Patent Publication Number: US-10760906-B2

Title: Apparatus and method for obtaining three-dimensional depth image

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2015-0151103, filed on Oct. 29, 2015 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Apparatuses and methods consistent with exemplary embodiments relate to obtaining depth images. 
     2. Description of the Related Art 
     In order to obtain a depth image of a subject, a time-of-flight (ToF) method may be used to measure the time taken for light (e.g., infrared light) to travel a distance from a viewpoint to the subject. A ToF depth camera using such a ToF method may obtain depth information of a subject in all pixels in real time, unlike other cameras for obtaining a depth image of a subject, such as a stereo camera or a structured light camera. 
     In a ToF method, a depth image may be obtained by using a phase difference between light emitted to a subject and light reflected from the subject. 
     SUMMARY 
     One or more exemplary embodiments provide apparatuses and methods of obtaining depth images with reduced motion blur. 
     Further, one or more exemplary embodiments provide apparatuses and methods of obtaining images that may increase frame rates of depth images. 
     According to an aspect of an exemplary embodiment, there is provided an apparatus for obtaining an image including: a light source configured to emit first light to a first region of an object for a first time period and emit second light to a second region of the object for a second time period, the first light and the second light respectively being reflected from the first region and the second region; and an image obtainer configured to obtain a first partial depth image based on the reflected first light, obtain a second partial depth image based on the reflected second light, and obtain a first depth image of the object based on the first partial depth image and the second partial depth image. 
     At least parts of the first time period and the second time period may not overlap each other. 
     The second time period may be subsequent to the first time period. 
     The first time period may be less than a reciprocal of a reference frame rate of an image sensor included in the image obtainer. 
     The first time period may be less than or equal to a value obtained by multiplying a ratio of the first region to the object by a reciprocal of a reference frame rate. 
     An amount of the first light emitted to the first region may be greater than a reference amount of light. 
     The amount of the first light emitted to the first region may be greater than or equal to a value obtained by multiplying a ratio of the object to the first region by the reference amount of light. 
     At least parts of the first region and the second region may not overlap each other. 
     The first region may be an upper region of the object, and the second region may be a lower region of the object. 
     The first light emitted from the light source may include a plurality of pieces of light with different phases. 
     A phase difference between adjacent two pieces of light from among the plurality of pieces of light may be 90°. 
     The first partial depth image may be obtained from a plurality of partial images with different phases. 
     The light source may be further configured to emit third light to the first region of the object for a third time period, the third light being reflected from the first region, and the image obtainer may be further configured to obtain a third partial depth image of the first region based on the reflected third light, and may obtain a second depth image of the object based on the second partial depth image and the third partial depth image. 
     At least parts of the second time period and the third time period may not overlap each other. 
     The third time period may be subsequent to the second time period. 
     A time interval may exist between the second time period and the third time period. 
     The image obtainer may include: an image sensor configured to obtain a plurality of first partial images based on the first light reflected from the first region and to obtain a plurality of second partial images based on the second light reflected from the second region; and a processor configured to obtain the first partial depth image from the plurality of first partial images, obtain the second partial depth image from the plurality of second partial images, and obtain the first depth image of the object based on the first partial depth image and the second partial depth image. 
     The image sensor may further configured to modulate a plurality of pieces of the reflected first light into signals with different gain waveforms. 
     According to an aspect of another exemplary embodiment, there is provided a method of obtaining an image including: emitting first light to a first region of an object for a first time period, the first light being reflected from the first region; emitting second light to a second region of the object for a second time period, the second light being reflected from the second region; obtaining a first partial depth image based on the reflected first light; obtaining a second partial depth image based on the reflected second light; and obtaining a first depth image of the object based on the first partial depth image and the second partial depth image. 
     At least parts of the first time period and the second time period may not overlap each other. 
     The second time period may be subsequent to the first time period. 
     The first time period may be less than a reciprocal of a reference frame rate of an image sensor configured to receive the reflected first light and the reflected second light. 
     The first time period may be less than or equal to a value obtained by multiplying a ratio of the first region to the object by the reciprocal of the reference frame rate. 
     An amount of the light emitted to the first region may be greater than a reference amount of light. 
     The amount of the light emitted to the first region may be greater than or equal to a value obtained by multiplying a ratio of the object to the first region by the reference amount of light. 
     At least parts of the first region and the second region may not overlap each other. 
     The first region may be an upper region of the object and the second region may be a lower region of the object. 
     The emitting the first light may include emitting a plurality of pieces of the first light to the first region, and the plurality of pieces of the first light may have different phases from each other. 
     A phase difference between adjacent two pieces of light from among the plurality of pieces of light may be 90°. 
     The method may further include: emitting third light to the first region of the object for a third time period, the third light being reflected from the first region; obtaining a third partial depth image of the first region based on the reflected third light reflected; and obtaining a second depth image of the object based on the second partial depth image and the third partial depth image. 
     At least parts of the second time period and the third time period may not overlap each other. 
     The third time period may be a time interval next to the second time period. 
     A time interval exists between the second time period and the third time period. 
     According to an aspect of another exemplary embodiment, there is provided a method of obtaining an image by a depth image sensor, the method including: alternately emitting light to each of a plurality of regions of an object with a predetermined time period, the light being reflected from each of the plurality of regions; repeatedly obtaining a partial depth image based on the reflected light when each of the predetermined time period ends; and generating a depth image of the object based on the partial depth image obtained while the predetermined time period occurs k number of times to collect the reflected light from all of the plurality of regions of the object, k corresponding to a number of the plurality of regions of the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will be more apparent by describing certain exemplary embodiments, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an apparatus for obtaining an image, according to an exemplary embodiment; 
         FIG. 2  is a flowchart of a method of obtaining a depth image, according to an exemplary embodiment; 
         FIG. 3  is a flowchart of a method of obtaining a plurality of depth images, according to an exemplary embodiment; 
         FIG. 4  is a reference diagram for explaining a method of obtaining a depth image, according to an exemplary embodiment; 
         FIG. 5  is a reference diagram for explaining a method of obtaining a depth image after an image sensor obtains an image of an entire object, as a comparative example; 
         FIG. 6A  shows a depth image obtained by using the method of  FIG. 5 ; 
         FIG. 6B  shows a depth image obtained by using the method of  FIG. 4 ; 
         FIG. 7  is a reference diagram for explaining a method of obtaining a depth image, according to another exemplary embodiment; and 
         FIGS. 8A and 8B  are diagrams for explaining a method of obtaining a depth image when a duty ratio is 50%, according to another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments are described in greater detail below with reference to the accompanying drawings. 
     In the following description, like drawing reference numerals are used for like elements, even in different drawings. The matters defined in the description, such as detailed construction and elements, are provided to assist in a comprehensive understanding of the exemplary embodiments. However, it is apparent that the exemplary embodiments can be practiced without those specifically defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the description with unnecessary detail. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
       FIG. 1  is a block diagram of an apparatus (e.g., camera)  100  for obtaining an image according to an exemplary embodiment. Referring to  FIG. 1 , the apparatus  100  may include a light source  110  configured to emit light to an object  10 , an image obtainer  120  configured to obtain a depth image of the object  10  by using light reflected from the object  10 , and a controller  130  configured to control operations of the light source  110  and the image obtainer  120 . 
     Also, the apparatus  100  may further include a first lens  140  configured to collect reflected light to obtain an image and a filter  150  configured to selectively transmit light having a predetermined wavelength and remove background light or stray light. 
     The light source  110  may be, for example, a light-emitting diode (LED) or a laser diode (LD) that may emit light having a near-infrared (NIR) wavelength of about 850 nm that is not seen by the human eye. However, a wavelength band and a type of the light source  110  are not limited. 
     The object  10  may have a plurality of regions and the light source  110  may sequentially emit light to the divided regions at time intervals. For example, the light source  110  may emit light to a first region  11  of the object  10  for a first time period, and may emit light to a second region  12  of the object  10  for a second time period. At least parts of the first region  11  and the second region  12  may not overlap each other. The first region  11  may be an upper region of the object  10  and the second region  12  may be a lower region of the object  10 . Also, at least parts of the first time period and the second time period may not overlap each other. For example, the second time period may be a time interval subsequent to the first time period. 
     Alternatively, the light source  110  may include a first light source  112  configured to emit light to the first region  11  of the object  10  and a second light source  114  configured to emit light to the second region  12  of the object  10 . Hence, the first and second light sources  112  and  114  may be switched on or off to alternately emit light according to a control signal of the controller  130 . For example, the first light source  112  may be switched on to emit light for the first time period and the second light source  114  may be switched on to emit light for the second time period. The first light source  112  may be turned off for the second time period, and the second light source  114  may be turned off for the first time period. 
     The light source  110  may perform, for example, amplitude modulation or phase modulation and may output resultant light according to a control signal of the controller  130 . A light emission signal applied from the light source  110  to the object  10  according to a control signal of the controller  130  may be a periodic continuous function with a predetermined period. For example, the light emission signal may have a specially defined waveform such as a sine waveform, a ramp waveform, or a square waveform, or may have a general waveform that is not defined. 
     The image obtainer  120  may include an image sensor  122  configured to receive light reflected from the object  10  and to obtain an image and a processor  124  configured to obtain a depth image including depth information of the object  10  from the image. 
     The image sensor  122  modulates light reflected from the object  10  according to a control signal received from the controller  130 . For example, the image sensor  122  may modulate an amplitude of reflected light by changing a gain according to a light modulation signal with a predetermined waveform applied from the controller  130 . To this end, the image sensor  122  has a variable gain. 
     The image sensor  122  may operate at a high light modulation speed of tens to hundreds of MHz in order to detect a travel time or a phase difference of light according to a distance. Examples of the image sensor  122  may include a phase image intensifier including a multi-channel plate (MCP), a GaAs-based solid-state modulator element, and a thin modulator element using an electroptic material 
     The image sensor  122  may obtain an image from detected reflected light. If only a distance to one point of the object  10  is to be measured, the image sensor  122  may include, for example, a single light sensor such as a photodiode or an integrator. However, if distances to a plurality of points of the object  10  are to be simultaneously measured, the image sensor  122  may include a two-dimensional (2D) or one-dimensional (1D) array of photodiodes or light detectors. For example, the image sensor  122  may be a charge-coupled device (CCD) image sensor or a contact image sensor (CIS) including a 2D array. 
     The image sensor  122  may divide pixels into pixels of a plurality of regions according to a control signal of the controller  130 . The divided regions of the image sensor  122  may correspond to divided regions of the light source  110 . For example, when the light source  110  divides the object  10  into an upper region and a lower region, the controller  130  may divide pixels of the image sensor  122  into pixels of the upper region and pixels of the lower region. 
     The controller  130  may synchronize the light source  110  with the image sensor  122 . In detail, the controller  130  may control the image sensor  122  so that a region of the image sensor  122  corresponding to the first region  11  operates when the light source  110  emits light to the first region  11 . The controller  130  may control the image sensor  122  so that a region of the image sensor  122  corresponding to the second region  12  operates when the light source  110  emits light to the second region  12  of the object  10 . 
     For example, when the light source  110  emits light to an upper region of the object  10 , an upper region of the image sensor  122  may receive light reflected from the upper region of the object  10 . When the light source  110  emits light to a lower region of the object  10 , a lower region of the image sensor  122  may receive light reflected from the lower region of the object  10 . 
     The processor  124  may obtain a depth image based on an output of the image sensor  122 . The processor  124  may obtain a first partial depth image based on an output of the image sensor  122  corresponding to the first region  11  of the object  10 , and may obtain a second partial depth image based on an output of the image sensor  122  corresponding to the second region  12  of the object  10 . The processor  124  may generate a depth image (e.g., depth map) by using the first partial depth image and the second partial depth image. The depth image may be obtained by combining the first and second partial depth images based on coordinate information of the image sensor  122 . The depth image may include information about a distance of a surface of the object  10  from the light source  110 . 
     The processor  124  may be, for example, a specific integrated circuit (IC), or software provided in the apparatus  100 . When the processor  124  is software, the processor  124  may be stored in an additional movable storage medium. 
       FIG. 2  is a flowchart of a method of obtaining a depth image according to an exemplary embodiment. Referring to  FIG. 2 , in operation S 210 , the light source  110  may emit light to the first region  11  of the object  10  for a first time period. When the light source  110  includes the first and second light sources  112  and  114 , the first light source  112  may emit light to the first region  11  of the object  10 . However, the present exemplary embodiment is not limited thereto. The light source  110  may emit light to the first region  11  of the object  10  by adjusting an illumination angle. The first region  11  may be a region of the object  10 , and may be, for example, an upper region of the object  10 . 
     First, the light source  110  may sequentially emit a plurality of pieces of light with predetermined periods and predetermined waveforms to the first region  11  of the object  10  under the control of the controller  130 . The plurality of pieces of light with the predetermined periods and the predetermined waveforms may be different from one another. The light source  110  may continuously and sequentially emit the plurality of pieces of light, or may sequentially emit the plurality of pieces of light at predetermined intervals. 
     For example, when four different pieces of light are used, the light source  110  may generate first light for a time t 1  and emit the first light to the first region  11  of the object  10 , may generate second light for a time t 2  and emit the second light to the first region  11  of the object  10 , may generate third light for a time t 3  and emit the third light to the first region  11  of the object  10 , and may generate fourth light for a time t 4  and emit the fourth light to the first region  11  of the object  10 . The first through fourth lights may be sequentially emitted to the first region  11  of the object  10 . Each of the first through fourth lights may have a continuous function with a specific period such as a sine waveform or a pulse waveform. For example, the first through fourth lights may be periodic waves with the same period, the same waveform, and different phases. 
     When a plurality of pieces of light, for example, n pieces of light, are used, a phase difference between adjacent pieces of light may be 360°/n, and a period of each light may be less than an operation time of the light source  110 . All of the n pieces of light may be sequentially emitted to the first region  11  of the object  10  within the operation time of the light source  110 . 
     Since the light source  110  emits light to a region of the object  10 , an amount of the emitted light may be greater than a reference amount of light. The reference amount of light may refer to an average minimum amount of light that the entire image sensor  122  may use to generate one frame image. Since the light source  110  emits light to a region of the object  10  and does not emit to remaining regions of the object  10 , an amount of light is greater than the reference amount of light, and thus an average amount of light for the object  10  may be the reference amount of light. For example, when the light source  110  emits light to a 1/N (N is a natural number equal to or greater than 2) region, the light source  110  may emit light that has an amount which is N times greater than the reference amount of light. Alternatively, when the light source  110  discontinuously emits light, the light source  110  may emit light that has an amount which is N times or more as great as the reference amount of light. 
     Also, the first time period may be less than a reciprocal of a reference frame rate of the image sensor  122 . The reciprocal of the reference frame rate refers to a time taken for the image sensor  122  to generate one frame image. Since the light source  110  emits light to a region of the object  10  and the image sensor  122  receives light reflected from the region of the object  10 , the first time period is less than the reciprocal of the reference frame rate. For example, when the light source  110  emits light to a 1/N (N is a natural number equal to or greater than 2) region of the object  10 , the light source  110  may emit light for a time obtained by multiplying 1/N by the reciprocal of the reference frame rate, and the image sensor  110  may receive light for a time obtained by multiplying 1/N by the reciprocal of the reference frame rate. Alternatively, when light is discontinuously emitted to the first region  11  of the object  10 , the light may be emitted to the first region  11  of the object  10  for a time that is shorter than a time obtained by multiplying 1/N by the reciprocal of the reference frame rate. 
     Also, in operation S 220 , the light source  110  may emit light to the second region  12  of the object  10  for a second time period. When the light source  110  includes the first and second light sources  112  and  114 , the second light source  114  may emit light to the second region  12  of the object  10 . However, the present exemplary embodiment is not limited thereto. The light source  110  may emit light to the second region  12  of the object  10  by changing an illumination angle. The light source  110  may adjust the illumination angle so that the emitted light travels toward the first region  11  and the second region  12  alternately. The second region  12  may be a region of the object  10  and the size of the second region  12  may be equal to the size of the first region  11 . Also, at least parts of the second region  12  and the first region  11  may not overlap each other. For example, the second region  12  may be a lower region. 
     The light source  110  may sequentially emit a plurality of pieces of light with predetermined periods and predetermined waveforms to the second region  12  of the object  10  under the control of the controller  130 . The plurality of pieces of light with the predetermined periods and the predetermined waveforms may be different from one another. The light source  110  may continuously and sequentially emit the plurality of pieces of light or may sequentially emit the plurality of pieces of light at predetermined intervals. 
     At least parts of the second time period and the first time period may not overlap each other. For example, the second time period may be a time interval subsequent to the first time period. However, the present exemplary embodiment is not limited thereto. Lengths of the first time period and the second time period may be the same. For example, since the light source  110  emits light to the second region  12  that is a region of the object  10 , an amount of the emitted light may be greater than the reference amount of light. The second time period may be less than the reciprocal of the reference frame rate of the image sensor  122 . A method performed by the light source  110  to emit light to the second region  12  of the object  10  is the same as a method performed by the light source  110  to emit light to the first region  11  of the object  10 , and thus a detailed explanation thereof will not be given. 
     In operation S 230 , the image obtainer  120  may obtain a first partial depth image by using light reflected from the first region  11 . Light emitted to the first region  11  of the object  10  is reflected from a surface of the object  10  to the lens  140 . In general, the first region  11  of the object  10  may have a plurality of surfaces that have distances, that is, depths, from the apparatus  100  are different from one another. For example, first light may be reflected from a surface of the first region  11  of the object  10  to generate a first reflected light, second light may be reflected from a surface of the first region  11  to generate a second reflected light, and likewise, an nth light may be reflected from a surface of the first region  11  of the object  10  to generate an nth reflected light. 
     The lens  140  focuses reflected light within a region of the image sensor  122 . The filter  150  may be disposed between the lens  140  and the image sensor  122 , and may pass light within a predetermined wavelength region (e.g., light within a wavelength region of interest) and filter out light out of the predetermined wavelength region (e.g., external light such as background light). For example, when the light source  110  emits light having a NIR wavelength of about 850 nm, the filter  150  may be an infrared (IR) band-pass filter through which light in a NIR wavelength band of about 850 nm passes. Accordingly, light incident on the image sensor  122  mainly includes light emitted from the light source  110  and reflected from the first region  11  of the object  10 , and also includes external light. Although the filter  150  is disposed between the lens  140  and the image sensor  122  in  FIG. 1 , positions of the lens  140  and the filter  150  may be switched with each other. For example, NIR light having passed through the filter  150  may be focused on the image sensor  122  by the lens  140 . 
     Next, the image sensor  122  modulates reflected light into a light modulation signal with a predetermined waveform. A period of a gain waveform of the image sensor  122  may be the same as a period of a waveform of light. In  FIG. 1 , the image sensor  122  may modulate the first reflected light reflected from the surface of the first region  11  of the object  10 , and then may sequentially modulate the second reflected light through the nth reflected light. An amplitude of each reflected light may be modulated by an amount obtained by multiplying the reflected light by the light modulation signal. A period of the light modulation signal is the same of that of reflected light. 
     The image sensor  122  may obtain an image of each reflected light by receiving light that has an amplitude modulated for a predetermined period of time. For example, the image sensor  122  obtains a first partial image by receiving the first reflected light reflected from the surface of the first region  11  and then modulated for a predetermined exposure time. An image obtained by the image sensor  122  is an image of a region of the object  10 , and thus is referred to as a partial image. Next, the image sensor  122  obtains a second partial image by receiving the second reflected light reflected from the surface of the first region  11  and then modulated for a predetermined exposure time. By repeatedly performing the above process, the image sensor  122  obtains an nth partial image by receiving the nth reflected light reflected from the surface of the first region  1  and then modulated for a predetermined exposure time. The image sensor  122  may sequentially obtain n different partial images in this manner. The first through nth partial images may be sub-frame images for forming an image having depth information. The n different partial images may have different phases. n may denote a positive integer. 
     A method of obtaining n different partial images by using n different pieces of light has been described. However, n different partial images may be obtained by using the same light and allowing the image sensor  122  to modulate a plurality of pieces of reflected light into signals with different gain waveforms. 
     For example, the image sensor  122  modulates reflected light into a first light modulation signal, modulates reflected light into a second light modulation signal that is different from the first light modulation signal, and modulates reflected light into an nth light modulation signal that are different from the first and second light modulation signals. The first through nth light modulation signals may be signals with different waveforms, or may be signals with the same period, the same waveform, and different phases. Accordingly, the image sensor  122  may obtain n differential partial images. 
     A plurality of partial images may have different phases due to modulation of light or reflected light. When there are n partial images, a phase difference between adjacent partial images may be 360°/n. For example, the image sensor  122  may obtain four partial images with phases of 0°, 90°, 180°, and 270°. However, the present exemplary embodiment is not limited thereto, and two or more partial images may be obtained. 
     The processor  124  may obtain a first partial depth image having depth information of the object  10  by using the plurality of partial images of the first region  11 . For example, assuming that depth information for a partial depth image is obtained by using four partial images with different phases, the processor  124  may obtain the first partial depth image having depth information as shown in Equation 1. 
     
       
         
           
             
               
                 
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     In Equation 1, R max  is a maximum distance of the object  10  captured by the apparatus  10  according to an exemplary embodiment and is determined by a speed of light and a modulation frequency of the light, and I 1 , I 2 , I 3 , and I 4  are first through fourth partial images. 
     In operation S 240 , the image obtainer  120  may obtain a second partial depth image by using light reflected from the second region  12 . A method of obtaining the second partial depth image is the same as a method of obtaining the first partial depth image, and thus a detailed explanation thereof will not be given. 
     In operation S 250 , the image obtainer  120  may obtain a first depth image of the object  10  by using the first and second partial depth images. The image obtainer  120  may obtain the first depth image by combining the first and second partial depth images based on coordinate information of pixels. Since the object  10  is divided into a plurality of regions and a depth image is obtained from partial depth images as described above, motion blur may be reduced. 
       FIG. 3  is a flowchart of a method of obtaining a plurality of depth images according to an exemplary embodiment. Referring to  FIG. 3 , in operation S 310 , the image obtainer  120  may obtain a first depth image by using first and second partial depth images. A method of obtaining the first depth image has been described above, and thus an explanation thereof will not be given. 
     In operation S 320 , the light source  110  may emit light to the first region  11  of the object  10  for a third time period. When the light source  110  includes the first and second light sources  112  and  114 , the first light source  112  may emit light to the first region  11  of the object  10 . However, the present exemplary embodiment is not limited thereto. The light source  110  may emit light to the first region  11  of the object  10  by changing an illumination angle. 
     At least parts of the third time period and the second time period may not overlap each other. For example, the third time period may be a time interval subsequent to the second time period. However, the present exemplary embodiment is not limited thereto. The third time period may be a time after a predetermined period of time elapses from the second time period. Lengths of the third time period and the second time period or the first time period may be the same. That is, the third time period may be less than the reciprocal of the reference frame rate of the image sensor  122 . This is because only some pixels of the image sensor  122  have to receive light. 
     An amount of the light emitted to the first region  11  for the third time period may be greater than the reference amount of light, and may be the same as an amount of the light emitted to the first region for the first time period or an amount of the light emitted to the second region  12  for the second time period. The amount of the light emitted for the third time period may be a value obtained by multiplying a ratio of the object  10  to the first region  11  by the reference amount of light. For example, when the first region  11  is ½ of the object  10 , an amount of the light emitted for the third time period may be two times greater than the reference amount of light. 
     In operation S 330 , the image obtainer  120  may obtain a third partial depth image by using light reflected from the first region  11  for the third time period. In detail, the image sensor  122  may obtain a plurality of partial images. For example, the plurality of partial images may be four partial images I 1 , I 2 , I 3 , and I 4  with phases of 0°, 90°, 180°, and 270°. However, the present exemplary embodiment is not limited thereto. The processor  124  may obtain the third partial depth image by using the four partial images. A method of obtaining the third partial depth image is the same as a method of obtaining the first partial depth image, and thus a detailed explanation thereof will not be given. 
     In operation S 340 , the processor  124  may obtain a second depth image of the object  10  by using the second and third partial depth images. Since the second partial depth image that is previously obtained and the third partial depth image that is newly obtained are combined with each other in order to obtain the second depth image, a frame rate of a depth image may be less than a reference frame rate of the image sensor  122 . For example, when the reference frame rate of the image sensor  122  is 60 frames per second (fps), although 1/60 seconds is taken to obtain an initial depth image, a frame rate of a next depth image may be 30 fps because a partial depth image is previously obtained and is used for the next depth image. Also, since a depth image is obtained by using a partial depth image, motion blur may be reduced. 
       FIG. 4  is a reference diagram for explaining a method of obtaining a depth image according to an exemplary embodiment. For convenience of explanation, the object  10  may be divided into an upper region H and a lower region L, and the light source  110  may alternately emit a plurality of pieces of light to the upper region H and the lower region L. The plurality of pieces of light may have predetermined periods and predetermined waveforms and may be different from one another. The light source  110  may continuously and sequentially emit the plurality of pieces of light. For example, the light source  110  may emit the plurality of pieces of light with the different phases, for example, four pieces of light whose phase difference between adjacent pieces of light is 90°, to the upper region H of the object  10  for a first time period and may emit four pieces of light with different phases to the lower region L of the object  10  for a second time period. Continuously, the light source  110  may alternately emit light to the upper region H and the lower region L. 
     The image obtainer  120  may be synchronized with the light source  110  according to a control signal of the controller  130 . Accordingly, the image sensor  122  may obtain partial images I 1 , I 2 , I 3 , and I 4  with phases of 0°, 90°, 180°, and 270° for the upper region H of the object  10  for the first time period. When T is a reciprocal of a frame rate of the image sensor  122 , the image sensor  122  may obtain each partial image in every time interval of T/2. Accordingly, the image sensor  122  may obtain four partial images for the first time period, that is, from 0 to 2T. 
     The processor  124  may obtain a first partial depth image PD 1  of the upper region H of the object  10  by using the four partial images I 1 , I 2 , I 3 , and I 4 . Next, the image sensor  122  may obtain partial images I 1 , I 2 , I 3 , and I 4  with phases of 0°, 90°, 180°, and 270° for the lower region L of the object  10  for the second time period, for example, from 2T to 4T. The processor  124  may obtain a second partial depth image PD 2  of the lower region L of the object  10  by using the four partial images I 1 , I 2 , I 3 , and I 4 . 
     Next, after the first and second time periods elapse, the processor  124  may obtain a first depth image D 1  of the object  10  by using the first partial depth image PD 1  of the upper region H and the second partial depth image PD 2  of the lower region L. 
     By using the same method, the light source  110  may emit a plurality of pieces of light with different phases to the upper region H of the object  10  for a third time period, for example, from 4T to 6T. The image sensor  122  may obtain partial images I 1 , I 2 , I 3 , and I 4  with phases of 0°, 90°, 180°, and 270° for the upper region H of the object  10  by being synchronized with the light source  110 . The processor  124  may obtain a third partial depth image PD 3  of the upper region H of the object  10  by using the four partial images I 1 , I 2 , I 3 , and I 4 . The processor  124  may obtain a second depth image D 2  of the object  10  by combining the second partial depth image PD 2  of the lower region L and the third partial depth image PD 3  of the upper region H. 
       FIG. 5  is a reference diagram for explaining a method performed by the image sensor  122  to obtain a depth image after obtaining an image of the entire object  10  according to a comparative example. When a reference frame rate of the image sensor  122  is 60 fps, as shown in  FIG. 5 , the image sensor  122  may obtain an image every 1/60 seconds, and may obtain a depth image by using four images after 1/50 seconds. Accordingly, a frame rate of the depth image is 15 fps. 
     When  FIGS. 4 and 5  are compared with each other, a frame rate of any of depth images sequent to a second depth image of  FIG. 4  is less than that of  FIG. 5 . This is because a partial depth image that is previously obtained and a partial depth image that is newly obtained are used. Even in  FIG. 5 , a depth image subsequent a second depth image may be obtained by using an image that is previously obtained, that is, an image obtained from 0 to 4T and an image that is newly obtained, that is, an image obtained from 4T to 5T. However, in this case, a lot of noise is included in the depth image, thereby making it difficult to obtain depth information. 
       FIG. 6A  shows a depth image obtained by using the method of  FIG. 5 . 
       FIG. 6B  shows a depth image obtained by using the method of  FIG. 4 . As shown in  FIGS. 6A and 6B , motion blur in  FIG. 6B  is less than motion blur in  FIG. 6A . 
     Although the object  10  is divided into two regions and the image obtainer  120  is synchronized with the light source  110  and obtains upper and lower partial depth images in  FIG. 4 , the present exemplary embodiment is not limited thereto. The apparatus  100  according to an exemplary embodiment may divide the object  10  into three or more regions and the image obtainer  120  may obtain three or more partial depth images of the object  10 . 
       FIG. 7  is a reference diagram for explaining a method of obtaining a depth image according to another exemplary embodiment. Referring to  FIG. 7 , the light source  110  may divide the object  10  into an upper region H, a central region, C, and a lower region L, and may sequentially emit a plurality of pieces of light to the upper region H through the lower region L. An amount of light of the light source  110  may be three times greater than a reference amount of light. 
     Also, the image obtainer  120  may obtain a partial image at a speed that is three times higher than a reference frame rate. For example, the processor  124  may obtain a first partial depth image PD 1  by using a partial image obtained from 0 to 4T/3, may obtain a second partial depth image PD 2  by using a partial image obtained from 4T/3 to 8T/3, and may obtain a third partial depth image PD 3  by using a partial image obtained from 8T/3 to 4T. Accordingly, the processor  124  may obtain a first depth image D 1  by combining the first through third partial depth images PD 1 , PD 2 , and PD 3 . Also, the image obtainer  120  may obtain a fourth partial depth image PD 4  of the upper region H of the object from 4T to 4T+4T/3 and may obtain a second depth image D 2  by combining the second through fourth partial depth images PD 2 , PD 3 , and PD 4 . 
     When the object  10  is divided into three regions and then each partial depth image is obtained, motion blur may be further reduced and a frame rate of a depth image may be further improved. Alternatively, the object  10  may be divided into four or more regions. 
     The object  10  has been vertically divided. However, the present exemplary embodiment is not limited thereto. The object  10  may be horizontally divided, and the image sensor  122  may be horizontally synchronized. The object  10  may be divided in other directions. 
     Also, the apparatus  100  may divide the object  10  into a plurality of regions and may discontinuously emit light to the plurality of regions. The image obtainer  120  may be synchronized with the light source  110  and may obtain a partial image only when light is emitted to a region of the object  10 . As described above, the light source  110  and the image sensor  122  may be synchronized with each other to emit light for a predetermined period of time, and the image sensor  122  may operate for the predetermined period of time to modulate reflected light. Since the image sensor  122  does not operate for a period of time for which light is not emitted to maintain light reception at a minimum level, the image sensor  122  may be prevented from receiving external light. A ratio of a time for which light is emitted to a time for which light is not emitted may be referred to as a duty ratio. When the duty ratio is less than 100%, it may mean that light is discontinuously emitted. 
       FIGS. 8A and 8B  are diagrams for explaining a method of obtaining a depth image when a duty ratio is 50% according to another exemplary embodiment. Referring to  FIG. 8A , the light source  110  may emit light to the object  10  for 2T from among 4T. For example, the light source  110  may emit four different pieces of light to the upper region H of the object  10  from 0 to T, and may emit four different pieces of light to the lower region L of the object  10  from T to 2T. An amount of each light emitted to the object  10  may be four times greater than the reference amount of light. This is because a duty ratio is 50% and light is emitted to only a ½ region of the object  10 . When light is discontinuously emitted in this manner, external light may be reduced. 
     Although light is emitted to the upper region H of the object  10  from 0 to T, light is emitted to the lower region L of the object from T to 2T, and light is not emitted from 2T to 4T in  FIG. 8A , the present exemplary embodiment is not limited thereto. As shown in  FIG. 8B , light may be emitted to the upper region H of the object  10  from 0 to T and light may be emitted to the lower region L from 2T to 3T, and light may not be emitted to the object  10  from T to 2T and from 3T to 4T. As shown in  FIG. 8B , since light is discontinuously emitted to a region of the object  10 , external light and motion blur may be reduced and a frame rate of a depth image may be improved. Although the object  10  is divided into two regions in  FIGS. 8A and 8B , the present exemplary embodiment is not limited thereto. 
     While not restricted thereto, an exemplary embodiment can be embodied as computer-readable code on a computer-readable recording medium. The computer-readable recording medium is any data storage device that can store data that can be thereafter read by a computer system. Examples of the non-transitory computer-readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, or DVDs), etc. The computer-readable recording medium can also be distributed over network-coupled computer systems so that the computer-readable code is stored and executed in a distributed fashion. Also, an exemplary embodiment may be written as a computer program transmitted over a computer-readable transmission medium, such as a carrier wave, and received and implemented in general-use or special-purpose digital computers that execute the programs. Moreover, it is understood that in exemplary embodiments, one or more units of the above-described apparatuses and devices can include circuitry, a processor, a microprocessor, etc., and may execute a computer program stored in a computer-readable medium. 
     The foregoing exemplary embodiments are merely exemplary and are not to be construed as limiting. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.