Abstract:
The present invention relates to a distance measuring apparatus and method having a structure enabling highly accurate distance measurement. In particular, the distance measuring apparatus according to the present invention comprises at least a measurement system for detecting, of measurement pulse light emitted into a measurement optical path toward a target, information concerning a reflected beam from the target. In order to prevent undesirable reflected beams, which form erroneous information, among reflected beams of the measurement pulse light from being received, an operation for detecting reflected beams of the measurement pulse light is inhibited for a predetermined period of time from the emitting time at which the measurement pulse light is emitted.

Description:
RELATED APPLICATIONS 
     This is a continuation application of International patent application Ser. No. PCT/JP98/03890 filed on Sep. 1, 1998, now pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a distance measuring apparatus for measuring a distance to a target by using pulse light. 
     2. Related Background Art 
     It has been known that optical systems for distance measuring apparatus using light are roughly classified into two kinds, i.e., biaxial and coaxial types, depending on the arrangement of their light-transmitting optical path through which measurement pulse light is propagated and light-receiving optical path through which a reflected beam of the measurement pulse light is propagated. The light-transmitting optical path and the light-receiving optical path constitute a measurement optical path. Of these two types of optical systems, the coaxial optical system is advantageous from the viewpoints of reducing the size of apparatus, eliminating parallax, and making the light-transmitting optical path coincide with a collimation optical system for focusing onto a target. It is due to the fact that the coaxial optical system has a configuration in which the light-transmitting axis for transmitting light to the target and the light-receiving axis for receiving the reflected beam from the target coincide with each other. 
     Also, this kind of distance measuring apparatus includes those employing a continuous modulation system in which a light source such as LED, semiconductor laser, or the like is continuously modulated, thereby the distance to the target is determined from the phase difference between the transmitted light and the received light; and those employing a pulse system in which a semiconductor laser is used as a light source, thereby the distance to the target is determined from the period of time between the time at which pulse light is transmitted and the time at which part of thus transmitted light is returned after being reflected by the target. 
     In order to respond to the demand for enhancing the range capable of distance measurement and the demand for the ability of non-prism measurement in which reflectors such as corner cube reflector are not used at the measuring point for saving labor and attaining efficient operation, the pulse system adapted to employ a large peak power is quite advantageous. 
     Meanwhile, in the distance measuring apparatus using a coaxial optical system, when the output of the light source is intensified in view of the demand for enhancing the range capable of distance measurement and the demand for the ability of non-prism measurement, reflections within the apparatus such as those at the inner side face of an objective lens and the inner face of a lens barrel may be received by a light-receiving device as flare, thereby yielding a large error in measured values. Though the pulse system is quite advantageous for the demand for enhancing the range capable of distance measurement and the like, the pulse light has a greater peak, thus increasing the magnitude of flare, thereby yielding a larger error in measured values in the pulse system. 
     Methods for eliminating the error in measured values caused by flare are disclosed, for example, in U.S. Pat. No. 4,113,381 and Japanese Utility Model Publication No.  3-21502 . These methods employ a configuration in which the emission optical system and the light-receiving optical system are optically separated from each other, so as to eliminate flare. 
     SUMMARY OF THE INVENTION 
     Having studied the foregoing prior art, the inventor has found the following problems. Namely, the optical systems of the prior art are configured such that a smaller-diameter lens barrel accommodating the emission optical system is disposed in front of a larger-diameter lens barrel accommodating the light-receiving optical system so as to be coaxially supported. Therefore, a collimation optical system for collimating the target cannot be disposed on the same axis as the optical axis of the measurement optical system (including the light-receiving optical system and emission optical system), since the collimation optical system is blocked by the smaller-diameter lens barrel disposed in front thereof. As a consequence, in the case of non-prism measurement in particular, it becomes difficult for a measurer to recognize the measuring point, thus yielding a large drawback in the distance measurement. Also, since the light source is disposed in front of a large-diameter lens which functions as a light-receiving lens, the wiring of the power supply to the light source and the like may obstruct the light-receiving lens, thus clearly indicating a structural problem which causes the quantity of received light to decrease. 
     In view of such conventional problems, it is an object of the present invention to provide a highly accurate distance measuring apparatus and method which enhance the range capable of distance measurement, improve the non-prism distance measuring ability, and are free from influences of flare. 
     The distance measuring apparatus according to the present invention comprises two measurement systems, i.e., one for detecting, of measurement pulse light emitted into a measurement optical path toward a target, information concerning a component reflected from the target, and the other for detecting information concerning reference pulse light emitted into a reference optical path; and a structure enabling highly accurate distance measurement by comparing the results of measurement obtained from these measurement systems. 
     Specifically, the distance measuring apparatus according to the present invention comprises an emitter for emitting measurement pulse light into a measurement optical path between the apparatus and a target; a receiver for receiving a reflected beam (reflected component) of the measurement pulse light emitted into the measurement optical path; a detector for specifying a receiving time at which the reflected beam of the measurement pulse light is received; and a controller for measuring the period of time between an emitting time at which the measurement pulse light is emitted and the receiving time of the reflected beam of the measurement pulse light and calculating an optical path length of the measurement optical path on the basis of thus obtained time information. In this configuration, in particular, when the calculated value obtained in the initial measurement operation (pre-measurement operation) for the measurement optical path is not greater than a predetermined value, the controller inhibits the detector from effecting the specifying operation for a predetermined period of time from the emitting time of the measurement pulse light in the next measurement operation (main measurement operation) for the measurement optical path. 
     The distance measuring apparatus according to the present invention can further comprise therein a reference optical path which is different from the measurement optical path, and a switching mechanism for causing the receiver to selectively receive one of the reflected beam of the measurement pulse light propagated through the measurement optical path and reference pulse light propagated through the reference optical path so as to selectively measure the respective optical path lengths of the measurement optical path and the optical path length of the reference optical path. 
     It is presumed that, of the unnecessary reflected beam (flare) generated within the apparatus, the part having a strong power (pulse light having a power greater than a threshold which is set for eliminating noise) propagates by a shorter distance than the reflected beam from the target does, thereby reaching a pulse receiving element faster than the latter component does. The distance measuring apparatus according to the present invention inhibits flare, which may be mistaken as the reflected beam from the target, from being received, and securely receives the reflected beam from the target. 
     The distance measuring method realized by the measurement distance measuring apparatus having the above-mentioned configuration thus carries out the pre-measurement operation for confirming occurrence of flare, and the main measurement operation for eliminating the influence of flare. 
     Strictly speaking, the measurement optical path refers to a path extending from a pulse generating source to a pulse receiving element by way of a target positioned outside the apparatus, whereas the reference optical path refers to a path disposed within the apparatus so as to extend from the pulse generating source to the pulse receiving element. The emitting time at which the measurement pulse light is emitted refers to the point of time when a control signal to be fed into a driving circuit for driving the pulse generating source is generated, whereas the receiving time at which the reflected beam of the measurement pulse light is received refers to the point of time when a received signal generated upon arrival of the pulse light at the pulse receiving element is detected. 
     In addition to the constituents mentioned above, the distance measuring apparatus according to the present invention can further comprise a collimator having an optical system for focusing an image of the target onto a predetermined position. In this case, when the distance value from the apparatus to the target corresponding to the in-focus position of the optical system in the collimator is not less than a predetermined value, the controller inhibits the detector from effecting the specifying operation for a predetermined period of time from the emitting time at which the measurement pulse light is emitted in the measurement operation for the measurement optical path. 
     Though it is sufficient for inhibiting the received signal from being detected only for a predetermined period of time from the emitting time of the measurement pulse light as mentioned above in the case where the target is relatively far, it may not be able to judge whether the received light is the reflected beam from the target or flare generated within the apparatus in the case where the target is located at a relatively short distance. As a consequence, in an apparatus having a collimator, the optical system of the collimator for focusing the target image onto a predetermined position is adjusted beforehand, and the controller judges whether to inhibit the operation of the detector for a predetermined period of time or not as mentioned above according to the in-focus position of the optical system. 
     In this configuration, a limit switch for detecting the position of the optical system of the collimator may be provided (which is configured, for example, such as to turn on when a predetermined optical element in the optical system moves to a predetermined position), thereby judging whether the target is far or near. 
     In the distance measuring method realized by the distance measuring apparatus comprising a collimator, the measurement operation is carried out on the basis of the result obtained upon the collimating operation. 
     In the distance measuring apparatus and method according to the present invention, as mentioned above, the optical path length of the measurement optical path is measured, and the optical path length of the reference optical path is selectively measured. By obtaining the difference between the length of the measurement optical path and the length of the reference optical path as the distance from the apparatus to the target, the distance measuring apparatus and method enable highly accurate distance measurement. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conceptual view for explaining a method of measurement; 
     FIG. 2 is a perspective view showing the exterior of a distance measuring apparatus; 
     FIG. 3 is a plan view showing a display DSP of the distance measuring apparatus shown in FIG. 2; 
     FIG. 4 is a block diagram showing a schematic configuration of the distance measuring apparatus (telescope unit TSU) according to the present invention; 
     FIG. 5 is a view showing a configuration of an optical system in the distance measuring apparatus according to the present invention; 
     FIGS. 6A to  6 D are timing charts for explaining a measuring method utilizing pulse light, in which FIGS. 6A and 6C show timings for generating an emission instructing signal S 102 , FIG. 6B shows a timing for generating a receive timing signal S 106  when measuring the reference optical path, and FIG. 6D shows a timing for generating the receive timing signal S 106  when measuring the measurement optical path; 
     FIGS. 7A to  7 D are timing charts showing signal generating timings in the case where the distance between the target and the distance measuring apparatus is short, in which FIG. 7A shows a timing for generating a received pulse signal S 105  before adjusting light quantity, FIG. 7B shows a timing for generating the received pulse signal S 105  after adjusting the light quantity, FIG. 7C shows a timing for generating an active state signal of a received signal detector  6 , and FIG. 7D shows a timing for generating the received pulse signal S 106 ; 
     FIGS. 8A to  8 D are timing charts showing signal generating timings in the case where the distance between the target and the distance measuring apparatus is long, in which FIG. 8A shows a timing for generating the received pulse signal S 105  before adjusting light quantity, FIG. 8B shows a timing for generating the received pulse signal S 105  after adjusting the light quantity, FIG. 8C shows a timing for generating the active state signal of the received signal detector  6 , and FIG. 8D shows a timing for generating the received pulse signal S 106 ; 
     FIGS. 9A to  9 D are timing charts showing individual signal generating timings when flare is ignored in the case where the distance between the target and the distance measuring apparatus is long, in which FIG. 9A shows a timing for generating the received pulse signal S 105  before adjusting light quantity, FIG. 9B shows a timing for generating the received pulse signal S 105  after adjusting the light quantity, FIG. 9C shows a timing for generating the active state signal of the received signal detector  6 , and FIG. 9D shows a timing for generating the received pulse signal S 106 ; 
     FIG. 10 is a flowchart for explaining a pre-measurement operation in the distance measuring apparatus according to the present invention; 
     FIG. 11 is a flowchart for explaining a measurement operation utilizing a limit switch; 
     FIG. 12 is a view showing a configuration of a received signal detector in the distance measuring apparatus shown in FIG. 4; 
     FIGS. 13A to  13 M are timing charts of individual signals when no flare occurs, in which FIG. 13A shows a timing for generating a pulse selecting signal S 107 , FIG. 13B shows a timing for generating a pulse selecting signal S 107 A, FIG. 13C shows a timing for generating an emission instructing signal S 102 , FIG. 13D shows a timing for outputting a signal S 601  from an AND circuit  66 , FIG. 13E shows a timing for outputting a signal S 602  from an AND circuit  67 , FIG. 13F shows a timing for outputting a signal S 602 A from a delay element  69 , FIG. 13G shows a timing for outputting a signal (active state signal) S 603  from an OR circuit  65 , FIGS. 13H and 13K show timings for generating the received pulse signal S 105 , FIG. 13I shows a timing for outputting a signal S 604  from a comparator  62 , FIG. 13J shows a timing for outputting a signal S 605  from a latch circuit  70 , FIG. 13L shows a timing for outputting a signal S 105 A from a delay element  64 , and FIG. 13M shows a timing for outputting a signal S 106  from a comparator  61 ; 
     FIG. 14 is a flowchart for more specifically explaining the measurement operation shown in the flowchart of FIG. 10; 
     FIG. 15 is a flowchart for more specifically explaining the measurement operation shown in the flowchart of FIG. 11; and 
     FIGS. 16A to  16 M are timing charts of individual signals when flare occurs, in which FIG. 16A shows a timing for generating the pulse selecting signal S 107 , FIG. 16B shows a timing for generating the pulse selecting signal S 107 A, FIG. 16C shows a timing for generating the emission instructing signal S 102 , FIG. 16D shows a timing for outputting the signal S 601  from the AND circuit  66 , FIG. 16E shows a timing for outputting the signal S 602  from the AND circuit  67 , FIG. 16F shows a timing for outputting the signal S 602 A from the delay element  69 , FIG. 16G shows a timing for outputting the signal (active state signal) S 603  from the OR circuit  65 , FIGS. 16H and 16K show timings for generating the received pulse signal S 105 , FIG. 16I shows a timing for outputting the signal S 604  from the comparator  62 , FIG. 16J shows a timing for outputting the signal S 605  from the latch circuit  70 , FIG. 16L shows a timing for outputting the signal S 105 A from the delay element  64 , and FIG. 16M shows a timing for outputting the signal S 106  from the comparator  61 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, the distance measuring apparatus according to the present invention will be explained with reference to FIGS. 1 to  5 ,  6 A to  6 D,  7 A to  7 D,  8 A to  8 D,  9 A to  9 D,  10  to  12 ,  13 A to  13 M,  14  and  15 , and  16 A to  16 M. Among the drawings, parts identical to each other will be referred to with numerals or letters identical to each other without repeating their overlapping explanations. 
     FIG. 1 is a conceptual view for explaining a method of measurement. First, in order to carry out measurement, a surveying instrument comprising a distance measuring apparatus TS attached onto a tripod TP is placed on a horizontal ground surface HL. When the ground surface HL is not horizontal, a non-depicted adjustment screw is turned such that the central vertical axis of the distance measuring apparatus TS becomes orthogonal to the horizontal plane. Such an adjustment screw mechanism is disclosed, for example, in U.S. Pat. No. 4,113,381. This surveying instrument measures a horizontal distance HD to a point to be measured P which is located halfway up a mountain MTN. 
     The distance measuring apparatus TS is a so-called total station also having a theodolite or transit function. The apparatus TS is mounted on the tripod TP so as to be rotatable about the vertical direction with respect to the ground surface, and has a telescope unit TSU which is rotatable about the horizontal direction with respect to the ground surface. Their rotation angles are measured by a rotary encoder as disclosed in the above-mentioned U.S. Pat. No. 4,113,381. 
     The telescope unit TSU emits measurement pulse light. Of the pulse light, the component reflected at the point to be measured P enters into the telescope unit TSU. Then, according to the period of time  2 T between the timing at which the pulse light is emitted and the timing at which its reflected beam (reflected component) is received, and the velocity of light c, the distance L from the telescope unit TSU to the point to be measured P is measured. The horizontal distance HD can be computed from the angle VA formed between the optical axis of the telescope unit TSU and a horizontal line (ground surface) and the distance L, and is obtained by an electronic circuit EC within the apparatus TS (see FIG.  4 ). Namely, when the correction explained later is not effected within the telescope unit TSU, the distances L and HD become: 
     
       
         
           L=c×T  
         
       
     
     
       
         
           HD =L×cosVA  
         
       
     
     Here, a prism for reflecting the emitted measurement pulse light may be disposed at the point to be measured P. 
     The horizontal rotation angle HA and vertical rotation angle VA measured by the rotary encoder of the distance measuring apparatus TS, and the horizontal distance HD obtained by the arithmetic controller are indicated on the display DSP of the distance measuring apparatus TS. 
     FIG. 2 is a perspective view showing the exterior of the distance measuring apparatus. Disposed within a support housing HSG′ having substantially a U-like form are a battery and the electronic circuit EC. The telescope unit TSU having an objective lens  305  on its front face includes a rotatable housing HSG, whereas a collimator ST is disposed on the upper face of the housing HSG. Disposed outside the U-shaped housing HSG′ are adjustment screws S, S′ which are used for changing the posture of the housing HSG′. The rotary housing HSG is disposed between the opposing outer surfaces of the U-shaped housing HSG′ and are rotatably supported thereby. Attached to the upper end portion of the U-shaped housing HSG′ is a handle HAND. The display DSP is disposed at the proximal end portion of the U-shaped housing HSG′. 
     FIG. 3 is a plan view showing the display DSP. The display DSP is constituted by a liquid crystal DSP 1  and an operation keyboard section KB. The display DSP is electrically connected to the electronic circuit EC within the U-shaped housing HSG′ and indicates, on its liquid crystal DSP 1 , the above-mentioned horizontal rotation angle HA, vertical rotation angle VA, horizontal distance HD, remaining battery power, and the like. The keyboard section KB has a power button for turning power on, function selecting buttons for selecting the starting of distance measurement and the starting of angle measurement, an enter button for determining the selected function, and an escape button for canceling various commands. The operator pushes these buttons when necessary. 
     FIG. 4 is a block diagram showing a configuration of the distance measuring apparatus according to the present invention. First, a basic measurement operation will be explained. An arithmetic controller  1  is constituted, for example, by a CPU, its peripheral circuits, and the like, and outputs a measurement instructing signal S 101  to a signal processor  2 . The signal processor  2  is constituted by a counter and the like. In response to the measurement instructing signal S 101 , the signal processor  2  outputs an emission instructing signal S 102  to an emitter  3  and starts counting time. The emission instructing signal S 102  is also outputted to a received signal detector  6 . Upon receiving the emission instructing signal S 102 , the received signal detector  6  causes a comparator therein to attain a signal detectable state (active state). 
     The emitter  3  is constituted by a semiconductor laser driving circuit  31 , a semiconductor laser  32  which is a light source, and an emission optical system  33 . The emission optical system  33  selectively introduces pulse light  103  into a measurement optical path as measurement pulse light  110  directed toward a target or into a reference optical path as reference pulse light  111 , thereby transmitting the pulse light into each optical path. 
     The pulse light llOa (reflected beam) reflected by the target in the measurement pulse light  110  emitted into the measurement optical path from the emission optical system  33 , or the reference pulse light  111  propagated through the reference optical path is received by a receiver  4 . The receiver  4  is constituted by a light-receiving optical system  41 , an APD (avalanche photodiode)  42  which is a highly sensitive light-receiving device, and an APD driving circuit  43 . The pulse light  104  (reference pulse light  111  or reflected beam llOa of the measurement pulse light  110 ) transmitted through the light-receiving optical system  41  is photoelectrically converted by the APD  42  into an APD output signal S 109 , which is outputted to a received signal amplifier  5 . 
     The APD output signal S 109  is amplified by the received signal amplifier  5  and, as a received pulse signal S 105  having a pulse height corresponding to the magnitude of the APD output signal S 109 , is outputted to a received signal detector  6 . When receiving a received pulse signal S 105  which is not smaller than a predetermined threshold level TL, the received signal detector  6  generates, from the received pulse signal S 105 , a receive timing signal S 106  for terminating the time measurement and outputs thus generated signal to the signal processor  2 . 
     The arithmetic controller  1  outputs a pulse selecting signal S 107  to the received signal detector  6  and instructs whether to delay the activation of the comparator within the received signal detector  6  or not. Namely, when the pulse selecting signal S 107  is at its high level “H” the activation is not delayed, thereby the received signal detector  6  attains an active state at the same moment when the emission instructing signal S 102  is outputted. By contrast, when the pulse selecting signal S 107  is at its low level “L” the activation of the comparator is delayed by a predetermined period of time, thereby the pulse selecting signal S 107  becomes the high level “H” after a predetermined period of time has elapsed since the outputting of the emission instructing signal S 102 . 
     Upon receiving the receive timing signal S 106 , the signal processor  2  terminates the time measurement, and acquires the time data extending from the emission instructing signal S 102  to the receive timing signal S 106 . Thus obtained time data are transferred to the arithmetic controller  1  as a signal S 112 , which is turned into distance data with reference to the velocity of light. Here, the time at which the pulse light (measurement pulse light  110  or reference pulse light  111 ) is emitted refers to the output timing for the emission instructing signal S 102 , whereas the time at which the pulse light (measurement pulse light  110  or reference pulse light  111 ) is received refers to the input timing for the receive timing signal S 106 . 
     Also, a limit switch  330  is provided for detecting whether the target is farther than a predetermined value or not. Namely, when a focusing lens  306 , which will be explained later, is moved to the farther distance side (toward the objective lens  305 ) in order to focus onto a target farther than the predetermined value, the limit switch  330  detects that the focusing lens  306  is farther than the predetermined value, and outputs a detection signal S 108  to the arithmetic controller  1 . 
     FIG. 5 is a block diagram showing a configuration of an optical system in the distance measuring apparatus according to the present invention. The pulse light emitted from the semiconductor laser  32 , which is a light source, is turned into a parallel luminous flux by a collimator lens  301  and then is made incident on a prism  302 . The prism  302  has, for example, a ratio of transmissivity T with respect to reflectivity R of {fraction (T/R )}={fraction (1/99+L )}, thereby greatly attenuating the pulse light transmitted to the reference optical path  350 . 
     The case where the reference optical path is selected by the arithmetic controller  1  (see FIG. 4) will now be explained. The arithmetic controller  1  sets a reference optical path opening state and a measurement optical path closing state by means of an optical path switching shutter  321  (indicated by solid line in FIG. 5) which is an optical path selecting means (switching mechanism). 
     The pulse light transmitted through the prism  302  is reflected by a mirror  312 . Thus reflected pulse light is attenuated to a predetermined level by a filter  313  which has already been set at the time of assembling and adjustment, and is transmitted therethrough. The pulse light transmitted through the filter  313  is then transmitted through a light quantity adjustment filter  314 . Usually, the light quantity for the reference optical path is not attenuated. The light quantity adjustment filter  314  is set such as to reduce the signal level of the reference optical path only when the signal level of the measurement optical path is smaller than that of the reference optical path. 
     Subsequently, the pulse light transmitted through the light quantity adjustment filter  314  is reflected by a mirror  315 . Thus reflected pulse light is transmitted through the optical path switching shutter  321  set to the reference optical path opening state, thereby becoming incident on a prism  310 . This prism  310  has a characteristic equivalent to that of the prism  302 . The pulse light transmitted through the prism  310  is made incident on the APD  42 , which is a light-receiving device, by way of a collimator lens  311 . 
     The reference optical path traveling only within the apparatus is installed for the purpose of correcting the fluctuation in delay time caused by changes in temperature of the received signal detector  6  and the like so as to enhance the accuracy in measurement and the purpose of making the reference point of distance measurement, when set on a measurement point of the apparatus, coincide with the mechanical center point positioned on a plumb line passing this measurement point. 
     The case where the measurement optical path is selected by the arithmetic controller  1  will now be explained. The arithmetic controller  1  sets a measurement optical path opening state and a reference optical path closing state by means of an optical path switching shutter  321 a (indicated by dashed line in FIG. 5) which is an optical path selecting means (switching mechanism). 
     The pulse light reflected by the prism  302  is transmitted through a collimator lens  303  and then is reflected by a dichroic mirror  304 . The dichroic mirror  304  has such a characteristic to reflect infrared light and transmit therethrough visible light. The pulse light  351  (measurement pulse light  110 ) reflected by the dichroic mirror  304  is transmitted through the objective lens  305 , thereby being emitted toward the target. 
     The reflected beam  352  (reflected beam llOa of the measurement pulse light  110 ) is collected by the objective lens  305 . The pulse light (reflected beam  352 ) collected by the objective lens  305  is transmitted through the focusing lens  306 , and is reflected by a dichroic mirror  307 , thereby becoming incident on an optical fiber  308 . Here,  308   a  indicates the entrance end face of the optical fiber  308 . 
     The pulse light propagated through the optical fiber  308  is turned into a parallel luminous flux by a collimator lens  309 , and is attenuated to a predetermined amount through the light quantity adjustment filter  314  driven by a light quantity adjustment filter driving motor  322 , thereby becoming incident on the prism  310 . Since the optical path switching filter  321   a  is set to the measurement optical path opening state at the time of selecting the optical path, the pulse light reflected by the prism is made incident on the APD  42 , which is a light-receiving element, with the aid of the collimator lens  311 . 
     When the above-mentioned high-power pulse light is transmitted to the measurement optical path, there is, for example, light reflected by an inner face  305   a  of the objective lens  305  so as to be made incident on the optical fiber  308  by way of the dichroic mirror  307 . Also, an unnecessary reflected beam occurs at the entrance end face  308   a  of the optical fiber  308 . Such a reflected beam is flare, which results in an error in measurement of the measurement optical path as mentioned above. 
     The flare and reflected pulse light  352  emitted out of the optical fiber  308  enter the APD  42 , which is a light-receiving device, with a time difference in proportion to the distance to the objective lens  305  or distance to the target with respect to the measurement pulse light  351  emitted into the measurement optical path. 
     Before starting the measurement, the focusing lens  306  is moved along the directions of D in the drawing while the target is being observed through a collimation optical system with an eye  319  of the measurer, thereby the target is brought into focus. The collimation optical system is constituted by an eyepiece  318 , a reticle (focusing screen)  317 , an erect prism (Porro prism), the focusing lens (anallatic lens)  306 , and the objective lens  305 . The limit switch  330  detects whether the target is placed at a position farther than a predetermined value or not by detecting the movement of the focusing lens  306  to the side farther than a predetermined position (toward the objective lens  305 ). 
     FIGS. 6A to  6 D are views for explaining a distance measuring method utilizing pulse light. FIGS. 6A and 6B show a relationship between the emission instructing signal S 102  and the receive timing signal S 106  in the case where the reference pulse light  111  is propagated through the reference optical path. FIGS. 6C and 6D show a relationship between the emission instructing signal S 102  and the receive timing signal S 106  in the case where the measurement pulse light  110  is propagated through the measurement optical path. 
     The reference optical path is disposed within the apparatus. The reference pulse light  111  is emitted from the emitter  3  shown in FIG. 4, and reaches the receiver  4  by way of the reference optical path. As mentioned above, the signal processor  2  measures the period of time between the emission instructing signal S 102  shown in FIG.  6 A and the receive timing signal S 106  shown in FIG. 6B, and the arithmetic controller  1  determines, from thus measured period of time and the velocity of light, a measured length L ref  (optical path length) of the reference optical path. 
     On the other hand, the emitter  3  emits the measurement pulse light  110  into the measurement optical path toward the target, and the reflected beam  110   a  reflected by the target reaches the receiver  4 . Subsequently, from the emission instructing signal S 102  shown in FIG.  6 C and the receive timing signal S 106  shown in FIG. 6D, in a manner similar to that mentioned above, a measured length L s , (optical path length) of the measurement optical path is determined. Then, on the basis of the difference between the measured length L s  , and the measured length L ref , the distance from the apparatus to the target is determined. 
     In general, the electronic circuit has a delay characteristic depending on temperature and the like. Therefore, the measured length L ref  of the reference optical path having a known length is subtracted from the measured length L s  of the measurement optical path, thereby canceling the error in delay caused by temperature and the like. 
     Measured in the time measurement in the measurement optical path is the period of time between the emission instructing signal S 102  shown in FIG.  6 C and the receive timing signal S 106 , shown in FIG. 6D, indicating that the received signal detector  6  has received a pulse signal which is not smaller than a threshold level. 
     Meanwhile, when flare is made incident on the light-receiving device  42  as relatively strong pulse light, the receive timing signal S 106  indicated by dashed line in FIG. 6D is generated by the flare, thereby an erroneous measured length L s  is determined. 
     In addition, the pulse signal caused by flare does not always have a magnitude as high as the threshold level detected by the received signal detector  6 . Therefore, before the distance measurement, light quantity adjustment is effected in order that the received light quantity of the reflected beam  110   a  propagated through the measurement optical path and the received light quantity of the reference pulse light  111  propagated through the reference optical path have levels equivalent to each other. In the case where the light quantity of the reflected beam  110   a  is small due to the fact that the distance from the apparatus to the target is large and so forth, the reflected beam  110   a  is not attenuated even when the light quantity adjustment is effected. As a consequence, flare is not attenuated, thus allowing the received signal detector  6  to detect the pulse signal caused by the flare. 
     By contrast, in the case where the light quantity of the reflected beam  110   a  is large due to the fact that the distance from the apparatus to the target is small and so forth, the reflected beam  110   a  is attenuated by the light quantity adjustment. Here, the flare may be attenuated similarly, thereby the received signal detector  6  may fail to detect the pulse signal caused by the flare. 
     Here, the light quantity adjustment is effected before the distance measurement since the delay characteristic of the electronic circuit generally differs depending on the power of the signal fed therein. The light quantity adjustment filter  314  effects the light quantity adjustment such that the reference pulse light  111  received by way of the reference optical path and the reflected beam  110   a  of the measurement pulse light  110  received by way of the measurement optical path are equal in power. 
     In the conventional example mentioned above, the structure of the optical system is improved so as to eliminate the flare itself. As a result, however, the received light quantity decreases, thus failing to increase the measurable distance. 
     Hence, in this embodiment, the error in measurement caused by flare is overcome by signal processing. Namely, a pulse light source capable of outputting a large peak power is employed, so as to allow the measurable range to increase and the non-prism measuring ability to improve. On the other hand, influences of the resultant flare, which may cause a large error in measurement, are eliminated as explained in the following. Here, when the reference pulse light  111  from the emitter  3  is made incident on the receiver  4  by way of the reference optical path, there is no flare caused by the reflection from the objective lens  305  or the like. 
     In the case where the measurement pulse light  110  from the emitter  3  is propagated through the measurement optical path, and then the reflected beam  110   a  thereof is made incident on the receiver  4  by way of the measurement optical path, the flare caused by the reflection of the objective lens  305  or the like is also received by the receiver  4 . In the following, influences of the flare will be explained in two cases, i.e., the cases where the target is near and far, respectively. 
     Case with Short Distance between Apparatus and Target (Case 1) 
     In Case  1 , the light quantity of the reflected beam  110   a  received after being propagated through the measurement optical path is greater than the light quantity of the reference pulse light  111  received after being propagated through the reference optical path. It corresponds to the case where the target is relatively near. 
     In this case, in order to match the light quantity of the reflected beam  110   a  received after being propagated through the measurement optical path to the light quantity of the reference pulse light  111  received after being propagated through the reference optical path, the light quantity adjustment filter  314  attenuates the light quantity of the reflected beam  110   a . At the same time, flare is attenuated, thereby the pulse signal caused by the flare is suppressed to a level which is not detected by the received signal detector  6 . 
     FIGS. 7A to  7 D are timing charts showing timings for generating signals in the case where the distance between the apparatus and the target is short. FIG. 7A shows a timing for generating a received pulse signal S 105  before light quantity adjustment. Since the target is near, the reflected pulse signal S 81  caused by the reflected beam from the target has a peak level higher than the flare pulse signal S 80  caused by flare, with both signals S 80  and S 81  exceeding the threshold level TL. 
     FIG. 7B shows the received pulse signal S 105  after the light quantity adjustment. The light quantity adjustment is effected such that the light quantity of the reflected pulse light  110   a  from the target corresponding to the maximum peak level of the received pulse signal S 105  equals the light quantity of the reference pulse light  111  propagated through the reference optical path. In addition, the reference pulse light  111  is attenuated by the filter so as to become about the same as the light quantity of the reflected pulse light from the target located at a long distance. As a consequence, the reflected pulse signal S 81  is suppressed to the level of the reflected pulse signal S 83  in FIG.  7 B. At this time, the flare pulse signal S 80  is similarly suppressed to the level of the flare pulse signal S 82 , with its peak level not exceeding the threshold level TL. 
     FIG. 7C shows an active state signal indicative of whether the received signal detector  6  is in an active state or not. As explained with reference to FIG. 4, in response to the emission instructing signal S 102 , the active state signal attains its high level “H” thereby activating the received signal detector  6 . When the received pulse signal S 105  exceeding the threshold level TL is inputted, the activated received signal detector  6  generates a receive timing signal S 106 . 
     In the above-mentioned case, though the flare pulse signal S 82  also exists as the received pulse signal S 105 , it is reduced as mentioned above, thereby failing to reach the above-mentioned threshold level TL. Hence, as shown in FIG. 7D, the receive timing signal S 106  is generated only in response to the reflected pulse signal S 83 . 
     Consequently, the distance computed by the arithmetic controller  1  coincides with the distance to the target. Though flare is received, there is no error in distance measurement generated by the flare. 
     Case with Long Distance between Apparatus and Target (Case 2) 
     In Case 2, the light quantity of the reflected beam  110   a  received after being propagated through the measurement optical path is equal to or less than the light quantity of the reference pulse light  111  received after being propagated through the reference optical path. Namely, it corresponds to the case where the target is far. In this case, even when the light quantity adjusting means causes the light quantity of the reflected beam  110   a  to coincide with the light quantity of the reference pulse light  111 , the pulse signal caused by flare is detected by the received signal detector  6 . 
     FIGS. 8A to  8 D are timing charts showing timings for generating signals in the case where the distance between the apparatus and the target is long. As with FIG. 7A, FIG. 8A shows the received pulse signal S 105  before light quantity adjustment. Since the target is far, the reflected pulse signal from the target has a low signal level, which is about the same as that of the flare pulse signal S 85 . 
     FIG. 8B shows the received pulse signal S 105  after the light quantity adjustment. The reflected pulse signal S 86  in FIG. 8A is originally at a low level, which is close to the level of the received signal from the reference optical path including the prism  302  for attenuating the pulse light from the semiconductor laser  32  and the filter  313 . Hence, the received light quantity is regulated only slightly upon the light quantity adjustment. As a consequence, the magnitude of the flare pulse signal S 87  hardly changes, thus remaining at a level which is about the same as that of the reflected pulse signal S 88 . 
     FIG. 8D shows the receive timing signal S 106 . In this case, since the flare pulse signal S 87  has a magnitude which is about the same as that of the reflected pulse signal S 88 , the receive timing signal S 106  also generates a receive timing signal S 89  at the timing of the flare pulse signal S 87 . FIG. 8C shows an active state of the received signal detector  6 . 
     The arithmetic controller  1  effects time measurement according to the receive timing signal S 89  that is initially detected. As a result, the time measured for the measurement optical path substantially equals the time measured for the reference optical path, thereby the measured length value L computed on the basis of thus measured time becomes a very short length, e.g., less than 1 m. It is due to the fact that flare is generated in the objective lens or the like within the apparatus, whereas the optical path length within the apparatus is less than 1 m in general. This measured length of 1 m will be referred to as a reference value. 
     Hence, in this embodiment, in the case where the measured length value level L becomes less than the reference value of 1 m as a result of computing, or the case where the measured time is not greater than a value at which the measured length value L is 1 m, the arithmetic controller  1  delays for a predetermined period of time the timing for activating the received signal detector  6 . 
     In another embodiment, when the limit switch  330  detects that the target to be focused on by the focusing optical system is farther than a predetermined value, the arithmetic controller  1  delays for a predetermined period of time the timing for activating the received signal detector  6 . 
     FIGS. 9A to  9 D show timing charts when the timing for activating the received signal detector  6  is delayed by a predetermined term tl in the case where the distance between the apparatus and the target is long. Namely, it is an embodiment in which the receive timing signal generated by flare is ignored. 
     FIGS. 9A and 9B are similar to FIGS. 8A and 8B. A flare pulse signal S 97  exists in FIG. 9B without being attenuated. The active state signal of the received signal detector  6  in FIG. 9C is set to its high level “H” with a delay of the predetermined term tl. As a consequence, the receive timing signal S 106  would not be generated in response to the flare pulse signal S 97 . The receive timing signal S 106  is generated, as a pulse signal S 99 , only at the timing for generating a reflected pulse signal S 98  based on the reflected beam from the target. 
     In the following, distance measuring operations according to embodiments (distance measuring method according to the present invention) will be explained with reference to flowcharts. FIG. 10 shows a schematic flowchart in the case where whether to ignore signals caused by flare or not is judged on the basis of the measured length value obtained upon pre-measurement which is a preliminary distance measurement operation. 
     When measurement is started, the reference optical path is initially measured (step ST 210 ). As a consequence, the optical path length L ref  of the reference optical path is determined. L ref  is usually less than 1 m. 
     Then, the optical path is switched to a measurement optical path, thereby pre-measurement for preliminarily measuring the optical path length of the measurement optical path is effected (step ST 220 ). Then, according to whether the resulting measured length L is, for example, shorter than 1 m or not, it is judged whether to ignore flare or not (step ST 230 ). If the measured length value L determined by the pre-measurement is less than 1 m, then the signal caused by the flare is ignored (step ST 240 ). If the measured length value L is 1 m or longer, then the processing of step ST 240  is not effected. The signal caused by the flare is ignored by delaying the active state signal of the received signal detector  6  for tl as mentioned above. 
     After it is judged whether to ignore the signal caused by the flare or not, a main measurement operation for the measurement optical path is effected (step ST 250 ). As a consequence, in the main measurement operation, even when flare exists, the signal caused by the flare is ignored, thereby measurement is effected on the basis of the reflected pulse light  110   a  from the target. 
     FIG. 11 shows a schematic flowchart in the case where whether the target is far or near is judged by means of the limit switch  330 , and then distance measurement is effected. This case is substantially the same as that of FIG. 10 except that collimation which will be explained later is effected (step ST 270 ) prior to measurement of the reference optical path (step ST 280 ), thereby judging whether the target is located farther or nearer than a predetermined position. The collimation is an operation for adjusting the position of the focusing lens along the optical axis, thereby forming an image of the target onto the reticle  317 . The limit switch  330  is disposed near the focusing lens as shown in FIG. 5, and whether the target is far or near is judged on the basis of the position of the focusing lens (step ST 290 ). 
     In the case where the target is far, flare would not be attenuated upon light quantity adjustment. Therefore, the active state signal of the received signal detector  6  is delayed so that the signal caused by the flare can be ignored (step ST 300 ). When the target is near, since flare is attenuated upon light quantity adjustment, the signal caused by the flare is not ignored. As with the case of FIG. 10, the length is thereafter measured in the measurement optical path (step ST 310 ). 
     Thus, even in the case where flare is not attenuated, the received signal detector  6  is activated later than the timing at which the flare is received. Hence, the distance to the target can be computed without being influenced by flare. Namely, no error would occur in the measured length value due to flare. 
     FIG. 12 is a block diagram showing a configuration of the received signal detector  6  according to an embodiment of the present invention. FIGS. 13A to  13 M show timing charts for measurement in a reference optical path and a measurement optical path which is free from influences of flare. FIG. 14 is a flowchart for explaining measurement operations for pre-measurement in this embodiment. With reference to these drawings, measurement operations in this embodiment of the present invention free from influences of flare will be explained in more detail. 
     First, the reference optical path is selected. This operation corresponds to steps ST 211  to ST 213  in the measurement flowchart of FIG.  14 . In the reference optical path set at step ST 211 , the reference pulse light  111  does not travel through the objective lens  305 , thereby no flare occurs. When selecting the reference optical path, the CPU of the arithmetic controller  1  sets the pulse selecting signal  107  (FIG. 13A) to its high level “H” (step ST 212 ). Here, when the emission instructing signal S 102  (FIG. 13C) of the signal processor  2  becomes its high level “H,” the output signal S 601  (FIG. 13D) of the AND circuit  66  and, subsequently, the output signal S 603  (FIG. 13G) of the OR circuit  65  attain their high levels “H,” thus placing the comparator  62  into an operable state. FIGS. 13A to  13 G show timing charts for these operations. FIGS. 13B and 13F show signals S 107 A and S 602 A, respectively. 
     The output S 603  of the OR circuit  65  in FIG. 13G is the active state signal of the received signal detector  6  mentioned above. When the pulse selecting signal S 107  in FIG. 13A is at its high level “H,” the active state signal S 603  in FIG. 13G attains its high level “H” together with the emission instructing signal S 102  in FIG.  13 C. 
     In the measurement at step ST 213 , the pulse light  104  (reference pulse light  111 ) received after being propagated through the reference optical path is photoelectrically converted by the APD  42 , and the resulting signal is fed into the received signal amplifier  5 . The received pulse signal S 105  (FIG. 13H) outputted from the received signal amplifier  5  is fed to the non-inverting input terminal of the comparator  62 . The inverting input terminal of the comparator  62  has been set to a predetermined level beforehand. Its purpose is to prevent erroneous operations from occurring due to circuit noise and the like. 
     When the received pulse signal S 105  at the above-mentioned predetermined level or higher is inputted, the comparator  62  outputs a pulse signal S 604  (FIG.  13 I). At the rising edge of the pulse signal S 604 , the latch circuit  70  latches the pulse signal S 604 , and the output S 605  (FIG. 13J) of the latch circuit  70  allows a comparator  61 , which is a receive timing determining means, to operate. The received pulse signal S 105  (FIG. 13K) is fed to the non-inverting input terminal of the comparator  61 , whereas the signal S 105 A (FIG. 13L) obtained when a delay element  64  having a predetermined delay amount delays the received pulse signal S 105  is fed to the inverting input terminal. 
     As a consequence, the output of the comparator  61  becomes a pulse signal S 106  (FIG. 13H) which attains its high level “H” at the rising part of the received pulse signal S 105  and its low level “L” at the crossing point between the received pulse signal S 105  and the delayed signal S 105 A, and is transmitted to the signal processor  2 . The signal processor  2  detects the falling edge of the pulse signal S 106 . As a result, the measured value L ref  in the reference optical path is determined (step ST 213 ). FIGS. 13H to  13 M show the timings for the foregoing operations. 
     Also, the CPU of the arithmetic controller  1  detects the peak level of the received pulse signal S 105  by means of a peak detecting circuit  63  within the received signal detector  6 . It is necessary for controlling the light quantity adjustment filter driving motor  322  used for driving the light quantity adjustment filter  314 . 
     Then, the measurement optical path is selected. Here, while the pulse selecting signal S 107  is kept at its high level “H” the measurement optical path is set (step ST 221  in FIG.  14 ). Then, a light quantity adjustment operation is effected (step ST 222 ), and pre-measurement is performed (steps ST 223  and ST 224 ). 
     Case with Short Distance between Apparatus and Target 
     In the case where the distance between the apparatus and the target is short, the light quantity of the reflected beam  110   a  from the target is so large that the output of the peak detecting circuit  63  is at the level of the reflected beam  110   a . As a consequence, the light quantity adjustment filter  314  moves to the denser side upon light quantity adjustment, thereby flare is attenuated together with the signal light. Hence, there is no substantial influence of the flare. 
     Consequently, the result L of the measurement value calculation (step ST 224 ) concerning the pre-measurement (step ST 223 ) becomes the optical length of the measurement optical path, thereby the measured value L is 1 m or longer. Therefore, the active stage signal is not delayed for ignoring the signal caused by flare, and the flow shifts to main measurement (step ST 252 ). 
     Case with Long Distance between Apparatus and Target 
     In the case where the distance between the apparatus and the target is long, the reflected beam  110   a  from the target has a small light quantity, thereby the output of the peak detecting circuit  63  is at the level of the signal caused by flare. As a consequence, the light quantity adjustment filter  314  hardly moves upon light quantity adjustment, thereby time measurement is effected with the signal caused by flare. 
     When the length L calculated as a result of the pre-measurement is less than 1 m, the optical path length measured for the measurement optical path substantially equals the optical path length measured for the reference optical path, thereby the measurement is judged to be effected with flare (step ST 230 ). Then, the pulse selecting signal S 107  is set to its low level “L” (step ST 241 ), a light quantity adjustment operation is effected (step ST 251 ), and then the flow shifts to a main measurement operation (step ST 252 ). When the length L is 1 m or longer (step ST 230 ), the flow directly shifts to the measurement operation (step ST 252 ) as mentioned above. 
     In another embodiment of the present invention, whether to ignore the signal caused by flare or not is judged not according to the result of measurement in the pre-measurement but according to the detection signal from the limit switch  330  obtained in response to the focusing operation of a collimator. 
     FIG. 15 shows a flowchart in the case using the limit switch  330  in synchronization with a collimator. This flowchart is substantially the same as the measurement flowchart according to pre-measurement shown in FIG. 14 except that the limit switch  330  judges whether the target is located farther or nearer than a predetermined position in this embodiment. 
     Namely, in the collimating step (step ST 270 ) prior to the measurement concerning the reference optical path (steps ST 281  to ST 283 ), as the focusing lens  306  of the focusing optical system is moved to the longer distance side (toward the objective lens  305  side of the limit switch  330 ), the limit switch  330  detects that the target is far (step S 290 ), and sets the pulse selecting signal S 107  to its low level “L” (step ST 301 ). Then, light quantity adjustment is effected (step ST 311 ), and the flow proceeds to a measurement operation (step ST 312 ). When there is no signal from the limit switch  330 , the flow directly proceeds to the measurement operation, of course. 
     FIGS. 16A to  16 M show timing charts for main parts of the received signal detector  6  in the case where flare is detected in the measurement optical path. The case where the measured value L in the pre-measurement is less than 1 m will be explained in detail with reference to FIGS. 12 and 16A to  16 M. The CPU within the arithmetic controller  1  sets the pulse selecting signal S 107  (FIG. 16A) to its low level “L.” As a consequence, the output signal S 107 A (FIG. 16B) of the inverting circuit  68  fed with the pulse selecting signal S 107  attains its high level “H.” Since the input signal S 107  is at the low level “L,” the output signal S 601  (FIG. 16D) of the AND circuit  66  maintains its low level “L” regardless of the emission instructing signal S 102  (FIG.  16 C). FIGS. 16A to  16 D show these timing charts. 
     Also, since the signal S 107 A is at the high level “H,” the output signal S 602  (FIG. 16E) of the AND circuit  67  depends on the emission instructing signal S 102  that is the other input signal. Further, since the input signal S 601  is at the low level “L,” the input signal S 603  (FIG. 16G) of the OR circuit  65  depends on the other input signal S 602 A (FIG.  16 F). The signal S 602 A is a signal obtained when the signal S 602  is delayed by a predetermined amount tl. This delay amount is provided by the delay element  69 . This signal S 602 A passes through the OR circuit  65 , thereby becoming the active state signal S 603 . 
     Thus, in the case where the pulse selecting signal S 107  in FIG. 16A is at the low level “L,” even when the emission instructing signal S 102  becomes the high level “H,” the active state signal S 603  in FIG. 16G does not attain its high level “H” together therewith but after the delay term tl caused by the delay element  69 . As a consequence, the signal caused by the flare occurring during the delay term tl can be ignored. 
     The delay amount caused by the delay element  69  is set to a timing at which the signal S 602 A changes from the low level “L” to the high level “H” when the measured value L is several 10 m. FIGS. 16E to  16 G show the foregoing timing charts. 
     In each of the above-mentioned cases, depending on the configuration of the delay element  69 , the above-mentioned delay amount can be set by the CPU in a programmable fashion. As a consequence, in the case where the light reflected from an object existing within the distance corresponding to thus set delay amount is received as flare, its influence can be eliminated. 
     Hence, when the received pulse signal S 105 X caused by flare exists as shown in FIG. 16H, the activation of the received signal detector  6  is delayed by the delay element  69 , thereby this signal would not be detected as the receive timing signal S 106 . Then, after the delay term tl caused by the delay element  69 , the comparators  61 ,  62  are activated, thereby the receive timing signal S 106  for the received pulse signal S 105 Y caused by the reflected pulse light from the target is detected as shown in FIG.  16 M. Here, FIGS. 16I,  16 J,  16 K, and  16 L show the output signal S 604  of the comparator  62 , the output signal S 605  of the latch circuit  70 , the received pulse signal S 105  identical to that shown in FIG. 16H, and the output signal S 105 A of the delay element  34 , respectively. 
     Thus, the received pulse signal S 105 X caused by flare is not used for time measurement. As a consequence, the time measurement is not influenced by the flare. As explained in the foregoing, these embodiments can provide a highly accurate distance measuring apparatus without being influenced by flare pulse signals caused by flare. 
     The foregoing explanation exemplifies a distance measuring apparatus having a coaxial optical system which is likely to receive flare. Though a distance measuring apparatus having a biaxial optical system receives no flare generated by the apparatus itself, it may receive the reflected light as flare from objects other than the target. When the present invention is applied to a distance measuring apparatus having a biaxial optical system, influences of such flare can be eliminated. 
     Industrial Applicability 
     As explained in the foregoing, in accordance with the present invention, whether the received signal is caused by flare or not is judged according to a measured length value which is calculated. If the received signal is caused by flare, then activation is delayed such that the received signal detector is in an inactive state when receiving flare, so as to eliminate influences thereof, and measurement is effected again. Consequently, while a high accuracy in measurement is realized without being influenced by flare, the measurement range is enhanced, thus allowing the non-prism measuring ability to improve. 
     Also, in the case where it is detected, at the same time when the collimation optical system is set to the in-focus position, that the target is located farther than a predetermined position, and the timing for activating the received signal detector is delayed, a distance measurement operation without error can be effected rapidly in a simple configuration. 
     From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.