Abstract:
A device for optically measuring distance, in particular a hand-held device, comprising an transmission unit ( 12 ) which is provided with a light source ( 17, 18 ) for emitting optical measuring radiation ( 13, 20, 22 ) towards a target object ( 15 ), and a capturing unit ( 14 ) which is arranged at a distance from the optical axis ( 38 ) of the transmission unit ( 12 ). The capturing unit ( 14 ) comprises at least one optical detector ( 54 ) for capturing optical radiation ( 16, 49, 50 ) reflected by the target object ( 15 ). The detector ( 54 ) of the capturing unit ( 14 ) comprises a plurality of light-sensitive surfaces ( 70, 72, 74; 170, 172, 174; 270, 272, 274; 370, 372; 470, 472; 570, 572 ) which are separated from each other and which are be activated separately. The invention also relates to a method for operating a device for optically measuring distance.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
   The invention described and claimed hereinbelow is also described in German Patent Application DE 10 2006 013 290.4 filed on Mar. 23, 2006. This German Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119(a)-(d). 
   BACKGROUND OF THE INVENTION 
   Optical distance-measuring devices as such have been known for a long time, and they are now sold commercially in large quantities. These devices emit a modulated light beam that is directed toward the surface of a desired target object whose distance from the device is to be determined. A portion of the returning light that has been reflected or scattered by the target object is detected by the device, and it is used to determine the distance in question. 
   The application range of distance-measuring devices of this type generally ranges from a few centimeters to several hundred meters. 
   Depending on the paths to be measured and the reflectance of the target object, different requirements result for the light source, the quality of the measurement beam, and the detector. 
   The optical distance-measuring devices known from the related art basically belong to two categories, depending on the configuration of the transmission and reception channels present in the device. 
   In one category, there are devices with which the transmission channel is located a certain distance away from the reception channel, so that the optical axes extend in parallel with each other but a distance away from each other. The other category includes monoaxial measuring devices with which the reception channel extends coaxially with the transmission channel. 
   The former, bi-axial measurement systems have the advantage that a complex beam-splitting system is not required to select the returning measurement signal, thereby also enabling, e.g., optical crosstalk from the transmission path directly into the reception path to be suppressed to a greater extent. 
   Bi-axial distance-measuring devices have the disadvantage, however, that detection problems may arise when performing close-range distance measurements, due to a parallax. In this case, the image of the target object on the detector surface—the image being located unambiguously on the detector even when target distances are great—moves increasingly further away from the optical axis of the reception path as the measurement distance decreases, and the beam cross-section in the detector plane changes markedly. 
   As a result, the measurement signal that is detected may approach zero in the close range of detection, i.e., when the distance between the target object and the measuring device is short, if no further measures are taken in the device. 
   Although measuring devices of this type may be optimized for a certain distance range, this requires that the measuring range that is actually accessible to the measuring device be limited substantially. 
   Publication DE 10 130 763 A1 makes known a device for optically measuring distance over a large measuring range that includes a transmission unit with a light source for emitting modulated, optical radiation toward a target object, and with which the receiving unit that includes an optical detector located in this measuring device—which serves to receive the optical radiation returning from the target object—is located on a reception axis, which is located a distance away from the optical axis. The active, photosensitive surface of the detector of the reception unit described in DE 10 130 763 A1 tapers in the direction of a beam displacement for decreasing target object distances that results due to a parallax of the returning measurement radiation. 
   Publication DE 10 051 302 A1 makes known a laser distance-measuring device for the short- and long-range that includes a special receiver with a transmission channel and a reception channel. The transmission channel is composed of a transmission lens, in whose focal point a laser light source is located. The reception channel is composed of a reception lens, in whose focal plane a receiver system is located. The optical axes of the transmission lens and the reception lens extend in parallel with each other for a finite distance. The receiver system of the laser distance-measuring device described in DE 100 51 302 A1 is a photodiode chip system with at least two active photodiode surfaces located on a straight line that intersects the optical axes of the transmission and reception lens of this device. 
   SUMMARY OF THE INVENTION  
   The object of the present invention is to ensure—based on a device for optically measuring distance according to the related art—that the most constant reception signal possible may be measured across the largest possible measuring range. 
   The inventive device for optically measuring distance includes a transmission unit with a light source for emitting optical radiation, in particular modulated optical measurement radiation, and a reception unit, which is located a distance away from the optical axis of this transmission unit and includes at least one optical detector. The detector of the reception unit advantageously includes a plurality of separated—electrically separated, in particular—photosensitive surfaces, which may be activated separately when the device is operated. 
   The photosensitive surface of an optical detector, which is typically composed of at least one semiconductor material, may be activated via the application of voltage. This means that light that strikes the photosensitive surface of the detector is converted into an electrical signal and may therefore be detected using a downstream electronic circuit. 
   To this end, the inventive device includes switching means with which one or more subregions of the photosensitive surface of the detector may be switched on or off. 
   When photodiode surfaces that are not used or are used only partially are switched off, the noise that is basically produced due to the extraneous light that strikes the detector may be markedly reduced, since only those photosensitive surfaces are used that receive the light that is essentially usable for the measurement. In particular, it is advantageously possible to activate only a single photosensitive surface. 
   In addition, the electrical capacitance of the photodiode and, therefore, of the detection system decreases, since the photodiode surface that is active during operation is smaller. The photodiode may therefore be operated at a higher frequency, which results in greater measurement accuracy of the inventive device for optically measuring distance. 
   It is therefore possible to expand the measuring range that is accessible to this measuring device in a simple, reliable manner. 
   SUMMARY OF THE INVENTION 
   Advantageously, only that photosensitive surface or those photosensitive surfaces of the plurality of photosensitive surfaces of the detector are active that receive the largest portion of returning optical measurement radiation. It is therefore possible to detect a measurement signal that is sufficiently strong, while also markedly reducing the portion of extraneous light that strikes the unused regions of the photosensitive surface of the detector. This results in a better signal-to-noise ratio in the detected measurement signal, thereby making it possible to greatly improve the resolution of the measuring device and the distance range that is accessible to the measuring device. 
   The inventive device includes means with which individual photosensitive surfaces of the detector may be switched on or off. Particular photosensitive regions may be activated, e.g., by guiding a connection to the outside, that is, out of the diode housing, for instance, when a common ground exists. Via the contacting of the connection, it is therefore possible to influence which of the surfaces is activated and should therefore be used. As an alternative, it is possible to integrate a multiplexer directly in the photodiode when several surfaces are involved. 
   In an advantageous embodiment, the envelope of the photosensitive surfaces of the detector tapers in the direction of a beam displacement for decreasing target object distances that results due to a parallax of the returning measurement radiation. The envelope is a hypothetical curve that may be placed around the edge of the photosensitive surfaces of the detector at a minimal distance. In this manner it may be ensured that adequate measurement signal strikes that portion of the detector that is active, for every object distance. In particular, the shape of the overall optical detection area is selected such that a signal with a sufficient amplitude is present on the detector surface, even at close range. This results in a further improvement of the ratio of useful light to extraneous light, thereby increasing the measurement accuracy of the device in the immediate close range for this reason as well, and therefore expanding the measuring range that is accessible to the device. 
   In an advantageous embodiment, the envelope may also expand in the direction of the beam displacement for decreasing target object distances that results due to a parallax of the returning measurement radiation. This may compensate, in particular, for the effect of the enlarged measurement beam diameter as object distances decrease. 
   The envelope of the photosensitive surfaces of the detector advantageously has a symmetry axis, which lies in the common plane of the optical axes of the transmission unit and the reception unit of the device. Given that the measurement beam that returns from the target object travels laterally in the common plane of the optical axes of the transmission unit and the reception unit as the object distance decreases, the detector will advantageously have an enlongated shape in this direction. In this manner, the dependence of the direction of the returning measurement signal on the distance of the measuring device from a target object is taken into account. The size of the photosensitive surfaces of the detector of the reception unit is selected such that a sufficient amount of the signal, e.g., also in the close range, still strikes the particular active subregion of the detector. 
   This also advantageously makes it possible to account for the dependence of the strength of the returning measurement signal on the distance of the measuring device from the target object. 
   Due to the inverse square law for the change in intensity as a function of the path covered, the returning measurement signal is much greater for the close range than it is for target objects located far away from the measuring device. 
   The expansion of the envelope of all photosensitive surfaces of the detector perpendicular to the common plane of the optical axes of the transmission unit and the reception unit may therefore decrease to the extent at which the light signal increases due to the shorter path at close range. This also has the advantage that, due to the reduced expansion of the detector, a sufficient amount of light from close range may still strike the detector, but it may not be possible for the light from close range to override the detector, given that its active photosensitive surfaces become smaller in this direction. 
   This effect is limited by the opposite effect, namely, that, due to the poorer focussing of the returning measurement beam bundle at shorter object distances, the diameter of the measurement beam bundle increases greatly, and the energy density of the returning measurement signal that occurs on a small detector surface could also possibly become too small. In a case such as this, a detector is advantageous with which the envelope of the photosensitive subregions of the detector do not taper in the direction of the beam displacement as object distances decrease, but expand instead. 
   In terms of the size of the surface of the detector and/or the size of the individual, sensitive subregions of the detector, it should only be ensured that the active surface, i.e., an active surface in the range of the detector in which light from distant target objects strikes the detector surface, is great enough to detect the entire signal, if possible, in this case, since remote measurement objects result in a relatively weak detection signal. This is also a consequence of the inverse square law, on which the detected intensity is based. 
   The lateral expansion of the active surfaces of the detector should therefore be large enough that a sufficient amount of light from the immediate close range of detection still reaches the particular active detection area. Due to the high signal level that results from short paths at close range, it is not necessary in this case to detect the entire signal intensity. 
   A further advantage of the claimed device is the fact that the electrical-capacitive properties of the detector of the measuring device are positively influenced, due to the inventive shape of the active detection areas. If the active detector area were too large, the electrical capacitance of the detector would increase, and the response characteristic over time, or, as an equivalent, the frequency response of the measurement system, would no longer correspond to the necessary requirements of the time and frequency resolution of the measurement system. 
   The inventive device for optically measuring distance may be advantageously realized by using a laser, in particular a laser diode, as the light source. Lasers and, in particular, laser diodes, are now available at favorable prices across the entire visible spectral range of electromagnetic waves. Due to their compact size and relatively high output powers, laser diodes are suited for use in devices for optically measuring distance, in particular in hand-held devices of this type. 
   The inventive device for optically measuring distance therefore makes it possible to generate the most constant reception and detection signal possible over a large measuring range of distances between the device and a target object. 
   Further advantages of the inventive device are presented in the drawing below, and in the associated description of an exemplary embodiment of the inventive device. 
   Exemplary embodiments of the inventive device are shown in the drawing. The description, associated figures, and the claims contain numerous features in combination. One skilled in the art will also consider these features—and, in particular, the features of various exemplary embodiments—individually, and will combine them to form further reasonable combinations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematicized view of an exemplary embodiment of an inventive device for optically measuring distance, 
       FIG. 2  shows a schematicized depiction of the variation of the measurement beam bundle in the detection plane when the distance of the object to be measured varies, 
       FIG. 3  shows a view of the detector surface of an inventive device, 
       FIG. 4  shows a view of an alternative embodiment of a detector for an inventive device, in a schematic depiction, 
       FIG. 5  shows a further embodiment of the detector of an inventive device, in a schematic illustration. 
       FIG. 6  shows a further embodiment of the detector of an inventive device, in a schematic illustration, 
       FIG. 7  shows a further embodiment of the detector of an inventive device, in a schematic illustration, 
       FIG. 8  shows a further embodiment of the detector of an inventive device, in a schematic illustration. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     FIG. 1  is a schematic view of an inventive device for optically measuring distance, including the most important components, whose function will be described. Inventive device  10  includes a housing  11 , in which a transmission device  12  for generating a measurement signal  13 , and a reception device  14  for detecting measurement signal  16  returning from a target object  15  are located. 
   Transmission device  12  includes a light source  17 , which is realized as a semiconductor laser diode  18  in the exemplary embodiment shown in  FIG. 1 . It is also possible to use other light sources in the inventive device. Laser diode  18  emits a laser beam  20  in the form of a light bundle  22  that is visible to the human eye. 
   Laser diode  18  is operated via a control device  24 , which generates a modulation of electrical input signal  19  of diode  18  via appropriate electronics. Via a modulation of the diode current that is carried out in the manner, it is ensured that optical measurement signal  13 —which is used to measure distance—is also modulated in a desired manner. 
   Laser beam bundle  20  then passes through collimation optics  26  designed as a lens  28 , which is depicted simply as a single lens  30  in  FIG. 1 . In this exemplary embodiment, lens  28  is optionally located on an adjustment device  32 , which serves basically to change the position of the lens in all three spacial directions, e.g., for adjustment purposes. 
   As an alternative, collimation optics  26  may be a component of laser diode  18 , or it may be fixedly connected therewith. 
   After passing through lens  28 , an, e.g., amplitude-modulated signal  13  results in the form of a parallel light bundle  37 , which propagates along optical axis  38  of transmission unit  12 , as depicted schematically in  FIG. 1 . A preferably switchable beam deflector  40  is also located in transmission branch  12  of the inventive device that makes it possible to redirect measurement signal  13  to reception unit  14  of device  10  directly, i.e., inside the device, and to avoid a target object. In this manner, a reference path  42  inside the device is created, which may be used to calibrate or compensate for the measurement system. 
   When a distance measurement is carried out using the inventive device, measurement beam  13  leaves housing  11  of the inventive device via an optical window  44  in front wall  45  of device  10 . The opening of the optical window may be secured, e.g., with a shutter  46 . To perform the measurement, measuring device  10  is pointed at a target object  15 , whose distance  48  from the measuring device is to be determined. Signal  16 , which is reflected or scattered on target object  15 , forms a returning ray bundle  49  or  50 , a certain portion of which returns to measuring device  10 . 
   Returning measurement radiation  16  is coupled into the measuring device through an entrance window  47  in front side  45  of device  10 . In the exemplary embodiment shown in  FIG. 1 , measurement radiation  16  is deflected to reception optics  52 . 
   Two returning measurement beam bundles  49  and  50  for two different target object distances  48  are sketched in  FIG. 1 , as an example and for purposes of illustration. For large object distances—with “large” in this case meaning large compared with the focal distance of reception optics  52 —signal  16  that is returning from the target object enters parallel to optical axis  51  of reception device  14 . In the exemplary embodiment depicted in  FIG. 1 , this case is represented by measurement beam bundle  49 . As the object distance decreases, returning signal  16  that enters the measuring device becomes increasingly slanted relative to axis  51  of reception unit  14 , due to a parallax. Beam bundle  50  is drawn in  FIG. 1  as an example of a returning measurement beam bundle of this type located within close range of the distance-measuring device. 
   Reception optics  52 , which are also depicted only schematically as a single lens in the exemplary embodiment in  FIG. 1 , collimates returning measurement signal  16  and focuses its beam bundle on photosensitive surface  66  of a reception detector  54 . Detector  54  includes—in order to detect the optical measurement radiation—at least one photodiode, e.g., a PIN diode, an APD (Avalanche Photo Diode), or at least one CCD chip, as the photosensitive element  66 . Of course, other surface detectors known to one skilled in the technical art may also be used as reception detectors. The surface detector is typically oriented such that its active photosensitive surface  66  is perpendicular to the optical axis of the reception branch. The incident optical signal is converted by reception detector  54  into an electrical signal  55 , and it is sent to the inventive device for further evaluation in an evaluation unit  36 . 
   Reception optics  52 —which are also mounted on adjustment device  53  in the exemplary embodiment in  FIG. 1 , but is not limited thereto—are located approximately at the distance of their focal width away from active surface  66  of the detector, so that incident radiation arriving from a target object located far away from the measuring device is focused exactly on the detector or the active photosensitive surfaces. 
   When the distances from the target object are small, it should be noted, however, that the image position of the measurement spot that is reflected or scattered on the target object is located increasingly further away from the focal point of the reception lens. For example, as the distance between the target object and the measuring device decreases, the returning measurement beam travels increasingly further away from the optical axis of the reception device, thereby deviating more and more from the optical axis of the transmission device. In addition, the returning measurement beam bundle is no longer focused exactly on the detector surface, due to the changed imaging conditions on the reception lens. As the target object distance decreases, the size of the measurement spot on the detector surface increases. 
   Additional components located in the measuring device that are not related to what is required to understand the inventive device will not be discussed further in this context. It should merely be noted that the measuring device also includes a control and evaluation unit  36 , of course. 
   The relationships between the distance of the target object from the measuring device and the position and size of the measurement spot on the detector surface are depicted schematically in  FIG. 2  as an overview.  FIG. 2  shows a top view of a detector surface  64  per the related art in the direction of view of measurement signal  16 , which is returning from the measurement object. Reference numeral  56  labels the common plane of optical axis  38  of transmission unit  12  and optical axis  51  of reception unit  14 . Measurement spot  58  of returning radiation  16  for very large object distances  48  is located on optical axis  51  of reception unit  14  and is focused on surface  64  of the detector, forming a small spot. Since detector  54  is located approximately at the distance of the focal width of reception optics  52 , light that comes from infinity, optically speaking, is focused directly on the detector surface, due to the principles of optical imagery. To illustrate the relationships, a “classical” detector surface  64  of a detector per the related art is shown as a dashed line in  FIG. 2 . 
   As distance  48  of measuring device  10  from target object  15  decreases, returning signal  16  strikes reception lens  52  at an increasing slant, so that the measurement spot on the detector surface also travels in the direction of arrow  61  in  FIG. 2 . 
   Measurement spot  60  for a short object distance  48  of target object  15  from measuring device  10 , which is also sketched in  FIG. 2 , has therefore traveled away from optical axis  51  of the reception device, and it is greatly enlarged in terms of its expansion, in particular its lateral expansion. 
   When measurement distance  48  of measurement object  15  from the measuring device is very short, a measurement spot  62  of returning measurement signal  16  appears in the detector plane, which is also markedly increased in size and also appears further away from optical axis  51  of reception unit  14 . A displacement of this type of the measurement spot to be detected with relative distance  48  of a measurement object  15  from measuring device  10  may result—for very short object distances—in returning signal  16  no longer striking the active surface of measurement receiver  54 , as indicated by dashed surface  64  of a “classical” measurement receiver shown in  FIG. 2 . 
   To account for the variation in size and position of the measurement spot in the detection plane of reception unit  14 , active photosensitive surface  66  of inventive detector  54  is designed accordingly and will be described in greater detail below. 
     FIG. 3  shows a first exemplary embodiment of photosensitive surface  66  of a detector of the inventive device. In this case, detector  54  of reception unit  14  includes a plurality of photosensitive surfaces  70 ,  72 , and  74 , which are separated from each other and, in entirety, form photosensitive surface  66  of the detector. In particular, the photosensitive surfaces of the detector are electrically separated from each other, thereby making it possible to actively switch only one of the photosensitive surfaces  70  through  74  at a time, that is, e.g., to apply a voltage signal to it, thereby enabling the incident light to be converted to an electrical signal. Subregions  70 ,  72 , and  74  of the detector may all have the same size, i.e., surface area, in particular, or they may be designed with different sizes. 
   To activate a photosensitive subregion of the detector, a connection for each surface may be guided out of the diode housing, for example, thereby making it possible to trigger and selectively use the particular photosensitive subelement via a contacting or triggering of a connection of this type. This is indicated via electrical connection lines  57  depicted symbolically in  FIGS. 3 through 8 . To this end, appropriate switching means are provided that make it possible to activate the preferred subregion or subregions of detector  54  depending on the control signal. As an alternative, when several surfaces are involved, a multiplexer could also be integrated directly in detector  54 , e.g., in a photodiode. 
   For very large object distances  48  between target object  15  and measuring device  10 , measuring spot  58  comes to rest entirely on photosensitive subregion  70 . In this case, i.e., for large measurement distances, only photosensitive surface  70  would be activated using appropriate switching means, thereby enabling it to function as a detector surface and convert the optical measurement signal into an electrical measurement signal. Subregions  72  and  74  of the detector, which are also present, are not activated. No voltage is applied to these photosensitive surfaces, for example. Light that strikes these surfaces therefore does not cause an electrical signal to be generated. If extraneous light from other objects that are located closer to the measuring device than object  15  to be measured at this time would enter the measuring device, this extraneous light would not be detected, because photosensitive surfaces  72  and  74  are not activated, i.e., they are not switched on. This extraneous light would therefore not contribute to increased background noise relative to the measurement signal from active surface  70  generated by measurement bundle  58 . 
   Active surface  70 , which has been activated in particular for very large measurement distances, advantageously has a lateral expansion in the detection plane such that it ensures that measurement spot  58  of measurement radiation  16  or  49  returning from a remote target object of this type is detected in entirety. A direction that is perpendicular to the measurement signal direction is the lateral direction in this case. The dimensions of photosensitive surfaces  70  should therefore be essentially the same or slightly larger than the dimensions of a measurement spot  58  for very large object distances. If—as object distance  48  decreases—the measurement spot now travels away from original reception axis  51 , in the direction of arrow  61 , then the diameter and/or the lateral expansion of the measurement spot increases, as illustrated in  FIG. 2 . The lateral direction is the direction perpendicular to direction  61 , in which the measurement beam bundle travels. 
   The surface detector has an elongated shape overall in direction  61  of a beam displacement, as target object distances  48  decrease. The expansion in the direction of travel of the measurement signal is greater—and is much greater, in particular—than it is in the orthogonal, i.e., lateral direction. 
   When the returning measurement beam bundle travels, a situation arises in which the measurement beam bundle passes at least partially over, e.g., parts of both photosensitive surface  70  and  72 , as indicated in  FIG. 3  using measurement spot  63 , which is shown as a dashed line. In a situation such as this, an appropriate measurement technique is used to detect which photosensitive surface  70  or  72  receives the larger portion of the reflected measurement beam bundle (bundle  63  in this case), so that, when a distance measurement is carried out in this configuration, only that photosensitive surface ( 70  or  72  in this case) may be activated that receives the largest portion of returning radiation. By switching off photodiode surfaces that are not used or are used only partially, the noise that is produced due to extraneous light may therefore be markedly reduced, since only those subregions of the detector are used that receive the useful light in an optimal manner. Those surfaces of the detector that have a relatively large portion of extraneous light are therefore switched off. In determining a distance from a target object  15 , only one photosensitive surface of the detector is therefore active, in this embodiment in particular. In alternative embodiments, several subregions may also be activated, in particular when the measurement signal strikes several subregions simultaneously and, e.g., the sum signal of two subregions contains less noise than the signal of the particular subregions that are considered individually. In this case, several subregions of the detector may also be activated, according to the present invention. 
   To determine that surface or subregion that has the highest portion of useful light and, therefore, the highest signal-to-noise ratio, a short test measurement may be carried out before the actual distance measurement is performed, which serves merely to determine the signal components on the individual photosensitive surfaces of the detector of the reception unit. While this test measurement is being carried out, it is possible to activate all or a majority of the photosensitive subregions of the optical detector, and to read them out individually, in particular, using switching means provided for this purpose. In this manner, it may be determined which photosensitive subregion is receiving the strongest light signal, in order to decide whether only a single surface should be activated, or whether several surfaces—which represent a true partial quantity of all available photosensitive surfaces—yield a better measurement signal, in particular a better signal-to-noise ratio. 
   When measurement distance  48  of a measurement object  48  from measuring device  10  is very short, and the measurement spot will therefore travel further in the direction of arrow  61  in  FIG. 3 , it is possible, e.g., according to the present invention, to only activate photosensitive surface  74 , and to switch off photosensitive surface  70 —which receives no light or only partial light—and to switch off photosensitive surface  72 , which receives only partial light. 
     FIG. 4  shows an alternative embodiment of inventive detector  54 , with which an envelope  165 , which may be placed or drawn around the photosensitive surfaces of the detector, tapers in direction  61 , i.e., in the direction of travel of the returning measurement beam bundle for a decreasing measurement object distance. 
   The expansion of photosensitive surfaces  170 ,  172  or  174  of the detector in the direction perpendicular to optical axis  51  of reception unit  14  is advantageously at least so great that the measurement beam returning from a target object  15  at close range still strikes photosensitive surface  174  at least partially. This means, in particular, that, when distances  48  to a target object  15  are short, photosensitive surface  174  used with distances of this type may also selected to be much smaller, given that the light intensity will be much greater, due to the inverse square law. This advantageously results in a reduction in the electrical capacitance of the detector, so that the response characteristic over time and/or, analogously, the frequency response of the measurement system may be markedly increased. 
   Envelope  165 , which may be placed or drawn around the photosensitive surfaces of the detector in the detector plane, therefore advantageously tapers in direction  61  of a beam displacement for decreasing target object distances  48 . An envelope  165  of this type is also shown in  FIG. 4 . The envelope basically follows the boundary of the photosensitive subregions, and the course of the envelopes in the direction of arrow  61 , i.e., in the direction of a beam displacement for decreasing object distances, is interpolated between two subregions. 
   The shapes of the photosensitive surfaces and their number within a detector may vary according to the embodiment. For instance,  FIG. 5  shows a detector with a plurality of rectangular photosensitive surfaces  270 ,  272 ,  274  of different sizes, whose envelope  265  tapers in direction  61  of a beam displacement for decreasing target object distances  48 . According to the present invention, photosensitive surfaces  270 ,  272 ,  274  may be activated—i.e., switched on or off—individually when a distance measurement is performed. 
     FIG. 6  shows a further embodiment with only two separate, photosensitive surfaces  370 ,  372 , which may also be activated individually using the principle described above. 
   Basically, the envelope, which may also be placed around the photosensitive surfaces of the detector, may also expand in the direction of decreasing object distances. An embodiment of this type with an envelope that expands in direction  61 —as shown, e.g., in FIGS.  7  and  8 —has the advantage that it accounts for the reduced energy density of the returning measurement signal for short measurement object distances. Due to a short measurement object distance, the returning measurement beam bundle is no longer optimally focused in the detection plane, since collimation optics  52  of a measuring device of this type are typically optimized for very large measurement object distances. Since the measurement spot increases rapidly in the detection plane for decreasing measurement object distances—refer to the illustration in FIG.  2 —a reduced energy surface density and/or intensity of the measurement signal results on the detector surface. A measurement spot  462  for short measurement object distances is shown in  FIG. 7  as an example. In particular when an envelope of detector subregions tapers (see  FIGS. 4 through 6 ), this behavior may result in only a small portion of the measurement spot striking the detector surface, and the detected measurement signal would therefore be relatively small. This effect of reduced energy density of the measurement signal on the detector surface may be offset by the fact that the detector surface expands in direction  61  as object distances decrease, or they expand after having been constricted, as depicted in the exemplary embodiment of an inventive detector shown in  FIG. 7 . Envelope  465  of photosensitive detector surfaces  470  and  472  and  474  widens in direction  61  as object distances decrease, after it has constricted in the region of the transition from subelement  470  to subelement  472 . Within the framework of the disclosure of the present invention, a shape of this type should also meet the criterium that the photosensitive surfaces of the detector are designed and located such that an envelope of these areas expands in the direction of a beam displacement as target object distances decrease. 
   With specific embodiments of an inventive device, the number and/or shape of the individual photosensitive surfaces that a detector may have may deviate from the exemplary embodiment depicted in  FIG. 7 , of course. For instance, subelement  472  could also be rectangular in design, while other subelements  470  and  474  of the photosensitive surface have the shape shown in  FIG. 7 . 
   Depending on the design of the measuring device, the effect of the inverse square law and the effect of a more or less poor focusing—which occur as measurement object distances decrease, and which have an opposing effect on the intensity of the measurement signal—should be weighed against each other, and the optimized shape of the photosensitive surfaces should be found, in particular for the envelope of the photosensitive surfaces. 
     FIG. 8  shows another possible embodiment of the inventive idea, with only two photosensitive detector subregions  570  and  572 , whose envelope  565  expands continually in direction  61 , however, as object distances decrease. 
   Independently of the shape of the envelopes of the photosensitive surfaces, they may be activated individually, so that the inventive detector may be operated with only one or more subregions. 
   The inventive device is not limited to the embodiments presented in the description. In particular, the inventive device is not limited to the shapes and numbers of individual photosensitive subregions of the detector.