Endoscope system

Abstract

Endoscope system (110), comprising: a light source unit (121) that alternately emits first illumination light and second illumination light in a predetermined cycle, the first illumination light and the second illumination light having different wavelength bands; an image generating means (126) for imaging an object illuminated with the first illumination light and for generating a first image signal and for imaging the object illuminated with the second illumination light and for generating a second image signal; a first image signal storage means (41) for successively storing generated first image signals; a second image signal storage means (42) for successively storing generated second image signals; an image evaluation value calculating means (45) for calculating image evaluation values ​​of first images based on the stored first image signals and image evaluation values ​​of second images based on the stored second image signals; an image signal selecting means (45) for selecting a first display image signal from a plurality of the first image signals stored in the first image signal storing means (41) and for selecting a second display image signal from a plurality of the second image signals stored in the second image signal storing means (42) independently of one another and on the basis of the calculated image evaluation values; and a display means (130) for simultaneously displaying a first image based on the first display image signal and a second image based on the second display image signal.

Description

Technical field

[0001] The present invention relates to an endoscope system for performing pause (freeze) processing on a moving image that has been captured. State of the art

[0002] An endoscope system including an endoscope unit, a processor unit, and a monitor is known as an endoscope system for observing an object in a body cavity such as a person's esophagus or intestine. Illumination light emitted from the processor unit is guided in an optical fiber bundle provided in the endoscope unit and emitted from a light distribution window provided at the distal end of the endoscope unit. The illumination light emitted from the light distribution window is reflected by an object in the body cavity and received as object light through an observation window provided at the distal end of the endoscope unit. The object light received through the observation window is converted into an electrical signal by an image sensor at a predetermined frame rate, and the electrical signal is output to the processor unit.The processor unit contains a signal processing circuit, and the received electrical signal is converted into an image signal by the signal processing circuit and sent to the monitor. The monitor displays a moving image based on the image signal received by the processor unit. Thus, a user (operator) using the endoscope system is able to observe the object by viewing the moving image displayed on the monitor. When the operator inputs a freeze command to the processor to pause (freeze) the moving image, the object is displayed on the monitor as a still image. This allows the operator to examine the object. However, depending on the timing of the freeze command, the still image displayed on the monitor may be blurred or defocused due to movement.

[0003] An endoscope system that addresses the above-mentioned problem and suppresses color shift in a still image is disclosed in Japanese Patent JP 3 887 453 B2 (hereinafter referred to as "Patent Document 1"). In the light source device for an endoscope disclosed in Patent Document 1, a frame in which a color shift is less than a predetermined value is selected from a moving image displayed at a predetermined frame rate and stored as a still image. The number of still images that can be stored is limited, and the still images are updated starting from the oldest image. In this endoscope system, when a freeze command is input, the still image of the frame having the least degree of color shift is selected and displayed among the stored still images.If a freeze command is entered, a still image with a slight color shift is displayed on the monitor.

[0004] Document JP 2012-239757 A discloses an endoscope with a light source configured to alternately emit white light and excitation light. The endoscope further comprises a freeze control circuit configured to select a white light observation image and a fluorescence observation image for simultaneous display on a screen. The freeze control circuit selects two images acquired at the same time.

[0005] For the state of the art, reference is also made to document JP 2014-220 690 A. Brief description of the invention

[0006] An endoscope system is known in which two types of illumination light with different wavelength bands are alternately emitted at a predetermined frame rate, and images of the object illuminated with the two types of illumination light are generated and displayed simultaneously on the same screen of a monitor. Now, consider the case where the processing method disclosed in Patent Document 1 is applied to this type of endoscope system. In this case, the manner in which motion blur and defocus occur in a still image changes according to the brightness and wavelength band of the illumination light illuminating the object, so the degree of motion blur and defocus occurring differs for the two types of object images corresponding to the two types of illumination light with different wavelength bands.For this reason, it is difficult to select a single image suitable for display as a still image, and there is a risk that image blur and defocus will occur in both types of object images.

[0007] The present invention has been made in light of the above-described circumstances, and its object is to provide an endoscope system adapted to display images of an object illuminated with two kinds of illumination light having different wavelength bands, and having the advantage of displaying a still image in which motion blur and defocus are suppressed.

[0008] To achieve the above-described object, an endoscope system according to an embodiment of the present invention comprises: a light source unit that alternately emits first illumination light and second illumination light in a predetermined cycle, the first illumination light and the second illumination light having different wavelength bands; image generation means for imaging an object illuminated with the first illumination light and generating a first image signal, and for imaging the object illuminated with the second illumination light and generating a second image signal; first image signal storage means for successively storing generated first image signals; second image signal storage means for successively storing generated second image signals;an image evaluation value calculation means for calculating image evaluation values ​​of first images based on the stored first image signals and / or image evaluation values ​​of second images based on the stored second image signals; an image signal selection means for selecting a first display image signal from a plurality of the stored first image signals and for selecting a second display image signal from a plurality of the stored second image signals based on the calculated image evaluation values; and a display means for simultaneously displaying a first image based on the first display image signal and a second image based on the second image signal.

[0009] In this configuration, the first display image signal and the second display image signal are selected independently of each other based on the result of image evaluation of the first image signals and / or the result of image evaluation of the second image signals. Accordingly, an image signal suitable for display as a still image is selected for the first display image signal and / or the second display image signal.

[0010] The endoscope system may further include a command receiving means for receiving a freeze command from a user. In this case, the image evaluation value calculating means weights each of the calculated image evaluation values ​​according to a time difference between a time at which an image corresponding to the image evaluation value was captured and a time at which the command receiving means received the freeze command.

[0011] The image evaluation value calculating means may perform the weighting by adding an addend to the image evaluation value or by multiplying the image evaluation value by a multiplier, wherein the smaller the time difference, the larger the addend or the multiplier.

[0012] Furthermore, the image signal selecting means may select the first display image signal from a plurality of stored image signals based on the image evaluation values ​​and select, as the second display image signal, the second image signal of the same frame as that of the first display image signal or the second image signal of a frame which is a predetermined number of frames before or a predetermined number of frames after the frame of the first display image signal.

[0013] In a case where a new first image signal is generated by the image generating means when a predetermined number of first image signals are stored, the first image signal storing means may store the newly generated first image signal by overwriting a first image signal having the oldest storage time among the stored first image signals, and in a case where a new second image signal is generated by the image generating means when a predetermined number of second image signals are stored, the second image signal storing means may store the newly generated second image signal by overwriting a second image signal having the oldest storage time among the stored second image signals.

[0014] According to the present invention, there is provided an endoscope system which is adapted to display an image of an object illuminated with two kinds of illumination light of different wavelength bands and which has the advantage of displaying a still image in which motion blur and defocus are suppressed. Brief description of the drawings Fig. 1 is a block diagram of an endoscope system according to an embodiment of the present invention. Fig. 2 is a block diagram of a light source unit according to the embodiment of the present invention. Fig. 3 is a front view of a filter changer according to the embodiment of the present invention. Fig. 4 is a diagram showing spectral characteristics according to the embodiment of the present invention. Fig. 5 is a diagram for describing the timing of changes in a field synchronization signal, a field signal, and illumination light according to the embodiment of the present invention, as well as the timing at which image signals are output based on image formation signals, and the timing at which storage processing and readout processing are performed on image signals in memories. Fig. 6 is a diagram showing a display screen of a monitor according to the embodiment of the present invention. Description of implementation examples

[0015] In the following, an embodiment of the present invention will be described with reference to the drawings.

[0016] Fig. 1 is a block diagram showing the structure of an endoscope system 100 according to an embodiment of the present invention. The endoscope system 100 of the present embodiment is a medical imaging system used to observe an object in a body cavity of a person. As shown in Fig. 1, the endoscope system 100 includes an endoscope unit 110, a processor unit 120, and a monitor 130.

[0017] The endoscope unit 110 has a connecting part 111 and an insertion tube 112. An image forming unit 10 and a light distribution lens 11, which function as an image forming means, are provided in the distal end portion of the introducer 112. Furthermore, an optical fiber bundle 13 and a plurality of wires 14 are provided in the endoscope unit 110, extending from the connecting part 111 to the distal end portion.

[0018] The processor unit 120 includes a light source unit 121, a light source driver circuit 122, an image processing unit 123, a controller 124, an input device 125, and an image sensor driver 126.

[0019] Fig. Figure 2 is a block diagram of the light source unit 121. The light source unit 121 includes a light source 20, a filter unit 21, a light adjusting unit 22, and a condenser lens 23. The light source 20 is driven by the light source driving circuit 122 and emits white illumination light. The illumination light emitted from the light source 20 enters the filter unit 21. The filter unit 21 has an IR (infrared) cut filter 25 and a filter changer 26. The illumination light entering the filter unit 21 passes through the IR cut filter 25 and the filter changer 26 in the specified order and then enters the light adjusting unit 22.

[0020] Fig. 3 is a front view showing the filter changer 26 from the light source 20. The filter changer 26 has the shape of a circular disk. As shown in Fig. As shown in Figure 3, the filter changer 26 includes a first optical filter 26a and a second optical filter 26b having different spectral characteristics and arranged side by side in the circumferential direction. These optical filters are in the shape of fans spread out over an angular range (here, an angular range of approximately 180°) corresponding to a predetermined field rate (1 / 60 second in the present embodiment).

[0021] The filter changer 26 is rotated by a motor 27 and extracts from white illumination light emitted by the light source 20 (cf. a Fig. 3 with a dashed line), one of two types of illumination light having different spectra, with timing synchronized with image formation performed by the image formation unit 10 described later. Here, Fig. 4 the spectral characteristics of the first optical filter 26a and the second optical filter 26b. In Fig. 4, the horizontal axis represents the wavelength (unit: nm), and the vertical axis represents the transmittance (unit: %). As in Fig. As shown in Figure 4, the transmittance Ta of the first optical filter 26a is approximately 100% with respect to light in the visible wavelength band from about 400 nm to about 700 nm. For this reason, the illumination light passing through the first optical filter 26a is white light. In contrast, the second optical filter 26b has a high transmittance Tb with respect to light with a wavelength around 450 nm (blue light) and light with a wavelength around 550 nm (green light), and a low transmittance Tb with respect to light of other wavelengths. For this reason, the illumination light passing through the second optical filter 26b is blue light and green light, which, when mixed, produce the color cyan. For the sake of simplicity of description, the illumination light passing through the first optical filter 26a is referred to as “normal light” and the illumination light passing through the second optical filter 26b is referred to as “narrowband light”.

[0022] In this way, the filter changer 26 alternately extracts normal light using the first optical filter 26a and narrowband light using the second optical filter 26b at a predetermined field rate while rotating. The rotational position and rotational phase of the filter changer 26 are controlled using a photointerrupter 28 to detect an opening (not shown) formed near the outer edge of the filter changer 26.

[0023] The light adjusting unit 22 has a variable aperture. The amount of illumination light entering the light adjusting unit 22 from the filter changer 26 is adjusted by the size of the aperture of the variable aperture. The illumination light adjusted by the light adjusting unit 22 is condensed onto an end surface of the optical fiber bundle 13 by the condenser lens 23 and enters the optical fiber bundle 13. The illumination light entering the optical fiber bundle 13 is guided to the distal end portion of the introducer 112. The illumination light guided to the distal end portion of the introducer 112 exits the exit end of the optical fiber bundle 13 and illuminates the object in a body cavity. The illumination light illuminating the object is reflected by the object and enters the image forming unit 10 as object light.

[0024] The image forming unit 10 includes an objective lens 10a and an image sensor 10b. The image sensor 10b is, for example, an interlaced single CCD (charge-coupled device) image sensor. The object light entering the image forming unit 10 is received by the image sensor 10b through the objective lens 10a. The object light received by the image sensor 10b is converted into an image signal at a predetermined field rate and sent to the image processing unit 123 of the processor unit 120 via one of the wires 14. For convenience of description, the image signal for an EVEN field output from the image sensor 10b is referred to as a "first image signal," and the image signal for an ODD field is referred to as a "second image signal."Furthermore, the timing for switching between the normal light and the narrow-band light by the filter changer 26 and the timing for switching between the image formation period (field period) in the image sensor 10b are synchronized with a field synchronization signal output from the image sensor driver 126. The first image signal related to the EVEN field is an image signal generated by imaging an object illuminated with the normal light, and the second image signal related to the ODD field is an image signal generated by imaging the object illuminated with the narrow-band light.

[0025] Furthermore, the image sensor driver 126 sends a partial image signal with the first image signal and the second image signal to the image processing unit 123. The partial image signal is a signal indicating whether the first image signal or the second image signal was sent from the image sensor 10b to the image processing unit 123.

[0026] The image processing unit 123 includes a pre-processing unit 40, a first memory 41, a second memory 42, an image composing unit 43, a post-processing unit 44, and an image evaluation processing unit 45.

[0027] The pre-processing unit 40 performs signal processing, such as AD conversion and gamma correction, on the first image signal and the second image signal received from the image sensor 10b. Of these image signals, which have been signal-processed by the pre-processing unit 40, the first image signal is sent to the first memory 41 and the second image signal is sent to the second memory 42.

[0028] The first memory 41, which functions as a first image signal storage means, and the second memory 42, which functions as a second image signal storage means, successively store the received image signals. Note that the number of image signals (number of frames) that can be stored in the first memory 41 and the second memory 42 is limited. Therefore, when the first memory and the second memory receive a new image signal while the maximum number of image signals is stored, the new image signal is stored by overwriting the oldest image signal among the stored image signals.

[0029] Fig. 5 is a diagram showing the temporal change of the field synchronization signal, the field signal of the illumination light, as well as the timing at which image signals are output from the image sensor 10b, and the timing at which storage processing and readout processing are performed on image signals in the first memory 41 and the second memory 42. In Fig. 5, time progresses from left to right. The field synchronization signal is a two-value voltage signal having a rectangular waveform, the voltage of which changes at a predetermined field rate (1 / 60 second in the present embodiment). The field signal is a two-value voltage signal having a rectangular waveform. As shown in Fig. As shown in Figure 5, the two values ​​of the "field signal" represent "EVEN" and "ODD". The timing at which the voltage of the field signal changes is synchronized with the rise of the voltage of the field synchronization signal. The Fig. 5, the “illumination light” refers to the type of illumination light that illuminates the object. As shown in Fig. As shown in Figure 5, when the “field signal” is “EVEN”, the object is illuminated with normal light, and when the “field signal” is “ODD”, the object is illuminated with narrow band light.

[0030] First image signals of the object illuminated with normal light are successively stored in the first memory 41 at the frame rate of the image sensor 10b (1 / 30 second in the present embodiment). Second image signals of the object illuminated with narrowband light are successively stored in the second memory 42 at the frame rate of the image sensor 10b. Fig. 5, the successively stored first image signals and second image signals are designated by different numbers. It should be noted that processing time is required for the electrical signal processing performed by circuits of the processor unit 120 and the endoscope unit 110, such as signal processing in the preprocessing unit 40 and write processing for writing image signals to the first memory and the second memory. For this reason, a discrepancy occurs between the exposure timing of the image sensor 10b and the output timing of the image signals (first image signal or second image signal). Fig. 5, it is assumed that this deviation is equivalent to a partial image. While, for example, in Fig. 5 While the "ODD" field signal and the "first image signal 1" are output from the image sensor 10b and the object is illuminated with the narrow-band light, the storage processing for storing the "first image signal 1" in the first memory 41 is performed, and the readout processing for reading out the "second image signal 0" stored in the immediately preceding field from the second memory 42 is performed. While the "EVEN" field signal and the "second image signal 1" are read out from the image sensor 10b and the object is illuminated with normal light, the readout processing for reading out the "first image signal 1" stored in the immediately preceding field from the first memory 41 is performed, and the storage processing for storing the "second image signal 2" in the second memory 42 is performed.It should be noted that the deviation between the timing of exposure to the illumination lights by the image sensor 10b and the timing of outputting image signals is not limited to being exactly equivalent to one field, and that this deviation varies with the processing time required in the circuits.

[0031] The image signals read from the first memory 41 and the second memory 42 are sent to the image composing unit 43. The image composing unit 43 composes the first image signal and the second image signal of the same frame and sends the composite image signal to the post-processing unit 44. The post-processing unit 44 processes the composite image signal received from the image composing unit 43, generates screen data for monitor display, and converts the generated screen data for monitor display into a predetermined video format signal. The converted video format signal is output to the monitor 130.

[0032] Fig. 6 shows an example of a display screen of the monitor 130. As in Fig.6, an image 131 based on the first image signal (hereinafter referred to as a "normal light observation image") and an image 132 based on the second image signal (hereinafter referred to as a "narrow band light observation image") are displayed side by side in the same screen on the monitor 130.

[0033] Next, a moving image freeze processing for the present embodiment will be described.

[0034] When a freeze command is input to the input device 125 by the user (operator) of the endoscope system 100 while a moving image is displayed on the monitor 130, the displayed moving image is paused (frozen) under the control of the controller 124, which functions as a command means. The frozen moving image is displayed on the monitor 130 as a still image. A first image signal and a second image signal for display as a still image are selected by the image evaluation processing unit 45 from the frame-by-frame image signals stored in the first memory 41 and the second memory 42. For convenience of description, the image signals selected by the image evaluation processing unit 45 are hereinafter referred to as "still image signals."

[0035] The image evaluation processing unit 45 functions as image evaluation value calculation means. When image signals (first image signal and second image signal) of individual images are stored in the memories (first memory 41 and second memory 42), the image evaluation processing unit 45 calculates evaluation values ​​for the stored image signals. Edge detection processing is applied in the image evaluation value calculation. The edge detection processing is a counting process designed to count the number of pixels corresponding to a clear (strong) edge.

[0036] The image evaluation processing unit 45 performs counting processing on the first image signal of each frame stored in the first memory 41. In this counting processing, the number of pixel values ​​in the first image signal that are greater than a predetermined threshold is counted for each frame. The counted number of pixels becomes the image evaluation value of the first image signal. The higher the image evaluation value, the less motion blur or defocus is present in the normal-light observation image.

[0037] The image evaluation value calculation performed on the first image signal is also performed by the image evaluation processing unit 45 on the second image signal of each frame stored in the second memory 42. The higher the image evaluation value of the second image signal, the less motion blur or defocus is present in the narrowband light observation image.

[0038] The image evaluation processing unit 45 operates as an image signal selecting means.

[0039] When a freeze command is input to the input device 125, the image evaluation processing unit 45 selects freeze image signals from the image signals (first image signal and second image signal) stored in the memories (first memory 41 and second memory 42) based on the calculated image evaluation values.

[0040] Next, consider the case where the image evaluation value is based only on motion blur and defocus. In this case, there is a risk of a large time difference occurring between the time at which the freeze command is issued (hereinafter referred to as the "freeze time") and the time at which the image corresponding to the still image signal selected based on the image evaluation values ​​is captured (hereinafter referred to as the "image capture time"). If this time difference is larger, cases may occur in which a still image corresponding to the time desired by the operator is not displayed on the monitor 130.

[0041] The still image displayed on the monitor 130 is more suitable the closer the recording time is to the freezing time. For this reason, the image evaluation processing unit 45 weights the image evaluation values ​​according to the image recording time. The image evaluation processing unit 45 performs addition processing, in which an addend is added to the image evaluation value, or multiplication processing, in which the image evaluation value is multiplied by a multiplier. This addend or multiplier is larger the smaller the time difference between the freezing time and the image recording time of the image signal. After the addition processing or the multiplication processing has been performed on the image evaluation values ​​of the image signal, the image evaluation processing unit 45 selects the image signal of the frame having the highest image evaluation value (ie,the still image signal). The processing performed by the image evaluation processing unit 45 to select the still image signal from the first image signals of multiple frames and the processing performed by the image evaluation processing unit 45 to select the still image from the second image signal of multiple frames are performed independently of each other.

[0042] Accordingly, the first image signal and the second image signal, each having little image blur and defocus, and also having a small time difference between the freezing time and the image capture time, are independently selected as still image signals. The image composing unit 43 and the post-processing unit 44 function as display means. The still image signals are processed by the image composing unit 43 and the post-processing unit 44 and sent to the monitor 130, so that a normal-light observation image (still image) and a narrow-band light observation image (still image) are displayed side by side on the display screen of the monitor 130.

[0043] Here, the normal light and the narrowband light have different wavelength bands and light intensities. When the wavelength bands and light intensities of the illumination light are different, the conditions for the occurrence of motion blur and defocus in an image change. For this reason, even in the same frame, the amounts of image blur and defocus in the normal-light observation image and the narrowband light observation image are not necessarily the same. However, in the present embodiment, the still image signal selection is performed separately for the first image signal and the second image signal.Therefore, even if the amounts of image blur and defocus are different between the first image signal and the second image signal, or the image pickup timings (frames) having little image blur or defocus are different, the image signal for an image having little image blur and defocus is selected as the still image signal for both the first image signal and the second image signal.

[0044] An illustrative example of an embodiment of the present invention has been described above. However, the embodiments of the present invention are not limited to the above-described embodiment; various changes may be made within the scope of the technical idea of ​​the present invention.

[0045] Although in the present embodiment, the image evaluation processing unit 45 performs image evaluation value calculation on both the first image signal and the second image signal, the present invention is not limited thereto. Thus, the image evaluation value calculation may be performed on either the first image signal or the second image signal.

[0046] The following describes the case where the image evaluation processing unit 45 performs image evaluation value calculation only on the first image signal. The image evaluation processing unit 45 performs image evaluation value calculation processing only on the first image signal of each frame stored in the first image memory. When an input command is supplied to the input device 125, the image evaluation processing unit 45 selects a still image signal from the first image signals of multiple frames based on the calculated image evaluation values. Subsequently, the second image signal of the same frame is selected as the selected first image signal by the image evaluation processing unit 45 as the still image signal.Accordingly, it is possible to reduce the amounts of motion blur and defocus in the normal-light observation image, and also to reduce the load of the image evaluation value calculation performed by the image evaluation processing unit 45. Note that the frame of the second image signal selected as the still image signal does not have to be the same frame as the selected first image signal. For example, the second image signal of the frame immediately before or after the frame of the selected first image signal may be selected as the still image signal.

[0047] The image evaluation value can also be calculated based on the difference between the image signals of two consecutive frames. For example, if a strong motion blur occurs in an observation image due to strong (fast) movement of the object, the difference between the image signals of two consecutive frames increases, so a low image evaluation value is calculated. If a weak motion blur occurs in an observation image due to slight movement of the object, the difference between the image signals of two consecutive frames decreases, and therefore a high image evaluation value is calculated.

[0048] Furthermore, although interlaced imaging is described as an example in the present embodiment, the interlaced imaging may be replaced by progressive imaging in another embodiment.

Claims

[1] Endoscope system (110), comprising: a light source unit (121) that alternately emits first illumination light and second illumination light in a predetermined cycle, the first illumination light and the second illumination light having different wavelength bands; an image generating means (126) for imaging an object illuminated with the first illumination light and for generating a first image signal and for imaging the object illuminated with the second illumination light and for generating a second image signal; a first image signal storage means (41) for successively storing generated first image signals; a second image signal storage means (42) for successively storing generated second image signals; an image evaluation value calculating means (45) for calculating image evaluation values ​​of first images based on the stored first image signals and image evaluation values ​​of second images based on the stored second image signals; an image signal selecting means (45) for selecting a first display image signal from a plurality of the first image signals stored in the first image signal storing means (41) and for selecting a second display image signal from a plurality of the second image signals stored in the second image signal storing means (42) independently of one another and on the basis of the calculated image evaluation values; and a display means (130) for simultaneously displaying a first image based on the first display image signal and a second image based on the second display image signal. [2] Endoscope system (110) according to claim 1, further comprising a command receiving means (125) for receiving a freeze command from a user, wherein the image evaluation value calculating means (45) weights each of the calculated image evaluation values ​​according to a time difference between a time at which an image corresponding to the image evaluation value was captured and a time at which the command receiving means (125) received the freeze command. [3] Endoscope system (110) according to claim 2, wherein the image evaluation value calculating means (45) performs the weighting by adding an addend to the image evaluation value or by multiplying the image evaluation value by a multiplier, where the addend or multiplier is larger the smaller the time difference is. [4] Endoscope system (110) according to one of claims 1 to 3, in which the image signal selection means (45) selects the first display image signal from a plurality of stored first image signals based on the image evaluation values, and selecting as the second display image signal the second image signal of the same frame as that of the first display image signal or the second image signal of a frame which is a predetermined number of frames before or a predetermined number of frames after the frame of the first display image signal. [5] Endoscope system (110) according to one of claims 1 to 4, in a case where a new first image signal is generated by the image generating means (126) when a predetermined number of first image signals are stored, the first image signal storing means (41) stores the newly generated first image signal by overwriting a first image signal having the oldest storage time among the stored first image signals, and in a case where a new second image signal is generated by the image generating means (126) when a predetermined number of second image signals are stored, the second image signal storing means (42) stores the newly generated second image signal by overwriting a second image signal having the oldest storage time among the stored second image signals.

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