Image sensor using SPAD array

Abstract

The image sensor of the present invention may include: an SPAD array including one or more SPAD elements; the multiplexer is used for receiving a signal output from the SPAD array; a controller that transmits a mode setting signal to the multiplexer; and a first counter and a second counter which output a digital signal based on a signal output by the SPAD array. The multiplexer may change a connection relationship between the first counter and the second counter according to the mode setting signal.

Description

Technical Field

[0001] The present invention relates to an image sensor using a SPAD array, and more particularly, to an image sensor capable of switching modes and improving image quality. Background Art

[0002] Image sensors are devices that construct images using light and can include a variety of elements that can sense light. Among the elements that can sense light, SPAD elements are elements that detect single photons through the avalanche phenomenon, and research on image sensors using SPAD elements has been actively conducted recently.

[0003] Image sensors sense light in a variety of ways depending on the mode. Each mode has its own advantages and disadvantages, and each mode needs to be selected according to the target object to be sensed. In addition, image sensors with new structures that can solve the problems existing in each mode are needed.

[0004] The present invention is derived from a research conducted by the Ministry of Science and ICT, the Ministry of Trade, Industry and Energy, the Ministry of Health and Welfare, and the Ministry of Food and Drug Safety as part of the development of a 20x20cm large-area hybrid X-ray video detector based on Global Shutter (project specific number: 1711138026; project number: KMDF_PR_20200901_0048-03; research project name: All-sector full-cycle medical device research and development project (R&D); project management organization: All-sector full-cycle medical device research and development project group; project execution agency: Yonsei University Industry-Academia Cooperation Group; research period: 2021.03.01~2022.02.28).

[0005] In addition, the Korean government, as the subject of the subject matter, has no property interest in all aspects of the present invention. Summary of the invention

[0006] Technical problem to be solved by the invention

[0007] One object of the present invention is to provide an image sensor that uses two or more counters to compensate for the shortcomings of each mode.

[0008] Means for solving technical problems

[0009] An image sensor of an embodiment may include: a SPAD array including more than one SPAD element; a multiplexer for receiving a signal output from the SPAD array; a controller for transmitting a mode setting signal to the multiplexer; and a first counter and a second counter for outputting a digital signal based on the signal output from the SPAD array. The multiplexer may change a connection relationship between the first counter and the second counter according to the mode setting signal.

[0010] Here, the mode setting signal may include: a first signal corresponding to a first mode, wherein the first mode is a mode in which the photon accumulation period of the first counter and the photon accumulation period (integration) of the second counter do not overlap; and a second signal corresponding to a second mode, wherein the second mode is a mode in which the second counter is run after the first counter is run.

[0011] Here, the first mode may be a mode in which one of the first counter and the second counter is in a photon accumulation period while the other is in a read out period; the second mode may be a mode in which the photon accumulation period of the first counter corresponding to the first SPAD element included in the SPAD array overlaps with the photon accumulation period corresponding to the second SPAD element included in the SPAD array.

[0012] Here, when the multiplexer obtains the first signal from the controller, the multiplexer can transmit the signal output by the SPAD array to the first counter and the second counter; when the multiplexer obtains the second signal from the controller, the multiplexer can only transmit the signal output by the SPAD array directly to the first counter.

[0013] Here, in the case where the multiplexer acquires the first signal, the first counter and the second counter are connected in parallel, and the second counter can acquire the signal output by the SPAD array from the multiplexer.

[0014] Here, in the case where the multiplexer acquires the second signal, the first counter and the second counter are connected in series, and the second counter can acquire the signal output by the SPAD array from the first counter.

[0015] Here, the first SPAD element and the second SPAD element may be located in different rows on the SPAD array.

[0016] Here, the SPAD array may be configured on a second wafer different from a first wafer, and the first wafer is a wafer on which the first counter and the second counter are configured.

[0017] Here, the controller may generate an image of the target reflecting light on the SPAD array based on the first result signal output by the first counter and the second result signal output by the second counter.

[0018] Here, the controller may change the mode setting signal based on a first image of the object at a first time point and a second image of the object at a second time point.

[0019] Effects of the Invention

[0020] According to an embodiment of the present invention, it is possible to provide an image sensor using two or more counters to compensate for the disadvantages of each mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 FIG. 1 is a diagram showing the structure of an image sensor using a conventional SPAD.

[0022] Figure 2 is a diagram showing a counter operation of a conventional image sensor in a global shutter mode and a constructed image.

[0023] Figure 3 is a diagram showing a counter operation of a conventional image sensor in a rolling shutter mode and a constructed image.

[0024] Figure 4 It is a diagram showing the structure of an image sensor according to one embodiment of the present invention.

[0025] Figure 5 This is a diagram showing the structure of an image sensor in a first mode according to an embodiment of the present invention.

[0026] Figure 6 It is a diagram for explaining the operation of the counter in the first mode.

[0027] Figure 7 This is a diagram showing the structure of an image sensor in a second mode according to an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The embodiments described in this specification are intended to clearly explain the concept of the present invention to those skilled in the art. Therefore, the present invention is not limited to the embodiments described in this specification, and the scope of the present invention should be interpreted as including modifications or variations that do not deviate from the concept of the present invention.

[0029] The terms used in this specification are selected as far as possible from the currently widely used common terms based on the functions in the present invention, but this may vary according to the intention of those skilled in the art, precedents or the emergence of new technologies. However, in contrast, if a specific term is defined as a certain meaning, the meaning of the term will be recorded separately. Therefore, the terms used in this specification should be interpreted based on the actual meaning of the term and the full content of this specification, rather than simply the name of the term.

[0030] The drawings attached to this specification are for the purpose of explaining the present invention, and the shapes shown in the drawings may be exaggerated as needed to help understand the present invention, so the present invention is not limited to the drawings.

[0031] In this specification, if it is considered that the detailed description of a well-known structure or function related to the present invention may obscure the gist of the present invention, the detailed description thereof will be omitted as necessary.

[0032] Figure 1 FIG. 1 is a diagram showing the structure of an image sensor using a conventional SPAD.

[0033] Reference Figure 1 An existing image sensor using a single photon avalanche diode (SPAD) may include: a SPAD array including more than one SPAD element; and a counter for receiving a result signal of the SPAD array to count photons.

[0034] Since the existing image sensor is fixed in one of the global shutter mode and the rolling shutter mode and uses a counter, there are various problems. Figure 2 as well as Figure 3 , the problems of existing image sensors are explained.

[0035] Figure 2 is a diagram showing a counter operation of a conventional image sensor in a global shutter mode and a constructed image.

[0036] Reference Figure 2 ,Since the existing image sensor uses a counter, there is a problem of being unable to accumulate photons when the counter is ,derived in the global shutter mode.

[0037] Specifically, refer to Figure 2 In part (a), the photon accumulation (integration) period T is repeated alternately in the counter. INT And the export (read out) period T READ , photon accumulation period T INT is the period of photon accumulation, and the derived period T READ This is the period during which the accumulated photons are read.

[0038] For example, Figure 2 In part (a), at 30fps, 0s to 16.5ms is the photon accumulation period T INT T, photon accumulation period T INT The next 16.5ms to 33ms is the export period T READAfter the lead-out period from 16.5ms to 33ms, the photon accumulation period T restarts from 33ms to 49.5ms INT Thus, in global shutter mode, the photon accumulation period and the photon export period of the counter are repeated alternately.

[0039] Figure 2 Part (b) shows an image generated based on the count result of the counter in the global shutter mode. Figure 2 Part (b) of Figure 3 Time and Figure 3 This is illustrated by comparing it with part (b) of .

[0040] Since photons cannot be accumulated during the counter derivation period, the image sensor takes a long time to generate an image frame. In other words, the image sensor has a low frame rate when operating in global shutter mode. To solve this problem in global shutter mode, rolling shutter mode is introduced.

[0041] Figure 3 is a diagram showing a counter operation of a conventional image sensor in a rolling shutter mode and a constructed image.

[0042] Reference Figure 3 , it is possible to confirm the counter operation and the generated image in the rolling shutter mode, which performs photon accumulation and derivation for each row of the SPAD array to address the low frame rate of the global shutter mode.

[0043] Specifically, refer to Figure 3 In part (a), the counter can divide the SPAD elements in the SPAD array into rows and accumulate and export photons for each row of elements.

[0044] For example, in the case of 30 fps, for the first line, the counter may allocate a portion of the 33 ms, that is, the first period, as the photon accumulation period T INT The remaining period of 33 ms except the first period, that is, the second period, is allocated as the derivation period T READ In addition, for the second row, the counter can allocate a portion of the 33 ms, that is, the third period, as the photon accumulation period T INT The remaining period of 33 ms except the third period, that is, the fourth period, is allocated as the derivation period T READ .

[0045] At this time, the derivation period of each row can be set to not overlap. That is, when the counter performs derivation on the first row, although it cannot accumulate photons for the first row, it can accumulate photons for the second row, thereby partially solving the problem of the global shutter that photons cannot be accumulated during the derivation period.

[0046] In rolling shutter mode, the counter accumulates and outputs photons by row, so the time loss is small compared to global shutter mode. However, in rolling shutter mode, the image generated by the image sensor may have different sensing times for each row. That is, motion artifacts and jello effects may occur. Therefore, when the target object of the image construction moves, image distortion may occur.

[0047] Specifically, if we compare Figure 2 Part (b) of Figure 3 In part (b), all of their targets are moving fans. For moving targets, even if the fan moves, the image sensor running in global shutter mode will not produce image distortion if the size of the fan blades is taken into account. This is because the global shutter mode generates an image based on the results sensed by all SPAD elements included in the SPAD array at the same time.

[0048] In contrast, for image sensors operating in rolling shutter mode, Figure 3 As shown in the figure in part (b), if the size of the fan blades is taken into account, it can be seen that image distortion occurs. This is because the rolling shutter mode divides the SPAD elements included in the SPAD array into rows, and the time point at which each row senses light is different.

[0049] As described above, since the global shutter mode and the rolling shutter mode each have problems, it is difficult for an image sensor that operates in a fixed mode to achieve high-quality images. In addition, in the global shutter mode, if the image sensor uses a counter, it will result in a low frame rate. Therefore, the image sensor of the present application proposes a solution to the above problems below.

[0050] Figure 4 It is a diagram showing the structure of an image sensor according to one embodiment of the present invention.

[0051] Reference Figure 4 An image sensor according to an embodiment of the present invention may include: a SPAD array 100 , a multiplexer (MUX), a first counter 300 , and a second counter 400 .

[0052] The SPAD array 100 may include more than one SPAD element. The SPAD array 100 may include a plurality of SPAD elements in a matrix. For example, the SPAD array 100 may be a SPAD matrix in the form of 8×8, 64×64, 100×100, 1024×1024, 4000×4000, etc., but is not limited thereto, and may be implemented in a variety of shapes such as a circular, elliptical, or honeycomb structure.

[0053] The SPAD array 100 may include more than one sub-arrays consisting of N SPAD elements. For example, the SPAD array 100 may include a plurality of sub-arrays, each of which is composed of SPAD elements in a 4×4 matrix.

[0054] When light is incident on the SPAD array 100, photons can be detected by an avalanche phenomenon. The SPAD array 100 can output a result signal corresponding to the photon detection. The result signal can be a pulse in an analog form. Therefore, a counter is required to count photons based on the pulse in the analog form.

[0055] The multiplexer 200 is connected to the SPAD array 100 and can receive the signal output from the SPAD array 100. At this time, the signal output from the SPAD array 100 is a result signal corresponding to the photon detection, and the result signal can be the pulse in the analog form described above.

[0056] The multiplexer 200 may receive the mode setting signal 500 through a controller (not shown). The controller may perform overall control of the image sensor. Alternatively, the controller may be an MCU (Microcontroller Unit) that generates an image based on the result signal of the SPAD array 100.

[0057] The multiplexer 200 may transmit the signal received from the SPAD array 100 to the counters 300 and 400. At this time, the signal transmission to the counters 300 and 400 may be different according to the mode setting signal 500. The mode setting signal will be described below together with the description of the controller.

[0058] The multiplexer 200 can change the connection relationship between the counters 300 and 400 according to the mode setting signal 500. Specifically, the multiplexer 200 can change the connection relationship between the first counter 300 and the second counter 400 to series or parallel according to the mode setting signal 500. Alternatively, the multiplexer 200 can change the signal applied to the second counter 400. Alternatively, the multiplexer 200 can change the subject of the signal input to the second counter 400.

[0059] For example, when the mode setting signal is the first signal, the multiplexer 200 may set the connection relationship of the counters to be connected in parallel to the first counter 300 and the second counter 400. At this time, the second counter 400 may receive the signal output from the SPAD array 100 through the multiplexer 200. That is, the main body of the signal input to the second counter 400 may be the multiplexer 200.

[0060] Specifically, the multiplexer 200 sets the input IN to a terminal connected to the SPAD array 100 , sets the output OUT to two terminals connected to the first counter 300 and the second counter 400 , and sets the connection relationship such that the first counter 300 and the second counter 400 are connected in parallel.

[0061] For another example, when the mode setting signal is the second signal, the multiplexer 200 sets the connection relationship of the counters to be a first counter 300 and a second counter 400 connected in series. At this time, the second counter 400 is connected in series with the first counter 300, so that the signal output by the SPAD array 100 is obtained through the first counter 300. That is, the subject that inputs the signal to the second counter 400 can be the first counter 300.

[0062] Specifically, the multiplexer 200 sets the input IN to a terminal connected to the SPAD array 100, and sets the output OUT to a terminal connected to the first counter 300, so that the signal output from the SPAD array 100 is directly transmitted only to the first counter 300. At this time, the output terminal of the first counter 300 is connected to the input terminal of the second counter 400, so that the second counter 400 indirectly obtains the signal output by the SPAD array 100.

[0063] Below, refer to Figures 5 to 7 , illustrating the operation of the multiplexer 200 corresponding to the mode setting signal 500.

[0064] The first counter 300 and the second counter 400 can count photons based on the signal output by the SPAD array 100 and output corresponding digital signals. The first counter 300 and the second counter 400 can be N-bit counters capable of counting to the Nth power of 2 (N is an integer greater than 1). When the first counter 300 and the second counter 400 are connected in series, they can operate as a 2N-bit counter.

[0065] Although the image sensor includes two counters as an example in this specification, it is not limited to this, and the image sensor may include multiple counters. In this case, the two counters form a pair to operate. That is, the two counters are connected in series or in parallel as a pair according to the mode setting signal.

[0066] The image sensor may include a controller that outputs a mode setting signal 500. The mode setting signal 500 may include a first signal corresponding to a global shutter mode and a second signal corresponding to a rolling shutter mode.

[0067] The global shutter mode may be a mode in which all SPAD elements included in the SPAD array 100 perform photon integration and readout at the same time to generate one frame. That is, the global shutter mode may be a mode in which the photon integration period and the readout period of the counter are alternately repeated.

[0068] The rolling shutter mode may be a mode in which photon accumulation and derivation are performed row by row for the SPAD elements included in the SPAD array 100 to generate one frame. That is, the rolling shutter mode may be a mode in which the photon accumulation period of the SPAD elements of the first row included in the SPAD array 100 partially overlaps with the photon accumulation period of the SPAD elements of the second row.

[0069] The controller can generate an image of the sensing object, ie, the target object, based on the output signal of the counter. At this time, the controller can determine and / or set the mode setting signal 500 according to the movement of the target object.

[0070] For example, when the target object is in motion, the controller may set the mode setting signal 500 to a first signal corresponding to the global shutter mode to minimize motion artifacts and gel phenomenon caused by dynamic motion.

[0071] For another example, when the target object is static, the controller may set the mode setting signal 500 to a second signal corresponding to the rolling shutter mode to increase the frame rate to generate an image.

[0072] As described above, the controller can determine the movement of the target object, so as to set or change the optimal mode for the target object. There are many ways for the controller to determine the movement of the target object.

[0073] For example, the controller uses light to sense the distance of the target object. When the distance of the target object changes by more than a certain level, it is determined that the movement of the target object is dynamic. Specifically, the image sensor includes another SPAD element for sensing the distance, and the distance of the target object can be sensed by the other SPAD element. However, it is not limited to this, and it is also possible not to configure another SPAD element, but to use one or more SPAD elements in the SPAD elements included in the SPAD array 100 to sense the distance of the target object.

[0074] For another example, the controller can determine the movement of the target object based on the pixel value difference between the first image generated at the first time point and the second image generated at the second time point. Specifically, the pixel at the first position included in the first image generated at the first time point is compared with the pixel at the first position included in the second image generated at the second time point. When the pixel value changes by more than a certain level, the controller can determine that the movement of the target object is dynamic.

[0075] The image sensor may be implemented by more than one wafer. This may vary depending on the configuration relationship between the SPAD array 100 and the counters 300, 400.

[0076] According to an embodiment, the SPAD array 100 and the counters 300 and 400 may be configured on one chip. In this case, each SPAD element included in the SPAD array 100 may be connected to two counters. However, this is not limited thereto, and each sub-array included in the SPAD array 100 may be connected to two counters.

[0077] According to another embodiment, the SPAD array 100 and the counters 300 and 400 may be disposed on different wafers. In this case, the first wafer on which the SPAD array 100 is disposed may be located above the second wafer on which the counters 300 and 400 are disposed.

[0078] The reason why the SPAD array 100 and the counters 300 and 400 are arranged on different wafers is to ensure the light receiving area of ​​the SPAD elements. That is, when the counters 300 and 400 are arranged between the SPAD elements included in the SPAD array 100, the light incident on the area where the counters are arranged cannot be sensed, resulting in light loss. Therefore, different wafers can be used to separate the area of ​​the SPAD array 100 for sensing light and the area where the counters perform circuit operations.

[0079] Figure 5 This is a diagram showing the structure of an image sensor in a first mode according to an embodiment of the present invention.

[0080] Reference Figure 5 In order to set the image sensor to the first mode, the controller may set the mode setting signal 500 to the first signal 510. At this time, the first mode may be a mode corresponding to the global shutter mode.

[0081] The multiplexer 200 may receive a first signal 510 from the controller. The multiplexer 200 may change the connection relationship between the first counter 300 and the second counter 400 based on the first signal 510. The multiplexer 200 may connect the first counter 300 and the second counter 400 in parallel so that the signal output by the SPAD array 100 can be directly transmitted to the first counter 300 and the second counter 400.

[0082] In the first mode, the first counter 300 and the second counter 400 can alternately perform the photon accumulation and derivation process for the output signal of the SPAD array 100. Since two counters are used, the problem that photon accumulation cannot be performed during the derivation period in the global shutter mode can be solved.

[0083] pass Figure 6 , the operation method of the first counter 300 and the second counter 400 is described.

[0084] Figure 6 It is a diagram for explaining the operation of the counter in the first mode.

[0085] Reference Figure 6 , the first counter 300 and the second counter 400 may alternately perform photon accumulation and derivation. That is, during the period when the first counter 300 performs photon accumulation, the second counter 400 may perform derivation, and during the period when the first counter 300 performs derivation, the second counter 400 may accumulate photons. Therefore, the photon accumulation period of the first counter 300 and the photon accumulation period of the second counter 400 may not overlap each other.

[0086] For example, Figure 6 As shown, when the frame rate is 30 fps, the first counter 300 can use the first interval from 0 seconds to 16.5 ms as the photon accumulation period T INT The second counter 400 can use the first interval as the deriving period T READ The photon accumulation results performed before the first interval are derived.

[0087] In addition, the first counter 300 can use the second interval of 16.5 ms to 33 ms as the derivation period T INT The photon accumulation result performed in the first interval is derived, and the second counter 400 can use the second interval as the photon accumulation period T INT And accumulate photons.

[0088] The image sensor of the present invention uses two counters, so that the counters can be derived alternately in the first mode corresponding to the global shutter mode, which can solve the existing problem of low frame rate. That is, the time for generating an image can be shortened compared to the case of using one counter.

[0089] Figure 7 This is a diagram showing the structure of an image sensor in a second mode according to an embodiment of the present invention.

[0090] Reference Figure 7 In order to set the image sensor to the second mode, the controller may set the mode setting signal 500 to the second signal 520. At this time, the second mode may be a mode corresponding to the rolling shutter mode.

[0091] The multiplexer 200 may receive a second signal 520 from the controller. The multiplexer 200 may change the connection relationship between the first counter 300 and the second counter 400 based on the second signal 520. The multiplexer 200 may connect the first counter 300 and the second counter 400 in series so that the signal output by the SPAD array 100 is directly transmitted to the first counter 300, and is indirectly transmitted to the second counter 400 through the first counter 300.

[0092] Since the second counter 400 receives an input signal from the first counter 300, the second counter 400 will perform a counting operation after the first counter 300 performs a counting operation. That is, the operation of the first counter 300 and the operation of the second counter 400 are not performed at the same time, and the operation of the second counter 300 can be performed after the operation of the first counter 300.

[0093] In the second mode, the first counter 300 and the second counter 400 are connected in series, so they can operate as a 2N-bit counter. Since two counters are used, 2 to the power of 2N can be counted, so the dynamic range of the image sensor can be increased. As a result, the image sensor can maintain a high frame rate in the rolling shutter mode, minimize motion artifacts, and improve image quality.

[0094] like Figure 3 As shown, the first counter 300 and the second counter 400 may perform photon accumulation and derivation in rows.

[0095] The method of the embodiment is implemented in the form of program commands that can be executed by various computer devices and can be stored in a computer-readable medium. The above-mentioned computer-readable medium may include program commands, data files, data structures, etc. alone or in combination. The program commands stored in the above-mentioned medium may be specially designed and configured for the embodiment, or they may be well known and used by computer software practitioners. Examples of computer-readable recording media include: magnetic media (magnetic media), such as hard disks, floppy disks and tapes; optical media (optical media), such as CDROM, DVD; magneto-optical media (magneto-optical media), such as floppy disks (floptical disk); and hardware devices, specially constructed in a way that can store and execute program commands, such as read-only memory (ROM), random access memory (RAM), flash memory. Examples of program commands include not only mechanical codes made by compilers, but also high-level language codes that can be executed by computers using interfaces, etc. The above-mentioned hardware devices can be configured as a structure that is operated by one or more software modules to perform the actions of the embodiment, and vice versa.

[0096] As described above, although the embodiments are described by limited embodiments and drawings, those skilled in the art can make various modifications and variations from the above description. For example, even if the described techniques are performed in a different order from the described method and / or the system, structure, device, circuit and other components are combined or combined in a different form from the described method, or replaced or substituted by other components or equivalents, practical results can be achieved.

[0097] Therefore, other implementations, other embodiments, and equivalents of the claims are all within the scope of the following claims.

Claims

1. An image sensor, wherein: include: A SPAD array includes more than one SPAD element. a multiplexer for receiving a signal output from the SPAD array, a controller that transmits a mode setting signal to the multiplexer, and A first counter and a second counter output a digital signal based on the signal output by the SPAD array; The multiplexer changes the connection relationship between the first counter and the second counter according to the mode setting signal.

2. The image sensor according to claim 1, in, The mode setting signal includes: a first signal corresponding to a first mode, wherein the first mode is a mode in which a photon accumulation period of the first counter and a photon accumulation period of the second counter do not overlap; and The second signal corresponds to a second mode, where the second mode is a mode in which the second counter is run after the first counter is run.

3. The image sensor according to claim 2, in, The first mode is a mode in which one of the first counter and the second counter is in a photon accumulation period while the other is in a photon extraction period. The second mode is a mode in which a photon accumulation period of the first counter corresponding to a first SPAD element included in the SPAD array overlaps with a photon accumulation period corresponding to a second SPAD element included in the SPAD array.

4. The image sensor according to claim 2, in, When the multiplexer obtains the first signal from the controller, the multiplexer transmits the signal output by the SPAD array to the first counter and the second counter. In the case that the multiplexer obtains the second signal from the controller, the multiplexer only transmits the signal output by the SPAD array directly to the first counter.

5. The image sensor according to claim 4, in, In the case where the multiplexer acquires the first signal, The first counter and the second counter are connected in parallel, The second counter obtains the signal output by the SPAD array from the multiplexer.

6. The image sensor according to claim 4, in, In the case where the multiplexer acquires the second signal, The first counter and the second counter are connected in series, The second counter obtains the signal output by the SPAD array from the first counter.

7. The image sensor according to claim 3, in, The first SPAD element and the second SPAD element are located in different rows on the SPAD array.

8. The image sensor according to claim 1, in, The SPAD array is configured on a second chip different from a first chip, and the first chip is a chip configured with the first counter and the second counter.

9. The image sensor according to claim 1, in, The controller generates an image of the target reflecting light on the SPAD array based on a first result signal output by the first counter and a second result signal output by the second counter.

10. The image sensor according to claim 9, in, The controller changes the mode setting signal based on a first image of the object at a first time point and a second image of the object at a second time point.

Images