Abstract:
A method for determining a distance from an object may include emitting an emission light beam from a light emitter, receiving a reception light beam at a light receiver, and determining the distance on the basis of a propagation time of the emission and reception light beams. The reception light beam may arise as a result of reflection of the emission light beam at the object. The emission light beam may be amplitude-modulated with a rectangular-waveform modulation signal. The modulation signal may have a multiplicity of rectangular pulses which occur in a multiplicity of groups. The groups may occur at varying temporal intervals with respect to one another and have changing numbers of rectangular pulses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation Application of International Application No. PCT/EP2009/050887, filed on Jan. 27, 2009 priority to which is claimed under 35 U.S.C. §120. Priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) is also claimed from German Application Nos. 10 2008 008 064.0, filed Feb. 1, 2008; and 10 2008 014 274.3, filed Mar. 3, 2008, the disclosures of which are also incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present application relates to methods and devices for determining a distance to an object. 
     BACKGROUND 
     A method and a device of this type are known from DE 40 27 990 C1. 
     At least an embodiment of the invention relates to a so-called laser scanner that is designed to measure a spatial region and/or an object three-dimensionally. Such a laser scanner is described e.g. in DE 103 61 870 A1. This known laser scanner has a measuring head that can be rotated about a vertical axis. The measuring head contains a rotor that can be rotated about a horizontal axis. The rotor emits an emission light beam and receives a reception light beam reflected from an object. (Reflection within the meaning of the present invention need not necessarily be a total reflection, but rather can also be a diffuse reflection or scattering of the emitted light beam.) The distance between the measuring head and the object is determined from the propagation time of the emission light beam and of the reception light beam. The rotation of the rotor and of the measuring head makes it possible to move the emission light beam by 360° in Azimut and by approximately 270° in elevation. In this way it is possible to measure virtually the entire space all around the known laser scanner. Typical applications for such laser scanners are the measurement of buildings (inside and/or outside), tunnels or the measurement of large objects such as ships&#39; hulls, for instance. 
     The propagation time of the emission and reception light beams can be determined in various ways. In principle, a distinction is drawn between pulse propagation time methods and CW (Continuous Wave) methods. In the pulse propagation time methods, the emission light beam contains only a short emission pulse for each measuring operation. The time until the reflected pulse arrives in the receiver is measured. In the CW methods, an (at least substantially) continuous emission light beam is emitted and the propagation time is determined on the basis of a phase shift between the emission and reception light beams. In this case, the emission light beam is typically amplitude-modulated with the aid of a modulation signal and the phase shift of the modulation signal in the emitted and received light beams is used for determining the propagation time. The higher the modulation frequency, the more accurately the distance can be determined here. However the unambiguity range decreases as the modulation frequency increases; since the phase shift between emission and reception light beams is repeated after a phase cycle of 360°. 
     DE 40 27 990 C1 cited in the introduction therefore proposes a distance measuring device with a modulated emission light beam according to the CW method, wherein the emission light beam is amplitude-modulated with a rectangular-waveform modulation signal having a first, relatively high modulation frequency, and wherein said emission light beam is interrupted after a specific number of periods of the modulation signal for a relatively long time period. This interruption can be interpreted as amplitude modulation with a second modulation signal having a second, lower modulation frequency. In other words, the emission light beam in this case is amplitude-modulated with a first, higher and with a second, lower modulation frequency, wherein the two different modulation frequencies determine the unambiguity range. The latter is significantly larger than when just one modulation frequency is used. 
     In DE 43 03 804 A1 the method according to DE 40 27 990 C1 is deemed to be disadvantageous insofar as the emission light intensity averaged over the duration of the entire signal period is reduced by the amplitude modulation with the lower, second modulation frequency. This is said to lead to a reduction of the signal/noise ratio and, consequently, to have the effect that objects having a low reflectivity can no longer be measured. In order to avoid this disadvantage, DE 43 03 804 A1 proposes modulating the emission light beam alternately with the higher first and the lower, second modulation frequency, that is to say that in each time interval the emission light beam is modulated with only one of the two modulation frequencies in each case. However, this method results in lengthened measuring times since each object has to be measured twice. The higher measuring time is disadvantageous particularly in the case of a laser scanner, because the emission light beam can then only be pivoted relatively slowly. 
     SUMMARY OF THE INVENTION 
     Against this background, in at least an embodiment of the present invention, the available light power is optimally utilized to obtain a large signal/noise ratio, which furthermore affords high measuring accuracies together with a large unambiguity range. 
     According to at least an embodiment of the invention, this object is achieved by means of a method and a device of the type mentioned in the introduction wherein the groups of rectangular pulses occur at varying temporal intervals with respect to one another and have changing numbers of rectangular pulses. 
     The novel method and the novel device are furthermore based on the principle of propagation time measurement according to the CW method, that is to say that the propagation time is determined on the basis of a phase shift of the modulation signal in the reception light beam relative to the modulation signal in the emission light beam. Furthermore, the novel method and the novel device use a rectangular-waveform modulation signal with which the emission light beam is amplitude-modulated. Though DE 40 27 990 C1 cited in the introduction has already considered a rectangular-waveform modulation signal for the amplitude modulation of a emission light beam, sinusoidal modulation signals are usually used in practice, by contrast. A rectangular-waveform modulation signal has the advantage over a sinusoidal modulation signal that the modulation signal remains at its respective maximum value for longer given the same signal amplitude (pulse peak) and the same modulation frequency. In other words, less light power is “wasted” for the edge rise in the case of a rectangular-waveform modulation signal. Consequently, the use of a rectangular-waveform modulation signal enables a higher signal/noise ratio than the use of a comparable sinusoidal modulation signal. The available light power can be utilized significantly better with a rectangular-waveform modulation signal. 
     In contrast to the method according to DE 40 27 990 C1, the emission light beam modulated with a rectangular waveform according to the novel method is not only suppressed periodically, however, the modulation signal itself is modulated in a manner such that the rectangular pulses occur at temporally varying intervals with respect to one another and with changing numbers of pulses. 
     In a particularly preferred exemplary embodiment, the modulation signal is a binary rectangular signal (in the manner of a digital 0-1 sequence), wherein the individual rectangular pulses occur with the varying mark-space ratio and varying pulse clustering. In other exemplary embodiments, the rectangular pulses can be pulse peaks of a non-binary rectangular-waveform modulation signal, e.g. the pulse peaks of a quaternary rectangular-waveform signal. In a preferred exemplary embodiment, the rectangular pulses of the modulation signal are distributed in such a way that the modulation signal itself is frequency-modulated, to be precise preferably according to a periodically recurring pattern. 
     It should be pointed out in this connection that a rectangular signal can never have an exactly rectangular waveform in reality, since unavoidable bandwidth limitations and overshoots in real circuits always lead to a deviation from the ideal rectangle. It generally holds true, however, that the luminous efficiency in the method and the device of at least an embodiment of the present invention is all the better, the closer the modulation signal comes to an ideal rectangular signal. 
     According to the novel method and the novel device, the amplitude of the emission light beam is modulated with a modulation signal which comprises a multiplicity of different modulation frequencies on account of its own changing properties. This multiplicity of different modulation frequencies includes not just the harmonic multiples that every rectangular-waveform signal comprises per se on account of the Fourier relationship. Over and above the harmonic multiples of a rectangular signal the novel modulation signal comprises a multiplicity of different modulation frequencies which, in particular, are lower than the first harmonic of the rectangular-waveform signal. Consequently, the novel modulation signal is a combined modulation signal with which the light emitter can be driven continuously. The different modulation frequencies contained in the novel modulation signal are preferably evaluated at least approximately simultaneously, such that in principle one measuring operation suffices for each distance measurement. On account of the high modulation frequencies contained in the combined modulation signal, the distance can be determined with high measuring accuracy. On the other hand, the modulation signal also contains lower modulation frequencies on account of the varying pulse groups, further, such that a large unambiguity range is obtained. 
     Furthermore, it has been found that the available light power can be utilized significantly better with the novel modulation signal than with comparable combined sinusoidal signals. The object mentioned above is therefore fully achieved. 
     In at least an embodiment of the invention, the temporal intervals change periodically. 
     In this configuration, the temporal intervals between the groups of rectangular pulses become longer and shorter according to a periodically repeating pattern. The periodically varying temporal intervals lead to a modulation frequency in the rectangular-waveform modulation signal which is low in comparison with the fundamental frequency of the rectangular-waveform modulation signal. The low modulation frequency enables a large unambiguity range. Furthermore, this configuration, on account of the “pauses” between the pulse groups, enables a higher peak loading of the light emitter given the same average light power, which results in a further improvement of the signal/noise ratio. 
     In a further configuration, the number of rectangular pulses per group changes periodically. 
     This configuration provides for a further “low” modulation frequency in the combined modulation signal and it can consequently contribute to a further enlargement of the unambiguity range. The combination of this configuration with the preceding configuration is particularly advantageous, wherein the periodically changing intervals and the periodically changing number of rectangular pulses per group occur with the same period. In this case, the larger temporal intervals between groups of rectangular pulses result from the smaller number of rectangular pulses per group. This configuration simplifies the practical realization and it enables a very good utilization of the available light power. 
     In a further configuration, the modulation signal is generated by addition of a first rectangular-waveform modulation signal having a first modulation frequency and a second rectangular-waveform modulation signal having a second modulation frequency, wherein the first modulation frequency is large relative to the second modulation frequency. Preferably, the first modulation frequency is at least five times greater than the second modulation frequency. 
     This configuration enables a very simple and cost-effective generation of the novel modulation signal and hence a simple and cost-effective realization of the novel device. Moreover, an addition of the first and second modulation signals reduces the number of unrequired “secondary frequencies” in the modulation signal in comparison with a multiplication, which is likewise conceivable in principle. Consequently, the available light power is concentrated on the useable and used modulation frequencies to a greater extent. 
     In a further configuration, a third rectangular-waveform modulation signal having a third modulation frequency is added to the first and second rectangular-waveform modulation signals, wherein the second and third modulation frequencies differ from one another, and wherein the first modulation frequency is also large relative to the third modulation frequency. It is particularly advantageous if the second and third modulation frequencies are approximately identical or close together. 
     In this configuration, the difference between the second and third modulation frequencies is significantly less than the difference between the second and first modulation frequencies or the difference between the third and first modulation frequencies. In a preferred exemplary embodiment, the first modulation frequency is approximately 125 MHz, the second modulation frequency is approximately 15 MHz and the third modulation frequency is approximately 13 MHz. 
     This configuration has the advantage that a third modulation frequency is available for signal evaluation, whereby the unambiguity range can be enlarged further. It is particularly advantageous if the second and third modulation frequencies are relatively close to one another, as in the preferred exemplary embodiment, because in such case a beat arises whose frequency corresponds to the difference between the second and third modulation frequencies. This difference is very small in comparison with the actual frequencies of the modulation signals. Consequently, the unambiguity range can be enlarged to a very great extent without the low beat frequency having to be made available separately. The choice and tuning of the individual circuit components of the novel device can be significantly simplified in this configuration. 
     In a further configuration, the second and third modulation signals have identical pulse amplitudes. 
     This configuration simplifies the signal evaluation and leads to a further improved luminous efficiency. It is particularly advantageous if the second and third modulation frequencies are so close to one another that a beat frequency is available for signal evaluation. 
     In a further configuration, the first modulation signal has a larger pulse amplitude than the second modulation signal. In a particularly preferred exemplary embodiment, the pulse amplitude of the first modulation signal is larger approximately by a factor of 2 than the pulse amplitudes of the second or third modulation signal, wherein the latter are identical. 
     This configuration contributes to enlarging the temporal intervals between the groups of rectangular pulses in the combined modulation signal, which at first sight results in a reduction of the average transmission power of the emission light beam. The pulse or peak power with which the light emitter is operated is advantageously increased in this case, however. This is possible without destruction of the light emitter by virtue of the larger intervals between the groups of rectangular pulses and contributes to again increasing the signal/noise ratio in the useful signal. 
     In a further configuration, all the rectangular pulses of the modulation signal have an at least substantially identical pulse amplitude. 
     In this configuration, the combined modulation signal is a binary signal such as is usually used for the representation of a 0-1 sequence in field of digital technology. As an alternative, the combined modulation signal could be a rectangular-waveform signal having a multiplicity (n&gt;2) of pulse amplitude values. The preferred configuration has the advantage that the combined modulation signal can be generated very simply and efficiently with the aid of digital circuits, wherein the modulation signals to be combined and the combined modulation signal are in this case provided digitally as 0-1 sequences. Furthermore, the maximum amplitude of the emission light beam can be utilized with each pulse which likewise contributes to an optimum utilization of the available light power. 
     In a further configuration, the rectangular-waveform modulation signal is generated as a binary rectangular-waveform modulation signal with the aid of a digital circuit. 
     As already indicated above, this configuration enables a very simple and cost-effective realization of the novel method and of the novel device. Furthermore, the combined modulation signal in this configuration can be varied very flexibly and be adapted to different environments and/or measuring tasks. 
     As an alternative to this, in other configurations, the rectangular-waveform modulation signal is generated from at least two sinusoidal signals having a different frequency, wherein the sinusoidal signals are in each case amplified and amplitude-limited. 
     In this configuration, the rectangular-waveform modulation signal is generated with the aid of analog circuit technology. This configuration enables a very simple and cost-effective realization of the novel device using circuit components which have hitherto been operated with sinusoidal signals. In particular, the novel method in this configuration can be integrated very easily into existing circuit concepts according to the prior art. 
     In a further configuration, the propagation time of the emission and reception light beams is determined on the basis of a phase difference of the modulation signal in the emission light beam and in the reception light beam, wherein there is measurement of the phase angle of the modulation signal in the emission light beam at the light emitter. 
     In this configuration—which is also intrinsically inventive—the phase angle of the modulation signal in the emission light beam is determined metrologically and this phase is used as a reference for determining the propagation time. In other words, here the phase angle instantaneously present in the emission light beam is used for determining the propagation time. It is particularly preferred in this case if the light emitter comprises a laser diode and if the phase angle of the control current that flows through the laser diode is measured. The phase angle of the control current can be determined in a simple manner and it represents the actual instantaneous phase angle of the modulation signal in the emission light beam with high accuracy. This configuration enables a very high measuring accuracy because a phase drift in the region of the light emitter is eliminated from the distance determination. 
     It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the combination respectively specified, but also in other combinations or by themselves, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, exemplary embodiments are shown which should not be construed to be limiting regarding the entire scope of the disclosure, and wherein the elements are numbered alike in several FIGURES: 
         FIG. 1  shows a laser scanner in accordance with an embodiment of the invention, 
         FIG. 2  shows a simplified illustration of a plurality of modulation signals which can be employed in the laser scanner in accordance with  FIG. 1 , 
         FIG. 3  shows a preferred exemplary embodiment of a modulation signal, 
         FIG. 4  shows the frequency spectrum of the modulation signal from  FIG. 3 , 
         FIG. 5  shows a circuit for generating the novel modulation signal in accordance with a further exemplary embodiment, and 
         FIG. 6  shows a circuit for generating the novel modulation signal in accordance with at least another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a laser scanner is designated in its entirety by the reference numeral  10 . The laser scanner  10  is a an embodiment of a device according to the present invention. However, the novel device and the novel method can also be employed in other apparatuses in which a distance from an object is to be determined with the aid of a emission light beam and a reception light beam. The invention is also not restricted to the use of light beams in the narrower sense (preferred wavelengths between 300 and 1000 nm), but rather can in principle also be realized with electromagnetic waves from a larger wavelength range as long as a quasi-optical propagation is present. The expression light beam as used here therefore encompasses such electromagnetic waves as well. 
     The laser scanner  10  comprises a light emitter  12  and a light receiver  14 , both of which are connected to an evaluation and control unit  16 . In the preferred exemplary embodiment, the light emitter  12  comprises a laser diode  13  (see illustration in  FIGS. 5 and 6 ) that is designed to emit a laser beam  18  in order to illuminate an object point at an object  20 . The laser beam  18  is amplitude-modulated here with a rectangular-waveform modulation signal, as is explained in more detail below with reference to  FIGS. 2 to 6 . In a preferred exemplary embodiment, the emission light beam has a wavelength of approximately 790 nm. 
     The laser beam  18  is deflected here via a minor  22  to the object  20 . The reference numeral  24  designates a reception light beam that is reflected from the object  20  and is deflected via the minor  22  to the receiver  14 . The evaluation and control unit  16  is designed to determine the distance from the laser scanner  10  to the illuminated point at the object  20  from the propagation time of the emitted laser beam  18  and of the received reflected beam  24 . A phase shift between the emission light beam  18  and the reception light beam  24  is determined and evaluated for this purpose. 
     The minor  22  is arranged here at the front end face of a cylinder  26  connected to a rotary drive  30  via a shaft  28 . With the aid of the rotary drive  30 , the mirror  22  can be rotated about an axis  32  of rotation. The respective rotary position of the minor  22  can be determined with the aid of an encoder  34 . The output signals of the encoder  34  are likewise fed to the evaluation and control unit  16 , though this is not illustrated here for reasons of clarity. 
     In the preferred exemplary embodiment, the axis  32  of rotation is arranged horizontally and the minor  22  is inclined at an angle of approximately 45° relative to the axis  32  of rotation. A rotation of the mirror  22  about the horizontal axis  32  therefore has the consequence that the emission light beam  18  is deflected along a vertical plane (elevation) perpendicular to the axis  32  of rotation. The emission light beam  18  forms as it were a fan with which the spatial region  36  is scanned in a vertical plane. 
     The laser scanner  10  here has a housing structure essentially having two housing parts  38 ,  40 . The housing parts  38 ,  40  are arranged on a common baseplate  42 . The emitter  12 , the receiver  14  and the evaluation and control unit  16  are accommodated in the housing part  38  illustrated on the left in  FIG. 1 . The housing part  40  illustrated on the right in  FIG. 1  accommodates the rotary drive  30  with the encoder  34  and the cylinder  26 , wherein the cylinder  26  with the minor  22  projects from the housing part  40 , such that the mirror  22  is arranged approximately centrally between the two housing parts  38 ,  40 . 
     The baseplate  42  is arranged on a rotary drive  44  seated on a stand  46 . The stand  46  is height-adjustable and has a scaling  48  in order to be able to perform a reproducible height setting. The reference numeral  50  designates a further encoder, with the aid of which the rotational position of the rotary drive  44  can be determined. The output signals of the encoder  50  are likewise fed to the evaluation and control unit  16  (not illustrated here). 
     The rotary drive  44  enables the laser scanner  10  to be rotated about a vertical axis  52  which together with the axis  32  of rotation defines an axial intersection point. The axial intersection point lies approximately centrally on the mirror  22  and, in preferred exemplary embodiments, defines the origin of a system of coordinates to which all the distance measuring values are referred. With the aid of the rotary drive  44 , the vertical “scanning fan” produced with the aid of the rotating mirror  22  can be rotated by up to 360° in azimuth. The emission light beam  18  can thus illuminate almost any object point in the vicinity of the laser scanner  10 . Shading takes place only toward the bottom by the baseplate  42 , such that the viewing angle of the laser scanner  10  is limited toward the bottom. 
     In this exemplary embodiment, the evaluation and control unit  16  comprises a microprocessor  54  and an FPGA (field programmable gate array)  56 . The FPGA  56  here generates a binary rectangular-waveform modulation signal with which the laser diode of the light emitter  12  is driven. The microprocessor  54  reads in digitized reception data from the light receiver  14  and determines the distance d between the laser scanner  10  and the object  20  on the basis of these data. In addition, the microprocessor  54  and the FPGA  56  communicate with one another, wherein the microprocessor  54  receives, inter alia, the phase information of the modulation signal for determining the propagation time. 
       FIG. 2  shows three modulation signals  60 ,  62 ,  64 , illustrated in an idealized fashion, against a time axis. The first modulation signal  60  is a rectangular-waveform modulation signal having a fundamental frequency of e.g. 125 MHz. The second modulation signal  62  is a rectangular-waveform signal having a fundamental frequency of 13 MHz, and the third modulation signal  64  is a rectangular-waveform signal having a fundamental frequency of 15 MHz. The reference numeral  66  represents a summation signal resulting from an addition of the three modulation signals  60 ,  62 ,  64 . The summation signal  66  is a rectangular-waveform signal having a number of rectangular pulses  68 ,  70  which succeed one another with the fundamental frequency of the first modulation signal  60 . The rectangular pulses  68 ,  70  of the summation signal  66  have different pulse heights, however, on account of the addition with the second and third modulation signals  62 ,  64 . The summation signal  66  is therefore a combined signal which contains further signal frequencies in addition to the basic frequency of the first modulation signal  60 . In particular, the summation signal  66  comprises a signal frequency corresponding to the difference between the basic frequencies of the second and third modulation signals  62 ,  64 . This further signal frequency is manifested in the periodic pattern with which the highest rectangular pulses  68  go beyond the threshold value indicated by the reference numeral  72 . Furthermore, the summation signal  66  contains a signal frequency corresponding to the average value of the fundamental frequencies of the two modulation signals  62 ,  64 . In the preferred exemplary embodiment, the summation signal therefore contains a signal frequency of approximately 2 MHz (15 MHz−13 MHz) and a signal frequency of approximately 14 MHz (15 MHz+13 MHz/2). The summation signal  66  is therefore suitable as a modulation signal for an amplitude modulation of the emission light beam  18 , wherein the relatively high signal frequency of 125 MHz supplies a fine phase for the accurate determination of the distance d, while the low signal frequency of 2 MHz supplies a coarse phase for a large unambiguity range. It goes without saying that these different signal frequencies and phase differences are evaluated correspondingly in the evaluation and control unit of the novel device, to be precise preferably in each individual measuring cycle. 
     In the exemplary embodiment in accordance with  FIG. 2 , the pulse amplitude of the first modulation signal  60  is twice as high as the pulse amplitude of the second and third modulation signals  62 ,  64 . This has the consequence that the summation signal  66  is a quaternary signal in which the rectangular pulses  68 ,  70  assume one of four possible pulse values. In principle, this quaternary signal  66  can be used as a modulation signal for the emission light beam. 
     In a particularly preferred exemplary embodiment, however, rather than the quaternary summation signal  66 , a binary modulation signal  74  is used, which results from the summation signal  66  by virtue of the fact that only the rectangular pulses  68  that extend beyond the pulse value designated by the reference numeral  72  are used. In other words, here only the “high” pulse peaks of the summation signal  66  are used, which are designated by the reference numeral  68 ′ in  FIG. 2 . The lower part of the signal  66  is “cut off”. As can be discerned with reference to  FIG. 2 , the temporal intervals PA between the rectangular pulses  68 ′ change periodically. Furthermore, the number of rectangular pulses  68 ′ per group  76  of rectangular pulses  68 ′ varies. The modulation signal  74  is therefore a frequency-modulated, rectangular-waveform, binary signal whose fundamental frequency corresponds to the fundamental frequency of the first modulation signal  60  (that is to say 125 MHz here). This fundamental frequency is frequency-modulated with the beat frequency resulting from the frequency difference between the second and third modulation signals  62 ,  64 . 
       FIG. 3  shows a modulation signal calculated with the aid of a digital circuit and corresponding to the modulation signal  74  from  FIG. 2 .  FIG. 4  shows the frequency spectrum of the modulation signal from  FIG. 3 . A first peak, indicating a high signal component at the fundamental frequency of 125 MHz, can be discerned from the reference numeral  80 . The reference numeral  82  designates further peaks lying at 375 MHz, 625 MHz, 875 MHz etc. These are odd-numbered multiples of the fundamental frequency that are typical of a rectangular-waveform signal. 
     Further peaks that occur on account of the combination with the second and third modulation signals  62 ,  64  can be discerned from the reference numerals  84 ,  86 . The further peaks  84 ,  86  characterize frequency components which are likewise contained in the combined modulation signal  74  and which, in at least an embodiment of the invention, are evaluated in addition to the fundamental frequency of the first modulation signal  60  in order to determine the propagation time of the emission light beam  18  and of the reception light beam  24  and consequently the distance d. In currently preferred exemplary embodiments, only the fundamental frequencies, but not the further harmonic frequencies  82 ,  88  are evaluated in order to determine a phase shift between the emission light beam  18  and the reception light beam  24 . In other exemplary embodiments, the harmonic frequencies, i.e. the frequencies at the peaks  82  and the frequencies  88  respectively grouped around them can also be evaluated as well. In the currently preferred exemplary embodiments, the harmonic frequencies  82 ,  88  are suppressed with the aid of a suitable input filter in the region of the light receiver  14 . It goes without saying that such an input filter (not illustrated here) can be omitted and/or must be modified if the harmonic frequency components are likewise intended to be evaluated. 
     In the currently preferred exemplary embodiment, the modulation signal  74  is generated as a binary rectangular-waveform modulation signal with the aid of a digital circuit in the form of the FPGA  56 . A computational specification and/or a table of values which represents the modulation signals  60 ,  62 ,  64  is stored in the FPGA  56  for this purpose. With the aid of said computational specification and/or a table of values, the FPGA  56  generates the binary pulse sequence that is fed as modulation signal  74  to the light emitter  12 . 
       FIG. 5  shows an alternative exemplary embodiment, in which the modulation signal for the light emitter  12  is generated in an analog manner. As can be discerned with reference to  FIG. 5 , the light emitter  12  comprises the laser diode  13  and a transistor  90 , through which flows a control current I with which the laser diode  13  is fed. By varying the control current I through the transistor  90 , an amplitude modulation of the laser light generated by the laser diode  13  is produced. The phase angle of the control current I is a measure of the phase angle of the modulation signal with which the emission light beam is modulated. Preferably, the phase angle of the control current I is measured by means of a phase detector  91  and reported as reference phase to the microprocessor  54 . In another preferred exemplary embodiment, a part of the emitted light beam is branched off by means of a signal splitter and the branched-off part is measured by means of a light-sensitive monitor diode. In this way, one obtains the phase angle of the modulation signal in the emitted light beam. In a particularly preferred exemplary embodiment, a communication channel of the FPGA  56  is used for transmitting the phase information. 
     In the exemplary embodiment in accordance with  FIG. 5 , the base of the transistor  90  is fed with a summation signal corresponding e.g. to the summation signal  66  from  FIG. 2 . The summation signal is generated by adding a first modulation signal  60 , a second modulation signal  62  and a third modulation signal  64  at a summation point  91 . The modulation signals  60 ,  62 ,  64  are generated with the aid of three sinusoidal signals  92 ,  94 ,  96 . Each of the three sinusoidal signals  92 ,  94 ,  96  is amplified with the aid of an amplifier  98  and subsequently “clipped” by means of a limiter  100 . In this way, the sinusoidal signals  92 ,  94 ,  96  become rectangular-waveform signals such as are illustrated in an idealized fashion in  FIG. 2 . 
       FIG. 6  show a further exemplary embodiment. Identical reference symbols designate the same elements as before. In this exemplary embodiment, the sinusoidal signals  92 ,  94 ,  96  are amplified with the aid of the amplifiers  98  in each case to such a great extent that the summation signal in each case drives the transistor  90  to saturation. In this exemplary embodiment, the transistor  90  itself acts as a limiter that generates the rectangular-waveform modulation signal from the sinusoidal modulation signals  92 ,  94 ,  96 . 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.