Abstract:
A shape measuring system reduced in size and cost and high in the light utilization efficiency, as well as a shape measuring method, are disclosed which can measure the distance up to an object accurately without being influenced by external conditions such as a change in reflectance of the surface of the object. A semiconductor laser emits an intensity-modulated illumination light. A plane sensor detects a combined light of both a reflected light from an object and a reference light and outputs a composite light detection signal. The semiconductor laser also emits an illumination light which is a stationary light not intensity-modulated. At this time, a shutter is closed. The plane sensor detects the reflected light from the object and outputs a detection signal thereof. For the composite light detection signal a distance calculator makes correction for eliminating the influence of reflectance of the object on the basis of a detection signal of a reflected stationary light and thereafter calculates the distance up to the object on the basis of the composite light detection signal after the correction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a shape measuring system and method wherein an intensity-modulated light is emitted toward an object and the distance up to the object is measured on the basis of a phase difference between the light reflected from the object and the emitted light. Particularly, the present invention is concerned with a shape measuring system of reduced size and cost and a shape measuring method both capable of measuring the distance up to an object accurately and independently of external conditions such as a change in reflectance of the object surface. 
     2. Description of the Prior Art 
     As methods for measuring a three-dimensional shape there have been proposed two methods which are a passive method and an active method. The passive method measures the shape of an object without radiating energy to the object, while the active method measures the shape of an object by radiating a certain energy to the object and detecting its reflection. 
     As an example of the passive method there is known a stereo method which measures distances at plural points up to an object. According to the stereo method, two cameras are disposed at a certain interval and the distance up to an object is determined by triangulation on the basis of a parallax of two images obtained. This method is advantageous in that a remote distance can be measured if only images can be picked up, but involves a serious problem that it is impossible to make a three-dimensional measurement for the whole of a smooth surface free of any pattern. In addition, since it is basically impossible to align the optical axes of two cameras with each other, there has been a drawback that there occurs an area (occlusion) incapable of measuring distance. 
     As to the active method, a light-stripe method is mentioned as an example of the method which measures distances at plural points up to an object. According to this light-stripe method, a slit light is radiated to an object at a certain angle and the distance up to the object is determined by triangulation on the basis of an image picked up at an angle different from the said radiation angle. This method is characteristic in that a relatively simple construction suffices, but it is required that the slit light be scanned in a very small angular unit, resulting in that the measurement time is long because an image is picked up at every angular unit. For solving this problem there has been proposed a structured light method which utilizes the light-stripe method. According to the structured light method, a pattern of a projection light is coded instead of radiating the slit light many times and it is thereby intended to measure a distance with a reduced number of projections. However, given that the number of samples in the horizontal direction is n, it is necessary that image pick-up be conducted log2 n times (nine times if n=512 points), thus giving rise to the problem that the measurement time becomes longer. Additionally, since it is basically impossible to align the optical axis of a projector and that of an image pick-up device with each other, a drawback has been encountered that there occurs an area (occlusion) incapable of measuring a distance. 
     In connection with the active method, as an example of the method capable of measuring distances at plural points by one image pick-up, there is known a phase distribution measuring method wherein an intensity-modulated light is radiated to an object and a phase distribution of reflected light is measured. 
     As examples of conventional phase distribution measuring methods are mentioned methods disclosed in Literature 1, “An article described on pages 126 to 134 of SPIE Vol. 2588 (1995) (A new active 3D-Vision system based on rf-modulation interferometry light),” Japanese Patent No. 2690673, and SPIE Vol. 2748 (1996), pp. 47-59, “The Emerging Versatility of a Scannless Range Imager.” 
     FIG. 10 illustrates the conventional shape measuring system described in Literature 1. This shape measuring system, indicated at  100 , comprises a modulation/demodulation signal generator  104  which applies an intensity modulation to light emitted from a light source  101 A through a condenser lens  102  to a plane modulator  103  using such a crystal as Pockels cell, a projection lens  106  which projects an intensity-modulated light  105   a  onto an object  6  planewise, the modulation/demodulation signal generator  104  which applies an intensity demodulation to a reflected light  105   b  incident on a plane demodulator  108  through a focusing lens  107  after being reflected by the object  6 , the plane demodulator  108  using such a crystal as Pockels cell, and a CCD camera  109  which picks up an intensity-demodulated light signal. According to this construction, light emitted from the light source  101 A is directed to the plane modulator  103  by the condenser lens  102  and is intensity-modulated in accordance with a signal produced from the modulation/demodulation signal generator  104 . Thereafter, the intensity-modulated signal  105   a  is projected planewise onto the object  6  by the projection lens  106 . The reflected light  105   b  from the object  6  is introduced into the plane demodulator  108  by the focusing lens  107  and is intensity demodulated in accordance with a signal provided from the modulation/demodulation signal generator  104 , then is focused on the CCD camera  109 . An intensity image picked up by the CCD camera  109  contains a phase information based on the distance up to the object  6 . By processing this intensity image with a computer  110  it is possible to obtain distance data of the object  6  at a single pick-up of image. 
     FIG. 11 illustrates the conventional shape measuring system disclosed in Japanese Patent No. 2690673. This shape measuring system is different in the following three points from the shape measuring system shown in FIG.  10 . The first point is the use of a semiconductor laser  101 B as a light source, the second point is that intensity modulation is performed directly by the semiconductor laser  101 B without the use of a modulator using such a crystal as Pockels cell, and the third point is that demodulation is performed by an image intensifier  111  without the use of a demodulator using such a crystal as Pockels cell. Light which has been subjected to intensity modulation in accordance with a signal provided from the modulation/demodulation signal generator  104  is radiated from the semiconductor laser  101 B and is then projected planewise onto the object  6  by the projection lens  106 . Reflected light  105   b  from the object  6  is focused on the image intensifier  111  by the focusing lens  107 . The signal from the modulation/demodulation signal generator  104  is converted to a high-voltage signal by a high-voltage drive circuit  112 , which signal is inputted to a gain controller terminal of the image intensifier  111 . Thus, the intensity-demodulated reflected light is picked up by the CCD camera  109 . A intensity image obtained in the CCD camera  109  contains phase information based on the distance up to the object  6 . By processing this intensity image with the computer  110  it is possible to obtain distance data of the object  6  at a single pick-up of image. 
     However, the shape measuring system shown in FIG. 10 is disadvantageous in that the cost thereof is very high because a modulator/demodulator using such a crystal as Pockels cell is used for each of the plane modulator  103  and the plane demodulator  108 . Moreover, since the modulator/demodulator using such a crystal is of a small aperture which is several millimeters or so, it is necessary that the light emitted from the light source  101 A and the light reflected by the object  6  be condensed in conformity with the said aperture by the condenser lens  107 , with consequent increase in system size. 
     The shape measuring system shown in FIG. 11 is also disadvantageous in that it is very expensive because the image intensifier  111  is used. Moreover, for driving the image intensifier  111  it is necessary to intensity-modulate a signal whose voltage is as high as several hundred volts, thus giving rise to a disadvantage that, the structure of a drive circuit used is complicated. Further, the whole of the system becomes larger in size because the image intensifier  111  is larger than the CCD camera  109 . 
     As a distance measuring method using neither the expensive modulator/demodulator nor the expensive and large-sized image intensifier, a phase distribution measuring method using a reference light is disclosed, for example, in Japanese Published Examined Patent Application No. Sho 59-30233. 
     FIG. 12 illustrates a conventional shape measuring system which adopts the method just mentioned above. This shape measuring system, indicated at  100 , comprises a light emitting element  123  adapted to emit light after intensity modulation at a predetermined frequency with a drive signal which is provided from a drive circuit  121  and which is based on a signal produced from an oscillator  120 , a beam splitter  125  adapted to transmit and reflect light which is incident thereon from the light emitting element  123  through a projection lens  124 , a light receiving element  127  adapted to receive a light reflected again by the beam splitter  125  after passage through the beam splitter  125  and after subsequent reflection by the object  6  and a light which has passed through the beam splitter  125  again after being reflected by the beam splitter  125  and after subsequent reflection by a reflecting mirror  128 , an amplifier  128  which amplifies an output signal provided from the light receiving element  127 , a wave detector  129  which detects an output signal provided from the amplifier  128 , and a level meter  130  which reads an amplitude from an output signal provided from the wave detector  129 . 
     In such a configuration, when from the light emitting element  123  is emitted a light after intensity modulation at a predetermined frequency with a drive signal which is provided from the drive circuit  121  and which is based on a signal produced from the oscillator  120 , the light is directed to the beam splitter  125  through the projection lens  124 . One light (illumination light) which has passed through the beam splitter  125  is reflected by an object  6  and then reflected by the splitter  125  and is introduced into the light receiving element  127  through the condenser lens  126 . The other light (reference light) reflected by the beam splitter  125  is then reflected by the reflecting mirror  128  which is disposed at a known distance, then passes through the beam splitter  125  and enters the light receiving element  127 . The illumination light and reference light are optically combined on the light receiving element  127  and the thus-combined light is converted its waveform into an electric signal, which is applied to the amplifier  128 . The amplitude of this waveform varies depending on the difference between the distance from the light receiving element  127  to the object  6  and the distance from the light receiving element  127  to the reflecting mirror  128 . The thus-amplified waveform signal is detected by the wave detector  129  and the amplitude thereof is read by the level meter  130 , whereby the distance up to the object  6  can be calculated. 
     According to the above conventional shape measuring system, however, since the illumination light and the reference light each pass through the beam splitter  125  twice in the section from the light emitting element  123  to the light receiving element  127 , the quantity of light incident on the light receiving element  127  decreases to one fourth, thus giving rise to the problem that the light utilization efficiency is poor. 
     Moreover, since the light incident on the light receiving element  127  is influenced by reflectance on the surface of the object  6  and the distance up to the object  6  is calculated on the basis of an output signal provided from the light receiving element  127 , there arises the problem that an accurate distance cannot be measured due to the difference in reflectance of the object surface. 
     The conventional shape measuring system in question further involves the following serious problems. For an object having concave and convex with respect to an optical axis it is impossible to make an accurate measurement of distance. Accurate distance cannot be measured where extraneous light is present although no problem arises in the dark. For the measurement of a three-dimensional shape it is necessary that light be scanned in two dimensions, resulting in increase of the measurement time. Thus, the measurement of distance is infeasible in ordinary environmental conditions. 
     SUMMARY OF THE INVENTION 
     Accordingly, present invention provides a shape measuring system which is small in size, low in cost and high in the light utilization efficiency. 
     The present invention also provides a shape measuring system and method capable of measuring the distance up to an object accurately without being influenced by external conditions such as a change in reflectance of an object surface. 
     The present invention also provides a shape measuring system wherein a light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring system comprising a light emitting unit that emits the light having been intensity-modulated at the predetermined frequency toward the object, a reflecting member that reflects in a predetermined direction a part of the light emitted from the light emitting unit, and a detecting unit that receives the light reflected from the object and also receives the light reflected by said reflecting member and that outputs a detection signal with the said phase difference reflected therein. 
     In the above construction, as the reflecting member there may be used, for example, a beam splitter which transmits and reflects the emitted light at a predetermined ratio or a reflecting mirror disposed at a position not obstructing the radiation of emitted light to the object. The use of a beam splitter as the reflecting member is advantageous in that the emitted light passes through the beam splitter only once and that therefore the quantity of the reflected light and that of the emitted light both received by the detecting unit are prevented from decrease by the beam splitter. Likewise, the use of a reflecting mirror as the reflecting member is advantageous in that the quantity of the reflected light and that of the emitted light both received by the detecting member are prevented from decrease by the reflecting mirror. 
     The present invention also provides a shape measuring system wherein a light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring system comprising a light emitting unit that emits the light having been intensity-modulated at the predetermined frequency or a stationary light not intensity-modulated toward the object, a reflecting member that reflects in a predetermined direction a part of the light emitted from the light emitting unit or a part of the stationary light, a detecting unit that receives the light reflected from the object and also receives the light reflected by the reflecting member and that outputs a composite detection signal produced by combining both received lights and with the said phase difference reflected therein, the detecting unit further receiving the stationary light reflected by the object and outputting a reflected stationary light detection signal, the detecting unit further receiving the stationary light from the reflecting member and outputting a stationary light detection signal, and a calculating unit that, in accordance with the said composite detection signal, the said reflected stationary light detection signal and the said stationary light detection signal, makes correction for eliminating external components such as a change in reflectance of the object and calculates the said distance. 
     According to this construction, the distance up to the object can be determined accurately by eliminating external components such as a change in reflectance of the object. 
     The present invention further provides a shape measuring system wherein light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring system comprising a light emitting unit that emits the light having been intensity-modulated at the predetermined frequency or a stationary light not intensity-modulated toward the object, a detecting unit that receives the light reflected from the object and also receives the emitted light and outputs a composite detection signal produced by combining both received lights and with the said phase difference reflected therein, and then receives the emitted stationary light after the reflection by the object and outputs a reflected stationary light detection signal, and further receives the stationary light and outputs a stationary light detection signal, and a calculating unit that, in accordance with the said composite detection signal, the said reflected stationary light detection signal and the said stationary light detection signal, makes correction for eliminating external components such as a change in reflectance of the object and calculates the said distance. 
     According to this construction, even in such an optical system as shown in FIG. 12, the distance up to the object can be determined accurately by removing an external component such as a change in reflectance of the object. 
     The present invention further provides a shape measuring method wherein a light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring method comprising a first step and a second step, the first step comprising emitting the light intensity-modulated at the predetermined frequency toward the object, detecting the reflected light and the emitted light, combining both lights into a composite detection signal with the phase difference reflected therein, emitting a stationary light not intensity-modulated toward the object, detecting the stationary light reflected from the object, converting the reflected stationary light thus detected into a reflected stationary light detection signal, and detecting the stationary light and converting it into a stationary light detection signal, and the second step comprising making correction for eliminating external components such as a change in reflectance of the object and calculating the said distance, in accordance with the said composite detection signal, the said reflected stationary light detection signal and the said stationary light detection signal. 
     According to this construction, the removal of external components such as a change in reflectance of the object permits the distance up to the object to be determined accurately. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of a shape measuring system and method according to the present invention will be described in detail based on the drawings: 
     FIG. 1 is a construction diagram of a three-dimensional shape measuring system according to the first embodiment of the present invention; 
     FIG. 2 is a block diagram showing a pixel circuit which constitutes a plane sensor used in the first embodiment; 
     FIG. 3 is a timing chart for explaining the operation of the plane sensor; 
     FIGS. 4A and 4B are diagrams showing, in terms of computer simulation, a change in amplitude of a composite light due to a phase lag of reflected light; 
     FIGS. 5A to  5 D are diagrams for explaining the operation of the three-dimensional shape measuring system according to the first embodiment; 
     FIG. 6 is a flow chart for explaining the operation of the three-dimensional shape measuring system according to the first embodiment; 
     FIG. 7 is a block diagram showing another example of plane sensor; 
     FIG. 8 is a timing chart for explaining the operation of the plane sensor shown in FIG. 7; 
     FIG. 9 is a construction diagram of a three-dimensional shape measuring system according to the second embodiment of the present invention; 
     FIG. 10 is a construction diagram of a conventional three-dimensional shape measuring system; 
     FIG. 11 is a construction diagram of a conventional three-dimensional shape measuring system; and 
     FIG. 12 is a construction diagram of a conventional three-dimensional shape measuring system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a three-dimensional shape measuring system according to the first embodiment of the present invention. This measuring system, indicated at  1 , comprises a modulation signal generator  2  which generates a modulation signal, a semiconductor laser  3  which emits a laser beam as an illumination light  4   a  in accordance with the modulation signal produced from the modulation signal generator  2 , a projection lens  5  which projects the illumination light  4   a  from the semiconductor laser  3  toward an object  6 , a focusing lens  7  which focuses reflected light  4   b  reflected by the object  6  onto a plane sensor  9  through an optical filter  8 , a half mirror  10  which allows a half of the illumination light  4   a  from the semiconductor laser  3  to pass therethrough and the remaining half thereof to be reflected thereby and conducted as a reference light  4   c  onto the plane sensor  9  through the optical filter  8 , a first shutter  11 A disposed between the semiconductor laser  3  and the projection lens  5 , a shutter  11 B disposed between the object  6  and the optical filter  8 , a third shutter  11 C disposed between the half mirror  10  and the optical filter  8 , a two-dimensional image memory  12  which stores an output signal from the plane sensor  9  as intensity information, a distance calculator  13  which calculates distance data on a surface shape of the object  6  in a two-dimensional manner on the basis of the intensity information stored in the image memory  12 , and a CPU  14  which controls various components of the system  1 . 
     In accordance with the modulation signal provided from the modulation signal generator  2  the semiconductor laser  3  emits an intensity-modulated signal as the illumination light  4   a , and in accordance with a stationary signal provided from the modulation signal generator  2  the semiconductor laser  3  emits as the illumination light  4   a  a stationary light which has not been intensity-modulated. This stationary light has a light intensity equal to an average intensity of the intensity-modulated illumination light  4   a.    
     As each of the first to third shutters  11 A,  11 B and  11 C, there may be used, for example, a single crystal plate having an electro-optical effect, the single crystal plate being disposed between a polarizer and an analyzer and provided at both ends thereof with transparent electrodes. Liquid crystal or a mechanical shutter may also be used. The shutters used in this embodiment are of the type which transmits incident light upon application (ON) of voltage. 
     FIG. 2 illustrates one pixel circuit which constitutes the plane sensor  9 . The plane sensor  9  has a plurality of pixels arranged in two dimensions. One pixel comprises: a photodiode  90 ; a first by-pass circuit change-over unit  91 A; a high pass filter (HPF)  92 , a peak holding circuit  93  which comprises a comparator  93   a , a diode  93   b  and a capacitor  93   c ; a current converter circuit  94 ; a second by-pass circuit change-over unit  91 B; a by-pass line  95  which is connected to both first and second by-pass circuit change-over units  91 A,  91 B and which by-passes both HPF  92  and peak holding circuit  93 ; a switch  96 ; and a charge storage circuit  97 . The plane sensor  9  has an amplitude detecting mode and a light quantity detecting mode. 
     FIGS. 3A to  3 D illustrate the operation of the plane sensor  9 . If the first and second by-pass circuit change-over units  91 A,  91 B are set on the side A, as shown in FIG. 3A, a signal Sa is outputted from the photodiode  90  and is then cut off its DC component V 0  by HPF  92  into a high-frequency signal Sb shown in FIG. 3B, which is inputted to the peak holding circuit  93 . From the peak holding circuit  93  is outputted a peak value signal Sc which is held at a peak value of amplitude, as shown in FIG.  3 C. The peak value signal Sc is very low in voltage and is difficult to detect, so is converted into an electric current by the converter circuit  94  and is then stored for a predetermined period of time in the charge storage circuit  97 . Voltage Sd stored in the charge storage circuit  97  increases linearly as in FIG.  3 D and if integration is performed for a sufficiently long period of time T 1  in comparison with the modulation frequency ω/2π of laser beam, there is obtained a voltage value V which is easily detectable. It is apparent that the voltage value V is proportional to the amplitude of the composite light. In a data transfer period T 2 , the voltage value V is transferred to the distance calculator  13 . From the charge storage circuit  97  is detected an amplitude of the intensity-modulated light reflected from the object  6  and there is obtained an image signal which contains phase data corresponding to the distance up to the object  6 . In a discharge period T 3 , the charge storage circuit  97  is grounded by the switch  96  and the stored charge is released, followed by re-start of storage. On the other hand, if the first and second by-pass circuit change-over units  91 A,  91 B are set on the side B, the output signal Sa from the photodiode  90  is inputted directly to the charge storage circuit  97  and an average luminance of the stationary light reflected from the object is detected, affording luminance data of the object  6 . With these circuits, the amplitude of a high-frequency component contained in the output signal Sa of the photodiode  90  can be detected in the form of voltage. 
     The operation of the system  1  will now be described in accordance with a flow chart of FIG. 6 with reference also to FIGS. 4 and 5. FIGS. 4A and 4B shows in terms of computer simulation that the amplitude of the composite light changes due to a phase lag of the reflected light  4   b . FIG. 5A shows an image pick-up condition using illumination light  4   a  and reference light  4   c , both having been subjected to intensity modulation, FIG. 5B shows an image pick-up condition using illumination light which is a stationary light and extraneous light, FIG. 5C shows an image pick-up condition using extraneous light alone, and FIG. 5D shows an image pick-up condition using only reference light that is a stationary light. 
     (1) Image pick-up using both illumination light and reference light 
     As shown in FIG. 5A, an image is picked up from the object  6  on condition that both illumination light  4   a  and reference light  4   c  have been subjected to intensity modulation (ST 1 ). More specifically, the CPU  14  provides a control signal to a current signal mixer  22  in the modulation signal generator  2 , thereby causing the current signal mixer  22  to combine the output of a modulation current source  20  and that of a direct current source  21  and generate the intensity-modulated illumination light  4   a  from the semiconductor laser  3 . Further, the CPU  14  provides control signals to the first to third shutters  11 A,  11 B and  11 C, thereby causing all of the shutters to open and all of the illumination light  4   a  generated from the semiconductor laser  3 , the reflected light  4   b  from the object  6  and the reference light  4   c  to pass therethrough. In more particular terms, the illumination light  4   a  from the semiconductor laser  3  passes through the first shutter  11 A and enters the half mirror  10  through the projection lens  5 . The illumination light  4   a  thus incident on the half mirror  10  is divided into a transmitting light and a reflected light. The illumination light  4   a  which has passed through the half mirror  10  is radiated to the object  6  and the reflected light  4   b  from the object  6  passes through the focusing lens  7  and the second shutter  11 B and is focused onto the plane sensor  9  through the optical filter  8 . The reference light  4   c  reflected by the half mirror  10  enters the plane sensor  9 . Thus, a composite light of both reflected light  4   b  and reference light  4   c  is incident on the plane sensor  9 . The CPU  14  further provides a control signal to the plane sensor  9  to set the light detecting mode in the plane sensor  9  to the amplitude detecting mode for detecting the amplitude of the intensity-modulated light. By picking up an image in this state, such amplitude information on the composite light of both reflected light  4   b  and reference light  4   c  as is represented by expression (6) to be described later is stored as intensity information (image data An) in the image memory  12 . 
     (2) Image pick-up using a stationary light as illumination light and extraneous light 
     An image of the object  6  is picked up by radiation of the illumination light  4   a  which is a stationary light and while the reference light  4   c  is shielded (ST 2 ), as shown in FIG.  5 B. More specifically, under control made by the CPU  14 , the current signal mixer  22  outputs only the output from the direct current source  21  and causes a stationary light as illumination light  4   a  to be generated from the semiconductor laser  3 . The CPU  14  provides control signals to the first to third shutters  11 A,  11 B and  11 C to open the shutters  11 A and  11 B and close the shutter  11 C, thereby causing the illumination light generated from the semiconductor laser  3  and the reflected light  4   b  from the object  6  to transmit and shielding the reference light  4   c . Further, the CPU  14  provides a control signal to the plane sensor  9  to set the light detecting mode in the plane sensor  9  to the light quantity detecting mode. By picking up an image in this state, such luminance information on the object  6  illuminated with the stationary light as is represented by expression (7) to be described later is stored in as intensity information (image data Bn) in the image memory  12  in a two-dimensional manner. 
     (3) Image pick-up with extraneous light alone 
     In the case where light having the wavelength of the illumination light  4   a  is contained in the extraneous light  4   d  (ST 3 ), there is performed an image pick-up step (ST 4 ) which follows, while if such light is not contained in the extraneous light  4   d , a jump is made to further steps (ST 5 ,  6 ). Here, as shown in FIG. 5C, the illumination light  4   a  is shielded and an image of the object  6  is picked up with the extraneous light  4   d  alone. More specifically, the CPU  14  provides a control signal to the current signal mixer  22 , causing the current signal mixer to output at least the output from the direct current source  21  and a stationary light or an intensity-modulated light to be generated as the illumination light  4   a  from the semiconductor laser  3 . Further, the CPU  14  provides control signals to the first to third shutters  11 A,  11 B and  11 C to open the second shutter  11 B and close the first shutter  11 A, thereby shielding the illumination light  4   a  generated from the semiconductor laser  3  and allowing the reflected light  4   b  from the object  6  to transmit. The CPU  14  further provides a control signal to the plane sensor  9  to set the light detecting mode in the plane sensor  9  to the light quantity detecting mode. By picking up an image in this state, luminance information on the object  6  illuminated with the extraneous light  4   d  alone, such as that represented by expression (8) to be described later, is stored as intensity information (image data Cn) in the image memory  12  in a two-dimensional manner. 
     (4) Image pick-up with reference light alone 
     Next, monitoring a semiconductor laser monitor output line  14   a , the CPU  14  performs the following image pick-up step (ST 6 ) if a change in laser output is larger than a preset threshold value (ST 5 ). On the other hand, if the change in laser output is smaller than the preset threshold value, the CPU  14  terminates the image pick-up. operation; provided, however, that intensity information (image data Dn) obtained by performing the following image pick-up operation only once at the time of start-up of the system  1  is stored beforehand in the image memory  12  and is used in the calculation of distance data which will be described later. Here, as shown in FIG. 5D, the reflected light  4   b  from the object  6  is shielded and only the reference light  4   c  is used for image pick-up. To be more specific, the CPU  14  provides a control signal to the current signal mixer  22 , thereby causing the current signal mixer to output only the output from the direct current source  21  and a stationary light to be generated as the illumination light  4   a  from the semiconductor laser  3 . Further, the CPU  14  provides control signals to the first to third shutters  11 A,  11 B and  11 C to open the first and third shutters  11 A,  11 C and close the second shutter  11 B, thereby shielding the reflected light  4   b  from the object  6  and allowing the reference light  4   c  to transmit. The CPU  14  further provides a control signal to the plane sensor  9  to set the light detecting mode in the plane sensor  9  to the light quantity detecting mode. By picking up an image in this state, such luminance information based on the reference light  4   c  of a stationary light as is represented by expression (9) to be described later is stored as intensity information (image data Dn) in the image memory  12  in a two-dimensional manner. 
     (5) Calculating distance data in a two-dimensional manner 
     On the basis of two to four sheets of image data An, Bn, Cn, Dn thus picked up, the distance calculator  13  calculates distance data in a two-dimensional manner in accordance with expression (12) which will be described later (ST 7 ). 
     A description will now be given in detail about the said calculation. Given that an angular frequency of modulation is ω and amplitude is  2 E, the intensity-modulated illumination light  4   a  generated from the semiconductor laser  3  is represented like the following expression (1): 
     
       
           I   0   =E (sin ω t +1)  (1) 
       
     
     Assuming that the distance up to the object  6  is 0-2.5 m, the modulation frequency required is 30 MHz. Given that the light transmittance in the half mirror  10  is a and the reflection coefficient at a certain point on the object  6  is Cn, the intensity of the reflected light  4   b  incident on point, n, as a point formed by focusing of the above point on the plane sensor  9  is expressed like the following expression (2) assuming that the intensity of the extraneous light  4   d  is e: 
     
       
           I   n   =d   l   c   n   ·aE {sin(ω t +φ n )+1 }+e   (2) 
       
     
     Where, d 1  stands for a constant which depends on the optical system (projection system and focusing system) in the system  1  and φ n  stands for a phase lag which is related to a time of flight of the light incident on the plane sensor  9  from the light source. If the distance of (semiconductor laser−object  6 )+(object  6 −plane sensor  9 ) is assumed to be L (C is the velocity of light):          φ   n     =       ω                 L     C                            
     On the other hand, assuming that the reflectance of the half mirror  10  is b and that the optical path length from the semiconductor laser  3  to the plane sensor  9  and the size of the plane sensor  9  are sufficiently small in comparison with the wavelength of the modulated wave, the intensity of the reference light  4   c  on the plane sensor  9  becomes uniform and the following expression (3) is established at point n on the plane sensor  9 : 
     
       
           R   n   =d   2   bE {(sin ω t +1}  (3) 
       
     
     Where, d 2  is a constant which depends on the optical system (focusing system) in the system  1 . 
     The light intensity P n  at point n on the plane sensor  9  is of a combined light of both reflected light  4   b  and reference light  4   c  and is represented like the following expression (4) as a result of addition of expressions (2) and (3):                      P   n     =                  I   n     +     R   n                   =                    d   1            c   n     ·   aE          {       sin        (       ω                 t     +     φ   n       )       +   1     }       +   e   +       d   2        bE        {       sin                 ω                 t     +   1     }                     =                    d   1            c   n     ·   aE          {       sin                 ω                 t                 cos                   φ   n       +     cos                 ω                 t                 sin                   φ   n       +   1     }       +   e   +       d   2        bE        {       sin                 ω                 t     +   1     }                     =                    (         d   1            c   n     ·   a       +       d   2        b       )        E     +   e   +       (         d   1            c   n     ·   aE                   cos                   φ   n       +       d   2        bE       )        sin                 ω                 t     +       d   1            c   n     ·   aE                   sin                   φ   n        cos                 ω                 t                   =                    (         d   1            c   n     ·   a       +       d   2        b       )        E     +   e   +             (         d   1            c   n     ·   aE                   cos                   φ   n       +       d   2        bE       )     2     +       (       d   1            c   n     ·   aE                   sin                   φ   n       )     2            sin                   (       ω                 t     +   θ     )                     =                    (         d   1            c   n     ·   a       +       d   2        b       )        E     +   e   +             (       d   1            c   n     ·   aE       )     2     +       (       d   2        bE     )     2     +     2        (       d   1            c   n     ·   aE       )          (       d   2        bE     )        cos                   φ   n              sin                   (       ω                 t     +   θ     )                       (   4   )                                
     Provided,          tan                 θ     =         d   1          c   n        aE                 sin                   φ   n             d   1          c   n        aE                 cos                   φ   n       +       d   2        bE                                
     FIG. 4A is with a relatively small distance up to the object  6 , that is, with a small phase lag (π/4 lag), in which the amplitude of the combined light, or composite light, is large, while FIG. 4B is with a relatively large distance up to the object  6 , that is, with a large phase lag (7π/8 lag), in which the amplitude of the composite light is small. As shown in the above expression (4), the composition light corresponds to the sum of the following DC component: 
     
       
         ( d   1   c   n   a+d   2   b ) E+e   (5) 
       
     
     and the following high-frequency component:                (       d   1            c   n     ·   a                   E     )     2     +       (       d   2        b                 E     )     2     +     2        (       d   1            c   n     ·   a                   E     )          (       d   2        b                 E     )        cos                   φ   n          sin        (       ω                 t     +   θ     )                                               
     Since d 1 c n  aE and d 2 bE appearing in the amplitude term are reflected light (including surface reflectance of the object)  4   b  and reference light  4   c  components both based on light not intensity-modulated, it is possible to make measurement beforehand as follows. 
     In the mage pick-up condition shown in FIG. 5A, if the amplitude of the intensity-modulated light incident on the plane sensor  9  is assumed to be 2A n , A n  can be written like the following expression (6):                A   n     =           (       d   1            c   n     ·   a                   E     )     2     +       (       d   2        b                 E     )     2     +     2        (       d   1            c   n     ·   a                   E     )          (       d   2        b                 E     )        cos                   φ   n                   (   6   )                                
     In the image pick-up condition shown in FIG. 5B, the intensity of light incident on the plane sensor  9  is represented as follows: 
     
       
         B n   =d   1   c   n   ·aE+e   (7) 
       
     
     In the image pick-up condition shown in FIG. 5C, the intensity of light incident on the plane sensor  9  is represented as follows: 
     
       
           C   n   =e   (8) 
       
     
     In the image pick-up condition shown in FIG. 5D, the intensity of light incident on the plane sensor  9  is represented like the following expression (9): 
     
       
           D   n   =d   2   bE   (9) 
       
     
     From the above expressions (6), (7), (8) and (9), the amplitude of the composite wave is represented like the following expression (10):                A   n     =           (       B   n     -     C   n       )     2     +     D   n   2     +     2        (       B   n     -     C   n       )          D   n        cos                   φ   n                   (   10   )                                
     Given that the distance between the semiconductor laser  3  as the light source and the object  6  and the distance between the object  6  and the plane sensor  9  are L and that the light velocity is C, the phase lag φ n  is represented like the following expression (11):                φ   n     =       ω                 L     C             (   11   )                                
     From the above expressions (10) and (11), the distance L is represented like the following expression (12) using the foregoing four kinds of image data An, Bn, Cn and Dn:              L   =       C   ω          cos     -   1            {         A   n   2     -       (       B   n     -     C   n       )     2     -     D   n   2         2        (       B   n     -     C   n       )          D   n         }               (   12   )                                
     It follows that the distance up to the object  6  can be calculated by detecting the four kinds of image data An, Bn, Cn and Dn. Since the reflection coefficient C n  of the object  6 , constants d 1 , d 2  based on the optical system, and extraneous light intensity, e, are not contained in the expression (12), it is possible to obtain distance information no matter what reflectance distribution the object to be measured may have and not matter what extraneous light  4   d  may be present in image pick-up. 
     According to the first embodiment described above, there are attained the following effects. 
     (a) Since the number of time in which the illumination light  4   a  emitted from the semiconductor laser  3  passes the half mirror  10  and the number of time in which the reference light  4   c  passes the half mirror  10  are each once, it is possible to suppress the decrease in the quantity of the reflected light and that of the reference light  4   c  both received by the plane sensor  9 . 
     (b) Since an expensive and large-sized unit so far used as a light demodulating unit such as a light intensity demodulator or an image intensifier is not needed, it is possible to obtain a phase distribution proportional to the distance up to an object and hence possible to provide a less expensive and small-sized three-dimensional shape measuring system. 
     (c) Since the optical filter  8  which transmits the light from the light source selectively is disposed on the front side of the plane sensor  9 , if there is used an infrared or ultraviolet light source as the light source, it is possible to measure a three-dimensional shape with a high accuracy and with a minimized error caused by the extraneous light  4   d.    
     (d) Since a phase distribution proportional to the distance up to the object  6  can be measured in terms of a voltage value, it is possible to measure a three-dimensional shape easily. 
     (e) Since both range image and intensity image can be obtained with a single plane sensor  9  and the pixels of these two images are in one-to-one correspondence to each other, it is possible to easily effect image processing for a intensity image using a range image. 
     (f) Since up to four sheets of images are picked up under different lighting conditions, distance information can be acquired no matter what reflectance distribution the object  6  may have and what extraneous light  4   d  may be present in image pick-up. 
     (g) Since both range image and intensity image are obtained, it is easy, for example, to prepare a composite image by combining an image picked up in a studio with a building image and a background image and cut out an object image from the composite image. 
     FIG. 7 shows an example of using a rectifier circuit  98  instead of the peak holding circuit  93  shown in FIG. 2, the rectifier circuit  98  comprising a resistor  98   a  and a diode  98   b . The operation of this plane sensor is the same as in FIG. 3 except that such a rectification signal S as shown in FIG. 8 is outputted from the rectifier circuit  98 . 
     FIG. 9 illustrates a shape measuring system according to the second embodiment of the present invention. In this second embodiment, instead of the half mirror  10  used in the first embodiment, a reflecting mirror  15  is disposed at a position deviated from an optical path (indicated with dotted lines in the same figure) of the illumination light  4   a  radiated to the object  6 , with other constructional points being the same as in the first embodiment. According to this second embodiment, since the quantity of the reflected light  4   b  incident on the plane sensor  9  is not reduced to half by the half mirror  10 , so that the output signal from the plane sensor  9  is enhanced and S/N ratio is improved. 
     The present invention is not limited to the above embodiments, but various modifications may be made. For example, although in the above embodiment a semiconductor laser is used as the light source, there may be used an ordinary type of a light source such as a xenon lamp or an electronic flash because a coherent light is not required in principle. 
     Although in the above embodiment both reflected light  4   b  and reference light  4   c  are received by a common plane sensor  9 , a modification may be made such that the reflected light  4   b  is received by the plane sensor, the reference light  4   c  is received by one or plural light receiving elements, and a composite signal obtained by combining respective output signals is outputted. 
     Although in the above embodiment both intensity-modulated reflected light  4   b  and intensity-unmodulated reflected light  4   b  of a stationary light are received by the same pixel on the plane sensor  9 , both may be received by separate pixels. 
     Although the half mirror  10  is used in the first embodiment, it may be substituted by a beam splitter which transmits and reflects the incident light at a predetermined ratio. 
     The present invention which makes correction for removing an external component such as a change in reflectance of the object  6  maybe applied to the conventional configuration shown in FIG.  12 . 
     Further, the first shutter  11 A may be disposed on the optical path between the half mirror  10  and the object  6 . 
     According to the present invention, as set forth above, since the quantity of the light emitted from the light emitting unit and that of the light reflected from an object can be prevented from being decreased by the reflecting member, the light utilization efficiency becomes higher. Moreover, since the distance up to the object is measured on the basis of a phase difference between an emitted light and a reflected light, it is not necessary to use an expensive modulator/demodulator or an expensive and large-sized image intensifier, so that it is possible to provide a small-sized and less expensive shape measuring system and method. 
     Further, since correction is made to eliminate external components such as a change in reflectance of an object in calculating the distance up to the object, it is possible to effect an accurate measurement of the distance up to the object.