Imaging device

Abstract

This imaging device is provided with: a photocathode having a photoelectric surface; a solid-state imaging element for detecting photoelectrons emitted from the photoelectric surface and outputting a detection signal; a vacuum housing for holding the photocathode and the solid-state imaging element in a vacuum state; and a signal processing unit for processing the detection signal output from the solid-state imaging element. The solid-state imaging element detects photoelectrons multiplied by electron bombardment multiplication by the incidence of accelerated photoelectrons, and outputs the detection signal. The signal processing unit performs binarization processing on the detection signal of each frame on the basis of a first threshold related to a signal strength set in advance so as to enable identification of the presence or absence of photons, and performs photon counting by adding the binarized signal of each frame after the binarization processing.

Description

Imaging device

[0001] The present disclosure relates to an imaging device.

[0002] Patent Document 1 describes a method of using a filter to prevent the fluorescence detection from being affected by the reflected excitation light, and Patent Document 2 describes a method of synchronizing the timing of generating and reading out the excitation and reflected light to prevent the fluorescence detection from being affected by the reflected excitation light.

[0003] Japanese Patent Laid-Open No. 9-140714 Japanese Patent Laid-Open No. 2001-190489

[0004] In the above-mentioned fluorescence detection and bioluminescence observation, which has an even smaller signal intensity, further improvement in the S / N ratio is required.

[0005] The present disclosure has been made in consideration of the above-described circumstances, and has an object to provide an imaging device that can achieve a further improvement in the S / N ratio.

[0006] An imaging device according to one aspect of the present disclosure is [1] "an imaging device comprising: a photoelectric conversion unit having a photocathode; a solid-state imaging element that detects photoelectrons emitted from the photocathode and outputs a detection signal; a vacuum housing that holds the photoelectric conversion unit and the solid-state imaging element in a vacuum state; and a signal processing unit that processes the detection signal output from the solid-state imaging element, wherein the solid-state imaging element detects photoelectrons multiplied by electron bombardment multiplication when accelerated photoelectrons are incident thereon and outputs a detection signal; and the signal processing unit performs binarization processing on the detection signal of each frame based on a first threshold value related to signal strength that is set in advance so that the presence or absence of photons can be identified, and performs photon counting by adding the binarized signals of each frame after the binarization processing."

[0007] In an imaging device according to an aspect of the present disclosure, a photoelectric conversion unit and a solid-state imaging element are held in a vacuum state, and photoelectrons multiplied by electron bombardment multiplication are detected in the solid-state imaging element, outputting a detection signal. In this way, by detecting photoelectrons multiplied by electron bombardment multiplication in a vacuum state, a detection signal with little multiplication fluctuation can be output. In the imaging device according to an aspect of the present disclosure, the detection signal is binarized based on a first threshold set to enable discrimination between the presence and absence of photons, and the binarized signals for each frame are added together to perform photon counting. As described above, the detection signal has little multiplication fluctuation, and noise-free signal components are appropriately multiplied, making it possible to appropriately discriminate between the presence and absence of photons through binarization. Then, by adding the binarized signals in which the presence or absence of photons has been appropriately discriminated, photon counting can be performed, thereby obtaining an image with an improved S / N ratio. As described above, the imaging device according to an aspect of the present disclosure can achieve a further improvement in the S / N ratio.

[0008] An imaging device according to one aspect of the present disclosure may be [2] "the imaging device according to [1], wherein the signal processing unit deletes the binarized signal of the oldest frame when adding the binarized signal of a new frame when the number of frames to be added reaches a predetermined upper limit of frames." This configuration makes it possible to always acquire the signal of the most recent frame (the frame with the upper limit of frames). This makes it possible to acquire information on the movement (movement information) of an object, for example, in bioluminescence observation.

[0009] An imaging device according to one aspect of the present disclosure may be [3] "an imaging device according to [2], wherein the signal processing unit performs a moving average process or an addition process on the detection signals of predetermined exclusion frames without performing the binarization process." With this configuration, signals relating to light of different properties can be acquired, for example, by a moving average process or an addition process, in addition to the process of adding the binarized signals described above. This makes it possible to acquire signals relating to light of different properties by switching between them within the same time period.

[0010] An imaging device according to one aspect of the present disclosure may be [4] "an imaging device according to any one of [1] to [3], wherein the signal processing unit multiplies a signal obtained by adding the binarized signals of each frame by a predetermined constant." For example, in bioluminescence observation, the signal amount is extremely small, and when the binarized signals of each frame are added to generate a single image, the signal amount may be insufficient. In this regard, multiplying the signal obtained by adding the binarized signals by a constant increases the signal amount, improves the S / N ratio, and generates image contrast. Note that this processing is possible only because the present method, which can appropriately separate noise components from signal components through binarization processing, is able to achieve this. In other words, if a signal contains a large amount of noise, multiplying it by a constant will not improve the S / N ratio. However, in the present method, the binarization processing results in almost no noise component in the signal, so multiplying it by a constant increases only the signal component and improves the S / N ratio.

[0011] An imaging device according to one aspect of the present disclosure may be [5] "the imaging device according to any one of [1] to [4], wherein the signal processing unit specifies the number of photons based on a second threshold related to signal strength that is set in advance so that the number of photons can be identified." In this way, in addition to the first threshold for identifying the presence or absence of photons, a second threshold is set so that the number of photons can be identified, and by specifying the number of photons based on the second threshold, it is possible to identify signals containing two or more photons. Note that this type of processing is possible only because of the present method, which has little signal multiplication fluctuation.

[0012] An imaging device according to one aspect of the present disclosure may be [6] "an imaging device according to any one of [1] to [5], wherein the signal processing unit excludes frames with an abnormal number of photons from the frames to be added based on a third threshold related to signal strength that is preset to enable identification of an abnormal number of photons." For example, in bioluminescence and the like, the amount of light emitted is extremely small, so it is almost impossible for a single frame to contain a signal with many photons (e.g., three or more photons). If such a large number of photons is detected, it is likely to be an abnormal value such as cosmic ray noise. By setting the third threshold so that signal strengths above a certain level are not counted as abnormal values, the influence of abnormal noise can be canceled. Note that this processing is possible only because of the present method, which has little signal multiplication fluctuation.

[0013] According to the imaging device according to the present disclosure, it is possible to achieve a further improvement in the S / N ratio.

[0014] FIG. 1 is a diagram schematically illustrating the configuration of an imaging device according to a first embodiment. FIG. 2 is a cross-sectional view of an electron bombardment imaging sensor. FIG. 3(a) is a graph showing a signal detected in an sCMOS, and FIG. 3(b) is a graph showing a signal detected in an EB-CMOS. FIG. 4 is a diagram illustrating the characteristics of an electron bombardment camera compared to an sCMOS camera and a cooled CCD camera. FIG. 5 is a diagram illustrating the characteristics of an electron bombardment camera compared to an imaging device that combines an image intensifier with an imaging element. FIG. 6(a) is a diagram illustrating a live image, FIG. 6(b) is a diagram illustrating a moving average image, and FIG. 6(c) is a diagram illustrating a photon counting image. FIG. 7 is a diagram illustrating an example of setting a second threshold. FIG. 8 is a diagram illustrating an example of setting a third threshold. FIG. 9 is a diagram schematically illustrating the configuration of an imaging device according to a modified example. FIG. 10 is a diagram schematically illustrating the configuration of an imaging device according to a modified example. FIG. 11 is a diagram schematically illustrating the configuration of an imaging device according to a second embodiment. FIG. 12 is a diagram explaining the rolling photon counting method. FIG. 13(a) is an image of movement information acquired by an imaging device, and FIG. 13(b) is an image of movement information acquired by a cooled CCD. FIG. 14 is a diagram explaining rolling photon counting according to a modified example. FIG. 15 is a diagram schematically showing the configuration of an imaging device according to a modified example. FIG. 16 is a diagram schematically showing the configuration of an imaging device according to a modified example. FIG. 17 is a diagram schematically showing the configuration of an imaging device according to a modified example. FIG. 18 is a diagram schematically showing the configuration for fluorescence observation of an imaging device according to a third embodiment. FIG. 19 is a diagram explaining switching between the rolling photon counting method and the rolling averaging method or the summation processing method. FIG. 20(a) is a diagram showing the results of bioluminescence observation, and FIG. 20(b) is a diagram showing the results of fluorescence observation.

[0015] [First embodiment] A first embodiment of the present invention will be described in detail below with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and duplicated explanations will be omitted.

[0016] FIG. 1 is a schematic diagram illustrating the configuration of an imaging device 1 according to a first embodiment. The imaging device 1 is an apparatus for performing bioluminescence observation, for example, which involves extremely low signal levels. In addition to bioluminescence, other luminescence phenomena include fluorescence and chemiluminescence. Bioluminescence is the generation and emission of light by living organisms. Fluorescence is the emission of light by living organisms that have absorbed energy from excitation light and become excited. Chemiluminescence is the emission of light by living organisms that have become excited through a chemical reaction. For example, in fluorescence observation, increasing the excitation intensity increases the fluorescence intensity, but this can lead to damage to living organisms (e.g., cells) and increased autofluorescence, resulting in reduced quantitative observation accuracy. In contrast, bioluminescence observation does not require excitation light and does not cause photodamage to living organisms (cells), enabling long-term imaging. Furthermore, bioluminescence observation suppresses background light due to autofluorescence, allowing stable acquisition of signal values ​​alone, ensuring quantitative observation accuracy. On the other hand, in bioluminescence observation, the amount of luminescence (signal amount) is extremely weak, and it may be difficult to improve the S / N ratio. The imaging device 1 according to this embodiment employs a method for improving the S / N ratio even in such weak bioluminescence observation (details will be described later).

[0017] 1, the imaging device 1 includes an objective lens 2, a filter 3, an electron-illuminated camera 10, a high-voltage controller 20, a personal computer (PC) 30, and a monitor 40. In the following description, the imaging device 1 is assumed to observe bioluminescence from a cell S.

[0018] The objective lens 2 is a lens that focuses bioluminescence from the cells S toward the electron-implanted imaging sensor 11 of the electron-implanted camera 10. The bioluminescence focused by the objective lens 2 passes through a filter 3 and enters the electron-implanted imaging sensor 11 of the electron-implanted camera 10.

[0019] The electronic flash camera 10 includes an electronic flash imaging sensor 11 , a driving unit 12 , a control unit 13 , a high-voltage power supply 14 , a signal processing unit 15 , and an interface 16 .

[0020] As shown in FIG. 2, the electron bombardment imaging sensor 11 includes a photocathode 111 (photoelectric conversion unit), a solid-state imaging element 112 having an electron bombardment multiplication function, and a vacuum housing 113. In this embodiment, the solid-state imaging element 112 is an EB-CMOS (Electron Bombarded-Complementary Metal-Oxide-Semiconductor). The vacuum housing 113 is a housing that holds the photocathode 111 and the solid-state imaging element 112 in a vacuum state. The vacuum housing 113 has an entrance window 113a through which light focused by the objective lens 2 enters. The light entering through the entrance window 113a reaches the photocathode 111's photocathode surface 111a.

[0021] The photocathode 111 is a photoelectric conversion unit having a photocathode 111a. The photocathode 111a converts incident light into photoelectrons. The photocathode 111 is made, for example, in the form of a semiconductor layer and may be a crystalline photocathode having a single crystal structure (e.g., a III-V semiconductor such as GaAs or a II-VI semiconductor such as CdTe). Alternatively, the photocathode 111 may be an alkali photocathode having a polycrystalline structure such as SbK2Cs, SbRb2Cs, SbCs3, SbNa3, SbNaKRbCs, SbNaKCs, or SbNa2KCs. The photocathode 111 is electrically connected to an electrode 118, and a predetermined potential is applied to the photocathode 111 (photocathode 111a) via the electrode 118. Photoelectrons emitted from the photocathode 111 photocathode surface 111 a are radiated into a vacuum and detected by the solid-state image sensor 112 .

[0022] The solid-state imaging element 112 is an imaging element that detects photoelectrons emitted from the photocathode 111a and outputs a detection signal. The solid-state imaging element 112 detects photoelectrons that are accelerated by energy imparted corresponding to the potential difference between the photocathode 111 and the solid-state imaging element, and multiplies the photoelectrons by electron bombardment multiplication. The solid-state imaging element 112 outputs a detection signal. The solid-state imaging element 112 is electrically connected to the lead pins 120 via bonding wires 119. The solid-state imaging element 112 is disposed opposite the photocathode 111 and is located at the end of the vacuum housing 113 opposite the end where the photocathode 111 is disposed. The solid-state imaging element 112 may be, for example, a backside illumination (BSI) CMOS. The detection signal output from the solid-state imaging element 112 is input to a signal processing unit 15 located outside the vacuum housing 113. The voltage applied to the solid-state imaging element 112 and the gain obtained as a detection signal are generally proportional to each other. A desired gain can be obtained by adjusting the potential difference between the photocathode 111 and the solid-state imaging element 112. The EB-CMOS (solid-state imaging element 112) may be replaced with an EB-CCD (Electron-Bombarded Charge-Coupled Device).

[0023] FIG. 3(a) is a graph showing signals detected by an sCMOS (scientific CMOS), and FIG. 3(b) is a graph showing signals detected by an EB-CMOS. The sCMOS in FIG. 3(a) is shown as an example configuration for comparison with the electron bombardment camera 10. In FIGS. 3(a) and 3(b), the horizontal axis represents time, and the vertical axis represents signal amount. In FIGS. 3(a) and 3(b), the areas with small signal amounts represent noise such as readout noise and dark current generated within the solid-state image sensor 112. The sCMOS shown in FIG. 3(a) suppresses dark current through cooling and reduces readout noise, thereby improving the S / N ratio. On the other hand, the sCMOS does not have a multiplication function and cannot amplify signal components. In contrast, as shown in FIG. 3(b), the EB-CMOS improves the S / N ratio by using the gain described above to amplify only signal components that do not contain noise (readout noise and dark current noise) resulting from the operation of the solid-state image sensor 112. Such an EB-CMOS can achieve a higher S / N ratio than an sCMOS in measurements under extremely low illumination (e.g., in the photon counting region where the average number of photons detected by one pixel per frame is less than a single photon).

[0024] Returning to Fig. 1, the drive unit 12 is a circuit for driving the electron bombardment imaging sensor 11. The control unit 13 is configured to change various settings in the electron bombardment imaging sensor 11, for example, changing the number of pixels and the field of view through processing such as binning and sub-array. The high-voltage power supply 14 is a power supply for applying voltage to the photocathode 111a and the solid-state image sensor 112 to generate electron bombardment multiplication gain. The voltage of the high-voltage power supply 14 is adjusted by a high-voltage controller 20.

[0025] The signal processing unit 15 processes the detection signal output from the solid-state imaging device 112. The signal processing unit 15 performs binarization processing on the detection signal of each frame based on a first threshold value related to signal strength that is set in advance so that the presence or absence of photons can be identified, and performs photon counting by adding up the binarized signals of each frame after the binarization processing.

[0026] 1, the signal processing unit 15 outputs a live signal, a binarized signal after binarization processing, and a photon counting signal obtained by adding the binarized signal. The live signal is a raw signal that has not undergone any special processing on the detection signal. Such a live signal is used, for example, when acquiring a moving average image (rolling average image).

[0027] The binarization process will be described with reference to FIGS. 4 and 5 . The signal processing unit 15 predetermines a luminance value (signal intensity) at the time of incident photons as a first threshold value, which is a luminance value that can be used to distinguish between the presence and absence of photons, taking into account the luminance value (signal intensity) at the time of incident photons. The first threshold value is set to a luminance value that cannot be reached in the case of "no photons" (when only noise is present) and is assumed to be reached in the case of "presence of photons." By performing binarization based on whether or not the signal exceeds the first threshold value, the presence or absence of photons can be appropriately distinguished for each frame. As shown in the bottom row of FIG. 4 , the signal processing unit 15 determines that frames with signal intensity greater than the first threshold have "photons present" and assigns a value of "1," and determines that frames with signal intensity less than the first threshold have "no photons" and assigns a value of "0." As shown in FIG. 4 , the signal contains dark current and readout noise. However, since the first threshold value is set to a luminance value that cannot be reached in the case of "no photons" and only noise, as described above, the presence or absence of photons can be appropriately distinguished even when noise is taken into account.

[0028] 4 is a diagram illustrating the characteristics of the electron-fired camera 10 compared with an sCMOS camera and a cooled CCD camera. The sCMOS camera does not have a multiplication function and is therefore unable to amplify only the signal component, making it impossible to distinguish between the presence and absence of photons using the first threshold value described above. The cooled CCD camera also cannot amplify only the signal component compared to noise such as readout noise, making it impossible to distinguish between the presence and absence of photons using the first threshold value described above. In contrast, the electron-fired camera 10 according to this embodiment amplifies only the signal component, making it possible to appropriately distinguish between the presence and absence of photons based on the first threshold value.

[0029] FIG. 5 illustrates the characteristics of the electron bombardment camera 10 compared to an imaging device that combines an image intensifier with an imaging element (CMOS camera). An image intensifier is a sensor with a signal multiplication function, and an imaging device configuration is conceivable in which the optical signal amplified by the image intensifier and output is captured by a CMOS camera. However, as shown in the upper diagram of FIG. 5 , the optical signal amplified by the image intensifier fluctuates significantly, and when captured by a CMOS camera, the presence or absence of photons may not be properly identified (i.e., the signal may not be properly binarized) using the first threshold. If proper binarization is not possible, the image may appear grainy with a low S / N ratio. In contrast, as shown in the lower diagram of FIG. 5 , the electron bombardment camera 10 detects photoelectrons multiplied by electron bombardment in a vacuum state, resulting in less multiplication fluctuation in the detection signal, allowing the presence or absence of photons to be properly identified using the first threshold. In the drawings following FIG. 5 (FIGS. 5, 7, and 8), the dark current and the readout noise are shown together without distinction.

[0030] FIG. 6(a) shows an example of a live image, FIG. 6(b) shows an example of a moving average image, and FIG. 6(c) shows an example of a photon counting image. The images shown in FIGS. 6(a) to 6(c) are images acquired by an electronic flash camera 10, and show examples of images acquired with an exposure time of 20 msec and 500 V. The live image shown in FIG. 6(a) is an image acquired from one frame of live signals (raw signals). Due to the low illumination, almost no signal is visible. When such images are added together, the resulting image has an extremely poor S / N ratio.

[0031] The moving average image shown in Figure 6(b) is an image acquired from a live signal. As mentioned above, because a live signal cannot acquire an appropriate image, a moving average can be adopted as a method for improving the S / N ratio. However, in an extremely low-light environment such as bioluminescence, even if about 15,000 frames are added, noise components are also acquired proportionally, and therefore the S / N ratio cannot be significantly improved as shown in Figure 6(b).

[0032] In contrast, the photon counting image shown in Figure 6(c) is obtained by canceling noise components (dark current, readout noise) and adding signal components. Therefore, by integrating signals for, for example, 15,000 frames, only the signal component (S) is added, thereby making it possible to achieve an extremely high S / N ratio.

[0033] Returning to FIG. 1 , the signal processing unit 15 may multiply the signal obtained by adding the binarized signals for each frame by a predetermined constant. As described above, a single image is acquired by adding the binarized signals for any number of frames. In this case, the dimmer the luminescence from the cell S, which is the bioluminescence sample, the less signal there is in the final image. Therefore, as a method for generating image contrast, the signal processing unit 15 multiplies the final signal obtained by adding the binarized signals by a predetermined constant. As described above, since the binarization process appropriately separates noise components and signal components, multiplying the signal obtained by adding the binarized signals by a predetermined constant can increase almost only the signal component. This improves the S / N ratio in the final image. The signal processing unit 15 may also multiply not only the final signal obtained by adding the binarized signals by a predetermined constant, but also a signal at any timing. For example, in bioluminescence, luminescence is extremely weak. Therefore, when binarizing such a weak signal, it is preferable to multiply the signal at any timing by a constant to check whether an appropriate signal is being detected at any stage. For this reason, the signal processing unit 15 may multiply the signal at any timing by a predetermined constant.

[0034] The signal processing unit 15 may also determine the number of photons based on a second threshold related to signal strength that is preset to enable identification of the number of photons. Photon counting using the above-described first threshold distinguishes between the presence and absence of photons. Such a first threshold can be considered a threshold for canceling noise components. In addition, by providing a second threshold, which is a threshold for distinguishing the number of photons, it becomes possible to distinguish between one photon and two photons. In the case of bioluminescence, it is difficult to imagine three or more photons entering one frame, so the second threshold is set to, for example, a luminance value that distinguishes between one photon and two photons. For this reason, as shown in FIG. 7 , the second threshold is set to a luminance value that is expected to be impossible to reach in the case of "one photon" and always reached in the case of "two photons." Furthermore, the second threshold is set to a luminance value greater than the first threshold. Whether the photon count is "1" or "2" can be distinguished based on whether the second threshold is exceeded.

[0035] Furthermore, the signal processing unit 15 may exclude frames with abnormal photon counts from the summation target based on a third threshold value related to signal strength that is preset to enable identification of abnormal photon counts. As described above, in the case of bioluminescence, it is difficult to imagine that three or more photons will be present in a single frame, and if three or more photons are detected, it is highly likely that the detected value is an abnormal value such as cosmic ray noise. Therefore, as shown in FIG. 8 , a brightness value that cannot be reached in the case of fewer than "3 photons" and is expected to be reached in the case of "3 photons or more" is set as the third threshold value, thereby enabling appropriate identification of frames with abnormal photon counts (noise). Then, the frames with abnormal photon counts can be appropriately excluded from the summation target. Here, the frames to be added refer to frames used to construct an image.

[0036] The signal processing unit 15 outputs the Live signal, the binary signal, and a photon counting signal obtained by adding the binary signal for each frame to the PC 30 via the interface 16. The interface 16 is a device for connecting the signal output from the signal processing unit 15 to a downstream device (here, the PC 30).

[0037] The PC 30 includes an image input board 31 and software 32. The image input board 31 has a function of converting signals from the signal processing unit 15 into images. Examples of the results of converting each signal into an image are shown in, for example, FIGS. 6(a) to 6(c). The software 32 performs various processes related to the images. The software 32 outputs images to a monitor 40. The monitor 40 is a display device that displays images.

[0038] (Operational Effects) Next, operational effects of the imaging device 1 according to this embodiment will be described.

[0039] The imaging device 1 according to this embodiment includes a photocathode 111 having a photocathode 111a, a solid-state imaging element 112 that detects photoelectrons emitted from the photocathode 111a and outputs a detection signal, a vacuum housing 113 that holds the photocathode 111 and the solid-state imaging element 112 in a vacuum state, and a signal processing unit 15 that processes the detection signal output from the solid-state imaging element 112. The solid-state imaging element 112 detects photoelectrons that are multiplied by electron bombardment multiplication when accelerated photoelectrons are incident on it, and the signal processing unit 15 outputs a detection signal. The signal processing unit 15 performs a binarization process on the detection signal for each frame based on a first threshold value related to signal strength that is set in advance so that the presence or absence of photons can be identified, and performs photon counting by adding up the binarized signals for each frame after the binarization process.

[0040] In the imaging device 1 according to this embodiment, the photocathode 111 and the solid-state imaging element 112 are maintained in a vacuum state, and the solid-state imaging element 112 detects photoelectrons multiplied by electron bombardment multiplication and outputs a detection signal. In this way, by detecting photoelectrons multiplied by electron bombardment multiplication in a vacuum state, a detection signal with little multiplication fluctuation can be output. The imaging device 1 according to this embodiment then performs binarization processing of the detection signal based on a first threshold value set to enable discrimination between the presence and absence of photons, and then adds up the binarized signals for each frame to perform photon counting. As described above, the detection signal has little multiplication fluctuation, and noise-free signal components are appropriately multiplied, making it possible to appropriately discriminate between the presence and absence of photons through binarization processing. Then, by adding up the binarized signals in which the presence or absence of photons has been appropriately discriminated and performing photon counting, a captured image with an improved S / N ratio can be obtained. As described above, the imaging device 1 according to this embodiment can achieve a further improvement in the S / N ratio.

[0041] The signal processing unit 15 may multiply the signal obtained by adding the binarized signals of each frame by a predetermined constant. For example, in bioluminescence observation, the signal amount is extremely small, so when the binarized signals of each frame are added to generate a single image, the signal amount may be insufficient. In this regard, multiplying the signal obtained by adding the binarized signals by a constant increases the signal amount, improves the S / N ratio, and generates image contrast. Note that this processing is possible only because the present method is able to appropriately separate noise components from signal components through binarization processing. In other words, if a signal contains a large amount of noise, multiplying it by a constant will not improve the S / N ratio. However, in the present method, the binarization processing results in the signal containing almost no noise components, so multiplying it by a constant increases only the signal component and improves the S / N ratio.

[0042] The signal processing unit 15 may identify the number of photons based on a second threshold value related to signal strength that is set in advance so that the number of photons can be identified. In this way, in addition to the first threshold value for identifying the presence or absence of photons, a second threshold value is set so that the number of photons can be identified, and by identifying the number of photons based on the second threshold value, it is possible to identify signals containing two or more photons. Note that this type of processing is possible only because the present method has little signal multiplication fluctuation.

[0043] The signal processing unit 15 may exclude frames with abnormal photon counts from the counting target based on a third threshold related to signal strength that is preset to enable identification of abnormal photon counts. For example, in bioluminescence and the like, the amount of light emitted is extremely low, so it is almost impossible for a signal containing many photons (e.g., three or more photons) to be included in one frame. If such a large number of photons is detected, it is likely to be an abnormal value such as cosmic ray noise. By setting the third threshold so that signal strengths above a certain level are considered abnormal and are not counted, the influence of abnormal noise can be canceled. Note that this type of processing is possible only because of the present method, which has little signal multiplication fluctuation.

[0044] While the imaging device 1 according to the first embodiment has been described above, the aspects of the imaging device according to the first embodiment are not limited to the above. Figures 9 and 10 are diagrams schematically showing the configuration of imaging devices according to modified examples.

[0045] The imaging device 1A shown in FIG. 9 includes a first signal processing unit 15A and a second signal processing unit 15B, which correspond to the signal processing units described above. The first signal processing unit 15A is provided inside the electronic flash camera 10. The second signal processing unit 15B is provided outside the electronic flash camera 10. The first signal processing unit 15A performs binarization processing on the detection signal output from the electronic flash imaging sensor 11 based on a first threshold value and outputs a binarized signal after the binarization processing. The second signal processing unit 15B acquires the binarized signal output from the first signal processing unit 15A via the interface 16, performs photon counting by adding the binarized signals for each frame, and outputs a photon counting signal. In this way, the signal processing unit may be separated into a configuration that performs binarization processing (first signal processing unit 15A) and a configuration that performs addition processing (second signal processing unit 15B).

[0046] 10 has a configuration equivalent to a signal processing unit, similar to that of the image capture device 1A, that is, a first signal processing unit 15A within the electronic flash camera 10 and a second signal processing unit 15B outside the electronic flash camera 10. In the image capture device 1B, the first signal processing unit 15A only outputs a detection signal (live signal) to a subsequent stage, while the second signal processing unit 15B performs binarization processing and addition processing. In this way, the functions to be borne by the two signal processing units may be selected as appropriate.

[0047] Second Embodiment Next, a second embodiment of the present disclosure will be described with reference to Figures 11 to 13. In the second embodiment, differences from the first embodiment will be mainly described, and descriptions that overlap with the first embodiment will be omitted.

[0048] The imaging device 101 according to the second embodiment includes a signal processing unit 115. The signal processing unit 115 is configured to be able to perform rolling photon counting in addition to the same processing as the signal processing unit 15 according to the first embodiment. Rolling photon counting will be described below.

[0049] In rolling photon counting, when the number of frames to be added reaches a preset upper limit, the signal processing unit 115 deletes the binarized signal of the oldest frame when adding the binarized signal of a new frame. For example, if 3000 frames are required to obtain a sufficient signal (upper limit: 3000 frames), when information on the 3001st frame is acquired and added, the information on the first frame is deleted, and when information on the 3002nd frame is acquired and added, the information on the second frame is deleted. By acquiring signals in this manner, it is possible to always acquire and add signals for the most recent 3000 frames.

[0050] 12 shows an example in which the time for one image (i.e., the upper limit number of frames) is assumed to be 4, and the oldest frame's binary signal is deleted each time the latest frame's binary signal is acquired. In this way, by always acquiring the latest maximum number of frames' worth of binary signals, it is possible to obtain information on the movement (movement information) of an object, for example, in bioluminescence observation.

[0051] Fig. 13(a) is an image of movement information acquired by the imaging device 101, and Fig. 13(b) is an image of movement information acquired by a cooled CCD camera. As shown in Fig. 13(b), a cooled CCD camera requires long exposure times to add signals, and acquires images in one frame only at specific moments (e.g., moments at 5-minute intervals). Therefore, images capturing the movement of, for example, a cell S, are discontinuous and scattered. In contrast, as shown in Fig. 13(a), the imaging device 101 allows the user to acquire the latest image at any timing, making it possible to capture the movement of the cell S over time.

[0052] (Operational Effects) Next, operational effects of the imaging device 101 according to this embodiment will be described.

[0053] In the imaging device 101, when the number of frames to be added reaches a predetermined upper limit, the signal processing unit 115 may delete the binarized signal of the oldest frame when adding the binarized signal of a new frame. This configuration allows the latest frame (the frame with the upper limit of frames) to be acquired at any time. This allows, for example, in bioluminescence observation, for example, to acquire information on the movement of an object (movement information) by saving images at a user-specified timing.

[0054] While the imaging device 101 according to the second embodiment has been described above, the aspects of the imaging device according to the second embodiment are not limited to the above. Figures 14 to 17 are diagrams illustrating imaging devices according to modified examples.

[0055] As shown in FIG. 14 , in rolling photon counting, a predetermined number of frames may be stored together rather than being stored one frame at a time. That is, for example, if 3,000 frames are required for image acquisition, images may be binarized and added every 1,000 frames and stored to collect the information for the 3,000 frames required for image acquisition. In FIG. 14 , the frames labeled Addition I to Addition IV each represent a frame group of 1,000 frames, and images may be stored in these frame groups. When adding and deleting are repeated using rolling photon counting, for example, 1,000 frames in Addition I may be deleted, and new 1,000 frames in Addition IV may be added and stored. According to this method, the number of images stored is 1 / 1000 of that required for frame-by-frame storage, thereby reducing the amount of data by 1 / 1000.

[0056] The imaging device 101A shown in FIG. 15 includes a first signal processing unit 115A and a second signal processing unit 115B, which correspond to the signal processing units described above. The first signal processing unit 115A is provided inside the electronic flash camera 10. The second signal processing unit 115B is provided outside the electronic flash camera 10. The first signal processing unit 115A performs binarization processing on the detection signal output from the electronic flash imaging sensor 11 based on a first threshold value and outputs a binarized signal after the binarization processing. The first signal processing unit 115A also performs photon counting by adding the binarized signals of each frame and outputs a photon counting signal. When the number of frames to be added reaches a predetermined upper limit, the second signal processing unit 115B performs rolling photon counting, which deletes the binarized signal of the oldest frame when adding the binarized signal of a new frame. In this way, the signal processing unit may be divided into a configuration that performs binarization processing and addition processing (first signal processing unit 115A) and a configuration that performs rolling photon counting processing (second signal processing unit 115B).

[0057] The imaging device 101B shown in FIG. 16 includes a first signal processing unit 115A and a second signal processing unit 115B, which correspond to the signal processing units described above. The first signal processing unit 115A is provided inside the electronic flash camera 10. The second signal processing unit 115B is provided outside the electronic flash camera 10. The first signal processing unit 115A performs binarization processing on the detection signal output from the electronic flash imaging sensor 11 based on a first threshold value and outputs the binarized signal after binarization processing. The second signal processing unit 115B acquires the binarized signal output from the first signal processing unit 15A via the interface 16, performs photon counting by adding the binarized signals of each frame, and outputs a photon counting signal. Furthermore, when the number of frames to be added reaches a predetermined upper limit, the second signal processing unit 115B performs rolling photon counting, in which the binarized signal of the oldest frame is deleted when adding the binarized signal of a new frame. In this way, the functions to be borne by the two signal processing units may be selected appropriately.

[0058] 17 includes a first signal processing unit 115A within the electronic flash camera 10 and a second signal processing unit 115B outside the electronic flash camera 10, which correspond to the signal processing units. In the imaging device 101C, the first signal processing unit 115A only outputs the detection signal (live signal) to the subsequent stage, while the second signal processing unit 115B performs binarization processing, addition processing, and photon counting processing. In this way, the functions to be performed by the two signal processing units may be selected as appropriate.

[0059] Third Embodiment Next, a third embodiment of the present disclosure will be described with reference to Figures 18 to 20. In the third embodiment, differences from the first and second embodiments will be mainly described, and descriptions that overlap with the first and second embodiments will be omitted.

[0060] In the imaging device according to the third embodiment, imaging is performed while switching between the above-described rolling photon counting and moving average processing or summation processing at any time. When performing bioluminescence observation, the signal processing unit of the imaging device may perform fluorescence observation by performing moving average processing or summation processing on the detection signals of predetermined excluded frames without performing binarization processing. In this case, the signal processing unit does not add the signals for fluorescence observation to the signals for bioluminescence observation. The signal processing unit stores the bioluminescence signals before fluorescence acquisition in memory and adds the bioluminescence signals after fluorescence acquisition, thereby saving the bioluminescence image even immediately after fluorescence acquisition.

[0061] Fig. 18 is a diagram schematically illustrating a configuration for fluorescence observation in an imaging device according to the third embodiment. The imaging device according to the third embodiment includes the configuration for fluorescence observation shown in Fig. 18 in addition to the configurations included in the imaging devices 1 and 101 described above. In the example shown in Fig. 18, the configuration for fluorescence observation includes a dichroic mirror 501 and an excitation filter 502.

[0062] The excitation filter 502 is an optical element for extracting light of a wavelength required for exciting a fluorescent substance from an excitation light source (e.g., a mercury lamp, not shown). The excitation filter 502 may be a bandpass filter that transmits only light of a specific wavelength and blocks other light. The dichroic mirror 501 is an optical element for separating the excitation light from the fluorescence. The dichroic mirror 501 reflects the light that has passed through the excitation filter 502 and guides it toward the cells S, and also transmits the fluorescence emitted from the cells S and guides it toward the filter 3.

[0063] Fig. 19 is a diagram illustrating switching between the rolling photon counting method and the rolling averaging method. In the example shown in Fig. 19, signals from the most recent four frames are added together in rolling photon counting. However, for predetermined excluded frames in which fluorescence observation is performed without photon counting, moving average processing or addition processing is performed without binarization processing for fluorescence observation, and the frames are excluded from the rolling photon counting addition target. In rolling photon counting, bioluminescence signals from before fluorescence acquisition are retained in memory, and bioluminescence signals from after fluorescence acquisition are added together, thereby acquiring a bioluminescence image even immediately after fluorescence acquisition.

[0064] FIG. 20(a) shows the results of bioluminescence observation, and FIG. 20(b) shows the results of fluorescence observation. As shown in FIGS. 20(a) and 20(b), the brightness value of fluorescence is much higher than that of bioluminescence. Therefore, if simultaneous observation is performed using, for example, a CCD, the image of luminescence will be blurred and will not be able to be properly detected. In this regard, by acquiring only fluorescence observation using moving average processing or addition processing separately from rolling photon counting, it is possible to acquire both bioluminescence and fluorescence images by switching between them during the same time period.

[0065] The present disclosure is not limited to the above-described embodiments. For example, image generation processing may be completed (fully performed) in the signal processing unit. In this case, the above-described PC 30, monitor 40, and other components may not be provided. Furthermore, the generated image does not necessarily need to be displayed in real time on the monitor, but may be recorded by being output to a USB or the like. This configuration allows for appropriate recording of generated images in a configuration that does not include the above-described PC 30, monitor 40, and the like. Furthermore, various signal processing processes, such as binarization, do not need to be performed within the above-described electronic flash camera 10, but may be performed, for example, by software built into the PC.

[0066] 1, 1A, 1B, 101, 101A, 101B, 101C...imaging device, 15, 15A, 15B, 115, 115A, 115B...signal processing unit, 111...photocathode (photoelectric conversion unit), 111a...photocathode, 112...solid-state imaging element, 113...vacuum housing.

Claims

1. An imaging device comprising: a photoelectric conversion unit having a photoelectric surface; a solid-state imaging device that detects photoelectrons emitted from the photoelectric surface and outputs a detection signal; a vacuum housing that holds the photoelectric conversion unit and the solid-state imaging device in a vacuum state; and a signal processing unit that processes the detection signal output from the solid-state imaging device, wherein the solid-state imaging device detects photoelectrons multiplied by electron bombardment multiplication when the accelerated photoelectrons are incident thereon and outputs the detection signal, and the signal processing unit performs binarization processing on the detection signal of each frame based on a first threshold value related to a signal intensity preset so as to enable identification of the presence or absence of photons, and performs photon counting by adding the binarized signals of each frame after the binarization processing.

2. The imaging device according to claim 1, wherein when the number of frames to be added reaches a preset upper limit number of frames, the signal processing unit deletes the binarized signal of the oldest frame when adding the binarized signal of a new frame.

3. The imaging device according to claim 2, wherein the signal processing unit performs a moving average process or an addition process on the detection signal of a predetermined excluded frame without performing the binarization process.

4. The imaging device according to any one of claims 1 to 3, wherein the signal processing unit multiplies a signal obtained by adding the binarized signals of each frame by a predetermined constant.

5. The imaging device according to any one of claims 1 to 4, wherein the signal processing unit specifies the number of photons based on a second threshold value related to a signal intensity preset so as to enable identification of the number of photons.

6. The imaging device according to any one of claims 1 to 5, wherein the signal processing unit excludes a frame having an abnormal number of photons from the addition target based on a third threshold value related to a signal intensity preset so as to enable identification of an abnormal number of photons.

Images