Observation device and observation method

Abstract

An observation device according to the present disclosure is for observing a cell suspension containing a plurality of cells. This observation device comprises: at least one irradiation unit that is disposed at a position opposing a cell suspension, and that irradiates the cell suspension with irradiation light that has at least one wavelength included in the ranges of 650-850 nm and 1060-1090 nm; at least one light-receiving unit that is disposed at a position opposing the cell suspension, and that receives emitted light emitted from the cell suspension, as a result of the cell suspension being irradiated with the irradiation light; and an analysis unit that is communicably connected to the light-receiving unit, and that obtains, on the basis of the intensity of the emitted light, a parameter relating to the cell concentration of the cells in the cell suspension.

Description

Observation device and observation method

[0001] This application claims priority to Japanese Patent Application No. 2024-005456, filed on January 17, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] For example, in the field of regenerative medicine, research is being conducted on disease treatment using cells or cell masses, and on drug discovery evaluation using cells as an alternative to laboratory animals. Cells used in these fields are produced by culturing and growing cells of human or animal origin. In the cell culturing process, cells are placed in a culture vessel together with a culture medium. The cells are grown by controlling and managing the environment within this culture vessel. The culturing is completed when a parameter such as the cell concentration or cell number of the cell suspension in the vessel reaches a target value.

[0003] Known methods for determining whether a cell concentration or cell number has reached a target value include, for example, a method in which a cell suspension is collected and poured into a counting board and the number of cells is counted by visual observation using a microscope, and a method in which the cell suspension is collected and poured into a cell counter and the number of cells is counted mechanically by image processing of the cell counter. Another method is, for example, a method in which the cell concentration is estimated using a turbidimeter (see, for example, Patent Document 1). This method measures the intensity of light emitted by irradiating a cell suspension with light, and estimates the cell concentration by utilizing the fact that the intensity of the measured light varies depending on the cell concentration.

[0004] JP 2023-144814 A

[0005] The observation device disclosed herein is an observation device for observing a cell suspension containing a plurality of cells, and includes at least one irradiation unit positioned opposite the cell suspension and irradiating the cell suspension with irradiation light having at least one wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, at least one light receiving unit positioned opposite the cell suspension and receiving emitted light emitted from the cell suspension when the cell suspension is irradiated with the irradiation light, and an analysis unit communicably connected to the light receiving unit and determining a parameter related to the cell concentration of the cells in the cell suspension based on the intensity of the emitted light.

[0006] FIG. 1 is a diagram showing the configuration of an observation device of a first embodiment. FIG. 2 is a diagram showing the hardware configuration of the analysis unit of FIG. 1. FIG. 3 is a diagram showing the functional configuration of the analysis unit of FIG. 1. FIG. 4 is a graph showing a calibration curve stored in the analysis unit of FIG. 1. FIG. 5 is a graph showing a light intensity change spectrum of a cell. FIG. 6 is a graph showing a light intensity change spectrum of a culture solution. FIG. 7 is a flowchart showing an example of an observation method performed using the observation device of FIG. 1. FIG. 8 is a diagram showing a modified example of the observation device of FIG. 1. FIG. 9 is a graph showing a calibration curve stored in the analysis unit of FIG. 8. FIG. 10 is a diagram showing a modified example of the observation device of FIG. 1. FIG. 11 is a diagram showing the configuration of an observation device of a second embodiment. FIG. 12 is a diagram showing the functional configuration of the analysis unit of FIG. 11. FIG. 13 is a graph showing a calibration curve stored in the analysis unit of FIG. 11. FIG. 14 is a graph for explaining how to determine the Rayleigh scattering coefficient in the analysis unit of FIG. 11. FIG. 15 is a graph for explaining a wavelength range suitable for observing the state of a cell suspension. Fig. 16 is a diagram showing a modified example of the observation device of Fig. 11. Fig. 17 is a graph for explaining how to determine the Rayleigh scattering coefficient in the analysis unit of Fig. 16. Fig. 18 is a diagram showing a modified example of the observation device of Fig. 11. Fig. 19 is a diagram showing a modified example of the observation device of Fig. 11. Fig. 20 is a diagram showing a modified example of the observation device of Fig. 11.

[0007] [Problem to be Solved by the Present Disclosure] The above-mentioned methods using a counting board or cell counter cannot count cells while they are three-dimensionally suspended in a cell suspension. Therefore, the cell suspension must be collected and injected into a container or a measuring unit in a device that encloses the cell suspension in a planar space. This method makes it difficult to count cells efficiently. The method using a turbidimeter irradiates the cell suspension with light and estimates the cell number based on the intensity of the emitted light obtained from the cell suspension. The intensity of this emitted light reflects a mixture of light absorption by the cells and culture medium and light scattering by the cells. Therefore, the intensity of absorption and scattering can be observed by observing the change or relative value of the intensity of the emitted light relative to the intensity of the irradiated light (hereinafter, the "change or relative value of the light intensity relative to the irradiated light intensity" will be referred to as "light intensity change"). Hereinafter, "absorption intensity" and "absorption intensity" refer to the "change in light intensity due to absorption," and "scattering intensity" and "scattering intensity" refer to the "change in light intensity due to scattering." Among these phenomena, the change in the intensity of emitted light due to scattering depends on the cell concentration of the cell suspension and can therefore be used to estimate the cell concentration. However, because it is difficult to distinguish between the change in light intensity due to scattering and the change in light intensity due to absorption from the obtained change in light intensity, it is difficult to accurately determine the cell concentration of the cell suspension using the light intensity.

[0008] The present disclosure provides an observation device and an observation method that can accurately and efficiently observe the state of a cell suspension.

[0009] [Effects of the Present Disclosure] According to the present disclosure, an observation device and an observation method are provided that can accurately and efficiently observe the state of a cell suspension.

[0010] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0011] (1) The observation device disclosed herein is an observation device for observing a cell suspension containing a plurality of cells. The observation device includes at least one irradiation unit positioned opposite the cell suspension and irradiating the cell suspension with irradiation light having at least one wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, at least one light receiving unit positioned opposite the cell suspension and receiving emitted light emitted from the cell suspension when the cell suspension is irradiated with the irradiation light, and an analysis unit communicably connected to the light receiving unit and configured to determine a parameter related to the cell concentration of cells in the cell suspension based on the intensity of the emitted light.

[0012] Generally, when a cell suspension is irradiated with light, phenomena such as light scattering and absorption can occur in the cell suspension. The intensity of light emitted from the cell suspension in response to light irradiation reflects a mixture of these phenomena. Therefore, the intensity of absorption and scattering can be observed by observing the change in light intensity. Of these phenomena, the change in light intensity due to scattering depends on parameters related to the cell concentration of the cell suspension. The change in light intensity due to absorption depends on chemical reactions in the cell suspension. Therefore, to accurately determine parameters related to the cell concentration, the change in light intensity due to scattering alone can be obtained. After extensive research into methods for obtaining the change in light intensity due to scattering alone, the inventors have found that irradiating a cell suspension with irradiation light having at least one wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm is effective. Within the wavelength ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, light absorption by the culture medium in which the cells are placed is sufficiently small compared to light scattering by the cells, so the effect of light absorption by the culture medium on the intensity of the emitted light can be ignored. In the wavelength ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, the absorption of light by cells is sufficiently small compared to the scattering of light by cells, so the effect of light absorption by cells on the intensity of emitted light can be ignored. Therefore, when cells are irradiated with irradiation light having at least one wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, parameters related to cell concentration can be accurately determined based on the amount of change in light intensity due to scattering alone, thereby enabling accurate observation of the state of the cell suspension. The above-mentioned observation device can determine parameters related to cell concentration by the simple task of irradiating a cell suspension containing suspended cells with irradiation light. This method allows for more efficient observation of the state of the cell suspension than methods that require the steps of collecting and injecting the cell suspension into the device.

[0013] (2) In the observation device described in (1) above, the irradiation unit may include a light source using a semiconductor light-emitting element that emits light having a wavelength within the range of 650 nm to 850 nm and 1060 nm to 1090 nm as irradiation light. In this case, using a semiconductor light-emitting element with stable light intensity as the light source reduces the risk of fluctuations in the intensity of the emitted light due to factors other than the state of the cell suspension (e.g., instability in the intensity of the irradiation light). This allows for more reliable observation of the state of the cell suspension based on the intensity of the emitted light.

[0014] (3) In the observation device described in (1) or (2) above, the light-receiving unit may be disposed on the opposite side of the cell suspension from the irradiation unit, and may receive a portion of the irradiation light that has passed through the cell suspension as emitted light. In this case, the position of the light-receiving unit relative to the irradiation unit can be easily adjusted.

[0015] (4) In the observation device described in any one of (1) to (3) above, the irradiation unit may irradiate the cell suspension with first irradiation light having a first wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, and second irradiation light having a second wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, but different from the first wavelength. The light receiving unit may receive first exit light and second exit light having different wavelengths emitted from the cell suspension by irradiation with the first irradiation light and second irradiation light. The analysis unit may calculate a parameter related to the cell concentration based on the intensity of the first exit light and the intensity of the second exit light. Light scattering that occurs in cells when light is irradiated onto the cell suspension includes Rayleigh scattering, which is caused by particles smaller than the wavelength of light, and Mie scattering, which is caused by particles larger than the wavelength of light. Of these, Rayleigh scattering is primarily caused by molecules constituting the cell membrane. If cells in a cell suspension adhere to each other to form cell clusters, the amount and structure of molecules constituting the cell membranes of the cells in the cell clusters remain unchanged, so the intensity of Rayleigh scattering obtained from the cell suspension does not change when irradiated with light. The size of the particles (cell clusters) formed by cell adhesion changes, so the intensity of Mie scattering obtained from the cell suspension may change. In this case, even though the number of cells in the cell suspension remains unchanged, it is expected that the intensity of light emitted from the cell suspension will change in response to changes in Mie scattering intensity. Such changes in light intensity can reduce the accuracy of estimating parameters related to cell concentration. Therefore, if only Rayleigh scattering, which is the scattering occurring in the cell suspension, can be observed, parameters related to cell concentration can be accurately determined without being affected by cell adhesion. While Rayleigh scattering intensity depends on the wavelength of light, Mie scattering intensity is either independent of the wavelength of light or does not depend significantly on the wavelength of light. Therefore, when observing the state of a cell suspension using light of multiple wavelengths, differences in wavelength dependence can be exploited to determine only the wavelength-dependent Rayleigh scattering intensity, enabling accurate determination of parameters related to cell concentration."Rayleigh scattering intensity" and "Rayleigh scattering intensity" mean "the amount of change in light intensity due to Rayleigh scattering," and "Mie scattering intensity" and "Mie scattering intensity" mean "the amount of change in light intensity due to Mie scattering."

[0016] (5) In the observation device described in (4) above, the irradiation unit may include a first light source that emits first irradiation light, a second light source that emits second irradiation light, and a switching unit that switches the optical path of the first irradiation light and the optical path of the second irradiation light so that the first irradiation light and the second irradiation light are selectively irradiated onto the cell suspension. The light receiving unit may include an optical sensor that selectively receives the first output light and the second output light emitted from the cell suspension. In this case, a single optical sensor can be used to distinguish and detect the intensities of the first output light and the second output light. As a result, there is no need to provide separate optical sensors for detecting the intensity of the first output light and the second output light, thereby avoiding a complex configuration of the light receiving unit.

[0017] (6) In the observation device described in (4) above, the irradiation unit may include a first light source that emits first irradiation light, a second light source that emits second irradiation light, and a combining unit that combines the first irradiation light and the second irradiation light to generate combined irradiation light and irradiates the cell suspension with the combined irradiation light. The light receiving unit may include a spectroscopic unit that separates, by wavelength, the combined output light emitted from the cell suspension by irradiation with the combined irradiation light, a first optical sensor that detects the intensity of the first output light separated from the combined output light, and a second optical sensor that detects the intensity of the second output light separated from the combined output light. In this case, the intensities of the first output light and the second output light can be detected simultaneously using the combined irradiation light and the combined output light, thereby making it possible to more efficiently observe the state of the cell suspension based on the intensities of the first output light and the second output light in a shorter time.

[0018] (7) In the observation device described in (4) above, the irradiation unit may include a light source that emits light including the first irradiation light and the second irradiation light, a first filter that transmits only the first irradiation light, and a second filter that transmits only the second irradiation light, and a filter switching unit that switches the position of the first filter and the position of the second filter so that the first filter or the second filter is positioned on the optical path of the light between the light source and the cell suspension. The light receiving unit may include an optical sensor that selectively receives the first emitted light and the second emitted light emitted from the cell suspension. In this case, for example, by using an inexpensive lamp light source as the light source, the cost of the observation device can be reduced compared to when an expensive laser light source is used as the light source.

[0019] (8) The present disclosure provides an observation method for observing a cell suspension containing a plurality of cells. The observation method includes the steps of irradiating the cell suspension with irradiation light having at least one wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, receiving light emitted from the cell suspension as a result of irradiation of the cell suspension with the irradiation light, and determining a parameter related to the cell concentration of the cells in the cell suspension based on the intensity of the emitted light. This observation method can achieve the same effects as the observation device described above.

[0020] (9) In the observation method described in (8) above, the step of irradiating the cell suspension may include irradiating the cell suspension with first irradiation light having a first wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, and second irradiation light having a second wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, and different from the first wavelength. The step of receiving the emitted light may include receiving first emitted light and second emitted light having different wavelengths, which are emitted from the cell suspension by irradiation with the first irradiation light and second irradiation light. The step of calculating the parameter related to the cell concentration may calculate the parameter related to the cell concentration based on the intensity of the first emitted light and the intensity of the second emitted light. In this case, the same effect as in (4) above can be obtained.

[0021] [Details of the embodiments of the present disclosure] Specific examples of the observation device and observation method of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0022] [First Embodiment] Fig. 1 is a diagram showing the configuration of an observation device 1 of a first embodiment. The observation device 1 is a device for observing the state of a cell suspension 3. The cell suspension 3 includes a plurality of cells 2 and a culture solution 7 in which the plurality of cells 2 are suspended. Each cell 2 may be a cell collected from an animal or a human, a stem cell collected from an animal or a human, a stem cell prepared from a cell collected from an animal or a human, or a cell differentiated from a stem cell. The cells 2 may be a mixture of a plurality of cell types.

[0023] Some or all of the cells 2 contained in the cell suspension 3 may be dispersed or aggregated together. That is, some or all of the cells 2 contained in the cell suspension 3 may be separated from one another or in contact with one another. Some or all of the cells 2 may form cell clusters (spheroids, organoids, assembloids, tissues, organs) formed by aggregation. A cell cluster is a cell accumulation in which multiple cells 2 are stacked in multiple layers. There may be multiple types of cells 2. A cell cluster may include various cell forms, such as two-dimensional and three-dimensional clusters. The cells 2 in the cell suspension 3 may float or fill the container 5.

[0024] The "state" of the cell suspension 3 observed by the observation device 1 is represented by a parameter related to the "cell concentration" of the cell suspension 3. The parameter related to the "cell concentration" may be the "cell concentration" itself or the "cell count" calculated from the "cell concentration." The "cell count" is calculated from the "cell concentration" and the volume of the container 5 containing the cell suspension 3 or the volume of the culture medium 7. Since the volume of the container 5 is a constant and the volume of the culture medium 7 can be known in advance, if the "cell concentration" can be calculated, the "cell count" can also be calculated. The parameter related to the "cell concentration" may be a parameter indicating either the "cell concentration" or the "cell count," or may include both a parameter indicating the "cell concentration" and a parameter indicating the "cell count." In this embodiment, the parameter related to the "cell concentration" will be described as indicating the "cell concentration" itself. The "cell count" refers to the total number of cells 2 contained in the cell suspension 3. The "cell concentration" refers to the number of cells 2 present in a unit volume of the cell suspension 3.

[0025] As shown in FIG. 1 , the observation device 1 includes, for example, an irradiation unit 10, a light-receiving unit 20, and an analysis unit 30. The irradiation unit 10 is disposed in a position facing the cell suspension 3. The light-receiving unit 20 is disposed in a position opposite the irradiation unit 10 with respect to the cell suspension 3. The cell suspension 3 is contained in a container 5. The container 5 is formed of, for example, a material that is transmissive to the irradiation light L1 emitted from the irradiation unit 10 and the emission light L2 incident on the light-receiving unit 20, such as a glass material or a resin material. The observation device 1 may also include a mounting table on which the container 5 is placed. The transmissive material is, for example, a material that has a transmittance of 20% or more for light of 650 nm to 850 nm and 1060 nm to 1090 nm when the material is 1 mm thick.

[0026] Each cell 2 is suspended in a culture solution 7 inside the container 5, for example. Each cell 2 may be obtained by culturing cells attached to a scaffold containing particles such as resin spheres, or by culturing cells suspended in a medium without a scaffold. Therefore, the cell suspension 3 may contain particles such as resin spheres as a scaffold for the cells 2. Hereinafter, a first direction in the vertical direction A1 along the normal to the bottom surface 5a of the container 5 will be referred to as "up," and a second direction facing the opposite direction will be referred to as "down." The direction perpendicular to the vertical direction A1 will be referred to as the horizontal direction A2.

[0027] The irradiation unit 10 includes, for example, a light source 13 and a lens 15. The light source 13 is disposed above the cell suspension 3 and faces the cell suspension 3 in the vertical direction A1. The light source 13 emits irradiation light L1 toward an irradiation area set in the cell suspension 3. The irradiation area is a region that constitutes part of the cell suspension 3 when viewed from above and is an area irradiated with the irradiation light L1 from the light source 13. The irradiation area may be, for example, the center of the cell suspension 3 when viewed from above. The optical axis of the irradiation light L1 emitted from the light source 13 is aligned along the vertical direction A1. In one example, the optical axis of the irradiation light L1 is perpendicular to the bottom surface 5a and parallel to the vertical direction A1. The light source 13 is, for example, a laser light source using a laser diode (LD). The irradiation light L1 has a wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. The ranges of 650 nm to 850 nm and 1060 nm to 1090 nm refer to ranges that include both the range of 650 nm to 850 nm and the range of 1060 nm to 1090 nm. Therefore, it can be said that the irradiation light L1 has one wavelength that is included in at least one of the ranges of 650 nm to 850 nm and the range of 1060 nm to 1090 nm.

[0028] The light source 13 is not limited to a laser light source using a laser diode. For example, the light source 13 may be a light source using a semiconductor light-emitting element such as a laser diode, a light-emitting diode (LED), or a superluminescent diode (SLD), a laser light source using an element other than a semiconductor light-emitting element, or a lamp light source. For example, when a lamp light source is used, the irradiation light L1 may have one or more wavelengths different from the wavelengths included in the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. In this case, the one or more wavelengths different from the wavelengths included in the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm may be wavelengths included in the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, or may be wavelengths outside the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm.

[0029] The lens 15 is disposed on the optical path of the irradiation light L1 between the light source 13 and the cell suspension 3. The lens 15 is, for example, a condensing lens that condenses the irradiation light L1 from the light source 13. The lens 15 may be a collimating lens that collimates the irradiation light L1. The irradiation light L1 emitted from the light source 13 passes through the lens 15 and is irradiated onto the cell suspension 3. A portion of the irradiation light L1 passes through the cell suspension 3 and is emitted as emitted light L2. In this embodiment, the emitted light L2 is the transmitted light of the irradiation light L1 that has passed through the cell suspension 3.

[0030] The light receiving unit 20 includes, for example, an optical sensor 23 and a lens 25. The optical sensor 23 is disposed below the cell suspension 3 and faces the light source 13 in the vertical direction A1, with the cell suspension 3 sandwiched between them. The optical sensor 23 is disposed on the opposite side of the cell suspension 3 from the light source 13 in the vertical direction A1. The optical sensor 23 is disposed on the optical path of the emitted light L2 emitted from the cell suspension 3, i.e., on an extension of the optical axis of the irradiated light L1. The optical sensor 23 receives the emitted light L2 emitted from the cell suspension 3. The optical axis of the emitted light L2 is, for example, perpendicular to the bottom surface 5a on which the cell suspension 3 is placed and parallel to the vertical direction A1.

[0031] The lens 25 is disposed on the optical path of the emitted light L2 between the cell suspension 3 and the optical sensor 23. The lens 25 is, for example, an objective lens. The lens 25 may be a collimating lens. The emitted light L2 emitted from the cell suspension 3 passes through the lens 25 and enters the optical sensor 23. The optical sensor 23 receives the emitted light L2 and outputs an electrical signal S corresponding to the intensity of the emitted light L2 to the analysis unit 30. If the illumination light L1 includes multiple wavelengths, a filter that transmits light of a specific wavelength may be disposed at any position between the light source 13 and the optical sensor 23.

[0032] As shown in FIG. 1 , the analysis unit 30 is communicatively connected to the light-receiving unit 20. The analysis unit 30 may be communicatively connected to the irradiation unit 10. The analysis unit 30 receives an electrical signal S from the light-receiving unit 20. The analysis unit 30 calculates the "cell concentration" of the cell suspension 3 based on the received electrical signal S. The analysis unit 30 may also calculate the "cell number" from the "cell concentration." The intensity of the emitted light L2 indicated by the electrical signal S directly correlates with the "cell concentration." Therefore, the analysis unit 30 may calculate only the "cell concentration" based on the electrical signal S, or may calculate the "cell number" by using the volume of the container 5 as a constant or by obtaining the volume of the culture solution 7 in advance. The analysis unit 30 may calculate both the "cell concentration" and the "cell number," or may calculate either the "cell concentration" or the "cell number."

[0033] 2 is a diagram showing the hardware configuration of the analysis unit 30. The analysis unit 30 is physically one or more computers. As shown in FIG. 2, the analysis unit 30 physically includes hardware such as one or more processors 301, a main memory device 302, an auxiliary memory device 303, an input device 304, an output device 305, and a communication device 306. The analysis unit 30 is configured by one or more physical computers configured by these physical hardware devices and software such as programs.

[0034] Fig. 3 is a diagram showing the functional configuration of the analysis unit 30. Fig. 4 is a graph showing a calibration curve G1 held by the analysis unit 30. The analysis unit 30 includes, as functional components, for example, an acquisition unit 31, a storage unit 33, and a conversion unit 35. Each functional component of the analysis unit 30 is implemented by executing a program on the hardware of the physical computer described above.

[0035] The acquisition unit 31 acquires the intensity of the output light L2 indicated by the electrical signal S. The acquisition unit 31 provides the conversion unit 35 with light intensity information D1 indicating the intensity of the output light L2.

[0036] The memory unit 33 stores conversion information D2 for determining the "cell concentration" of the cell suspension 3 based on the intensity of the emitted light L2. The conversion information D2 includes, for example, a calibration curve G1 showing the relationship between the amount of change in light intensity obtained from the light intensity information D1 and the "cell concentration" of the cell suspension 3. As shown in FIG. 4, the calibration curve G1 is a graph plotting the relationship between the amount of change in light intensity and the "cell concentration" of the cell suspension 3. The amount of change in light intensity is the relative ratio between the intensity of the irradiated light L1 and the intensity of the emitted light L2, and refers to the amount of change in the intensity of the emitted light L2 relative to the intensity of the irradiated light L1. The amount of change in light intensity is defined as the value obtained by dividing the intensity of the emitted light L2 by the intensity of the irradiated light L1 (-10 Log (intensity of emitted light L2 / intensity of irradiated light L1)).

[0037] As shown in Figure 4, the amount of change in light intensity increases as the "cell concentration" increases and decreases as the "cell concentration" decreases. Therefore, if the magnitude of the amount of change in light intensity is known, the "cell concentration" can be estimated. By preparing a sample of cell suspension 3 with a known "cell concentration" (or a sample with a reference "cell concentration") in advance and irradiating the sample with irradiation light L1 using observation device 1, a calibration curve G1 showing the relationship between the amount of change in light intensity and the "cell concentration" of the sample can be created in advance.

[0038] As shown in FIG. 3 , the storage unit 33 stores the calibration curve G1 thus prepared in advance and provides the conversion information D2 including the calibration curve G1 to the conversion unit 35. Instead of or in addition to the calibration curve G1, the conversion information D2 may include a conversion formula (conversion formula) showing the relationship between the amount of change in light intensity and the "cell concentration." In addition to the calibration curve G1, the conversion information D2 may also include information necessary for converting the light intensity information D1 to the "cell concentration." For example, the conversion information D2 may include the intensity of the irradiated light L1 used to calculate the amount of change in light intensity from the light intensity information D1, or the volume of the container 5 or the volume of the culture medium 7 required to calculate the "cell number" from the "cell concentration."

[0039] The conversion unit 35 uses the light intensity information D1 and the conversion information D2 to determine the "cell concentration" of the cell suspension 3. First, the conversion unit 35 calculates the amount of change in light intensity, which is the relative ratio between the intensity of the emitted light L2 and the intensity of the irradiated light L1. Next, the conversion unit 35 converts the calculated amount of change in light intensity into the "cell concentration" of the cell suspension 3 by referring to the calibration curve G1. The analysis unit 30 may output the observation results, including the "cell concentration" of the cell suspension 3 determined by the conversion unit 35, using an output device 305, such as a monitor or a printer.

[0040] In the observation device 1 described above, the irradiation light L1 is irradiated onto a partial irradiation area of ​​the cell suspension 3. Therefore, strictly speaking, the intensity of the emitted light L2 from the irradiation area depends on the "cell concentration" (or "cell number") in that irradiation area. Since the cells 2 in the cell suspension 3 are usually uniformly distributed, the same intensity of the emitted light L2 is obtained regardless of which irradiation area of ​​the cell suspension 3 is irradiated with the irradiation light L1. Therefore, in order to determine the "cell concentration" of the cell suspension 3, it is not necessary to irradiate multiple areas of the cell suspension 3 with the irradiation light L1; it is sufficient to irradiate a partial area of ​​the cell suspension 3 with the irradiation light L1. However, if the distribution of the cells 2 in the cell suspension 3 is not uniform, the irradiation light L1 may be irradiated onto multiple areas of the cell suspension 3.

[0041] The reason why the irradiation light L1 in the wavelength ranges of 650 nm to 850 nm and 1060 nm to 1090 nm is used to irradiate the cell suspension 3 will be explained below with reference to FIGS.

[0042] Fig. 5 is a graph showing the spectrum of change in light intensity of cell 2. The vertical axis of Fig. 5 represents the change in light intensity of cell 2, and the horizontal axis represents wavelength [nm]. The vertical axis of Fig. 5 can also be said to represent the scattering intensity and absorption intensity of cell 2. Fig. 6 is a graph showing the spectrum of change in light intensity of culture solution 7. The vertical axis of Fig. 6 represents the change in light intensity of culture solution 7, and the horizontal axis represents wavelength [nm]. Since no scattering occurs in the culture solution 7, the vertical axis of Fig. 6 can also be said to represent the absorption intensity of the culture solution 7.

[0043] When light is irradiated onto the cell suspension 3, the cells 2 and the culture solution 7 absorb the light, causing it to be scattered by the cells 2. The intensity of the light emitted from the cell suspension 3 when light is irradiated onto the cell suspension 3 reflects a mixture of multiple phenomena, namely, light scattering and light absorption. Of these phenomena, the amount of change in light intensity due to scattering depends on a parameter related to the "cell concentration" of the cell suspension 3. Therefore, if the amount of change in light intensity due only to scattering can be obtained, it becomes possible to quantitatively determine the "cell concentration" of the cell suspension 3.

[0044] As shown in graph G2 of Figure 5, in the wavelength range of 1300 nm or more, there are many peaks where the change in light intensity of the cells 2 changes sharply. These peaks are caused by light absorption by the cells 2. Therefore, in wavelength ranges where many peaks exist, the effect of light absorption on the intensity of light emitted from the cell suspension 3 is significant, making it difficult to obtain the amount of change in light intensity due to scattering alone. In wavelength range R1 of 1300 nm or less, it can be seen that there are no peaks and the amount of change in light intensity changes extremely gradually. In such wavelength range R1, the effect of light absorption on the intensity of light emitted from the cell suspension 3 is sufficiently small compared to the effect of scattering that it can be ignored, making it easy to obtain the amount of change in light intensity due to scattering alone.

[0045] As shown in graph G3 of FIG. 6 , within the wavelength range R1 of 1300 nm or less (see FIG. 5 ), in the wavelength ranges R2 of 650 nm to 850 nm and 1060 nm to 1090 nm, the change in light intensity due solely to light absorption by the culture solution 7 is significantly smaller than the change in light intensity due to light scattering by the cells 2 shown in FIG. 5 . Therefore, by irradiating the cell suspension 3 with light in the wavelength ranges R2 of 650 nm to 850 nm and 1060 nm to 1090 nm, the effect of light absorption by the culture solution 7 can be ignored. In other words, the risk that the intensity of light emitted from the cell suspension 3 will be affected by light absorption by the culture solution 7 can be reduced. The culture solution 7 may contain water, amino acids, vitamins, salts, and glucose. An indicator such as phenol red may be added to the culture solution 7. If the effect of light absorption by the culture solution 7 can be ignored, it is possible to obtain a change in light intensity that reflects only scattering in the cell suspension 3. Ultraviolet light having a wavelength shorter than 400 nm is not suitable for irradiating the cell suspension 3 because it may damage the cells 2 .

[0046] Therefore, when the cell suspension 3 is irradiated with irradiation light L1 having wavelengths within the wavelength ranges R2 of 650 nm to 850 nm and 1060 nm to 1090 nm, as described above, the influence of absorption of the irradiation light L1 by the cells 2 and the culture solution 7 can be ignored, making it possible to obtain the intensity of the output light L2 that reflects only the scattering of the irradiation light L1 in the cell suspension 3. As a result, the "cell concentration" of the cell suspension 3 can be quantitatively determined based on the intensity of the output light L2 obtained. Thus, in this embodiment, the wavelength ranges R2 of 650 nm to 850 nm and 1060 nm to 1090 nm are selected as wavelength ranges in which the influence of absorption of the irradiation light L1 by the cell suspension 3 and the culture solution 7 can be ignored, taking into account the light intensity change spectrum of the cells 2 and the light intensity change spectrum of the culture solution 7.

[0047] The steps of the observation method carried out using the observation device 1 described above will be described with reference to Fig. 7. Fig. 7 is a flowchart showing an example of each step of the observation method.

[0048] First, the irradiation unit 10 disposed above the cell suspension 3 irradiates the cell suspension 3 with irradiation light L1 in the wavelength ranges of 650 nm to 850 nm and 1060 nm to 1090 nm (step S11). A portion of the irradiation light L1 irradiated onto the cell suspension 3 passes through the cell suspension 3 and is emitted from the cell suspension 3 as emitted light L2.

[0049] Next, the light receiving unit 20 disposed below the cell suspension 3 receives the emitted light L2 emitted from the cell suspension 3 (step S12). The light receiving unit 20 outputs an electrical signal S corresponding to the intensity of the received emitted light L2 to the analysis unit 30.

[0050] Next, the analysis unit 30 determines the "cell concentration" of the cell suspension 3 based on the intensity of the emitted light L2 indicated by the electrical signal S (step S13). Specifically, the conversion unit 35 of the analysis unit 30 calculates the amount of change in light intensity by dividing the intensity of the emitted light L2 by the intensity of the irradiated light L1. The conversion unit 35 then converts the calculated amount of change in light intensity into the "cell concentration" of the cell suspension 3 by referring to the calibration curve G1 shown in FIG. 4. Using the "cell concentration" determined in this manner, it becomes possible to quantitatively observe the state of the cell suspension 3.

[0051] The effects obtained by the first embodiment described above will be explained. As described above, in the light intensity change spectrum of the culture solution 7, the wavelength ranges R2 (see FIG. 6 ) of 650 nm to 850 nm and 1060 nm to 1090 nm are ranges in which light absorption by the culture solution 7 is sufficiently small compared to light scattering by the cells 2 shown in FIG. 5 . Therefore, when irradiating the cell suspension 3 with irradiation light L1 having wavelengths within the wavelength ranges R2 of 650 nm to 850 nm and 1060 nm to 1090 nm, the effect of light absorption by the culture solution 7 on the intensity of the emitted light L2 can be ignored. In the light intensity change spectrum of the cells 2, the wavelength ranges R2 of 650 nm to 850 nm and 1060 nm to 1090 nm are included in the wavelength range R1 (see FIG. 5 ) in which there are no sharp peaks caused by light absorption by the cells 2. Therefore, the effect of light absorption by the cell suspension 3 on the intensity of the emitted light L2 is sufficiently small compared to scattering by the cells 2 and can be ignored.

[0052] Therefore, when the cell suspension 3 is irradiated with irradiation light L1 having at least one wavelength within the wavelength range R2 of 650 nm to 850 nm and 1060 nm to 1090 nm, the intensity of the emitted light L2 resulting only from scattering can be obtained, allowing for accurate determination of the "cell concentration" of the cell suspension 3. In this embodiment, the "cell concentration" of the cell suspension 3 can be determined by the simple task of irradiating the cell suspension 3 with irradiation light L1 while the cells 2 remain suspended in the culture solution 7. This method allows for more efficient observation of the state of the cell suspension 3 than a method that requires the steps of collecting and injecting the cell suspension 3 into an apparatus.

[0053] As in this embodiment, the irradiation unit 10 may include a light source 13 using a laser diode that emits light having a wavelength within the range of 650 nm to 850 nm and 1060 nm to 1090 nm as the irradiation light L1. The light source 13 may also be a light source using a semiconductor light-emitting element such as a laser diode, a light-emitting diode, or a superluminescent diode. In this case, by using light from a light source using a semiconductor light-emitting element with stable light intensity as the irradiation light L1, the risk of fluctuations in the intensity of the emitted light L2 due to factors other than the state of the cell suspension 3 (e.g., instability in the intensity of the irradiation light) can be reduced. This allows for more reliable observation of the state of the cell suspension 3 based on the intensity of the emitted light L2.

[0054] As in this embodiment, the light receiving unit 20 may be disposed on the opposite side of the cell suspension 3 from the irradiation unit 10, and may receive as output light L2 a portion of the irradiation light L1 that has passed through the cell suspension 3. In this case, the position of the light receiving unit 20 relative to the irradiation unit 10 can be easily adjusted.

[0055] The observation device and observation method of the present disclosure are not limited to the first embodiment described above, and various other modifications are possible.

[0056] <Variation 1> Fig. 8 is a diagram showing a variation of the observation device 1. As in the observation device 1A shown in Fig. 8, the light-receiving unit 20 may be disposed in a position facing the cell suspension 3 in the left-right direction A2. In this case, the light-receiving unit 20 receives output light L2 emitted from the cell suspension 3 in the left-right direction A2 by irradiation with the irradiation light L1 from the light source 13 of the irradiation unit 10. In this case, the output light L2 is scattered light of the irradiation light L1 scattered in the left-right direction A2 in the cell suspension 3. The light-receiving unit 20 is disposed on the optical path of the output light L2 extending from the cell suspension 3 in the left-right direction A2. The light-receiving unit 20 receives the output light L2 through a lens 25 and outputs an electrical signal S indicating the intensity of the output light L2 to the analysis unit 30. The optical axis of the output light L2 is, for example, parallel to the left-right direction A2 and perpendicular to the up-down direction A1 in which the irradiation light L1 extends.

[0057] FIG. 9 is a graph showing a calibration curve G10 stored in the analysis unit 30. The calibration curve G10 is a graph plotting the relationship between the amount of change in light intensity and the "cell concentration" of the cell suspension 3. The amount of change in light intensity is the relative ratio between the intensity of the irradiated light L1 and the intensity of the emitted light L2, and means the amount of change in the intensity of the emitted light L2 relative to the intensity of the irradiated light L1. The amount of change in light intensity is defined as the value obtained by dividing the intensity of the emitted light L2 by the intensity of the irradiated light L1 (-10 Log (intensity of emitted light L2 / intensity of irradiated light L1)). As shown in FIG. 9, the amount of change in light intensity decreases as the "cell concentration" increases, and increases as the "cell concentration" decreases. Therefore, if the magnitude of the amount of change in light intensity is known, the "cell concentration" can be estimated.

[0058] The analysis unit 30 calculates the amount of change in light intensity, which is the relative ratio between the intensity of the emitted light L2 and the intensity of the irradiated light L1, and then, by referring to the calibration curve G10, converts the calculated amount of change in light intensity into the "cell concentration" of the cell suspension 3. The observation device 1A shown in Figure 8 can also quantitatively determine the "cell concentration" based on the intensity of the emitted light L2 from the cell suspension 3, thereby achieving the same effect as the first embodiment described above.

[0059] <Variation 2> Fig. 10 is a diagram showing a variation of the observation device 1. As shown in Fig. 10, the observation device 1B may include a first light receiving unit 20A and a second light receiving unit 20B. The first light receiving unit 20A has the same configuration as the light receiving unit 20 shown in Fig. 8. That is, the first light receiving unit 20A is disposed in a position facing the cell suspension liquid 3 in the left-right direction A2, and receives, as first emitted light L2A, light scattered by the cell suspension liquid 3 out of the illumination light L1 irradiated onto the cell suspension liquid 3. The first light receiving unit 20A outputs a first electrical signal SA indicating the intensity of the first emitted light L2A to the analysis unit 30.

[0060] The second light receiving unit 20B has the same configuration as the light receiving unit 20 of the first embodiment described above. That is, the second light receiving unit 20B is disposed in a position facing the irradiation unit 10 in the up-down direction A1 with the cell suspension 3 interposed therebetween, and receives, as second emitted light L2B, transmitted light that has passed through the cell suspension 3 out of the irradiation light L1 irradiated onto the cell suspension 3. The second light receiving unit 20B outputs a second electrical signal SB indicating the intensity of the second emitted light L2B to the analysis unit 30.

[0061] The analysis unit 30 determines the "cell concentration" of the cell suspension 3 based on the intensity of the first outgoing light L2A and the intensity of the second outgoing light L2B. For example, the analysis unit 30 converts a first amount of change in light intensity obtained from the intensity of the first outgoing light L2A into a "cell concentration" by referring to the calibration curve G10 shown in FIG. 9 . The analysis unit 30 converts a second amount of change in light intensity obtained from the intensity of the second outgoing light L2B into a "cell concentration" by referring to the calibration curve G1 shown in FIG. 4 . The analysis unit 30 may determine a more accurate "cell concentration" by averaging the "cell concentration" obtained based on the intensity of the first outgoing light L2A and the "cell concentration" obtained based on the intensity of the second outgoing light L2B.

[0062] 10, the "cell concentration" can also be quantitatively determined based on the intensity of the first emitted light L2A and the intensity of the second emitted light L2B, and therefore the same effect as in the first embodiment can be obtained. In the observation device 1B, by acquiring the intensity of both the first emitted light L2A and the intensity of the second emitted light L2B, the "cell concentration" can be more accurately determined based on these intensities.

[0063] Second Embodiment An observation device according to a second embodiment will be described below. In the following description of the second embodiment, descriptions of parts that overlap with the first embodiment will be omitted as appropriate, and differences from the first embodiment will be mainly described.

[0064] Fig. 11 is a diagram showing the configuration of an observation device 101 of the second embodiment. The observation device 101 of this embodiment includes, for example, an irradiation unit 110, a light receiving unit 120, and an analysis unit 130. The light receiving unit 120 has the same configuration as the light receiving unit 20 of the first embodiment, for example. That is, as shown in Fig. 11 , the light receiving unit 120 includes a lens 125 having the same configuration as the lens 25, and an optical sensor 123 having the same configuration as the optical sensor 23.

[0065] The irradiation unit 110 has a different configuration from the irradiation unit 10 of the first embodiment. The irradiation unit 110 includes a first light source 113A, a second light source 113B, a lens 115, and a switching unit 117. The lens 115 has the same configuration as the lens 15 of the first embodiment. The first light source 113A and the second light source 113B are disposed above the cell suspension 3 and are adjacent to each other along the left-right direction A2. Each of the first light source 113A and the second light source 113B is, for example, a laser light source using a laser diode (LD). Each of the first light source 113A and the second light source 113B may be a light source using a semiconductor light-emitting element such as a laser diode, a light-emitting diode (LED), or a superluminescent diode (SLD), or may be a laser light source or a lamp light source using an element other than a semiconductor light-emitting element.

[0066] The first light source 113A emits first irradiation light L1A having a first wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. The second light source 113B emits second irradiation light L1B having a second wavelength different from the first wavelength. The second wavelength is, for example, a wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm that is different from the first wavelength. The first irradiation light L1A and the second irradiation light L1B may each have multiple wavelengths. That is, the first irradiation light L1A may have a wavelength different from the first wavelength, and the second irradiation light L1B may have a wavelength different from the second wavelength.

[0067] The switching unit 117 is disposed between the first light source 113A, the second light source 113B, and the lens 115. The switching unit 117 is optically coupled to the first light source 113A, the second light source 113B, and the lens 115 using lenses and optical fibers. The switching unit 117 switches the optical path of the first irradiation light L1A and the optical path of the second irradiation light L1B so that the first irradiation light L1A from the first light source 113A and the second irradiation light L1B from the second light source 113B are selectively irradiated onto the cell suspension 3. The switching unit 117 sequentially irradiates the cell suspension 3 with the first irradiation light L1A and the second irradiation light L1B, for example. The switching unit 117 is, for example, an optical switch that changes the direction of light by sliding a mirror or prism. The switching unit 117 may be, for example, an optical switch that switches the direction of light by using a dielectric having an electro-optic effect, such as LiON3 (lithium nitrate) or LiTaO3 (lithium tantalate).

[0068] When the cell suspension 3 is irradiated with the first irradiation light L1A, a portion of the first irradiation light L1A passes through the cell suspension 3 and is emitted from the cell suspension 3 as first outgoing light L2A. The first outgoing light L2A has the same wavelength as the first wavelength of the first irradiation light L1A. When the cell suspension 3 is irradiated with the second irradiation light L1B, a portion of the second irradiation light L1B passes through the cell suspension 3 and is emitted from the cell suspension 3 as second outgoing light L2B. The second outgoing light L2B has the same wavelength as the second wavelength of the second irradiation light L1B.

[0069] The optical sensor 123 selectively receives the first emitted light L2A and the second emitted light L2B emitted from the cell suspension 3. For example, the optical sensor 123 sequentially receives the first emitted light L2A and the second emitted light L2B. The optical sensor 123 outputs to the analysis unit 130 a first electrical signal SA corresponding to the intensity of the first emitted light L2A and a second electrical signal SB corresponding to the intensity of the second emitted light L2B.

[0070] The analysis unit 130 quantitatively determines the "cell concentration" of the cell suspension 3 based on the intensity of the first outgoing light L2A and the intensity of the second outgoing light L2B. Generally, when light is irradiated onto the cell suspension 3, light is absorbed by the cells 2 and the culture solution 7, causing light scattering in the cells 2. Among these phenomena, light scattering that occurs in the cells 2 includes Rayleigh scattering caused by particles smaller than the wavelength of light, and Mie scattering caused by particles larger than the wavelength of light. Since the size of the cells 2 is about 10 μm, Mie scattering occurs when light hits the cells 2. Rayleigh scattering occurs when light hits molecules that make up the cell membrane of the cells 2.

[0071] In this way, the Rayleigh scattering intensity generated due to the molecular structure inside the cells 2 depends on parameters related to the "cell concentration," such as the "cell concentration" and "cell number" of the cell suspension 3. Let us assume that the cells 2 in the cell suspension 3 have adhered to each other to form a cell cluster. In this case, the amount and structure of the molecules that make up the cell membrane of the cells 2 in the cell cluster do not change, so the Rayleigh scattering intensity obtained from the cell suspension 3 by light irradiation does not change. The size of the particles (cell clusters) formed by the adhesion of the cells 2 to each other changes, so the Mie scattering intensity obtained from the cell suspension 3 changes.

[0072] Therefore, even if the same number of cells 2 are present in the cell suspension 3, the Rayleigh scattering intensity does not change depending on whether the cells 2 are present in an adhered state or in a separate state without adhering to each other, but the Mie scattering intensity does. Therefore, if the "cell concentration" of the cell suspension 3 is estimated based on the scattering intensity of a mixture of Rayleigh scattering and Mie scattering, the correct "cell concentration" may not be obtained. Resin spheres that can be used as a scaffold material for culturing cells 2 have a homogeneous molecular structure, so when light hits such resin spheres, only Mie scattering occurs. Even when such a scaffold material is present, the "cell concentration" obtained based on the scattering intensity may vary depending on the Mie scattering intensity.

[0073] Therefore, if only Rayleigh scattering can be observed among the scattering occurring in the cell suspension 3, the "cell concentration" can be accurately estimated without being affected by adhesion between the cells 2 or by the scaffold material. While Rayleigh scattering intensity depends on the wavelength of light, Mie scattering intensity does not depend on the wavelength of light, or does not depend significantly on the wavelength of light. Therefore, when observing the state of the cell suspension 3 using light of multiple wavelengths, it is possible to determine only the wavelength-dependent Rayleigh scattering intensity by utilizing the differences in wavelength dependence.

[0074] Fig. 12 is a diagram showing the functional configuration of the analysis unit 130. Fig. 13 is a graph showing a calibration curve G12 held by the analysis unit 130. The analysis unit 130 includes, as functional components, an acquisition unit 131, a storage unit 133, and a conversion unit 135, for example.

[0075] The acquisition unit 131 acquires the intensity of the first output light L2A indicated by the first electrical signal SA and the intensity of the second output light L2B indicated by the second electrical signal SB. The acquisition unit 131 provides the conversion unit 135 with first light intensity information D11 indicating the intensity of the first output light L2A and second light intensity information D12 indicating the intensity of the second output light L2B.

[0076] The storage unit 133 stores conversion information D21 for determining the "cell concentration" of the cell suspension 3 based on the intensity of the first emitted light L2A and the intensity of the second emitted light L2B. The conversion information D21 includes a calibration curve G12 showing the relationship between the Rayleigh scattering intensity obtained from the first light intensity information D11 and the second light intensity information D12 and the "cell concentration" of the cell suspension 3. The "Rayleigh scattering coefficient" is a parameter that indicates the relationship between the Rayleigh scattering intensity and wavelength, and can be determined from the wavelength dependence of the Rayleigh scattering intensity. There is a proportional relationship in which the larger the Rayleigh scattering coefficient, the greater the Rayleigh scattering intensity. Therefore, the "cell concentration" can be quantitatively determined by determining either the Rayleigh scattering intensity or the Rayleigh scattering coefficient. This embodiment illustrates an example in which the "cell concentration" is determined using the Rayleigh scattering coefficient. In addition to the calibration curve G12, the conversion information D21 may also include information necessary for converting the first light intensity information D11 and the second light intensity information D12 into the "cell concentration." For example, the conversion information D21 may include the intensity of the irradiated light L1A for calculating the first amount of change in light intensity from the first light intensity information D11, or the intensity of the irradiated light L1B for calculating the second amount of change in light intensity from the second light intensity information D12. The conversion information D21 may include the volume of the container 5 or the volume of the culture solution 7 required to calculate the "cell number" from the "cell concentration." As shown in Fig. 13, the calibration curve G12 is a graph plotting the relationship between the Rayleigh scattering coefficient and the "cell concentration."

[0077] FIG. 14 is a graph illustrating how to calculate the Rayleigh scattering coefficient. Graph G11 in FIG. 14 shows the relationship between the amount of change in light intensity and the wavelength λ of light when there is no effect of absorption and the effects of Rayleigh scattering and Mie scattering. In FIG. 14, the vertical axis represents the amount of change in light intensity, and the horizontal axis represents the wavelength λ divided by the fourth power. FIG. 14 also shows a plot P11 illustrating the relationship between the first amount of change in light intensity P(A) and the first wavelength λ(A), and a plot P12 illustrating the relationship between the second amount of change in light intensity P(B) and the second wavelength λ(B). The first amount of change in light intensity P(A) is the relative ratio between the intensity of the first emitted light L2A and the intensity of the first irradiation light L1A, and is defined as −10 Log (intensity of first emitted light L2A / intensity of first irradiation light L1A). The second light intensity change amount P(B) is the relative ratio between the intensity of the second emitted light L2B and the intensity of the second irradiation light L1B, and is defined as −10 Log (intensity of the second emitted light L2B / intensity of the second irradiation light L1B).

[0078] 14, the wavelength-dependent Rayleigh scattering intensity is expressed by the product of the slope of the line connecting plot P11 and plot P12 (i.e., the Rayleigh scattering coefficient) and one-fourth power of the wavelength λ. Therefore, the Rayleigh scattering coefficient Pr1 can be calculated using the following equation (1). Once the Rayleigh scattering coefficient Pr1 is calculated, it is also possible to calculate the Rayleigh scattering intensity using the above relationship.

[0079] 12 again. The storage unit 133 pre-stores conversion information D21 including a calibration curve G12 showing the relationship between the Rayleigh scattering coefficient and the "cell concentration." A sample of the cell suspension 3 with a known "cell concentration" (or a sample with a reference "cell concentration") is prepared in advance, and the first irradiation light L1A and the second irradiation light L1B are irradiated onto the sample using the observation device 101, thereby creating in advance the calibration curve G12 showing the relationship between the Rayleigh scattering coefficient and the "cell concentration" of the sample.

[0080] The conversion unit 135 calculates the "cell concentration" of the cell suspension 3 using the first light intensity information D11, the second light intensity information D12, and the conversion information D21. Specifically, the conversion unit 135 calculates a first light intensity change P(A), which is the relative ratio between the intensity of the first outgoing light L2A and the intensity of the first irradiation light L1A, and calculates a second light intensity change P(B), which is the relative ratio between the intensity of the second outgoing light L2B and the intensity of the second irradiation light L1B. Next, the conversion unit 135 calculates the Rayleigh scattering coefficient Pr1 shown in Equation (1) using the first light intensity change P(A) and the second light intensity change P(B). Thereafter, the conversion unit 135 converts the Rayleigh scattering coefficient Pr1 into the "cell concentration" by referring to the calibration curve G12. This allows the "cell concentration" to be quantitatively calculated based on the intensity of the first outgoing light L2A and the intensity of the first irradiation light L1A.

[0081] FIG. 15 is a graph illustrating a wavelength range suitable for observing the state of the cell suspension 3. Graph G13, represented by a solid line in FIG. 15, shows the wavelength dependence of the amount of change in light intensity, reflecting the effects of Rayleigh scattering and Mie scattering by the cells 2 and light absorption by the cells 2 and the culture solution 7. Graph G14, represented by a dotted line in FIG. 15, shows the wavelength dependence of the amount of change in light intensity when there is no effect of light absorption by the cells 2 and the culture solution 7 (i.e., when there is only the effect of Rayleigh scattering and Mie scattering by the cells 2). As shown in FIG. 15, graph G13 includes wavelength ranges R11 and R12 in which there is a peak where the amount of change in light intensity increases sharply, and wavelength ranges R21 and R22 in which there is no such peak. In the wavelength ranges R11 and R12 in which such peaks exist, it may be difficult to accurately determine the Rayleigh scattering coefficient due to the influence of the peaks. Therefore, to accurately determine the Rayleigh scattering coefficient, it is appropriate to use wavelengths included in the wavelength ranges R21 and R22 in which there are no peaks.

[0082] In this embodiment, the first wavelength λ(A) of the first irradiation light L1A and the second wavelength λ(B) of the second irradiation light L1B are both within wavelength ranges R2 (see FIG. 6 ) of 650 nm to 850 nm and 1060 nm to 1090 nm. In graph G13 of FIG. 15 , wavelengths of 1060 nm to 1090 nm are within wavelength range R21, in which no peak exists, and wavelengths of 650 nm to 850 nm are within wavelength range R22, in which no peak exists. Therefore, when the cell suspension 3 is irradiated with the first irradiation light L1A and the second irradiation light L1B, a change in light intensity is obtained that is not affected by absorption of the irradiation light L1 by the cells 2 and the culture solution 7, and the Rayleigh scattering coefficient can be accurately determined based on the change in light intensity.

[0083] The effects obtained by the second embodiment described above will be explained. As described above, when light is irradiated onto a cell suspension 3, light scattering occurs from particles such as cells 2. These include Rayleigh scattering, which occurs due to particles smaller than the wavelength of light, and Mie scattering, which occurs due to particles larger than the wavelength of light. If cells 2 adhere to each other in the cell suspension 3 to form cell clusters, the amount and structure of molecules constituting the cell membranes of the cells 2 in the cell clusters do not change, so the Rayleigh scattering intensity obtained from the cell suspension 3 does not change due to light irradiation. The size of the particles (cell clusters) formed by the adhesion of cells 2 changes, so the Mie scattering intensity obtained from the cell suspension 3 changes. In this case, it is expected that the intensity of light emitted from the cell suspension 3 will change even though the number of cells 2 in the cell suspension 3 remains unchanged. Such changes in the intensity of the emitted light can reduce the accuracy of estimating the "cell concentration." Resin spheres that can be used as scaffolds for culturing cells 2 have a homogeneous molecular structure, so only Mie scattering occurs when light hits these resin spheres. Such scaffold materials can also be a factor that reduces the accuracy of estimating "cell concentration."

[0084] Therefore, if only Rayleigh scattering, which is part of the scattering occurring in the cell suspension 3, can be observed, the "cell concentration" can be accurately determined without being affected by adhesion between the cells 2 or by the scaffold material. While Rayleigh scattering intensity depends on the wavelength of light, Mie scattering intensity does not depend on the wavelength of light, or does not depend significantly on the wavelength of light. Therefore, when observing the state of the cell suspension 3 using light of multiple wavelengths, differences in wavelength dependence can be utilized to determine only the wavelength-dependent Rayleigh scattering intensity, making it possible to accurately determine the "cell concentration."

[0085] As in the present embodiment, the irradiation unit 110 may include a first light source 113A, a second light source 113B, and a switching unit 117. The light receiving unit 120 may include an optical sensor 123. In this case, the intensity of the first emitted light L2A and the intensity of the second emitted light L2B can be distinguished and detected using a single optical sensor 123. As a result, it is not necessary to separately prepare an optical sensor for detecting the intensity of the first emitted light L2A and an optical sensor for detecting the intensity of the second emitted light L2B, which can avoid the configuration of the light receiving unit 120 from becoming complicated.

[0086] The observation device and observation method of the present disclosure are not limited to the second embodiment described above, and various other modifications are possible.

[0087] <Variation 1> Fig. 16 is a diagram showing a variation of the observation device 101. As in the observation device 101A shown in Fig. 16, the light receiving unit 120 may be disposed in a position facing the cell suspension liquid 3 in the left-right direction A2. In this case, the light receiving unit 120 sequentially receives first emitted light L2A and second emitted light L2B (hereinafter simply referred to as "emitted light L2A, L2B") emitted from the cell suspension liquid 3 in the left-right direction A2 by irradiation with first irradiation light L1A and second irradiation light L1B (hereinafter simply referred to as "irradiation light L1A, L1B") from the irradiation unit 110. The emitted light L2A and L2B are scattered light of the irradiation light L1A and L1B scattered by the cell suspension liquid 3. The light receiving unit 120 is disposed on the optical path of the emitted light L2A and L2B extending from the cell suspension liquid 3 in the left-right direction A2.

[0088] The light receiving unit 120 receives the emitted light beams L2A and L2B through the lens 125 and outputs a first electrical signal SA indicating the intensity of the first emitted light beam L2A and a second electrical signal SB indicating the intensity of the second emitted light beam L2B to the analyzing unit 130. The optical axes of the emitted light beams L2A and L2B are, for example, parallel to the left-right direction A2 and perpendicular to the up-down direction A1 in which the illumination light beams L1A and L1B extend. As in the second embodiment, the analyzing unit 130 calculates the Rayleigh scattering coefficient based on the intensities of the emitted light beams L2A and L2B.

[0089] FIG. 17 is a graph illustrating how to calculate the Rayleigh scattering coefficient. In FIG. 17, the vertical axis represents the amount of change in light intensity, and the horizontal axis represents the wavelength λ divided by the fourth power. FIG. 17 shows a plot P21 illustrating the relationship between the first amount of change in light intensity P(A) and the first wavelength λ(A) of the first emitted light L2A, and a plot P22 illustrating the relationship between the second amount of change in light intensity P(B) and the second wavelength λ(B) of the second emitted light L2B. The first amount of change in light intensity P(A) is the relative ratio between the intensity of the first emitted light L2A and the intensity of the first illumination light L1A, and is defined as −10 Log (Intensity of the first emitted light L2A / Intensity of the first illumination light L1A). The second amount of change in light intensity P(B) is the relative ratio between the intensity of the second emitted light L2B and the intensity of the second illumination light L1B, and is defined as −10 Log (Intensity of the second emitted light L2B / Intensity of the second illumination light L1B).

[0090] In graph G15 shown in FIG. 17 , the wavelength-dependent Rayleigh scattering intensity is expressed by the product of the slope of the line connecting plots P21 and P22 (i.e., the Rayleigh scattering coefficient) and one-fourth power of the wavelength λ. Therefore, the Rayleigh scattering coefficient Pr1 can be calculated using the above-described formula (1). After calculating the Rayleigh scattering coefficient Pr1, the analysis unit 130 converts the Rayleigh scattering coefficient Pr1 into a "cell concentration" by referring to the calibration curve G12 shown in FIG. 13 . Therefore, the observation device 101A can also accurately calculate the "cell concentration" based on the intensities of the emitted light beams L2A and L2B, which have different wavelengths, thereby achieving the same effect as the second embodiment.

[0091] <Variation 2> Figure 18 is a diagram showing a variation of the observation device 101. As shown in Figure 18, the observation device 101B may include a first light receiving unit 120A and a second light receiving unit 120B. The first light receiving unit 120A has the same configuration as the light receiving unit 120 shown in Figure 16. That is, the first light receiving unit 120A is disposed in a position facing the cell suspension liquid 3 in the left-right direction A2, and receives, as output light L2A, L2B, light scattered by the cell suspension liquid 3 out of the illumination light L1A, L1B irradiated onto the cell suspension liquid 3. The first light receiving unit 120A outputs, to the analysis unit 130, a first electrical signal SA indicating the intensity of the first output light L2A and a second electrical signal SB indicating the intensity of the second output light L2B.

[0092] The second light receiving unit 120B has the same configuration as the light receiving unit 120 of the second embodiment described above. That is, the second light receiving unit 120B is disposed in a position facing the irradiating unit 110 in the up-down direction A1, sandwiching the cell suspension 3 therebetween, and receives, of the irradiation light L1A, L1B irradiated onto the cell suspension 3, transmitted light that has passed through the cell suspension 3 as third emitted light L2C and fourth emitted light L2D (hereinafter simply referred to as "emitted light L2C, L2D"). The second light receiving unit 120B outputs, to the analyzing unit 130, a third electrical signal SC indicating the intensity of the third emitted light L2C and a fourth electrical signal SD indicating the intensity of the fourth emitted light L2D.

[0093] The analysis unit 130 determines the "cell concentration" of the cell suspension 3 based on the intensities of the emitted light beams L2A, L2B, L2C, and L2D. For example, the analysis unit 130 determines a Rayleigh scattering coefficient using the emitted light beams L2A and L2B, and then converts the Rayleigh scattering coefficient into a "cell concentration" by referring to the calibration curve G12 shown in FIG. 13. Similarly, the analysis unit 130 determines a Rayleigh scattering coefficient using the emitted light beams L2C and L2D, and then converts the Rayleigh scattering coefficient into a "cell concentration" by referring to the calibration curve G12 shown in FIG. 13. The analysis unit 130 may calculate a more accurate "cell concentration" by averaging the two "cell concentrations" determined in this manner.

[0094] 18, the "cell concentration" can also be quantitatively determined based on the intensities of the emitted light beams L2A, L2B, L2C, and L2D, thereby achieving the same effect as in the second embodiment described above. The observation device 101B acquires the intensities of both the emitted light beams L2A and L2B and the emitted light beams L2C and L2D, thereby enabling the "cell concentration" to be more accurately determined based on these intensities.

[0095] <Modification 3> Figure 19 is a diagram showing a modification of the observation device 101. As in the observation device 101C shown in Figure 19, the irradiation unit 110A may include a multiplexing unit 119 instead of the switching unit 117. The multiplexing unit 119 is disposed between the first light source 113A and the second light source 113B and the lens 115. The multiplexing unit 119 is optically coupled to the first light source 113A, the second light source 113B, and the lens 115 using lenses and optical fibers. The multiplexing unit 119 multiplexes the first irradiation light L1A from the first light source 113A and the second irradiation light L1B from the second light source 113B, and irradiates the cell suspension 3 with the multiplexed irradiation light L3. The multiplexed irradiation light L3 is multiplexed light including the first irradiation light L1A and the second irradiation light L1B, and has a first wavelength λ(A) and a second wavelength λ(B). When the cell suspension 3 is irradiated with the combined irradiation light L3, a portion of the combined irradiation light L3 passes through the cell suspension 3 and is emitted from the cell suspension 3 as combined output light L4. The combined output light L4 has a wavelength corresponding to the first wavelength λ(A) and the second wavelength λ(B) (e.g., the same wavelength as the first wavelength λ(A) and the second wavelength λ(B)). The multiplexing unit 119 is, for example, a dichroic mirror. The multiplexing unit 119 may also be an optical fiber multiplexer such as a WDM coupler.

[0096] The light receiving unit 120C includes a spectroscopic unit 129, a first optical sensor 123A, and a second optical sensor 123B instead of the optical sensor 123. The first optical sensor 123A and the second optical sensor 123B are disposed below the cell suspension 3 and are adjacent to each other in the left-right direction A2. The spectroscopic unit 129 is disposed between the lens 125 and the first and second optical sensors 123A and 123B. The spectroscopic unit 129 is optically coupled to the lens 125, the first optical sensor 123A, and the second optical sensor 123B using lenses and optical fibers. The spectroscopic unit 129 splits the combined output light L4 into wavelengths. For example, the spectroscopic unit 129 splits the combined output light L4 into a first output light L2A having a first wavelength λ(A) and a second output light L2B having a second wavelength λ(B). The spectroscopic unit 129 is, for example, a dichroic mirror, or may be an optical fiber spectrometer such as a WDM coupler.

[0097] The first optical sensor 123A receives the first outgoing light L2A separated from the combined outgoing light L4 by the spectroscopic unit 129 and outputs a first electrical signal SA indicating the intensity of the first outgoing light L2A. The second optical sensor 123B receives the second outgoing light L2B separated from the combined outgoing light L4 by the spectroscopic unit 129 and outputs a second electrical signal SB indicating the intensity of the second outgoing light L2B.

[0098] 19 also allows the "cell concentration" of the cell suspension 3 to be quantitatively determined based on the intensities of the first emitted light L2A and the second emitted light L2B, thereby achieving the same effect as in the second embodiment. The observation device 101C makes it possible to simultaneously detect the intensities of the first emitted light L2A and the second emitted light L2B using the combined irradiation light L3 and the combined emitted light L4, thereby enabling the state of the cell suspension 3 to be observed more efficiently in a shorter time based on the intensities of the first emitted light L2A and the second emitted light L2B.

[0099] <Modification 4> FIG. 20 is a diagram showing a modification of the observation device 101. As shown in FIG. 20, the observation device 101D may include an irradiation unit 110B instead of the irradiation unit 110. The irradiation unit 110B includes a light source 113C, a lens 115, and a filter switching unit 118. The light source 113C is a halogen lamp (lamp light source) that emits irradiation light L1 containing multiple wavelengths. At least two wavelengths contained in the irradiation light L1 are within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. Other wavelengths contained in the irradiation light L1 may be within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm, or may be outside the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm.

[0100] The lens 115 is optically coupled to the light source 113C using an optical fiber such as a bundle fiber. The filter switching unit 118 is disposed on the optical path of the irradiation light L1 between the lens 115 and the cell suspension 3. The filter switching unit 118 holds a first filter 118A and a second filter 118B, and positions either the first filter 118A or the second filter 118B in the optical path of the irradiation light L1. The first filter 118A is a bandpass filter that transmits only a first wavelength λ(A) among the multiple wavelengths contained in the irradiation light L1. The second filter 118B is a bandpass filter that transmits only a second wavelength λ(B) among the multiple wavelengths contained in the irradiation light L1. As shown in the upper right portion of FIG. 20 , when the filter switching unit 118 is viewed in the up-down direction A1, each of the first filter 118A and the second filter 118B is, for example, circular.

[0101] 20 , when the filter switching unit 118 arranges the first filter 118A in the optical path of the irradiation light L1, the first irradiation light L1A having a first wavelength λ(A) that has passed through the first filter 118A is irradiated onto the cell suspension 3. When the filter switching unit 118 arranges the second filter 118B in the optical path of the irradiation light L1, the second irradiation light L1B having a second wavelength λ(B) that has passed through the second filter 118B is irradiated onto the cell suspension 3. The filter switching unit 118 interchanges the arrangement of the first filter 118A and the arrangement of the second filter 118B so that the first irradiation light L1A and the second irradiation light L1B are selectively irradiated onto the cell suspension 3. As described above, the first wavelength λ(A) is within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. The second wavelength λ(B) is a wavelength that is different from the first wavelength (A) and falls within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm.

[0102] When the cell suspension 3 is irradiated with the first irradiation light L1A, a portion of the first irradiation light L1A passes through the cell suspension 3 and is emitted from the cell suspension 3 as first outgoing light L2A. When the cell suspension 3 is irradiated with the second irradiation light L1B, a portion of the second irradiation light L1B passes through the cell suspension 3 and is emitted from the cell suspension 3 as second outgoing light L2B. The light receiving unit 120 selectively receives the first outgoing light L2A and the second outgoing light L2B emitted from the cell suspension 3. The light receiving unit 120 outputs a first electrical signal SA corresponding to the intensity of the first outgoing light L2A and a second electrical signal SB corresponding to the intensity of the second outgoing light L2B to the analysis unit 130.

[0103] 20 also allows the "cell concentration" of the cell suspension 3 to be quantitatively determined based on the intensity of the first emitted light L2A and the intensity of the second emitted light L2B, thereby achieving the same effect as in the second embodiment. When a broadband halogen lamp is used as in the observation device 101D, the device can be constructed more inexpensively than when a laser light source is used.

[0104] The present disclosure is not limited to the above-described embodiments and modifications, and various other modifications are possible. For example, the above-described embodiments and modifications may be combined with each other to the extent that no contradictions exist, depending on the required purpose and effect. The observation device of the present disclosure is not limited to the configurations of the above-described embodiments and modifications.

[0105] For example, the observation device may include multiple irradiation units and multiple light-receiving units. In this case, the multiple irradiation units may be arranged two-dimensionally when viewed from above, and may irradiate multiple irradiation regions set in the cell suspension with irradiation light, respectively. The multiple irradiation regions may be multiple regions set by dividing the cell suspension when viewed from above, and may be regions to be irradiated with irradiation light. The multiple irradiation units may be arranged above the multiple irradiation regions, respectively, and may vertically face the multiple irradiation regions one-to-one. The multiple light-receiving units may be arranged two-dimensionally when viewed from above. The multiple light-receiving units may be arranged below the multiple irradiation regions, and may vertically face the multiple irradiation units one-to-one, with the cell suspension sandwiched between them.

[0106] In the above-described embodiments and modifications, the irradiating light irradiated to the cell suspension contains at least one wavelength within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. However, all wavelengths of the irradiating light may be within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. That is, the irradiating light may contain only wavelengths within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm. In this case, a decrease in measurement accuracy due to the inclusion of wavelengths outside the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm can be avoided. The analysis unit may handle only information (irradiating light intensity or emitted light intensity) obtained from wavelengths within the ranges of 650 nm to 850 nm and 1060 nm to 1090 nm.

[0107] DESCRIPTION OF SYMBOLS 1, 1A, 1B, 101, 101A, 101B, 101C, 101D... Observation device 2... Cell 3... Cell suspension 5... Container 5a... Bottom surface 7... Culture solution 10, 110, 110A, 110B... Irradiation unit 13... Light source (laser light source) 15, 25, 115, 125... Lens 20, 120... Light receiving unit 20A, 120A... First light receiving unit 20B, 120B... Second light receiving unit 23, 123... Optical sensor 30, 130... Analysis unit 31, 131... Acquisition unit 33, 133... Memory unit 35, 135... Conversion unit 113A... First light source 113B... Second light source 113C... Light source 117... Switching unit 118... Filter switching unit 118A... First filter 118B...Second filter 119...Combining section 123A...First optical sensor 123B...Second optical sensor 129...Spectroscopic section 301...Processor 302...Main memory device 303...Auxiliary memory device 304...Input device 305...Output device A1...Up / down direction A2...Left / right direction D1...Light intensity information D2, D21...Conversion information D11...First light intensity information D12...Second light intensity information G1, G10, G12...Calibration curve L1...Irradiation light L1A...First irradiation light L1B...Second irradiation light L2...Emitted light L2A...First emitted light L2B...Second emitted light L2C...Third emitted light L2D...Fourth emitted light L3...Combined irradiation light L4...Combined emitted light P(A)...First light intensity change amount P(B)...Second light intensity change amount Pr1...Rayleigh scattering coefficient R1, R2, R11, R21, R22...wavelength range λ...wavelength λ(A)...first wavelength λ(B)...second wavelength S...electrical signal SA...first electrical signal SB...second electrical signal

Claims

1. An observation device for observing a cell suspension containing a plurality of cells, comprising: at least one irradiation unit disposed at a position facing the cell suspension and irradiating the cell suspension with irradiation light having at least one wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less; at least one light receiving unit disposed at a position facing the cell suspension and receiving the emitted light emitted from the cell suspension by the irradiation of the irradiation light onto the cell suspension; and an analysis unit communicably connected to the light receiving unit and obtaining a parameter related to the cell concentration of the cells in the cell suspension based on the intensity of the emitted light.

2. The observation device according to claim 1, wherein the irradiation unit includes a light source using a semiconductor light emitting element that emits light having one wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less as the irradiation light.

3. The observation device according to claim 1 or 2, wherein the light receiving unit is disposed at a position on the opposite side of the irradiation unit with the cell suspension interposed therebetween, and receives a part of the irradiation light transmitted through the cell suspension as the emitted light.

4. The irradiation unit irradiates the cell suspension with first irradiation light having a first wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less, and second irradiation light having a second wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less and different from the first wavelength; the light receiving unit receives first emitted light and second emitted light having different wavelengths emitted from the cell suspension by the irradiation of the first irradiation light and the second irradiation light; and the analysis unit obtains a parameter related to the cell concentration based on the intensity of the first emitted light and the intensity of the second emitted light. The observation device according to any one of claims 1 to 3.

5. The irradiation unit includes a first light source that emits the first irradiation light, a second light source that emits the second irradiation light, and a switching unit that switches the optical path of the first irradiation light and the optical path of the second irradiation light so that the first irradiation light and the second irradiation light are selectively irradiated onto the cell suspension. The light receiving unit includes a photosensor that selectively receives the first emitted light and the second emitted light emitted from the cell suspension. The observation device according to claim 4.

6. The irradiation unit includes a first light source that emits the first irradiation light, a second light source that emits the second irradiation light, and a multiplexing unit that multiplexes the first irradiation light and the second irradiation light to generate multiplexed irradiation light and irradiates the multiplexed irradiation light onto the cell suspension. The light receiving unit includes a spectroscopic unit that spectroscopically analyzes the multiplexed emitted light emitted from the cell suspension for each wavelength by the irradiation of the multiplexed irradiation light, a first photosensor that detects the intensity of the first emitted light spectroscopically analyzed from the multiplexed emitted light, and a second photosensor that detects the intensity of the second emitted light spectroscopically analyzed from the multiplexed emitted light. The observation device according to claim 4.

7. The irradiation unit includes a light source that emits light including the first irradiation light and the second irradiation light, a first filter that transmits only the first irradiation light, and a second filter that transmits only the second irradiation light, and a filter switching unit that switches the position of the first filter and the position of the second filter so that the first filter or the second filter is disposed on the optical path of the light between the light source and the cell suspension. The light receiving unit includes a photosensor that selectively receives the first emitted light and the second emitted light emitted from the cell suspension. The observation device according to claim 4.

8. An observation method for observing a cell suspension containing a plurality of cells, the method comprising: irradiating the cell suspension with irradiation light having at least one wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less; receiving emitted light emitted from the cell suspension by the irradiation of the irradiation light onto the cell suspension; and obtaining a parameter related to the cell concentration of the cells in the cell suspension based on the intensity of the emitted light.

9. In the step of irradiating the cell suspension, the cell suspension is irradiated with first irradiation light having a first wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less, and second irradiation light having a second wavelength included in the range of 650 nm or more and 850 nm or less and 1060 nm or more and 1090 nm or less and different from the first wavelength. In the step of receiving the emitted light, the first emitted light and the second emitted light having different wavelengths, which are emitted from the cell suspension by the irradiation of the first irradiation light and the second irradiation light, are received. In the step of obtaining a parameter related to the cell concentration, a parameter related to the cell concentration is obtained based on the intensity of the first emitted light and the intensity of the second emitted light. The observation method according to claim 8.

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