Patent Publication Number: US-10310084-B2

Title: Range imaging apparatus and range imaging method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. continuation application of PCT International Patent Application Number PCT/JP2015/000015 filed on Jan. 6, 2015, claiming the benefit of priority of Japanese Patent Application Number 2014-004479 filed on Jan. 14, 2014, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates to a range imaging apparatus and a range imaging method using a time-of-flight (TOF) system in which distance information of a subject is obtained based on the time difference arising between signal light and reflection light when the signal light is emitted toward, reaches and reflects off the subject, and the reflection light is received. 
     2. Description of the Related Art 
     In recent years, with the size reduction of equipment and the advent of products for general consumers, more attention has been directed toward apparatuses and methods for generating an image having distance-measuring information of a subject (in the following, referred to as a range image) not only for use in the traditional fields of gaming and amusement but also in the application and technological development in various fields such as medical care and advertisements. 
     The methods of measuring the distance of a subject for range imaging include several methods such as range imaging based on parallax of images picked up by a plurality of cameras; and a method of illuminating a subject with a known random dot pattern and generating a range image based on a distortion of the dot pattern. As one example of these methods, there is a TOF system in which infrared light or a laser beam is emitted toward a subject and the distance to the subject is calculated (measured) based on measurement time the infrared light or the laser beam has taken to reach the subject, reflect off the subject and be received. 
     Obviously, the control flexibility increases in keeping with the accuracy of measuring the distance to the subject. The TOF system utilizes a signal amount obtained by exposure of light reflecting off the subject. Thus, by appropriately adjusting the emission of signal light for illumination and the exposure of reflection light to be received according to the distance from an imaging point to the subject and a surface reflectivity of the subject, it is possible to prevent received light signal from being saturated and an S/N ratio from declining, so that the accuracy of distance measurement will improve and the range imaging will be performed with enhanced accuracy. 
     In one conventional technology, when a highly accurate synthetic range image is to be generated, a plurality of range images with different exposures are obtained and compared pixel by pixel so as to generate the synthetic range image (see Japanese Unexamined Patent Application Publication No. 2012-225807). 
     In the other conventional technology, the light emission amount (power consumption) is adjusted to a minimum level necessary for measuring the distance to the subject based on distribution characteristics of distance values of the obtained range image (see Japanese Unexamined Patent Application Publication No. 2011-179997). 
     SUMMARY 
     In the range image synthesizing method disclosed in Japanese Unexamined Patent Application Publication No. 2012-225807, when a highly accurate synthetic range image is to be generated, a plurality of range images with different exposures are obtained and compared pixel by pixel so as to generate the synthetic range image. Thus, there is a problem that an increase in the number of pixels inevitably brings about longer generation time, so that it takes considerable time to generate the synthetic range image. 
     In the method of adjusting light amount disclosed in Japanese Unexamined Patent Application Publication No. 2011-179997, the light emission amount (power consumption) is adjusted to a minimum level necessary for measuring the distance to the subject based on the distribution characteristics of distance values of the obtained range image. This makes it difficult to obtain a highly accurate distance information. In addition, when a plurality of subjects are present at different distances (depths), since the light emission amount is set to a minimum level, the distance information for all the subjects cannot be obtained. 
     With the foregoing in mind, the present disclosure controls emission of signal light and exposure of reflection light according to a position of a subject and, obtains highly accurate distance information in a short time. In other words, the present disclosure relates to a range image for one or more subjects, and provides range imaging with high accuracy. 
     In order to achieve the above, a range imaging apparatus according to one aspect of the present disclosure includes a light illuminator that emits signal light from a light source toward a subject; a light receiver that receives reflection light to obtain a signal amount, the reflection light resulting from reflection of the signal light emitted by the light illuminator off the subject; a range image generator that calculates a distance value to the subject based on a time difference between illumination start time and light reception start time, and generates a range image; a distance value distribution analyzer that analyzes a distribution characteristic of the distance value calculated by the range image generator; an illumination controller that adjusts output of the signal light emitted by the light illuminator; an exposure controller that adjusts light reception by the light receiver; and a signal adjuster that calculates an adjustment value for the output and the light reception based on the distribution characteristic, and controls the illumination controller and the exposure controller. The signal amount includes a plurality of signal amounts, the light illuminator emits the signal light toward the subject, the light receiver obtains the plurality of signal amounts at different timings, and the range image generator calculates the distance value to the subject based on a ratio between the plurality of signal amounts. The signal light emitted to generate the range image of the subject includes first signal light and second signal light, the first signal light being for calculating the adjustment value for output and light reception of the second signal light, and the second signal light being for generating the range image. The signal adjuster controls at least one of the illumination controller and the exposure controller for the second signal light based on the distribution characteristic obtained through emission and light reception of the first signal light. 
     For example, the signal adjuster may control the illumination controller and the exposure controller by adjusting as the adjustment value at least one of illumination intensity, an illumination period, an illumination count, an exposure-free period, exposure start time, an exposure period, and an exposure count of the second signal light based on the distribution characteristic of the distance value obtained through the emission and the light reception of the first signal light. 
     According to the present disclosure, it becomes possible to provide range imaging for one or more subjects with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present disclosure. 
         FIG. 1  is a block diagram illustrating a configuration of a range imaging apparatus according to Embodiment 1 and Embodiment 2 of the present disclosure; 
         FIG. 2  is an exemplary timing chart of a TOF system when signal amounts S 0  and S 1  are obtained in the stated order and each signal amount is obtained once; 
         FIG. 3  is an exemplary timing chart of a TOF system when signal amounts S 0 , S 1 , and BG are obtained in the stated order and each signal amount is obtained once; 
         FIG. 4  is an exemplary timing chart of a TOF system when signal amounts S 1 , S 0 , and BG are obtained in the stated order and each signal amount is obtained once; 
         FIG. 5A  is an exemplary timing chart of a TOF system when signal amounts S 0  and S 1  are obtained in the stated order and each signal amount is obtained N times (N is a natural number); 
         FIG. 5B  is a timing chart following  FIG. 5A ; 
         FIG. 5C  is a timing chart following  FIG. 5B ; 
         FIG. 6A  is an exemplary timing chart of a TOF system when signal amounts S 0 , S 1 , and BG are obtained in the stated order and each signal amount is obtained N times (N is a natural number); 
         FIG. 6B  is a timing chart following  FIG. 6A ; 
         FIG. 6C  is a timing chart following  FIG. 6B ; 
         FIG. 6D  is a timing chart following  FIG. 6C ; 
         FIG. 7A  is an exemplary timing chart of a TOF system when signal amounts S 1 , S 0 , and BG are obtained in the stated order and each signal amount is obtained N times (N is a natural number); 
         FIG. 7B  is a timing chart following  FIG. 7A ; 
         FIG. 7C  is a timing chart following  FIG. 7B ; 
         FIG. 8  is a flowchart illustrating a procedure in Embodiment 1 and Embodiment 2 of the present disclosure; 
         FIG. 9  illustrates an example of measuring the distance to a subject; 
         FIG. 10  is an exemplary timing chart for measuring the distance to a subject; 
         FIG. 11  illustrates an example of a generated range image; 
         FIG. 12A  illustrates an example of distribution characteristics of distance values of a subject; 
         FIG. 12B  illustrates an example of distribution characteristics of distance values of a subject; 
         FIG. 13  is an exemplary timing chart when signal amounts S 0  and S 1  are obtained in the stated order using second signal light (adjustment light) and each signal amount is obtained once in Embodiment 1 of the present disclosure; 
         FIG. 14  is an exemplary timing chart when signal amounts S 0 , S 1 , and BG are obtained in the stated order using second signal light (adjustment light) and each signal amount is obtained once; 
         FIG. 15  is an exemplary timing chart when signal amounts S 1 , S 0 , and BG are obtained in the stated order using second signal light (adjustment light) and each signal amount is obtained once; 
         FIG. 16  is an exemplary flowchart illustrating a procedure different from the procedure shown in  FIG. 8 ; 
         FIG. 17  is an exemplary flowchart when second signal light (light for distance measurement) is readjusted by alternate emissions of the first signal light (adjustment light) and the second signal light (light for distance measurement); 
         FIG. 18  is an exemplary flowchart when second signal light (light for distance measurement) is readjusted by emission of the first signal light (adjustment light) after a lapse of a predetermined period; 
         FIG. 19  is an exemplary flowchart when second signal light (light for distance measurement) is readjusted based on the result of comparing distribution characteristics of distance values calculated with first signal light (adjustment light) and distribution characteristics of distance values calculated with second signal light (light for distance measurement); 
         FIG. 20  is an exemplary flowchart when emission of first signal light (adjustment light) is adjusted in the case where a signal is saturated at the time of calculating distribution characteristics of distance values using the first signal light (adjustment light); 
         FIG. 21  is an exemplary timing chart when signal amounts S 0  and S 1  are obtained in the stated order using second signal light (adjustment light) and each signal amount is obtained once in Embodiment 2 of the present disclosure; 
         FIG. 22  is an exemplary timing chart when signal amounts S 0 , S 1 , and BG are obtained in the stated order using second signal light (adjustment light) and each signal amount is obtained once; and 
         FIG. 23  is an exemplary timing chart when signal amounts S 1 , S 0 , and BG are obtained in the stated order using second signal light (adjustment light) and each signal amount is obtained once. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following is a detailed description of embodiments of the present disclosure, with reference to accompanying drawings. It should be noted that the present disclosure is not limited to the following description and that any person skilled in the art should readily appreciate the possibility of modifying the form and details thereof in various ways without departing from the essence and scope of the present disclosure. Thus, the present disclosure should not be interpreted as limited to the following embodiments. In the configuration of the present disclosure below, the same portions or the portions having similar functions appearing in different figures are assigned the same reference signs, and the descriptions thereof will not be repeated. 
     Embodiment 1 
       FIG. 1  is a block diagram illustrating a schematic configuration of range imaging apparatus  100  according to one embodiment of the present disclosure. Range imaging apparatus  100  illustrated in  FIG. 1  is a range imaging apparatus of a TOF system, and includes light illuminator  101  that emits signal light toward a subject and calculates a signal amount, light receiver  102  that receives reflection light, which is light resulting from reflection of the signal light off the subject, range image generator  103  that calculates the distance from range imaging apparatus  100  to the subject based on time difference between the emitted light and the reflection light and generates a range image whose pixels each have the calculated distance as a distance value, distance value distribution analyzer  104  that analyzes distribution characteristics of the distance values, signal adjuster  105  that determines an adjustment value of light emission and exposure, illumination controller  106  that adjusts the light emission, and exposure controller  107  that adjusts the exposure. The signal adjuster controls at least one of illumination controller  106  and exposure controller  107  based on the distribution characteristics obtained through emission and reception of the signal light. 
     Light that is emitted by light illuminator  101  can include infrared light, a laser beam and so on. An image sensor used in light receiver  102  can include a charge coupled device (CCD) sensor, a complementary metal-oxide semiconductor (CMOS) sensor and so on. 
     How to determine the adjustment value can include a method of determining the adjustment value based on a maximum value, an average value, threshold setting, and a local maximum value, etc. of the signal amount indicated by the distribution characteristics analyzed by distance value distribution analyzer  104 , and a method of providing a signal amount adjustment table in signal adjuster  105  and determining the adjustment value in correspondence with the result of distribution characteristics. 
     Examples of a parameter for adjusting illumination controller  106  can include intensity, light emission period and illumination count of signal light for illumination. Examples of a parameter for adjusting exposure controller  107  can include exposure period, exposure timing and so on. Furthermore, examples of a method for adjusting a parameter can include changing a shutter speed of an image sensor, using a diaphragm mechanism, and using a filter capable of changing light transmittance. 
     The present disclosure is characterized by calculating a distance value to the subject in the TOF system, and adjusting signal light based on the distribution characteristics of the calculated distance value. For convenience of description, two kinds of signal light, namely, first signal light and second signal light will be used below. The first signal light is signal light used for adjusting light emission and exposure of the second signal light, and the second signal light is signal light used for generating a highly accurate range image corresponding to the position and number of the subject. 
     First, how to measure the distance to the subject in the TOF system will be described. In the TOF system, when light illuminator  101  emits light (illumination light) toward the subject and light reflecting off the subject (reflection light) reaches light receiver  102 , a time period from when the illumination light reaches and then reflects off the subject to when the reflection light reaches light receiver  102  is detected, thus calculating (measuring) the distance from range imaging apparatus  100  to the subject. Distance L from range imaging apparatus  100  to the subject is expressed by L=(c×ΔT)/2, where c indicates the speed of light (about 3.0×108 m/s) and ΔT indicates a time period from when light illuminator  101  emits the signal light to when the reflection light reaches light receiver  102 . 
     A specific driving method to which the present disclosure is to be applied will be described with reference to a timing chart in  FIG. 2 .  FIG. 2  is an exemplary timing chart of illumination light  201 , reflection light  202  and exposure  203 . 
     As illustrated in the timing chart of illumination light  201 , time T 1  indicates a first illumination start time, time T 3  indicates a first illumination end time, and a period from time T 1  to time T 3  indicates a first light emission period. Time T 6  indicates a second illumination start time, time T 8  indicates a second illumination end time, and a period from time T 6  to time T 8  indicates a second light emission period. Although the first light emission period and the second light emission period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     As illustrated in the timing chart of reflection light  202 , time T 2  indicates a first reflection start time, time T 4  indicates a first reflection end time, and a period from time T 2  to time T 4  indicates a first reflection period. Time T 7  indicates a second reflection start time, time T 9  indicates a second reflection end time, and a period from time T 7  to time T 9  indicates a second reflection period. Although the light emission period and the reflection period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     As illustrated in the timing chart of exposure  203 , time T 1  indicates a first exposure start time, time T 5  indicates a first exposure end time, and a period from time T 1  to time T 5  indicates a first exposure period (a time period during which the light receiver is exposed to the entire reflection light amount). Time T 8  indicates a second exposure start time, time T 10  indicates a second exposure end time, and a period from time T 8  to time T 10  indicates a second exposure period (a time period during which the light receiver is exposed to the delayed reflection light amount). The first exposure period and the second exposure period are equal in length, and the exposure period is 2T, which is twice as long as the light emission period T. However, there is no particular limitation to this. 
     Furthermore, as illustrated in the timing chart of exposure  203 , a period from time T 2  to time T 4  indicates a first reflection light reception period, and the signal amount obtained during this period (the signal amount obtained during the time period for the exposure to the entire reflection light amount) is referred to as “signal amount S 0 ”. In addition, a period from time T 8  to time T 9  indicates a second reflection light reception period, and the signal amount obtained during this period (the signal amount obtained during the time period for the exposure to the delayed reflection light amount) is referred to as “signal amount S 1 ”. 
     Herein, the exposure indicates that the light receiver accumulates all of the reflection light resulting from signal light emitted toward and reflecting off the subject that is present within a target distance range. The reception indicates that the light receiver accumulates the reflection light resulting from the signal light emitted toward and reflecting off the subject that is present at a certain distance within the target distance range. 
     In this chart, the first exposure start time is at the same timing as the first illumination start time, and the second exposure start time is at the same timing as the second illumination end time. However, there is no particular limitation to this. 
     In the following, the method for calculating distance L to the subject using signal amount S 0  obtained by the first illumination, reflection and exposure and signal amount S 1  obtained by the second illumination, reflection and exposure as above will be explained with reference to calculation formulae. 
     When T indicates the illumination period (light emission period), ΔT indicates a period from when signal light is emitted toward, reaches and reflects off the subject to when the signal light is received, c indicates the speed of light, and K indicates a constant of proportionality of the signal light, distance L from a light source to the subject can be expressed by formula 5, which is derived from formulae 1 to 4. 
     
       
         
           
             
               
                 
                   
                     [Formula 1] 
                   
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       FIG. 3  is an exemplary timing chart of illumination light  301 , reflection light  302  and exposure  303 . Irrespective of the illumination light, signal amount S 0  and signal amount S 1  also include a signal amount generated from ambient light. Thus, it may be possible to obtain signal amount BG of non-illumination period (a period from time T 11  to time T 12 ) as illustrated in the timing chart of  FIG. 3 , and subtract signal amount BG from signal amount S 0  and signal amount S 1 , thereby calculating distance L to the subject as shown in formula 6. 
     
       
         
           
             
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       FIG. 4  is an exemplary timing chart of illumination light  401 , reflection light  402  and exposure  403 . It may be possible to set the exposure start time of signal amount S 1  earlier than the first illumination start time as illustrated in the timing chart of  FIG. 4 , and when obtaining the signal light, calculate distance L to the subject by the calculation formula shown in formula 12, which is derived from formulae 7 to 11. The calculation formula at the time of considering the signal amount generated from ambient light is shown by formula 13. 
     
       
         
           
             
               
                 
                   
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     In this manner, since light speed c and light emission period T are known, when signal amount S 0  and signal amount S 1  are obtained, it is possible to calculate distance L to the subject. 
     Moreover, in the obtaining methods indicated by formulae 5 and 6, it is conceivable to obtain each of signal amounts (S 0 , S 1 , and BG) plural times and use a total value thereof. 
     With regard to the obtaining method indicated by formula 5, the case of obtaining each signal amount plural times will be explained with reference to the timing charts illustrated in  FIG. 5A  to  FIG. 5C .  FIG. 5A  to  FIG. 5C  are exemplary timing charts of illumination light  501 , reflection light  502  and exposure  503 .  FIG. 5A  to  FIG. 5C  illustrate the case of emitting signal light N times (N is a natural number) for obtaining each signal amount. A period from time T 1  to time T 20  is a reception period of signal amount S 0 , and a period from time T 21  to time T 40  is a reception period of signal amount S 1 . Signal amount S 0  used for calculating the distance value is a sum of signal amount S 0  obtained from time T 2  to time T 4 , signal amount S 0  obtained from time T 7  to time T 9 , signal amount S 0  obtained from time T 12  to time T 14 , signal amount S 0  obtained from time T 17  to time T 19 , and so on. In other words, it is a sum of signal amounts S 0  obtained N times. Signal amount S 1  used for calculating the distance value is a sum of signal amount S 1  obtained from time T 23  to time T 24 , signal amount S 1  obtained from time T 28  to time T 29 , signal amount S 1  obtained from time T 33  to time T 34 , signal amount S 1  obtained from time T 38  to time T 39 , and so on. In other words, it is a sum of signal amounts S 1  obtained N times. Signal amount S 0  and signal amount S 1  of first signal light (adjustment light) obtained in this manner based on the case illustrated in  FIG. 5A  to  FIG. 5C  may be applied to formula 5. 
     Additionally, timing charts when considering the signal amount generated by ambient light included in signal amount S 0  and signal amount S 1  will be illustrated in  FIG. 6A  to  FIG. 6D .  FIG. 6A  to  FIG. 6D  are exemplary timing charts of illumination light  601 , reflection light  602  and exposure  603 .  FIG. 6A  to  FIG. 6D  illustrate the case of emitting signal light N times (N is a natural number) for obtaining each signal amount. The description from time T 1  to time T 40  is similar to that of  FIG. 5A  to  FIG. 5C , and thus will be omitted. A period from time T 41  to time T 48  is a reception period of signal amount BG. Signal amount BG used for calculating the distance value is a sum of signal amount BG obtained from time T 41  to time T 42 , signal amount BG obtained from time T 43  to time T 44 , signal amount BG obtained from time T 45  to time T 46 , signal amount BG obtained from time T 47  to time T 48 , and so on. In other words, it is a sum of signal amounts BG obtained N times. Signal amounts S 0 , S 1 , and BG of first signal light (adjustment light) obtained in this manner based on the case illustrated in  FIG. 6A  to  FIG. 6D  may be applied to formula 6. 
     Furthermore, it is also conceivable to obtain a signal amount plural times in the case of setting the exposure start time of signal amount S 1  earlier than the first illumination start time described with reference to the timing chart of  FIG. 4  (see  FIG. 7A  to  FIG. 7C ).  FIG. 7A  to  FIG. 7C  are exemplary timing charts of illumination light  701 , reflection light  702  and exposure  703 . Calculation formulae at this time are formulae 12 and 13. 
     Now, referring to  FIG. 8  to  FIG. 15 , an operation procedure of range imaging apparatus  100  according to Embodiment 1 will be described by way of a specific example in which a range image illustrated in  FIG. 11  of subject  903  illustrated in  FIG. 9  is generated.  FIG. 8  is a flowchart illustrating the operation procedure of range imaging apparatus  100  according to Embodiment 1. 
     Light illuminator  101  emits the first signal light (adjustment light) toward the subject, and light receiver  102  receives the reflection light resulting from the reflection of the adjustment light off the subject, thus obtaining each of signal amounts (S 0 , S 1 , and BG) (S 801 ). 
     Then, the operation of obtaining each of the signal amounts N times (N is a natural number) is repeated (S 802 ). The timing chart with regard to the emission and reception of signal light and the obtainment of signal amounts is illustrated in  FIG. 10 .  FIG. 10  is an exemplary timing chart of illumination light  1001 , reflection light  1002  and exposure  1003 . This figure illustrates the operation of N=1 and the case of obtaining signal amount S 0  and signal amount S 1 . 
     Light receiver  102  includes a plurality of light receiving elements, which can each obtain a signal amount. Accordingly, range image generator  103  calculates distance information (a distance value) based on a plurality of signal amounts that are obtained by light receiver  102  (S 803 ), and further generates range image  1100  configured by a plurality of distance values as illustrated by  FIG. 11 . 
     Distance value distribution analyzer  104  calculates distribution characteristics of the distance values as illustrated by  FIG. 12A  and  FIG. 12B  based on the range image generated by range image generator  103 , and stores the distribution characteristics therein (S 804 ). 
     Based on the distribution characteristics of the distance values calculated by distance value distribution analyzer  104 , signal adjuster  105  calculates adjustment values of illumination controller  106  and exposure controller  107  (S 805 ). More specifically, signal adjuster  105  calculates a nearest distance (N) based on the result of the distribution characteristics, calculates an exposure-free period (Δα) based on the nearest distance (N), calculates exposure start time (EXS) based on the exposure-free period (Δα), and calculates illumination count (EC) and exposure count (EXC) based on the exposure-free period (Δα). 
     The method for determining the adjustment values can include a method of determining an adjustment value based on a maximum value, an average value and threshold setting, and a local maximum value, etc., of a signal amount indicated by the distribution characteristics, and a method of providing a signal amount adjustment table in signal adjuster  105  and determining the adjustment value in correspondence with the result of distribution characteristics. 
     In the following, each of the adjustment values will be described more specifically. 
     The nearest distance (N) is the nearest depth at which a subject is assumed to be present. The nearest distance (N) varies depending on the distance from an imaging device such as a camera to the subject. In other words, the nearest distance (N) decreases as the subject lies closer to the imaging device. 
     The exposure-free period (Δα) is a period during which exposure is not performed in the depth range where no subject is present. The exposure-free period (Δα) is calculated based on the light speed c and the depth range where no subject is present. For example, when it is determined based on the calculation result of the distribution characteristics that no subject is present in the depth range of 0 m to 1.11 m, the exposure-free period (Δα) can be calculated to be 7.4 ns from the formula 1.11=(3.0×108×Δα)/2, with light speed c being 3.0×108 m/s. 
     The exposure start time (EXS) is time of starting exposure of reflection light, and calculated based on the exposure-free period (Δα). The exposure start time (EXS) gets later with an increase in the exposure-free period (Δα), and vice versa. 
     The illumination count (EC) is the number of times each signal amount is obtained, and calculated based on the exposure-free period (Δα). The presence of the exposure-free period (Δα) avoids unnecessary exposure, but reduces the signal amount to be obtained, leading to deterioration of an S/N ratio. Accordingly, by providing the illumination count (EC) to control the number of times each signal amount is obtained, the deterioration of the S/N ratio is prevented. 
     The exposure count (EXC) is the number of times the reflection light is exposed, and calculated based on the exposure-free period (Δα). Furthermore, the exposure count (EXC) is twice as large as the illumination count (EC). 
     In this manner, the exposure-free period (Δα) and the exposure start time (EXS) are set to receive only the necessary reflection light, thereby improving the resolution of the signal amount. Moreover, by providing the illumination count (EC) and the exposure count (EXC), it is possible to prevent the deterioration of the S/N ratio with a decrease in the signal amount that is obtained. 
     Based on the adjustment values determined by signal adjuster  105 , signal adjuster  105  controls illumination controller  106  and exposure controller  107 , thus adjusting the emission and reception of the second signal light (light for distance measurement) (S 806 ). The illumination count (EC) is the adjustment value of illumination controller  106 , and the exposure start time (EXS) and the exposure count (EXC) are the adjustment values of exposure controller  107 . In addition to the above-noted adjustment values, the illumination intensity (E) may be adjusted to the limit to which the signal amount is saturated. 
     Next, the distance measurement of a subject using the second signal light (light for distance measurement) will be described. 
     After the second signal light (light for distance measurement) is adjusted, light illuminator  101  emits the second signal light (light for distance measurement) toward the subject. Similar to the first signal light (adjustment light), light receiver  102  is exposed to reflection light of the second signal light and obtains the signal amount (S 807 ). 
     With reference to the timing chart in  FIG. 13 , a specific driving method using the second signal light (light for distance measurement) will be described.  FIG. 13  is an exemplary timing chart of illumination light  1301 , reflection light  1302  and exposure  1303 . Although  FIG. 13  represents the case in which it is determined that the illumination count (EC)=1, the same will apply to the case of emitting the signal light N times. 
     As illustrated in the timing chart of illumination light  1301 , time T 1  indicates a first illumination start time, time T 4  indicates a first illumination end time, and a period from time T 1  to time T 4  indicates a first light emission period. Time T 8  indicates a second illumination start time, time T 11  indicates a second illumination end time, and a period from time T 8  to time T 11  indicates a second light emission period. Although the first light emission period and the second light emission period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     As illustrated in the timing chart of reflection light  1302 , time T 3  indicates a first reflection start time, time T 6  indicates a first reflection end time, and a period from time T 3  to time T 6  indicates a first reflection period. Time T 10  indicates a second reflection start time, time T 13  indicates a second reflection end time, and a period from time T 10  to time T 13  indicates a second reflection period. Although the light emission period and the reflection period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     As illustrated in the timing chart of exposure  1303 , time T 2  indicates a third exposure start time, time T 4  indicates a third exposure end time, and a period from time T 2  to time T 4  indicates a third exposure period (an exposure period of signal amount S 0  for the first time). Time T 5  indicates a fourth exposure start time, time T 7  indicates a fourth exposure end time, and a period from time T 5  to time T 7  indicates a fourth exposure period (an exposure period of signal amount S 0  for the second time). Time T 12  indicates a fifth exposure start time, time T 14  indicates a fifth exposure end time, and a period from time T 12  to time T 14  indicates a fifth exposure period (an exposure period of signal amount S 1  for the first time). Time T 15  indicates a sixth exposure start time, time T 16  indicates a sixth exposure end time, and a period from time T 15  to time T 16  indicates a sixth exposure period (an exposure period of signal amount S 1  for the second time). Although the third exposure period to the sixth exposure period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     Moreover, as illustrated in the timing chart of exposure  1303 , a period from time T 3  to time T 4  indicates a third reflection light reception period, and a period from time T 5  to time T 6  indicates a fourth reflection light reception period. Now, a sum of the signal amounts obtained in the third reflection light reception period and the fourth reflection light reception period is replaced by S 0  in formulae 1 to 13. Furthermore, a period from time T 12  to time T 13  indicates a fifth reflection light reception period, and a period from time T 15  to time T 16  indicates a sixth reflection light reception period. Now, a sum of the signal amounts obtained in the fifth reflection light reception period and the sixth reflection light reception period is replaced by S 1  in formulae 1 to 13. 
     The third exposure period and the fourth exposure period correspond to the first exposure period in  FIG. 2 . The fifth exposure period and the sixth exposure period correspond to the second exposure period in  FIG. 2 . 
     Although the period from time T 1  to time T 2 , the period from time T 4  to time T 5 , the period from time T 8  to time T 9 , the period from time T 11  to time T 12 , and the period from time T 14  to time T 15  are the exposure-free period (Δα) and equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     In this chart, the third exposure start time (time T 2 ) is timing at which the exposure-free period (Δα) has passed since the first illumination start time (time T 1 ), the fourth exposure start time (time T 5 ) is timing at which the exposure-free period (Δα) has passed since the first illumination end time (time T 4 ), the fifth exposure start time (time T 12 ) is timing at which the exposure-free period (Δα) has passed since the second illumination end time (time T 11 ), and the sixth exposure start time (time T 15 ) is timing at which the exposure-free period (Δα) has passed since the fifth exposure end time (time T 14 ). However, there is no particular limitation to these. 
     The above-described processing is repeated N times (the time set by the illumination count (EC)), thus obtaining each of the signal amounts (S 808 ). 
     Next, a distance value is calculated based on the obtained signal amounts (S 809 ). With reference to the exemplary timing chart illustrated in  FIG. 13 , the calculation of the distance value will be described using the formulae. When T indicates the light emission period, c indicates the light speed, ΔT indicates a period from when signal light is emitted and reaches the subject to when the signal light reflects off the subject and is received by light receiver  102 , Δα indicates the exposure-free period, Δβ indicates the difference between ΔT and Δα (Δβ=ΔT−Δα), and Δγ indicates the difference between T and ΔT (Δγ=T−ΔT), distance L to the subject can be expressed by formula 23, which is derived from formulae 14 to 22. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       14 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     S 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   = 
                   
                     K 
                     × 
                     
                       ( 
                       
                         Δβ 
                         × 
                         Δγ 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       15 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     S 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     K 
                     × 
                     Δβ 
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       16 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     c 
                     × 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         T 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       17 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         
                           ( 
                           
                             Δα 
                             + 
                             Δβ 
                           
                           ) 
                         
                       
                       2 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           ∵ 
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T 
                           
                         
                         = 
                         
                           Δα 
                           + 
                           Δβ 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       18 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         c 
                         × 
                         Δβ 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       19 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           Δβ 
                         
                         2 
                       
                       × 
                       
                         
                           Δβ 
                           + 
                           Δγ 
                         
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       20 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               Δβ 
                               + 
                               Δγ 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         Δβ 
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       21 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               Δα 
                               + 
                               Δβ 
                               + 
                               Δγ 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         Δβ 
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       22 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               T 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         Δβ 
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             ∵ 
                             T 
                           
                           = 
                           
                             Δα 
                             + 
                             Δβ 
                             + 
                             Δγ 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       23 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               T 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         
                           S 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         
                           S 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           0 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   23 
                   ) 
                 
               
             
           
         
       
     
     Similarly to the first signal light (adjustment light), signal amount S 0  and signal amount S 1  that are obtained based on the emission of the second signal light (light for distance measurement) include the signal amount caused by reflection of ambient light off surrounding objects. Thus, it is also possible to calculate the distance value by subtracting signal amount BG obtained during an unilluminated period from signal amount S 0  and signal amount S 1 . This processing is illustrated by the timing chart in  FIG. 14 , and can be expressed by formula 24.  FIG. 14  is an exemplary timing chart of illumination light  1401 , reflection light  1402 , and exposure  1403 . 
     
       
         
           
             
               [ 
               
                 Formula 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 24 
               
               ] 
             
             ⁢ 
             
                 
             
           
         
       
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               T 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           - 
                           BG 
                         
                         
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                           - 
                           BG 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   24 
                   ) 
                 
               
             
           
         
       
     
     Moreover, the timing chart in  FIG. 15  illustrates the case of adopting a method of obtaining a signal amount by making the exposure start time of signal amount S 1  earlier than the first illumination start time.  FIG. 15  is an exemplary timing chart of illumination light  1501 , reflection light  1502 , and exposure  1503 . In  FIG. 15 , distance L to the subject can be calculated by formula 35, which is derived from formulae 25 to 34. The calculation formula at the time of considering the signal amount generated from ambient light is shown by formula 36. Variables used in these formulae are similar to those in formulae 14 to 24. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       25 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     S 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   = 
                   
                     K 
                     × 
                     
                       ( 
                       
                         Δβ 
                         + 
                         Δγ 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   25 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       26 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     S 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     K 
                     × 
                     Δγ 
                   
                 
               
               
                 
                   ( 
                   26 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       27 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     c 
                     × 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         T 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   27 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       28 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         
                           ( 
                           
                             Δα 
                             + 
                             Δβ 
                           
                           ) 
                         
                       
                       2 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           ∵ 
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T 
                           
                         
                         = 
                         
                           Δα 
                           + 
                           Δβ 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   28 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       29 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         c 
                         × 
                         Δβ 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   29 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       30 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           Δβ 
                         
                         2 
                       
                       × 
                       
                         
                           Δβ 
                           + 
                           Δγ 
                         
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   30 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       31 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               Δβ 
                               + 
                               Δγ 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         Δβ 
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   31 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       32 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               Δβ 
                               + 
                               Δγ 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         
                           Δβ 
                           + 
                           Δγ 
                           - 
                           Δγ 
                         
                         
                           Δβ 
                           + 
                           Δγ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   32 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       33 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               Δβ 
                               + 
                               Δγ 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             Δγ 
                             
                               Δβ 
                               + 
                               Δγ 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   33 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       34 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               T 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             Δγ 
                             
                               Δβ 
                               + 
                               Δγ 
                             
                           
                         
                         ) 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             ∵ 
                             T 
                           
                           = 
                           
                             Δα 
                             + 
                             Δβ 
                             + 
                             Δγ 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   34 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       35 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               T 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               0 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   35 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       36 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               T 
                               - 
                               Δα 
                             
                             ) 
                           
                         
                         2 
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               
                                 S 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                               - 
                               BG 
                             
                             
                               
                                 S 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 0 
                               
                               - 
                               BG 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   36 
                   ) 
                 
               
             
           
         
       
     
     In the above formulae, light emission period T and the exposure-free period (Δα) are known. Thus, similarly to the first signal light (adjustment light), by obtaining signal amount S 0 , signal amount S 1 , and signal amount BG, range image generator  103  can calculate distance L to the subject and generate the range image (S 809 , S 810 ). 
     Additionally, as in the flowchart (S 1601  to S 1610 ) illustrated in  FIG. 16 , a method of repeating the generation of the range image using the second signal light (light for distance measurement) is also conceivable (i.e., repeating the processes indicated by S 1607  to S 1610 ). 
     The following is a description of readjustment of the second signal light (light for distance measurement) with reference to  FIG. 17  to  FIG. 19 . 
     In order to follow the movement of range imaging apparatus  100  and subject  903 , the readjustment of the second signal light (light for distance measurement) is necessary. When readjusting the second signal light (light for distance measurement), light illuminator  101  and light receiver  102  are set to have values similar to the first signal light (adjustment light) using illumination controller  106  and exposure controller  107 . There are several methods of determining timing of making the readjustment. 
     Method 1: as in S 1701  to S 1711  in  FIG. 17 , the emission of the first signal light (adjustment light) and that of the second signal light (light for distance measurement) are alternated, thereby readjusting the second signal light (light for distance measurement). Similarly to the method described with reference to  FIG. 8 , after the emission and exposure of the second signal light (light for distance measurement) are adjusted based on the result obtained from the first signal light (adjustment light), the range image is generated using the second signal light (light for distance measurement) (S 1701  to S 1710 ). Subsequently, the emission and exposure are adjusted to the first signal light (adjustment light) (S 1711 ). Using the first signal light (adjustment light), the adjustment of the second signal light (light for distance measurement) is made again. 
     Method 2: as in S 1801  to S 1812  in  FIG. 18 , the first signal light (adjustment light) is emitted at fixed time intervals, thereby readjusting the second signal light (light for distance measurement). Similarly to the method described with reference to  FIG. 8 , the emission and exposure of the second signal light (light for distance measurement) are adjusted based on the result obtained from the first signal light (adjustment light) (S 1801  to S 1806 ). From when the adjustment of the second signal light (light for distance measurement) is ended, time measurement is started. If a predetermined period has not elapsed since the adjustment, the range image is generated repeatedly using the second signal light (light for distance measurement) (S 1808  to S 1811 ). If a predetermined period has elapsed, then the process returns to the emission and exposure of the first signal light (adjustment light) (S 1812 ). Using the first signal light (adjustment light), the adjustment of the second signal light (light for distance measurement) is made again. 
     Method 3: as in S 1901  to S 1914  in  FIG. 19 , the distribution characteristics calculated using the first signal light (adjustment light) and those calculated using the second signal light (light for distance measurement) are compared. If these two distribution characteristics are different, the second signal light (light for distance measurement) is readjusted. Similarly to the method described with reference to  FIG. 8 , the distribution characteristics of the distance value are calculated using the first signal light (adjustment light) (S 1901  to S 1904 ), and then stored (S 1905 ). Thereafter, the second signal light (light for distance measurement) is adjusted, and the distribution characteristics of the distance value are calculated using the second signal light (light for distance measurement) (S 1906  to S 1911 ). The distribution characteristics calculated using the first signal light (adjustment light) and those calculated using the second signal light (light for distance measurement) are compared (S 1912 ). If histograms of these two are not very different, the range image is generated (S 1913 ). If they are very different, then the process returns to the emission and exposure of the first signal light (adjustment light) (S 1914 ). Using the first signal light (adjustment light), the adjustment of the second signal light (light for distance measurement) is made again. For example, when  FIG. 12A  illustrates the distribution characteristics calculated using the first signal light and  FIG. 12B  illustrates the distribution characteristics calculated using the second signal light, frequencies of range  10 - 11  and range  11 - 12  are higher in  FIG. 12B  than in  FIG. 12A , so that it is assumed that another subject should have entered here. In this case, when a predetermined value is set and the difference in frequency in a specific range exceeds the predetermined value, it is considered that the distribution characteristics are very different. Alternatively, when a range with the highest frequency shifts from range  8 - 9  to range  9 - 11 , it is assumed that the subject should have moved. 
     The three methods described above can be performed independently or in combination. 
     Finally, with regard to the calculation of the distribution characteristics using the first signal light (adjustment light), the case where the signal amount is saturated will be explained. 
     As in S 2001  to S 2013  in  FIG. 20 , similarly to the method described with reference to  FIG. 8 , the distribution characteristics of the distance value are calculated using the first signal light (adjustment light) (S 2001  to S 2004 ), and whether or not the signal amount is saturated is determined (S 2005 ). It should be noted that this determination is made based on whether or not a signal output from the light receiver is output at its maximum value. This is because the signal output from the light receiver is set to be maximal at a charge lower than or equal to a maximum charge that can be received by the light receiver. If the signal amount is saturated, an illumination intensity of the first signal light (adjustment light) is reduced (S 2006 ). Then, the distribution characteristics are calculated again using the first signal light (adjustment light). If the illumination intensity is adjusted equal to or more than predetermined times, it is determined that reflectivity of the subject is abnormally high, and the process is ended (S 2007 ). If the signal amount is not saturated, the distance value using the first signal light (adjustment light) is adjusted using the second signal light (light for distance measurement). The range image is generated using the adjusted second signal light (light for distance measurement) (S 2008  to S 2013 ). 
     Additionally, the method of readjusting the second signal light (light for distance measurement) may be combined with the method described above. 
     The processing as described above makes it possible to measure the distance to the subject and generate a highly accurate range image according to a position of the subject. 
     Embodiment 2 
     The following is a description of Embodiment 2, with reference to the accompanying drawings. It should be noted that the description overlapping Embodiment 1 will be omitted. 
     Embodiment 2 is different from Embodiment 1 in the number of the adjustment values calculated by signal adjuster  105  in  FIG. 1 , and similar to Embodiment 1 in that the distance to the subject is measured using the TOF system and that the distance to the subject is measured (the range image is generated) using the first signal light (adjustment light) and the second signal light (light for distance measurement). 
     Now, a specific procedure will be described. First, the procedure is similar to the method described above in Embodiment 1, and follows the procedure illustrated in  FIG. 8 . Light illuminator  101  emits first signal light (adjustment light) toward a subject, light receiver  102  receives reflection light resulting from reflection of the first signal light (adjustment light) off the subject, and signal amount S 0  and signal amount S 1  (further signal amount BG as needed) are obtained N times (N is a natural number). Based on the obtained signal amounts, range image generator  103  generates a range image. Based on the range image, distance value distribution analyzer  104  calculates distribution characteristics of distance values. Based on the calculated distribution characteristics, signal adjuster  105  calculates adjustment values of illumination controller  106  and exposure controller  107 . 
     Based on the distribution characteristics of the distance values calculated by distance value distribution analyzer  104 , signal adjuster  105  calculates the adjustment values of illumination controller  106  and exposure controller  107 . More specifically, signal adjuster  105  calculates a farthest distance (F) and a nearest distance (N) based on the distribution characteristics, calculates an illumination intensity (E) based on the farthest distance (F), calculates an exposure-free period (Δα) based on the nearest distance (N), calculates exposure start time (EXS) based on the exposure-free period (Δα), calculates an illumination period (T) based on the farthest distance (F) and the nearest distance (N), calculates an exposure period (EXT) based on the illumination period (T), and calculates illumination count (EC) and exposure count (EXC) based on the illumination intensity (E) and the illumination period (T). 
     Similarly to Embodiment 1, the method for determining the adjustment values can include a method of determining an adjustment value by a maximum value, an average value and threshold setting, and a local maximum value, etc., of a signal amount indicated by the distribution characteristics, and a method of providing a signal amount adjustment table in signal adjuster  105  and determining the adjustment value in correspondence with the result of distribution characteristics. 
     Hereinafter, each of the adjustment values will be described. 
     The farthest distance (F) is the farthest depth at which a subject is assumed to be present, whereas the nearest distance (N) is the nearest depth at which a subject is assumed to be present. The farthest distance (F) and the nearest distance (N) vary depending on the distance from an imaging device such as a camera to the subject. In other words, the nearest distance (N) decreases as the subject lies closer, and the farthest distance (F) increases as the subject lies farther. 
     The illumination intensity (E) is the intensity of signal light for illumination, and calculated based on the farthest distance (F). When the subject lies farther, the farthest distance (F) increases, and thus the illumination intensity (E) is raised so as to prevent the deterioration of the S/N ratio. On the other hand, when the subject lies closer, the farthest distance (F) decreases, and thus the illumination intensity (E) is reduced so as to prevent the saturation of the signal amount obtained from reflection light. It is assumed that the illumination intensity (E) should be adjusted to a saturation limit. 
     The exposure-free period (Δα) is a period for adjusting exposure timing of reflection light, and calculated based on the nearest distance (N). The exposure-free period (Δα) shortens with a decrease in the nearest distance (N), and vice versa. 
     The exposure start time (EXS) is time of starting exposure of reflection light, and calculated based on the exposure-free period (Δα). The exposure start time (EXS) gets later with an increase in the exposure-free period (Δα), and vice versa. 
     The illumination period (T) is a period during which signal light is emitted, and calculated based on an absolute value of the difference between the farthest distance (F) and the nearest distance (N). When the difference between the farthest distance (F) and the nearest distance (N) has a small value and the illumination period (T) is short, the depth range for distance measurement is narrow. On the other hand, when the difference between the farthest distance (F) and the nearest distance (N) has a large value and the illumination period (T) is long, the depth range for distance measurement is wide. 
     The exposure period (EXT) is a period during which reflection light resulting from the reflection of signal light off the subject is exposed. The exposure period (EXT) increases in keeping with the illumination period (T), and vice versa. Additionally, the exposure period (EXT) is twice as long as the illumination period (T). 
     The illumination count (EC) is the number of times each signal amount is obtained, and calculated based on the illumination intensity (E) and the illumination period (T). The adjustment of the illumination intensity (E) and the illumination period (T) avoids unnecessary exposure, but reduces the signal amount to be obtained, leading to deterioration of an S/N ratio. Accordingly, by providing the illumination count (EC) to control the number of times each signal amount is obtained, the deterioration of the S/N ratio is prevented. 
     A total illumination amount is a product of the illumination intensity (E), the illumination period (T) and the illumination count (EC). Since the total illumination amount varies depending on the reflectivity of the subject, it is adjusted to be close to its limit where a light reception amount of a pixel receiving the largest amount of light is saturated, for example. 
     The exposure count (EXC) is the number of times of exposure to reflection light, and calculated based on the illumination intensity (E) and the illumination period (T). Furthermore, the exposure count (EXC) is equal to the illumination count (EC). 
     The above adjustment values are used to adjust the second signal light (light for distance measurement). The illumination intensity (E), the illumination period (T), and the illumination count (EC) are values adjusted by illumination controller  106 , and the exposure period (EXT), the exposure start time (time of day when exposure is started) (EXS), and the exposure count (EXC) are values adjusted by exposure controller  107 . 
     In this manner, the depth at which the subject is present is determined based on the distribution characteristics of the distance value so that only the reflection light at that depth is exposed, thereby making it possible to reduce the signal amount to be obtained. On the other hand, the reduction of the signal amount brings about the deterioration of the S/N ratio, lowering the accuracy of the distance value. In view of this, the illumination intensity (E), the illumination period (T), and the illumination count (EC) have to be determined. Moreover, depending on the distribution characteristics of the distance value obtained using the first signal light (adjustment light), the first signal light (adjustment light) and the second signal light (light for distance measurement) may have the same illumination intensity (E), illumination period (T), illumination count (EC), exposure period (EXT), exposure start time (EXS), and exposure count (EXC). 
     It should be noted that the adjustment values described above may be adjusted alone or in combination. 
     Next, the distance measurement of a subject using the second signal light (light for distance measurement) will be described. 
     Similarly to Embodiment 1, light illuminator  101  emits the adjusted second signal light (light for distance measurement) toward the subject, light receiver  102  receives the reflection light resulting from the reflection of the second signal light (light for distance measurement) off the subject, and range image generator  103  calculates the range image of the subject based on the received signal amount. 
     With reference to the timing chart in  FIG. 21 , a specific driving method using the second signal light (light for distance measurement) will be described.  FIG. 21  is an exemplary timing chart of illumination light  2101 , reflection light  2102 , and exposure  2103 . Although  FIG. 21  represents the case in which it is determined that the illumination count (EC)=the exposure count (EXC)=1, the same will apply to the case of emitting the signal light N times (N is a natural number). 
     As illustrated in the timing chart of illumination light  2101 , time T 1  indicates a first illumination start time, time T 3  indicates a first illumination end time, and a period from time T 1  to time T 3  indicates a first light emission period. Time T 7  indicates a second illumination start time, time T 9  indicates a second illumination end time, and a period from time T 7  to time T 9  indicates a second light emission period. Although the first light emission period and the second light emission period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     As illustrated in the timing chart of reflection light  2102 , time T 4  indicates a first reflection start time, time T 5  indicates a first reflection end time, and a period from time T 4  to time T 5  indicates a first reflection period. Time T 10  indicates a second reflection start time, time T 12  indicates a second reflection end time, and a period from time T 10  to time T 12  indicates a second reflection period. Although the light emission period and the reflection period are equal in length, namely, time period T in this chart, there is no particular limitation to this. 
     As illustrated in the timing chart of exposure  2103 , time T 2  indicates a first exposure start time, time T 6  indicates a first exposure end time, and a period from time T 2  to time T 6  indicates a first exposure period (a reception period of signal light S 0 ). Time T 11  indicates a second exposure start time, time T 14  indicates a second exposure end time, and a period from time T 11  to time T 14  indicates a second exposure period (a reception period of signal light S 1 ). Although the first exposure period and the second exposure period are equal in length, namely, time period 2T in this chart, there is no particular limitation to this. 
     Furthermore, as illustrated by exposure  2103 , a period from time T 4  to time T 5  indicates a first reflection light reception period (a reception period of signal light S 0 ), and S 0  indicates a signal amount obtained during the first reflection light reception period. Moreover, a period from time T 11  to time T 12  indicates a second reflection light reception period (a reception period of signal light S 1 ), and S 1  indicates a signal amount obtained during the second reflection light reception period. 
     In addition, a period from time T 1  to time T 2  and a period from time T 9  to time T 11  are the exposure-free period (Δα) and equal in length. However, there is no particular limitation to this. 
     In this chart, the first exposure start time (time T 2 ) is timing at which the exposure-free period (Δα) has passed since the first illumination start time (time T 1 ), and the second exposure start time (time T 11 ) is timing at which the exposure-free period (Δα) has passed since the second illumination end time (time T 9 ). However, there is no particular limitation to these. 
     Now, the following is a description of how to calculate the distance value when exposure-free period Δα is provided in the second signal light (light for distance measurement). Distance L to the subject can be expressed by formula 44, which is derived from formulae 37 to 43, where T indicates the light emission period, c indicates the light speed, ΔT indicates a period from when signal light is emitted and reaches the subject to when the signal light reflects off the subject and is received by light receiver  102 , Δα indicates the exposure-free period, and Δβ indicates the difference between ΔT and Δα (Δβ=ΔT−Δα). 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       37 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       S 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                     
                     = 
                     
                       K 
                       × 
                       T 
                     
                   
                 
               
               
                 
                   ( 
                   37 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       38 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       S 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     = 
                     
                       K 
                       × 
                       Δβ 
                     
                   
                 
               
               
                 
                   ( 
                   38 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       39 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     = 
                     
                       c 
                       × 
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           T 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   39 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       40 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     = 
                     
                       
                         
                           c 
                           × 
                           
                             ( 
                             
                               Δα 
                               + 
                               Δβ 
                             
                             ) 
                           
                         
                         2 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             ∵ 
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               T 
                             
                           
                           = 
                           
                             Δα 
                             + 
                             Δβ 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   40 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       41 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     = 
                     
                       
                         
                           c 
                           × 
                           Δα 
                         
                         2 
                       
                       + 
                       
                         
                           c 
                           × 
                           Δβ 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   41 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       42 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     = 
                     
                       
                         
                           c 
                           × 
                           Δα 
                         
                         2 
                       
                       + 
                       
                         
                           
                             c 
                             × 
                             Δβ 
                           
                           2 
                         
                         × 
                         
                           T 
                           T 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   42 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       43 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     = 
                     
                       
                         
                           c 
                           × 
                           Δα 
                         
                         2 
                       
                       + 
                       
                         
                           
                             c 
                             × 
                             T 
                           
                           2 
                         
                         × 
                         
                           Δβ 
                           T 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   43 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       44 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     = 
                     
                       
                         
                           c 
                           × 
                           Δα 
                         
                         2 
                       
                       + 
                       
                         
                           
                             c 
                             × 
                             T 
                           
                           2 
                         
                         × 
                         
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   44 
                   ) 
                 
               
             
           
         
       
     
     Moreover, similarly to the first signal light (adjustment light), signal amount S 0  and signal amount S 1  include the signal amount of the reflection light resulting from reflection of ambient light off surrounding objects. Thus, it is also possible to calculate the distance value by subtracting signal amount BG obtained during an unilluminated period from signal amount S 0  and signal amount S 1 . This processing is illustrated by the timing chart in  FIG. 22 .  FIG. 22  is an exemplary timing chart of illumination light  2201 , reflection light  2202 , and exposure  2203 . Distance L to the subject can be expressed by formula 45. 
     
       
         
           
             
               [ 
               
                 Formula 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 45 
               
               ] 
             
             ⁢ 
             
                 
             
           
         
       
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           T 
                         
                         2 
                       
                       × 
                       
                         
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           - 
                           BG 
                         
                         
                           
                             S 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                           - 
                           BG 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   45 
                   ) 
                 
               
             
           
         
       
     
       FIG. 23  is an exemplary timing chart of illumination light  2301 , reflection light  2302 , and exposure  2303 . It may be possible to set the exposure start time of signal amount S 1  earlier than the first illumination start time as illustrated in the timing chart of  FIG. 23 , and when obtaining the signal light, calculate distance L to the subject by the calculation formula shown in formula 56, which is derived from formulae 46 to 55. The calculation formula in the case of considering the signal amount generated from ambient light is shown by formula 57. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       46 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     S 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   = 
                   
                     K 
                     × 
                     T 
                   
                 
               
               
                 
                   ( 
                   46 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       47 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     S 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     K 
                     × 
                     Δβ 
                   
                 
               
               
                 
                   ( 
                   47 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       48 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     c 
                     × 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         T 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   48 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       49 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         
                           ( 
                           
                             Δα 
                             + 
                             Δβ 
                           
                           ) 
                         
                       
                       2 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           ∵ 
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T 
                           
                         
                         = 
                         
                           Δα 
                           + 
                           Δβ 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   49 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       50 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         c 
                         × 
                         Δβ 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   50 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       51 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           Δβ 
                         
                         2 
                       
                       × 
                       
                         T 
                         T 
                       
                     
                   
                 
               
               
                 
                   ( 
                   51 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       52 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           T 
                         
                         2 
                       
                       × 
                       
                         Δβ 
                         T 
                       
                     
                   
                 
               
               
                 
                   ( 
                   52 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       53 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           T 
                         
                         2 
                       
                       × 
                       
                         
                           Δβ 
                           + 
                           T 
                           - 
                           T 
                         
                         T 
                       
                     
                   
                 
               
               
                 
                   ( 
                   53 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       54 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           T 
                         
                         2 
                       
                       × 
                       
                         
                           T 
                           - 
                           
                             ( 
                             
                               T 
                               - 
                               Δβ 
                             
                             ) 
                           
                         
                         T 
                       
                     
                   
                 
               
               
                 
                   ( 
                   54 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       55 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           T 
                         
                         2 
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               T 
                               - 
                               Δβ 
                             
                             T 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   55 
                   ) 
                 
               
             
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       56 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   L 
                   = 
                   
                     
                       
                         c 
                         × 
                         Δα 
                       
                       2 
                     
                     + 
                     
                       
                         
                           c 
                           × 
                           T 
                         
                         2 
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               0 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   56 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           [ 
                           
                             Formula 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             57 
                           
                           ] 
                         
                         ⁢ 
                         
                             
                         
                       
                     
                     
                       
                           
                       
                     
                   
                   
                     
                       
                         L 
                         = 
                         
                           
                             
                               c 
                               × 
                               Δα 
                             
                             2 
                           
                           + 
                           
                             
                               
                                 c 
                                 × 
                                 T 
                               
                               2 
                             
                             × 
                             
                               ( 
                               
                                 1 
                                 - 
                                 
                                   
                                     
                                       S 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                     - 
                                     BG 
                                   
                                   
                                     
                                       S 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       0 
                                     
                                     - 
                                     BG 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                     
                       
                           
                       
                     
                   
                 
               
               
                 
                   ( 
                   57 
                   ) 
                 
               
             
           
         
       
     
     As described above, values necessary for calculating distance value L to the subject are signal amounts (S 0 , S 1 , and BG), light emission period T, and exposure-free period Δα. Since light emission period T and exposure-free period Δα are known, it is possible to measure the distance to the subject and generate the range image by obtaining the signal amounts (S 0 , S 1 , and BG) similarly to the first signal light (adjustment light). 
     Furthermore, similarly to Embodiment 1, the readjustment of the second signal light (light for distance measurement) and the case where the signal amount is saturated in the calculation of the distribution characteristics using the first signal light (adjustment light) may also be added to the processing in Embodiment 2. Since they have been already described in Embodiment 1, the redundant description will be omitted here. 
     The processing as described above makes it possible to measure the distance to the subject and generate a highly accurate range image according to a position of the subject. 
     The present disclosure can be provided not only as a range imaging apparatus including individual components in Embodiment 1 and Embodiment 2 but also as a program that causes a computer to execute a range imaging method including steps corresponding to the individual components included in the range imaging apparatus. In addition, such a program can be distributed via a recording medium represented by a USB or via the Internet. 
     It is needless to say that the embodiments described above are merely an example. In other words, a concept for which a patent protection is sought is a generic concept resulting from abstraction of the embodiments described above. It is needless to say that such a generic concept may be carried out (implemented or realized) through the above-described exemplary embodiments or through some other embodiments that are partially or entirely different from the above-described embodiments. 
     INDUSTRIAL APPLICABILITY 
     The range imaging apparatus disclosed herein adjusts the emission and exposure of the second signal light (light for distance measurement) based on the distribution characteristics of the distance value calculated based on the range image obtained using the first signal light (adjustment light), thereby generating a highly accurate range image, and can be used in various equipment such as digital audio visual (AV) equipment and a game machine that require gesture recognition and object recognition. The range imaging apparatus according to the present disclosure is applicable to not only the equipment described above but also any range imaging apparatuses capable of obtaining necessary distribution characteristics of distance information. 
     Although only some exemplary embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure.