Blood flow measurement device

Abstract

Provided is a blood flow measurement device exhibiting excellent measurement accuracy. The blood flow measurement device comprises: a light source unit that projects near-infrared radiation onto an object; and a light-receiving unit that receives scattered light resulting from the near-infrared radiation emitted from the light source unit being scattered by the object. The blood flow measurement device further comprises: a first polarizing element that is disposed on a front surface of the light source unit, that includes a layer formed of a liquid crystal compound, and that changes the polarization state of the near-infrared radiation; and a second polarizing element that is disposed on a front surface of the light-receiving unit, that includes a layer formed of a liquid crystal compound, and that changes the polarization state of the near-infrared radiation.

Description

blood flow measuring device

[0001] The present invention relates to a blood flow measuring device.

[0002] It is known that measuring blood flow in the brain, muscles, organs, etc. of a living body can be applied to diagnosing bodily functions, health management, and mediating information between living bodies and devices. In particular, with regard to the brain, a device has been proposed that provides a cerebral blood flow measurement device called a headset with a near-infrared irradiator and a near-infrared detector to detect changes in blood flow on the brain surface, and processes the detected data in a data processor to obtain information indicating the brain's activity state.

[0003] For example, Patent Document 1 describes a blood flow measuring device that includes a first main body portion, a second main body portion, and a hinge, where the first main body portion has a first housing including a first bottom surface, a light source that irradiates near-infrared rays from the first bottom surface to the outside of the first housing, and a first light receiving portion that receives near-infrared rays from the first bottom surface side outside the first housing, the second main body portion has a second housing including a second bottom surface and a second light receiving portion that receives near-infrared rays from the second bottom surface side outside the second housing, and the hinge connects the first main body portion and the second main body portion by changing the angle between the first bottom surface and the second bottom surface.

[0004] Japanese Patent Application Laid-Open No. 2020-054649

[0005] Such blood flow measuring devices obtain information about blood flow by, for example, detecting near-infrared rays that are partially absorbed and scattered by blood vessels (blood). Because the near-infrared rays irradiated for measurement are scattered, the detected near-infrared rays are weak. Furthermore, because the irradiated near-infrared rays are reflected by areas other than the measurement area, such as the surface of the body, the near-infrared rays reflected by areas other than the measurement area are detected as noise components. Conventional blood flow measuring devices have had problems with low signal-to-noise ratios and poor measurement accuracy because it is difficult to distinguish between the near-infrared rays to be detected and the near-infrared rays that constitute noise components.

[0006] An object of the present invention is to solve the problems of the prior art and to provide a blood flow measuring device with excellent measurement accuracy.

[0007] In order to solve this problem, the present invention has the following configuration. [1] A blood flow measurement device including a light source unit that irradiates an object with near-infrared rays and a light receiving unit that receives scattered light generated when the near-infrared rays emitted from the light source unit are scattered by the object, the blood flow measurement device further including: a first polarizing element that is arranged in front of the light source unit and that changes the polarization state of the near-infrared rays, the first polarizing element including a layer formed using a liquid crystal compound; and a second polarizing element that is arranged in front of the light receiving unit and that changes the polarization state of the near-infrared rays, the second polarizing element including a layer formed using a liquid crystal compound. [2] The blood flow measurement device according to [1], wherein the layer formed using a liquid crystal compound contained in the first polarizing element is a linear polarizer. [3] The blood flow measurement device according to [2], wherein the first polarizing element further includes a λ / 4 plate. [4] The blood flow measurement device according to [3], wherein the λ / 4 plate exhibits reverse wavelength dispersion. [5] The blood flow measuring device according to [1], wherein the first polarizing element has a first linear polarizer, a retardation layer, and a second linear polarizer in this order, and at least one of the first linear polarizer and the second linear polarizer is a layer formed using a liquid crystal compound. [6] The blood flow measuring device according to [5], wherein the retardation plate exhibits reverse wavelength dispersion. [7] The blood flow measuring device according to any one of [1] to [6], wherein the layer formed using the liquid crystal compound contained in the first polarizing element has a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane. [8] The blood flow measuring device according to any one of [1] to [7], wherein the liquid crystal compound is a rod-shaped liquid crystal compound or a discotic liquid crystal compound.

[0008] According to the present invention, it is possible to provide a blood flow measuring device with excellent measurement accuracy.

[0009] FIG. 1 is a diagram conceptually showing an example of a blood flow measuring device of the present invention. FIG. 2 is a conceptual diagram showing a part of an example of a blood flow measuring device of the present invention. FIG. 3 is a conceptual diagram showing a part of another example of the blood flow measuring device of the present invention. FIG. 4 is a conceptual diagram showing a part of another example of the blood flow measuring device of the present invention. FIG. 5 is a diagram conceptually showing a liquid crystal diffraction element included in a first polarizing element of the blood flow measuring device shown in FIG. 5. FIG. 6 is a plan view of the liquid crystal diffraction element shown in FIG. 7. FIG. 8 is a conceptual diagram for explaining the action of the liquid crystal diffraction element shown in FIG. 8. FIG. 9 is a diagram schematically showing an example of an exposure device that exposes an alignment film of the liquid crystal diffraction element shown in FIG. 6. FIG. 10 is a diagram conceptually showing another example of the liquid crystal diffraction element. FIG. 11 is a diagram conceptually showing another example of the liquid crystal diffraction element.

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A blood flow measuring device according to the present invention will be described in detail below with reference to preferred embodiments shown in the accompanying drawings.

[0011] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0012] [Blood flow measuring device] The blood flow measuring device of the present invention is a blood flow measuring device comprising a light source unit that irradiates an object with near-infrared rays and a light receiving unit that receives scattered light generated when the near-infrared rays emitted from the light source unit are scattered by the object, and further comprising: a first polarizing element that is arranged in front of the light source unit and includes a layer formed using a liquid crystal compound, and that changes the polarization state of the near-infrared rays; and a second polarizing element that is arranged in front of the light receiving unit and includes a layer formed using a liquid crystal compound, and that changes the polarization state of the near-infrared rays.

[0013] Fig. 1 conceptually shows an example of a blood flow measuring device of the present invention. The blood flow measuring device 100 shown in Fig. 1 is a device that acquires information about blood flow by irradiating a living body with near-infrared rays and detecting the near-infrared rays reflected by the living body. The blood flow measuring device 100 shown in Fig. 1 includes a control unit 102, a light source unit 104, a first polarizing element 106, a light receiving unit 108, a second polarizing element 110, and a housing 112.

[0014] <Control Unit> The control unit 102 functions as a support substrate that supports the light source unit 104 and the first polarizing element 106, as well as the light receiving unit 108 and the second polarizing element 110, and also performs measurement control and data processing in the blood flow measuring device 100. That is, the control unit 102 controls the timing of irradiation of near-infrared rays by the light source unit 104, the amount of light, etc., and also performs various processes on data obtained by receiving light by the light receiving unit 108 to calculate the amount of change in blood flow, the pulse rate, etc. The pulse rate corresponds to the heart rate.

[0015] The control unit 102 has a processor such as a CPU (Central Processing Unit) or a DSP (Digital Signal Processor) and a memory, and executes processing using computer programs, firmware, etc. that are executablely deployed on the memory. The control unit 102 may be a dedicated hardware circuit, FPGA (Field Programmable Gate Array), etc. that activates the light source unit 104 and the light receiving unit 108 and executes cooperative processing with each component.

[0016] As shown in FIG. 1, the control unit 102 includes a light source unit 104 and a light receiving unit 108 arranged on the surface of the control unit 102 at a predetermined distance d in the planar direction.

[0017] In the illustrated example, the control unit 102 is configured to also function as a support substrate that supports the light source unit 104 and the first polarizing element 106, as well as the light receiving unit 108 and the second polarizing element 110, but this is not limited to this, and the support substrate that supports the light source unit 104 and the first polarizing element 106, as well as the light receiving unit 108 and the second polarizing element 110, and the control unit 102 may be separate components.

[0018] <Light Source Unit> The light source unit 104 is for irradiating near-infrared rays to the living body S. The light source unit 104 includes a near-infrared light source that irradiates near-infrared rays. The near-infrared light irradiated by the light source unit 104 preferably has a wavelength of 650 nm to 1400 nm.

[0019] As the near-infrared light source, for example, LEDs (Light Emitting Diodes), LDs (Laser Diodes), etc. can be used.

[0020] The light source unit 104 basically irradiates unpolarized near-infrared light. Note that, when the near-infrared light source has a linear polarizer and irradiates linearly polarized near-infrared light, this linear polarizer is considered to be the linear polarizer included in the first polarizing element in the present invention.

[0021] Furthermore, the light source unit 104 may irradiate two or more types of near-infrared rays having different wavelengths. For example, the light source unit 104 may irradiate near-infrared rays having wavelengths of 780 nm and 830 nm. Such a light source unit 104 may be configured to include a plurality of light sources that irradiate near-infrared rays of different wavelengths, or may be configured to irradiate near-infrared rays of different wavelengths by combining a light source that irradiates near-infrared rays over a wide wavelength range with a filter that transmits light in a specific wavelength range.

[0022] <Light Receiving Unit> The light receiving unit 108 receives (detects) near-infrared light reflected inside the body of the living organism S. The light receiving unit 108 includes, for example, a photoelectric conversion element such as a photodiode or phototransistor that outputs a current according to the amount of near-infrared light received, an amplifier circuit that amplifies the output current of the photoelectric conversion element, an AD (Analog-to-digital) converter, etc. The light receiving unit 108 converts the received light into a voltage signal and outputs it as a light detection signal.

[0023] The size of the light receiving unit 108 is not limited as long as it can receive (detect) near-infrared rays reflected inside the body of the living body S, but it is preferable to increase the area and the angle of acceptance in order to obtain high detection sensitivity.

[0024] Furthermore, when the light source unit 104 emits near-infrared rays of two or more different wavelengths, it is preferable that the light receiving unit 108 receives (detects) near-infrared rays for each wavelength. In this case, the light receiving unit 108 may be configured to have a combination of a filter that transmits one wavelength range and blocks the other wavelength range and a photoelectric conversion element, or a combination of a filter that transmits the other wavelength range and blocks one wavelength range and a photoelectric conversion element.

[0025] <First Polarizing Element> The first polarizing element 106 is disposed in front of the light source unit 104, i.e., on the irradiation surface, and is an element that changes the polarization state of near-infrared light emitted from the light source unit 104. The first polarizing element 106 includes a layer formed using a liquid crystal compound. The first polarizing element 106 converts the polarization state of the near-infrared light emitted from the light source unit 104 into linearly polarized or circularly polarized light of a desired polarization state, and allows the light to enter the body of the living organism S. The configuration of the first polarizing element 106 will be described in detail later.

[0026] <Second Polarizing Element> The second polarizing element 110 is disposed on the front surface of the light receiving unit 108, i.e., on the light receiving surface, and is an element that changes the polarization state of near-infrared light that is scattered within the body of the living organism S and enters the light receiving unit 108. The second polarizing element 110 includes a layer formed using a liquid crystal compound. The second polarizing element 110 converts the polarization state of the near-infrared light scattered within the body of the living organism S into linearly polarized or circularly polarized light of a certain polarization state, and allows the light to enter the light receiving unit 108. The configuration of the second polarizing element 110 will be described in detail later.

[0027] In addition to the above-mentioned components, the blood flow measuring device 100 may also have a housing 112 for accommodating the respective components, a holding mechanism such as a band for attaching the device to the head, arm, leg, etc. of a user (living body S), etc. The blood flow measuring device 100 is attached to the head, arm, leg, etc. of the user (living body S) by the holding mechanism with the irradiation surface facing the living body S so that near-infrared rays from the light source unit 104 are irradiated into the living body S, and with the light receiving surface facing the living body S so that the light receiving unit 110 receives near-infrared light scattered within the living body S.

[0028] The operation of the blood flow measuring device 100 will now be described. The blood flow measuring device 100, which is attached to the head, arm, leg, or the like of a living body S, irradiates near-infrared rays from a light source unit 104. The near-infrared rays irradiated from the light source unit 104 are incident on a first polarizing element 106, and the polarization state thereof is changed by the first polarizing element 106 before they enter the living body S. The irradiated near-infrared rays are partially absorbed and scattered, for example, near the cerebral cortex of the brain or near blood vessels in the arm, etc. Some of the scattered near-infrared rays travel toward the light receiving unit 108 and are incident on a second polarizing element 110. The second polarizing element 110 changes the polarization state of the incident near-infrared rays and causes them to enter the light receiving unit 108. The light receiving unit 108 receives the near-infrared rays, converts them into electrical signals, and outputs the electrical signals. The electrical signals (data) output from the light receiving unit 108 are transmitted to a control unit 102. The control unit 102 performs various processes on the received data to calculate blood flow change amounts, pulse rate, and the like.

[0029] For example, in the cerebral cortex of the brain, blood flow varies depending on the brain's activity state. As a result, the amount of oxygenated hemoglobin and the amount of oxygen-unbound hemoglobin in the blood in the cerebral cortex vary depending on the brain's activity state. Furthermore, the absorption or scattering characteristics of near-infrared light near the cerebral cortex change due to changes in the amount of hemoglobin and the amount of oxygen. As a result, the amount of near-infrared light received by the light-receiving unit 108 changes. Therefore, the control unit 102 can obtain information about blood flow near the cerebral cortex (such as changes in blood flow and pulse rate) from data on the amount of near-infrared light received by the light-receiving unit 108.

[0030] Furthermore, changes in the absorption or scattering characteristics of near-infrared rays due to changes in the amount of hemoglobin and oxygen, etc., vary depending on the wavelength. Therefore, for example, in the cerebral cortex of the brain, the change in the amount of near-infrared rays received by the light-receiving unit 108 varies depending on the brain's activity state. That is, the ratio of the amount of light received by the light-receiving unit 108 for each wavelength varies depending on the brain's activity state. Therefore, by configuring the light source unit 104 to irradiate near-infrared rays of two or more different wavelengths and configuring the light-receiving unit 108 to receive light for each wavelength and obtain data on the amount of light for each wavelength, information on blood flow (such as changes in blood flow and pulse rate) can be obtained from data on the ratio of the amount of light received at two (or three or more) wavelengths.

[0031] Here, in a blood flow measurement device that irradiates near-infrared rays into a living body and receives near-infrared rays scattered near blood vessels to obtain blood flow information, because the scattered near-infrared rays are received, the amount of near-infrared light received by the light-receiving unit is weak, approximately 1 / 100 to 1 / 1000 of the amount of the irradiated near-infrared rays. Furthermore, the near-infrared rays irradiated from the light source unit are also reflected by areas other than the measurement area, such as the surface of the body and the interfaces of organs. When such near-infrared rays reflected by areas other than the measurement area are received by the light-receiving unit, they become unwanted noise components. Conventional blood flow measurement devices have difficulty distinguishing between the near-infrared rays to be detected and the near-infrared rays that constitute noise components, resulting in a low signal-to-noise ratio and poor measurement accuracy.

[0032] In contrast, the blood flow measuring device 100 of the present invention has a first polarizing element 106 on the front surface of the light source unit 104, which includes a layer formed using a liquid crystal compound and changes the polarization state of near-infrared light, and also has a second polarizing element 110 on the front surface of the light receiving unit 108, which includes a layer formed using a liquid crystal compound and changes the polarization state of near-infrared light.

[0033] The first polarizing element 106 changes the near-infrared light emitted from the light source unit 104 into a predetermined linearly polarized or circularly polarized light. A portion of the polarized light changed by the first polarizing element 106 enters the living body S and is scattered near the blood vessels. At this time, the near-infrared light to be measured is depolarized by the scattering and becomes a polarization state different from the predetermined polarization state, for example, unpolarized. A portion of the scattered near-infrared light to be measured enters the second polarizing element 110. The second polarizing element 110 changes, for example, the unpolarized near-infrared light to a predetermined linearly polarized or circularly polarized light. The polarized near-infrared light to be measured changed by the second polarizing element 110 is received by the light receiving unit 108.

[0034] On the other hand, a portion of the polarized light changed to a predetermined polarization state by the first polarizing element 106 is reflected from areas other than the measurement area, such as the surface of the body and the interfaces of organs. Because polarization is not eliminated by reflection, the polarized light reflected from areas other than the measurement area remains in a certain polarization state. When the near-infrared light reflected from areas other than the measurement area travels toward the light-receiving unit 108, it is incident on the second polarizing element 110 arranged in front of the light-receiving unit 108. As will be described later, the second polarizing element 110 is configured to block polarized light reflected from areas other than the measurement area, i.e., near-infrared light that becomes a noise component, thereby reducing the amount of light received by the light-receiving unit 108.

[0035] In this way, the blood flow measuring device 100 of the present invention can distinguish between the near-infrared light to be detected and the near-infrared light that is a noise component and cut out the noise component, thereby improving the signal-to-noise ratio and improving measurement accuracy.

[0036] Here, in the blood flow measuring device 100 of the present invention, the first polarizing element 106 and the second polarizing element 110 include layers formed using a liquid crystal compound. In the first polarizing element 106 and the second polarizing element 110, the layers formed using a liquid crystal compound are layers for changing the polarization state of incident near-infrared light. Specifically, as described below, the layers formed using a liquid crystal compound are linear polarizers or liquid crystal diffraction elements. The layers formed using a liquid crystal compound can be layers that change the polarization state of near-infrared light with high efficiency. Furthermore, in the case of linear polarizers, they can be made into absorption-type linear polarizers that do not reflect near-infrared light, thereby suppressing the generation of reflected light that could become noise. Therefore, in the blood flow measuring device 100 of the present invention, by configuring the first polarizing element 106 and the second polarizing element 110 to include layers formed using a liquid crystal compound, the above-mentioned effect of cutting noise components by blocking reflected light of near-infrared light whose polarization state has been changed by the first polarizing element 106 with the second polarizing element 110 can be properly exhibited.

[0037] There is no particular limitation on the distance d from the light source unit 104 to the light receiving unit 108. The depth from the surface of the living body S at which blood flow information is obtained varies depending on the distance d, so the distance d can be set depending on the depth at which blood flow information is desired to be obtained.

[0038] The configurations of the first polarizing element 106 and the second polarizing element 110 will be described below.

[0039] FIG. 2 is a conceptual diagram showing a portion of an example of a blood flow measurement device of the present invention. The blood flow measurement device 100a shown in FIG. 2 has a light source unit 104, a first polarizing element 106a, a light receiving unit 108, and a second polarizing element 110a. Note that the control unit, housing, and the like are not shown in the blood flow measurement device 100a shown in FIG. 2. Furthermore, in the blood flow measurement device 100a, the light source unit 104 and the light receiving unit 108 have the same configurations as the light source unit 104 and the light receiving unit 108 described in the blood flow measurement device 100 shown in FIG. 1, and therefore, description thereof will be omitted. This also applies to FIGS. 3 to 5 described below.

[0040] In the blood flow measurement device 100a shown in FIG. 2, the first polarizing element 106a has a linear polarizer 120 as a layer formed using a liquid crystal compound. In a preferred embodiment, the second polarizing element 110a has a linear polarizer 122 as a layer formed using a liquid crystal compound. The linear polarizer 120 of the first polarizing element 106a and the linear polarizer 122 of the second polarizing element 110a are arranged so that their transmission axes are substantially perpendicular to each other. For example, in the example shown in FIG. 2, the transmission axis of the linear polarizer 120 of the first polarizing element 106a may be set to transmit linearly polarized light that oscillates in the left-right direction in the figure, and the transmission axis of the linear polarizer 122 of the second polarizing element 110a may be set to transmit linearly polarized light that oscillates in a direction perpendicular to the plane of the paper in the figure.

[0041] In this blood flow measurement device 100a, when near-infrared light is irradiated from the light source unit 104, the linear polarizer 120 of the first polarizing element 106a converts the near-infrared light into linearly polarized light that oscillates, for example, horizontally in the figure. The near-infrared light linearly polarized by the linear polarizer 120 (first polarizing element 106a) is incident on the living body S. The near-infrared light irradiated on the living body S is partially absorbed and scattered near blood vessels. At this time, the linearly polarized near-infrared light is depolarized and becomes unpolarized. Some of the scattered near-infrared light travels toward the light receiving unit 108 and enters the second polarizing element 110a. The linear polarizer 122 of the second polarizing element 110a converts the incident near-infrared light into linearly polarized light that oscillates in a direction perpendicular to the paper surface and transmits it. The light receiving unit 108 receives the linearly polarized near-infrared light, converts it into an electrical signal, and outputs it to the control unit. The control unit performs various processes on the received data to calculate blood flow change, pulse rate, etc.

[0042] On the other hand, a portion of the near-infrared light converted to linear polarized light by the linear polarizer 120 (first polarizing element 106a) is reflected from areas other than the measurement area, such as the surface of the body and the interfaces of organs. Since the polarization is not eliminated, the light remains linearly polarized, vibrating horizontally in the figure, and is incident on the linear polarizer 122 (second polarizing element 110a). Since the linear polarizer 122 of the second polarizing element 110a has a transmission axis perpendicular to the paper surface, it absorbs the linearly polarized light vibrating horizontally in the figure without transmitting it. This allows the linearly polarized light reflected from areas other than the measurement area, i.e., the near-infrared light that becomes a noise component, to be blocked, thereby suppressing reception of the noise component by the light receiving unit 108.

[0043] Furthermore, as described above, by forming the linear polarizers 120 and 122 using a liquid crystal compound, it is possible to obtain a polarizer with a high degree of polarization with respect to near-infrared light. Furthermore, since it is possible to make the linear polarizers 120 and 122 an absorptive linear polarizer that does not reflect near-infrared light, it is possible to suppress the generation of reflected light that can become noise.

[0044] There are no particular limitations on the orientation of the transmission axes of the linear polarizer 120 and the linear polarizer 122, as long as the transmission axis of the linear polarizer 120 and the transmission axis of the linear polarizer 122 are approximately perpendicular to each other. The transmission axis of the linear polarizer 120 of the first polarizing element 106a is preferably oriented so that the transmitted linearly polarized light becomes p-polarized with respect to the skin surface of the living body S. This makes it possible to suppress reflection on the skin surface.

[0045] Furthermore, in the blood flow measuring device 100a, it is preferable that the near-infrared rays emitted from the light source unit 104 are configured to be incident in a direction inclined with respect to the skin surface of the living body S and directed toward the light receiving unit 108. This allows the amount of near-infrared rays scattered near blood vessels to be received by the light receiving unit 108 to be increased, thereby improving the signal-to-noise ratio and measurement accuracy.

[0046] There are no particular limitations on the method for tilting the near-infrared rays emitted from the light source unit 104 with respect to the skin surface of the living body S, and the light source unit 104 may be arranged on the control unit 102 (support substrate) so that the emission direction of the light source unit 104 is tilted with respect to the control unit 102 (main surface of the support substrate). Alternatively, the light source unit 104 may be configured to have a diffraction element or the like, or the first polarizing element 106a may be configured to have a diffraction element.

[0047] The linear polarizer formed using a liquid crystal compound will be described in detail later.

[0048] Fig. 3 is a conceptual diagram showing a part of another example of the blood flow measuring device of the present invention. The blood flow measuring device 100b shown in Fig. 3 has a light source unit 104, a first polarizing element 106b, a light receiving unit 108, and a second polarizing element 110b. Note that in the blood flow measuring device 100b shown in Fig. 3, the control unit, the housing, etc. are not shown.

[0049] In the blood flow measuring device 100b shown in Fig. 3, the first polarizing element 106b has a linear polarizer 120 as a layer formed using a liquid crystal compound. Furthermore, the first polarizing element 106b has a λ / 4 plate 124 on the side of the linear polarizer 120 opposite the light source unit 104 side. In a preferred embodiment, the second polarizing element 110b has a linear polarizer 122 as a layer formed using a liquid crystal compound. Furthermore, the second polarizing element 110b has a λ / 4 plate 125 on the side of the linear polarizer 122 opposite the light receiving unit 108. In other words, the first polarizing element 106b and the second polarizing element 110b include circular polarizing plates composed of a linear polarizer and a λ / 4 plate.

[0050] The λ / 4 plate 124 of the first polarizing element 106b is disposed so as to convert the near-infrared light, which has been converted into linearly polarized light by the linear polarizer 120, into circularly polarized light. That is, the λ / 4 plate 124 is disposed so that the slow axis is at an angle of approximately 45° (or −45°) with respect to the transmission axis of the linear polarizer 120. Therefore, the first polarizing element 106b converts the near-infrared light emitted from the light source unit 104 into circularly polarized light.

[0051] The λ / 4 plate 125 of the second polarizing element 110b converts circularly polarized light incident from the λ / 4 plate 125 side into linearly polarized light. The λ / 4 plate 125 is also positioned so that its slow axis is at 45° (or −45°) with respect to the transmission axis of the linear polarizer 122. This second polarizing element 110b transmits one of right-handed and left-handed circularly polarized light and blocks the other circularly polarized light. Specifically, the second polarizing element 110b transmits circularly polarized light with the same rotation direction as the circularly polarized light output from the first polarizing element 106b and blocks circularly polarized light with the opposite rotation direction. Therefore, for example, the second polarizing element 110b is disposed so that the direction of the transmission axis of the linear polarizer 122 is the same as the direction of the transmission axis of the linear polarizer 120 of the first polarizing element 106b, and the direction of the slow axis of the λ / 4 plate 125 is the same as the direction of the slow axis of the λ / 4 plate 124 of the first polarizing element 106b. Alternatively, the second polarizing element 110b is disposed so that the direction of the transmission axis of the linear polarizer 122 is orthogonal to the direction of the transmission axis of the linear polarizer 120 of the first polarizing element 106b, and the direction of the slow axis of the λ / 4 plate 125 is orthogonal to the direction of the slow axis of the λ / 4 plate 124 of the first polarizing element 106b. Hereinafter, an example will be described in which the second polarizing element 110b is arranged so that the direction of the transmission axis of the linear polarizer 122 is the same as the direction of the transmission axis of the linear polarizer 120 of the first polarizing element 106b, and the direction of the slow axis of the λ / 4 plate 125 is the same as the direction of the slow axis of the λ / 4 plate 124 of the first polarizing element 106b.

[0052] In this blood flow measurement device 100b, when near-infrared light is irradiated from the light source unit 104, the linear polarizer 120 of the first polarizing element 106b converts the near-infrared light into linearly polarized light that oscillates, for example, in the left-right direction in the figure. The near-infrared light linearly polarized by the linear polarizer 120 enters the λ / 4 plate 124 and is converted into circularly polarized light. For example, assume that the near-infrared light is converted into right-circularly polarized light by the λ / 4 plate 124. That is, the first polarizing element 106b converts the incident near-infrared light into circularly polarized light. The near-infrared light converted into right-circularly polarized light enters the living body S. The near-infrared light irradiated into the living body S is partially absorbed and scattered near the blood vessels. At this time, the near-infrared light is depolarized from right-circularly polarized light to unpolarized light. A portion of the scattered near-infrared light travels toward the light receiving unit 108 and enters the second polarizing element 110b. The near-infrared light is incident on the λ / 4 plate 125 of the second polarizing element 110b but is unpolarized, and therefore enters the linear polarizer 122 as unpolarized light. The linear polarizer 122 converts the incident near-infrared light into linearly polarized light that oscillates, for example, in the left-right direction in the figure, and transmits it. The light receiving unit 108 receives the linearly polarized near-infrared light, converts it into an electrical signal, and outputs it to the control unit. The control unit performs various processes on the received data to calculate the amount of change in blood flow, pulse rate, etc.

[0053] Meanwhile, a portion of the near-infrared light converted to right-handed circularly polarized light by the first polarizing element 106b (linear polarizer 120 and λ / 4 plate 124) is reflected from areas other than the measurement area, such as the surface of the body and the interfaces of organs. At this time, the polarization is not eliminated, and the circularly polarized light is reversed in its rotation direction upon reflection, becoming left-handed circularly polarized light and entering the λ / 4 plate 125 of the second polarizing element 110b. Because the slow axis of the λ / 4 plate 125 is oriented in the same direction as the slow axis of the λ / 4 plate 124 of the first polarizing element 106b, the left-handed circularly polarized light entering the λ / 4 plate 125 is converted into linearly polarized light oscillating in a direction perpendicular to the paper surface in the figure. This linearly polarized light enters the linear polarizer 122. Because the linear polarizer 122 has a transmission axis in the horizontal direction in the figure, it absorbs linearly polarized light oscillating in a direction perpendicular to the paper surface without transmitting it. This makes it possible to block circularly polarized light reflected at areas other than the measurement area, i.e., near-infrared light that becomes a noise component, and to prevent the light receiving unit 108 from receiving the noise component.

[0054] Note that even when the second polarizing element 110b is configured so that the transmission axis of the linear polarizer 122 is perpendicular to the transmission axis of the linear polarizer 120 of the first polarizing element 106b and the slow axis of the λ / 4 plate 125 is perpendicular to the slow axis of the λ / 4 plate 124 of the first polarizing element 106b, it is still possible to block circularly polarized light reflected outside the measurement area. Specifically, a portion of the near-infrared light converted to right-handed circular polarization by the first polarizing element 106b is reflected outside the measurement area, such as the surface of the body and the interface between organs, and becomes left-handed circularly polarized light, which then enters the λ / 4 plate 125 of the second polarizing element 110b. Because the slow axis of the λ / 4 plate 125 is perpendicular to the slow axis of the λ / 4 plate 124 of the first polarizing element 106b, the left-handed circularly polarized light incident on the λ / 4 plate 125 is converted into linearly polarized light oscillating in the left-right direction in the figure. This linearly polarized light is incident on the linear polarizer 122. Since the linear polarizer 122 has a transmission axis perpendicular to the paper surface in the drawing, it absorbs linearly polarized light vibrating in the left-right direction without transmitting it. This makes it possible to block circularly polarized light reflected outside the measurement area, i.e., near-infrared light that becomes a noise component, and to suppress reception of noise components by the light receiving unit 108.

[0055] Here, circularly polarized light has a higher transmittance through living bodies than non-polarized light. Therefore, a configuration using circular polarizers as the first polarizing element 106b and the second polarizing element 110b, as in the blood flow measurement device 100b, can be configured to allow circularly polarized light to enter a living body, and the amount of light scattered near blood vessels can be increased, thereby further improving the S / N ratio.

[0056] The λ / 4 plate will be described in detail later.

[0057] 2 and 3, the first polarizing element and the second polarizing element have the function of changing the polarization state of the near-infrared light, but they may also have the function of controlling the direction of the near-infrared light. An example in which the first polarizing element and the second polarizing element also have the function of controlling the direction of the near-infrared light will be described with reference to FIGS. 4 and 5.

[0058] Fig. 4 is a conceptual diagram showing a part of another example of the blood flow measuring device of the present invention. The blood flow measuring device 100c shown in Fig. 4 has a light source unit 104, a first polarizing element 106c, a light receiving unit 108, and a second polarizing element 110c. Note that in the blood flow measuring device 100c shown in Fig. 4, the control unit, the housing, etc. are not shown.

[0059] In the blood flow measuring device 100c shown in Fig. 4, the first polarizing element 106c has, from the light source unit 104 side, a first linear polarizer 120a, a retardation layer 126, and a second linear polarizer 120b, in this order. The first linear polarizer 120a and the second linear polarizer 120b correspond to the layers formed using the liquid crystal compound of the present invention. In addition, as a preferred embodiment, the second polarizing element 110c has, from the light receiving unit 108 side, a first linear polarizer 122a, a retardation layer 127, and a second linear polarizer 122b, in this order. The first linear polarizer 122a and the second linear polarizer 122b correspond to the layers formed using the liquid crystal compound of the present invention.

[0060] In the first polarizing element 106c, the first linear polarizer 120a and the second linear polarizer 120b are arranged so that their transmission axes are substantially perpendicular to each other. In the following description, the first linear polarizer 120a has a transmission axis in the horizontal direction in the figure, and the second linear polarizer 120b has a transmission axis in the direction perpendicular to the paper surface.

[0061] The retardation layer 126 is configured to act as a λ / 2 plate for near-infrared light of a wavelength emitted from the light source unit 104, which is incident from a direction tilted at a certain angle with respect to the main surface of the retardation layer 126. The retardation layer 126 is disposed so that the slow axis is at an angle of approximately 45° (or −45°) with respect to the transmission axis of the first linear polarizer 120a.

[0062] Similarly, in the second polarizing element 110c, the first linear polarizer 122a and the second linear polarizer 122b are arranged so that their transmission axes are substantially perpendicular to each other. In the following description, the first linear polarizer 122a has a transmission axis in the horizontal direction in the figure, and the second linear polarizer 122b has a transmission axis in the direction perpendicular to the paper surface.

[0063] The retardation layer 127 is configured to act as a λ / 2 plate for near-infrared light of a wavelength emitted from the light source unit 104, which is incident from a direction inclined at a certain angle with respect to the main surface of the retardation layer 127. The retardation layer 127 is disposed so that the slow axis is at an angle of approximately 45° (or −45°) with respect to the transmission axis of the first linear polarizer 122a.

[0064] In this blood flow measurement device 100c, when near-infrared light is irradiated from the light source unit 104, the first linear polarizer 120a of the first polarizing element 106c converts the near-infrared light into linearly polarized light that oscillates, for example, horizontally in the figure. The near-infrared light that has been linearly polarized by the first linear polarizer 120a is incident on the retardation layer 126. The retardation layer 126 imparts a phase difference to the incident linearly polarized near-infrared light. Here, linearly polarized light that enters the retardation layer 126 from a direction tilted at a certain angle α is given a phase difference of λ / 2, and its oscillation direction is rotated by 90°. In other words, the linearly polarized light that enters the retardation layer 126 is converted into linearly polarized light that oscillates in a direction perpendicular to the paper surface in the figure. On the other hand, linearly polarized light that enters from a direction tilted at an angle other than this angle α and from a direction perpendicular to the main surface has a phase difference that is different from λ / 2, and therefore the amount of rotation of the oscillation direction is different from 90°. The linearly polarized light whose vibration direction has been rotated by the retardation layer 126 is incident on the second linear polarizer 120b. Since the second linear polarizer 120b has a transmission axis perpendicular to the paper surface, linearly polarized light incident from a direction tilted at an angle α is transmitted through the second linear polarizer 120b, while linearly polarized light incident from a direction tilted at an angle other than the angle α and from a direction perpendicular to the main surface is blocked by the second linear polarizer 120b. Therefore, the propagation direction of the near-infrared light that has passed through the first polarizing element 106c is tilted at the angle α.

[0065] As described above, the first polarizing element 106c can change the polarization state of the near-infrared light emitted from the light source unit 104 and control the traveling direction of the near-infrared light.

[0066] The near-infrared light converted into linearly polarized light enters the living body S. The near-infrared light irradiated into the living body S is partially absorbed and scattered near the blood vessels. At this time, the near-infrared light is depolarized from linearly polarized light to become unpolarized. Some of the scattered near-infrared light travels toward the light receiving unit 108 and enters the second polarizing element 110c. The second linear polarizer 122b of the second polarizing element 110c converts the incident near-infrared light into linearly polarized light that vibrates in a direction perpendicular to the paper surface. The near-infrared light linearly polarized by the second linear polarizer 122b enters the retardation layer 126. The retardation layer 126 imparts a phase difference to the incident linearly polarized near-infrared light. Here, linearly polarized light that enters the retardation layer 126 from a direction tilted by a certain angle β is given a phase difference of λ / 2, and its vibration direction is rotated by 90°. In other words, the linearly polarized light that enters the retardation layer 126 is converted into linearly polarized light that vibrates left and right in the figure. On the other hand, linearly polarized light incident from a direction tilted at an angle other than this angle β and from a direction perpendicular to the main surface has a phase difference other than λ / 2, and therefore the rotation of the vibration direction is different from 90°. The linearly polarized light whose vibration direction has been rotated by the retardation layer 126 is incident on the first linear polarizer 122a. Since the first linear polarizer 122a has a transmission axis in the horizontal direction in the figure, linearly polarized light incident from a direction tilted at an angle β is transmitted through the first linear polarizer 122a, while linearly polarized light incident from a direction tilted at an angle other than this angle β and from a direction perpendicular to the main surface is blocked by the first linear polarizer 122a. Therefore, the propagation direction of the near-infrared light passing through the second polarizing element 110c is tilted at an angle β. The near-infrared light passing through the second polarizing element 110c is incident on the light receiving unit 108. The light receiving unit 108 receives the linearly polarized near-infrared light, converts it into an electrical signal, and outputs it to the control unit. The control unit performs various processes on the received data to calculate the amount of change in blood flow, pulse rate, and the like.

[0067] On the other hand, a portion of the near-infrared light converted to linear polarized light by the first polarizing element 106c is reflected from areas other than the measurement area, such as the surface of the body and the interfaces of organs. Since the polarization is not eliminated, the light remains linearly polarized, vibrating in a direction perpendicular to the paper surface in the figure, and is incident on the second linear polarizer 122b of the second polarizing element 110c. The second linear polarizer 122b of the second polarizing element 110c has a transmission axis perpendicular to the paper surface, and therefore transmits linearly polarized light vibrating in a direction perpendicular to the paper surface. The near-infrared light converted to linear polarized light by the second linear polarizer 122b is incident on the retardation layer 126. The retardation layer 126 imparts a phase difference to the linearly polarized near-infrared light that has entered. Here, linearly polarized light incident on the retardation layer 126 from a direction tilted at a certain angle β is given a phase difference of λ / 2 and its vibration direction is rotated by 90°. However, light reflected outside the measurement area is incident from a direction tilted at an angle other than the angle β, and therefore the phase difference caused by the retardation layer is deviated from λ / 2, resulting in a rotation of the vibration direction that is deviated from 90°. The linearly polarized light whose vibration direction has been rotated by the retardation layer 126 is incident on the first linear polarizer 122a. Because the first linear polarizer 122a has a transmission axis in the left-right direction, linearly polarized light incident from a direction tilted at an angle other than the angle β is blocked by the first linear polarizer 122a. This makes it possible to block linearly polarized light reflected outside the measurement area, i.e., near-infrared light that becomes a noise component, and to suppress reception of noise components by the light receiving unit 108.

[0068] By using the first polarizing element 106c to control the direction of travel of the near-infrared light so that the azimuth direction is toward the light receiving unit 108 (second polarizing element 110c) and is inclined at a predetermined angle relative to the perpendicular to the main surface of the first polarizing element 106c, the amount of near-infrared light that is irradiated into the living body S and scattered near the blood vessels and that heads toward the light receiving unit 108 (second polarizing element 110c) can be increased, thereby further improving the signal-to-noise ratio.

[0069] 4, the first polarizing element 106c and the second polarizing element 110c have a first linear polarizer, a retardation layer, and a second linear polarizer in this order, but the present invention is not limited to this, and either the first polarizing element 106c or the second polarizing element 110c may have a first linear polarizer, a retardation layer, and a second linear polarizer in this order. In this case, the other polarizing element may be, for example, a linear polarizer, and linearly polarized light reflected in areas other than the measurement area may be blocked by the second polarizing element.

[0070] 4, the first linear polarizer and the second linear polarizer in the first polarizing element 106c and the second polarizing element 110c are configured so that their transmission axes are orthogonal to each other, but they may also be configured so that their transmission axes are parallel. When the transmission axes of the first linear polarizer and the second linear polarizer are parallel, for example, if the direction in which the refractive index of the retardation layer is zero, i.e., the direction of the optical axis of the retardation layer is inclined in the range of 20 degrees to 60 degrees with respect to the main surface, the amount of light directed toward the light receiving unit 108 (second polarizing element 110c) can be increased, thereby further improving the S / N ratio, which is preferable.

[0071] In the example shown in FIG. 4, the second linear polarizer in the first polarizing element 106c and / or the second polarizing element 110c may have its absorption axis perpendicular to the surface. In this case, the absorption axes of the first linear polarizer and the second linear polarizer can be orthogonal or parallel only to near-infrared light incident from an oblique direction. This reduces the range of angles through which obliquely reflected light from the measurement area is transmitted. As a result, the axial angle relationship between the polarizer and the retardation layer can be set such that the transmitted light decreases as soon as the angle changes slightly from the maximum transmission angle. This allows measurements to be made that place more weight on reflected light at the required angle, resulting in measurements with less noise.

[0072] Fig. 5 is a conceptual diagram showing a part of another example of the blood flow measuring device of the present invention. The blood flow measuring device 100d shown in Fig. 5 has a light source unit 104, a first polarizing element 106d, a light receiving unit 108, and a second polarizing element 110d. Note that in the blood flow measuring device 100d shown in Fig. 5, the control unit, the housing, etc. are not shown.

[0073] 5, the first polarizing element 106d has, as a layer formed using a liquid crystal compound, an optically anisotropic layer having a liquid crystal orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane. In a preferred embodiment, the second polarizing element 110d has, as a layer formed using a liquid crystal compound, an optically anisotropic layer having a liquid crystal orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane.

[0074] An optically anisotropic layer having a liquid crystal orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane is a liquid crystal diffraction element that diffracts incident near-infrared light. This liquid crystal diffraction element diffracts the right-handed and left-handed circularly polarized components of the incident near-infrared light in different directions. The liquid crystal diffraction element will be described in detail later.

[0075] In this blood flow measuring device 100d, when near-infrared light is emitted from the light source unit 104, the liquid crystal diffraction element 128 of the first polarizing element 106d diffracts, for example, a right-handed circularly polarized component of the near-infrared light in a direction inclined at a predetermined angle with respect to the normal to the principal surface of the liquid crystal diffraction element 128, with the azimuth direction toward the light receiving unit 108 (second polarizing element 110d). The near-infrared light converted to right-handed circularly polarized light by the liquid crystal diffraction element 128 (first polarizing element 106d) is incident on the living body S. The near-infrared light irradiated into the living body S is partially absorbed and scattered near the blood vessels. At this time, the near-infrared light is depolarized from right-handed circularly polarized light to become unpolarized. A portion of the scattered near-infrared light travels toward the light receiving unit 108 and enters the second polarizing element 110d. The liquid crystal diffraction element 128 of the second polarizing element 110d diffracts and transmits the right-handed or left-handed circularly polarized component of the unpolarized near-infrared light incident from an oblique direction toward the light receiving unit 108. The light receiving unit 108 receives circularly polarized near-infrared light, converts it into an electrical signal, and outputs it to the control unit. The control unit performs various processes on the received data to calculate the amount of change in blood flow, pulse rate, etc.

[0076] Meanwhile, some of the near-infrared light converted to right-handed circularly polarized light by the first polarizing element 106d is reflected from areas other than the measurement area, such as the surface of the body and the interfaces of organs. At this time, the polarization is not depolarized, and the circularly polarized light is reversed in its rotation direction upon reflection, becoming left-handed circularly polarized light, which then enters the liquid crystal diffraction element 128 of the second polarizing element 110d. The liquid crystal diffraction element 128 does not diffract the incident left-handed circularly polarized light toward the light receiving unit 108, but diffracts the right-handed circularly polarized light resulting from depolarization at the measurement area toward the light receiving unit 108. This makes it possible to block the circularly polarized light reflected from areas other than the measurement area, i.e., the near-infrared light that becomes a noise component, and to suppress reception of the noise component by the light receiving unit 108.

[0077] 5, the first polarizing element 106d and the second polarizing element 110d are configured to include a liquid crystal diffraction element, but this is not limited thereto, and either the first polarizing element 106d or the second polarizing element 110d may be configured to include a liquid crystal diffraction element. For example, if the first polarizing element is a liquid crystal diffraction element, the second polarizing element may be configured to include, for example, a circular polarizing plate (linear polarizer + λ / 4 plate) to block circularly polarized light reflected in areas other than the measurement area.

[0078] <Linear Polarizer> The linear polarizers 120, 120a, 120b, 122, 122a, and 122b are layers formed using a liquid crystal compound, and are absorption polarizers that absorb linearly polarized light vibrating in the absorption axis direction of incident light and transmit linearly polarized light vibrating in the transmission axis direction.

[0079] The liquid crystal compound may be a rod-shaped liquid crystal compound or a discotic liquid crystal compound. The liquid crystal compound may have a polymerizable group. Examples of liquid crystal compounds having a polymerizable group (polymerizable liquid crystal compounds) include the compounds exemplified as polymerizable liquid crystal compounds described in the optically anisotropic layer section below. The liquid crystal compound may be a thermotropic liquid crystal compound or a lyotropic liquid crystal compound. A lyotropic liquid crystal compound is a liquid crystal compound that exhibits the property of undergoing a phase transition between an isotropic phase and a liquid crystal phase when dissolved in a solvent and the temperature or concentration is changed. Examples of lyotropic liquid crystal compounds include non-colored lyotropic liquid crystal compounds (e.g., rod-shaped compounds and plate-shaped compounds) described in paragraphs

[0026] to

[0091] of WO 2021 / 200987.

[0080] The linear polarizer is preferably formed using a liquid crystal composition containing a liquid crystal compound and a dichroic material, the liquid crystal compound being as described above.

[0081] A dichroic substance is a compound having different absorbance in the long axis direction of the molecule and absorbance in the short axis direction. The dichroic substance preferably has a maximum absorption wavelength in the near-infrared region. More specifically, the maximum absorption wavelength of the dichroic substance is preferably located in the wavelength range of 700 to 1600 nm, preferably in the wavelength range of 700 to 1200 nm, and more preferably in the wavelength range of 700 to 900 nm. In other words, the dichroic substance is preferably a so-called near-infrared absorbing dye. The dichroic substance may or may not exhibit liquid crystallinity (e.g., lyotropic liquid crystallinity). The type of dichroic substance is not particularly limited, but is preferably a cyanine dye, an oxonol dye, a boron complex dye, a phthalocyanine dye, a squarylium dye, a metal complex dye, a diimmonium dye, or a rylene dye.

[0082] When a linear polarizer is produced using the liquid crystal composition, a method can be used in which the liquid crystal composition is applied, and the resulting coating film is subjected to an alignment treatment, if necessary, to produce a linear polarizer. The method for applying the liquid crystal composition is not particularly limited, and known methods such as spin coating and bar coating can be used. The substrate on which the liquid crystal composition is applied may have an alignment film on its surface. By providing the alignment film, the liquid crystal compound is aligned according to the alignment regulating force of the alignment film.

[0083] The formed coating film is subjected to an alignment treatment as necessary. As the alignment treatment, an optimum method depending on the type of liquid crystal compound used can be used. For example, when the liquid crystal compound is a thermotropic liquid crystal compound, and when the above-mentioned alignment film is used, the liquid crystal compound can be aligned by performing a heat treatment on the coating film. Furthermore, when the liquid crystal compound is a lyotropic liquid crystal compound, by employing a coating method that applies shear to the liquid crystal composition, such as wire bar coating, two treatments, coating of the compound and alignment, can be performed simultaneously.

[0084] The formed coating film may be subjected to a curing treatment as necessary. In particular, when the liquid crystal compound has a polymerizable group, the polymerizable group can be polymerized by a heat treatment or a light irradiation treatment.

[0085] By carrying out the above procedure, the dichroic material is also aligned along the alignment of the liquid crystal compound, and a linear polarizer having predetermined properties is obtained.

[0086] <λ / 4 Plate> The λ / 4 plates 124 and 125 function as λ / 4 plates for the wavelength of incident light, and can convert linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light. There are no particular limitations on the λ / 4 plates as long as they can convert incident linearly polarized light into circularly polarized light and incident circularly polarized light into linearly polarized light, and any conventionally known λ / 4 plate can be used.

[0087] In the present invention, from the viewpoint of wide angle characteristics and wide wavelength dispersion, the λ / 4 plate is preferably a layer formed using a liquid crystal compound.

[0088] The wide-angle characteristic refers to the range of angles (the angle of incident light relative to the perpendicular to the main surface of the λ / 4 plate) at which a λ / 4 plate functions as a λ / 4 plate when near-infrared light is incident on the λ / 4 plate from an oblique direction. From the viewpoint of being able to function as a λ / 4 plate over a wider angle range, the λ / 4 plate is preferably a laminate of a layer formed using a rod-shaped liquid crystal compound (e.g., a layer formed by fixing horizontally aligned rod-shaped compounds) and a layer formed using a discotic liquid crystal compound (e.g., a layer formed by fixing vertically aligned discotic liquid crystal compounds). Examples of the layers constituting such a laminate include the layers described in Japanese Patent No. 6,975,074 and Japanese Patent No. 6,640,847. Alternatively, the λ / 4 plate is preferably a laminate of a layer formed by horizontally aligning rod-shaped liquid crystal compounds (e.g., a layer formed by fixing horizontally aligned rod-shaped compounds) and a layer formed by vertically aligning rod-shaped liquid crystal compounds (e.g., a layer formed by fixing vertically aligned rod-shaped compounds). Examples of the layers constituting such a laminate include the layers described in WO 2019 / 159960.

[0089] Wavelength dispersion refers to the wavelength range exhibiting quarter-wave characteristics. As described above, the blood flow measurement device of the present invention preferably uses near-infrared rays of two or more different wavelengths. In this case, the λ / 4 plate preferably exhibits quarter-wave characteristics for all wavelengths, and preferably exhibits so-called reverse wavelength dispersion (a characteristic in which in-plane retardation increases as the measurement wavelength increases). From the viewpoint of wavelength dispersion, the λ / 4 plate is preferably a layer formed using a reverse dispersion liquid crystal compound. Examples of layers formed using a reverse dispersion liquid crystal compound include the layers described in International Publication No. 2019 / 159960. The λ / 4 plate may also be a laminate of a λ / 4 plate and a λ / 2 plate. Examples of the layers constituting such a laminate include the layers described in Japanese Patent No. 6,975,074 and Japanese Patent No. 6,640,847. The λ / 4 plate may also include a layer formed by fixing a liquid crystal compound that is twisted and aligned along a helical axis extending along the thickness direction. Examples of the layer including a layer in which a liquid crystal compound is fixed in a twisted orientation along a helical axis extending along the thickness direction include the layer described in WO 2021 / 033631.

[0090] As described above, the λ / 4 plate may be formed using a liquid crystal compound. The liquid crystal compound may be a rod-shaped liquid crystal compound or a discotic liquid crystal compound. The liquid crystal compound may have a polymerizable group. Examples of liquid crystal compounds having a polymerizable group (polymerizable liquid crystal compounds) include the compounds exemplified as polymerizable liquid crystal compounds described in the optically anisotropic layer below. As described above, the liquid crystal compound may be a liquid crystal compound with normal wavelength dispersion or a liquid crystal compound with reverse wavelength dispersion. The method for producing a λ / 4 plate formed using a liquid crystal compound is not particularly limited, and known methods can be used. For example, a method may be used in which a liquid crystal composition containing a liquid crystal compound is applied to a substrate having an alignment film, the coating film is subjected to an alignment treatment (e.g., a heat treatment), and, if necessary, a curing treatment is further performed.

[0091] <Retardation Layer> The retardation layer changes the state of incident polarized light by imparting a phase difference (optical path difference) to two orthogonal polarized light components. In the present invention, the retardation layer is a layer formed by arranging birefringent materials such as liquid crystal compounds in the same direction.

[0092] As explained in Fig. 4, the retardation layer used in the polarizing element for controlling the direction of near-infrared rays preferably functions as a λ / 2 plate for near-infrared rays incident from a direction tilted at a certain angle, from the viewpoint of transmitting near-infrared rays in a direction tilted at a certain angle. From this point of view, the retardation layer preferably has liquid crystal compounds obliquely aligned with respect to the main surface.

[0093] As described above, the blood flow measuring device of the present invention preferably uses near-infrared rays of two or more different wavelengths. In this case, the retardation layer preferably exhibits a predetermined retardation for each wavelength, and preferably exhibits so-called reverse wavelength dispersion.

[0094] The retardation layer may be formed using a liquid crystal compound. The liquid crystal compound may be a rod-shaped liquid crystal compound or a discotic liquid crystal compound. The liquid crystal compound may have a polymerizable group. Examples of liquid crystal compounds having a polymerizable group (polymerizable liquid crystal compounds) include compounds exemplified as polymerizable liquid crystal compounds described in the optically anisotropic layer below. The method for producing a retardation layer formed using a liquid crystal compound is not particularly limited, and known methods can be used. For example, a method may be used in which a liquid crystal composition containing a liquid crystal compound is applied to a substrate having an alignment film, the coating film is subjected to an alignment treatment (for example, a heat treatment), and, if necessary, a curing treatment is further applied.

[0095] <Liquid Crystal Diffraction Element> A liquid crystal diffraction element has an optically anisotropic layer in which liquid crystal compounds are oriented in a predetermined arrangement, and refracts near-infrared light by diffraction.

[0096] The optically anisotropic layer of the liquid crystal diffraction element will be described with reference to Figures 6 and 7. The optically anisotropic layer shown in Figures 6 and 7 is a layer formed by fixing a liquid crystal phase in which a liquid crystal compound is aligned, and has a liquid crystal alignment pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane.

[0097] As conceptually shown in Figure 6, in the optically anisotropic layer, the liquid crystal compound 40 is not twisted and rotated in a spiral in the thickness direction, and the liquid crystal compound 40 at the same position in the plane direction is oriented so that its optical axis 40A faces the same direction.

[0098] <<Liquid Crystal Alignment Pattern of Optically Anisotropic Layer>> The optically anisotropic layer has a liquid crystal alignment pattern in which the direction of the optical axis 40A derived from the liquid crystal compound 40 changes while continuously rotating in one direction within the plane of the optically anisotropic layer. The optical axis 40A derived from the liquid crystal compound 40 is the axis in which the refractive index of the liquid crystal compound 40 is highest, that is, the so-called slow axis. For example, when the liquid crystal compound 40 is a rod-shaped liquid crystal compound, the optical axis 40A is aligned with the long axis direction of the rod shape. In the following description, the optical axis 40A derived from the liquid crystal compound 40 is also referred to as the "optical axis 40A of the liquid crystal compound 40" or the "optical axis 40A."

[0099] Fig. 7 conceptually shows a plan view of the optically anisotropic layer. The plan view is a view of the optically anisotropic layer in Fig. 6 as seen from above, i.e., a view of the optically anisotropic layer as seen from the thickness direction (i.e., the lamination direction of each layer (film)). In Fig. 7, in order to clearly show the structure of the optically anisotropic layer, only the liquid crystal compound 40 on the surface is shown.

[0100] As shown in FIG. 7 , the liquid crystal compound 40 constituting the optically anisotropic layer on the surface has a liquid crystal orientation pattern in which the orientation of the optic axis 40A changes while continuously rotating along a predetermined direction indicated by arrow D (hereinafter referred to as alignment axis D) within the plane of the optically anisotropic layer. In the illustrated example, the liquid crystal orientation pattern is such that the optic axis 40A of the liquid crystal compound 40 changes while continuously rotating clockwise along the alignment axis D direction. The liquid crystal compound 40 constituting the optically anisotropic layer is two-dimensionally aligned along the alignment axis D and a direction perpendicular to this direction (alignment axis D direction). In the following description, the direction perpendicular to the alignment axis D direction will be referred to as the Y direction for convenience. That is, the arrow Y direction is the direction perpendicular to the direction in which the orientation of the optic axis 40A of the liquid crystal compound 40 changes while continuously rotating within the plane of the optically anisotropic layer. Therefore, in FIGS. 8 and 9 described below, the Y direction is perpendicular to the paper surface.

[0101] The expression "the orientation of the optical axis 40A of the liquid crystal compound 40 changes while continuously rotating in the direction of the alignment axis D (a predetermined direction)" specifically means that the angle formed between the optical axis 40A of the liquid crystal compound 40 aligned along the alignment axis D and the alignment axis D direction varies depending on the position in the alignment axis D direction, and the angle formed between the optical axis 40A and the alignment axis D direction sequentially changes from θ to θ+180° or θ−180° along the alignment axis D direction. The difference in angle between the optical axes 40A of the liquid crystal compound 40 adjacent to each other in the alignment axis D direction is preferably 45° or less, more preferably 15° or less, and even more preferably a smaller angle.

[0102] In the present invention, the rotation direction of the optical axis 40A of the liquid crystal compound in the direction of the alignment axis D is such that the angle formed by the optical axes 40A of the liquid crystal compounds 40 adjacent to each other in the direction of the alignment axis D becomes smaller. Therefore, in the optically anisotropic layers shown in Figures 6 and 7, the optical axis 40A of the liquid crystal compound 40 rotates rightward (clockwise) along the direction of the arrow of the alignment axis D.

[0103] On the other hand, the liquid crystal compound 40 forming the optically anisotropic layer has the same orientation of the optical axis 40A in the Y direction perpendicular to the direction of the alignment axis D, i.e., the Y direction perpendicular to the direction in which the optical axis 40A continuously rotates. In other words, the liquid crystal compound 40 forming the optically anisotropic layer has the same angle between the optical axis 40A of the liquid crystal compound 40 and the direction of the alignment axis D in the Y direction.

[0104] In the optically anisotropic layer, the liquid crystal compounds aligned in the Y direction have an equal angle between their optical axes 40A and the alignment axis D (one direction in which the optical axes of the liquid crystal compounds 40 rotate). A region in which the liquid crystal compounds 40, with the same angle between their optical axes 40A and the alignment axis D, are arranged in the Y direction, is referred to as region R. In this case, the in-plane retardation (Re) value in each region R is preferably half the wavelength, i.e., λ / 2. These in-plane retardations are calculated by the product of the refractive index difference Δn associated with the refractive index anisotropy of region R and the thickness of the optically anisotropic layer. Here, the refractive index difference associated with the refractive index anisotropy of region R in the optically anisotropic layer is a refractive index difference defined by the difference between the refractive index in the direction of the slow axis in the plane of region R and the refractive index in the direction perpendicular to the slow axis. That is, the refractive index difference Δn due to the refractive index anisotropy of region R is equal to the difference between the refractive index of liquid crystal compound 40 in the direction of optical axis 40A and the refractive index of liquid crystal compound 40 in the direction perpendicular to optical axis 40A in the plane of region R. In other words, the refractive index difference Δn is equal to the refractive index difference of liquid crystal compound 40.

[0105] In the optically anisotropic layer, in the liquid crystal orientation pattern of the liquid crystal compound 40, the length (distance) over which the optical axis 40A of the liquid crystal compound 40 rotates 180° in the direction of the alignment axis D, in which the optical axis 40A continuously rotates and changes in the plane, is defined as the length Λ of one period of the liquid crystal orientation pattern. That is, the distance between the centers of two liquid crystal compounds 40 in the direction of the alignment axis D, which are at the same angle with respect to the direction of the alignment axis D, is defined as the length Λ of one period. Specifically, as shown in FIG. 7 , the distance between the centers of two liquid crystal compounds 40 in the direction of the alignment axis D, in which the direction of the optical axis 40A coincides with the direction of the alignment axis D, is defined as the length Λ of one period. In the following description, this length Λ of one period is also referred to as "one period Λ." In the liquid crystal orientation pattern of the optically anisotropic layer, this one period Λ is repeated in one direction along the alignment axis D, i.e., in which the orientation of the optical axis 40A continuously rotates and changes.

[0106] When circularly polarized light enters such an optically anisotropic layer, the light is refracted and the direction of the circular polarization is converted. This action is conceptually shown in Figures 8 and 9. Assume that the product of the refractive index difference of the liquid crystal compound and the thickness of the optically anisotropic layer is λ / 2. As shown in Figure 8, when the product of the refractive index difference of the liquid crystal compound and the thickness of the optically anisotropic layer is λ / 2, when left-handed circularly polarized incident light L1 enters the optically anisotropic layer, the incident light L1 is given a phase difference of 180° as it passes through the optically anisotropic layer, and the transmitted light L2 is converted to right-handed circularly polarized light. Furthermore, because the liquid crystal orientation pattern formed in the optically anisotropic layer is a periodic pattern in the direction of the alignment axis D, the transmitted light L2 travels in a direction different from the traveling direction of the incident light L1. In this way, the left-handed circularly polarized incident light L1 is converted to right-handed circularly polarized transmitted light L2, which is tilted at a certain angle toward the alignment axis D with respect to the incident direction. In the example shown in FIG. 8, the transmitted light L2 is diffracted so as to travel in a lower right direction.

[0107] On the other hand, as shown in Figure 9, when the product of the refractive index difference between the liquid crystal compounds in the optically anisotropic layer and the thickness of the optically anisotropic layer is λ / 2, when right-handed circularly polarized incident light L4 enters the optically anisotropic layer, the incident light L4 is given a phase difference of 180° as it passes through the optically anisotropic layer and is converted into left-handed circularly polarized transmitted light L5. Furthermore, because the liquid crystal alignment pattern formed in the optically anisotropic layer is a periodic pattern in the direction of the alignment axis D, the transmitted light L5 travels in a direction different from the propagation direction of the incident light L4. At this time, the transmitted light L5 travels in a different direction from the transmitted light L2, that is, in the direction opposite to the direction of the arrow of the alignment axis D with respect to the incident direction. In this way, the incident light L4 is converted into left-handed circularly polarized transmitted light L5 that is tilted at a certain angle in the direction opposite to the direction of the alignment axis D with respect to the incident direction. In the example shown in Figure 9, the transmitted light L5 is diffracted to travel in a downward and leftward direction.

[0108] As described above, the optically anisotropic layer can adjust the refraction angles of the transmitted light beams L2 and L5 depending on the length of one period Λ of the formed liquid crystal orientation pattern. Specifically, the shorter the period Λ of the liquid crystal orientation pattern, the stronger the interference between the lights that have passed through adjacent liquid crystal compounds 40, and therefore the greater the refraction of the transmitted light beams L2 and L5.

[0109] Furthermore, by reversing the direction of rotation of the optical axis 40A of the liquid crystal compound 40, which rotates along the alignment axis D, the azimuth direction of refraction of transmitted light can be reversed. That is, in the examples shown in FIGS. 8 and 9 , the direction of rotation of the optical axis 40A pointing toward the alignment axis D is clockwise, but by changing this direction of rotation to counterclockwise, the azimuth direction of refraction of transmitted light can be reversed. Specifically, in FIGS. 8 and 9 , when the direction of rotation of the optical axis 40A pointing toward the alignment axis D is counterclockwise, left-handed circularly polarized light incident on the optically anisotropic layer from above in the figure passes through the optically anisotropic layer, and the transmitted light is converted to right-handed circularly polarized light, and is diffracted to travel in the lower left direction in the figure. Similarly, right-handed circularly polarized light incident on the optically anisotropic layer from above in the figure passes through the optically anisotropic layer, and the transmitted light is converted to left-handed circularly polarized light, and is diffracted to travel in the lower right direction in the figure.

[0110] <<Method for forming optically anisotropic layer>> The method for forming the optically anisotropic layer includes, for example, a step of applying a liquid crystal composition containing the prepared liquid crystal compound onto an alignment film, and a step of curing the applied liquid crystal composition.

[0111] The liquid crystal composition may be prepared by a conventionally known method. The liquid crystal composition may be applied by various known methods used for applying a liquid, such as printing methods such as inkjet printing and scroll printing, as well as spin coating, bar coating, gravure coating, and spray coating. The coating thickness of the liquid crystal composition (coating thickness) may be appropriately determined so as to obtain an optically anisotropic layer of a desired thickness, depending on the composition of the liquid crystal composition.

[0112] Here, as described below, an alignment pattern is formed on the alignment film, so that the liquid crystal compound of the liquid crystal composition applied onto the alignment film is aligned along the alignment pattern (anisotropic periodic pattern) of the alignment film.

[0113] The liquid crystal composition is dried and / or heated as necessary, and then cured. The liquid crystal composition may be cured by a known method such as photopolymerization or thermal polymerization. Photopolymerization is preferred for polymerization. Ultraviolet light is preferably used for light irradiation. The irradiation energy is 20 mJ / cm. 2 ~50 J / cm 2 is preferred, and 50 to 1500 mJ / cm 2 is more preferable. To promote the photopolymerization reaction, light irradiation may be performed under heating conditions or in a nitrogen atmosphere. The wavelength of the ultraviolet light to be irradiated is preferably 250 to 430 nm. When heating is performed, the heating temperature is preferably 200°C or less, more preferably 130°C or less. By curing the liquid crystal composition, the liquid crystal compounds in the liquid crystal composition are fixed in a state aligned along the alignment pattern of the alignment film (liquid crystal alignment pattern). This results in the formation of an optically anisotropic layer having a liquid crystal alignment pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane. Note that the liquid crystal compound does not need to exhibit liquid crystallinity when the optically anisotropic layer is completed. For example, the polymerizable liquid crystal compound may be polymerized by the curing reaction and lose its liquid crystallinity.

[0114] Alternatively, the optically anisotropic layer may be formed by coating the liquid crystal composition on the alignment film in multiple layers. Multilayer coating is a method in which a first layer of the liquid crystal composition is first coated on the alignment film, heated, cooled, and then cured with ultraviolet light to form a liquid crystal fixed layer. Then, subsequent layers are coated on top of the liquid crystal fixed layer, and similarly heated, cooled, and then cured with ultraviolet light. This process is repeated until the desired thickness is achieved to form the optically anisotropic layer. By forming the liquid crystal layer by multilayer coating, the total thickness of the liquid crystal layer can be increased. Furthermore, even when the total thickness of the liquid crystal layer is increased, the alignment direction of the alignment film is reflected from the lower surface to the upper surface of the liquid crystal layer.

[0115] <Liquid crystal composition for forming an optically anisotropic layer> An example of a material used to form an optically anisotropic layer is a liquid crystal composition containing a liquid crystal compound. The liquid crystal compound is preferably a polymerizable liquid crystal compound. The liquid crystal composition used to form the liquid crystal layer may further contain a surfactant and a chiral agent.

[0116] --Polymerizable Liquid Crystal Compound-- The polymerizable liquid crystal compound may be a rod-shaped liquid crystal compound or a discotic liquid crystal compound. Examples of rod-shaped polymerizable liquid crystal compounds include rod-shaped nematic liquid crystal compounds. Preferred rod-shaped nematic liquid crystal compounds include azomethines, azoxy compounds, cyanobiphenyls, cyanophenyl esters, benzoic acid esters, cyclohexanecarboxylic acid phenyl esters, cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines, alkoxy-substituted phenylpyrimidines, phenyldioxanes, tolanes, and alkenylcyclohexylbenzonitriles. Not only low-molecular-weight liquid crystal compounds but also high-molecular-weight liquid crystal compounds can be used.

[0117] A polymerizable liquid crystal compound can be obtained by introducing a polymerizable group into a liquid crystal compound. Examples of the polymerizable group include an unsaturated polymerizable group, an epoxy group, and an aziridinyl group, with an unsaturated polymerizable group being preferred, and an ethylenically unsaturated polymerizable group being more preferred. The polymerizable group can be introduced into the molecule of the liquid crystal compound by various methods. The number of polymerizable groups in the polymerizable liquid crystal compound is preferably 1 to 6, more preferably 1 to 3. Examples of polymerizable liquid crystal compounds are described in Makromol. Chem. , Vol. 190, p. 2255 (1989), Advanced Materials Vol. 5, p. 107 (1993), U.S. Pat. No. 4,683,327, U.S. Pat. No. 5,622,648, U.S. Pat. No. 5,770,107, WO 95 / 22586, WO 95 / 24455, WO 97 / 00600, WO 98 / 23580, WO 98 / 52905, JP-A Nos. 1-272551, 6-16616, 7-110469, 11-80081, and 2001-328973. Two or more polymerizable liquid crystal compounds may be used in combination. The alignment temperature can be lowered by using two or more types of polymerizable liquid crystal compounds in combination.

[0118] As an exception to this, polymerizable liquid crystal compounds that can be used include cyclic organopolysiloxane compounds such as those disclosed in JP-A-57-165480. Furthermore, as the polymer liquid crystal compounds, polymers having mesogenic groups exhibiting liquid crystallinity introduced into the main chain, side chain, or both the main chain and side chain, polymeric cholesteric liquid crystals having cholesteryl groups introduced into the side chain, liquid crystalline polymers such as those disclosed in JP-A-9-133810, and liquid crystalline polymers such as those disclosed in JP-A-11-293252 can be used.

[0119] --Discotic Liquid Crystal Compound-- As the discotic liquid crystal compound, for example, those described in JP-A Nos. 2007-108732 and 2010-244038 can be preferably used.

[0120] The amount of the polymerizable liquid crystal compound added to the liquid crystal composition is preferably 75 to 99.9 mass %, more preferably 80 to 99 mass %, and even more preferably 85 to 90 mass %, based on the solid content mass (mass excluding the solvent) of the liquid crystal composition.

[0121] --Surfactant-- The liquid crystal composition used in forming the liquid crystal layer may contain a surfactant. The surfactant is preferably a compound that can function as an alignment control agent that contributes to the stable or rapid alignment of the liquid crystal compound 40 in the liquid crystal layer 102. Examples of the surfactant include silicone surfactants and fluorine surfactants, with fluorine surfactants being a preferred example.

[0122] Specific examples of surfactants include the compounds described in paragraphs

[0082] to

[0090] of JP-A No. 2014-119605, the compounds described in paragraphs

[0031] to

[0034] of JP-A No. 2012-203237, the compounds exemplified in paragraphs

[0092] and

[0093] of JP-A No. 2005-99248, the compounds exemplified in paragraphs

[0076] to

[0078] and paragraphs

[0082] to

[0085] of JP-A No. 2002-129162, and fluorine (meth)acrylate polymers described in paragraphs

[0018] to

[0043] of JP-A No. 2007-272185, etc. One type of surfactant may be used alone, or two or more types may be used in combination. As the fluorine-based surfactant, the compounds described in paragraphs

[0082] to

[0090] of JP-A-2014-119605 are preferred.

[0123] The amount of the surfactant added in the liquid crystal composition is preferably 0.01 to 10% by mass, more preferably 0.01 to 5% by mass, and even more preferably 0.02 to 1% by mass, based on the total mass of the liquid crystal compound.

[0124] --Polymerization initiator-- When the liquid crystal composition contains a polymerizable compound, it preferably contains a polymerization initiator. In an embodiment in which the polymerization reaction is caused to proceed by ultraviolet irradiation, the polymerization initiator used is preferably a photopolymerization initiator that can initiate the polymerization reaction by ultraviolet irradiation. Examples of photopolymerization initiators include α-carbonyl compounds (described in U.S. Pat. Nos. 2,367,661 and 2,367,670), acyloin ethers (described in U.S. Pat. No. 2,448,828), α-hydrocarbon-substituted aromatic acyloin compounds (described in U.S. Pat. No. 2,722,512), polynuclear quinone compounds (described in U.S. Pat. Nos. 3,046,127 and 2,951,758), combinations of triarylimidazole dimers and p-aminophenyl ketones (described in U.S. Pat. No. 3,549,367), acridine and phenazine compounds (described in JP-A No. 60-105,667 and U.S. Pat. No. 4,239,850), and oxadiazole compounds (described in U.S. Pat. No. 4,212,970). The content of the photopolymerization initiator in the liquid crystal composition is preferably 0.1 to 20% by mass, more preferably 0.5 to 12% by mass, based on the content of the liquid crystal compound.

[0125] Crosslinking Agent The liquid crystal composition may optionally contain a crosslinking agent to improve the film strength and durability after curing. Suitable crosslinking agents are those that cure under ultraviolet light, heat, moisture, or the like. The crosslinking agent is not particularly limited and can be appropriately selected depending on the purpose. Examples include polyfunctional acrylate compounds such as trimethylolpropane tri(meth)acrylate and pentaerythritol tri(meth)acrylate; epoxy compounds such as glycidyl (meth)acrylate and ethylene glycol diglycidyl ether; aziridine compounds such as 2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate] and 4,4-bis(ethyleneiminocarbonylamino)diphenylmethane; isocyanate compounds such as hexamethylene diisocyanate and biuret-type isocyanate; polyoxazoline compounds having an oxazoline group in the side chain; and alkoxysilane compounds such as vinyltrimethoxysilane and N-(2-aminoethyl)3-aminopropyltrimethoxysilane. Furthermore, known catalysts can be used depending on the reactivity of the crosslinking agent, which can improve productivity in addition to improving film strength and durability. These may be used alone or in combination of two or more. The content of the crosslinking agent is preferably 3 to 20% by mass, more preferably 5 to 15% by mass, based on the mass of the solid content of the liquid crystal composition. When the content of the crosslinking agent is within the above range, the effect of improving the crosslink density is easily obtained, and the stability of the liquid crystal phase is further improved.

[0126] --Other Additives-- If necessary, polymerization inhibitors, antioxidants, ultraviolet absorbers, light stabilizers, colorants, metal oxide fine particles, and the like may be added to the liquid crystal composition within a range that does not impair optical performance, etc.

[0127] The liquid crystal composition is preferably used as a liquid when forming an optically anisotropic layer. The liquid crystal composition may contain a solvent. The solvent is not limited and can be appropriately selected depending on the purpose, but organic solvents are preferred. The organic solvent is not limited and can be appropriately selected depending on the purpose, and examples thereof include ketones, alkyl halides, amides, sulfoxides, heterocyclic compounds, hydrocarbons, esters, and ethers. These may be used alone or in combination of two or more. Among these, ketones are preferred when considering the environmental impact.

[0128] The liquid crystal diffraction element may also have layers other than the optically anisotropic layer, such as a support and an alignment film.

[0129] (Support) As the support for supporting the alignment film and the optically anisotropic layer, various sheet-like materials (films, plates) can be used as long as they can support the alignment film and the optically anisotropic layer. The support preferably has a transmittance of 50% or more, more preferably 70% or more, and even more preferably 85% or more for diffracted light (near infrared light).

[0130] There is no limitation on the thickness of the support, and the thickness that can support the alignment film and the optically anisotropic layer may be appropriately set depending on the application of the liquid crystal diffraction element, the material forming the support, etc. The thickness of the support is preferably 1 to 1000 μm, more preferably 3 to 250 μm, and even more preferably 5 to 150 μm.

[0131] The support may be a single layer or a multilayer. Examples of single-layer supports include supports made of glass, triacetyl cellulose (TAC), polyethylene terephthalate (PET), polycarbonate, polyvinyl chloride, acrylic, polyolefin, etc. Examples of multilayer supports include those that include any of the above-mentioned single-layer supports as a substrate and have another layer provided on the surface of this substrate.

[0132] (Alignment Film) An alignment film is formed on the surface of the support. The alignment film is used to orient the liquid crystal compound 40 into a predetermined liquid crystal alignment pattern when forming an optically anisotropic layer. As described above, in the present invention, the optically anisotropic layer has a liquid crystal alignment pattern in which the direction of the optical axis 40A (see FIG. 7) derived from the liquid crystal compound 40 changes while continuously rotating along one in-plane direction. Therefore, the alignment film is formed so that the optically anisotropic layer can form this liquid crystal alignment pattern. In the following description, "the direction of the optical axis 40A rotates" will also be simply referred to as "the optical axis 40A rotates."

[0133] Various known alignment films can be used, including, for example, a rubbed film made of an organic compound such as a polymer, an obliquely evaporated film of an inorganic compound, a film having microgrooves, and a film formed by accumulating LB (Langmuir-Blodgett) films made of organic compounds such as ω-tricosanoic acid, dioctadecylmethylammonium chloride, and methyl stearate by the Langmuir-Blodgett method.

[0134] The alignment film formed by rubbing treatment can be formed by rubbing the surface of the polymer layer several times in a certain direction with paper or cloth. Preferred materials for the alignment film include polyimide, polyvinyl alcohol, polymers having polymerizable groups as described in JP-A-9-152509, and materials used to form alignment films 32 as described in JP-A-2005-97377, JP-A-2005-99228, and JP-A-2005-128503.

[0135] The alignment film is preferably a so-called photo-alignment film, which is formed by irradiating a photo-alignable material with polarized or non-polarized light. That is, the alignment film is preferably a photo-alignment film formed by applying a photo-alignment material onto a support. The polarized light can be irradiated perpendicularly or obliquely to the photo-alignment film, and the non-polarized light can be irradiated obliquely to the photo-alignment film.

[0136] Examples of photo-alignment materials used in the alignment film that can be used in the present invention include those disclosed in JP-A-2006-285197, JP-A-2007-76839, JP-A-2007-138138, JP-A-2007-94071, JP-A-2007-121721, JP-A-2007-140465, JP-A-2007-156439, and JP-A-2007 azo compounds described in JP-A-133184, JP-A-2009-109831, Japanese Patent Nos. 3,883,848 and 4,151,746; aromatic ester compounds described in JP-A-2002-229039; maleimides having photo-orientable units described in JP-A-2002-265541 and JP-A-2002-317013; / or alkenyl-substituted nadimide compounds, photocrosslinkable silane derivatives described in Japanese Patent Nos. 4205195 and 4205198, photocrosslinkable polyimides, photocrosslinkable polyamides and photocrosslinkable polyesters described in JP-T-2003-520878, JP-T-2004-529220 and JP-T-4162850, and photodimerizable compounds described in JP-A-9-118717, JP-A-10-506420, JP-A-2003-505561, WO 2010 / 150748, JP-A-2013-177561 and JP-A-2014-12823, particularly cinnamate compounds, chalcone compounds and coumarin compounds are exemplified as preferred examples. Among these, azo compounds, photocrosslinkable polyimides, photocrosslinkable polyamides, photocrosslinkable polyesters, cinnamate compounds, and chalcone compounds are preferably used.

[0137] There is no limitation on the thickness of the alignment film, and it may be set appropriately depending on the material of which the alignment film is formed so as to obtain the necessary alignment function. The thickness of the alignment film is preferably 0.01 to 5 μm, and more preferably 0.05 to 2 μm.

[0138] There is no limitation on the method for forming the alignment film, and various known methods can be used depending on the material for forming the alignment film. One example is a method in which an alignment film is applied to the surface of a support, dried, and then exposed to laser light to form an alignment pattern.

[0139] Fig. 10 conceptually shows an example of an exposure device that exposes an alignment film to light to form an alignment pattern. The exposure device 60 shown in Fig. 10 includes a light source 64 equipped with a laser 62, a λ / 2 plate 65 that changes the polarization direction of laser light M emitted by the laser 62, a beam splitter 68 that splits the laser light M emitted by the laser 62 into two beams MA and MB, mirrors 70A and 70B that are respectively arranged on the optical paths of the two split beams MA and MB, and λ / 4 plates 72A and 72B. The light source 64 emits linearly polarized light P0. The λ / 4 plate 72A converts the linearly polarized light P0 (beam MA) into right-handed circularly polarized light P R The λ / 4 plate 72B converts the linearly polarized light P0 (light beam MB) into left-handed circularly polarized light P L are converted to , respectively.

[0140] A support 30 having an alignment film 32 before an alignment pattern is formed is placed in an exposure unit, and two light beams MA and MB are caused to intersect and interfere on the alignment film 32, and the alignment film 32 is irradiated with the interference light for exposure. This interference causes the polarization state of the light irradiating the alignment film 32 to periodically change in the form of interference fringes. This results in an alignment film having an alignment pattern in which the alignment state periodically changes (hereinafter also referred to as a pattern alignment film). In the exposure device 60, the period of the alignment pattern can be adjusted by changing the crossing angle α of the two light beams MA and MB. That is, in the exposure device 60, in an alignment pattern in which the optical axis 40A derived from the liquid crystal compound 40 continuously rotates along one direction, the length of one period in which the optical axis 40A rotates 180° in one direction of rotation of the optical axis 40A can be adjusted by adjusting the crossing angle α. By forming an optically anisotropic layer on the alignment film 32 having such an alignment pattern in which the alignment state changes periodically, it is possible to form an optically anisotropic layer having a liquid crystal alignment pattern in which the optical axis 40A derived from the liquid crystal compound 40 rotates continuously along one direction. In addition, by rotating the optical axes of the λ / 4 plates 72A and 72B by 90°, respectively, the rotation direction of the optical axis 40A can be reversed.

[0141] As described above, the patterned alignment film has an alignment pattern that aligns liquid crystal compounds in an optically anisotropic layer formed on the patterned alignment film, so that the orientation of the optical axis of the liquid crystal compound changes while continuously rotating along at least one in-plane direction. If the axis of the patterned alignment film is the axis along which the liquid crystal compound is aligned, then the patterned alignment film can be said to have an alignment pattern in which the orientation of the alignment axis changes while continuously rotating along at least one in-plane direction. The alignment axis of the patterned alignment film can be detected by measuring absorption anisotropy. For example, when the patterned alignment film is irradiated with linearly polarized light while rotating and the amount of light transmitted through the patterned alignment film is measured, the direction in which the amount of light is maximized or minimized is observed to gradually change along one in-plane direction.

[0142] In the present invention, the alignment film is provided as a preferred embodiment but is not an essential component. For example, by forming an alignment pattern on the support by a method of rubbing the support or a method of processing the support with laser light or the like, it is possible to configure the optically anisotropic layer to have a liquid crystal alignment pattern in which the direction of the optical axis 40A derived from the liquid crystal compound 40 changes while continuously rotating along at least one direction in the plane. That is, in the present invention, the support may function as an alignment film.

[0143] In the optically anisotropic layers shown in Figures 6 and 7, the optical axes of the liquid crystal compounds aligned in the thickness direction are aligned in the same direction, but this is not limiting. As in the optically anisotropic layer 36b shown in Figure 11, the layer may have an in-plane region in which the optical axes of the liquid crystal compounds are twisted along the thickness direction. In this case, the twist angle across the entire thickness direction in the region having a twist in the thickness direction is 10° to 360°.

[0144] In this way, when the optically anisotropic layer has a liquid crystal orientation pattern in which the direction of the optical axis 40A changes while continuously rotating in-plane along the alignment axis D, and the liquid crystal compound 40 has a twisted structure in the thickness direction, in a cross section parallel to the alignment axis D, a line segment connecting liquid crystal compounds 40 oriented in the same direction in the thickness direction is inclined with respect to the main surface of the optically anisotropic layer, and in an image obtained by observing a cross section of the optically anisotropic layer cut in the thickness direction along the alignment axis D with a scanning electron microscope (SEM), the striped pattern of bright and dark areas observed is inclined with respect to the main surface. This can increase the diffraction efficiency of the diffraction element.

[0145] In order to form an optically anisotropic layer in which the liquid crystal compound is twistedly aligned in the thickness direction, a chiral agent may be added to the liquid crystal composition for forming the optically anisotropic layer.

[0146] --Chiral Agents (Optically Active Compounds)--Chiral agents have the function of inducing a helical structure in a liquid crystal phase. Chiral agents can be selected according to the purpose, as the direction of helical twist and helical twisting power (HTP) they induce vary depending on the compound. There are no particular limitations on the chiral agent, and known compounds (e.g., those described in "Liquid Crystal Device Handbook," Chapter 3, Section 4-3, Chiral Agents for TN (Twisted Nematic) and STN (Super Twisted Nematic)," p. 199, edited by the 142nd Committee of the Japan Society for the Promotion of Science, 1989), isosorbide, and isomannide derivatives can be used. Chiral agents generally contain an asymmetric carbon atom, but axially asymmetric or planar asymmetric compounds without an asymmetric carbon atom can also be used as chiral agents. Examples of axially asymmetric or planar asymmetric compounds include binaphthyl, helicene, paracyclophane, and their derivatives. The chiral agent may have a polymerizable group. When both the chiral agent and the liquid crystal compound have a polymerizable group, a polymer having a repeating unit derived from the polymerizable liquid crystal compound and a repeating unit derived from the chiral agent can be formed by a polymerization reaction between the polymerizable chiral agent and the polymerizable liquid crystal compound. In this embodiment, the polymerizable group of the polymerizable chiral agent is preferably the same type of group as the polymerizable group of the polymerizable liquid crystal compound. Therefore, the polymerizable group of the chiral agent is also preferably an unsaturated polymerizable group, an epoxy group, or an aziridinyl group, more preferably an unsaturated polymerizable group, and even more preferably an ethylenically unsaturated polymerizable group. The chiral agent may also be a liquid crystal compound.

[0147] When the chiral agent has a photoisomerizable group, it is possible to form a desired twisted alignment corresponding to the emission wavelength by irradiating the chiral agent with actinic rays or the like through a photomask after coating and alignment. The photoisomerizable group is preferably an isomerization site of a compound exhibiting photochromic properties, an azo group, an azoxy group, or a cinnamoyl group. Specific compounds that can be used include those described in JP-A-2002-80478, JP-A-2002-80851, JP-A-2002-179668, JP-A-2002-179669, JP-A-2002-179670, JP-A-2002-179681, JP-A-2002-179682, JP-A-2002-338575, JP-A-2002-338668, JP-A-2003-313189, and JP-A-2003-313292.

[0148] The content of the chiral dopant in the liquid crystal composition is preferably 0.01 to 200 mol %, more preferably 1 to 30 mol %, based on the molar amount of the liquid crystal compound.

[0149] The optically anisotropic layer may have regions with different twist states (twist angles and twist directions) in the thickness direction. In such a configuration, in an image obtained by observing with a scanning electron microscope a cross section of the optically anisotropic layer cut in the thickness direction along one direction in which the orientation of the optical axis of the liquid crystal compound changes while rotating continuously, bright and dark areas extending from one main surface to the other main surface are observed, and the dark areas have one or more inflection points.

[0150] An example of such an optically anisotropic layer is shown in Figure 12. In Figure 12, a bright region 42 and a dark region 44 are shown superimposed on a cross section of the optically anisotropic layer 36c. In the following description, an image obtained by observing a cross section cut in the thickness direction along one direction in which the optical axis rotates, using an SEM, will also be simply referred to as a "cross-sectional SEM image."

[0151] 12, the optically anisotropic layer 36c has two inflection points where the angle changes in the dark portion 44. That is, the optically anisotropic layer 36c can be said to have three regions in the thickness direction, namely, region 37a, region 37b, and region 37c, according to the inflection points of the dark portion 44.

[0152] The optically anisotropic layer 36c has a liquid crystal orientation pattern in which the optical axis originating from the liquid crystal compound 40 rotates clockwise toward the left in the in-plane direction as viewed from above in the figure at any position in the thickness direction. Furthermore, one period of the liquid crystal orientation pattern is constant in the thickness direction.

[0153] As shown in FIG. 12 , in the lower region 37a in the thickness direction, the liquid crystal compound 40 is twisted in a clockwise (right-handed) spiral manner from top to bottom in the thickness direction. In the middle region 37b in the thickness direction, the liquid crystal compound 40 is not twisted in the thickness direction, and the liquid crystal compound 40 stacked in the thickness direction has the same optical axis. That is, the liquid crystal compound 40 present at the same position in the in-plane direction has the same optical axis. In the upper region 37c in the thickness direction, the liquid crystal compound 40 is twisted in a counterclockwise (left-handed) spiral manner from top to bottom in the thickness direction. That is, in the optically anisotropic layer 36c shown in FIG. 12 , the liquid crystal compound 40 in the region 37a, region 37b, and region 37c has a different twist state in the thickness direction.

[0154] In an optically anisotropic layer having a liquid crystal orientation pattern in which the optical axes derived from the liquid crystal compounds continuously rotate in one direction, the bright and dark areas in a cross-sectional SEM image of the optically anisotropic layer are observed to connect liquid crystal compounds with the same orientation. As an example, FIG. 12 shows that dark areas 44 are observed to connect liquid crystal compounds 40 whose optical axes are oriented perpendicular to the paper surface. In the bottom region 37a in the thickness direction, the dark areas 44 are inclined toward the upper left in the figure. In the middle region 37b, the dark areas 44 extend in the thickness direction. In the top region 37c, the dark areas 44 are inclined toward the upper right in the figure. That is, the optically anisotropic layer 36c shown in FIG. 12 has two inflection points at which the angle of the dark areas 44 changes. Furthermore, in the top region 37c, the dark areas 44 are inclined toward the upper right, and in the bottom region 37b, the dark areas 44 are inclined toward the upper left. That is, the inclination direction of the dark portion 44 is different between the region 37c and the region 37a.

[0155] 12, the dark portion 44 of the optically anisotropic layer 36c has one inflection point where the tilt direction turns back to the opposite direction. Specifically, in the dark portion 44 of the optically anisotropic layer 36c, the tilt direction in the region 37c is opposite to the tilt direction in the region 37b. Therefore, the inflection point located at the interface between the region 37c and the region 37b is the inflection point where the tilt direction turns back to the opposite direction. In other words, the optically anisotropic layer 36c has one inflection point where the tilt direction turns back to the opposite direction.

[0156] In addition, in the optically anisotropic layer 36c, the regions 37c and 37a have, for example, the same thickness, and as described above, the liquid crystal compound 40 has different twist states in the thickness direction. Therefore, as shown in Figure 1, the bright region 42 and the dark region 44 in the cross-sectional SEM image have a substantially C-shape. Therefore, in the optically anisotropic layer 36c, the shape of the dark region 44 is symmetrical with respect to the center line in the thickness direction.

[0157] Such an optically anisotropic layer 36c, i.e., an optically anisotropic layer 36c having bright and dark regions 42 and 44 extending from one surface to the other in a cross-sectional SEM image, and the dark regions 44 having one or more inflection points, can reduce the wavelength dependency of the diffraction efficiency and diffract light with the same diffraction efficiency regardless of the wavelength.Furthermore, the wide-angle characteristics of the optically anisotropic layer 36c are improved, and light can be diffracted with the same diffraction efficiency regardless of the incident angle.

[0158] 12, the dark portion 44 has two inflection points, but the present invention is not limited thereto. The dark portion 44 may have one inflection point, or three or more inflection points. For example, when the dark portion 44 of the optically anisotropic layer has one inflection point, the dark portion 44 may be composed of the region 37c and the region 37a shown in FIG. 12, or the region 37c and the region 37b, or the region 37b and the region 37a. Alternatively, when the dark portion 44 of the optically anisotropic layer has three inflection points, the dark portion 44 may have two alternating regions 37c and two alternating regions 37a shown in FIG. 12.

[0159] The period Λ in the optically anisotropic layer may be appropriately set depending on the refraction angle of transmitted light. The period Λ is preferably about 1 to 3 times the wavelength of the near-infrared light emitted from the light source. By setting the period Λ in this range, the refraction angle can be set to the oblique incident and outgoing angles shown by α and β in Figure 1.

[0160] The blood flow measuring device of the present invention has been described in detail above, but the present invention is not limited to the above examples, and various improvements and modifications may of course be made within the scope of the gist of the present invention.

[0161] 30 Support 32 Alignment film 36, 36b to 36c Liquid crystal diffraction element (optically anisotropic layer) 37a to 37c Region 60 Exposure device 62 Laser 64 Light source 65 λ / 2 plate 68 Beam splitter 70A, 70B Mirror 72A, 72B λ / 4 plate 100, 100a to 100d Blood flow measurement device 102 Control unit 104 Light source unit 106, 106a to 106d First polarizing element 108 Light receiving unit 110, 110a to 106d Second polarizing element 112 Housing 120, 122 Linear polarizer 120a, 122a First linear polarizer 120b, 122b Second linear polarizer 124, 125 λ / 4 plate 126, 127 Retardation layer 128 Liquid crystal diffraction element S Living body R Region Λ One period D Arrangement axis L1, L4 Incident light L2, L5 Transmitted light M Laser light MA, MB Light ray P O Linear polarized light P R Right circular polarization P L Left circularly polarized light α crossing angle

Claims

1. A blood flow measurement device comprising a light source unit that irradiates an object with near-infrared light, and a light receiving unit that receives scattered light generated by scattering of the near-infrared light emitted from the light source unit by the object, a first polarizing element that is disposed on the front surface of the light source unit and includes a layer formed using a liquid crystal compound, and that changes the polarization state of the near-infrared light, and a second polarizing element that is disposed on the front surface of the light receiving unit and includes a layer formed using a liquid crystal compound, and that changes the polarization state of the near-infrared light.

2. The blood flow measurement device according to claim 1, wherein the layer formed using the liquid crystal compound included in the first polarizing element is a linear polarizer.

3. The blood flow measurement device according to claim 2, wherein the first polarizing element further includes a λ / 4 plate.

4. The blood flow measurement device according to claim 3, wherein the λ / 4 plate exhibits inverse wavelength dispersion.

5. The first polarizing element has, in this order, a first linear polarizer, a retardation layer, and a second linear polarizer, and at least one of the first linear polarizer and the second linear polarizer is a layer formed using the liquid crystal compound. The blood flow measurement device according to claim 1.

6. The blood flow measurement device according to claim 5, wherein the retardation layer exhibits inverse wavelength dispersion.

7. The blood flow measurement device according to any one of claims 1 to 6, wherein the layer formed using the liquid crystal compound included in the first polarizing element has a liquid crystal alignment pattern in which the direction of the optical axis derived from the liquid crystal compound continuously changes while rotating along at least one direction in the plane.

8. The blood flow measurement device according to any one of claims 1 to 6, wherein the liquid crystal compound is a rod-like liquid crystal compound or a disc-like liquid crystal compound.