that, overall, a non-periodic temporal profile of the lamp current intensity results.However, it can also be provided that the deviation from the predetermined commutation pattern by the direct current phases is periodic in terms of the temporal sequence and the formation of the direct current phases, in particular with regard to the phase duration, the temporal profile of the current intensity during this phase duration and the polarity. In particular, it is further preferred that the temporal profile of the current intensity during a direct current phase corresponds in terms of amount to the temporal profile of the section of the predetermined commutation pattern from which there is a deviation at this point in time due to the direct current phase. If, for example, one or more segments of a color wheel are to be illuminated with an increased light intensity, the corresponding current segments of the commutation pattern of the lamp current have a disproportionately higher current intensity than other current segments.By having a DC phase exhibit the same temporal progression as the corresponding commutation scheme section, it is advantageously ensured that the temporal intensity progression is maintained during illumination of the sequentially illuminated color wheel segments. This means that when a DC phase deviates from the specified commutation scheme, the polarity of the lamp current is preferably deviated from the specified commutation scheme, but not the magnitude of the current.

[0030] As an alternative to a symmetrical design of the direct current phases, as described above, i.e. with the same phase duration and alternating polarity, it can also be provided that the direct current phases are distributed asymmetrically between both electrodes, i.e. that, for example, the time period that an electrode is in an anodic phase during the direct current phases is shorter or longer than the time period that this electrode is in a cathodic phase during the direct current phases. In other words, the durations and / or time intervals of the direct current phases can be dimensioned such that one electrode is in an anodic phase significantly more frequently or for a longer time than the other electrode. Due to the geometry of the structure of a projection arrangement, in particular, for example,Because the electrodes of the discharge lamps are not at the same distance from a color wheel, the color wheel's reflection effects on the electrodes can lead to varying average heating of the respective electrodes over the course of the projection system's operating life. This uneven heating effect can be advantageously counteracted by an asymmetrical design of the DC phases with respect to the electrodes.

[0031] In a further advantageous embodiment of the invention, the average frequency of the lamp current is at least 180 Hz, and in particular is an integer multiple of 60 Hz. As already mentioned, the positive effect of the described embodiment of the direct current phases on the stabilization of the electrode tips is not limited to a specific operating frequency of the discharge lamp. Therefore, an average frequency of the lamp current can advantageously be selected, which can improve the flicker behavior of the discharge lamp. As described in more detail in the European application of the same applicant with the application number 13185019.0, in a so-called2X control of the color wheel, i.e. when the color wheel is controlled in such a way that it rotates completely around the axis of rotation twice in 16.67 ms, commutation schemes which have an even number of commutations in this time interval of 16.67 ms are particularly advantageous with regard to flicker behavior, i.e. the lamp current has a mean frequency of 60 Hz, 120 Hz, 180 Hz, etc., in particular an even multiple of 60 Hz. Since boundary conditions must be met for even stress on both electrodes, some of these operating frequencies are unsuitable, especially multiples of 120 Hz, which is why mean frequencies of 60 Hz, 180 Hz, 300 Hz, etc. have proven to be particularly advantageous.One advantage of medium frequencies greater than or equal to 180 Hz, particularly those that are an integer multiple of 60 Hz, is that they provide more degrees of freedom with regard to the arrangement of the commutations in a commutation scheme and, in particular, that, in contrast to a frequency of 60 Hz, asymmetrical waveforms can be used to operate the discharge lamp. Frequency-modulated waveforms of the lamp current of this type also increase the service life of the discharge lamp compared to symmetrical waveforms. This particularly advantageous embodiment allows the discharge lamp to be operated both flicker-free and with a particularly long service life. The same also applies to a so-called3X control of the color wheel, except that with regard to the flicker behavior, commutation schemes with an odd number of commutations in the time interval 16.67 ms are particularly advantageous, which is why medium frequencies of 30 Hz, 90 Hz, 150 Hz, 210 Hz, etc., i.e. odd multiples of 30 Hz, are preferred for a 3X control of the color wheel, whereby here too additional boundary conditions that must be placed on the commutation scheme are preferably taken into account.

[0032] Another major advantage of the inventive designs is that they allow for optimization not only of the discharge lamp's operation at normal power, but also of other discharge lamp operating modes. Examples include operating the discharge lamp in Eco mode with a fixed Eco power in the range of 50%-85% of normal power, or operating in the so-called Dynamic Dimming mode, in which the lamp power is modulated frame-by-frame depending on the brightness of the image content. The lamp power can be varied between 30%-100% of normal power.The application of the method according to the invention and its embodiments when operating the discharge lamp at a reduced operating power compared to the normal power is particularly advantageous, since such dimmed operating modes are typically highly susceptible to flicker, and this severe flickering in these operating modes could previously only be reduced or eliminated at the expense of a significantly reduced lamp life. However, the invention now makes it possible to ensure flicker-free operation even for such dimmed operating modes of a discharge lamp, guaranteeing a long lamp life.Since, as already described, in such dimmed operating modes the DC phases are less effective than in normal operation due to the reduced current intensity of the lamp current, it is particularly advantageous to adapt the duration and / or frequency of the DC phases to such operating modes, in particular by extending the durations of the DC phases compared to normal operation and / or by having the DC phases occur at shorter time intervals.

[0033] In a further advantageous embodiment of the invention, the time intervals between any two consecutive commutations are at least partially different according to the specified commutation scheme. The average frequency, particularly in commutation schemes with different time intervals between any two commutations, is defined as half the number of commutations in a specific time interval, particularly in the periodicity interval of the commutation scheme, divided by the length of the time interval, i.e., particularly by the length of the periodicity interval of the commutation scheme. The temporal sequence of the commutations is preferably coordinated with a specified color wheel of a projection arrangement.The commutation scheme is synchronized with the color wheel at a given rotational frequency of the color wheel in such a way that commutations of the lamp current only occur when the area of ​​the color wheel illuminated by the discharge lamp is located precisely between two color segments, in a so-called blind region. The lengths of the current half-waves of the commutation scheme, i.e. the intervals between two commutations, are determined by the lengths of individual or multiple color wheel segments of the color wheel to be illuminated, particularly at high medium frequencies where commutation occurs multiple times during one color wheel revolution and possibly even after each color segment. If the color segments of a color wheel to be illuminated by the discharge lamp are of different sizes, the current half-waves of the specified commutation scheme of the lamp current will also be of different lengths.The resulting asymmetric, frequency-modulated waveforms are, as already mentioned, also particularly advantageous with regard to the lifetime of the discharge lamp.

[0034] The projection arrangement according to the invention comprises a discharge lamp and a ballast for the discharge lamp, which is designed, during operation of the projection arrangement, to provide the discharge lamp with a lamp current in the form of an alternating current having a medium frequency and a predetermined waveform, which has a predetermined commutation pattern. The predetermined commutation pattern is predetermined by a predetermined temporal sequence of commutations of the lamp current. The ballast is further designed to provide the lamp current in such a way that, at least at one predetermined time interval, the predetermined commutation pattern of the lamp current is repeatedly deviated from by at least one direct current phase with a predetermined time duration, wherein the at least one predetermined time interval is a maximum of 50 s.

[0035] The features mentioned for the method according to the invention and its embodiments enable the inventive projection arrangement to be further developed with additional specific features. Furthermore, the features and feature combinations mentioned for the method according to the invention and its embodiments, and their advantages, apply equally to the inventive projection arrangement.

[0036] Further advantages, features and details of the invention emerge from the claims, the following description of preferred embodiments and from the drawings. Short description of the drawings

[0037] The invention will be explained in more detail below using exemplary embodiments. The figures show: Fig. 1 a schematic representation of a color wheel for a projection arrangement according to an embodiment of the invention; Fig. 2 a schematic representation of a possible commutation scheme for a lamp current in the course of time, which is supplied to the discharge lamp by a ballast, in particular in coordination with the Fig. 1 color wheel shown; Fig. 3 a schematic representation of the commutation scheme according to Fig. 2 in a longer time interval; and Fig. 4 a schematic representation of a commutation scheme of a lamp current with a direct current phase for operating a discharge lamp according to an embodiment of the invention, wherein in particular the current intensity changes that briefly occur during the conversion of the direct current phase by double commutations are not shown. Preferred embodiment of the invention

[0038] Fig. 1 shows a schematic representation of a color wheel 10 for a projection arrangement according to an embodiment of the invention. This color wheel 10 has, by way of example, six color segments 12a, 12b, 12c, 12d, 12e, and 12f, in particular a red segment 12a, a yellow segment 12b, a white segment 12c, a cyan segment 12d, a blue segment 12e, and a green segment 12f. Between each two color segments 12a, 12b, 12c, 12d, 12e, and 12f there is a so-called blind region 14, which is also referred to as a spoke. This color wheel 10 is illuminated by a discharge lamp, in particular a high-pressure gas discharge lamp, which is supplied with an alternating current I according to a predetermined commutation scheme 16a, 16b (cf. Fig. 2, Fig. 3 and Fig. 4). The light from the lamp is directed onto a predetermined area of ​​the color wheel 10, so that upon rotation of the color wheel 10, the color segments 12a, 12b, 12c, 12d, 12e, and 12f are illuminated sequentially. Since a commutation K (cf. Fig. 2) of the lamp current I entails a brief intensity fluctuation, commutations K of the lamp current I only occur between two color segments 12a, 12b, 12c, 12d, 12e, and 12f, i.e., only when the area onto which the light is radiated is located in a blind area 14 of the color wheel 10. For this purpose, the rotation of the color wheel 10 is synchronized with the commutation scheme 16a, 16b of the lamp current I. The lamp current I does not necessarily have to be commutated in every blind area 14 of the color wheel 10.

[0039] To illustrate this, Fig. 2 a schematic representation of a possible commutation scheme 16a for a lamp current I in the course of time, which is supplied to the discharge lamp by a ballast, in particular in this case in coordination with the Fig. 1. In this case, a periodicity interval of the commutation scheme 16a is shown in particular, with a period T which, as an example, corresponds to the duration of two revolutions of the color wheel 10. However, a periodic temporal profile of the lamp current I would also be conceivable, which has a shorter or longer period T, so that the current profile only repeats after, for example, a 3, 4, 5, 6, etc. rotation of the color wheel. Typically, a color wheel 10 rotates twice in T = 16.67 ms, i.e. at a frequency of 120 Hz, with a so-called 2X control, and correspondingly three times in T = 16.67 ms with a 3X control. The commutation scheme 16a is predetermined by a predetermined temporal sequence of commutations K.The commutation scheme 16a thus has, between every two commutations K, a current segment 18a, 18b, 18c, 18d, and 18e combined, and 18f, within which a direct current with a predetermined, time-constant current intensity is supplied to the discharge lamp. The current intensities in the individual current segments 18a, 18b, 18c, 18d, and 18e combined, and 18f, can vary. The heights of these current half-waves are oriented towards the desired weighting of individual color wheel segments 12a, 12b, 12c, 12d, 12e, and 12f. In this example, the current segments 18b and 18c each have a higher current intensity than the other current segments 18a, 18d, and 18e combined, and 18f. As a result, the corresponding color wheel segments, here the yellow segment 12b and the white segment 12c, are given greater weight.

[0040] In this example, the first current segment 18a shown is synchronized with the red segment 12a of the color wheel 10, i.e. the first current segment 18a corresponds to the red segment 12a of the color wheel 10, the second current segment 18b to the yellow segment 12b of the color wheel 10 and the third current segment 18c to the white segment 12c of the color wheel 10. The fourth current segments 18d and 18e correspond to the light blue 12d and blue segment 12e of the color wheel 10, i.e. in the blind area 14 between the light blue 12d and blue segment 12e in this example no commutation K of the lamp current I takes place. The fifth current segment 18f corresponds to the green segment 12f of the color wheel 10. Following the fifth current segment 18f, the scheme is repeated with opposite polarity. This design ensures that both electrodes are subjected to the same load over a longer period of time.In particular, this current waveform inversion of the two halves of the periodicity interval fulfills several necessary conditions for uniform development of the electrode tips. This is because each electrode spends the same amount of time in both the anodic and cathodic phases, and the average current across the anodic phases corresponds to the average current across the cathodic phases, i.e., the average current across the periodicity interval is zero. In particular, each electrode undergoes a specific temporal current waveform and then repeats the same current waveform with opposite polarity. Furthermore, in accordance with the . Fig. 1, the time intervals between the commutations K vary. Furthermore, the commutation scheme shown has an average frequency of 300 Hz, i.e., 10 commutations K take place in the period T=16.67 ms. The maximum average frequency for a 6-segment color wheel with a 2X control is therefore 360 ​​Hz, if commutation occurs only in blind areas 14.

[0041] Fig. 3 shows a schematic representation of the commutation scheme 16a according to Fig. 2 over a longer period of time. The repetition of the periodicity intervals is particularly noticeable.

[0042] The waveform of the lamp current, in particular the design of the commutation scheme, is crucial for the lamp performance of the discharge lamp, i.e. for its service life and its flicker behavior. The design of the commutation scheme has a major influence on the formation of the electrode tips of the discharge lamp. The commutation scheme is also subject to fundamental conditions, such as that, in synchronization with a color wheel, commutation of the lamp current is only possible in blind areas between two color segments, and that the lamp current is designed such that both electrodes of the lamp are equally stressed. The invention now advantageously makes it possible to reduce the susceptibility of the discharge lamp to flicker and also increase the service life of the lamp by controlling the discharge lamp with a suitable lamp current.For this purpose, the specified commutation scheme, such as the one shown here, is used at predefined time intervals of less than 50 seconds. Fig. 3, by direct current phases. In particular, direct current phases are generated at regular intervals by performing double commutations which, with a suitable length, i.e. between 5 ms and 50 ms, and repetition rate, i.e. with a time interval of preferably 1 s to 25 s, bring about the temperature modulation of the electrode tips necessary to stabilize the geometry and position of the electrode tips. To achieve an advantageous effect of this method, both the average lamp frequency and the length of the DC phases and their frequency can be optimized, whereby the selection options with regard to the average lamp frequency are generally severely limited by the requirements for flicker-free operation and by a specific color wheel design.However, there are no significant limitations on the parameters for the DC phases, making this method ideal for use in the widely used 1-chip DLP projectors.

[0043] Fig. 4 shows a schematic representation of a commutation scheme 16b of a lamp current I in time course, in particular according to the Fig. 2 and Fig. 3, but with a direct current phase 20 for operating a discharge lamp according to an embodiment of the invention. This direct current phase 20 deviates from the predetermined commutation scheme 16a for a predetermined period of time. This is achieved by the fact that during this direct current phase 20, at the times at which, according to the commutation scheme as shown in Fig.2 a commutation K would take place, a double commutation (not explicitly shown) takes place, so that effectively no commutation K of the lamp current I takes place during this time period. These double commutations are designed as two consecutive commutations which are so closely spaced that, in synchronization with a color wheel, a double commutation only takes place during or within a blind area of ​​the color wheel. The time interval between these DC phases is particularly important for improving lamp performance. Surprisingly, it has been shown that a relatively short time interval between such phases, i.e. in the range between 1s and 25s, can bring about a stabilization of the tip position and even active growth of the electrode tips.This effect is surprising because DC phases are usually used to shorten the length of the electrode tips and to space them as far apart as possible in order to avoid excessive stress on the electrodes.

[0044] Furthermore, as an embodiment of the invention, it is preferred to allow the length of the DC phases to last between 5 ms and 100 ms. Furthermore, the average frequency of the lamp current should be selected such that flicker-free operation can also be advantageously guaranteed. As already described and further explained in the European application with application number 13185019.0, this is made possible by the implementation of such commutation schemes, in which the change between the anodic and cathodic electrode phases for each color segment of a color wheel occurs at a frequency above the perception threshold of the human eye.

[0045] These measures can, of course, be implemented equally for differently designed color wheels and, for example, a 3X control system. The phase length and repetition rate of the DC phases in the aforementioned time ranges are particularly relevant here. In combination with a suitably selected average frequency of the lamp current, this can achieve enormous advantages in terms of flicker behavior and the service life of the discharge lamp, regardless of the design and control of the color wheel.

Claims

[1] Method for operating a discharge lamp with two electrodes for a projection arrangement, in which the discharge lamp is supplied with a lamp current (I) in the form of an alternating current with a medium frequency and with a predetermined waveform, which has a predetermined commutation scheme (16a; 16b), during operation, wherein the predetermined commutation scheme (16a; 16b) is predetermined by a predetermined temporal sequence of commutations (K) of the lamp current (I), wherein the commutation scheme (16a; 16b) has a current segment (18a, 18b, 18c, 18d and 18e taken together, 18f) between each two commutations (K), wherein the absolute values ​​of the current intensities in at least two of the current segments (18a, 18b, 18c, 18d and 18e taken together, 18f) are different, and wherein at least one predetermined time interval is repeated from the given commutation scheme (16a;16b) is deviated by at least one direct current phase (20) with a predetermined time duration, ; characterized by that at least one specified time interval is a maximum of 50 seconds. [2] Method according to claim 1, characterized by that the at least one predeterminable time interval is a maximum of 25 s, preferably a maximum of a single-digit second range and particularly preferably in the millisecond range, wherein the at least one predeterminable time interval is in particular at least 5 milliseconds. [3] Method according to one of the preceding claims, characterized by that the at least one direct current phase (20) lasts between 5 milliseconds and 100 milliseconds. [4] Method according to one of the preceding claims, characterized by that at least one parameter of the at least one direct current phase (20) is predetermined by a measured current intensity of the lamp current (I). [5] Method according to claim 4, characterized byin that the at least one parameter represents a duration of the at least one direct current phase (20) and / or the at least one predeterminable time interval, wherein in particular for a first measured current intensity which is smaller in magnitude than a second measured current intensity, the at least one parameter is predetermined such that the duration of the at least one direct current phase (20) is longer and / or that the at least one predeterminable time interval is shorter than for the second measured current intensity. [6] Method according to one of the preceding claims, characterized by in that the at least one direct current phase (20) is provided by at least one commutation (K) being carried out as a double commutation according to the commutation scheme (16a; 16b). [7] Method according to one of the preceding claims, characterized bythat a deviation from the predetermined commutation scheme (16a; 16b) is achieved by a plurality of successive direct current phases (20) which have at least one predetermined time interval from one another, wherein two temporally successive direct current phases (20) in each case have an opposite polarity. [8] Method according to one of the preceding claims, characterized by that a deviation from the predetermined commutation scheme (16a; 16b) is achieved by a plurality of successive direct current phases (20) which have the at least one predetermined time interval from one another, wherein the direct current phases (20) each have the same phase duration. [9] Method according to one of the preceding claims, characterized bythat the predetermined commutation scheme (16a; 16b) is designed such that a current intensity profile of the lamp current (I) according to this commutation scheme (16a; 16b) is designed such that a predetermined time interval (T) has on average no direct current component. [10] Method according to one of the preceding claims, characterized by that the predetermined commutation scheme (16a; 16b) is periodic, wherein in particular the temporal course of the current intensity according to this periodic commutation scheme is repeated periodically several times during the at least one predetermined time interval. [11] Method according to one of the preceding claims, characterized by that the average frequency of the lamp current (I) is at least 180 Hz and, in particular, is an integer multiple of 60 Hz. [12] Method according to one of the preceding claims, characterized bythat time intervals between two consecutive commutations (K) according to the predetermined commutation scheme (16a; 16b) are at least partially unequal. [13] Projection arrangement, comprising a discharge lamp and a ballast for the discharge lamp, which is designed, during operation of the projection arrangement, to provide the discharge lamp with a lamp current (I) in the form of an alternating current having a medium frequency and with a predetermined waveform, which has a predetermined commutation scheme (16a; 16b), wherein the predetermined commutation scheme is predetermined by a predetermined time sequence of commutations (K) of the lamp current (I), wherein the commutation scheme (16a;16b) has a current segment (18a, 18b, 18c, 18d and 18e taken together, 18f) between each two commutations (K), wherein the absolute values ​​of the current intensities in at least two of the current segments (18a, 18b, 18c, 18d and 18e taken together, 18f) are different, wherein the ballast is designed to provide the lamp current (I) in such a way that at least one DC phase (20) with a predetermined time duration repeatedly deviates from the predetermined commutation pattern (16a; 16b) of the lamp current (I) at least at one predetermined time interval; characterized by that at least one specified time interval is a maximum of 50 seconds.

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