A two-dimensional rangefinding sensor of the present invention consisting of an illuminating device for projecting light beams, which undergoes luminance modulation at a predetermined frequency for a predetermined duration with a predetermined cycle time, onto an object, an image-formation optical system for forming an image of the object illuminated with light beams which have undergone the luminance modulation and which have been projected from the illuminating device, a two-dimensional image sensor mounted on an image-formation plane of the image-formation optical system, a driving means for performing a modulation driving operation on an electrode terminal, which is operative to determine the sensitivity of the two-dimensional image sensor at the frequency, and a reading means for reading a signal corresponding to a signal charge generated in each of picture elements of the image sensors. Thereby, a range-data image of a three-dimensional object can be obtained by using a two-dimensional image sensor without mechanically scanning with illumination light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a two-dimensional rangefinding 
sensor using a two-dimensional image sensor for extracting a "range image" 
of a three-dimensional object together with a "luminance image" thereof. 
More particularly, the present invention relates to a two-dimensional 
rangefinding sensor using a two-dimensional image sensor that can be 
utilized as "eyes" of mobile units such as a vehicle and a robot, which 
are used during the mobile units autonomously run on a celestial body and 
on the ground, or utilized as a device for inputting a static range-data 
image of a three-dimensional object. 
2. Description of the Related Art 
Hitherto, various methods, for example, (1) what is called a 
stereo-matching method and (2) what is called a slit(-light) projection 
method (namely, a silicon rangefinder method) and other methods have been 
known as methods for obtaining range images of three-dimensional objects 
by utilizing area sensors. In the case of the stereo-matching method, a 
range image is obtained by performing a correlation operation on a 
plurality of luminance images which have been obtained by picking up an 
object from a plurality of directions of visual lines (or lines of sight), 
respectively. This method is disclosed in "Handbook of Image Input 
Techniques" (edited by Yuji Kiuchi and published by the Nikkan Kogyo in 
1992). Next, this stereo-matching method will be further described by 
referring to FIGS. 1A and 1B that illustrate the principle of a "binocular 
stereo vision method", by which output signals of two television cameras 
provided in such a manner as to be set away from each other are analyzed 
and thus information concerning the distance therefrom to an object. In 
FIG. 1A, reference numeral 1001 designates an object; 1002 and 1003 
television cameras; and 1004 a signal processing system. FIG. 1B 
illustrates an enlarged view of a circled portion in FIG. 1A together with 
a coordinate system. If the object 1001 is at infinity, in each of the 
television cameras, an image point corresponding to a same point on the 
object 1001 coincides with what is called a "camera center", namely, a 
point of intersection of the photosensitive surface and the optical axis 
of a camera lens. Generally, in the case of a stereoscopic image, the 
object 1001 is not at infinity, so that the image point deviates from such 
a "camera center". Thus, if deviation distances a and b are known, the 
position or location P(x, y) of an object is obtained by the following 
equations (1) and (2): 
EQU x=a.multidot.d/(a+b) (1) 
EQU y=d.multidot.c/(a+b) (2) 
where c designates the distance between the lens surface and the 
photosensitive surface; and d the distance between the camera centers of 
the television cameras. Generally, television cameras using charge coupled 
devices (CCDs) are used as input devices. Although this method should be 
applied to all cases in principle under appropriate illumination 
conditions, extremely high computational complexity is necessary for 
extracting from stereoscopic-image signals by performing a signal 
processing thereon. Moreover, in such a case, the calculation accuracy is 
not sufficiently high. 
Meanwhile, in the case of the slit projection method, as shown in FIG. 2, 
by irradiating the object 1101 with stripe light 1105, which is obtained 
from laser light emitted by a laser 1102 through a cylindrical lens 1103 
and a mirror 1104, and further forming a projection slit image modified 
correspondingly to the three-dimensional structure of the object 1101 on 
what is called a two-dimensional image sensor 1106. Incidentally, in FIG. 
2, reference numeral 1107 denotes a base line. However, in the case of 
using this technique, it is necessary for inputting three-dimensional 
information into the two-dimensional image sensor to scan the position of 
the projection slit by means of the mirror 1104 and to take out the 
information from the two-dimensional image sensor 1106 at each scan. 
Therefore, in order to realize the 256-pixel spatial resolution (or 
resolving power) in the scanning direction of the projection stripe light, 
the information should be captured (or read) 256 times from the 
two-dimensional image sensor 1106. When the frame rate of the 
two-dimensional image sensor 1106 is 1/30 seconds, it takes about 8.5 
seconds to read the information from the two-dimensional image sensor 
1106. 
Meantime, Kanade et al. of Carnegie Mellon University have proposed a 
method that uses an image sensor, which employs a circuit configuration 
designed specifically for rangefinding, instead of the aforementioned 
two-dimensional image sensor, whereby information corresponding to a full 
scan of the two-dimensional image sensor can be read only one time. This 
method, however, requires scanning with the projection stripe light 
similarly as the method described by referring to FIG. 2, in spite of the 
restriction that the special image sensor should be used. Incidentally, 
the slit projection method of FIG. 2 is described in detail in, for 
example, the article "Integrated Sensor and Range-Finding Analog Signal 
Processing", A. Gruss et al, IEEE J. Solid-State Circuits, Vol. 26, No. 3, 
March 1991, pp. 184-191. 
The aforesaid conventional stereo-matching method requires performing 
operations and signal processing on enormous amounts of data and signals, 
respectively. In contrast, in the case of the slit projection method, 
there has been caused a problem that even in the case where the 
two-dimensional image sensor itself has the ability or capability of 
directly capturing a range image, some mechanical scanning portion is 
required, similarly as in the case of mechanically scanning the object 
with the stripe light. In view of reducing the size and weight of a sensor 
and increasing the operating speed thereof and realizing a 
maintenance-free sensor, it is preferable that the sensor is not provided 
with such a moving part. This is strongly demanded, for instance, in the 
case where the sensor is used in outer space. 
The present invention is accomplished to resolve the problems concerning 
the conventional methods for obtaining a range image of a 
three-dimensional object by utilizing conventional area sensors. 
SUMMARY OF THE INVENTION 
It is, accordingly, a primary object of the present invention to provide a 
two-dimensional rangefinding sensor that can directly obtain a range image 
of an object without mechanical scan using illumination light. 
To achieve the foregoing object, in accordance with the present invention, 
there is provided a two-dimensional rangefinding sensor of the present 
invention which comprises: an illuminating device for projecting light 
beams, which undergoes luminance modulation (namely, brilliance modulation 
or intensity modulation) at a predetermined frequency for predetermined 
duration with a predetermined cycle (or repetition) time, onto an object; 
an image-formation optical system (namely, an imaging optical system) for 
forming an image of the object illuminated with light beams which have 
undergone the luminance modulation and have been projected from the 
illuminating device; a two-dimensional image sensor mounted on an 
image-formation plane of the image-formation optical system; a driving 
means for performing a modulation driving operation on an electrode 
terminal, which is operative to determine the sensitivity of the 
two-dimensional image sensor, at the frequency; and a reading means for 
extracting a signal corresponding to a signal charge generated in each of 
picture elements of the two-dimensional image sensor. 
In the case of the two-dimensional rangefinding sensor of the present 
invention having the aforementioned configuration, a light beam undergoing 
luminance modulation performed at a predetermined frequency is emitted 
from the illuminating device. Subsequently, an image of the object 
illuminated with this light beam is formed on the two-dimensional image 
sensor through the image-formation optical system. At that time, in 
reflected illumination light received on each of the picture elements 
built in the two-dimensional image sensor, a phase shift corresponding to 
the three-dimensional structure of the object is caused. Thus, the sensor 
is constructed in such a manner that the light-sensing sensitivity of the 
two-dimensional image sensor can be modulated at the predetermined 
frequency. Thereby, in the case of a picture element at which the phase of 
the received illumination light matches the phase of the sensitivity of 
the two-dimensional image sensor, a large amount of signal charge is 
stored therein. In contrast, in the case of a picture element at which the 
phase of the received illumination light does not match the phase of the 
sensitivity of the two-dimensional image sensor, only a small amount of 
signal charge is stored therein. Therefore, two dimensional range 
information concerning the three-dimensional object is directly 
represented by taking out the signal charge stored in each of the picture 
elements of the two-dimensional image sensor by the reading means. 
Further, it is another object of the present invention to achieve a two 
dimensional rangefinding operation with higher accuracy at a higher speed 
in the two-dimensional rangefinding sensor recited in the appended claim 
1. 
To attain this object of the present invention, in accordance with the 
present invention, there is provided another two-dimensional rangefinding 
sensor that employs a CMD (charge modulation device) image sensor as the 
two-dimensional image sensor and further uses the gate electrode or the 
substrate electrode of the CMD image sensor as an electrode terminal for 
sensitivity modulation. Thereby, the sensitivity modulation can be 
achieved at a high frequency. 
Moreover, it is a further object of the present invention to achieve a 
sensitivity modulating operation at a further higher speed in the 
two-dimensional rangefinding sensor recited in the appended claim 2, which 
uses the CMD image sensor. 
To attain this object of the present invention, in accordance with the 
present invention, there is provided still another two-dimensional 
rangefinding sensor, wherein a p-channel layer and an n-channel layer are 
formed on a p-type substrate of a CMD image sensor by performing an 
epitaxial growth method, wherein the resistivity of the substrate is 
lowered by setting the impurity concentration of the p-type substrate in 
such a way as to be higher than the impurity concentration of the 
p-channel layer. Alternatively, there is provided yet another 
two-dimensional rangefinding sensor, wherein the back surface portion of 
the substrate of a CMD image sensor is partly removed by performing a 
lapping method, thereby reducing the thickness of the substrate. Thus, the 
maximum modulation frequency of the CMD image sensor can be increased and 
the accuracy of the range information can be enhanced by lowering the 
resistivity of the substrate or by reducing the thickness of the 
substrate. 
Furthermore, it is still another object of the present invention to enhance 
the operating accuracy of the two-dimensional rangefinding sensor recited 
in the appended claim 1 by realizing a long-time integration in the 
two-dimensional image sensor. 
To attain this object of the present invention, in accordance with the 
present invention, there is provided still another two-dimensional 
rangefinding sensor, wherein a cooling unit is provided in the back 
surface portion of the two-dimensional image sensor with the intention of 
reducing a dark current. Thereby, the long-time integration can be 
achieved. 
Additionally, it is yet another object of the present invention to change 
the configuration of the two-dimensional rangefinding sensor recited in 
the appended claim 1 in such a way as to be able to deal with a weak 
incidence signal. 
To attain this object of the present invention, in accordance with the 
present invention, there is provided still another two-dimensional 
rangefinding sensor, wherein an electron-bombarded two-dimensional 
amplified MOS intelligent imager (AMI) image sensor is used as a 
two-dimensional image sensor. The electron-bombarded AMI image sensor has 
the high-sensitivity characteristics. Thus this two-dimensional 
rangefinding sensor can deal with a weak incidence signal. 
Besides, it is another object of the present invention to change the 
configuration of the two-dimensional rangefinding sensor recited in the 
appended claim 1 in such a manner as to be able to obtain two-dimensional 
range information with little off-set and preferable signal-to-noise ratio 
(S/N). 
To attain this object of the present invention, in accordance with the 
present invention, there is provided another two-dimensional rangefinding 
sensor, wherein a laminated two-dimensional AMI image sensor is used as a 
two-dimensional image sensor. In the case of using this laminated 
two-dimensional AMI image sensor, there can be obtained a signal with 
little off-set and preferable S/N. 
In addition, it is still another object of the present invention to provide 
a two-dimensional rangefinding sensor which can simultaneously perform 
both of a modulation-mode image pickup and a luminance-mode image pickup 
in a single image pickup operation to thereby obtain two-dimensional range 
information in a short period of time. 
To attain this object of the present invention, in accordance with the 
present invention, there is provided still another two-dimensional 
rangefinding sensor, which comprises first and second two-dimensional 
image sensors for receiving a light beam representing an image of an 
object, a beam splitter for splitting an image-formation light beam 
obtained through image-formation optical system into light beams to be 
respectively sent to the first and second two-dimensional image sensors, 
and a sensitivity modulation driving means for modulating the sensitivity 
of the first two-dimensional image sensor. Thus, both of a modulation-mode 
image pickup and a luminance-mode image pickup can be simultaneously 
performed in a single image pickup operation to thereby obtain 
two-dimensional range information in a short period of time. 
Further, to attain this object of the present invention, in accordance with 
the present invention, there is provided yet another two-dimensional 
rangefinding sensor that comprises a single two-dimensional CMD image 
sensor in which picture elements, each of which is operative to perform an 
image pickup operation in an ordinary direct-current-component-mode-like 
luminance mode, and picture elements, each of which is operative to 
perform an image pickup operation in a modulation mode, are disposed in an 
intermixed manner. Thus, both of a modulation-mode image pickup and a 
luminance-mode image pickup can be simultaneously performed in a single 
image pickup operation to thereby obtain two-dimensional range information 
in a short period of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, before describing practical preferred embodiments of the present 
invention, the fundamental configuration of a two-dimensional rangefinding 
sensor of the present invention will be described hereinbelow by referring 
to FIG. 3. The two-dimensional rangefinding sensor of the present 
invention consists of: an illuminating device 2 for projecting light 
beams, which undergoes luminance modulation (namely, brilliance modulation 
or intensity modulation) at a predetermined frequency for predetermined 
duration with a predetermined cycle (or repetition) time, onto an object 1 
(to be illuminated); an image-formation optical system (namely, an imaging 
optical system) 3 for forming an image of the object 1 illuminated with 
light beams which have undergone the luminance modulation and have been 
projected from the illuminating device 2; a two-dimensional image sensor 4 
mounted on an image-formation plane of the image-formation optical system 
3; and a processing unit 5 that has a driving means for performing a 
modulation driving operation on an electrode terminal, which is operative 
to determine the sensitivity of the two-dimensional image sensor 4, at the 
frequency and further has a reading means for extracting a signal 
corresponding to a signal charge generated in each of picture elements of 
the two-dimensional image sensor 4. 
In the case of the two-dimensional rangefinding sensor of the present 
invention having the aforementioned configuration, a light beam undergoing 
luminance modulation performed at a predetermined frequency is emitted 
from the illuminating device 2. Subsequently, an image of the object 1 
illuminated with this light beam is formed on the two-dimensional image 
sensor 4 through the image-formation optical system 3. Then, the 
two-dimensional image sensor 4 is driven by the processing circuit 5. Thus 
information stored in the image sensor 4 is read out therefrom. At that 
time, in illumination light received on each of the picture elements built 
in the two-dimensional image sensor 4, a phase shift corresponding to the 
three-dimensional structure of the object 1 is caused. Thus, the sensor is 
constructed in such a manner that the light-sensing sensitivity of the 
two-dimensional image sensor 4 can be modulated at the predetermined 
frequency. Thereby, in the case of a picture element at which the phase of 
the received illumination light matches the phase of the sensitivity of 
the two-dimensional image sensor 4, a large amount of signal charge is 
stored therein. In contrast, in the case of a picture element at which the 
phase of the received illumination light does not match the phase of the 
sensitivity of the two-dimensional image sensor 4, only a small amount of 
signal charge is stored therein. In other words, the detection of the 
phase of the received illumination light is performed at each picture 
element. The amounts of electric charge respectively stored in the picture 
elements directly represents range information corresponding to the 
three-dimensional object. 
The two-dimensional image sensor is provided with a photoelectric 
conversion portion that uses p-n photodiodes or metal-oxide-semiconductor 
(MOS) photodiodes, which are usually put into a reverse-bias condition, as 
picture elements. Therefore, the sensitivity modulation can be realized by 
modulating the bias conditions, which are established for this 
photoelectric conversion portion, in synchronization with the 
luminance-modulation frequency of the illuminating device 2. Thus, the 
object is irradiated with light beams which have undergone the luminance 
modulation and are emitted from the illuminating device 2. Then, a 
two-dimensional image is formed on the two-dimensional image sensor 4 from 
reflected light. Subsequently, the bias terminal established 
correspondingly to the photoelectric conversion portion of the 
two-dimensional image sensor 4 is modulated and driven in synchronization 
with the modulation frequency of the illuminating device 2. Thereby, the 
degree of the photoelectric conversion by the two-dimensional image sensor 
4 is modulated. Consequently, what is called a phase detection can be 
achieved at each of the picture elements. Namely, the range information 
concerning the three-dimensional object can be obtained without a 
mechanical scanning portion as being represented by the amounts of 
electric charge stored in each of the picture elements of the 
two-dimensional image sensor. In other words, the range information can be 
obtained as being corresponding to a luminance image. 
Next, a practical embodiment will be described hereinafter. FIG. 4 is block 
diagram for illustrating the configuration of a two-dimensional 
rangefinding sensor embodying the present invention, namely, the first 
embodiment of the present invention. In this figure, reference numeral 11 
designates a sensitivity modulation driver for modulating the sensitivity 
of the two-dimensional image sensor:; 12 a two-dimensional image sensor 
driver; 13 a two-dimensional image sensor; 14 a control signal generator; 
15 a signal processor; 16 an optical band-pass filter which eliminates the 
influence of background light and allows only light, whose wavelength is 
close to the wavelength of reflected light of the wavelength band of a 
light source undergoing luminance modulation, to pass therethrough; 17 an 
image-formation optical system; 18 a light source driver; 19 a light 
source; and 20 an object. 
In the case of this embodiment, a two-dimensional CMD image sensor using 
CMDs (namely, charge modulation devices) as picture elements is employed 
as the two-dimensional image sensor 13. FIG. 5 is a diagram for roughly 
illustrating the structure of a CMD, which composes a single picture 
element serving as a unit for receiving light, of a two-dimensional CMD 
image sensor. In this figure, reference numeral 21 designates a p.sup.- 
-substrate; 22 an n.sup.- -channel layer formed by performing an epitaxial 
growth method; 23 an n.sup.+ source diffusion layer; 24 an n.sup.+ drain 
diffusion layer; 25 a gate insulation film; 26 a gate electrode; and 27 a 
source electrode. 
The CMD picture element having such a configuration operates as follows. 
Namely, signal light 28 having been incident thereon from an upper portion 
of the gate electrode 26 is transmitted by the gate electrode 26 and the 
gate insulation film 25 in sequence. Thus, hole-electron pairs are 
generated in the n.sup.- -channel layer 22. The generated holes are stored 
in the interface between the insulation film and a semiconductor under the 
gate electrode 26 to which a negative voltage is applied. Then, the 
potential barrier against electrons, which are present in a region between 
the drain diffusion layer 24 and the source diffusion layer 23, is lowered 
owing to the stored holes. As a result, a source-drain current modulated 
by the stored hole flows therethrough. Consequently, a signal current 
depending on the quantity of incident light can be obtained 
non-destructively. 
Next, an operation of the first embodiment of FIG. 4 using the 
two-dimensional CMD image sensor of such a configuration will be described 
hereinbelow by referring to a timing chart of FIG. 6. In this figure, 
.PHI..sub.S denotes a measurement starting pulse; .PHI..sub.SM a waveform 
of a control signal used for modulating the sensitivity of the CMD image 
sensor 13; .PHI..sub.LD a waveform of a control signal used for driving 
the light source 19; .PHI..sub.ref a waveform of return light that is 
emitted from the light source 19 and is thereafter reflected by the object 
20, which is at a finite distance therefrom, and forms an image on the 
light receiving surface of the CMD ; .PHI..sub.RD a group of driving 
pulses for reading data from the CMD image sensor 13; and .PHI..sub.DATA a 
waveform of a signal representing read data. 
At a point of time to shown in FIG. 6, all signal charges have been 
preliminarily discharged from the CMD image sensor 13. On the other hand, 
the light source 19 is turned off. Subsequently, when a measurement 
starting pulse .PHI..sub.S is outputted, the light source driver 18 starts 
the luminance modulation of the light source 19 in response to a control 
signal .PHI..sub.LD issued from the control signal generator 14 (see the 
term (namely, the time period) T.sub.1 of FIG. 6). Moreover, in this time 
period, the sensitivity modulation driver 11 for the modulation of the 
sensitivity of the CMD image sensor 13 commences performing such 
sensitivity modulation at a same frequency as in the case of the luminance 
modulation, in response to a control signal .PHI..sub.SM issued from the 
control signal generator 14. Here, for simplicity of description, it is 
assumed that the luminance modulation and the sensitivity modulation are 
conducted in a period T.sub.0 by using rectangular waves whose duty ratio 
is 50%, that the lowest level of the modulated luminance is set at a level 
0 of the luminance and that the lowest level of the modulated sensitivity 
is set at a level 0 of the sensitivity. Considering the case that the 
object 20 is at a distance z from the sensor, light projected from the 
light source 19 travels the distance z. Then, the light is projected onto 
the object 20 and is further reflected thereon. Subsequently, the 
reflected light travels the distance z once more. Finally, an image is 
formed on the CMD image sensor 13 from this light. Thus, as shown in the 
enlarged timing chart of FIG. 7, there is a time lag t.sub.d which is 
given by the following equation (3), between the emission of the light 
from the light source 19 and the incidence of the light coming from the 
object 20, which is at the distance z away therefrom, onto the CMD image 
sensor 13: 
EQU t.sub.d =2.multidot.z/c (3) 
where c designates the velocity of light. 
Therefore, the ratio of the number of signal charges, which can be 
generated in a picture element of the two-dimensional CMD image sensor 13 
for one period (or cycle) of the luminance modulation term, to the number 
of signal charges, which can be generated therein for the same term in the 
case that the object is at the distance z=0 (namely, in this case, the 
ratio .theta.=1), is obtained by the following equation (4): 
EQU .eta.=1-2.multidot.t.sub.d /T.sub.0 (4) 
Namely, when an output of the picture element is measured, the value of the 
output thereof is proportional to the ratio .theta.. Moreover, as is 
understood from the equation (4), the time lag t.sub.d can be found from 
the known period T.sub.0 and the measured ratio .theta.. Furthermore, the 
distance z therefrom to the object can be calculated by using the equation 
(3). 
Thus, when the term T.sub.1, expires, the signal charges given by the 
following equation (5) are stored in each of the picture elements of the 
CMD image sensor 13: 
EQU H.sub.1 (x,y)=k.multidot..eta.(z).multidot.I(x,y).multidot.T.sub.1(5) 
where k designates a proportional constant; and I(x,y) the quantity of 
light reflected by the object. Incidentally, I(x,y) includes the influence 
of illuminance of the light source, which decreases in proportion to the 
distance therefrom to the object. 
After the expiration of the term T.sub.1, a pixel signal outputted from 
each of the picture elements is read by driving the CMD image sensor 13. 
Further, the read signal is stored in a memory (not shown) provided in the 
signal processor 15. Moreover, simultaneously with the reading of the 
signal, or upon completion of the reading thereof, the signal charges 
stored in each of the picture elements are cleared. Subsequently, the 
light source driver 18 starts the luminance modulation of the light source 
19 again in response to a control signal .PHI..sub.LD sent from the 
control signal generator 14 (see the term T.sub.2 of FIG. 6). On the other 
hand, the sensitivity modulation driver 11 maintains the sensitivity of 
the CMD image sensor 13 at a constant value or level. In this case, 
because of the fact that the modulation of the sensitivity of the CMD 
image sensor 13 is not performed differently from the case of the term 
T.sub.1, the factor relating to the sensitivity modulation in the equation 
(5) becomes a constant. Thus, the signal charges given by the following 
equation (6) are stored in each of the picture elements of the CMD image 
sensor 13 when the term T.sub.2 expires: 
EQU H.sub.2 (x,y)=(k/2).multidot.I(x,y).multidot.T.sub.2 (6) 
After the expiration of the term T.sub.2, each pixel signal is read by 
driving the CMD image sensor 13. Further, the read signals are stored in a 
memory (not shown) other than the aforementioned memory, to which the 
signals are written after the expiration of the term T.sub.1, among 
memories (not shown) provided in the signal processor 15. These memories 
store the information represented by the equations (5) and (6). Thus, 
two-dimensional range information can be easily obtained from the 
following equation (7): 
EQU .eta.(z)=(1/2).multidot.{H.sub.1 (x,y)/T.sub.1 }/{H.sub.2 (x,y)/T.sub.2 }(7 
) 
Namely, in the case of this embodiment, the luminance modulation is 
performed in a two-dimensional manner. Moreover, a light-receiving 
operation thereof is conducted on the two-dimensional image sensor. In 
accordance with the system of this embodiment of the present invention, 
the two-dimensional range information and the luminance information are 
obtained from continuous fields or frames. Although the exemplary sensor 
using the two-dimensional CMD image sensors has been described in the 
foregoing description of this embodiment, the gist of the system of this 
embodiment is that the sensitivity of the two-dimensional image sensor is 
modulated in synchronization with the frequency at which the luminance 
modulation is performed on the light source. Therefore, the 
two-dimensional image sensor to be utilized is not limited to the CMD 
image sensor. It is obvious that two-dimensional solid-state image sensors 
using CCDs, whose sensitivity depends on the substrate potential or the 
electrode potential thereof being present on the light receiving surface, 
or the like can be applied to, namely, employed in the sensor of this 
embodiment. Moreover, in the foregoing description of this embodiment, 
there has been disclosed the sensor adapted to perform the luminance 
modulation on the light source similarly as at the time of performing the 
sensitivity modulation, even in the case of exposing the image sensor 
without performing the sensitivity modulation. 
However, needless to say, the luminance modulation may be omitted in the 
case of exposing the image sensor without performing the sensitivity 
modulation. The above is the description of the principles or essentials 
of the first embodiment of the present invention. 
Next, a second embodiment of the present invention will be described 
hereinbelow. In the case of this embodiment, the sensitivity modulation is 
performed by using CMD image sensor disclosed in Japanese Patent Laid-Open 
No. 60-206063/1986 Official Gazette as the two-dimensional image sensor of 
the first embodiment of the present invention. As a result of employing 
such a CMD image sensor, the second embodiment of the present invention 
can achieve the sensitivity modulation at a higher frequency in comparison 
with the case of employing an ordinary image sensor that uses CCDs or the 
like. The second embodiment of the present invention, therefore, can 
accomplish a high-accuracy two-dimensional rangefinding operation. 
FIGS. 8 and 9 are diagrams respectively showing the internal potential 
distribution of the CMD image sensor in the case that the gate potential 
of CMDs composing the CMD image sensor is set at (-1) volt (V) and the 
internal potential distribution thereof in the case that the gate 
potential of the CMDs is set at (-6) V. Hereinafter, the principle of the 
method for modulating the light-sensing sensitivity of the CMD will be 
described by referring to these diagrams for showing the internal 
potential distributions of the CMD image sensor. Incidentally, in these 
figures, reference numeral 31 designates a gate electrode; 32 a drain 
diffusion layer; 33 the center of the source diffusion layer; and 34 a 
silicon surface; 35 the direction of the depth of a substrate. The hole of 
a hole-electron pair, which is generated between each of saddle points A 
and B of these figures and the gate electrode, travels according to the 
potential gradient in the substrate and is stored in a portion under the 
gate electrode. In other words, as a result of the behavior of the hole 
generated in a region between the saddle point A or B of potential and the 
gate electrode, the volume of a portion acting as a light receiving region 
is small in the case that V.sub.G of FIG. 8 is (-1) V. This is because the 
saddle point A of potential is close to the (silicon) surface. Thus, the 
ratio of the number of holes stored in the portion under the gate 
electrode to the number of all holes generated therein is small. In 
contrast, in the case of FIG. 9 in which V.sub.G is (-6) V, the saddle 
point B is formed in a deeper portion in comparison with the case that 
V.sub.G is (-1) V. Thus, the volume of a portion acting as a light 
receiving region, which is indicated by dashed curve, is large. 
Consequently, the ratio of the number of holes stored in the portion under 
the gate electrode to the number of all holes generated therein is large. 
Namely, if the gate potential V.sub.G is modulated between (-1) V and (-6) 
V while an image of the object is formed and the two-dimensional image 
sensor performs an image pickup operation, the light-sensing sensitivity 
is also modulated. Thereby, the two-dimensional rangefinding sensor as 
above described can be realized. 
Meanwhile, there is a method for modulating the light-sensing sensitivity 
of the CMD image sensor other than the method of modulating the gate 
potential. For example, the light-sensing sensitivity can be modulated to 
a necessary extent by modulating the substrate potential V.sub.SUB. 
When V.sub.SUB =-20 V, the potential distribution is similar to that of 
FIG. 8. In contrast, when V.sub.SUB =-2 V, the potential distribution is 
similar to that of FIG. 9. Similarly as in the case of changing the gate 
potential, the internal potential gradient of the substrate changes 
according to the substrate voltage. Thus, when V.sub.SUB =-20 V, the 
saddle point of potential is formed in a portion near to the surface. 
Consequently, the volume of a portion acting as a light receiving region 
is small. Moreover, the ratio of the number of holes stored in the portion 
under the gate electrode to the number of all holes generated therein is 
small. In contrast, in the case that V.sub.SUB is (-2) V, the saddle point 
is formed in a deeper portion in comparison with the case that V.sub.SUB 
is (-20) V. Thus, the volume of a portion acting as a light receiving 
region, is large. Consequently, the ratio of the number of holes stored in 
the portion under the gate electrode to the number of all holes generated 
therein is large. Namely, if the substrate potential V.sub.SUB is 
modulated between (-2) V and (-20) V while an image of the object is 
formed and the two-dimensional image sensor performs an image pickup 
operation, the light-sensing sensitivity is modulated, similarly as in the 
case of modulating the gate potential. Thereby, the two-dimensional 
rangefinding sensor as above described can be realized. 
Further, another example of the method for performing the sensitivity 
modulation by using the CMD image sensor is the combination of the 
aforementioned gate-potential modulation and the aforesaid 
substrate-potential modulation. In the case of this method, the 
sensitivity modulation is achieved by performing a bias modulation between 
a high-sensitivity state, in which the gate potential V.sub.G is (-6) V 
and the substrate voltage V.sub.SUB is (-2) V, and a low-sensitivity state 
in which the gate potential V.sub.G is (-1) V and the substrate voltage 
V.sub.SUB is (-20) V. In accordance with this method, the mutually 
potentiating effects of the gate-potential modulation and the 
substrate-voltage modulation can be obtained. Thus, a high degree of the 
sensitivity modulation effects can be realized in comparison with the 
sensitivity modulation effects obtained in the case of performing the bias 
modulation of the gate potential and that of the substrate voltage 
individually. It is demonstrated by simulation and experiment conducted by 
the applicant of the present application that when incident light has a 
wavelength .lambda. of 660 nanometers (nm), the bias modulation using the 
combination of V.sub.G (=-6 V/-1 V) and V.sub.SUB (=-2 V/-20 V) increases 
the sensitivity modulation effects about three-fold. 
Next, the practical configuration of the entire two-dimensional CMD image 
sensor of such a type will be described with reference to the circuit 
diagram of FIG. 11. First, CMDs 41-11, 41-12, . . . , 41-mn, which 
respectively constitute picture elements, are placed in a matrix-like 
arrangement. Further, a video bias V.sub.D (&gt;0) is applied in common to 
the drain of each of the CMDs. The gate terminals of the CMDs of the group 
disposed in X-direction are connected to row lines 42-1, 42-2, . . . , 
42-m, respectively. Moreover, the source terminals of the CMDs of the 
group disposed in Y-direction are connected to column lines 43-1, 43-2, . 
. . , 43-n, respectively. The column lines 43-1, 43-2, . . . , 43-n are 
connected to a signal line 45 through column selecting transistors 44-1, 
44-2, . . . , 44-n, respectively. The signal line 45 is connected to a 
pre-amplifier (not shown) of the current-voltage conversion type, whose 
input (terminal) is further virtually grounded. Moreover, video signals 
are read out at the output terminal of the pre-amplifier as a time series 
of signals. Moreover, the row lines 42-1, 42-2, . . . , 42-m are connected 
to a vertical scanning circuit 46. Furthermore, signals .PHI..sub.G1, 
.PHI..sub.G2, . . . , .PHI..sub.Gm are applied to the row lines 42-1, 
42-2, . . . , 42-m, respectively. The gate terminals of the column 
selecting transistors 44-1, 44-2, . . . , 44-n are connected to a 
horizontal scanning circuit 47. Further, signals .PHI..sub.S1, 
.PHI..sub.S2, . . . .PHI..sub.Sn are applied to the column lines 44-1, 
44-2, . . . , 44n, respectively. Incidentally, each of the CMDs is formed 
on the same substrate to which the substrate voltage V.sub.SUB is applied. 
Additionally, the vertical scanning circuit 46 and the horizontal scanning 
circuit 47 are driven in response to start pulses of a single kind and to 
clock pulses of two kinds. 
FIGS. 12 and 13 are diagrams showing the waveforms of signals for 
illustrating an operation of the two-dimensional CMD image sensor of FIG. 
11. Incidentally, FIG. 13 is a diagram for showing the waveforms of 
signals, which is continued from FIG. 12. In these figures, a signal G-n 
is shown as a typical example of signals applied to the row lines 42-1, 
42-2, . . . , 42-m. The voltage represented by the row-line pulse can be a 
readout gate voltage V.sub.RD, a reset voltage V.sub.RS, an overflow 
voltage V.sub.OF or an accumulation voltage V.sub.AC. In the case of a 
pulse corresponding to a row line which is not selected, the voltage 
represented by such a pulse is the accumulation voltage V.sub.AC during a 
horizontal effective period of a video signal. Further, during a 
horizontal blanking period thereof, the voltage represented by such a 
pulse is the overflow voltage V.sub.OF. In the case of a pulse 
corresponding to a row line which is selected, the voltage represented by 
such a pulse is the read-out gate voltage V.sub.RD during a horizontal 
effective period of a video signal. Further, during a horizontal blanking 
period thereof, the voltage represented by such a pulse is the reset 
voltage V.sub.RS. Thus, after a start pulse .PHI..sub.VST is inputted, the 
row lines are selected in sequence according to the two-phase clock pulses 
.PHI..sub.V1 and .PHI..sub.V2 inputted to the vertical scanning circuit 
46. Further, during the selection of the row line, a start pulse 
.PHI..sub.HST is inputted and signals corresponding to the picture 
elements are read out to the signal line 45 serially according to the 
two-phase clock pulses .PHI..sub.H1 and .PHI..sub.H2 inputted to the 
horizontal scanning circuit 47. 
Next, a practical operation of the two-dimensional rangefinding sensor 
using such a two-dimensional CMD image sensor will be described 
hereinbelow. In the term 1 of FIG. 12, the two-dimensional CMD image 
sensor performs an ordinary reading operation. This operation serves as a 
reset operation of clearing the signal charge of the two-dimensional 
rangefinding sensor. Therefore, this ordinary operation is not always 
necessary, and another driving method may be employed as long as the reset 
of the signal charge can be achieved by performing such a driving method. 
Moreover, in the case that this sensor is continuously operated as the 
two-dimensional rangefinding sensor, an operation of reading a signal in 
the term 5 (to be described later) can be simultaneously utilized as the 
operation of resetting the signal charge. 
Next, in the term 2, input pulses to each of the scanning circuits 46 and 
47 are fixed or kept in a state by which each picture element is put into 
a light receiving condition. Thereafter, for a time period T.sub.1, (the 
condition of) the light source is modulated in response to a clock pulse 
having a frequency f. Then, the object is illuminated with light emitted 
from such a light source. Further, the substrate voltage V.sub.SUB of the 
CMD image sensor is modulated in synchronization with the clock pulse 
having the frequency f. Subsequently, in the term 3, the ordinary reading 
operation is performed on the CMD image sensor, so that signals 
accumulated therein as a result of the illumination modulation and the 
sensitivity modulation performed in the term 2 are read out therefrom. 
Incidentally, H.sub.1 designates a read-out signal at that time. 
Moreover, in the subsequent term 4, input pulses to each of the scanning 
circuits 46 and 47 are fixed in the state by which each picture element is 
put into the light receiving condition, similarly as in the case of the 
operation performed in the term 2. Thereafter, for a time period T.sub.2, 
the light source is modulated in response to a clock pulse having a 
frequency f. Then, the object is illuminated with light emitted from such 
a light source. However, this time, the sensitivity modulation is not 
performed differently from the case of the operation performed in the term 
2. Further, the substrate voltage V.sub.SUB of the CMD image sensor is set 
at a direct-current (DC) voltage, at which an ordinary image pickup 
condition is realized. Subsequently, in the term 5, the ordinary reading 
operation is performed on the CMD image sensor, so that signals 
accumulated in each picture element in consequence of the illumination 
modulation performed in the term 4 are read out therefrom. Incidentally, 
H.sub.2 designates a read-out signal at that time. 
Further, data read out as a result of the sequence of these operations are 
given by the following equations (8), (9), (10), (11) and (12): 
EQU H.sub.i =h.sub.1 .multidot.t+h(z).multidot.t+h.sub.2 .multidot.t(8) 
EQU h.sub.i =k.sub.1 .multidot.I(x,y) (9) 
EQU h(z)=k.sub.2 .multidot.I(x,y).multidot.f(z) (10) 
where i=1, 2 and h, designates a DC component; h.sub.2 the current level of 
a dark current; h(z) a modulation detection component; t time; k.sub.1, 
k.sub.2 proportional constants; I(x,y) two-dimensional luminance 
information concerning an object; and f(z) range information concerning 
the object. 
EQU H.sub.1 =T.sub.1 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 f(z)}+h.sub.2 
! (11) 
EQU H.sub.2 =T.sub.2 .multidot.{I(x,y)k.sub.1 +h.sub.2 } (12) 
The aforementioned operations will be described hereunder by considering 
the correspondence relation between this embodiment and the first 
embodiment of FIGS. 4 to 7. First, a sensitivity modulation pulse 
.PHI..sub.SM is applied from the control signal generator 14 to the 
sensitivity modulation driving portion 11. Then, the pulse driving of the 
voltage V.sub.SUB applied to the substrate of the CMD image sensor 1 is 
carried out in response to the pulse .PHI..sub.SM. On the other hand, a 
light-source modulation pulse .PHI..sub.LD is applied from the control 
signal generator 14 to the light-source modulation driver 18, so that the 
light source 19 is driven in response to the pulse. The substrate 
application voltage V.sub.SUB is modulated in a period (or cycle) T.sub.0 
for a duration T, between (-2) V and (-20) V according to the pulse 
.PHI..sub.SM of FIG. 6. The light source 19 undergoes luminance modulation 
in a period T.sub.0 for the duration T.sub.1 in synchronization with the 
substrate application voltage V.sub.SUB. In addition, luminance modulation 
is also performed on the light source 19 in the same period To for the 
duration T.sub.2. 
The two-dimensional CMD image sensor 13 has two image capture modes. 
Namely, one of the modes is a range(-data) image capture mode, and the 
other is a luminance image capture mode. The image capture mode used for 
the duration T.sub.1 is the former, namely, the range image capture mode. 
The image capture mode used for the duration T.sub.2 is the latter, 
namely, the luminance image capture mode. The substrate application 
voltage V SUB can change from (-2) V to (-20) V in the range image capture 
mode. As can be seen from FIGS. 8 and 9, the thickness of a photoelectric 
conversion layer in the case of setting the substrate application voltage 
V.sub.SUB at (-2) V and further setting the gate potential V.sub.G at (-6) 
V is far larger than that of the photoelectric conversion layer in the 
case of setting the substrate application voltage V.sub.SUB at (-20) V and 
further setting the gate potential V.sub.G at (-1) V. In other words, the 
case of setting the substrate application voltage V.sub.SUB at (-2) V and 
further setting the gate potential V.sub.G at (-6) V corresponds to the 
case of FIG. 9. Further, the case of setting the substrate application 
voltage V.sub.SUB at (-20) V and further setting the gate potential 
V.sub.G at (-1) V corresponds to the case of FIG. 8. The luminance 
modulation of the light source is performed in such a way that the highest 
light output is obtained when setting the substrate application voltage 
V.sub.SUB at (-2) V and further setting the gate potential V.sub.G at (-6) 
V, and that the lowest light output is obtained when setting the substrate 
application voltage V.sub.SUB at (-20) V and further setting the gate 
potential V.sub.G at (-1) V. In the luminance image capture mode, the 
sensor is established in such a manner that the high sensitivity state 
obtained by setting the substrate application voltage V.sub.SUB at (-2) V 
and further setting the gate potential V.sub.G at (-6) V is maintained and 
that the substrate application voltage V.sub.SUB is maintained at a 
constant value for the duration T.sub.2. 
FIG. 14 is a graph for illustrating the relations among the relative 
sensitivity R of the substrate, the thickness T .mu.m of the photoelectric 
conversion layer and the voltage level V.sub.SUB of the substrate. When 
the substrate voltage V.sub.SUB increases negatively, the thickness T of 
the photoelectric conversion layer decreases and the relative sensitivity 
(namely, the relative light absorptance) R of the substrate becomes lower. 
Conversely, when the substrate (application) voltage V.sub.SUB is close to 
0 (V), the thickness T of the photoelectric conversion is large and the 
relative sensitivity R is high. 
Meanwhile, in the aforesaid range image capture mode, range image 
information concerning an object is affected by the luminance information 
concerning the object, as will be described hereinbelow. Thus, there is 
the necessity of making some compensation for the range information. 
Practically, if T denotes an integral time, the number H of holes stored 
in the gate of each of the picture elements of the two-dimensional CMD 
image sensor is obtained from the following equations (13), (14) and (15): 
EQU H=h.sub.1 .multidot.T+h(z).multidot.T+h.sub.2 .multidot.T (13) 
EQU h.sub.1 =k.sub.1 .multidot.I(x,y) (14) 
EQU h(z)=k.sub.2 .multidot.I(x,y).multidot.f(z) (15) 
where h.sub.1 designates a DC component; h.sub.2 the current level of a 
dark current; h(z) a modulation detection component; k.sub.1, k.sub.2 
proportional constants; I(x,y) two-dimensional luminance information 
concerning an object; and f(z) range information concerning the object. 
The range information f(z) is given by the following equation (16) obtained 
from the equations (3) and (4): 
EQU f(z)=1-4.multidot.z/(T.sub.0 .multidot.c) (16) 
where T.sub.0 designates a single period of the luminance modulation. 
An output obtained in each of the image capture modes is given by the 
following equations (17) and (18): 
EQU H.sub.1 =T.sub.1 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 
.multidot.f(z)}+h.sub.2 ! (17) 
EQU H.sub.2 =T.sub.2 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 
.multidot.1}+h.sub.2 ! (18) 
where H.sub.1 designates an output obtained in the range image capture 
mode; and H.sub.2 an output obtained in the luminance image capture mode. 
Namely, H.sub.1 and H.sub.2 correspond to the range image capture mode and 
the luminance image capture mode, respectively. Further, H.sub.1 and 
H.sub.2 correspond to data represented by a signal .PHI..sub.DATA read out 
in the terms T.sub.10 and T.sub.20, respectively. 
The range information f(z) given by the equation (16) is modified as the 
following equation (19) by using the equations (17) and (18): 
EQU f(z)=(T.sub.2 /T.sub.1).multidot.(H.sub.1 -h.sub.2 
.multidot.T.sub.1)/(H.sub.2 -h.sub.2 
.multidot.T.sub.2).multidot.(1+k.sub.1 /k.sub.2)-k.sub.1 /k.sub.2(19) 
In the equation (19), (k.sub.1 /k.sub.2) denotes the ratio of the intensity 
of the DC component to the intensity of the modulation component in the 
case that the luminance modulation of the light source and the sensitivity 
modulation of the image sensor are carried out. Further, as is understood 
from this equation, the range information is obtained as the difference 
between the first term and the second term of the right side of the 
equation (19). Thus, unless the intensity of the modulation component is 
higher than that of the DC component, the accuracy of the range 
information is deteriorated owing to a cancelling error (namely, to the 
degradation of the S/N). Therefore, in order to limit the off-set output 
of the dark current of the image sensor to a low level, it is necessary to 
enhance the rangefinding accuracy by eliminating the influence of 
background light which is similar to that of the DC component. As 
countermeasures against this, it is preferable that the sensor is provided 
with an optical band-pass filter adapted to transmit only light of the 
wavelength band in the vicinity of the wavelength of light emitted from 
the illumination light source and further adapted to cut off the 
background light. Further, in the case of using the optical band-pass 
filter 16, it is desirable that an object is illuminated with light 
emitted from a light source, whose energy is concentrated on a specific 
wavelength band. Thus, a light-emitting diode (LED) or a laser diode (LD) 
is usually used as the light source. 
Incidentally, to enhance the rangefinding accuracy, judging from the 
principle of the detecting method, it is preferable that the number of 
iteration of the cycle or period of the sensitivity modulation is set at a 
large value. Thus, the integral time (namely, a time period between a 
moment, at which the picture-element charge is reset, and another moment 
at which the picture-element charge is readout) becomes inevitably long. 
In the equation (19), h.sub.2 is a factor representing the generation rate 
of the dark current. Therefore, if the integral time T.sub.1 or T.sub.2 
becomes long, the accuracy of the range information is degraded owing to 
the cancelling error. Consequently, when rangefinding with high accuracy, 
the factor h.sub.2 representing the generation rate of the dark current 
should be reduced. Improvements for realizing this will be described in 
the aftermentioned description of a third embodiment of the present 
invention. In the foregoing description of the second embodiment, the 
problems of the intensity of the modulation component and the dark current 
have been discussed. These problems do not related only to the sensors 
using the CMD image sensors but are common to all of the two-dimensional 
rangefinding sensors of the present invention. 
Generally, when measuring distances of several to ten meters by means of 
the range finder, the period T.sub.0 of modulation signals is 0.1 .mu.m or 
so. However, in the case where the range or distance is 1 m or so, 
high-accuracy three-dimensional image input is strongly demanded. In this 
case, the period T.sub.0 of modulation signals is 10 ns or so. Thus, the 
sensitivity modulation should be achieved at the frequency of the order of 
100 megahertz (MHz). Consequently, there is the necessity of achieving the 
sensitivity modulation of the two-dimensional image sensor, which is the 
essential element (or principle) of the present invention, at a high 
speed. In the case of ordinary solid-state image sensors, the typical one 
of which is a CCD, the photoelectric conversion region of the picture 
element is not completely depleted, charges generated by the incidence of 
light in a semiconductor substrate are diffused therein according to the 
gradient of the density of electric charges. Therefore, even if trying to 
achieve the sensitivity modulation by the change of the volume of the 
photoelectric conversion region at a high speed through bias modulation, 
high-speed sensitivity modulation cannot be realized because of the 
low-speed re-arrangement of the diffused charges. In contrast with this, 
in the case of using the CMD image sensor, the photoelectric conversion 
region of the picture element is completely depleted, so that all of the 
charges generated by the incidence of light in a semiconductor substrate 
are moved in the presence of the electric field therein. Thus, the 
movement of the charges due to the diffusion in accordance with the 
gradient of the density of electric charges does not occur at all. 
Consequently, the high-speed sensitivity modulation, which is impossible 
for the ordinary solid-state image sensors (typically, a CCD) whose 
photoelectric conversion region is not completely depleted, can be 
achieved. Namely, a characteristic aspects of this embodiment reside in 
that the CMD image sensor is used as the solid-state image sensor, that 
the high-speed sensitivity modulation is performed by modulating the 
substrate bias and that a two-dimensional rangefinding sensor, which can 
rangefind with high rangefinding accuracy, is realized. 
Next, the third embodiment of the present invention will be described with 
reference to FIG. 15. In this figure, same reference numerals designate 
same or corresponding functional components illustrated in FIG. 3. 
Further, the description of such components is omitted herein, for 
simplicity of description. As stated in the description of the second 
embodiment, it is preferable for enhancing the rangefinding accuracy that 
the number of iteration of the cycle or period of the sensitivity 
modulation is set at a large value. Thus, the integral time (namely, a 
time period between a moment, at which the picture-element charge is 
reset, and another moment at which the picture-element charge is readout) 
becomes inevitably long. Consequently, the dynamic range is lowered owing 
to the dark current. This affects actual ranging accuracy. The third 
embodiment of the present invention is created to resolve such a problem. 
As illustrated in FIG. 15, a cooling unit 6 such as a Peltier device is 
provided in a rear portion of the two-dimensional solid-state image 
sensor. Thus, the two-dimensional dark current can be reduced by cooling 
the device by means of the cooling unit. With such a configuration, the 
long-time integration, which is impossible for the ordinary device, can be 
achieved. This results in realizing a tow-dimensional rangefinding sensor, 
whose rangefinding accuracy is enhanced. 
Next, a fourth embodiment of the present invention will be described 
hereinafter. This embodiment is a two-dimensional rangefinding sensor 
using CMDs as light receiving picture-elements, which is able to achieve 
the sensitivity modulation at a higher speed. First, a sectional view of 
the configuration of a CMD picture element is illustrated in FIG. 16. The 
configuration of this CMD picture element will be briefly described again 
by referring to this figure. Reference numeral 51 designates a p.sup.- 
-substrate; 52 an n.sup.- -channel layer; 53 an n.sup.+ source diffusion 
layer; 54 an n.sup.+ drain diffusion layer; 55 a gate insulation film; 56 
a gate poly-silicon electrode; 57 a source electrode; and 58 a drain 
electrode. The light receiving operation of this CMD picture element has 
been described with reference to FIG. 5. Therefore, the description of 
such an operation is omitted herein. 
Meanwhile, in the case of the CMD image sensors using CMD picture elements, 
a p.sup.- -substrate, whose impurity concentration is about 
6.times.10.sup.13 cm.sup.-3, is used without any change as the 
back-gate-electrode substrate of the CMD picture element. Further, an 
n.sup.- -channel layer, whose impurity concentration is about 
1.times.10.sup.13 cm.sup.-3 and thickness is 3 to 10 .mu.m, is formed 
thereon. Namely, as illustrated in FIGS. 17A and 17B, a high-resistance 
n.sup.- -channel layer, which is X.sub.1 in thickness, is formed on the 
p.sup.- -substrate, whose impurity concentration is constant substrate 
concentration C.sub.SUB. Then, in the case of modulating the sensitivity 
of the CMD picture element by changing the potential V.sub.SUB applied to 
the p.sup.- -substrate 51, the maximum modulation frequency f.sub.MAX is 
given by the following equation (20): 
EQU f.sub.MAX =1/(C.multidot.R.sub.SUB) (20) 
where C designates the capacitance of the surface depletion layer of the 
CMD, which is obtained by the following equation (21). Namely, 
EQU C=k.sub.si .multidot..di-elect cons..sub.o /X.sub.dep (21) 
where k.sub.si =11.8 and .di-elect cons..sub.o =8.86.times.10.sup.-14 F/cm. 
Additionally, in the case of the CMD picture element, X.sub.dep is 10 
.mu.m on average. When calculating by using these numerical values, the 
capacitance C of the surface depletion layer is found as follows: 
EQU C=1.0.times.10.sup.-9 F/cm (22) 
Further, in the case that the concentration in the p.sup.- -substrate is 
6.times.10.sup.-13 cm.sup.-3, the resistivity .rho. is 300 .OMEGA.cm. 
Furthermore, the resistance per unit area R.sub.SUB is expressed by the 
following equation (23): 
EQU R.sub.SUB =.rho..multidot.t.sub.SUB (23) 
Incidentally, the thickness t.sub.SUB of the substrate is 500 82 m in the 
case of a conventional silicon substrate. Then, the following equation 
(24) is obtained by substituting this value in the equation (23). 
EQU R.sub.SUB =15 .OMEGA.cm.sup.2 (24) 
Thus, f.sub.MAX is 64 MHz by the equation (20). 
Meanwhile, in the case of the sensitivity modulation method, the findable 
range L is given by the distance which light can travel in a time period 
corresponding to a half of the modulation frequency f.sub.MAX. Namely, L 
is obtained by the following equation (25): 
EQU L=c/f.sub.MAX /2 (25) 
As a result of substituting c=3.times.10.sup.8 m/s and f.sub.MAX =64 MHz in 
the equation (25), L is obtained by the following equation (26): 
EQU L=2.34 m (26) 
In the field in which the inputting of three-dimensional information is 
demanded, high-accuracy measurement of a short range, which is far shorter 
than the aforementioned value of L, is sometimes required. In accordance 
with this embodiment, there is provided a two-dimensional CMD image sensor 
which meet such a requirement. To obtain higher range resolving power and 
rangefind with high accuracy, f.sub.MAX should be increased. Further, 
f.sub.MAX is given by the following equation (27) by using the equation 
(20), (21) and (22): 
EQU f.sub.MAX =1/(C.multidot.R.sub.SUB)=X.sub.dep /(k.sub.si .di-elect 
cons..sub.o .multidot..rho..multidot.t.sub.SUB) (27) 
where X.sub.dep, k.sub.si and .di-elect cons..sub.o are values inherent in 
the CMD and thus cannot be changed largely. Namely, f.sub.MAX can be 
practically increased by lowering the resistivity .rho. or by reducing the 
thickness t.sub.SUB of the substrate. 
First, the process for lowering the resistivity .rho. will be described 
hereunder. The inventor of the present invention has previously proposed a 
method of producing a CMD by forming both of the n.sup.- -channel layer 
and the p.sup.- -substrate by performing the epitaxial (growth) method, in 
Japanese Patent Laid-Open. No. 3-114260/1991. FIG. 18A illustrates a 
sectional view of the configuration of the substrate of the CMD image 
sensor obtained by performing such a producing method. Namely, a p.sup.- 
-channel layer 61 having the concentration C.sub.SUB and an n.sup.- 
-channel layer 62 having the concentration C.sub.epi are formed serially 
by performing the epitaxial method on the p-type substrate 60 having the 
concentration C.sub.SUB '. An example of the concentration distribution in 
the case of such a substrate is illustrated in FIG. 18B. Because the 
p.sup.- -channel layer 61 and the n.sup.- -channel layer 62 are formed by 
performing the epitaxial method, the concentration C.sub.SUB ' of the 
p-type substrate 60 can take an arbitrary value. However, in the case of 
FIG. 18B, the concentration C.sub.SUB ' is set in such a manner that 
C.sub.SUB '&gt;C.sub.SUB, for the purpose of increasing f.sub.MAX. Namely, in 
consequence of setting the concentration C.sub.SUB ' in such a way that 
this inequality condition (C.sub.SUB '&gt;C.sub.SUB) is satisfied, the 
resistivity .rho. of the substrate is reduced. Thus, as is seen from the 
equation (27), f.sub.MAX is enhanced. For example, the resistivity .rho. 
can be reduced to 1 .OMEGA.cm if this embodiment is established in such a 
way that C.sub.SUB =2.times.10.sup.16 cm.sup.-3 instead of setting 
.rho.=300 .OMEGA.cm (C.sub.SUB =6.times.10.sup.13 cm.sup.-3), similarly as 
in the conventional case. Namely, in comparison with the conventional case 
that .rho.=300 .OMEGA.cm, the maximum modulation frequency f.sub.MAX can 
be improved by a factor of 300. 
Next, the reduction in thickness t.sub.SUB of the substrate will be 
described hereinbelow. In the case of the conventional method of producing 
a CMD image sensor, a substrate, whose thickness t.sub.SUB is 500 .mu.m 
when completed, is used without any change. However, the thickness 
t.sub.SUB (=500 .mu.m) of the substrate can be reduced by performing the 
back(-surface) lapping method in the last process of the method of 
producing the CMD image sensor. For example, thickness t.sub.SUB of the 
substrate can be reduced to 100 .mu.m if a part, whose thickness is 400 
.mu.m, of the substrate, whose thickness t.sub.SUB is 500 .mu.m, is 
removed from the back surface thereof by the lapping method. Consequently, 
in this case, the maximum modulation frequency f.sub.MAX can be improved 
by a factor of 5 in comparison with the conventional case that the 
thickness of the substrate is not reduced. 
Thus, a two-dimensional rangefinding sensor, whose modulation frequency can 
be increased, namely, range-information resolving power (or accuracy) can 
be enhanced, is able to be realized if at least one of the following two 
techniques (1) and (2) is applied to a two-dimensional 
variable-sensitivity CMD image sensor as above described: 
(1) An n.sup.- -channel layer and a p.sup.- -channel layer are formed on a 
p-substrate by using the epitaxial growth method, and the impurity 
concentration of the p-substrate is increased in such a manner as to be 
higher than the concentration of the p.sup.- -channel layer, and thereby, 
the resistivity .rho. is lowered. 
(2) The thickness t.sub.SUB of the substrate is reduced by using the back 
lapping method. 
Needless to say, the maximum modulation frequency f.sub.MAX can be further 
enhanced by using both of the techniques (1) and (2). 
Next, a fifth embodiment of the present invention, which utilizes an 
electron-bombarded AMI image sensor as the two-dimensional image sensor, 
will be described hereinbelow. Incidentally, the technical details of the 
electron-bombarded AMI image sensor are described in an article entitled 
"An Electron-Bombarded Amorphous Si/AMI Image Intensifier", T. Kawamura et 
al., Proceedings. Conference on Photoelectronic Image Devices, London, 
September 1991, pp. 175-182, published by IOP Publishing Ltd., 1992. As 
illustrated in FIG. 19, in the case of the electron-bombarded AMI image 
sensor of this embodiment, an aluminum electrode 71, a nitride film 72, an 
amorphous silicon hydride layer 73 and a nitrogen-doped amorphous silicon 
hydride layer 74 are superposed on a conventional AMI image sensor 75 in 
such a manner that the amorphous silicon hydride layer 73 is placed in the 
central portion of the laminated layers. Further, the top aluminum 
electrode 71 is bombarded with an electron flow 76 by using a device, 
which is similar to a conventional invertor-type image intensifier, 
similarly as in the case of an electron-bombarded AMI image tube of FIG. 
20. Namely, the electron flow 76 emitted from a photoelectric film is 
accelerated to a high velocity and then the accelerated electron flow is 
injected into the amorphous silicon hydride layer 73 so that a large 
number of hole-electron pairs are generated. Thereby, the amount of signal 
current flowing through the AMI image sensor 75 is increased. 
Consequently, high sensitivity can be achieved. Incidentally, in FIG. 20, 
reference numeral 81 designates an AMI image sensor; 82 an amorphous 
silicon layer; 83 a ceramic package; 84 an electrode; 85 a glass tube; 86 
a fiber plate; 87 a photoelectric film; and 88 an anode: 
Referring next to FIG. 21, there is shown a graph for illustrating an 
example of the relation between the acceleration voltage for accelerating 
a bombarding electron flow in the case of the electron-bombarded AMI image 
sensor and the obtained electron flow. As is seen from this figure, for 
instance, the signal current is about 5 nA when the acceleration voltage 
is 4 kV. Further, the signal current is about 20 nA when the acceleration 
voltage is 10 kV. The ratio of the sensitivities in both acceleration 
voltages is a factor of 4. Thus, if the electron-bombarded AMI image 
sensor is used as the two-dimensional image sensor and the acceleration 
voltage is modulated in synchronization with the luminance modulation 
frequency of the light source in the two-dimensional rangefinding sensor 
described in the foregoing description of the first embodiment, the 
sensitivity can be modulated in synchronization with the luminance 
modulation frequency, thereby, desirable characters of the two-dimensional 
rangefinding sensor can be obtained. The characteristic feature of the 
fifth embodiment resides in that the high-sensitivity characteristics of 
the electron-bombarded AMI image sensor can be utilized. However, 
generally, in the case of the two-dimensional rangefinding sensor of the 
present invention, the intensity of light reflected from a distant object 
decreases inversely in proportion to the square of the distance between 
the object and the sensor. Thus, the signal is liable to be weak. This 
affects the measurement accuracy. Therefore, it is extremely important for 
operating or utilizing the sensor that large signal current can be 
obtained by employing the electron-bombarded AMI image sensor in this 
embodiment. 
Next, a sixth embodiment of the present invention, which a laminated AMI 
image sensor is used as the two-dimensional image sensor of the 
two-dimensional rangefinding sensor, will be described hereinbelow. The 
technical details of the laminated AMI image sensor are described in an 
article entitled "An Amplified MOS Imager overlaid with an Amorphous Se 
Photoconductive Layer on its Surface", Ando et al., NHK Broadcasting 
Technical Research Laboratories R & D, No. 32, pp. 28-36, August 1994. In 
the case of the laminated AMI image sensor employed in this embodiment, as 
illustrated in FIG. 22, a first inter-layer insulation film 107, a second 
layer aluminum electrode 109, a second inter-layer insulation film 106, a 
third layer aluminum electrode 105, a fourth layer aluminum electrode 104, 
an amorphous silicon photoconductive film 103, a CeO film 102 and a 
transparent electrode (ITO) 101 are superposed on an AMI of the 
configuration which is similar to that of a conventional AMI and is 
produced by forming an n-type diffusion layer 113 or the like in a p-type 
well 111 formed on an n-type silicon substrate 112 and by performing an 
interconnecting process on the surface thereof by using poly-silicon 110 
and a first aluminum electrode 108. 
Next, an operation of the laminated AMI image sensor having such a 
configuration will be described hereinbelow. When applying a positive 
voltage to the transparent electrode (ITO) 101 provided in the upper 
portion relative to the AMI provided in the lower portion, holes travel 
toward the AMI among hole-electron pairs generated by photons having 
impinged on the amorphous silicon photoconductive film 103. Thereafter, 
the holes reach the signal electrode of the AMI and are stored therein as 
signal charges. Conversely, when applying a negative voltage to the 
transparent electrode (ITO) 101 provided in the upper portion relative to 
the AMI provided in the lower portion, electrons travel toward the AMI 
among hole-electron pairs generated by photons having impinged on the 
amorphous silicon photoconductive film 103. Thereafter, the electrons 
reach the signal electrode of the AMI and are stored therein as signal 
charges. Hence, if the voltage applied to the transparent electrode (ITO) 
101 is switched between positive and negative voltages during an image 
pickup operation in this laminated AMI image sensor, the charge caused to 
reach the signal electrode of the AMI provided in the lower portion is 
also switched between a hole and an electron. If a constant number of 
photons arrive there like DC component during the image pickup operation 
is performed switching the applied voltage between positive and negative 
voltages, a total amount of the charges generated due to the photons comes 
closer to 0 after stored therein, as illustrated in FIG. 23A. This is 
because holes and electrons successively reach the signal electrode of the 
AMI provided in the lower portion. In contrast, if the number of photons 
coming from the object changes during the image pickup operation, the 
total amount of the stored charge changes correspondingly to the change in 
the number of photons, as illustrated in FIG. 23B. 
Next, the configuration of the two-dimensional rangefinding sensor using 
the laminated AMI image sensor will be described hereunder with reference 
to FIG. 24. The luminance modulation of the light source 207 is conducted 
by the light-source driver 206 at a predetermined frequency. Then, an 
image of a three-dimensional object 208 illuminated with illumination 
light, which is emitted from this light source 207, is formed on the 
laminated AMI image sensor 201 through the image-formation optical system 
209. Further, the laminated AMI image sensor 201 is driven by the control 
signal generator 204 and the image sensor driver 202. Then, the 
information of the image sensor may be readout. At that time, the 
sensitivity modulation driver 203 changes the potential of the transparent 
electrode of the laminated AMI image sensor 201 in response to a signal 
sent from the control signal generator 204. Thereby, the sensitivity is 
modulated. Moreover, there is caused a shift in the phase of the 
illumination light received on each picture element, which corresponds to 
the structure of the three-dimensional object 208. Thus, a large number of 
signal charges are stored in a picture element on which the phase of the 
received illumination light matches the light-receiving sensitivity of the 
laminated AMI image sensor 201. In contrast, only a small number of signal 
charges are stored in a picture element on which the phase of the received 
illumination light does not match the light-receiving sensitivity of the 
laminated AMI image sensor 201. In other words, the detection of the phase 
of the received illumination light is performed at each picture element. 
The amount of electric charge stored therein as the result of the 
detection represents range information concerning the three-dimensional 
object 208 directly. Incidentally, in FIG. 24, reference numeral 205 
designates a signal processor for processing an image pickup signal sent 
from the image sensor 201. 
Furthermore, in the case of the sensor of the present invention, the 
electric charge generated by the photons coming like DC components is 
integrated within a storing time period. However a total amount of the 
integrated charges becomes nearly to 0, because the sensitivity of the 
image sensor is modulated in an alternating current manner as above 
described. Thus, only the electric charge generated by the photons, which 
are caused by the illumination light from the modulated light source to 
impinge on each picture element of the image sensor, is stored therein. 
Therefore, this embodiment has a characteristic feature in that a signal, 
which has a preferable S/N and little offset, can be obtained. Moreover, 
in the case of this embodiment, the influence of the direct-current-like 
reflected light is canceled in the stage in which the electric charge is 
obtained by the photoelectric conversion. Thus, the optical band-pass 
filter 16, which is required by the first embodiment of FIG. 4, for 
eliminating the influence of incident light having a wavelength other than 
that of the illumination light coming from the light source becomes 
unnecessary. Consequently, the configuration of the optical system is 
simplified. This corresponds to the fact that the ideal condition that 
k.sub.1 =0 is realized in the equation (19) which is derived in the 
description of the second embodiment and gives the range information. 
Next, a seventh embodiment of the present invention will be described with 
reference to FIG. 25. In this figure, reference numeral 301 designates a 
two-dimensional CMD image sensor; 302 a sensitivity modulation driver for 
modulating the sensitivity of the image sensor 301; 303 an image sensor 
driver; 304 a control signal generator; 305 a signal processor; 306 a 
light-source driver; 307 a light source; 308 an object; 309 an 
image-formation optical system; and 310 an optical band-pass filter for 
eliminating the influence of background light and for transmitting only 
light, whose wavelength is in the proximity of the wavelength of reflected 
light of the wavelength band of the light source 307 having undergone the 
luminance modulation 
FIG. 26 is a circuit diagram for illustrating the entire configuration of 
the two-dimensional CMD image sensor 301 that is a composing element of 
this embodiment of the present invention. This device is different in the 
vertical scanning circuit 405 from the two-dimensional CMD image sensor 
used in the second embodiment of FIG. 11. This circuit 405 has two 
terminals V.sub.AC1 and V.sub.AC2 for determining potential to be applied 
to each of the gate selecting lines 401-1, 401-2, . . . 401-m within the 
exposure time. Potential to be applied to each of gate selecting lines is 
set at one of the terminals V.sub.AC1 and V.sub.AC2 according to signals 
supplied to a control terminal CNTL. Here, it is assumed for brevity of 
description that for example, the potential corresponding to the terminal 
V.sub.AC1 is applied to the odd-numbered gate selecting lines and the 
potential corresponding to the terminal V.sub.AC2 is applied to the 
even-numbered gate selecting lines. Incidentally, in FIG. 26, reference 
characters 400-11, 400-21, . . . , 400-mn designate CMD picture elements; 
402-1, 402-2, . . . , 402-n designate column lines; 403-1, 403-2, . . . , 
403-n designate column selecting transistors; 404 a signal line; and 406 a 
horizontal scanning circuit. 
Next, an operation of the two-dimensional rangefinding sensor using the CMD 
image sensor 301 having such a configuration will be described by 
referring to FIG. 27. In this figure, .PHI..sub.LD designates the waveform 
of a signal for driving the light source 307; and .PHI..sub.SM the 
waveform of a sensitivity modulation control signal. Further, it is 
assumed that a signal having the waveform for causing the sensitivity 
modulation is applied at the terminal V.sub.AC1 of the CMD image sensor 
301 in synchronization with the sensitivity modulation control signal 
.PHI..sub.SM and that a DC voltage signal having the waveform for causing 
the light receiving operation in a direct-current component manner is 
applied at the terminal V.sub.AC2 thereof. Further, signals G-M and G-Y 
represent the potential of the gate selecting lines adjoining to each 
other. In the term T.sub.1, the light source 307 starts the luminance 
modulation in response to the luminance modulation signal .PHI..sub.LD. 
Then, the sensitivity modulation control signal .PHI..sub.SM is input to 
the sensitivity modulation driver 302 in synchronization with the signal 
.PHI..sub.LD. Subsequently, the sensitivity modulation driver 302 
modulates the sensitivity of the picture elements connected to the 
odd-numbered gate selecting lines of the CMD image sensor 301. The signal 
G-M represents the potential of the odd-numbered gate selecting lines. 
Further, for the term T.sub.1, the potential is modulated in order to 
realize the sensitivity modulation during the light receiving operation. 
On the other hand, the signal G-Y represents the potential of the 
even-numbered gate selecting lines. Furthermore, a DC voltage is applied 
thereto in order to cause the picture element to perform the normal 
direct-current-component-like light receiving operation when receiving 
light. In FIG. 27, reference characters DATA represent data to be read 
from the CMD image sensor 301 for the term T.sub.2 ; and DATA-M data 
corresponding to the sensitivity modulation light-receiving operation. On 
the other hand, the data represented by the signal DATA-Y corresponding to 
the even-numbered gate selecting lines are data corresponding to the 
ordinary direct-current-component-like light receiving operation. 
Here, data H.sub.1 (DATA-M), which corresponds to the sensitivity 
modulation, and data H.sub.2 (DATA-Y), which corresponds to the ordinary 
luminance in the case that the sensitivity modulation is not conducted, 
are obtained as data for adjacent picture elements arranged upwardly (or 
downwardly), on which the image of almost the same point on the object is 
formed, of the CMD image sensor 301. These data are given by the following 
equations (28) and (29) correspondingly to the aforementioned equations 
(17) and (18), respectively. 
EQU H.sub.1 =T.sub.1 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 
.multidot.f(z)}+h.sub.2 ! (28) 
EQU H.sub.1 =T.sub.1 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 
.multidot.1}+h.sub.2 ! (29) 
Thus, the range information is obtained by the following equation (30) 
correspondingly to the equation (19). 
EQU f(z)=(H.sub.1 -h.sub.2 .multidot.T.sub.1)/(H.sub.2 -h.sub.2 
.multidot.T.sub.1).multidot.(1+k.sub.1 /k.sub.2)-k.sub.1 /k.sub.2(30) 
In the case of the aforesaid examples (typically, the second embodiment), 
an image pickup operation in the sensitivity modulation mode and another 
image pickup operation in the luminance modulation mode should be 
performed, namely, the image pickup operations should be performed two 
times. In contrast, in the case of the seventh embodiment, the sensor is 
provided with a single two-dimensional CMD image sensor in which picture 
elements, each of which is operative to perform an image pickup operation 
in an ordinary direct-current-component-mode-like luminance mode, and 
picture elements, each of which is operative to perform an image pickup 
operation in a sensitivity modulation mode, are disposed in an intermixed 
manner. Thereby, both of a modulation-mode image pickup and a 
luminance-mode image pickup can be simultaneously performed in a single 
image pickup operation to thereby obtain two-dimensional range information 
in a short period of time in comparison with the aforementioned examples 
(typically, the second embodiment). 
Next, an eighth embodiment of the present invention will be described 
hereinafter with reference to FIG. 28. In this figure, reference numeral 
501 designates a first two-dimensional image sensor; 502 a second 
two-dimensional image sensor; and 503 a beam splitter constituted by a 
semi-transparent mirror or a prism or the like. The first and second image 
sensor 501 and 502 are disposed at places on the emission surface of the 
beam splitter 503, which are optically equivalent places. Moreover, 
reference numeral 504 designates a sensitivity modulation driver for 
modulating the sensitivity of the first image sensor 501; 505 an image 
sensor driver; 506 a control signal generator; 507 a signal processor; 508 
a light-source driver; 509 a light source; 510 an object; 511 an 
image-formation optical system; and 512 an optical band-pass filter for 
eliminating the influence of background light and for transmitting only 
light, whose wavelength is in the proximity of the wavelength of reflected 
light of the wavelength band of the light source 509 having undergone the 
luminance modulation. 
Next, an operation of the two-dimensional rangefinding sensor having this 
configuration will be described hereunder by referring to FIG. 29. In this 
figure, .PHI..sub.LD designates the waveform of a signal for driving the 
light source 509; SM-A the waveform of a signal for modulating the 
sensitivity of the first image sensor 501; and SM-B the waveform of a 
signal for modulating the sensitivity of the second image sensor 502. 
However, in the case of this embodiment, the sensitivity of the second 
image sensor 502 is not modulated but is performed in the ordinary image 
pickup mode. Further, reference characters DATA-A and DATA-B represent 
data to be read from the first image sensor 501 and data to be read from 
the second image sensor 502, respectively. In the term T.sub.1, the light 
source 509 starts the luminance modulation in response to the luminance 
modulation signal .PHI..sub.LD. Then, the sensitivity modulation control 
signal .PHI..sub.SM is input to the sensitivity modulation driver 504 in 
synchronization with the signal .PHI..sub.LD. Subsequently, the 
sensitivity modulation driver 504 modulates the sensitivity of the first 
image sensor 501. On the other hand, the sensitivity of the second image 
sensor 502 is not modulated but is operated in the ordinary image pickup 
mode. In the term T.sub.2, the illumination by means of the light source 
509 is stopped. Moreover, the first image sensor 501 and the second image 
sensor 502 are driven in order to read the data, respectively, and 
furthermore, output the corresponding data DATA-A and DATA-B, 
respectively. 
Here, these data are given by the following equations (31) and (32) 
correspondingly to the aforementioned equations (17) and (18), 
respectively. 
EQU H.sub.1 =T.sub.1 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 
.multidot.f(z)}+h.sub.2 ! (31) 
EQU H.sub.1 =T.sub.1 .multidot.I(x,y).multidot.{k.sub.1 +k.sub.2 
.multidot.1}+h.sub.2 ! (32) 
Thus, the range information is obtained by the following equation (33) 
correspondingly to the equation (19). 
EQU f(z)=(H.sub.1 -h.sub.2 .multidot.T.sub.1)/(H.sub.2 -h.sub.2 
.multidot.T.sub.1).multidot.(1+k.sub.1 /k.sub.2)-k.sub.1 /k.sub.2(33) 
In the case of the aforesaid examples (typically, the second embodiment), 
an image pickup operation in the sensitivity modulation mode and another 
image pickup operation in the luminance modulation mode should be 
performed, namely, the image pickup operations should be performed two 
times. In contrast, in the case of the eighth embodiment, light beams 
obtained by splitting (the optical path of) the light coming from the 
object by means of the beam splitter are sent to the two image sensors. 
Moreover, two images are formed onto the two image sensors simultaneously. 
Thereby, both of a modulation-mode image pickup and a luminance-mode image 
pickup can be simultaneously performed in a single image pickup operation 
to thereby obtain two-dimensional range information in a short period of 
time in comparison with the aforementioned examples (typically, the second 
embodiment). Furthermore, in the case of the aforementioned seventh 
embodiment of the present invention, the picture elements of a single 
two-dimensional image sensor are divided into a group of the picture 
elements for the sensitivity modulation mode and another group of the 
picture elements for the luminance modulation mode. Thus, the spacial 
resolving power of the seventh embodiment becomes half of that of the 
examples previously described (typically, the second embodiment). In 
addition, the seventh embodiment has an evil effect of using the data 
obtained at adjoining positions, which are different from each other in 
the strict sense, for the calculation of the range information. In the 
case of the eighth embodiment, a two-dimensional rangefinding sensor, 
which resolves such problems, can be provided. 
As above described in the foregoing description of the embodiments, in 
accordance with the present invention, the sensitivity of the 
two-dimensional image sensor is modulated in synchronization with the 
luminance modulation frequency of the illumination light. Thus, the 
distribution of the signal charge on the two-dimensional image sensor 
corresponding to the two-dimensional range information, in other words, 
the range(-data) image corresponding thereto can be directly obtained 
without mechanical scanning with illumination light. 
Although the preferred embodiments of the present invention have been 
described above, it should be understood that the present invention is not 
limited thereto and that other modifications will be apparent to those 
skilled in the art without departing from the spirit of the invention. The 
scope of the present invention, therefore, should be determined solely by 
the appended claims.