Patent Publication Number: US-9841580-B2

Title: Distance measuring apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a distance measuring apparatus, and in particularly, to a distance measuring apparatus for use in a digital still camera, a digital video camera, and so on. 
     BACKGROUND ART 
     A phase-contrast distance measuring technique is known as a distance measuring technique applicable to digital still cameras and video cameras. 
     PTL 1 discloses a solid-state imaging device in which some of pixels have a ranging function using a phase contrast method. The individual pixels have light receiving portions each including a microlens and a photoelectric conversion section. The light receiving portions have the characteristic of having sensitivity to a beam that is incident thereon at a small angle range through the microlens by guiding light incident at a specific angle to the photoelectric conversion sections. With this configuration, beams that have passed through partial regions on the pupil of the image-forming optical system are detected. Two images generated from the beams that have passed through different regions on the pupil of the camera lens are acquired by different light receiving portions. The distance between the two images is estimated, and the defocusing amount is calculated by triangulation using a stereo image to perform ranging. 
     This method does not need to move the lens to measure the range, in contrast to the conventional contrast method, thus allowing high-speed high-accuracy ranging. 
     The signals acquired by the light receiving portions of the pixels can be used as imaging signals for generating an image, thus allowing imaging and ranging at the same time. 
     PTL 2 discloses a focus detection apparatus equipped with an imaging device having a pair of pixel groups that receive beams that have passed through different pupil regions of an image-capturing optical system and output a first image signal and a second image signal, respectively. 
     The focus detection apparatus includes a calculation unit that subtracts a value obtained by multiplying a second image signal by a second factor from a value obtained by multiplying a first image signal by a first factor to generate a first correct image signal and that subtracts a value obtained by multiplying a first image signal by a fourth factor from a value obtained by multiplying a second image signal by a third factor to generate a second correct image signal; and a focus detection unit that determines the defocusing amount on the basis of the phase difference between the first and second correct image signals. 
     CITATION LIST 
     Patent Literature 
     PTL 1 Japanese Patent Laid-Open No. 2002-314062 PTL 2 U.S. Pat. No. 8,159,599 
     SUMMARY OF INVENTION 
     Technical Problem 
     With the configuration disclosed in PTL 1, the light receiving portions are low-sensitivity light receiving portions having sensitivity only to beams that enter in a relatively narrow angle range. Ranging a low-luminance subject with such light receiving portions causes low signal intensity and an insufficient S/N ratio, thus making it difficult to achieve high-accuracy ranging. 
     In contrast, using high-sensitivity light receiving portions having sensitivity to beams that enter in a wide angle range increases the signal intensity and improves the S/N ratio of the signals. These light receiving portions detect beams that have passed through a wide region of the pupil. This makes pupil division indefinite and the base length short, thus making it difficult to achieve high-accuracy ranging. 
     The configuration disclosed in PTL 2 may have room for improvement in focusing accuracy for a low-luminance subject, although the configuration increases the focusing accuracy with a simple calculation. 
     In consideration of the above problem, the present invention provides a distance measuring apparatus capable of measuring a distance to a low-luminance subject with high accuracy by using high-sensitivity light receiving portions that causes indefinite pupil division. 
     Solution to Problem 
     A distance measuring apparatus according to an aspect of the present invention includes an optical system forming an image of a subject; an imaging device acquiring an electrical signal from a beam that has passed through an exit pupil of the optical system; and a calculation unit calculating a distance to the subject on the basis of the electrical signal. The imaging device includes a signal acquisition unit that acquires, on a plurality of locations on the imaging device, a first electrical signal mainly based on a beam that has passed through a first region off the center of the exit pupil in a predetermined direction, a second electrical signal mainly based on a beam that has passed through a second region off the center of the exit pupil in a direction opposite to the predetermined direction, and a third electrical signal different from the second electrical signal on the basis of a beam that has passed through a region eccentric from the first region in the direction opposite to the predetermined direction. The calculation unit performs a signal correction process for generating a first corrected signal by subtracting the third electrical signal from the first electrical signal in a predetermined proportion and a distance calculation process for calculating the distance by using the first corrected signal. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     Advantageous Effects of Invention 
     According to an embodiment of the present invention, a distance measuring apparatus capable of measuring a distance to a low-luminance subject with high accuracy can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view showing, in outline, a distance measuring apparatus according to an embodiment of the present invention; 
         FIG. 1B  is a schematic diagram illustrating an example of an imaging device; 
         FIG. 1C  is a schematic diagram illustrating an example of the overall configuration of the distance measuring apparatus; 
         FIG. 2A  is a graph showing the characteristics of the distance measuring apparatus; 
         FIG. 2B  is a diagram of an exit pupil according to an embodiment of the present invention; 
         FIG. 3  is a flowchart for the distance measuring apparatus according to an embodiment of the present invention; 
         FIG. 4A  is a graph showing the angle characteristics of light receiving portions; 
         FIG. 4B  is a graph showing the angle characteristics of imaginary light receiving portions; 
         FIG. 4C  is a diagram illustrating pupil divided regions and the centers of gravity of the pupils; 
         FIG. 5  is a flowchart for the distance measuring apparatus according to an embodiment of the present invention; 
         FIG. 6  is a cross-sectional view showing, in outline, an imaging device of a distance measuring apparatus according to a first embodiment of the present invention; 
         FIG. 7A  is a diagram illustrating an example of the imaging device of the distance measuring apparatus according to the first embodiment of the present invention; 
         FIG. 7B  is a cross-sectional view showing, in outline, the imaging device of the distance measuring apparatus according to the first embodiment of the present invention; 
         FIG. 8  is a cross-sectional view showing, in outline, an imaging device of the distance measuring apparatus according to the first embodiment of the present invention; 
         FIG. 9A  is a diagram illustrating an example of an imaging device of a distance measuring apparatus according to a second embodiment of the present invention; 
         FIG. 9B  is a graph showing the angle characteristics of the light receiving portions; 
         FIG. 9C  is a diagram showing pupil divided regions on the exit pupil; and 
         FIG. 10  is a diagram illustrating an example of an imaging device of a distance measuring apparatus according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Basic Configuration 
     Referring to  FIGS. 1A to 1C  and  FIGS. 2A and 2B , a distance measuring apparatus  100  according to an embodiment of the present invention will be described. 
     The distance measuring apparatus  100  according to an embodiment of the present invention includes an optical system  101  that forms an image of a subject, an imaging device  102  that acquires an electrical signal by using a beam that has passed through an exit pupil of the optical system  101 , and a calculation unit that calculates a distance to the subject on the basis of the electrical signal. 
       FIG. 1A  is a schematic diagram that mainly illustrates the image-forming optical system  101  and the imaging device  102  constituting the distance measuring apparatus  100  according to an embodiment of the present invention.  FIG. 1B  is a schematic diagram illustrating an example of the imaging device  102 .  FIG. 1C  is a schematic diagram illustrating an example of the overall configuration of the distance measuring apparatus  100  according to an embodiment of the present invention. 
     As shown in  FIG. 1A , the image-forming optical system  101  forms an image of an external subject on the surface of the imaging device  102 . 
     As shown in  FIG. 1B , the imaging device  102  includes a plurality of pixels  103 ,  104 ,  105 , . . . . The pixel  103  includes light receiving portions  106  and  107 , the pixel  104  includes a light receiving portion  108 , and the pixel  105  includes a light receiving portion  109  serving as signal acquisition units for acquiring ranging and correcting electrical signals. The pixels  103 ,  104 , and  105  each include a reading unit (not shown) that converts electrical charge accumulated in each of the light receiving portions  106  to  109  to electrical signals and outputs the electrical signals to a distance calculation unit  111 . The reading unit is constituted by, for example, a floating diffusion portion, a gate electrode, and wires. 
     As shown in  FIG. 1C , the distance measuring apparatus  100  includes the distance calculation unit  111  for calculating a distance to the subject by using the acquired signals, in addition to the optical system  101  and the imaging device  102 . The distance calculation unit  111  is constituted by a signal processing substrate including, for example, a CPU and a memory. The distance measuring apparatus  100  may have a recording unit  112  for recording read signals or calculation results.  FIG. 1C  illustrates the distance measuring apparatus  100 ; alternatively, it may be a digital camera including a driving mechanism for focusing of the optical system  101 , a shutter, and a display, such as a liquid crystal display, for checking images. 
     The imaging device  102  that constitutes the distance measuring apparatus  100  of an embodiment of the present invention includes a signal acquisition unit that acquires, on a plurality of locations on the imaging device, a first electrical signal mainly based on a beam that has passed through a first region off the center of the exit pupil in a predetermined direction, a second electrical signal mainly based on a beam that has passed through a second region off the center of the exit pupil in a direction opposite to the predetermined direction, and a third electrical signal different from the second electrical signal on the basis of a beam that has passed through a region eccentric from the first region in the direction opposite to the predetermined direction. 
     The calculation unit  111  performs a signal correction process for generating a first corrected signal by subtracting the third electrical signal from the first electrical signal in a predetermined proportion and a distance calculation process for calculating the distance by using the first corrected signal. 
     These features will be described in detail later. 
     Definition of Center of Gravitation Angle, Center of Gravity of Pupil, and, Pupil Divided Region 
     In an embodiment of the present invention, the distance between the image-forming optical system  101  and the imaging device  102  is larger than the size of each pixel. This causes beams that have passed through different positions on an exit pupil  120  of the image-forming optical system  101  to be incident on the surface of the imaging device  102  at different incident angles. 
     The light receiving portions  106  and  107  receive beams from a predetermined angle range  121  depending on the shape of the exit pupil  120  and the positions of the light receiving portions  106  and  107  on the imaging device  102 . The sensitivity characteristics of the light receiving portions  106  and  107  to the beams incident at different angles are referred to as angle characteristics. 
     An angle that is the center of gravity of the sensitivity of a light receiving portion in the angle range of a beam incident on the light receiving portion is referred to as a center-of-gravity angle. The center-of-gravity angle can be calculated by using Exp. 1. 
     In Exp. 1, θ is an angle that a pupil dividing direction (in this embodiment, the x-axis direction) forms with the z-axis in a plane including the z-axis, θg is the center-of-gravity angle, t(θ) is the sensitivity of the light receiving portion, and integration is performed on the angle range of beams incident on the light receiving portion. 
     
       
         
           
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     A position on the exit pupil  120  through which a beam incident on the light receiving portion at a center-of-gravity angle θg is referred to as the center of gravity of the pupil. A region on the exit pupil  120  which includes the center of gravity of the pupil and through which a beam incident from an angle range in which the sensitivity of the light receiving portion is high is referred to as a pupil divided region. 
       FIG. 2A  shows the angle characteristics of the individual light receiving portions  106  to  109  to beams incident at different angles in the x-z plane, in which the horizontal axis indicates an angle that an incident beam forms with the x-axis in the x-z plane, and the vertical axis indicates the sensitivity. Reference signs  106   ga ,  107   ga ,  108   ga , and  109   ga  denote the center-of-gravity angles of the light receiving portions  106 ,  107 ,  108 , and  109 , respectively. 
     Broken lines denoted by reference signs  106   t ,  107   t ,  108   t , and  109   t  in  FIG. 2B  indicate regions through which the beams received by the light receiving portions  106 ,  107 ,  108 , and  109 , pass through, respectively. Reference signs  106   d ,  107   d ,  108   d , and  109   d  denote the pupil divided regions of the light receiving portions  106 ,  107 ,  108 , and  109 , respectively. Reference signs  106   gp ,  107   gp ,  108   gp , and  109   gp  denote the centers of gravity of the pupils of the light receiving portions  106 ,  107 ,  108 , and  109 , respectively. 
     The light receiving portions  106  and  107  are configured to have sensitivity to beams incident on the light receiving portions  106  and  107  in a wide angle range and to receive beams that have passed through the substantially whole regions  106   t  and  107   t  of the exit pupil  120 . 
     The light receiving portion  106  is configured such that the pupil divided region  106   d  is off the center of the exit pupil  120  in the +x direction. In other words, the light receiving portion  106  mainly receives a beam that has passed through a first region off the center of the exit pupil  120  in a predetermined direction. The light receiving portion  106  acquires a first electrical signal on the basis of the beam. Thus, the light receiving portion  106  constitutes a signal acquisition unit. The light receiving portion  106  can be referred to as a first light receiving portion. 
     The light receiving portion  107  is configured such that the pupil divided region  107   d  is off-centered in the −x direction opposite to the pupil divided region  106   d . In other words, the light receiving portion  107  mainly receives a beam that has passed through a second region off the center of the exit pupil  120  in the direction opposite to the predetermined direction. The light receiving portion  107  acquires a second electrical signal on the basis of the beam. Thus, the light receiving portion  107  constitutes a signal acquisition unit. The light receiving portion  107  can be referred to as a second light receiving portion. 
     A direction in which the pupil divided regions  106   d  and  107   d  are connected is referred to as a pupil dividing direction (in this embodiment, the x-direction). The light receiving portions  106  and  107  are disposed at a plurality of locations in the pupil dividing direction on the imaging device  102 . The light receiving portions  106  and  107  can receive beams that have passed through different pupil regions at the plurality of locations in the pupil dividing direction. Signals acquired by the light receiving portions  106  and  107  can be used as ranging signals or image signals. 
     The light receiving portion  108  is configured such that it has sensitivity to a beam that has passed through the pupil region  108   t  in the pupil region  106   t  and that the pupil divided region  108   d  is eccentric from the pupil divided region  106   d  to the −x direction (the direction opposite to the eccentric direction of the pupil divided region  106   d ). In other words, the light receiving portion  108  acquires a third electrical signal different from the second electrical signal mainly on the basis of a beam that has passed through a region eccentric from the first region to the direction opposite to the predetermined direction. Thus, the light receiving portion  108  constitutes a signal acquisition unit. The light receiving portion  108  can be referred to as a third light receiving portion. 
     The light receiving portion  109  is configured such that it has sensitivity to a beam that has passed through the pupil region  109   t  in the pupil region  107   t  and that the pupil divided region  109   d  is eccentric from the pupil divided region  107   d  to the +x direction (the direction opposite to the eccentric direction of the pupil divided region  107   d ). In other words, the light receiving portion  109  acquires a fourth electrical signal different from the first electrical signal mainly on the basis of the beam that has passed through a region eccentric from the second region in the predetermined direction. Thus, the light receiving portion  109  constitutes a signal acquisition unit. The light receiving portion  109  can be referred to as a fourth light receiving portion. 
     The light receiving portions  108  and  109  are disposed at a plurality of locations in the pupil dividing direction on the imaging device  102  and in the vicinity of the light receiving portions  106  and  107 , respectively. 
     The signals acquired by the light receiving portions  106  to  109  can be used as correcting signals. The light receiving portion  108  is configured to receive beams more than the light receiving portion  107 , and the light receiving portion  109  is configured to receive beams more than the light receiving portion  106  so that they have high sensitivity. Thus, the light receiving portions  106  to  108  can acquire correcting signals having a higher signal-to-noise (S/N) ratio than that of the ranging signals. 
     The signals acquired at the plurality of locations by the light receiving portions  106  to  108  are referred to as signals S 106 , S 107 , S 108 , and S 109 . 
     The distance calculation unit  111  calculates the distance to the subject in accordance with the flowchart shown in  FIG. 3 . 
     Step  131  is a signal correction process, in which a corrected signal CS 106  is generated by subtracting the signal S 108  from the signal S 106  at individual locations on the imaging device  102  in a predetermined proportion, as expressed as Exp. 2. In other words, the third electrical signal is subtracted from the first electrical signal in a predetermined proportion to generate a first corrected signal. 
     As expressed by Exp. 4, a corrected signal CS 107  is generated by subtracting the signal S 109  from the signal S 107  in a predetermined proportion at individual locations on the imaging device  102 . In other words, the fourth electrical signal is subtracted from the second electrical signal in a predetermined proportion to generate a second corrected signal. 
     In Exp. 2 and Exp. 4, α and β are correction factors, which are real numbers larger than 0. Such a signal correction process is performed at a plurality of locations in the pupil dividing direction on the imaging device  120  to generate the corrected signals CS 106  and CS 107 .
 
 CS   106   =S   106   −α·S   108   (Exp. 2)
 
     This can be simply expressed as Exp. 3.
 
 S   1   ′=S   1   −αS   3   (Exp. 3)
 
where S 1 ′ is the first corrected signal, S 1  and S 3  are the first and third electrical signals, and α is a correction factor.
 
 CS   107   =S   107   −β·S   109   (Exp. 4)
 
     This can be simply expressed as Exp. 5.
 
 S   2   ′=S   2   −βS   4   (Exp. 5)
 
where S 2 ′ is the second corrected signal, S 2  and S 4  are the second and fourth electrical signals, and β is a correction factor.
 
     In step  132 , the distance to the subject is calculated from the corrected signal CS 106  (first corrected signal) and corrected signal CS 107  (second corrected signal) pair. This includes a distance calculation process for calculating the distance by using the first corrected signal S 1 ′. 
     The gap length of the signal pair can be calculated using a known method. For example, the gap length can be obtained by calculating the correlation while shifting one of the signal pair to find a gap length at the highest correlation. Furthermore, a defocusing amount is obtained from the thus-acquired gap length by a known method to determine the distance to the subject. 
     Using the distance measuring apparatus  100  equipped with the light receiving portions  106  to  109  and the calculation unit described above allows the distance to a low-luminance subject to be measured with high accuracy. 
     Principle 
     The reason that high-accuracy ranging can be achieved by the distance measuring apparatus  100  of an embodiment of the present invention will be described. 
     The defocusing amount (distance) can be calculated from the gap length of the signal pair. 
     A gap length at a given defocusing amount depends on the base length. The base length depends on the distance between the centers of gravity of the pupils of light receiving portions that generate a signal pair. 
     The gap length increases as the distance between the centers of gravity of the pupils (base length) increases. This allows the gap length to be determined with high accuracy, thus enabling high-accuracy ranging. 
     In the signal correction process of step  131 , the corrected signal CS 106  (first corrected signal) corresponds to a signal acquired by an imaginary light receiving portion having an angle characteristic  106   a ′ that is obtained by subtracting the angle characteristic  108   a ×α from the angle characteristic  106   a . This also applies to the corrected signal CS 107  (second corrected signal). 
       FIGS. 4A and 4B  are graphs showing the angle characteristics of the light receiving portions  106  to  109  and the angle characteristics  106   a ′ and  107   a ′ of imaginary light receiving portions  106   a ′ and  107   a ′ corresponding to the corrected signal CS 106  (first corrected signal) and the corrected signal CS 107  (second corrected signal).  FIG. 4C  is a diagram illustrating the pupil divided regions  106   d ,  106   d   7 ,  107   d , and  107   d ′ and the centers of gravity of the pupils,  106   gp ,  106   gp ′,  107   gp , and  107   gp′.    
     As shown in  FIG. 4A , the center-of-gravity angle  108   ga  is smaller than the center-of-gravity angle  106   ga . Thus, the angle characteristic  106   a ′ of the imaginary light receiving portion  106 ′ corresponding to the corrected signal CS 106  is such that the sensitivity particularly at a small angle is lower than that of the angle characteristic  106   a , as shown in  FIG. 4B . 
     Thus, the center of the gravity angle  106   ga ′ of the angle characteristic  106   a ′ is larger to the + side than the center-of-gravity angle  106   ga . As shown in  FIG. 4C , the pupil divided region  106   d ′ corresponding to the corrected signal CS 106  is eccentric to the +x direction from the pupil divided region  106   d.    
     Also for the corrected signal CS 107 , the center of the gravity angle  107   ga ′ of the angle characteristic  107   a ′ is larger to the − side than the center-of-gravity angle  107   ga , and the pupil divided region  107   d ′ is eccentric to the −x direction from the pupil divided region  107   d.    
     The distance between the pupil divided regions  106   d ′ and  107   d ′ is lager than the distance between the pupil divided regions  106   d  and  107   d  before correction. 
     The distance  123  between the centers of gravity of the pupils  106   gp ′ and  107   gp ′ corresponding to the base length is larger than the base length  122  before correction. The gap length between the corrected signals CS 106  and CS 107  increases from that before correction, which allows the gap length to be obtained with high accuracy, thus allowing high-accuracy ranging. The distance measuring apparatus  100  of an embodiment of the present invention allows high-accuracy ranging of a low-luminance subject by using correcting signals with a high S/N ratio. 
     In the above-described signal correction process, if the correcting signals have noise (random noise), the corrected signals CS 106  and CS 107  are also given noise in a predetermined proportion in Exp. 3 or 5. 
     Insufficient S/N ratios of the corrected signals cause insufficient measurement accuracy of the image gap length. 
     This offsets the improving effect due to the image-gap-length increasing effect described above, thus reducing the ranging accuracy improving effect. 
     In particular, an extremely low-luminance subject causes much noise in the ranging signal S 106  (first electrical signal) and the ranging signal S 107  (second electrical signal). 
     Using a signal having a lower S/N ratio than that of a ranging signal as a correcting signal significantly increases an image-gap-length determination error, thus extremely reducing the improving effect of the signal correction process. 
     Thus, a correcting signal having a high S/N ratio or an S/N ratio higher than that of a ranging signal may be used. 
     In the distance measuring apparatus  100  of an embodiment of the present invention, the configurations of the signal acquisition units and the distance calculation unit  111  are not limited thereto. 
     The two light receiving portions  108  and  109  are provided as correcting signal acquisition units; alternatively, only the light receiving portion  108  may be provided, and signals acquired by the light receiving portion  108  may be used both as the correcting signals S 108  and S 109 . 
     An example configuration in which a pupil is divided in the x-direction has been shown; alternatively, the pupil may be divided in the y-direction or in a slanting direction, and light receiving portions having pupil divided regions may be disposed in such a direction to perform ranging. This configuration allows ranging of a subject having different contrasts in such a direction. 
     An example in which the distance calculation unit  111  generates a pair of corrected signals CS 106  and CS 107  is shown above; alternatively, a corrected signal of one of the ranging signals S 106  and S 107  may be generated, and the corrected signal and the other ranging signal may be used to calculate the distance. This configuration can also provide the above advantages, thus allowing high-accuracy ranging. 
     Range of Correction Factor 
     In step  131  of  FIG. 3 , it is preferable that the correction factors α and β be within the ranges expressed by Exp. 6 and Exp. 8. In Exp. 6 and Exp. 8, t 106 , t 107 , t 108 , and t 109  are the ratio of beams incident from locations on the exit pupil  120  to electrical signals converted by the individual light receiving portions  106  to  109 . 
     Min( ) is a function for obtaining the minimum value in ( ),x and y are coordinates on the exit pupil  120 , which are within the pupil region  108   t  of the light receiving portion  108  in Exp. 6, and within the pupil region  109   t  of the light receiving portion  109  in Exp. 8. 
     
       
         
           
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0&lt;α≦min[ T   1 ( p )/ T   3 ( p )]  (Exp. 7)
 
where p is coordinates on the exit pupil  120 , and T 1  and T 3  are the ratio of beams incident on the signal acquisition units from the coordinate positions on the exit pupil  120  to the first and third electrical signals S 1  and S 3  converted.
 
     
       
         
           
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     This can be simply expressed as Exp. 9.
 
0&lt;β≦min[ T   2 ( p )/ T   4 ( p )]  (Exp. 9)
 
where p is coordinates on the exit pupil  120 , and T 2  and T 4  are the ratio of beams incident on the signal acquisition units from the coordinate positions on the exit pupil  120  to the second and fourth electrical signals S 2  and S 4  converted.
 
     Setting the correction factors α and β in this manner causes the angle characteristics  106   a ′ and  107   a ′ to have no negative sensitivity. 
     The light receiving portions  106  to  109  acquire, as signals, values obtained by integrating the beams incident thereon at different angles depending on the sensitivity. 
     If part of the angle characteristics has negative sensitivity, a beam received at negative sensitivity shows a negative value, and the negative value is offset with the value of a beam received with positive sensitivity. Thus, the corrected signal lacks information on part of the beams. 
     This causes an error in the gap length determined from the corrected signal, thus causing a ranging error. Setting the correction factors α and β within the ranges expressed by Exp. 6 and Exp. 8 can decrease the error in the corrected signal to reduce a gap length detection error, thus allowing higher-accuracy ranging. 
     Disposition of Light Receiving Portions 
     In the distance measuring apparatus  100  of an embodiment of the present invention, the light receiving portions  106  to  109  that acquire the ranging signals and the correcting signals may be disposed next to each other on the imaging device  102 . 
     Disposing the light receiving portions  106  to  109  at separate positions causes beams coming from different subjects to be incident thereon. The difference among the incident beams causes errors in corrected signals generated from signals acquired by the light receiving portions  106  to  109  in the signal correction process, thus causing an error in the result of ranging. 
     The light receiving portions  106  to  109  may be disposed close to each other, within three pixels, or within the same pixel. 
     Distance Calculation Using Corrected Base Length 
     Calculating the distance using a corrected base length W′ in the distance calculation process in step  132  of  FIG. 3  allows higher-accuracy ranging. 
     The corrected base length W′ can be obtained by calculating the distance  123  between the centers of gravity of the pupils  106   gp ′ and  107   gp ′ of the imaginary light receiving portions  106   d ′ and  107   d ′ corresponding to the corrected signals CS 106  and CS 107  in  FIG. 4C  (corrected-base-length calculation process). 
     First, the centers of gravity angles  106   ga ′ and  107   ga ′ are obtained from the angle characteristics of the light receiving portions  106  and  107  and the correction factors α and β, and the centers of gravity of the pupils  106   gp ′ and  107   gp ′ are calculated from the centers of gravity angles  106   ga ′ and  107   ga ′ and the positional information on the exit pupil  120 . 
     Next, the distance  123  between the centers of gravity of the pupils  106   gp ′ and  107   gp ′ on the exit pupil  120  corresponding to the corrected base length W′ is calculated by using Exp. 10.
 
 W′=| 106 gp′− 107 gp′|   (Exp. 10)
 
     The corrected base length  123  thus obtained and the gap length calculated in step  131  are used to obtain the defocusing amount by using Exp. 11, thereby calculating the distance to the subject. 
     In Exp. 11, ΔL is the defocusing amount, r is the gap length, L is the distance between the exit pupil  120  and the imaging device  102 , and W′ is the corrected base length  123 . 
     
       
         
           
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                     . 
                     
                         
                     
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     By calculating the defocusing amount from the gap length by using a higher-accuracy base length, the distance can be calculated with higher accuracy. 
     In the distance calculation process of step  132 , the distance may be calculated by another method. 
     For example, a transformation coefficient for connecting the gap length to the defocusing amount may be calculated in advance, and the calculated gap length and the transformation coefficient may be used to calculate the defocusing amount. 
     This can omit a calculation for calculating the base length depending on image-capturing conditions and the position of the light receiving portions on the image plane, thus allowing high-speed ranging. 
     Adjustment of Correction Factor 
     In the signal correction process of step  131 , adjusting the correction factors α and β depending on the positions of the light receiving portions  106  to  109  on the imaging device  102  and image-acquisition conditions (correction-factor adjusting process) allows hither-accuracy ranging. 
     For the image-forming optical system  101  like a zoom lens, the distance from the exit pupil  120  to the imaging device  102  changes depending on the state of zooming. Eclipse due to changes in the aperture of the image-forming optical system  101  or the angle of view, if present, will change the shape of the exit pupil  120 . 
     These changes will change the angle ranges of beams incident on the light receiving portions  106  to  109 , thus changing the numerical ranges of the correction factors α and β in Exp. 6 and Exp. 8. Selecting optimum correction factors α and β depending on the positions of the light receiving portions  106  to  109  on the imaging device  102  and the image-acquisition conditions allow higher-accuracy ranging under the conditions. 
     Image Modification Process 
     The distance calculation unit of the distance measuring apparatus  100  according to an embodiment of the present invention may further perform a signal modification process for modifying the image form of the corrected signals CS 106  and CS 108 . 
     The corrected signals CS 106  and CS 107  can be described using the light distribution f of the subject and corrected line spread functions L 106 ′ and L 107 ′, as in Exp. 12 and Exp. 13, where x is a position on the imaging device  102  in the pupil dividing direction. 
     
       
         
           
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     Let L 106 , L 107 , L 108 , and L 109  be line spread functions determined depending on the point spread function of the image-forming optical system  101 , the angle characteristics of the light receiving portions  106  to  109 , and the shape and position of the exit pupil  120 . 
     Corrected line spread functions L 106 ′ and L 107 ′ in Exp. 12 and Exp. 13 can be described as in Exp. 14 and Exp. 16, where α and β are the correction factors used in step  131 .
 
 L′   106    [x]=L   106   [x]−αL   108   [x]   (Exp. 14)
 
     This can be further simplified to Exp. 15.
 
 L   1   ′=L   1   −αL   3   (Exp. 15)
 
where L 1 ′ can be referred to as a first modification filter, and L 1  and L 3  can be referred to as line spread functions determined depending on the signal acquisition units for the first and third electrical signals S 1  and S 3  and the optical system  101 .
 
 L′   107   [x]=L   107   [x]−βL   109   [x]   (Exp. 16)
 
     This can be further simplified to Exp. 17.
 
 L   2   ′=L   2   −βL   4   (Exp. 17)
 
where L 2 ′ can be referred to as a second modification filter, and L 2  and L 4  can be referred to as line spread functions determined depending the signal acquisition units for the second and fourth electrical signals S 2  and S 4  and the optical system  101 .
 
     The line spread function L is determined depending on the position and shape of the exit pupil  120  and the angle characteristics of the light receiving portions  106  to  109 . Eclipse due to the lens frame of the image-forming optical system  101 , is possible, will make the line spread functions L 106  and L 107  differ from each other and the line spread functions L 108  and L 109  differ from each other. 
     The corrected line spread functions L 106 ′ and L 107 ′ differ from each other, and the image shapes of the corrected signals CS 106 ′ and CS 107 ′ differ from each other. 
     Next, a flowchart including the signal modification process will be shown in  FIG. 5 . 
       FIG. 5  is a flowchart similar to  FIG. 3  but differs from  FIG. 3  in that it has a signal modification processing step  133 . 
     Step  133  is a signal modification process for modifying the image shapes of the corrected signals CS 106  and CS 107 . 
     First, a temporary defocusing amount is calculated from the pair of signals S 106  and S 107  or the corrected signals CS 106  and CS 107  using a known unit. 
     Next, the corrected line spread functions L 106 ′ and L 107 ′ shown in Exp. 14 and Exp. 16 are created as image modification filters on the basis of the temporary defocusing amount and the known information about the angle characteristics of the individual light receiving portions  106  to  109  and the known information on the exit pupil  120 . 
     The corrected signals CS 106  and CS 107  are subjected to convolution integral by the image modification filters L 107 ′ and L 106 ′ to create a modified signal MS 106  (first modified signal) and a modified signal MS 107  (second modified signal), respectively. The reference point for the convolution integral is set at the centers of weight of the image modification filters L 107 ′ and L 106 ′. 
     The image shapes of the modified signals MS 106  and MS 107  are expressed as Exp. 18 and Exp. 19. 
     
       
         
           
             
                 
             
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     As shown in Exp. 18 and Exp. 19, the image shapes of the modified signals MS 106  and MS 107  are determined depending on the corrected line spread functions L 106 ′ and L 107 ′ and are substantially the same. 
     In step  132 , the defocusing amount and the distance to the subject are calculated using the modified signals MS 106  and MS 107  by a known unit. 
     The ranging calculation including the signal modification process according to an embodiment of the invention allows the image shapes of the corrected signals CS 106  and CS 107  to be modified, thus reducing gap-length calculation errors due to a difference in image shape. This can improve the gap length determination accuracy, allowing higher-accuracy ranging. 
     The image modification filters L 106 ′ and L 107 ′ are functions that change depending on the defocusing amount. 
     An image modification filter may be created again on the basis of the defocusing amount obtained in step  132 , and the image modification filter may be used to create a modified image to find a defocusing amount. 
     Creating the image modification filter on the basis of a defocusing amount closer to a correct value reduces the shape error of the image modification filter. This reduces the shape error of the modified signals MS 106  and MS 107 , which improves the accuracy of calculation of the gap length and the defocusing amount, thus improving the ranging accuracy. 
     The signal modification process is not limited to that of this embodiment. For example, the signal modification process may be performed by deriving inverse functions L 106 ′−1 and L 107 ′−1 of the corrected line spread functions L 107 ′ and L 106 ′ in Exp. 14 and Exp. 16 and by performing convolution integral on the corrected signals CS 106  and CS 107  by the inverse functions L 106 ′−1 and L 107 ′−1. 
     This description shows a process example in which the signal modification process uses one-dimensional line spread functions in the pupil dividing direction (x-direction); alternatively, a two-dimensional point spread function (x-y direction) may be used for the modification. This allows a modification process considering a direction (y-direction) perpendicular to the pupil dividing direction caused by defocusing, thus allowing higher-accuracy modified signals to be created. Alternatively, another signal modification process may be performed on the basis of the information on the exit pupil  120  and the characteristics of the signal acquisition units. 
     Image-Acquisition System, AF, and Distance Image 
     The result of ranging of the distance measuring apparatus  100  of an embodiment of the present invention can be used, for example, to detect the focus of the image-forming optical system  101 . The distance measuring apparatus  100  of an embodiment of the present invention allows the distance to the subject to be measured at high speed and with high accuracy, thus allowing the gap length between the subject and the focal position of the image-forming optical system  101  to be determined. Controlling the focal position of the image-forming optical system  101  allows the focal position to coincide with the subject at high speed and with high accuracy. 
     Alternatively, disposing such light receiving portions all over the imaging device  102  and calculating the distance using signals acquired for a plurality of regions on the imaging device  102  allows a distance image to be acquired. 
     The distance measuring apparatus  100  of an embodiment of the present invention can constitute image acquisition units, such as a digital still camera and a digital video camera, and the focus of the optical system  101  can be detected on the basis of the distance measurement result of the distance measuring apparatus  100 . 
     Other Configurations 
     Specific examples of the imaging device of the distance measuring apparatus  100  of an embodiment of the present invention include solid-state imaging devices, such as a CMOS sensor (complementary metal-oxide semiconductor sensor) and a charge-coupled device (CCD) sensor. 
     The calculation unit  111  of the distance measuring apparatus  100  can be constituted by an integrated circuit in which semiconductor devices are integrated; for example, an integrated circuit (IC), a large scale integrated circuit (LSI), a system LSI, a microprocessing unit (MPU), and a central processing unit (CPU). 
     The present invention further includes a program in addition to the distance measuring apparatus  100 . 
     A program according to an embodiment of the present invention causes a computer for calculating, in a distance measuring apparatus including an optical system forming an image of a subject and an imaging device acquiring an electrical signal from a beam that has passed through an exit pupil of the optical system, a distance to a subject by using a first electrical signal mainly based on a beam that has passed through a first region off the center of the exit pupil in a predetermined direction, a second electrical signal mainly based on a beam that has passed through a second region off the center of the exit pupil in a direction opposite to the predetermined direction, and a third electrical signal different from the second electrical signal, the third electrical signal being based on a beam that has passed through a region eccentric from the first region in the direction opposite to the predetermined direction, to execute predetermined processes. 
     Here, the distance measuring apparatus  100  shown in  FIG. 1C  includes the optical system  101 , the imaging device  102 , the calculation unit  111 , and the recording unit  112 . 
     The calculation unit  111  constituted by a microprocessing unit, a central processing unit, or the like can be regarded as a computer. 
     A program according to an embodiment of the present invention causes the computer corresponding to the calculation unit to execute a signal correction process for generating a first corrected signal by subtracting the third electrical signal from the first electrical signal in a predetermined proportion and a distance calculation process for calculating the distance by using the first corrected signal. 
     The program according to an embodiment of the present invention allows a distance measuring apparatus or an image-acquisition unit to perform high-accuracy ranging by being installed in a computer of a distance measuring apparatus or an image-acquisition unit, such as a camera, including a predetermined image-forming optical system, a predetermined imaging device, and the computer. 
     The program of an embodiment of the present invention can be distributed via the Internet, in addition to a recording medium. 
     Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiments of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiments. The computer may comprise one or more of a central processing unit (CPU), microprocessing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     The present invention will be described in detail below with reference to specific examples. 
     First Embodiment 
     Example configurations of signal acquisition units of the distance measuring apparatus  100  according to a first embodiment of the present invention will be shown in  FIG. 6 . 
     In  FIG. 6 , the light receiving portions  106  and  107  are configured such that a microlens  210  is provided in the pixel  103  and photoelectric conversion sections  206  and  207  are provided in a substrate  201  formed of a semiconductor or the like. 
     The light receiving portions  108  and  109  are configured such that a microlens  211  is provided in each of the pixels  104  and  105 , and photoelectric conversion sections  208  and  209  are provided in the semiconductor substrate  201 , respectively. The pixels  103  to  105  each include a reading unit (not shown) that outputs electrical charge, as electrical signals, accumulated in the individual light receiving portions  106  to  109  to the distance calculation unit  111 . 
     Beams that have passed through the exit pupil  120  and are incident on the light receiving portions  106  to  109  pass through the microlens  210  or  211  and are guided to the photoelectric conversion sections  206  to  209 , respectively. 
     In the light receiving portions  106  and  107 , beams  202  and  203  that have passed through the pupil divided regions  106   d  and  107   d  having different exit pupils  120  ( FIG. 2B ) are particularly efficiently received by the photoelectric conversion section  206  and the photoelectric conversion section  207 , respectively. 
     This configuration can easily achieve the light receiving portions  106  and  107  having the high-sensitivity pupil divided regions  106   d  and  107   d  ( FIG. 2B ) that are eccentric in different directions on the exit pupil  120  ( FIG. 1A ). 
     The light receiving portions  108  and  109  efficiently receive beams  204  and  205  that have passed through the pupil divided regions  108   d  and  109   d  in the vicinity of the center of the exit pupil  120  ( FIG. 2B ) with photoelectric conversion sections  208  and  209 , respectively. 
     This configuration can easily achieve the light receiving portions  108  and  109  that mainly receive beams coming from the pupil divided regions  108   d  and  109   d  eccentric from the pupil divided regions  106   d  and  107   d  in the opposite direction, respectively. This configuration can easily achieve the light receiving portions  108  and  109  having higher sensitivity than the light receiving portions  106  and  107 . Adjusting the curvatures of the microlens  210  and  211  and the distances to the photoelectric conversion sections  206  to  209  allows control of the angle characteristics and the pupil divided regions  106   d  to  109   d  of the light receiving portions  106  to  109 . This configuration allows ranging signals to be obtained from the light receiving portions  106  and  107  and correcting signals to be obtained from the light receiving portions  108  and  109 , and the distance to be calculated by the distance calculation unit  111  described above. 
     In the distance measuring apparatus  100  of an embodiment of the present invention, the configuration in which a plurality of photoelectric conversion sections are disposed in a pixel, like the pixel  103 , allows beams to be received with high sensitivity by the photoelectric conversion sections  206  and  207 , thus allowing high-quality image signals to be acquired. The signals acquired by the light receiving portions  108  and  109  can be used as image signals for the pixels  104  and  105 , respectively. 
     Other example configurations of the light receiving portions  106  to  109  constituting the distance measuring apparatus  100  of the first embodiment will be shown in  FIGS. 7A and 7B . 
     Referring to  FIG. 7A , the light receiving portions  106  and  107  are disposed in different pixels  220  and  221 , respectively. 
     The light receiving portions  106  and  107  each include the microlens  210 , a photoelectric conversion section  222  or  223 , and a light shield  224  or  225 . The light receiving portions  108  and  109  have the same configuration as that in  FIG. 6 . The pixels  222  and  223  each include a reading unit (not shown) that outputs electrical charge, as electrical signals, accumulated in the light receiving portions  106  and  107  to the distance calculation unit  111 . 
     The photoelectric conversion sections  222  and  223  of the light receiving portions  106  and  107  receive beams that have passed through the exit pupil  120 . The light receiving portion  106  blocks the beam  203  and mainly receives the beam  202 . The light receiving portion  107  blocks the beam  202  and mainly receives the beam  203 . 
     This configuration allows the ranging signals signal S 106  and S 107  to be acquired. The correcting signals S 108  and S 109  can be acquired by the light receiving portions  108  and  109 , respectively, as in the above. 
     Using these signals allows high-accuracy ranging with the above-described method. This configuration can increase the distance between the photoelectric conversion sections  222  and  223 , thus making it easy to manufacture the light receiving portions  106  and  107  of an imaging device having small pixels. 
     Waveguide Type 
     The pixels  103  to  105  constituting the distance measuring apparatus  100  may each include a waveguide shown in  FIG. 8 . 
     The pixels  103  to  105  each include a waveguide composed of a core  230  and a clad  231  at the light incident side (+z side) of the substrate  201 . The pixel  103  includes the photoelectric conversion sections  206  and  207 . The pixels  104  and  105  include the photoelectric conversion sections  208  and  209 , respectively. The pixels  103  to  105  each include a reading unit (not shown) that outputs electrical charge, as electrical signals, accumulated in the light receiving portions  106  to  109  to the distance calculation unit  111 . 
     The core  230  and the clad  231  are formed of a transparent material having an imaging wavelength range. The core  230  is formed of a material having a higher refractive index than that of the clad  231 . 
     This allows light to be enclosed and propagated in the core  230 . The beams that have passed through the exit pupil  120  and are incident on the individual pixels  103  to  105  propagate through the waveguides to the photoelectric conversion sections  206  to  209 . 
     In the pixel  103 , the beam  202  is particularly efficiently received by the photoelectric conversion section  206 , and the beam  203  is particularly efficiently received by the photoelectric conversion section  207 . In the pixels  104  and  105 , the beams  204  and  205  are efficiently received by the photoelectric conversion sections  208  and  209 , respectively. 
     This configuration allows incident light to be efficiently received also with an imaging device having small pixel size. 
     Alternatively, a back-illuminated type in which a waveguide composed of a core and a clad is provided in the substrate  104  is possible. 
     This configuration allows light incident from the back of the substrate  104  (light propagating in the +z direction) to be detected. Wires and so on can be disposed on the front of the substrate  104 , thus preventing interference with propagation of incident light due to the wires and so on. Furthermore, this configuration reduces spatial restriction due to the wires and so on, thus allowing incident light to be efficiently guided to the photoelectric conversion sections  206  to  209 . 
     Second Embodiment 
     An example configuration of a signal acquisition unit of the distance measuring apparatus  100  according to a second embodiment will be shown in  FIGS. 9A to 9C . 
     In  FIG. 9A , an imaging device  200  includes the pixels  103  each including the light receiving portion  106  and the light receiving portion  107 . 
     The light receiving portions  106  and  107  have the same configuration as shown in  FIG. 6 . 
     The pixels  103  each include a reading unit (not shown) that converts electrical charge accumulated in the light receiving portions  106  and  107  to electrical signals and outputs the signals to the distance calculation unit  111  ( FIG. 1C ). 
     The second embodiment uses the electrical signals acquired by the light receiving portions  106  and  107  as the ranging signals S 106  and S 107 . 
       FIGS. 9B and 9C  are diagrams showing the angle characteristics of the light receiving portions  106  and  107  and the pupil divided regions  106   d  and  107   d  on the exit pupil  120 . 
     The light receiving portions  106  and  107  generate a signal (in the second embodiment, referred to as an added signal S 250 ) based on a beams that has mainly passed through a region  250   d  including the pupil divided regions  106   d  and  107   d . For example, the added signal S 250  is generated by adding the ranging signals S 106  and S 107  acquired by the light receiving portions  106  and  107 , respectively. Alternatively, the added signal S 250  is generated by the reading unit on the basis of the amount of electrical charge accumulated in the light receiving portions  106  and  107 . The added signal S 250  is used as a correcting signal of the ranging signal S 106  or S 107 . 
     Ranging by the above method using these signals allows a low-luminance subject to be measured with high accuracy. 
     The pupil region  250   d  has its center of gravity  250   ga  between the centers of gravity  106   ga  and  107   ga  of the pupil divided regions  106   d  and  107   d.    
     The pupil region  250   d  is a region eccentric to the direction opposite to the eccentric direction of the pupil region  106   d  in which the added signal S 250  can be used as a correcting signal for the ranging signal S 106 . The pupil region  250   d  is also a region eccentric to the direction opposite to the eccentric direction of the pupil region  107   d , in which the added signal S 250  can be used as a correcting signal for the ranging signal S 107 . 
     The added signal S 205  includes the signal components of the ranging signals S 106  and S 107 , that is, twice the ranging signals S 106  and S 107 . The noise component thereof is smaller than the twice, because the random noises of the ranging signals S 106  and S 107  are averaged. In other words, the S/N ratio of the added signal S 250  is higher than the S/N ratios of the individual signals S 106  and S 107 . 
     Using such a correcting signal for the signal correction process can suppress a drop in the S/N ratios of the corrected signals, thus allowing high-accuracy ranging of a low-luminance subject, as in the above. 
     Generating the signals from the electrical signals S 106  and S 107  acquired by the light receiving portions  106  and  107  eliminates the need for a light receiving portion for acquiring a correcting signal, thus simplifying the configuration. 
     Furthermore, this configuration allows the ranging signals S 106  and S 107  and the correcting signal S 250  to be acquired at the same pixel, and errors in the corrected signal CS 106  and CS 107  to be reduced for the above reason, thus allowing higher-accuracy ranging. 
     Disposing such pixels  103  all over the imaging device  102  allows a high-resolution and high-accuracy distance image and a high-quality image to be acquired at the same time. 
     In the distance measuring apparatus  200  of the second embodiment of the present invention, the configuration of the light receiving portions  106  and  107  is not limited thereto. 
     The pixels  220  and  221  shown in  FIGS. 7A and 7B  or the pixel  103  shown in  FIG. 8  may be disposed all over the imaging device  102 , and the electrical signals acquired by the light receiving portions  106  and  107  included in the pixels  220  and  221  or the pixels  103  to  105  may be used as ranging signals and correcting signals. This can provide the same advantages as those described above. 
     Third Embodiment 
     Example configurations of signal acquisition units of the distance measuring apparatus  100  according to a third embodiment will be shown in  FIG. 10 . 
     An imaging device  300  shown in  FIG. 10  includes pixels  220  and the pixels  104 . The pixels  220  each include the light receiving portion  106 , and the pixels  104  each include the light receiving portion  108 . 
     The angle characteristics and the pupil divided regions  106   d  and  108   d  of the light receiving portions  106  and  108  are the same as above. The pixels  220  and  104  each include a reading unit (not shown) that outputs electrical charge, as electrical signals, accumulated in the light receiving portions  106  and  108  to the distance calculation unit  111 . 
     The third embodiment uses a signal acquired by the light receiving portion  106  (first light receiving portion) as the ranging signal S 106  (first electrical signal) and a signal obtained by subtracting the ranging signal S 106  from a signal acquired by the light receiving portion  108  as the ranging signal S 107  (second electrical signal). The third embodiment uses the signal acquired by the light receiving portion  108  as a correcting signal for the ranging signal S 106  or S 107 . Thus, the light receiving portion  108  can be referred to as a fifth light receiving portion. 
     Ranging by the above method using these signals allows a low-luminance subject to be measured with high accuracy. The light receiving portion  108  is configured to receive beams more than the light receiving portion  106  to have high sensitivity, as described above, thus allowing correcting signals having a high S/N ratio to be acquired. 
     Such a configuration can reduce the number of light receiving portions for acquiring ranging signals and correcting signals, thus simplifying the configuration of the imaging device  102 . In particular, using the light receiving portion  108  having higher sensitivity than the light receiving portion  107  can improve the quality of the signals, thus enhancing the image quality, as compared with the configuration in  FIGS. 7A and 7B . 
     REFERENCE SIGNS LIST 
       100  distance measuring apparatus 
       101  optical system 
       102  imaging device 
       111  calculation unit 
       120  exit pupil 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-248624, filed Nov. 12, 2012, which is hereby incorporated by reference herein in its entirety.