Particle diameter distribution measurement device, particle diameter distribution measurement method, and particle diameter distribution measurement program

Abstract

A particle diameter distribution measurement device 100 comprising an imaging means 3 that images particles in a cell 1 and an analysis unit 41 that calculates a particle diameter distribution by calculating diffusion velocity due to Brownian motion of the particles in imaging data obtained by the imaging means 3, said particle diameter distribution measurement device 100 being provided with: a data group acquisition unit 43 that acquires, at each of a plurality of focus positions in the cell 1, an imaging data group which comprises a plurality of pieces of imaging data obtained by the imaging means 3; a variable value calculation unit 44 that calculates a variable value of a prescribed evaluation parameter obtained from each of the plurality of pieces of imaging data constituting the imaging data groups; and a focus position determination unit 45 that, on the basis of variable values respectively corresponding to the plurality of focus positions, determines a measurement focus position to be used during measurement.

Description

Particle size distribution measuring device, particle size distribution measuring method, and particle size distribution measuring program

[0001] The present invention relates to a particle size distribution measuring device, a particle size distribution measuring method, and a particle size distribution measuring program.

[0002] As disclosed in Patent Document 1, a conventional particle size distribution measuring device uses a measuring method called particle trajectory analysis (PTA).

[0003] This measurement method measures particle size distribution by calculating the diffusion rate of particles due to Brownian motion based on image data obtained by imaging particles in a cell.

[0004] To ensure measurement accuracy, it is desirable to obtain imaging data with many particles in focus, but currently, operators have to manually adjust the focus position, which is time-consuming and labor-intensive.

[0005] Therefore, the inventors attempted to apply an existing autofocus function that is installed in cameras and the like to provide the device with a function for automatically adjusting the focus.

[0006] One such existing autofocus function is to convert the contrast of an image into a numerical value by first differentiation or the like, and to focus on the position where the contrast is maximum.

[0007] However, the Brownian motion of particles is essentially discontinuous, in other words, it is motion that cannot be differentiated at any timing, so the particles are constantly flickering, and even if the existing autofocus function described above is used, the operation to maximize contrast often does not converge.

[0008] This is true not only for contrast, but also for any parameter used in autofocus. In other words, since various parameters appearing in an image can change in a short period of time, even if you try to shorten the interval between images by using a high-speed camera, there is no guarantee that the image data from the previous image and the next image will be similar; in fact, they may change drastically, and the focus position may not be determined.

[0009] As a result, even if the focus function is applied to the PTA method, the success rate is very low, and in reality, it does not lead to a reduction in time.

[0010] Patent Publication No. 2020-204604

[0011] Therefore, the present invention solves all of the above problems at once, and its main object is to enable the focus position to be adjusted with a high success rate in particle size distribution measurement by the PTA method.

[0012] That is, the particle size distribution measuring device according to the present invention is a particle size distribution measuring device comprising: an imaging means for imaging particles in a cell; and an analysis unit for calculating a particle size distribution by calculating the diffusion rate of particles due to Brownian motion based on imaging data obtained by the imaging means, and is characterized by comprising: a data group acquiring unit for acquiring an imaging data group consisting of a plurality of imaging data obtained by the imaging means at each of a plurality of focus positions in the cell; a fluctuation value calculating unit for calculating a fluctuation value of a predetermined evaluation parameter obtained from each of the plurality of imaging data constituting the imaging data group; and a focus position determining unit for determining a measurement focus position to be used during measurement based on the fluctuation value corresponding to each of the plurality of focus positions.

[0013] As described above, various parameters included in the imaging data can change over a short period of time, but the degree of this temporal change should be greater at out-of-focus focus positions and smaller at in-focus focus positions. In light of this, the particle size distribution measuring device according to the present invention determines the measurement focus position based on the fluctuations of the evaluation parameters corresponding to each of the multiple focus positions. In other words, the measurement focus position is determined using the essence of Brownian motion, namely, temporal fluctuations. As a result, according to the present invention, it is possible to achieve a focused focus position with a high success rate, significantly reducing the effort required for focusing.

[0014] However, since the Brownian motion of particles is essentially discontinuous, it is conceivable that the fluctuation value may accidentally become small even at an out-of-focus focus position. Therefore, in order to prevent such a focus position from being determined as the measurement focus position, it is preferable that the focus position determination unit determine the measurement focus position from within a stable range that includes multiple focus positions where the fluctuation value is below the threshold.

[0015] In order to bring the measurement focus position closer to the in-focus position, it is preferable that the focus position determination unit determines the focus position at which the fluctuation value is smallest in the stable range, or the focus position in the center of the stable range, as the measurement focus position.

[0016] In order to make the evaluation parameter a value that varies over time, it is preferable that the evaluation parameter be the contrast of the image shown by the imaging data, the number of particles appearing in the image, the intensity of each pixel that constitutes the image, or the number of pixels in the image that exceed a predetermined intensity.

[0017] The fluctuation value is a value indicating the degree of time fluctuation of the evaluation parameter, so that the measurement focus position can be determined by utilizing the essence of Brownian motion, that is, time fluctuation.

[0018] Here, if a distribution graph with one axis representing the evaluation parameter value and the other axis representing frequency has a wide tail on the larger value side, indicating a distribution prone to outliers, two possible ways of calculating the variation value are to either remove the outliers or to calculate the variation value assuming the presence of outliers. However, outlier processing to remove outliers varies greatly depending on the sample, which can lead to complication of the program. Therefore, it is preferable that the variation value calculation unit calculates the variation value by fitting a predetermined function to a distribution graph with one axis representing the evaluation parameter value and the other axis representing frequency. With this configuration, even if the distribution described above is prone to outliers, it is possible to calculate a plausible variation value without using potentially complication of outlier processing.

[0019] Preferably, the variation value calculation unit fits the predetermined function to the distribution graph corresponding to the focus position included in a search region set in a part of the depth direction of the cell. With this configuration, it is not necessary to move the focus position over the entire region of the depth direction of the cell, and the focus position can be adjusted in a shorter time.

[0020] Incidentally, in measurements using the PTA method, after measuring the particle size distribution, the particles in the cell are stirred or the imaging position is changed, and the particle size distribution is measured again. This process is repeated to increase the number of measurements and improve measurement accuracy. In this case, it is currently assumed that the focus is not significantly shifted after stirring, and measurements are continued without changing the focus position before stirring. However, to further improve measurement accuracy, it is preferable to readjust the focus position after stirring. However, manually adjusting the focus every time stirring is performed requires a great deal of time and effort.

[0021] Therefore, if the particle size distribution measuring device is configured to further include a stirring means for stirring the particles in the cell, the effects of the present invention will be more pronounced, and the measurement accuracy can be further improved in a short time and without much effort.

[0022] It is preferable to provide a focus adjustment unit that adjusts the focus of the imaging means to the measurement focus position. With this configuration, an autofocus function that automatically adjusts the focus to the measurement focus position can be achieved.

[0023] In the configuration including the stirring means and focus adjustment unit described above, in order to automate the measurement, it is preferable that the focus adjustment unit aligns the focus of the imaging means, the analysis unit calculates the particle size distribution, and the stirring means stirs the particles in the cell repeatedly.

[0024] A particle size distribution measuring method according to the present invention is a particle size distribution measuring method using a particle size distribution measuring device including an imaging means for imaging particles in a cell and an analysis unit for calculating a particle size distribution by calculating a diffusion rate of the particles due to Brownian motion based on imaging data obtained by the imaging means, the method comprising: a data group acquiring step for acquiring an imaging data group consisting of a plurality of imaging data obtained by the imaging means at each of a plurality of focus positions in the cell; a fluctuation value calculating step for calculating a fluctuation value of a predetermined evaluation parameter obtained from each of the plurality of imaging data constituting the imaging data group; and a focus position determining step for determining a measurement focus position to be used during measurement based on the fluctuation value corresponding to each of the plurality of focus positions.

[0025] The particle size distribution measurement program according to the present invention is a program used in a particle size distribution measurement device comprising an imaging means for imaging particles in a cell and an analysis unit for calculating a particle size distribution by calculating the diffusion rate of particles due to Brownian motion based on imaging data obtained by the imaging means, and is characterized in that it causes a computer to perform the functions of: a data group acquisition unit for acquiring an imaging data group consisting of a plurality of imaging data obtained by the imaging means at each of a plurality of focus positions within the cell; a fluctuation value calculation unit for calculating a fluctuation value of a predetermined evaluation parameter obtained from each of the plurality of imaging data constituting the imaging data group; and a focus position determination unit for determining a measurement focus position to be used during measurement based on the fluctuation value corresponding to each of the plurality of focus positions.

[0026] Such a particle size distribution measurement method and program can provide the same effects as those of the particle size distribution measurement device described above.

[0027] According to the present invention as described above, in particle size distribution measurement by the PTA method, it is possible to adjust the focus position with a high success rate, and the effort required for focus adjustment can be significantly reduced.

[0028] Fig. 1 is a schematic diagram showing a particle size distribution measuring device according to one embodiment of the present invention; Fig. 2 is a functional block diagram showing functions of an information processing device according to the embodiment; Fig. 3 is a schematic diagram for explaining an autofocus function in the embodiment; Fig. 4 is an experimental result showing the variation in evaluation parameters in the embodiment; Fig. 5 is a distribution graph showing the variation in evaluation parameters in the embodiment; Fig. 6 is a schematic diagram showing a particle size distribution measuring device according to another embodiment; Fig. 7 is a flowchart showing the operation of a particle size distribution measuring device according to another embodiment.

[0029] Hereinafter, an embodiment of a particle size distribution measuring device according to the present invention will be described with reference to the drawings.

[0030] <Device Configuration> As shown in FIG. 1 , the particle size distribution measuring device 100 in this embodiment includes a light irradiation unit 2 that irradiates particles in a cell 1 with light of an excitation wavelength, an imaging means 3 that images the particles by detecting scattered light from the particles in the cell 1, and an information processing device 4 that analyzes the imaging data obtained by the imaging means 3.

[0031] The light irradiation unit 2 has a plurality of light sources 21, 22, and 23 that irradiate light of different excitation wavelengths. The light sources 21, 22, and 23 are, for example, laser light sources. When detecting scattered light from particles and imaging the particles, it is necessary to irradiate light of an excitation wavelength appropriate for the particle diameter. In this embodiment, three light sources 21, 22, and 23 are provided to accommodate particles of various diameters. Note that one light source may irradiate light including three excitation wavelengths. Furthermore, the number of light sources may be three or more, or may be one.

[0032] The first light source 21 irradiates light of a predetermined first excitation wave λ1, the second light source 22 irradiates light of a predetermined second excitation wavelength λ2, and the third light source 23 irradiates light of a predetermined third excitation wavelength λ3. The light emitted from each of these light sources 21, 22, and 23 is guided to the cell 1 via an irradiation optical system 24 including reflecting mirrors 241 and 242, half mirrors 243 and 244, and a condenser lens 245.

[0033] These light sources 21, 22, and 23 are controlled by a control unit (not shown) so as to simultaneously irradiate light of the respective excitation wavelengths λ1, λ2, and λ3, although these light sources 21, 22, and 23 can also be controlled so as to individually irradiate light.

[0034] The imaging means 3 captures images of the particles by detecting scattered light from the particles in the cell 1, and in this embodiment is an imaging camera such as a CCD camera, and outputs image data as imaging data. Note that in Fig. 1, the light irradiation direction of the light irradiation unit 2 and the imaging direction of the imaging means 3 are arranged orthogonal to each other, but this is not limited to this.

[0035] The information processing device 4 is a computer equipped with a CPU, a memory, a display, various input / output devices, etc., and is connected to the imaging means 3 by wire or wirelessly.

[0036] The information processing device 4 performs at least the function of an analysis unit 41 as shown in FIG. 2 by executing a particle size distribution measurement program stored in the memory.

[0037] The analysis unit 41 acquires imaging data from the imaging means 3 and calculates the particle size distribution based on this imaging data using the PTA method. More specifically, the analysis unit 41 calculates the particle size distribution by calculating the diffusion speed of particles due to Brownian motion based on the imaging data.

[0038] The particle size distribution measuring device 100 of this embodiment is provided with an autofocus function that automatically adjusts the focus of the imaging means 3 to the particles in the cell 1 .

[0039] Specifically, the information processing device 4 described above is configured to function as a focus adjustment unit 42, a data group acquisition unit 43, a fluctuation value calculation unit 44, and a focus position determination unit 45, as shown in FIG. 2.

[0040] Below, these functions will be explained, and the operation of the information processing device 4 will be explained with reference to the flowchart shown in FIG.

[0041] The focus adjustment unit 42 moves the focus of the imaging means 3 to multiple focus positions within the cell 1, and specifically is configured to move the focus of the imaging means 3 toward and away from the cell 1 according to a predetermined rule.

[0042] The focus adjustment unit 42 of this embodiment is configured to move the focus of the imaging means 3 from the front side to the back side of the cell 1, or from the back side to the front side, in increments of a predetermined distance (S1 in FIG. 3). Note that the focus adjustment unit 42 may move the focus of the imaging means 3 over the entire depth direction of the cell 1, but here it is configured to move only to a partial depth direction region (hereinafter referred to as the search region). Note that this search region may be an unchanging range that is preset during production of the device, or may be a range that can be changed by the user, or may be a range that is automatically changed depending on measurement conditions such as the solvent in the cell.

[0043] The data group acquisition unit 43 acquires an imaging data group consisting of a plurality of pieces of imaging data obtained by the imaging means 3 at each of a plurality of focus positions within the cell 1 .

[0044] More specifically, when the focus of the imaging means 3 is moved by the focus adjustment unit 42, the imaging means 3 captures images of the inside of the cell 1 multiple times at each of the focus positions to which it is moved, and the data group acquisition unit 43 acquires an imaging data group consisting of these multiple imaging data (S2 in Figure 3).

[0045] However, it is not necessary to capture multiple images of the inside of cell 1 at each focus position; the data group acquisition unit 43 may capture an image of the inside of cell 1 once at each focus position and move each focus position multiple times, thereby acquiring a group of imaging data at each of the multiple focus positions.

[0046] There is no particular limit to the number of pieces of imaging data included in the imaging data group, but if it is too small, the probability of success of the autofocus function may decrease, and if it is too large, the time required for autofocusing will increase, so it is desirable that the number be, for example, several tens to several hundreds.

[0047] Here, the multiple imaging data included in a certain imaging data group were taken inside cell 1 at the same focus position, but the timing (time) of the imaging was different and the particles were undergoing Brownian motion, so differences appear in the images shown by each imaging data.

[0048] Specifically, various parameters (hereinafter also referred to as evaluation parameters) that can be obtained from each imaging data, such as the contrast of the image shown by each imaging data, the number of particles in the image, the intensity of each pixel that makes up the image, or the number of pixels in the image that exceed a predetermined intensity, will differ due to differences in the imaging timing of each imaging data.

[0049] Therefore, the variation value calculation unit 44 calculates the variation value of a predetermined evaluation parameter obtained from each of the plurality of imaging data constituting the imaging data group (S3 in FIG. 3).

[0050] This evaluation parameter is a parameter that can be obtained from the imaging data, and its value varies over time, in other words, varies depending on the timing of imaging of the imaging data, and the variation value indicates the degree of this variation over time, in other words, the variation in the value of the evaluation parameter.

[0051] Furthermore, the degree of change over time in the evaluation parameter, that is, the variation in the evaluation parameter, should be large at an out-of-focus position and small at an in-focus position.

[0052] As described above, various evaluation parameters can be used. In this embodiment, however, the evaluation parameter will be described as a contrast such as the RMS contrast (Root Mean Square Contrast) of the image represented by the imaging data.

[0053] Furthermore, the variation value may be any value that serves as an index of the variation in the values ​​of the evaluation parameters, such as standard deviation, variance, or residual sum of squares. In this embodiment, the width of the distribution of the evaluation parameters (hereinafter also referred to as distribution width) will be used as the variation value.

[0054] In other words, the fluctuation value calculation unit 44 of this embodiment performs image processing or image analysis on each of the multiple imaging data that make up the imaging data group, thereby obtaining the image contrast from each of the imaging data as an evaluation parameter, and calculates the distribution width of these contrasts as a fluctuation value.

[0055] The contrast acquired by the variation value calculation unit 44 may be the contrast of the entire image represented by the imaging data, or may be the contrast of a predetermined partial region in the image.

[0056] 4 is a graph in which contrast, which is an evaluation parameter obtained from each of a plurality of pieces of imaging data, is plotted on a graph with the horizontal axis representing the focus position and the vertical axis representing the value of the evaluation parameter. As can be seen from this graph, the values ​​of the evaluation parameter obtained from each of the plurality of imaging data included in a certain imaging data group vary to some extent.

[0057] For this reason, when the values ​​of the evaluation parameters obtained from each of the multiple imaging data included in a certain imaging data group are plotted on a graph as shown in Figure 5, with one axis representing the value of the evaluation parameter and the other axis representing frequency, a distribution graph with a wide spread is obtained.

[0058] Therefore, the fluctuation value calculation unit 44 of this embodiment is configured to calculate the fluctuation value by fitting a predetermined function to the distribution graph described above.

[0059] More specifically, examples of the predetermined function include an equation representing a probability density distribution such as a Gumble distribution or a Frechet distribution, and the fluctuation value calculation unit 44 fits these functions to a distribution graph and obtains the distribution width from the parameters included in the function after the fitting.

[0060] The fluctuation value calculation unit 44 may fit a predetermined function to a distribution graph corresponding to all groups of imaging data obtained in the above-mentioned search area, in other words, a distribution graph corresponding to all focus positions to which the focus of the imaging means 3 moves in the search area, or may fit a predetermined function only to a distribution graph corresponding to some of the focus positions.

[0061] Then, the fluctuation value calculation unit 44 compares each of the fluctuation values ​​(distribution widths) obtained after fitting with a threshold value, and identifies a range (hereinafter referred to as a stable range) that includes multiple focus positions where the fluctuation value is below the threshold value (see Figure 4).

[0062] The focus position determining unit 45 determines the measurement focus position to be used during measurement based on the fluctuation values ​​corresponding to each of the plurality of focus positions (S4 in FIG. 3).

[0063] The focus position determination unit 45 determines a measurement focus position from within the above-mentioned stable range, and one of the focus positions included in the stable range is set as the measurement focus position. Note that the focus position determination unit 45 may be configured to determine multiple focus positions included in the stable range as measurement focus positions and output these focus positions in a selectable manner.

[0064] The focus position determination unit 45 determines the focus position where the fluctuation value is smallest in the stable range or the focus position at the center of the stable range as the measurement focus position. Note that the center of the stable range may be the center of the stable range itself, or may be a position (center of gravity) shifted from the center toward the focus position where the fluctuation value is smallest.

[0065] The measurement focus position determined by the focus position determination unit 45 is output to the above-mentioned focus adjustment unit 42, and the focus adjustment unit 42 adjusts the focus of the imaging means 3 to the measurement focus position before measuring the particle size distribution (S5 in FIG. 3), and then the analysis unit 41 calculates the particle size distribution (S6 in FIG. 3).

[0066] <Effects of this embodiment> According to the particle size distribution measuring device 100 of this embodiment configured as described above, the measurement focus position is determined based on the fluctuation values ​​of the evaluation parameters corresponding to each of the multiple focus positions. In other words, the measurement focus position is determined using the essence of Brownian motion, namely, temporal fluctuation. Therefore, it is possible to automatically adjust the in-focus focus position with a high success rate, and it is possible to significantly reduce the effort required for adjusting the focus.

[0067] Furthermore, since the contrast of the image indicated by the imaging data is used as the evaluation parameter and the standard deviation is used as the fluctuation value, the fluctuation value can be calculated without using complex arithmetic processing.

[0068] However, since the Brownian motion of particles is essentially discontinuous, it is conceivable that the fluctuation value may accidentally become small even at an out-of-focus focus position. In contrast, in this embodiment, the focus position determination unit 45 determines the measurement focus position from within the stable range, so it is possible to prevent a focus position where the fluctuation value accidentally becomes small from being determined as the measurement focus position.

[0069] Furthermore, the focus position determination unit 45 determines the focus position with the smallest fluctuation value in the stable range or the focus position in the center of the stable range as the measurement focus position, so the measurement focus position can be brought closer to the in-focus position.

[0070] Furthermore, the fluctuation value calculation unit 44 calculates the fluctuation value by fitting a predetermined function to a distribution graph in which one axis represents the value of the evaluation parameter and the other axis represents the frequency, so that even if the distribution is prone to outliers, it is possible to calculate a plausible fluctuation value without using outlier processing.

[0071] In addition, since the fluctuation value calculation unit 44 fits a predetermined function to a distribution graph corresponding to the focus position included in the search area, it is not necessary to move the focus position over the entire depth direction of cell 1, and the focus position can be adjusted in a shorter time.

[0072] Other Embodiments The present invention is not limited to the above-described embodiments.

[0073] For example, the particle size distribution measuring device 100 may further include a stirring means 5 for stirring the particles in the cell 1, as shown in FIG.

[0074] 7, the information processing device 4 may include a control unit that repeats each of the following operations multiple times: adjusting the focus position of the imaging means 3 using an autofocus function (T1), calculating the particle size distribution using the analysis unit 41 (T2), and stirring the particles in the cell 1 using the stirring means 7 (T3). With this configuration, the number of measurements can be increased (i.e., the number of particles to be measured can be increased) in the measurement by the PTA method, and the focus of the imaging means 3 can be automatically adjusted each time stirring is performed, thereby further improving the measurement accuracy in a short time and without much effort.

[0075] Furthermore, the evaluation parameter is not limited to contrast, but may be the number of particles appearing in the image represented by the imaging data, or the intensity of each pixel constituting the image.

[0076] The fluctuation value is not limited to the standard deviation, but may be the variance or the residual sum of squares.

[0077] Furthermore, the particle size distribution measuring apparatus 100 may be configured so that the focus of the imaging means 3 can be manually changed, for example, to enable fine adjustment of the focus position after completion of an autofocus operation. In this case, for example, a focus position change button may be displayed on a display or the like, and the focus of the imaging means 3 may be manually changed by operating this button.

[0078] Furthermore, the particle size distribution measuring apparatus 100 according to the present invention does not necessarily need to be provided with an autofocus function using the focus adjustment unit 42, and an operator may manually adjust the focus to the measurement focus position determined by the focus position determination unit 45.

[0079] Furthermore, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit of the present invention.

[0080] According to the present invention described above, in particle size distribution measurement by the PTA method, the focus position can be automatically adjusted with a high success rate.

[0081] REFERENCE SIGNS LIST 100 Particle size distribution measuring device 1 Cell 2 Light irradiation unit 3 Imaging means 4 Information processing device 41 Analysis unit 42 Focus adjustment unit 43 Data group acquisition unit 44 Fluctuation value calculation unit 45 Focus position determination unit

Claims

1. 1. A particle size distribution measuring device comprising: an imaging means for imaging particles in a cell; and an analysis unit for calculating a particle size distribution by calculating a diffusion rate due to Brownian motion of particles based on imaging data obtained by the imaging means, a data group acquisition unit that acquires an imaging data group consisting of a plurality of imaging data obtained by the imaging means at each of a plurality of focus positions within the cell; a variation value calculation unit that calculates a variation value of a predetermined evaluation parameter obtained from each of the plurality of imaging data that constitute the imaging data group; a focus position determination unit that determines a measurement focus position to be used during measurement based on the fluctuation values ​​corresponding to each of the plurality of focus positions.

2. 2. The particle size distribution measuring device according to claim 1, wherein the focus position determining unit determines the measurement focus position from within a stable range including a plurality of focus positions where the fluctuation value is below a threshold value.

3. 3. The particle size distribution measuring device according to claim 2, wherein the focus position determining unit determines, as the measurement focus position, a focus position in the stable range where the fluctuation value is minimum, or a focus position in the center of the stable range.

4. 3. The particle size distribution measuring device according to claim 1, wherein the evaluation parameter is a contrast of an image represented by the imaging data, the number of particles appearing in the image, the intensity of each pixel constituting the image, or the number of pixels exceeding a predetermined intensity included in the image.

5. 2. The particle size distribution measuring device according to claim 1, wherein the fluctuation value is a value indicating the degree of time fluctuation of the evaluation parameter.

6. 3. The particle size distribution measuring device according to claim 1, wherein the fluctuation value calculation unit calculates the fluctuation value by fitting a predetermined function to a distribution graph having one axis representing the value of the evaluation parameter and the other axis representing frequency.

7. 7. The particle size distribution measuring device according to claim 6, wherein the fluctuation value calculation unit fits the predetermined function to the distribution graph corresponding to the focus position included in a search region set in a part of the cell in the depth direction.

8. The particle size distribution measuring apparatus according to claim 1 or 2, further comprising a stirring means for stirring the particles in the cell.

9. 3. The particle size distribution measuring device according to claim 1, further comprising a focus adjusting unit that adjusts the focus of the imaging means to the measurement focus position.

10. Further comprising a stirring means for stirring particles in the cell, 10. The particle size distribution measuring device according to claim 9, wherein the focus adjustment unit adjusts the focus of the imaging unit, the analysis unit calculates the particle size distribution, and the stirring unit stirs the particles in the cell.

11. 1. A particle size distribution measuring method using a particle size distribution measuring device comprising: an imaging means for imaging particles in a cell; and an analysis unit for calculating a particle size distribution by calculating a diffusion rate due to Brownian motion of the particles based on imaging data obtained by the imaging means, a data group acquisition step of acquiring an imaging data group consisting of a plurality of imaging data obtained by the imaging means at each of a plurality of focus positions within the cell; a fluctuation value calculation step of calculating a fluctuation value of a predetermined evaluation parameter obtained from each of a plurality of pieces of imaging data constituting the imaging data group; a focus position determination step of determining a measurement focus position to be used during measurement based on the fluctuation values ​​corresponding to each of the plurality of focus positions.

12. 1. A program used in a particle size distribution measuring device comprising: an imaging means for imaging particles in a cell; and an analysis unit for calculating a particle size distribution by calculating a diffusion rate due to Brownian motion of particles based on imaging data obtained by the imaging means, a data group acquisition unit that acquires an imaging data group consisting of a plurality of imaging data obtained by the imaging means at each of a plurality of focus positions within the cell; a variation value calculation unit that calculates a variation value of a predetermined evaluation parameter obtained from each of the plurality of imaging data that constitute the imaging data group; and a focus position determination unit that determines a measurement focus position to be used during measurement based on the fluctuation values ​​corresponding to each of the plurality of focus positions.