Optical measurement device

Abstract

An optical measurement device according to the present disclosure comprises: a light source (1) that irradiates a light scattering body with pulsed laser light; an ultrasonic source (2) that outputs ultrasonic waves to a measurement position at a predetermined depth in the light scattering body; a detector (3) that detects scattered light, which has a scattering angle of 90 degrees or more and passes through a region in the light scattering body including the measurement position and an imaging surface on the surface of the light scattering body, among the laser light; a control device (4) that controls the light source (1) and the ultrasonic source (2); and a base part (6) that is disposed in a space extending from the ultrasonic source to the light scattering body, covers the imaging surface, and is composed of a material having a predetermined acoustic impedance value and a predetermined light transmittance.

Description

Optical measurement device

[0001] The present disclosure relates to an optical measurement device, and more specifically to a technique for improving spatial resolution and measurement accuracy in measurement by ultrasound-modulated optical tomography.

[0002] Many optical measurement methods have been developed as minimally invasive techniques for measuring tissues inside light scattering media such as living bodies. For example, by irradiating light from outside the body and measuring light that propagates through the living body and exits therefrom, biological information such as morphological information and metabolic information (e.g., blood oxygen saturation) of in-vivo tissues can be obtained. However, since in-vivo tissues are light scattering media, light irradiated from outside the body diffuses in the in-vivo tissues, resulting in poor spatial resolution and inability to measure deep regions. Accordingly, as disclosed in Elson, Daniel S., et al. "Ultrasound-mediated optical tomography: a review of current methods." Interface Focus 1.4 632-648 (2011) (Non-Patent Document 1), an optical measurement device based on ultrasound-modulated optical tomography (UOT), which combines ultrasound that propagates with low scattering in living bodies and light, has been developed. In UOT, biological information is obtained by measuring light modulated by ultrasound.

[0003] As disclosed in Japanese Unexamined Patent Application Publication No. 2010-71692 (Patent Document 1), UOT includes a method of detecting laser light transmitted through a sample (transmission method), and as disclosed in Japanese Patent No. 5672104 (Patent Document 2), a method of detecting, among laser light emitted from a light source and incident on a measurement object, light scattered inside the measurement object and back to the light source side (backscattering method). In the transmission method, laser light transmitted through the measurement object is targeted for measurement, whereas in the backscattering method, light scattered toward the light source side is targeted for measurement, so the backscattering method can be applied regardless of the thickness of the measurement object.

[0004] Japanese Unexamined Patent Application Publication No. 2010-71692 Japanese Patent No. 5672104

[0005] Elson, Daniel S., et al. "Ultrasound-mediated optical tomography: a review of current methods." Interface Focus 1.4 632-648 (2011).

[0006] In the backscattering method, the detector is positioned to detect light scattered towards the light source. Placing the light source, detector, and ultrasonic source at different distances increases the size of the optical measurement device. A smaller optical measurement device makes it easier for the operator to carry and increases the freedom of movement for the subject to whom the device is attached. Therefore, there is a need for optical measurement devices in which the light source, detector, and ultrasonic source are positioned in close proximity.

[0007] Here, the size of the area where the ultrasound is focused is proportional to the diameter of the ultrasound source and the distance from the ultrasound source to the focus point (focal length). Therefore, by using an ultrasound source with a short focal length, the ultrasound can be focused to a smaller area, improving the spatial resolution of UOT measurements. However, when using an ultrasound source with a short focal length, it is necessary to place it close to the surface of the object being measured, which may cause the ultrasound source to overlap with the imaging surface of the detector. In this case, the amount of scattered light that the detector can receive decreases, which may reduce the measurement accuracy of the UOT measurement. As described above, placing the light source, detector, and ultrasound source in close proximity may reduce the spatial resolution and measurement accuracy in UOT measurements.

[0008] This disclosure has been made in light of these circumstances, and its purpose is to improve spatial resolution and measurement accuracy in measurements using ultrasonic modulated optical tomography.

[0009] An optical measuring device according to an aspect of the present disclosure comprises: a light source that irradiates a light scatterer with pulsed laser light; an ultrasonic source that outputs ultrasonic waves to a measurement position at a predetermined depth within the light scatterer; a detector that detects scattered light from the laser light having a scattering angle of 90 degrees or more and passing through a region within the light scatterer including the measurement position and the imaging surface of the light scatterer; a control device that controls the light source and the ultrasonic source; and a base that is arranged in the space from the ultrasonic source to the light scatterer, covers the imaging surface, and is made of a material having a predetermined acoustic impedance value and a predetermined light transmittance.

[0010] According to this disclosure, it is possible to improve the spatial resolution and measurement accuracy in measurements using ultrasonic modulated optical tomography.

[0011] This is a schematic diagram showing the configuration of the optical measuring device according to this embodiment. This is a schematic diagram showing the configuration of the optical measuring device according to Comparative Example 1. This is a schematic diagram showing the configuration of the optical measuring device according to Comparative Example 2. This is a schematic diagram showing the configuration of the optical measuring device according to a modified example. This is a diagram for explaining the focusing of light by the base lens. This is a diagram for explaining the change in the light acquisition angle by the base lens. This is a diagram for explaining the imaging of the detector lens and the power of the acquired light. This is a diagram for explaining the microlens array.

[0012] The embodiments of this disclosure will be described in detail below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and their descriptions will not be repeated.

[0013] [Configuration of the Optical Measurement Device] Figure 1 shows the configuration of an optical measurement device 10 according to an embodiment. Referring to Figure 1, the optical measurement device 10 comprises a laser source 1, an ultrasonic source 2, a camera 3, a control device 4, an analysis device 5, and a base 6. The optical measurement device 10 can detect modulated light generated by irradiating a measurement area affected by ultrasonic waves with laser light, and acquire morphological and physiological information of the object being observed. In practice, the optical measurement device 10 acquires an image of a speckle pattern, which is an aggregate of speckle grains formed by the multiple interference of numerous light waves multiple-scattered inside the object being observed, using the camera 3, and extracts the signal component of the modulated light contained in the speckle pattern using the analysis device 5. Note that the living organism 80 in the embodiment is an example of a light scatterer in this disclosure, and the light scatterer to be measured in this disclosure is not limited to living organisms.

[0014] The optical measurement device according to this embodiment can be applied, for example, to an optical measurement device that minimally invasively measures a subject's brain activity using near-infrared spectroscopy (NIRS). In addition to optical measurement devices that measure brain activity using near-infrared spectroscopy, the optical measurement device according to this embodiment can also be applied to measurement devices that measure blood oxygen saturation, etc. Oxygen saturation can be estimated, for example, by using the relationship between the absorption spectra of oxygenated hemoglobin (hemoglobin bound to oxygen) and deoxygenated hemoglobin (hemoglobin not bound to oxygen), based on the results measured using lasers of different wavelengths.

[0015] Specifically, brain function measurement using near-infrared light can provide information derived from changes in the light absorption characteristics of brain tissue. Near-infrared light is particularly permeable to water and hemoglobin among infrared rays. The wavelength of the near-infrared laser light related to this disclosure is, for example, 700 nm to 850 nm. Near-infrared light irradiated from the scalp passes through brain tissue, scatters within the biological tissue, and is emitted from the scalp. Near-infrared spectroscopy utilizes the fact that hemoglobin in the blood has characteristic absorption bands in this wavelength range to continuously detect quantitative changes in oxygenated hemoglobin, deoxygenated hemoglobin, and total hemoglobin in the blood within the biological tissue. This makes it possible to evaluate the activity level of local brain regions. For example, a brain region with decreased oxygenated hemoglobin can be estimated to be a region where oxygen consumption is increased and activity is high. Furthermore, a brain region with increased oxygenated hemoglobin can be estimated to have increased blood volume to supply oxygen as activity increases. Furthermore, the optical measurement device according to this embodiment can also handle changes in local cerebral blood volume.

[0016] The control device 4 and the analysis device 5 can be configured on a single computer (not shown), and external devices such as memory and printers can be connected as needed. Furthermore, the laser source 1, ultrasonic source 2, camera 3, and base 6 can be integrated, and these can be integrated with the control device 4 so that they can be worn by the subject.

[0017] The laser source 1 is a light source that irradiates the biological tissue 80, which is the object of observation, with laser light, and is, for example, a semiconductor laser element. In this embodiment, the laser source 1 is controlled to generate pulsed laser light. The laser source 1 irradiates the biological tissue 80 with laser light in the near-infrared wavelength range (for example, 780, 805, 830 nm, etc.), which has high penetrability to the biological tissue 80. As shown in Figure 1, the laser light irradiated from the laser source 1 onto the biological tissue 80 is scattered by the tissue within the biological tissue 80 and reaches the measurement position. The laser source 1 can also irradiate pulsed laser light of a duration of several nanoseconds to several microseconds. The laser source 1 in this embodiment corresponds to the light source in this disclosure.

[0018] The ultrasonic source 2 is an ultrasonic generator that outputs ultrasonic waves towards a measurement position at a predetermined depth within the living body 80. The ultrasonic source 2 is equipped with a focuser 21 for focusing the emitted ultrasonic waves at the measurement position within the living body 80. The ultrasonic waves are focused at the point where the ultrasonic waves emitted from the multiple ultrasonic transducers of the ultrasonic source 2 are in focus. In the ultrasonic focus region where the ultrasonic waves are focused, the ultrasonic waves and laser light interact strongly, generating modulated light. Therefore, by photodetecting the modulated light generated by this interaction, it is possible to obtain information at the measurement position within the living body 80. The ultrasonic waves emitted from or output from the ultrasonic source 2 may be continuous ultrasonic waves or pulsed ultrasonic waves.

[0019] Camera 3 includes an image sensor 31 for detecting laser light from the measurement position and a lens 32 for forming an image on the image sensor 31. In the optical measurement device 10, because it is necessary to photograph speckle patterns, an image sensor 31 such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) sensor, which is a multi-element photodetector, is used instead of a single-element photodetector such as a photomultiplier tube. By using a CCD sensor or CMOS sensor for the image sensor 31, the manufacturing cost of the optical measurement device 10 can be reduced. The pixel size of the image sensor 31 is preferably smaller than the average size of the speckle grain to be photographed. In this embodiment, camera 3 corresponds to the detector in this disclosure.

[0020] Image sensors 31, such as CCD sensors, typically have frame rates ranging from tens to hundreds of fps. In contrast, the frequency of ultrasound is very fast, at several MHz, and image sensors 31 cannot keep up with the changes in ultrasound. Therefore, the optical measuring device 10 uses a stroboscopic method to irradiate the tissue within the living organism 80, where the ultrasound is being emitted, with pulsed laser light, and captures a speckle pattern with the exposed image sensor 31.

[0021] Furthermore, it is also possible to set the exposure time of the image sensor 31 to be longer than the period of the ultrasound and use the acquired speckle pattern for measurement. In this case, the pulse duration of the laser light is limited by the distance the ultrasound travels within that duration. If this duration is increased, the spatial resolution in the direction of ultrasound propagation may decrease, so it is preferable to keep it to a few microseconds or less.

[0022] In this embodiment, camera 3 detects scattered light that has been scattered by the living organism 80, has a scattering angle of 90 degrees or more, and has passed through the imaging surface on the surface of the living organism 80. In this disclosure, "scattering angle" refers to the angle between the laser light incident on the living organism 80 and the laser light emitted from the living organism 80. For example, consider the case where laser light incident on the living organism 80 is scattered twice by the living organism 80. Even if the angle between the laser light incident on the living organism 80 and the laser light scattered by the first scattering is less than 90 degrees, if the angle between the laser light incident on the living organism 80 and the laser light emitted from the living organism 80 is 90 degrees or more due to the second scattering, then the scattering angle of the laser light can be said to be 90 degrees or more. The light detected by camera 3 is a combination of laser light that has passed through the region where ultrasound is present and been modulated by ultrasound (signal), and laser light that has not passed through the region where ultrasound is present and has not been modulated by ultrasound (background).

[0023] Camera 3 receives light that has passed through the imaging surface of the biological tissue 80. Note that, in order to acquire the speckle pattern, the lens 32 of camera 3 does not need to form an image on the imaging surface; it does not need to form an image on the imaging surface, and camera 3 does not need to include the lens 32. The amount of light received by the image sensor in cases where an image is formed on the imaging surface and cases where it is not will be described later.

[0024] In this embodiment, the camera 3 is positioned to detect scattered light that is scattered by the living body 80 and has a scattering angle of 90 degrees or more. Therefore, the laser source 1 and the camera 3 are positioned in the same direction with respect to the living body 80, and the camera 3 does not detect light that is irradiated from the laser source 1, passes through the living body 80, and is emitted from the living body 80 in the same direction as the laser light irradiated from the laser source 1.

[0025] Furthermore, if the laser source 1, ultrasonic source 2, and camera 3 are placed at separate locations, the size of the optical measuring device 10 will increase accordingly. Therefore, it is preferable that the laser source 1, ultrasonic source 2, and camera 3 be placed in close proximity. For example, the laser source 1, ultrasonic source 2, and camera 3 are placed on the same plane. Note that the irradiation surface of the laser light, the irradiation surface of the ultrasonic waves, and the imaging surface of the camera 3 are small relative to the living body 80 and can therefore be considered as planes. Thus, it can be said that the irradiation surface of the laser light, the irradiation surface of the ultrasonic waves, and the imaging surface of the camera 3 are on the same plane.

[0026] The control device 4 controls the timing of irradiating laser light from the laser source 1 and the timing of emitting or outputting ultrasound from the ultrasound source 2. When outputting pulsed ultrasound from the ultrasound source 2, a delay time is required for the pulsed ultrasound incident from the surface of the living body 80 to reach the measurement position. Therefore, the control device 4 controls the timing of emitting ultrasound from the ultrasound source 2 so that the pulsed ultrasound reaches the measurement position in sync with the timing when the pulsed laser light emitted from the laser source 1 reaches the measurement position.

[0027] The analysis device 5 extracts the modulated signal component modulated by ultrasound based on the detection signal of the laser light detected by the camera 3. Specifically, it compares the speckle pattern obtained when ultrasound is irradiated with the speckle pattern obtained when ultrasound is not irradiated to acquire information about the biological tissue 80 in the ultrasound-irradiated area.

[0028] The base 6 is positioned in the space between the ultrasonic source 2 and the living body 80 and has a shape that covers the imaging surface. The base 6 includes an observation surface 61 and a laser beam passage 62. The base 6 serves as a medium for propagating ultrasound from the ultrasonic source 2 to the living body 80. It also has a shape that covers the imaging surface, and scattered light emitted from the imaging surface passes through the observation surface 61 of the base 6 before being detected by the camera 3.

[0029] The base 6 is preferably made of a material with an acoustic impedance closer to the acoustic impedance of the biological tissue 80 than to the acoustic impedance of air. When ultrasound enters layers with different acoustic impedances, it is reflected at the interface. This causes the ultrasound to attenuate. As described above, by making the base 6 of a material with an acoustic impedance closer to the acoustic impedance of the biological tissue 80 than to the acoustic impedance of air, it is possible to prevent the ultrasound from being reflected and attenuated at the interface. Furthermore, the base 6 is preferably made of a material with high light transmittance, for example, a material with a light transmittance of 90% or more. The material of the base 6 is, for example, a resin such as cycloolefin polymer (COP) or acrylic.

[0030] The observation surface 61 is the surface through which scattered light emitted from the imaging surface passes before being detected by the camera 3. The observation surface 61 is configured to be perpendicular to the optical axis of the lens 32. Furthermore, it is preferable to polish the observation surface 61 to improve its surface accuracy in order to prevent light scattering on the observation surface 61, and to apply an anti-reflection (AR) coating to prevent light reflection on the observation surface 61.

[0031] The laser beam path 62 is the space through which the laser beam emitted from the laser source 1 passes. The laser beam path 62 corresponds to the optical path of the laser beam. The area around the laser beam path 62 is configured to have a lower light transmittance compared to the material of the base 6. This prevents stray light from the laser beam emitted from the laser source 1 from passing through the base 6 without entering the living body 80 and being detected by the camera 3. If laser beam that has not passed through the living body 80 enters the camera 3, the detection sensitivity of the modulated light may decrease. Therefore, by preventing stray light, the detection sensitivity in measurements in UOT can be improved. For example, the laser beam path 62 is a cavity, and black paint is applied around the cavity to prevent the laser beam from entering the base 6. Note that the laser beam path 62 does not need to be a cavity; it may be filled with a material that allows laser beams to pass through, such as COP and resins such as acrylic.

[0032] [Comparative Example] Many light-based methods have been developed as minimally invasive techniques for measuring tissues within light-scattering bodies such as living organisms. For example, by irradiating light from outside the body and measuring the light that propagates through the body and is emitted, biological information such as morphological information and metabolic information (such as blood oxygen saturation) of tissues within the body can be obtained. However, since tissues within the body are light-scattering media, the light irradiated from outside the body is diffused by the tissues within the body, resulting in poor spatial resolution and inability to measure deep tissues. Therefore, optical measurement devices using UOT, which combine ultrasound that propagates through living organisms with low scattering and light, have been developed. In UOT, biological information is obtained by measuring light modulated by ultrasound.

[0033] UOT (Ultraviolet Optical Threshold) has two methods: the transmission method, which detects light that has passed through the object being measured, and the backscattering method, which detects light that has been scattered within the object and then scattered back towards the light source after being irradiated from a light source and incident on the object. In the backscattering method, the detector measures scattered light with a scattering angle of 90 degrees or more. For example, consider measuring a target area at a depth of 15 mm from the surface of an object with a thickness of 120 mm. In the backscattering method, the laser light incident on the object passes through the target area, is scattered, and exits the object. Therefore, the laser light only needs to propagate for a length of about 30 mm inside the object. On the other hand, in the transmission method, the laser light needs to pass through the target area and through the object, so it needs to propagate for a length of about 120 mm, which is the thickness of the object. As the propagation length increases, the laser light attenuates, which may reduce the measurement accuracy of the UOT. Therefore, the backscattering method, whose measurement accuracy does not depend on the thickness of the object being measured, can measure a wider range of objects compared to the transmission method.

[0034] In the backscattering method, the detector is positioned to detect light scattered towards the light source. If the light source, detector, and ultrasonic source are placed far apart, the size of the optical measurement device increases accordingly. Considering the ease of transporting the device by the operator and the freedom of movement for the subject when attached, there is a need to place the light source, detector, and ultrasonic source in close proximity to minimize the size of the optical measurement device.

[0035] Figure 2 shows the configuration of the optical measuring device 10A according to Comparative Example 1. The optical measuring device 10A differs from the optical measuring device 10 in that it does not have a base portion 6.

[0036] In UOT (Ultrasonic Optical Threshold), biological information is acquired by measuring modulated light in the ultrasonic focusing region. Therefore, the smaller the ultrasonic focusing region, the higher the spatial resolution of the UOT measurement. The size of the region where the ultrasound is focused is proportional to the distance from the end face of the ultrasonic source 2 to the focusing position. For this reason, in the optical measurement device 10A, the closer the ultrasonic source 2 is to the living body 80, the higher the spatial resolution of the UOT measurement. On the other hand, as shown in Figure 2, when the ultrasonic source 2 approaches the living body 80, a part of the ultrasonic source 2 may enter between the camera 3 and the living body 80. This reduces the imaging surface that the camera 3 can detect light from, which may decrease the measurement accuracy of the UOT. For this reason, if the laser source 1, ultrasonic source 2, and camera 3 are placed in close proximity to reduce the size of the optical measurement device 10A, the spatial resolution and measurement accuracy may decrease.

[0037] Therefore, in order to place the ultrasonic source 2 at a distance from the living body 80 while preventing attenuation of the ultrasonic waves, it is conceivable to place a base made of a predetermined material in the space through which the ultrasonic waves propagate from the ultrasonic source 2. The predetermined material is a material that can propagate the ultrasonic waves output from the ultrasonic source 2 to the living body 80 without attenuation, and is, for example, a material having an acoustic impedance that is closer in value to the acoustic impedance of the living body 80 than to the acoustic impedance of air. Figure 3 is a diagram showing the configuration of the optical measuring device 10B according to Comparative Example 2. The optical measuring device 10B differs from the optical measuring device 10 in that it has a base 6A with a different shape from the base 6.

[0038] By positioning the base 6A between the ultrasonic source 2 and the living body 80, attenuation of the ultrasound during propagation from the ultrasonic source 2 to the living body 80 can be prevented. Therefore, the ultrasonic source 2 can be positioned further away from the living body 80 compared to the optical measuring device 10A. However, scattered light emitted from the living body 80 may be refracted at the interface of the base 6A. This refracted light may distort the speckle pattern obtained by the camera 3. Therefore, in the optical measuring device 10B shown in Figure 3, it is necessary to set the imaging surface so that it does not overlap with the base 6A, and the imaging surface may not be able to be sufficiently widened.

[0039] [Optical measuring device according to this embodiment] The optical measuring device 10 according to this embodiment is used in UOT to detect scattered light from laser light irradiated by a light source, which has a scattering angle of 90 degrees or more and has passed through the imaging surface of a living body 80, and is arranged in the space from the ultrasonic source 2 to the living body 80, covers the imaging surface, and comprises a base 6 made of a material having a predetermined acoustic impedance value and a predetermined light transmittance.

[0040] It is preferable that the predetermined acoustic impedance value is closer to the acoustic impedance value of the living body 80 than to the acoustic impedance value of air. By having the predetermined acoustic impedance value closer to the acoustic impedance value of the living body 80 than to the acoustic impedance value of air, the sound pressure of the ultrasound incident on the living body 80 can be increased. By increasing the sound pressure of the ultrasound, the laser light can be modulated more at the measurement position within the living body 80. This makes it easier for the camera 3 to detect the modulated light. Furthermore, by having the predetermined acoustic impedance value closer to the acoustic impedance value of the living body 80 than to the acoustic impedance value of air, the size of the area in which the ultrasound is focused can be reduced, thereby improving the spatial resolution in measurements by UOT.

[0041] The specified light transmittance is not limited to a specific value as long as scattered light is transmitted, but for example, it is 90% or more.

[0042] According to the optical measuring device 10 of this embodiment, by arranging the base 6 in the space from the ultrasonic source 2 to the living body 80, it is possible to prevent attenuation of the ultrasonic waves output from the ultrasonic source 2. This makes it possible to improve the spatial resolution in UOT.

[0043] Furthermore, in the optical measuring device 10 according to this embodiment, the imaging surface is covered by the base portion 6. Therefore, scattered light is refracted at the interface of the base portion 6, preventing distortion of the speckle pattern. As a result, the imaging surface can be enlarged compared to the optical measuring device 10B according to Comparative Example 2, and the measurement accuracy of measurements in UOT can be improved.

[0044] [Modification] In measurement by UOT, the greater the amount of detected light, the higher the measurement accuracy. To increase the amount of detected light, it is possible to increase the distance between the light scatterer and the detector, but this increases the size of the detector, which may lead to an increase in the overall size of the optical measurement device. The base of the optical measurement device according to the modification includes a lens. This makes it possible to collect more light onto the image sensor without increasing the size of the optical measurement device.

[0045] FIG. 4 is a diagram showing the configuration of an optical measurement device 10C according to a modification. A base 6B included in the optical measurement device 10C includes a lens 63 in addition to an observation surface 61 and a laser beam passage 62.

[0046] The lens 63 collects the scattered light that has passed through the imaging surface onto the camera 3. It is preferable that the optical axis of the lens 63 coincides with the optical axis of the lens 32 of the camera 3. Note that in FIG. 4, the lens 63 is provided on the observation surface 61, but the base 6B does not necessarily need to include the observation surface 61. Furthermore, the lens 63 may be integrated with the base 6B or may be separable. When the lens 63 is integrated with the base 6B, it is assumed that it is formed of the same material as the base 6B, but the material of the lens 63 is not limited as long as it can collect the scattered light that has passed through the imaging surface onto the camera 3. It is preferable that the surface of the lens 63 is AR-coated.

[0047] According to the optical measurement device 10C according to the modification, scattered light can be collected without increasing the size of the camera 3. This makes it possible to improve the UOT measurement accuracy and reduce the size of the optical measurement device.

[0048] Note that in the modification described above, an example in which the base 6B includes one lens has been described, but the number of lenses included in the base 6B is not limited to one. For example, the base 6B may include a microlens array in which a plurality of fine lenses are integrated. Adopting a microlens array makes it possible to increase the curvature of a curved surface. This makes it possible to collect more scattered light and improve the measurement accuracy in UOT.

[0049] The following example shows how to verify the amount of light detected by an optical measuring device using a ray tracing model based on Monte Carlo simulation.

[0050] Using Figures 5 and 6, we will explain the difference in the amount of light detected by an optical measuring device depending on the presence or absence of a base and the presence or absence of a lens at the base. Figure 5 is a diagram to illustrate each model, and Figure 6 is a diagram to explain the light acquisition angle due to the refraction of light on the surface of the sample, which is the object to be measured, facing the sensor in Figures 5(A) to (C).

[0051] As shown in Figure 5, the light intensity of the laser beam detected by the sensor and the magnification of the light source are determined in three patterns: (A) without a base, (B) with a base but without a lens, and (C) with a base 6B. The created model is set to have an F-number and magnification close to the actual imaging conditions. The wavelength of the light is 785 nm, and the optical properties of the sample to be measured are set to a scattering coefficient μs = 2.00 mm⁻¹, an absorption coefficient μa = 0.04 mm⁻¹, anisotropy parameter g = 0.631, and refractive index n = 1.37. The sample is illuminated with a uniform surface light source, and the light scattered within the sample is set to form an image on the sensor. The material of the base is COP. The sensor's entrance aperture is kept constant according to the required specifications in order to determine the particle size of the speckle pattern to be captured. The position of the sample is adjusted so that it is imaged on the surface of the sample facing the sensor. The magnification is determined by assuming that there is a surface light source on the sample surface, and from the ratio of the size of the surface light source to the size of the image of the surface light source formed on the sensor.

[0052] In Figure 5, in case (A), the amount of light received by the sensor is 6.02e-4W, and the magnification is 0.32. In case (B), the amount of light received by the sensor is 5.57e-4W, and the magnification is 0.32. In case (C), the amount of light received by the sensor is 6.57e-4W, and the magnification is 0.42.

[0053] Comparing (A) and (B), the amount of light detected by the sensor in (B) is less than in (A). This is thought to be because, as shown in Figure 6, the angle of light acquisition from the sample becomes smaller due to the refraction at the interface between the air and the base. In addition, Fresnel reflection occurs due to the difference in refractive index between the base and the air, and the amount of light transmitted through the base is thought to be another reason why the amount of light detected by the sensor in (B) is lower than in (A).

[0054] (C) shows a greater amount of light detected by the sensor compared to (A) and (B). As shown in Figure 6, the inclusion of a lens in the base increases the angle of light acquisition from the sample due to the difference in refractive index between the sample and the base. This is thought to increase the amount of light received by the sensor. Furthermore, the inclusion of a lens in the base reduces the difference in refractive index between the base and the sample to lessen the difference in refractive index between the sample and air, thus reducing the effect of Fresnel reflection between the sample and the base. Additionally, applying an AR coating to the lens portion of the base can suppress Fresnel reflection at the interface between the base and air. Therefore, when the base includes a lens, the amount of light received by the sensor increases compared to when there is no base or when there is a base but it does not include a lens. An increase in the amount of light received by the sensor indicates improved measurement accuracy in UOT measurements.

[0055] Figure 7 illustrates the difference in the amount of light received by the sensor when acquiring a speckle pattern using UOT, depending on whether the sensor is in focus on the sample surface or not.

[0056] As shown in Figure 7, there is no difference in light intensity when the sensor is in focus on the sample surface compared to when it is not. This indicates that imaging on the sample surface is not necessary when acquiring the speckle pattern. This shows that it is possible to position the sensor and the sample closer than the position where imaging on the sample surface is required. The closer the sensor and sample are positioned, the smaller the device can be made.

[0057] Figure 8 illustrates the amount of light received by the sensor when a microlens array is provided on the observation surface of the base. As shown in Figure 8, the amount of light received by the sensor is analyzed when a microlens array with r=1 and half width 0.5 is installed. As a result, the amount of light increases by 16% compared to when there is no base. In addition, when the base includes a microlens array, the distance between the sensor and the sample can be shortened by about 10 mm compared to when there is no base. Therefore, by providing a base that includes a microlens array, the optical measurement device can be made more compact.

[0058] [Aspects] The above-described exemplary embodiments will be understood by those skilled in the art to be specific examples of the following aspects.

[0059] (Section 1) An optical measuring device in one embodiment may include: a light source that irradiates a light scatterer with pulsed laser light; an ultrasonic source that outputs ultrasonic waves to a measurement position at a predetermined depth within the light scatterer; a detector that detects scattered light from the laser light having a scattering angle of 90 degrees or more and passing through a region within the light scatterer including the measurement position and an imaging surface on the surface of the light scatterer; a control device that controls the light source and the ultrasonic source; and a base that is arranged in the space from the ultrasonic source to the light scatterer, covers the imaging surface, and is made of a material having a predetermined acoustic impedance value and a predetermined light transmittance.

[0060] According to the optical measuring device described in paragraph 1, the spatial resolution and measurement accuracy in measurements using UOT can be improved.

[0061] (Paragraph 2) In the optical measuring device described in Paragraph 1, the predetermined acoustic impedance value may be closer to the acoustic impedance value of the light scatterer than to the acoustic impedance value of air.

[0062] According to the optical measuring device described in Section 2, the difference in acoustic impedance between the base and the light scatterer is reduced, which suppresses the reflection of ultrasound at the interface when ultrasound propagates from the base to the light scatterer. This increases the sound pressure of the ultrasound incident on the object being measured, making it easier to detect modulated light. Furthermore, the size of the area where the ultrasound is focused can be reduced, improving the spatial resolution in measurements using UOT.

[0063] (Paragraph 3) In the optical measuring device described in Paragraph 1 or Paragraph 2, the predetermined light transmittance may be 90% or more.

[0064] According to the optical measuring device described in paragraph 3, it is possible to prevent the attenuation of scattered light incident on the base, thereby improving the measurement accuracy in measurements using UOT.

[0065] (Paragraph 4) In the optical measuring device described in any one of paragraphs 1 to 3, the material may include a cycloolefin polymer or acrylic.

[0066] According to the optical measuring device described in paragraph 4, ultrasonic waves can be propagated from the ultrasonic source to the light scatterer without attenuation, and the attenuation of scattered light can be prevented. This improves the spatial resolution and measurement accuracy in measurements using UOT.

[0067] (Clause 5) In the base of the optical measuring device described in any one of paragraphs 1 to 4, the light transmittance of the side surface of the optical path of the laser light from the light source to the light scatterer may be less than the light transmittance of the base.

[0068] According to the optical measuring device described in paragraph 5, it is possible to prevent laser light emitted from a light source from passing through the base and being detected by the detector without entering the light scatterer.

[0069] (Clause 6) In the optical measuring device described in any one of paragraphs 1 to 5, the detector includes a first lens, and the base may have an observation surface perpendicular to the optical axis of the first lens.

[0070] According to the optical measuring device described in paragraph 6, scattered light emitted from the base to the detector is prevented from being reflected or refracted at the interface of the base, thereby increasing the amount of light detected by the detector.

[0071] (Clause 7) In the optical measuring device described in paragraph 6, the observation surface may have a predetermined surface accuracy.

[0072] According to the optical measuring device described in paragraph 7, scattered light emitted from the base to the detector is prevented from being refracted at the interface of the base, thereby increasing the amount of light detected by the detector.

[0073] (Clause 8) In the optical measuring device described in any one of paragraphs 1 to 7, the detector includes a first lens, the base includes a second lens, and the optical axis of the second lens may coincide with the optical axis of the first lens.

[0074] According to the optical measuring device described in paragraph 8, the amount of light focused by the detector can be increased, thereby improving the measurement accuracy in UOT measurements. Furthermore, since the amount of light focused by the detector can be maintained even when the size of the detector is reduced, the size of the optical measuring device can be reduced.

[0075] (Clause 9) In the optical measuring device described in paragraph 8, the second lens may have an anti-reflective coating.

[0076] According to the optical measuring device described in paragraph 9, reflection in the second lens can be prevented, and the amount of light focused by the detector can be increased.

[0077] (Clause 10) In the optical measuring device described in any one of paragraphs 1 to 9, the base may include a microlens array that focuses the scattered light onto the detector.

[0078] According to the optical measuring device described in paragraph 10, the amount of light collected by the detector can be increased, thereby improving the measurement accuracy in UOT.

[0079] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than by the description of the embodiments above, and all modifications within the meaning and scope of the claims are intended to be included. Furthermore, each technique in the embodiments is intended to be practiced individually or, as far as possible, in combination with other techniques in the embodiments.

[0080] 1. Laser source, 2. Ultrasonic source, 3. Camera, 4. Control device, 5. Analysis device, 6, 6A, 6B. Base, 10, 10A, 10B, 10C. Optical measuring device, 21. Focuser, 31. Image sensor, 32, 63. Lens, 61. Observation surface, 62. Laser beam path.

Claims

1. An optical measuring device comprising: a light source that irradiates a light scatterer with pulsed laser light; an ultrasonic source that outputs ultrasonic waves to a measurement position at a predetermined depth within the light scatterer; a detector that detects scattered light from the laser light having a scattering angle of 90 degrees or more, passing through a region within the light scatterer including the measurement position and an imaging surface on the surface of the light scatterer; a control device that controls the light source and the ultrasonic source; and a base disposed in the space from the ultrasonic source to the light scatterer, covering the imaging surface, and made of a material having a predetermined acoustic impedance value and a predetermined light transmittance.

2. The optical measuring device according to claim 1, wherein the predetermined acoustic impedance value is closer to the acoustic impedance value of the light scatterer than the acoustic impedance value of air.

3. The optical measuring device according to claim 1, wherein the predetermined light transmittance is 90% or more.

4. The optical measuring device according to claim 1, wherein the material comprises a cycloolefin polymer or acrylic.

5. The optical measuring device according to claim 1, wherein, in the base portion, the optical transmittance of the side surface of the optical path of the laser light from the light source to the light scatterer is smaller than the optical transmittance of the base portion.

6. The optical measuring device according to claim 1, wherein the detector includes a first lens, and the base has an observation surface perpendicular to the optical axis of the first lens.

7. The optical measuring device according to claim 6, wherein the observation surface has a predetermined surface accuracy.

8. The optical measuring device according to claim 1, wherein the detector includes a first lens, the base includes a second lens, and the optical axis of the second lens coincides with the optical axis of the first lens.

9. The optical measuring device according to claim 8, wherein the second lens has an anti-reflective coating.

10. The optical measuring device according to claim 1, wherein the base includes a microlens array for focusing the scattered light onto the detector.

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