Abstract:
A distance measuring method for performing distance measurement by projecting a distance measuring light to an object to be measured and by receiving a reflected light, comprising: a step of projecting for scanning the distance measuring light which has at least one luminous flux with a predetermined spreading angle; a step of emitting the light by pulsed light emission at least two times during a period when the luminous flux traverses the object to be measured; a step of measuring a distance by receiving the reflected light at least two times; and a step of averaging the results of the distance measurement.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a distance measuring method and a distance measuring device for measuring a distance by projecting a laser beam as a distance measuring light to an object to be measured and by receiving a reflection light from the object to be measured. In particular, the present invention relates to a distance measuring method and a distance measuring device for projecting the distance measuring light for scanning. 
   When a distance to an object to be measured is measured by projecting a laser beam as a distance measuring light toward the object to be measured and by using a reflected light from the object to be measured, two methods are known as a distance measuring method: a collimation type electro-optical distance measuring method for performing distance measurement to the object to be measured on 1:1 basis, and a scanning type electro-optical distance measuring method for performing distance measurement by receiving a reflection light from the object to be measured while projecting a distance measuring light for scanning. 
   When a laser beam is emitted from a light source, there is intensity distribution normally within a light emission plane due to interference. As shown in  FIG. 10 , in a cross-section  3 ′ of a distance measuring luminous flux of the laser beam, i.e. a distance measuring light  3 , speckle patterns  4  occur due to the intensity distribution. Therefore, the projected distance measuring light  3  also contains speckle patterns  4  caused by the intensity distribution. For this reason, when a distance is measured according to a reflected light from the center of the luminous flux of the laser beam projected to the object to be measured or when a distance is measured according to a reflected light from the edge region of the luminous flux, difference occurs in the measured distance due to the speckle patterns. 
   In the collimation type electro-optical distance measuring method as described above, measurement is made by collimating the central portion of the luminous flux of the distance measuring light. As a result, it is a measurement in a fixed condition, and the measurement can be repeatedly performed. Therefore, the values of the measured distance are averaged, and this makes it possible to reduce fluctuation of the values of measured distance caused by the speckle patterns. 
   As disclosed in the Japanese Patent Publication No. 2580148 or in JP-A-2000-162517, the influence of the speckle pattern is eliminated by homogenizing phase unevenness and light intensity unevenness of the distance measuring luminous flux. 
   On the other hand, according to the scanning type electro-optical distance measuring method, the distance measuring light  3  moves with respect to the object  2  to be measured as shown in  FIG. 11 , and the center of the object  2  to be measured is not always at the center of the cross-section  3 ′ of the distance measuring luminous flux. The object  2  to be measured may be in the edge region of the distance measuring luminous flux cross-section  3 ′. As a result, when the distance is measured, the results of the measurement may include the result of distance measurement at the center of the distance measuring luminous flux and the result of distance measurement at the edge region of the distance measuring luminous flux.  FIG. 11  shows a prism for distance measurement installed on a pole  1  as an object  2  to be measured, and the distance measuring light  3  is projected to scan within a scanning surface  5 . 
   When a distance is measured in the edge region of the distance measuring luminous flux cross-section  3 ′, a weighted position varies when the distance measuring light is detected according to the light amount in the edge region and due to the influence of speckle patterns. As a result, deviation occurs in the timing of detection. 
   An electro-optical distance measuring device measures a distance according to a phase difference between a reflected distance measuring light  6  and an internal reference light or according to the time obtained from deviation in time. If there is phase deviation in the reflected distance measuring light  6 , serious error may occur in the value of the measured distance. 
   Because the distance measuring light  3  is continuously projected for scanning, measurement cannot be repeatedly performed under condition that the object  2  to be measured is collimated, and the influence of the speckle pattern cannot be excluded. Therefore, when distance is measured by the scanning type electro-optical distance measuring method, measurement is made for one time based on partial reflection from the object to be measured of the single distance measuring luminous flux, and the measured distance value is under the influence of the speckle pattern of the distance measuring luminous flux. 
   Further, if the distance measuring light used for distance measurement does not have a certain predetermined light intensity, S/N ratio of the reflected light from the object to be measured is decreased, and this results in the problem of lower measurement accuracy. In case the measurement can be performed repeatedly, the measured values of the repeated measurement can be averaged, and this increases the measurement accuracy. However, in the scanning type electro-optical distance measurement with a single measuring operation, it is necessary to increase light intensity (light amount) of the distance measuring light. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a scanning type electro-optical distance measuring method and a scanning type electro-optical distance measuring device, by which it is possible to average the influence of speckle pattern on a luminous flux cross-section of a distance measuring light and to increase measurement accuracy without increasing the light intensity. 
   To attain the above object, the present invention provides a distance measuring method for performing distance measurement by projecting a distance measuring light to an object to be measured and by receiving the reflected light, comprising a step of projecting for scanning the distance measuring light which has at least one luminous flux with a predetermined spreading angle, a step of emitting the light by pulsed light emission at least two times during a period when the luminous flux traverses the object to be measured, a step of measuring a distance by receiving a reflected light at least two times, and a step of averaging the results of the distance measurement. Also, the present invention provides a distance measuring method for performing distance measurement by projecting a distance measuring light to an object to be measured and by receiving a reflected light, comprising a step of projecting for scanning the distance measuring light which has two or more luminous fluxes with a predetermined spreading angle overlapped on each other in a direction perpendicular to a rotating direction, a step of emitting the light by pulsed light emission at least two times during a period when the luminous flux traverses the object to be measured, a step of measuring a distance by receiving the reflected light at least two times, and a step of averaging the results of the distance measurement. Further, the present invention provides the distance measuring method as described above, wherein the number of pulsed light emissions is variable, and the number of light emissions increases with the increase of the distance to the object to be measured. Also, the present invention provides the distance measuring method as described above, wherein the pulsed light emission has a light emission frequency f and the scanning speed is a rotational angular speed Ω, and the values of the light emission frequency f and the rotational angular speed Ω are set up so that the reflected light from the object to be measured is received at least two times. Further, the present invention provides the distance measuring method as described above, wherein the distance measuring light is initially projected for scanning with an initial light emission frequency f0 and an initial rotational angular speed Ω0 and the reflected distance measuring light from the object to be measured is received, scanning information including at least the number of light receiving is acquired, and the light emission frequency f and the rotational angular speed Ω are set up based on scanning information. Also, the present invention provides the distance measuring method as described above, wherein the distance measuring light is projected by partial emission within a range of distance measuring light emission including the object to be measured and within a range of dummy emission before the range of light emission of the distance measuring light. 
   Further, the present invention provides a distance measuring device, which comprises a light emitting means for emitting a distance measuring light by pulsed light emission at a predetermined light emission frequency f, an optical system for projecting the distance measuring light with at least one luminous flux having a spreading angle φ, a photodetection means for receiving a reflected distance measuring light from an object to be measured, a scanning means for projecting the distance measuring light for scanning at a rotational angular speed Ω, and an arithmetic operation control unit for setting up the light emission frequency f, the spreading angle φ, and the rotational angular speed Ω so that the light is emitted by pulsed light emission for two or more times during a period when the luminous flux of the distance measuring light traverses the object to be measured, and so that the photodetection means receives the reflected distance measuring light from the object to be measured two or more times, and for calculating a distance to the object to be measured based on the result of photodetection. Also, the present invention provides the distance measuring device as described above, wherein the arithmetic operation control unit controls the light emission frequency f of the light emitting means and the rotational angular speed Ω of the scanning means so that the expression fφ/Ω≧2 is satisfied. Further, the present invention provides the distance measuring device as described above, wherein the arithmetic operation control unit controls the light emission frequency f according to the amount of photodetection of the reflected distance measuring light received by the photodetection means. Also, the present invention provides the distance measuring device as described above, wherein the arithmetic operation control unit controls the rotational angular speed Ω according to the amount of photodetection of the reflected distance measuring light received by the photodetection means. Further, the present invention provides the distance measuring device as described above, wherein the arithmetic operation control unit judges measurement accuracy of the measured distance according to the number of photodetection of the reflected distance measuring light received by the photodetection means. Also, the present invention provides the distance measuring device as described above, wherein the optical system comprises a luminous flux splitting optical member, and the projected distance measuring light is an aggregate of split luminous fluxes. Further, the present invention provides the distance measuring device as described above, wherein the arithmetic operation control unit controls the light emitting means so that light is emitted within a range of light emission of the distance measuring light including the object to be measured and within a dummy light emission range before the range of light emission of the distance measuring light. 
   According to the present invention, a distance measuring method for performing distance measurement by projecting a distance measuring light to an object to be measured and by receiving the reflected light comprises a step of projecting for scanning the distance measuring light which has at least one luminous flux with a predetermined spreading angle, a step of emitting the light by pulsed light emission at least two times during a period when the luminous flux traverses the object to be measured, a step of measuring a distance by receiving the reflected light at least two times, and a step of averaging the results of the distance measurement. Also, a distance measuring method for performing distance measurement by projecting a distance measuring light to an object to be measured and by receiving the reflected light comprises a step of, projecting for scanning the distance measuring light which has two or more luminous fluxes with a predetermined spreading angle overlapped on each other in a direction perpendicular to a rotating direction, a step of emitting the light by pulsed light emission at least two times during a period when the luminous flux traverses the object to be measured, a step of measuring a distance by receiving the reflected light at least two times, and a step of averaging the results of the distance measurement. As a result, the measurement can be performed repeatedly when the distance is measured in scanning mode, and this makes it possible to increase measurement accuracy. 
   Also, according to the present invention, the distance measuring light is initially projected for scanning with an initial light emission frequency f0 and an initial rotational angular speed Ω0 and the reflected distance measuring light from the object to be measured is received, scanning information including at least the number of light receiving is acquired, and the light emission frequency f and the rotational angular speed Ω are set up based on the scanning information. Thus, adequate values of the light emission frequency f and the rotational angular speed Ω can be set up promptly, and distance measurement can be achieved with high efficiency. 
   Further, according to the present invention, the distance measuring light is projected by partial emission within a range of distance measuring light emission including the object to be measured and within a range of dummy emission before the range of light emission of the distance measuring light. As a result, power consumption can be decreased. 
   Also, according to the present invention, the distance measuring device comprises a light emitting means for emitting a distance measuring light by pulsed light emission at a predetermined light emission frequency f, an optical system for projecting the distance measuring light with at least one luminous flux having a spreading angle φ, a photodetection means for receiving a reflected distance measuring light from an object to be measured, a scanning means for projecting the distance measuring light for scanning at a rotational angular speed Ω, and an arithmetic operation control unit for setting up the light emission frequency f, the spreading angle φ, and the rotational angular speed Ω so that the light is emitted by pulsed light emission for two or more times during a period when the luminous flux of the distance measuring light traverses the object to be measured, and so that the photodetection means receives the reflected distance measuring light from the object to be measured two or more times, and for calculating a distance to the object to be measured based on the result of photodetection. As a result, measurement can be performed repeatedly when the distance is measured in scanning mode, and this makes it possible to have higher measurement accuracy. 
   Further, according to the present invention, the arithmetic operation control unit controls the light emission frequency f according to the amount of photodetection of the reflected distance measuring light received by the photodetection means. Also, the arithmetic operation control unit controls the rotational angular speed Ω according to the amount of photodetection of the reflected distance measuring light received by the photodetection means. Thus, it is possible to have the measurement accuracy as desired without increasing light intensity of the distance measuring light. 
   Also, according to the present invention, the arithmetic operation control unit judges measurement accuracy of the measured distance according to the number of photodetection of the reflected distance measuring light received by the photodetection means. Thus, the accuracy of the result of measurement can be judged adequately and, it is possible to have measurement accuracy to match the measuring condition as necessary. 
   Further, according to the present invention, the optical system comprises a luminous flux splitting optical member, and the projected distance measuring light is an aggregate of split luminous fluxes. As a result, the problem of speckle patterns of the light emitting means can be eliminated, and this contributes to the increase of the measurement accuracy. 
   Also, according to the present invention, the arithmetic operation control unit controls the light emitting means so that light is emitted within a range of light emission of the distance measuring light including the object to be measured and within a dummy light emission range before the range of light emission of the distance measuring light. Thus, power consumption can be reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematical drawing to show general features of an embodiment of the present invention; 
       FIG. 2  is a schematical block diagram to show the embodiment of the present invention; 
       FIG. 3  is a schematical drawing of a distance measuring light projecting unit in the embodiment of the present invention; 
       FIG. 4  is a cross-sectional view of a rotary irradiation unit in the embodiment of the present invention; 
       FIG. 5  is a drawing to explain a measuring condition of the embodiment of the present invention; 
       FIG. 6  is a drawing to explain a relation between a distance measuring light and an object to be measured in the embodiment of the present invention; 
       FIG. 7  is a drawing to explain a relation between the distance measuring light and the object to be measured in the embodiment of the present invention; 
       FIG. 8  is a drawing to explain a relation between split luminous fluxes and the object to be measured in the embodiment of the present invention; 
       FIG. 9  is a drawing to explain a relation between the split luminous fluxes and the object to be measured in the embodiment of the present invention; 
       FIG. 10  is a drawing to explain a relation between a distance measuring light and the object to be measured in a conventional example; and 
       FIG. 11  is a drawing to explain a relation between the distance measuring light and the object to be measured in a conventional example. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Description will be given below on the best mode for carrying out the present invention referring to the attached drawings. 
   First, referring to  FIG. 1  to  FIG. 4 , description will be given on general features of a scanning type distance measuring device, in which the present invention is carried out. 
   In an embodiment of the present invention, an object  2  to be measured is a prism for distance measurement mounted on a pole  1 , which is held by an operator. A photodetection sensor device  8  for receiving a laser beam  9  for distance measurement reference plane (to be described later) is mounted on the pole  1 . 
   A distance measuring device  11  as shown in  FIG. 1  comprises a distance measuring unit (to be described later) and a distance measurement reference plane setting unit (not shown). The distance measuring device  11  projects the laser beam  9  for distance measurement reference plane by rotary irradiation and can project a distance measuring light  13  by rotary irradiation. A horizontal reference plane  12  can be set up by the laser beam  9  for distance measurement reference plane, and distances to the objects  2  to be measured at a plurality of points can be measured by the distance measuring light  13 . The distance measuring device  11  measures a distance by receiving a reflection light from the object  2  to be measured of the distance measuring light  13  projected for scanning. Thus, it is necessary that the object  2  to be measured is within a projection range of the distance measuring light  13 . The distance measuring light  13  has a spreading as required in upper and lower directions. 
   The distance measurement reference plane setting unit forms the horizontal reference plane  12  by projecting the laser beam  9  for distance measurement reference plane, which comprises three or more fan-shaped laser beams  9   a ,  9   b  and  9   c , of which at least one is tilted. As a laser device for projecting three or more fan-shaped laser beams  9   a ,  9   b  and  9   c , of which one is tilted, by rotary irradiation, a rotary laser device is suggested in JP-A-2004-212058. 
   The laser beam  9  for distance measurement reference plane comprises a plurality of fan-shaped laser beams  9   a ,  9   b  and  9   c , of which at least one is tilted, and the laser beam  9  is projected by rotary irradiation. The object  2  to be measured comprises the photodetection sensor device  8 . When the photodetection sensor device  8  receives two or more fan-shaped laser beams, a time difference between the fan-shaped laser beams is obtained. As a result, based on the time difference and a tilt angle of the fan-shaped laser beam, depression and elevation angles with respect to the horizontal reference plane  12  having the distance measuring device  11  at the center are obtained. Also, based on the depression or elevation angle, the distance measurement reference plane can be set up. 
     FIG. 2  shows general features of the distance measuring device  11  of the embodiment of the present invention. 
   A light emitting element  15  is a laser diode, for instance. The light emitting element  15  is driven by a light emission driving circuit  16  for distance measurement, and a laser beam is emitted by pulsed light emission at a predetermined light emission frequency f. The emitted laser beam enters an incident end surface of an optical fiber  18  for projection via a relay lens  17 . The laser beam projected from the optical fiber  18  for projection is collimated by a condenser lens  19 . The laser beam is then deflected by a mirror  21  and is projected on a distance measuring light optical axis  22  as the distance measuring light  13 . 
   On the distance measuring light optical axis  22 , there are provided a luminous flux splitting optical member and a first deflection optical member. The luminous flux splitting optical member is a diffraction grating  23  or an array lens which is an aggregate of small convex lenses. The first deflection optical member is a deflection mirror  24  or a prism. 
   By the diffraction grating  23 , the distance measuring light  13  is split to a certain required number of luminous fluxes aligned in upper and lower directions (In  FIG. 2 , the direction is shown in a direction perpendicular to the distance measuring light optical axis  22 .). The splitting condition is shown in  FIG. 3 . The distance measuring light  13  is turned to an aggregate of split luminous fluxes  13   a  diffracted by the diffraction grating  23 . Each of the split luminous fluxes  13   a  has a spreading angle φ, and the adjacent split luminous fluxes  13   a  are overlapped on each other as required. In  FIG. 3 , radius portions of the split luminous fluxes  13   a  are overlapped on each other. The distance measuring light  13  which has been turned to an aggregate of the split luminous fluxes  13   a  has a spreading S as required in upper and lower directions. The spreading S determines the range of projection of the distance measuring light  13  as described above. For instance, in case the measured distance is 28 meters, the spreading S is set in such a manner that the light is projected within a range of 5 meters in upper and lower directions around the distance measuring light optical axis  22 . After splitting, the distance measuring light  13  is deflected by the deflection mirror  24  so that the distance measuring light  13  is directed toward the object  2  to be measured. 
   A reflected distance measuring light  13 ′ reflected by the object  2  to be measured runs along the distance measuring light optical axis  22  and enters the deflection mirror  24 , and is deflected by the deflection mirror  24 . The reflected distance measuring light  13 ′ thus deflected enters an optical fiber  26  for photodetection via a condenser lens  25 . The reflected distance measuring light  13 ′ is received by a photodetection element  28  such as a photo-diode via a relay lens  27 . A photodetection signal outputted from the photodetection element  28  is processed as required by a photodetection circuit  29  and the signal is inputted to an arithmetic operation control unit  30  as described later. 
   Although not shown in the figure, a part of the laser beam from the light emitting element  15  is split, and an internal reference light optical path is formed so that the part of the laser beam enters the photodetection element  28 . An internal reference light after passing though the internal reference light optical path and the reflected distance measuring light  13 ′ are mechanically or electrically changed over (or separated from each other), and are received by the photodetection element  28 . Through comparison of the internal reference light with the reflected distance measuring light  13 ′, a distance to the object  2  to be measured is calculated and measured. 
   The light emitting element  15 , the light emission driving circuit for distance measurement  16 , the optical fiber for projection  18 , the optical fiber for photodetection  26 , the photodetection element  28 , the photodetection circuit  29 , and the internal reference light optical path (not shown) make up together a distance measuring unit  20 . 
   The laser beam  9  for distance measurement reference plane is emitted from a light emitting element  31  such as a laser diode. After passing through an optical member such as a diffraction grating  32 , the laser beam  9  for distance measurement reference plane is split to form a plurality of fan-shaped beams, and at least one of the fan-shaped beams is tilted with respect to a horizontal plane. The fan-shaped beams are deflected in a direction parallel to the distance measuring light optical axis  22  by a second deflection optical member, e.g. a pentagonal prism  33 , and the fan-shaped beams are projected as the laser beam  9  for distance measurement reference plane. The laser beam  9  for distance measurement reference plane is received by the photodetection sensor device  8 . The light emitting element  31  is driven by a light emission driving circuit for reference  34 . The driving conditions of the light emission driving circuit for reference  34  and the light emission driving circuit for distance measurement  16  are controlled respectively by the arithmetic operation control unit  30 . 
   A storage unit  55  is connected with the arithmetic operation control unit  30 . In the storage unit  55 , there are stored a sequence program for carrying out the measurement of the distance measuring device  11 , a calculation program for calculating the measured distance and the like, and a program for judging accuracy of a measurement result to match the measurement condition. Further, the results of calculation by the arithmetic operation control unit  30  are stored in the storage unit  55 . 
   The deflection mirror  24  and the pentagonal prism  33  are held so that these can be integrally rotated by driving means as required, e.g. by a motor (to be described later). The deflection mirror  24 , the pentagonal prism  33 , and the driving means make up together a rotary projecting unit  36 . 
     FIG. 4  shows general features of the rotary projecting unit  36  of the distance measuring device  11 . In  FIG. 4 , the same component as shown in  FIG. 1  is referred by the same symbol. 
   A projection window  38  in a cylindrical shape is mounted on a lower plate  37 , and the projection window  38  is made of transparent glass, etc. An upper plate  39  is mounted on an upper end of the projection window  38 , and an intermediate plate  41  is arranged inside the projection window  38 . 
   A prism holder  43  in a cylindrical shape is installed on the lower plate  37  and on the intermediate plate  41  via bearings  42 . The prism holder  43  is rotatably mounted around an optical axis (see  FIG. 2 ) of the condenser lens  25 . Inside the prism holder  43 , there is provided the pentagonal prism  33  as a deflection optical member, and a projection window  45  for reference laser beam is arranged on a portion facing to the pentagonal prism  33  of a cylindrical surface of the prism holder  43 . 
   The pentagonal prism  33  deflects the laser beam  9  for distance measurement reference plane after passing through the diffraction grating  32  and projects the laser beam  9  for distance measurement reference plane in a direction parallel to the distance measuring light optical axis  22 . 
   Above the prism holder  43 , a mirror holder  44  is provided integrally and concentrically with the prism holder  43 . Inside the mirror holder  44 , the deflection mirror  24  is held. On a portion facing to the deflection mirror  24  of a cylindrical surface of the mirror holder  44 , a projection window  50  for distance measurement is provided. 
   A lens holder  46  is installed on the upper plate  39 , and the condenser lens  25  is held by the lens holder  46 . As described above, the optical axis of the condenser lens  25  is aligned with the rotation center of the mirror holder  44 . Along the optical axis of the condenser lens  25 , there are arranged the mirror  21  which is smaller than the diameter of the condenser lens  25  and an incident end surface of the optical fiber  26  for photodetection. On the optical axis deflected by the mirror  21 , an exit end surface of the optical fiber  18  for exit is provided. 
   On an upper end of the prism holder  43 , a scanning gear  47  is fixed. On the intermediate plate  41 , there are provided a scanning motor  48  and an encoder  49 . A driving gear  51  is arranged on an output shaft of the scanning motor  48 , and a driven gear  52  is engaged on an input shaft of the encoder  49  respectively. The driving gear  51  and the driven gear  52  are engaged with the scanning gear  47 . 
   When the scanning motor  48  is driven, the prism holder  43  and the mirror holder  44  are rotated. The laser beam  9  for distance measurement reference plane projected from the pentagonal prism  33  is projected by rotary irradiation and sets up a distance measurement reference plane. The distance measuring light  13  from the deflection mirror  24  is projected by rotary irradiation for the purpose of distance measurement. 
   The encoder  49  detects rotation angles of the pentagonal prism  33  and the deflection mirror  24 . A detection signal from the encoder  49  is processed by a signal processing circuit  54  as shown in  FIG. 2 . After the required signal processing such as amplification, A/D conversion, etc., the signal is inputted to the arithmetic operation control unit  30 . Based on the detection results of the encoder  49 , the arithmetic operation control unit  30  detects a projecting direction of the laser beam  9  for distance measurement reference plane and the distance measuring light  13 . 
   The scanning motor  48  is driven by a motor driving unit  53 . The arithmetic operation control unit  30  controls the scanning motor  48  to a predetermined speed via the motor driving unit  53 . For instance, based on the results of detection by the encoder  49 , rotational angular speed of the prism holder  43  by the scanning motor  48  is calculated by the arithmetic operation control unit  30 . Based on the results of calculation, rotation speed of the scanning motor  48  is controlled so that rotational angular speed of the prism holder  43 , i.e. the scanning speed of the distance measuring light  13 , is turned to the rotational angular speed Ω. 
   When the reflected distance measuring light  13 ′ reflected by the object  2  to be measured is received by the photodetection element  28  via the optical fiber  26  for photodetection, the angle is detected by the encoder  49 , and the detected angle is stored in the storage unit  55  via the arithmetic operation control unit  30 . The spreading angle φ, the light emission frequency f, and the rotational angular speed Ω are stored in the storage unit  55 . 
   The measurement by the distance measuring device  11  is carried out under the condition that the laser beam  9  for distance measurement reference plane and the distance measuring light  13  are projected, and that the prism holder  43  and the mirror holder  44  are rotated at a constant speed by the scanning motor  48 . 
   The distance measuring light  13  projected from the optical fiber  18  for projection is continuously projected by rotary irradiation. When the light is projected to the objects  2  to be measured at a point as required, the reflected distance measuring light from the object  2  to be measured enters the deflection mirror  24  and enters the optical fiber  26  for photodetection via the condenser lens  25 . The lights are received by the photodetection element  28  via the optical fiber  26  for photodetection and a distance to the object  2  to be measured is measured. Based on a signal from the encoder  49  when the light is received by the photodetection element  28 , a projecting direction of the distance measuring light  13  is detected. The distance measurement result is matched with the projecting direction, and the results are stored in the storage unit  55 . Because the projecting direction is detected, the object  2  to be measured which has been measured is identified at the same time. 
   When the laser beam  9  for distance measurement reference plane is detected by the photodetection sensor device  8 , the depression or elevation angle of the object  2  to be measured with respect to the distance measuring device  11  is detected. From the measured distance of the object  2  to be measured and the depression or elevation angle, a height of the object  2  to be measured with respect to the horizontal reference plane is determined. 
   Now, referring to  FIG. 5  to  FIG. 7 , description will be given below on operation of the distance measurement of the present invention. To simplify the explanation, the luminous flux in  FIG. 6  and  FIG. 7  shows only one of the split luminous fluxes  13   a.    
   The distance measuring light  13  which is projected from the optical fiber  18  for projection, is split to a plurality of split luminous fluxes  13   a  by the diffraction granting  23 , and the split luminous fluxes  13   a  are projected along the distance measuring light optical axis  22 . Therefore, the distance measuring light  13  projected from the distance measuring device  11  is averaged by overlapping of the plurality of split luminous fluxes  13   a  and the distance measuring light  13  is turned to the luminous fluxes, which receive relatively lower influence from the speckle patterns. In the case of the present invention, if the distance measuring light  13  has spreading as required in upper and lower directions and has sufficiently strong light intensity, the diffraction grating  23  may not be used. Because of the overlapping of the plurality of split luminous fluxes  13   a , even when there is the influence from the speckle patterns on the distance measuring light  13 , the speckle patterns within the cross-sections of the luminous fluxes of the distance measuring light can be averaged (equalized) and the measurement accuracy can be improved without increasing the light intensity. 
   As described above, when the measuring light  13  is projected by rotary irradiation under the condition that the light emission frequency of the distance measuring light  13  is f, the rotational angular speed of the distance measuring light  13  is Ω, and the spreading angle of the split luminous fluxes  13   a  is φ, the relation between the object  2  to be measured and the split luminous flux  13   a  is as shown in  FIG. 6 .  FIG. 6  shows relative moving relation between the object  2  to be measured and the split luminous flux  13   a . A condition is shown where the split luminous flux  13   a  is turned on and off at the same position and the object  2  to be measured is moving at the rotational angular speed of Ω. 
   At the position where the light is projected to the object  2  to be measured, a luminous flux of the split luminous flux  13   a  has a spreading cross-section with a diameter D, and the object  2  to be measured has an area with a diameter d. 
   The distance measuring light  13  is projected by pulsed light emission at the light emission frequency of f and is rotated at the rotational angular speed of Ω, and the amount of moving (rotation angle) between pulses is Ω/f (&lt;φ). Therefore, when the distance measuring light  13  traverses the object  2  to be measured, this means that the reflected distance measuring light  13 ′ enters the distance measuring device  11  by “n” times, where φ/(Ω/f)=fφ/Ω=n. A portion corresponding to a diameter “d” of the object  2  to be measured is a luminous flux of the reflected distance measuring light  13 ′. In this way, the distance is measured at many points in a lateral direction. As a result, the measured distance values are averaged, and this contributes to the increase of the accuracy of distance measurement. 
     FIG. 7  shows a case where the distance measuring light  13  projected by pulsed light emission traverses the object  2  to be measured, which is at standstill state. This shows the condition where the reflected distance measuring light  13 ′ can be received by “n” times. 
   The measurement by the distance measuring device  11  is repeated by “n” times. By repeating the measurement, the measured distance values are averaged, and the distance measurement accuracy can be increased. Further, as shown in  FIG. 6 , the object  2  to be measured reflects the luminous fluxes at a portion with the speckle pattern and at a portion without the speckle pattern, and the influence of the speckle pattern is averaged as the result of the measurement repeated by “n” times. This makes it possible to obtain the measurement results with high accuracy. 
   The number of repeating “n” is correlated with the measurement accuracy. If the relation between the number of repeating “n” and the measurement accuracy is obtained in advance and is stored in the storage unit  55 , the measurement accuracy when measurement is repeated by the number of “n” times can be determined. Or, if the measurement accuracy is set up to match the conditions of distance measurement, the number of repeating “n” to attain the preset measurement accuracy can be obtained. 
   The relation between the diameter D of the split luminous flux  13   a  and the diameter d of the object  2  to be measured is determined by the spreading angle φ of the split luminous flux  13   a  and a distance to be measured L. When the value of the spreading angle φ and the value of the distance to be measured L are increased, D&gt;&gt;d. The light intensity of the reflected distance measuring light  13 ′ (i.e. the light amount of the reflected distance measuring light  13 ′ received by the photodetection element  28 ) is decreased. Because the reflected distance measuring light  13 ′ is limited to a small portion on the cross-section of the split luminous flux  13   a , the influence of the speckle pattern is increased (See  FIG. 6 ). Therefore, when the spreading angle φ and the distance to be measured L are large and when D&gt;&gt;d, the value of Ω/f should be made smaller. That is, the light emission frequency f should be increased and the rotational angular speed Ω should be reduced. The light emission from the light emitting element  15  should be controlled and the rotation speed of the scanning motor  48  should be controlled so that the number of repeating “n” is increased. By increasing the value of the number of repeating “n”, it is possible to compensate the decrease of the photodetection amount each time. Also, when the measurement is repeated by “n” times, the reflected distance measuring lights  13 ′ are a plurality of portions on the cross-section of the split luminous fluxes  13   a . Some of the reflected distance measuring light  13 ′ contain the speckle patterns  4  while some others do not. As a result, the speckle patterns are averaged. 
   When the distance to be measured L is decreased and the value of d/D increases, the light intensity of the reflected distance measuring light  13 ′ increases, and a larger area of the luminous flux is used at a same time as the reflected distance measuring light  13 ′. Therefore, the influence of the speckle pattern is decreased. Thus, the number of repeating “n” can be smaller. In order to have an average effect of the results of the measurement, the light emission frequency f and the rotational angular speed Ω are preferably set to the values so that the equation n≧2 is satisfied. 
   By adequately controlling the values of the light emission frequency f and the rotational angular speed Ω to match the spreading angle φ and the distance to be measured L, it is possible to perform distance measurement at high accuracy without changing the light emission intensity of the light emitting element  15 . 
   The values of the light emission frequency f and the rotational angular speed Ω may be set to fixed values to cope with the most extreme measurement condition. Or, the values of the light emission frequency f and the rotational angular speed Ω may be changed to match the distance to be measured. 
   Now, description will be given on operation in case where the values of the light emission frequency f and the rotational angular speed Ω are changed according to the distance to be measured. 
   In the storage unit  55 , there are set up and inputted initial operating conditions at the start of the measurement, e.g. the light emission frequency f0 and the rotational angular speed Ω0. It is supposed that the spreading angle φ is a fixed value determined by an optical system of the distance measuring device  11  and the light emission intensity of the light emitting element  15  is a constant value. 
   When the measurement is started by the distance measuring device  11 , the measurement is started under the initial conditions at first. That is, the measurement is made with the light emission frequency f0 of the distance measuring light  13  and with the rotational angular speed Ω0 of the distance measuring light  13 . 
   When the distance measuring light  13  scans over the object  2  to be measured, the number of light receiving (photodetection) by the photodetection element  28  of the reflected distance measuring light  13 ′ and the photodetection intensity of the reflection distance measuring light  13 ′ are detected. 
   The arithmetic operation control unit  30  determines an adequate value of the number of measurement repeating “n” according to the photodetection intensity and changes the light emission frequency f or the rotational angular speed Ω so that the number of light receiving is turned to n. For instance, the number of measurement repeating “n” can be increased by increasing the light emission frequency f. Or, the number of measurement repeating “n” can be increased when the rotational angular speed Ω is decreased without changing the light emission frequency f. When the distance to the object  2  to be measured is increased, peripheral speed is increased. In this case, too, the light emission frequency f is increased so that the number of light emission is increased and the number of light receiving (photodetection) is turned to “n”. 
   When adequate values of the light emission frequency f and the rotational angular speed Ω are calculated, based on the results of the calculation, the arithmetic operation control unit  30  controls the light emitting conditions of the light emitting element  15  via the light emission driving circuit for distance measurement  16  and controls the scanning motor  48  via the motor driving unit  53  so that a predetermined rotational speed is attained. 
   The measuring operation is carried out under the condition that the light emitting element  15  and the scanning motor  48  are controlled. 
   In the above, description has been given on a single split luminous flux  13   a . Now, referring to  FIG. 8 , description will be given on the distance measuring light  13 , which is an aggregate of the split luminous fluxes  13   a . In  FIG. 8 , the same component as in  FIG. 6  is referred by the same symbol. 
   The distance measuring light  13  is an aggregate of the split luminous fluxes  13   a  aligned in an up-to-bottom direction. The split luminous fluxes  13   a  are overlapped on each other by the portions as necessary. 
   When the object  2  to be measured is within a spreading S of the distance measuring light  13 , at least one of the split luminous fluxes  13   a  traverses the object  2  to be measured. By the object  2  to be measured, the split luminous flux  13   a  is reflected as the reflected distance measuring light  13 ′. The reflected distance measuring light  13 ′ enters the optical fiber  26  for photodetection via the condenser lens  25  and the reflected distance measuring light  13 ′ is received by the photodetection element  28 . 
     FIG. 8  shows a case where the light emitting element  15  is turned on and off by 8 times when the object  2  to be measured traverses diameter portions of the split luminous fluxes  13   a.    
   In the embodiment as given above, the distance measuring light  13  is divided in upper and lower directions by a luminous flux splitting optical member such as the diffraction grating  23 , an array lens, etc. There may be provided a scanning means for reciprocally rotating the mirror  21  within a predetermined range, and reciprocal scanning may be performed so that the spreading of the distance measuring light  13  will be S. 
     FIG. 9  shows a case where the distance measuring light  13  comprises the split luminous fluxes  13   a  overlapped on each other two times or three times or more and the measurement is made as the distance measuring light  13  traverses the object  2  to be measured.  FIG. 9  (A) shows distance measurement in condition that the split luminous fluxes  13   a  are overlapped two times or three times or more.  FIG. 9  (B) shows separately the distance measuring condition of each of the overlapped split luminous fluxes  13   a .  FIG. 9  (C) shows the distance measuring conditions of each of the split luminous fluxes  13   a  by completely overlapping on each other. A plurality of reflection lights are obtained in upper and lower directions of the overlapped distance measuring luminous fluxes. As a result, the measured distance is an average of the values in upper and lower directions. 
   As the light emitting condition of the distance measuring light when the distance is measured, the light may be projected over total circumference or the light may be partially projected to cover only a portion within a predetermined angle including the object  2  to be measured. 
   In the case of partial projection, the light is projected by one turn or by several turns over total circumference at the initiation of the measurement. The position of the object  2  to be measured is detected, and a direction (position) of the partial projection and the range (angle) of distance measuring light emission is set up. Also, a dummy light emission range is arranged before the range of distance measuring light emission so that unstable light emitting condition of the light emitting element immediately after the start of the light emission does not give influence on the measurement. 
   By projecting the distance measuring light in the mode of partial light projection, energy can be saved, and consumption of battery can be reduced. By dummy light emission, stable measurement can be attained.