Abstract:
An identification system for a light radiation source ( 103 ) having a control circuit ( 107 ) for communicating with an identification circuit ( 108 ) associated with the light radiation source, wherein the identification circuit is arranged for storing data relating to the light radiation source. During operation, the control circuit communicates with the identification circuit via a signal path comprising at least a portion of a first electric wire ( 112 ) provided for energizing the light radiation source such that it is used as a first transmitting antenna for communicating with the identification circuit. The operation of the light radiation source is controlled in dependence on the data retrieved from the identification circuit. Depending on the result of the identification, operation of the light radiation source can be authorized or prevented, thus blocking the use of an incorrect radiation source for a given application.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to an identification system for a light radiation source, comprising a control circuit for communicating with an identification circuit associated with the light radiation source, wherein the identification circuit is arranged for storing data relating to the light radiation source, and wherein, during operation, the control circuit communicates with the identification circuit via a signal path comprising at least a portion of a first electric wire provided for energizing the light radiation source. 
         [0002]    The invention further relates to a method of operating a light radiation source, in which said identification system is used. 
         [0003]    The invention further relates to a light radiation source suitable for cooperating with said identification system. 
         [0004]    The invention further relates to a solarium comprising said identification system. 
         [0005]    The invention further relates to a UV disinfection system comprising said identification system. 
       BACKGROUND ART 
       [0006]    The dimensions and electrical contacts of a light radiation source, e.g. a lamp for general illumination purposes or for special lighting purposes, are often standardized. The term “light” refers to any magnetic radiation having a wavelength between 250 and 900 nm. However, the wattage, spectral distribution, and degree of efficiency in producing light radiation may be different for identical looking lamps, and hence the radiation intensity is different as well. In case of replacement of a lamp that has become unusable, the operation of a system for general illumination or for special lighting purposes is negatively affected if a lamp with either too high or too low a radiation intensity is installed. For example, a timer is often set in order to choose the degree of tanning in the case of a solarium. When the lamps of a solarium are replaced, installing lamps with a too high UV radiation output would cause sunburn of the user&#39;s skin. UV lamps used for disinfection purposes with a too low UV radiation output will result in an insufficient degree of disinfection. When using UV or infrared lamps, for example for medical purposes, a too low or too high luminous intensity of the lamps after incorrect lamp replacement may result in an improper treatment of the patient. An incorrect replacement of lamps for general illumination purposes may result in an insufficient level of illumination on the one hand or a too high power consumption and possibly damage to the lampholder on the other hand. 
         [0007]    In mercury vapor discharge lamps, mercury constitutes the primary component for the generation of ultraviolet (UV) light. A luminescent layer comprising a luminescent material may be present on an inner wall of the discharge vessel to convert UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes or for the illumination of display devices. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of a low-pressure mercury vapor discharge lamp is usually circular and comprises both elongate and compact embodiments. Generally, the tubular discharge vessel of a compact fluorescent lamp comprises a collection of relatively short straight parts having a relatively small diameter; the straight parts being connected together by means of so-called bridge parts or via bent parts. Generally, means for maintaining a discharge in the discharge space are electrodes arranged in the discharge space. 
         [0008]    The Canadian patent application CA 2 403 463 A1 describes a method and a device for operating a UV-radiation source. A UV-radiation source is disclosed, having an identification element that can be interrogated electrically and that is connected to the electrical connections of the radiation source, in parallel to the heating coil. Before a start of operation of the UV-radiation source, the UV lamp is identified by the identification element and, if the result of the identification is negative, operation of the lamp is prevented. Operation of the UV-radiation source is thus prevented if a mistake was made during lamp replacement. 
         [0009]    It is a disadvantage of the prior art identification system that a proper functioning of the identification system is prohibited if the impedance of the lamp electrode is relatively low, for example with high signal frequencies in the range of 3-30 MHz, since the identification element cannot be interrogated reliably any more. 
       DISCLOSURE OF INVENTION 
       [0010]    It is an object of the invention to provide an identification system for lamps that does not require a substantial modification of the power-supply circuit for energizing the lamp. This object is achieved with an identification system according to the invention, characterized in that the at least portion of the first electric wire is used during operation as a first transmitting antenna for communicating with the identification circuit. The control circuit and the identification circuit communicate via an existing electric wire and the existing electrical contacts provided for energizing the lamp. With the electric wire acting as an antenna, there is no need to modify the current-supply wiring or the electrical contacts. Furthermore, identification of the light radiation source is still possible in cases in which an electrode of the light radiation source has a relatively low impedance. In addition, identification of the lamp is even possible in the case of a relatively long electric wire having a relatively high inductance compared with a short electric wire, which is the case, for example, when electromagnetic ballasts are used for energizing a discharge lamp. 
         [0011]    An embodiment of the identification system according to the invention is characterized in that the identification system is further arranged for controlling the operation of the light radiation source in dependence on data retrieved from the identification circuit. Only if the result of the identification is positive, the lamp is switched on, thus preventing an improper operation of a lighting device, for example after incorrect lamp replacement. 
         [0012]    An embodiment of the identification system is characterized in that the identification system is further arranged to store information on or corresponding to the number of operating hours of the light radiation source. As the efficiency of a lamp decreases with an increasing number of operating hours, this information can be used to identify lamps that have to be replaced. 
         [0013]    An embodiment of the identification system according to the invention is characterized in that the identification system is further arranged to generate a signal to alter the energizing of the light radiation source in dependence on the number of operating hours in order to maintain a substantially constant radiation output. If the decrease in efficiency of a lamp as a function of the number of operation hours is known, the supply of energy can be adapted in dependence on the number of operating hours in order to maintain a substantially constant light output. 
         [0014]    An embodiment of the identification system according to the invention is characterized in that the identification system is further arranged to store information on or corresponding to the number of start-ups of the light radiation source. As the lifetime of a lamp is dependent on the number of start-ups, lamps that are near the end of their lifetime can be identified and replaced. 
         [0015]    An embodiment of the identification system according to the invention is characterized in that the control circuit is arranged to communicate with the identification circuit by means of a modulated antenna current signal. 
         [0016]    An embodiment of the identification system according to the invention is characterized in that the control circuit is coupled to a conducting ground, which provides a relatively simple manner of creating an antenna current signal, during operation of the system. 
         [0017]    An embodiment of the identification system according to the invention is characterized in that the control circuit is coupled to the first electric wire and that the control circuit is coupled to a second electric wire provided for energizing the light radiation source, such that the second electric wire acts as a second transmitting antenna cooperating with the first transmitting antenna. If no conducting ground is available that can be used to provide a reference potential, this dipole antenna arrangement provides an alternative in using the first electric wire as an antenna for reading the identification circuit. 
         [0018]    An embodiment of the identification system according to the invention is characterized in that the control circuit is arranged to communicate with the identification circuit by means of a modulated antenna voltage signal. 
         [0019]    An embodiment of the identification system according to the invention is characterized by an electromagnetic ballast for energizing the light radiation source via the first electric wire, and a signal-passing device coupled to the first electric wire in parallel to the electromagnetic ballast for passing a signal used for communication between the control circuit and the identification circuit. This embodiment allows the use of an electromagnetic ballast for energizing the light radiation source. 
         [0020]    An embodiment of the identification system according to the invention is characterized by a signal-blocking device, coupled in series with the electromagnetic ballast, for preventing the signal used for communicating with the identification circuit from proceeding in the direction of a power supply for energizing the electromagnetic ballast. This prevents the antenna signal from being coupled to the outside world. 
         [0021]    According to the invention, a method of operating a light radiation source using an identification system according to claim  1  comprises the following steps: reading-out of data stored in the identification circuit; comparison of the read-out data with reference data; and authorization, prevention, interruption, or alteration of the operation of the light radiation source. If, for example, an incorrect lamp is installed, or the lamp has already been used for a given period of time, this method renders it possible to identify and control the operation of the light radiation source, preventing an improper operation thereof. 
         [0022]    According to the invention, a light radiation source suitable for cooperating with an identification system according to claim  1  comprises an identification circuit arranged to communicate with the control circuit via at least a portion of the first electric wire. The identification circuit may be used to store, for example, information on the manufacturer of the lamp, the type of lamp, and/or its wattage. 
         [0023]    An embodiment of the light radiation source according to the invention is characterized in that the identification circuit is magnetically coupled to the first electric wire via a current transformer. A magnetic coupling between the electric wire and the identification circuit removes the need to modify the current-supply wiring and the electrical contacts. The magnetic coupling provides a read-out of the identification circuit via a modulated antenna current signal. 
         [0024]    An embodiment of the light radiation source according to the invention is characterized in that the current transformer comprises a magnetic core surrounding the first electric wire and a coil wound around the magnetic core, which coil is coupled to the identification circuit, providing a relatively simple manner of magnetic coupling. 
         [0025]    An embodiment of the light radiation source according to the invention is characterized in that the identification circuit is coupled via a first output to the first electric wire and via a second output to a conducting element of the light radiation source. This embodiment renders it possible to feed the voltage induced in the first electric wire to the identification circuit and to read the data from the identification circuit through measuring of the modulated antenna voltage signal. 
         [0026]    An embodiment of the light radiation source according to the invention is characterized in that the light radiation source is a mercury vapor discharge lamp for generating UV radiation for suntanning purposes or for disinfection purposes. Identification of these lamps can prevent physical injury caused by the use of an incorrect type of lamp and can prevent an insufficient degree of disinfection. 
         [0027]    According to the invention, a solarium comprises an identification system for a light radiation source according to claim  1 . 
         [0028]    An embodiment of the solarium according to the invention is characterized in that, during operation, the first transmitting antenna operates against a reference potential created by a conducting metal part of the solarium. As a result, the antenna signal is generated with respect to the metal part. At least a portion of the internal wiring of the solarium acts as the first transmitting antenna. 
         [0029]    According to the invention, a UV disinfection system comprises an identification system for a light radiation source according to claim  1 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  schematically shows a mercury vapor discharge lamp having an identification circuit and an identification system according to the invention. 
           [0031]      FIG. 2  schematically shows a mercury vapor discharge lamp having an identification circuit and an alternative identification system according to the invention. 
           [0032]      FIG. 3  schematically shows a mercury vapor discharge lamp having an identification circuit and a further alternative identification system according to the invention. 
           [0033]      FIG. 4  is a flowchart of a method of operating a mercury vapor discharge lamp according to the invention, using an identification system as shown in  FIG. 1 . 
           [0034]      FIG. 5  schematically shows an alternative mercury vapor discharge lamp having an identification circuit and an identification system according to the invention. 
           [0035]      FIG. 6  schematically shows a further alternative mercury vapor discharge lamp having an identification circuit and an identification system according to the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]      FIG. 1  schematically shows a mercury vapor discharge lamp  103  having an identification circuit  108 , and an identification system having a controller  107 . The lamp  103  is connected to the power grid via connector  102 , and the lamp is powered via ballast circuit  101 . The ballast circuit  101  may be an electronic ballast or an electromagnetic ballast. The ballast circuit  101  is coupled in series with the electric wire  112  in one direction. The lamp  103  is, for example, a UV radiation source in a solarium, a disinfection installation lamp, or a device for medical treatment, or alternatively a visible light radiation source for general illumination purposes, or for liquid crystal display backlighting applications. In an alternative embodiment, the controller  107  is integrated into the ballast circuit  101 . The ballast circuit  101  and the controller  107  are connected to the lamp  103  via an electric wire  112 . The electric wire may comprise sockets, not shown in  FIG. 1 , as well as electrical contacts of the lamp  103 , not shown in  FIG. 1 , that are inserted into the sockets. The lamp  103  comprises two electrodes  113  and  114 , coupled to the electric wire  112 . A starter  105  is coupled in series with the first and the second electrode  113  and  114 , via electric wire  112 . The identification circuit  108  is integrated into the lamp  103 . The identification circuit is an integrated circuit having a digital memory for storage of information that can be used for identification of the lamp, for example in the form of a radio frequency identification chip. The information may be related to the type of the lamp, the manufacturer, a serial number, the wattage, the lumen output, to name a few, or a combination thereof. The identification circuit  108  operates at a frequency of 13.56 MHz, in accordance with the ISO-15693 or ISO-18000 standard of the International Organization for Standardization. In other embodiments, however, the identification circuit  108  may operate at a different frequency. The identification circuit  108  is magnetically coupled to the electric wire  112  via a current transformer, comprising a magnetic core  109  through which the electric wire  112  is passed and around which a coil  117  is wound. The coil  117  is connected to identification circuit  108 . The controller  107  is coupled to a conducting ground  106  providing a reference potential and to the electric wire  112 . In the case of a solarium, the conducting ground  106  is the metal housing of a solarium that is usually, but not necessarily, connected to ground. The first electric wire  112  is capacitively coupled to the conducting ground  106 . In operation, prior to activating the ballast circuit  101  for powering the electrodes  113  and  114 , the information stored in the memory of the identification circuit  108  is read by the controller  107  via a portion of the electric wire  112 , i.e. that part of the electric wire  112  that goes through the magnetic core  109 , that acts as an antenna. The controller  107  applies a voltage to the electric wire  112 , which induces an antenna current signal in the electric wire  112  due to the capacitive coupling of the electric wire  112  with the conducting ground  106 . The magnetic core  109  senses the antenna current signal and generates an electric field, which generates an electric current through coil  117 , activating and energizing the identification circuit  108 . The identification circuit  108  modulates the antenna current signal, and this modulated antenna current signal is communicated to the controller  107  via the electric wire  112 , allowing the controller  107  to read data from the identification circuit  108 . The antenna current signal is bypassed across the ballast circuit  101  via bypass circuit  110 , for example in the form of a capacitor, that is coupled to the electric wire  112  in parallel with the ballast circuit  101 , since the ballast circuit  101  acts as a blocker for the high-frequency antenna current signal. A signal blocker  111 , for example in the form of an inductor, prevents coupling of the antenna current signal to the outside world. The information read from the identification circuit  108  by controller  107  is compared by the controller  107  with reference data. For example, information related to the type of lamp read from the identification circuit  108  is compared with reference data on the type or types of lamps that are allowable for the particular purpose. This reference data is stored in a memory, not shown in  FIG. 1 , of the controller  107 . If the lamp  103  is identified as allowable, the controller  107  generates a signal in order to activate the ballast circuit  101  for powering the lamp  103 . A heating voltage is applied to the electrodes  113  and  114  via electric wire  112 . After a gas discharge has been initiated in the lamp  103 , the lamp  103  is powered normally to maintain this gas discharge in the lamp  103 . If the lamp  103  is identified as not allowable, however, the controller  107  does not generate a signal to activate the ballast circuit  101  for powering the lamp  103 . The controller may also generate a signal to warn the user that the lamp  103  should be replaced by a correct type of lamp. An advantage of this embodiment, wherein a portion of the first electric wire  112  is used as an antenna, over the use of the first electric wire  112  as a current loop for reading the identification circuit  108  is that the use of the latter, prior art identification system in a grounded metal device, such as a solarium, is limited to devices with a relatively small length of the first electric wire  112  because of the considerable amount of parasitic capacitances and inductances that are present. 
         [0037]    Referring to  FIG. 2  now, showing an alternative embodiment, the identification system comprises an electronic ballast circuit  201  coupled in series with the electric wire  112  in both directions. Two bypass circuits  202  and  203  are placed in parallel to the ballast circuit  201 , in both directions of the electric wire  112 , for bypassing the antenna current signal across the ballast circuit  201 . In operation, the information stored in the memory of the identification circuit  108  is read by a controller  107  via the electric wire  112  that acts as an antenna in an identical manner as described for the embodiment shown in  FIG. 1 , before the ballast circuit  201  for powering the electrodes  113  and  114  is activated. If the lamp  103  is identified as allowable, the controller  107  will generate a signal in order to activate the ballast circuit  201  for powering the lamp  103 . If the lamp  103  is identified as not allowable, the controller  107  will not generate such a signal. The controller  107  may generate a signal to warn the user that the lamp  103  should be replaced by a correct type of lamp. 
         [0038]    Referring to  FIG. 3 , in a further alternative embodiment, the controller  107  is coupled to the electric wire  112  in both directions in parallel to the signal blocker  111 , as well as to the electric wire  301  in both directions. The electric wire  112  acts as a first antenna, and the electric wire  301  acts as a second antenna. In operation, the information stored in the memory of the identification circuit  108  is read by the controller  107 , using a portion of the electric wire  112 , before the ballast circuit  101  for powering the electrodes  113  and  114  is activated. The controller  107  applies a voltage to both the electric wire  112  and the electric wire  301 , such that the electric wire  112  and the electric wire  301  behave as a dipole antenna. As a result, an antenna current signal is generated in the electric wire  112 . Magnetic core  109  senses the antenna current signal and generates an electric field, which generates an electric current through coil  117 , activating and energizing the identification circuit  108 . The identification circuit  108  modulates the antenna current signal, and this modulated antenna current signal is communicated to the controller  107  via the electric wire  112 , allowing the controller  107  to read data from the identification circuit  108 . The antenna current signal is bypassed across the ballast circuit  101  via bypass circuit  110  that is coupled to the electric wire  112  in parallel to the ballast circuit  101 , since the ballast circuit  101  acts as a blocker to the high-frequency antenna current signal. A signal blocker  111 , for example in the form of an inductor, prevents a coupling of the antenna current signal to the electric wire  301 . The information read from the identification circuit  108  by controller  107  is compared with reference data. For example, information related to the type of the lamp read from the identification circuit  108  is compared with reference data on the type or types of lamps that are allowable for the particular purpose. This reference data is stored in a memory, not shown in  FIG. 3 , of the controller  107 . If the lamp  103  is identified as allowable, a controller  107  will generate a signal in order to activate the ballast circuit  101  for powering the lamp  103 . A heating voltage is applied to the electrodes  113  and  114  via electric wire  112 . After a gas discharge has been initiated in the lamp  103 , the lamp  103  is powered normally to maintain a gas discharge in the lamp  103 . If the lamp  103  is identified as not allowable, the controller  107  will not generate a signal to activate the ballast circuit  101  for powering the lamp  103 . The controller  107  may generate a signal to warn the user that the lamp  103  should be replaced by a correct type of lamp. 
         [0039]    In an alternative embodiment, the controller  107  keeps track of the time during which the lamp  103  is energized by the ballast circuit  101  and stores this information at regular time intervals in the memory of the identification circuit  108 , for example in the form of the number of hours that the lamp has been in operation. Prior to activating the lamp  103  again, the number of operating hours as well as a reference value are read by the controller  107  from the identification circuit  108 , and these values are compared by the controller  107 . Alternatively, the number of operating hours and/or the reference value is stored in a memory of the controller  107  itself. If the number of operating hours exceeds the relevant reference value, the controller  107  will prevent operation of the lamp  103 . As the efficiency of a lamp decreases over time, and this decrease is generally fairly constant over time, it can be prevented in this way that the efficiency of the lamp will be too low for a given application. In a further alternative embodiment, the controller  107  keeps track of the number of start-ups of the lamp  103  and stores this information in the identification circuit  108 . Prior to activating the lamp  103 , the number of start-ups is read by the controller  107  from the identification circuit  108  and compared with a reference value. If the number of start-ups exceeds the reference value, the controller  107  will prevent operation of the lamp  103 . Alternatively, both the number of operating hours and number of start-ups may be recorded by the controller  107  and stored in the identification circuit  108  or a memory of the controller itself, and operation of the lamp  103  is prevented if one of these parameters exceeds a reference value. 
         [0040]    In a further alternative embodiment, the controller  107  stores information on the operating time of the lamp in the identification circuit  108  at regular time intervals and uses this information together with known information on the decrease of the efficiency of the lamp  103  over time to calculate an adapted voltage and/or current intensity for the lamp  103 , such that the lumen output of the lamp  103  remains fairly constant over time. The controller  107  sends the adapted value of the voltage and/or current intensity to the ballast circuit  101 , which applies the new value(s) to the lamp  103  via the electric wires  112 . Information on the decrease of the lamp efficiency over time may be stored in the identification circuit  108  and read by the controller  107 , or it is stored in a memory of the controller  107  itself. 
         [0041]      FIG. 4  is a flowchart of a method of operating a low-pressure mercury vapor discharge lamp, using an identification system as shown in  FIG. 1 . In a first step  401 , a controller  107  is activated without energizing the lamp  103  through switching-on of the main current supply of the identification system. In step  403 , controller  107  is activated to apply a voltage to the electric wire  112 , and data are read from the identification circuit  108 , for example on the serial number of the lamp  103 . In step  405 , the controller  107  compares the read data with reference data, and if the result of the comparison is negative, operation of the lamp is prevented in step  407  in that the ballast circuit  101  is not activated to energize the lamp  103 . In an alternative embodiment, the control signal may also generate a signal to notify the user of the negative result of the comparison. If the result of the comparison is positive, the controller  107  generates a signal in step  409  in order to authorize the ballast circuit  101  to energize the lamp  103 . In step  411 , a heating voltage is applied to the electrodes  113  and  114 . After a gas discharge has been initiated in the lamp  103 , the lamp  103  is powered in step  413  in a normal way to maintain a gas discharge in the lamp. 
         [0042]    In an alternative embodiment, the controller  107  keeps tracks of the operating time of the lamp  103 , for example in the form of the number of operating hours. The number of operating hours is stored in the identification circuit  108 . The controller  107  updates the number of operating hours in an additional step, not shown in  FIG. 4 , at regular time intervals, for example every hour. If the number of operating hours exceeds a reference value, the controller  107  activates the ballast circuit  101  in a further additional step, not shown in  FIG. 4 , to interrupt the operation of the lamp  103  by switching off the power supply to the lamp. 
         [0043]    In another alternative embodiment, the controller  107  alters the operation of the lamp  103  in another further additional step, not shown in  FIG. 4 , by determining a value of the voltage and/or current intensity such that the lumen output of the lamp  103  remains fairly constant, taking into account the number of operating hours of the lamp  103  and the decrease in efficiency of the lamp over time. The controller  107  sends the adapted value of the voltage and/or current intensity to the ballast circuit  101 , which applies the new value(s) to the lamp  103 . 
         [0044]      FIG. 5  shows an alternative embodiment of a mercury vapor discharge lamp  103  and an identification system. The identification circuit  108  is connected via a positive output to the electric wire  112  and via a negative output to the lamp cap  501 . In an alternative embodiment, the negative output is coupled to a different conducting part of the lamp  103 . The lamp cap  501  provides a sufficiently large reference ground for the identification circuit  108  to guarantee a sufficient capacitive coupling with the conductive ground  106 . In operation, the controller  107  applies a voltage to the electric wire  112 . The capacitive coupling of the identification circuit  108  with the ground  106  creates a voltage difference. The voltage induced in the electric wire  112  is fed to the identification circuit  108 . The identification circuit  108  modulates the antenna voltage signal, and this modulated antenna voltage signal is communicated to the controller  107  via the electric wire  112 , allowing the controller  107  to read data from the identification circuit  108 . This data is compared with reference data, and if the lamp  103  is allowable, a signal is generated to activate the ballast circuit  101  to start operation of the lamp  103 , otherwise operation of the lamp  103  is prevented. 
         [0045]      FIG. 6  shows a further alternative embodiment of a mercury vapor discharge lamp  103  and an identification system. The lamp  103  has an identification circuit  108  that is coupled via both of its outputs to the electric wire  112 . Parallel to the identification circuit  108 , a coil  601  is coupled to the electric wire  112 . In operation, the controller  107  applies a voltage to the electric wire  112 , which induces an antenna current signal in the electric wire  112  owing to the capacitive coupling of the electric wire  112  with the conducting ground  106 . The coil  601  blocks the antenna current signal, and the antenna current flows through the identification circuit  108 . The identification circuit  108  modulates the antenna current signal, and this modulated antenna current signal is communicated to the controller  107  via the electric wire  112 , allowing the controller  107  to read data from the identification circuit  108 . This data is compared with reference data, and if the lamp  103  is allowable, a signal is generated to activate the ballast circuit  101  to start operation of the lamp  103 , otherwise operation of the lamp  103  is prevented. The current for energizing the lamps flows through the coil  601  during normal operation of the lamp  103 . 
         [0046]    In an alternative embodiment, the lamp  103  is an incandescent lamp, for example for general illumination purposes, or an infrared lamp for medical treatment. In general, an incandescent lamp has only one electrode, and the identification circuit  108  can be integrated in the incandescent lamp as shown in  FIG. 1  for electrode  114 . 
         [0047]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.