Patent Publication Number: US-2017360296-A1

Title: Optical measurement apparatus for eyeball

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-119015 filed Jun. 15, 2016. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an optical measurement apparatus for an eyeball. 
     SUMMARY 
     According to an aspect of the invention, an optical measurement apparatus for an eyeball includes: 
     a light reflecting unit that reflects light in a direction where the light passes across an anterior chamber of the eyeball; and 
     a switching unit that switches an incident position of the light to the light reflecting unit so as to inhibit the light from being moved from a state in which the light passes across the anterior chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIGS. 1A and 1B  are views illustrating an example of a configuration of an optical measurement apparatus for an eyeball to which a first exemplary embodiment is applied, in which  FIG. 1A  is a view obtained when the eyeball is viewed from the top side, and  FIG. 1B  is a view obtained when the eyeball is viewed from the front side; 
         FIGS. 2A and 2B  are views illustrating an exemplary configuration of an optical measurement apparatus for an eyeball to which a second exemplary embodiment is applied, in which  FIG. 2A  is a view obtained when the eyeball is viewed from the top side, and  FIG. 2B  is a view when the eyeball is viewed from the front side; 
         FIGS. 3A and 3B  are views illustrating an exemplary configuration of an optical measurement apparatus for an eyeball to which a third exemplary embodiment is applied, in which  FIG. 3A  is a view obtained when the eyeball is viewed from the top side, and  FIG. 3B  is a view of the eyeball obtained when the eyeball is viewed from the front side; 
         FIG. 4  is a view for explaining a method of measuring, by the optical measurement apparatus, a rotation angle (optical rotation) of a vibrating surface caused by an optically active substance included in an aqueous humor in an anterior chamber; 
         FIGS. 5A and 5B  are views illustrating an exemplary configuration of an optical measurement apparatus for an eyeball to which a fourth exemplary embodiment is applied, in which  FIG. 5A  is a view obtained when the eyeball is viewed from the top side, and  FIG. 5B  is a view obtained when the eyeball is viewed from the front side; 
         FIGS. 6A and 6B  are views illustrating an exemplary configuration of an optical measurement apparatus for an eyeball to which a fifth exemplary embodiment is applied, in which  FIG. 6A  is a view obtained when the eyeball is viewed from the top side, and  FIG. 6B  is a view obtained when the eyeball is viewed from the front side; and 
         FIGS. 7A and 7B  are views illustrating an exemplary configuration of an optical measurement apparatus for an eyeball to which a sixth exemplary embodiment is applied, in which  FIG. 7A  is a view obtained when the eyeball is viewed from the top side, and  FIG. 7B  is a view obtained when the eyeball is viewed from the front side. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. Further, the accompanying drawings illustrate an eyeball such that the eyeball is greater than other members (such as an optical system to be described below) or smaller than the other members (such as the optical system to be described below) in order to clearly define a relationship between the eyeball and an optical path. 
     (Background of Measuring Glucose Concentration of Aqueous Humor) 
     First, a background of measuring the glucose concentration of an aqueous humor will be described. 
     It is recommended that a type 1 diabetic patient and a type 2 diabetic patient (measurement subject), who require insulin therapy, perform a self-blood glucose monitoring. To perform the self-blood glucose monitoring, the measurement subject measures his/her blood glucose level by him/herself at home or the like in order to precisely control the blood glucose. 
     A self-blood glucose monitor, which is currently distributed, measures the glucose concentration in the blood by puncturing a fingertip or the like by using a needle and then collecting a minute amount of blood. It is frequently recommended that the patient performs the self-blood glucose monitoring after each meal or before going to bed, and it is required to perform the self-blood glucose monitoring once to several times a day. In particular, in the case of intensive insulin therapy, it is required to perform the measurement much more times. 
     For this reason, an invasive blood glucose level monitoring method using a puncture-type self-blood glucose monitor easily causes a deterioration in incentives in respect to the self-blood glucose monitoring of the measurement subject due to the pain caused by a test (puncture by a pin prick) when collecting the blood. For this reason, in some cases, it is difficult to efficiently treat diabetes. 
     Therefore, a non-invasive blood glucose level monitoring method is being developed which does not requires the puncture, instead of the invasive blood glucose monitoring method such as the puncture. 
     As a non-invasive blood glucose level monitoring method, a near infrared spectroscopy method, an optoacoustic spectroscopy method, a method of using optical rotation, and the like are discussed. Further, these methods estimate a blood glucose level based on a glucose concentration. 
     The near infrared spectroscopy and the optoacoustic spectroscopy detect light absorption spectrum or acoustic vibration of the blood in a blood vessel of a finger. However, cell substances such as erythrocytes and leukocytes are present in the blood. For this reason, the near infrared spectroscopy method and the optoacoustic spectroscopy method are greatly affected by light scattering. Further, the near infrared spectroscopy method and the optoacoustic spectroscopy method are also affected by tissue at a circumference of the blood vessel as well as the blood in the blood vessel. Therefore, in these methods, it is difficult to separate signals because it is necessary to detect a signal with respect to a glucose concentration based on signals associated with enormous substances such as protein, amino acid, and the like. 
     Meanwhile, the aqueous humor in the anterior chamber has almost the same substances as serum, and includes protein, glucose, ascorbic acid, and the like. However, the aqueous humor does not include cell substances such as erythrocyte and leukocytes unlike the blood, and thus the aqueous humor is less affected by light scattering. Therefore, the aqueous humor is suitable for the optical measurement of a glucose concentration. 
     Therefore, the concentration of optically active substances including glucose may be optically measured by using the aqueous humor. 
     In addition, protein, glucose, ascorbic acid, and the like included in the aqueous humor are optically active substances, and have optical rotation. Therefore, the concentration of optically active substances including glucose may be optically measured by using the optical rotation. 
     Because the aqueous humor is a tissue fluid for transporting the glucose, it is considered that the glucose concentration in the aqueous humor is associated with the glucose concentration in the blood. Further, it is reported that in the measurement using a rabbit, a length of time required to transport the glucose from the blood to the aqueous humor (transportation delay time) is within 10 minutes. 
     As described above, the glucose concentration in the blood is obtained by measuring the glucose concentration in the aqueous humor. 
     By the way, in a technique of optically measuring the concentration of an optically active substance such as glucose included in an aqueous humor, the following two optical paths may be set. 
     One is an optical path in which light is caused to enter the eyeball at an angle approximately perpendicular to the eyeball, that is, in a front-back direction, the light is caused to be reflected by an interface between a cornea and the aqueous humor or an interface between the aqueous humor and a crystalline lens, and the reflected light is received (detected). The other is an optical path in which light is caused to enter the eyeball at an angle approximately parallel to the eyeball, and the light, which is caused to pass through across the anterior chamber, is received (detected). 
     In the case of the optical path like the former optical path in which the light is caused to enter the eyeball at an angle perpendicular to the eyeball, there is a concern that the light may reach a retina. In particular, in a case in which a laser having a high coherency is used as a light source, there is a concern that the light may reach the retina. 
     On the contrary, in the case of the optical path like the latter optical path in which light is caused to enter the eyeball at an angle approximately parallel to the eyeball, and to pass through the anterior chamber while traversing the anterior chamber, the light is inhibited from reaching the retina. 
     The concentration of the optically active substance or the optical rotation depends on the length of the optical path, and the longer the optical path is, the higher the optical rotation is. Therefore, since the light passes through across the anterior chamber, the length of the optical path may be set to be long. 
     As described above, the optical path in which the light is caused to pass through across the anterior chamber is adopted here. 
     First Exemplary Embodiment 
     &lt;Optical Measurement Apparatus  1 &gt; 
       FIGS. 1A and 1B  are views illustrating an exemplary configuration of an optical measurement apparatus  1  for an eyeball to which a first exemplary embodiment is applied.  FIG. 1A  is a view obtained when an eyeball  10  is viewed from the top side (a cross-sectional view in upward and downward directions), and  FIG. 1B  is a view obtained when the eyeball  10  is viewed from the front side. Further, it is assumed that the eyeball  10  illustrated in  FIGS. 1A and 1B  is a left eye.  FIGS. 1A and 1B  illustrate, by arrows, an inside-outside direction which indicates an inside of a face (nose side) and an outside of the face (ear side), an front-back direction which indicates front and back sides of the face, and an up-down direction which indicates the upper and lower sides of the face. 
     The optical measurement apparatus  1  for an eyeball (hereinafter, referred to as an “optical measurement apparatus  1 ”) includes an optical system  20  which is used to measure a property of an aqueous humor in an anterior chamber  13  (to be described below) of the eyeball (subject&#39;s eye)  10  of a measurement subject (test subject), a signal processor  30  which processes signals obtained by the optical system  20 , and a controller  40  which controls the optical system  20 . 
     The optical measurement apparatus  1  to which the first exemplary embodiment is applied measures the concentration of an optically active substance included in the aqueous humor based on the light intensity of transmitted light transmitted through the aqueous humor. 
     First, the structure of the eyeball  10  will be described. 
     As illustrated in  FIG. 1A , the eyeball  10  has a substantially spherical external shape, and has a vitreous body  11  at the center thereof. Further, a rear half of the eyeball  10  is omitted from  FIG. 1A . Further, a crystalline lens  12 , which serves as a lens, is buried in a part of the vitreous body  11 . The anterior chamber  13  is present at the front side of the crystalline lens  12 , and a cornea  14  is present at the front side of the anterior chamber  13 . The anterior chamber  13  and the cornea  14  protrude convexly from the spherical shape. 
     The peripheral portion of the crystalline lens  12  is surrounded by an iris  17 , and a pupil  15  is present at the center of the iris  17 . The vitreous body  11  is covered by a retina  16 , except for a portion in contact with the crystalline lens  12 . Further, the retina  16  is covered by a sclera  18 . That is, an outside of the eyeball  10  is covered by the cornea  14  and the sclera  18 . 
     The anterior chamber  13  is a region surrounded by the cornea  14  and the crystalline lens  12 . The anterior chamber  13  has a circular shape when viewed from the front side (see  FIG. 1B ). Further, the anterior chamber  13  is filled with the aqueous humor. 
     As illustrated in  FIG. 1B , the surface of the eyeball  10  is covered by an upper eyelid  19   a  and a lower eyelid  19   b.    
     Next, the optical system  20  will be described. 
     As illustrated in  FIG. 1A , the optical system  20  includes a light emitting system  20 A which emits light toward the anterior chamber  13  of the eyeball  10 , and a light receiving system  20 B which receives light passing through the anterior chamber  13 . 
     First, the light emitting system  20 A includes a light source  21 , a collimator lens  22 , a deflector  23 , and a mirror  27  as an example of a light reflecting unit. 
     The light source  21  may be a light source having a wide wavelength width like a light emitting diode (LED) or a lamp, or may be a light source having a narrow wavelength width like a laser. In addition, the light source  21  may have plural LEDs, lamps, or lasers. Further, the light source  21  may use plural wavelengths. 
     The collimator lens  22  converts a light beam, which is projected from the light source  21  and has an area, into parallel light beams each having a small diameter. Because the anterior chamber  13  is a small region surrounded by the cornea  14  and the crystalline lens  12 , it is more desirable that the light beams to be transmitted through the anterior chamber  13  have small diameters. 
     At this time, when the light projected from the light source  21  has a small diameter, it is not necessary to use the collimator lens  22 . 
     The deflector  23  refers to a member that deflects a direction in which the light travels, and for example, the deflector  23  includes a mirror  231 , and a driving device  232  which changes the inclination of a reflecting surface of the mirror  231 . The mirror  231  may be a galvano mirror or a polygon mirror. In the case of the galvano mirror, the inclination of a reflecting surface is changed by rotating the reflecting surface about an axis provided on the reflecting surface. In the case of the polygon mirror, the inclination of a reflecting surface is changed by rotating a polyhedral mirror. The galvano mirror or the polygon mirror deflects the light in a one-dimensional direction because the reflecting surface is inclined in one direction (one-dimensional direction). 
     The mirror  231  may be a mirror configured with micro electro mechanical systems (MEMS). In a case in which the reflecting surface is configured to be inclined with respect to a point, the reflecting surface is inclined in one direction and a direction orthogonal to the one direction. Therefore, because the reflecting surface is inclined in a two-dimensional direction, the light is deflected in the two-dimensional direction. 
     The inclination of the mirror  231  is controlled by the driving device  232 . In a case in which the mirror  231  is the galvano mirror or the polygon mirror, the driving device  232  includes, for example, a motor, and a circuit that controls the motor. In addition, in a case in which the mirror  231  is configured with the MEMS, the driving device  232  is a driving circuit that is configured integrally with the mirror  27  and supplies an electric potential to plural electrodes that control the inclination of the mirror  27  by using an electrostatic force. 
     The mirror  231  is an example of a reflective member, and the driving device  232  is an example of an angle change unit. 
     The mirror  27  reflects the light deflected by the deflector  23  so that the light passes across the anterior chamber  13 . In the first exemplary embodiment, similar to the deflector  23 , the mirror  27  is connected to a driving device  28 . The mirror  27  is a galvano mirror, a polygon mirror, or a mirror configured with the MEMS. Further, the inclination of the mirror  27  is changed by the driving device  28  so as to change the reflection angle with respect to the incident light. 
     Here, the deflector  23  and the driving device  28  are an example of a switching unit. 
     The light receiving system  20 B includes a detector  29 . Here, the detector  29  is a light receiving element such as, for example, a silicon diode. The detector  29  converts the intensity of the light passing through the anterior chamber  13  into an electrical signal. 
     The signal processor  30  receives the electrical signal from the detector  29  and processes the electrical signal so as to calculate the concentration of the optically active substance included in the aqueous humor. 
     As described above, the controller  40  controls the optical system  20  and the signal processor  30 . 
     Next, a relationship between the eyeball  10  and the optical system  20  will be described. 
     First, as illustrated in  FIG. 1A , the optical system  20  is set with respect to the eyeball  10  such that light projected from the light emitting system  20 A is caused to enter the light receiving system  20 B through an optical path indicated by an optical path α. That is, as illustrated in  FIG. 1A , the optical path α passes through the central portion of the anterior chamber  13  when viewing the eyeball  10  in a cross-sectional view in the up-down direction. Further, as illustrated in  FIG. 1B , the optical path α passes through the central portion of the anterior chamber  13  even when viewing the eyeball  10  from the front side. 
     The optical path α is an optical path suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 . 
     In addition, an optical path β illustrated in  FIG. 1A  is an optical path which is too much forward of the eyeball  10  and is reflected by the surface of the cornea  14 . The optical path β does not pass through the aqueous humor in the anterior chamber  13 . In addition, an optical path γ is an optical path which is too much rearward of the eyeball  10  and is blocked by the iris  17  or the sclera  18 . The optical path γ does not pass through the aqueous humor in the anterior chamber  13 . 
     An optical path δ illustrated in  FIG. 1B  is an optical path which is too much upward of the eyeball  10 , and a length by which the optical path δ passes through the aqueous humor in the anterior chamber  13  is short. Further, if an optical path is further too much upward of the eyeball  10  as compared with the optical path δ, the optical path is blocked by the upper eyelid  19   a  and does not pass through the aqueous humor in the anterior chamber  13 . 
     An optical path ε is an optical path which is much downward of the eyeball  10 , and a length by which the optical path ε passes through the aqueous humor in the anterior chamber  13  is short. Further, if an optical path is further too much downward of the eyeball  10  as compared to the optical path ε, the optical path is blocked by the lower eyelid  19   b  and does not pass through the aqueous humor in the anterior chamber  13 . 
     The terms “optical paths α, β, γ, δ, and ε” are used to explain the states and positions of the optical paths with respect to the anterior chamber  13  of the eyeball  10 . 
     However, in some cases, the optical paths may deviate because a relative position relationship between the eyeball  10  and the optical system  20  or a shape of the cornea  14  is changed over time, and as a result, a state of the optical path α may not be maintained. Further, the eyeball  10  may be moved relative to the optical system  20 , and the optical system  20  may be moved relative to the eyeball  10 . Hereinafter, for convenience, the description will be made assuming that the eyeball  10  is moved relative to the optical system  20 . 
     In a case in which an optical path, which is in the state of the optical path α, is brought into the state of the optical path β or the optical path γ or brought into a state of the optical path δ or the optical path ε with respect to the eyeball  10 , that is, in a case in which the optical path slightly deviates, the state of the optical path may return back to the state of the optical path α by displacing (moving) or switching the optical path. That is, it is not necessary to set the optical system  20  again with respect to the eyeball  10 . 
     For example, as illustrated in  FIG. 1A , it is assumed that an optical path, which is in the state of the optical path α, is brought into the state of the optical path β because the eyeball  10  moves rearward. In this case, a new optical path may be set at a position of the optical path γ. Therefore, based on a control by the controller  40 , the deflector  23  sets an optical path to a position of the optical path γ by switching an incident position to the mirror  27 . That is, an incident position of light is switched to the mirror  27  in order to set an optical path to the position of the optical path γ from the position of the optical path α, so that the position of the optical path γ is reset to the state of the optical path α which is suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 . 
     Similarly, it is assumed that an optical path, which is in the state of the optical path α, is brought into the state of the optical path γ because the eyeball  10  moves forward. In this case, a new optical path may be set at the position of the optical path β. Therefore, based on a control by the controller  40 , the deflector  23  sets an optical path to the position of the optical path β by switching an incident position to the mirror  27 . That is, the incident position of light is switched to the mirror  27  in order to set an optical path to the position of the optical path β from the position of the optical path α, so that the position of the optical path β is reset to the state of the optical path α which is suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 . 
     As illustrated in  FIG. 1B , it is assumed that an optical path, which is in the state of the optical path α, is brought into the state of the optical path ε because the eyeball  10  moves upward. In this case, a new optical path may be set at the position of the optical path δ. Therefore, based on a control by the controller  40 , the deflector  23  sets an optical path to the position of the optical path δ by switching an incident position to the mirror  27 . That is, the incident position of light is switched to the mirror  27  in order to set an optical path to the position of the optical path δ from the position of the optical path α, so that the position of the optical path δ is reset to the state of the optical path α which is suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 . 
     Similarly, it is assumed that an optical path, which is in the state of the optical path α, is brought into the state of the optical path δ because the eyeball  10  moves downward. In this case, a new optical path may be set at the position of the optical path ε. Therefore, based on a control by the controller  40 , the deflector  23  sets an optical path to the position of the optical path ε by switching the incident position to the mirror  27 . That is, the incident position of light is switched to the mirror  27  in order to set an optical path to the position of the optical path ε from the position of the optical path α, so that the position of the optical path ε is reset to the state of the optical path α which is suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 . 
     In this case, the incident angle of light is changed by changing the inclination (incident angle of light) of the mirror  231  of the deflector  23  and the inclination (incident angle of light) of the mirror  27 . 
     In  FIGS. 1A and 1B , the optical paths run in parallel. This is because a relative positional relationship between the light emitting system  20 A and the light receiving system  20 B of the optical system  20  is maintained. The optical paths may not necessarily run in parallel. 
     The mirror  231  or the mirror  27  is described as being a plane mirror, but may be a concave mirror, a convex mirror, a spherical mirror, a parabolic mirror, and the like. 
     As described above, in the optical measurement apparatus  1  of the first exemplary embodiment, even though an optical path, which is in the state of the optical path α, deviates because a relative position relationship between the eyeball  10  and the optical system  20 , a shape of the cornea  14 , or the like is changed over time, the optical path is reset to the state of the optical path α, which is suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 , by switching the incident position of light to the mirror  27 . That is, the optical path is set to pass across the anterior chamber  13 . 
     The state in which the optical path deviates from the state of the optical path α may be easily detected by the signal processor  30  that receives a signal from the detector  29 . Therefore, based on the signal from the signal processor  30 , the controller  40  may control the inclination (incident angle of light) of the mirror  231  of the deflector  23  and the inclination (incident angle of light) of the mirror  27 . 
     The incident angle of the mirror  231  of the deflector  23  and the incident angle of the mirror  27  may be changed not only in the inside-outside direction and but also in the up-do direction. In a case in which the incident angle is changed in the inside-outside direction, the incident position of light is switched to the mirror  27  in a one-dimensional direction in the front-back direction (among the optical paths α, β, and γ). In addition, in a case in which the incident angle is changed in the up-down direction, the incident position of light is switched to the mirror  27  in a one-dimensional direction in the up-down direction (among the optical paths α, δ, and ε). In a case in which the incident angle is changed in the front-back direction and the up-down direction, the incident position of light is switched to the mirror  27  in a two-dimensional direction in the front-back direction (among the optical paths α, β, and γ) and the up-down direction (among the optical paths α, δ, and ε). 
     Second Exemplary Embodiment 
     In the first exemplary embodiment, the incident angle of light is changed by the mirror  27  in addition to the mirror  231  of the deflector  23 . 
     In the second exemplary embodiment, the incident angle of light to the mirror  27  is fixed. 
       FIGS. 2A and 2B  are views illustrating an exemplary configuration of the optical measurement apparatus  1  for an eyeball to which the second exemplary embodiment is applied.  FIG. 2A  is a view obtained when the eyeball  10  is viewed from the top side (a cross-sectional view in the upward and downward directions), and  FIG. 2B  is a view obtained when the eyeball  10  is viewed from the front side. The parts similar to those of the optical measurement apparatus  1  to which the first exemplary embodiment is applied are designated by the same reference numerals, and description thereof will be omitted. 
     In the optical measurement apparatus  1  for an eyeball to which the second exemplary embodiment is applied, a telecentric optical system  24  including a telecentric fθ lens is provided between the deflector  23  and the mirror  27 . Further, the mirror  27  does not have the driving device  28  provided in the first exemplary embodiment. Here, the deflector  23  and the telecentric optical system  24  are an example of a switching unit. 
     The telecentric fθ lens is a lens that condenses an incident light beam in a direction perpendicular to a flat surface. That is, as illustrated in  FIGS. 2A and 2B , even though light beams are reflected by the mirror  231  of the deflector  23  and then obliquely enter the telecentric optical system  24 , the light beams are projected from the telecentric optical system  24  in parallel with each other. 
     Therefore, even though an incident angle (inclination) of the mirror  27  is fixed, the optical paths, which run toward the eyeball  10 , are changed to be parallel to each other so that the optical paths run in parallel with each other by switching an incident position to the mirror  27 . 
     Therefore, an incident position to the mirror  27  may be switched by controlling the reflection angle of the mirror  231  of the deflector  23 . That is, the control of switching an incident position to the mirror  27  is simplified. 
     Because the mirror  27  is provided to be close to the eyeball  10 , in the optical measurement apparatus  1  to which the first exemplary embodiment is applied, a dynamic force is applied to the measurement subject when the mirror  27  is moved (rotated) to change the incident angle of the mirror  27 . However, because the incident angle of the mirror  27  is fixed in the optical measurement apparatus  1  to which the second exemplary embodiment is applied, a dynamic force is inhibited from being applied to the measurement subject. 
     Because the operation of switching an incident position to the mirror  27  is identical to that described in the first exemplary embodiment except that the incident angle (inclination) of the mirror  27  is fixed, a description thereof will be omitted. 
     Third Exemplary Embodiment 
     In the first exemplary embodiment and the second exemplary embodiment, the concentration of the optically active substance included in the aqueous humor is measured based on a change in intensity of the light transmitted through the aqueous humor in the anterior chamber  13 . 
     In the third exemplary embodiment, the concentration of the optically active substance such as glucose included in the aqueous humor is measured by using an optical rotation. 
       FIGS. 3A and 3B  are views illustrating an exemplary configuration of the optical measurement apparatus  1  for an eyeball to which the third exemplary embodiment is applied.  FIG. 3A  is a view obtained when the eyeball  10  is viewed from the top side (a cross-sectional view in the upward and downward directions), and  FIG. 3B  is a view obtained when the eyeball  10  is viewed from the front side. The parts similar to those of the optical measurement apparatus  1  to which the second exemplary embodiment is applied (the optical measurement apparatus  1  to which the first exemplary embodiment is applied, except for some parts) are designated by the same reference numerals, and descriptions thereof will be omitted. 
     The optical measurement apparatus  1  to which the third exemplary embodiment is applied has a polarization controller  25  in addition to the parts of the optical measurement apparatus  1  to which the second exemplary embodiment is applied. The polarization controller  25  is an example of a polarization control part. 
     The polarization controller  25  includes a polarizer and a waveplate. Further, the polarization controller  25  extracts a predetermined polarized light (linearly polarized light, elliptically polarized light, circularly polarized light, etc.) from light projected from the light source  21 . 
     When the light is reflected by the mirror  27 , the reflectivity of a component (P) parallel to an incident surface and the reflectivity of a component (S) perpendicular to the incident surface depend on the refractive index and the incident angle of the mirror  27 , respectively. For this reason, when the polarized light enters the mirror  27 , the polarization state of the reflected light is changed by the incident angle in some cases. For example, in a case in which the linearly polarized light enters the mirror  27 , the reflected light may also be linearly polarized at a certain incident angle, and the reflected light may be elliptically polarized at another incident angle. 
     Therefore, an incident angle to the mirror  27  may be fixed. 
     Therefore, similar to the second exemplary embodiment, the optical measurement apparatus  1  to which the third exemplary embodiment is applied is configured such that it is not necessary to consider a change in polarization state caused by a change in incident angle to the mirror  27  by using the telecentric optical system  24  including the telecentric fθ lens. 
     Similarly, when the polarized light passes through the lens, a polarization state is changed. Therefore, the polarization controller  25  is provided at a subsequent stage of the telecentric fθ lens of the telecentric optical system  24 , that is, between the telecentric fθ lens and the mirror  27 . 
     Here, the deflector  23  and the telecentric optical system  24  are also an example of a switching unit. 
     The detector  29  includes an analyzer or the like to detect an angle of optical rotation, as described below. 
     In a case in which a refractive index of the mirror  27 , a polarization state (a direction of a vibrating surface, linear polarization, and elliptical polarization) of incident light, and an incident angle is already known, the polarization state of the reflected light may be calculated. Therefore, concentration of the optically active substance may be measured by using optical rotation by providing the polarization controller  25  in the optical measurement apparatus  1  for an eyeball to which the first exemplary embodiment is applied. 
     Because the operation of switching an incident position of light to the mirror  27  is similar to those described in the first exemplary embodiment and the second exemplary embodiment except that the concentration of the optically active substance is measured by using the optical rotation, a description thereof will be omitted. 
     (Calculation of Concentration of Optically Active Substance) 
       FIG. 4  is a view explaining a method of measuring, by the optical measurement apparatus  1 , a rotation angle (optical rotation) of a vibrating surface caused by an optically active substance included in the aqueous humor in the anterior chamber  13 . Here, for easy description, the optical path is configured not to be bent, and the telecentric optical system  24  and the mirror  27  are omitted. 
     It is assumed that the polarization controller  25  of the optical system  20  has a polarizer  251 , and the detector  29  has a compensator  291 , an analyzer  292 , and a light receiving element  293 . 
     Shapes of polarized lights, when viewed in a traveling direction of light, among the light source  21 , the polarizer  251  of the polarization controller  25 , the anterior chamber  13 , and the compensator  291 , the analyzer  292 , and the light receiving element  293  of the detector  29 , which are illustrated in  FIG. 4 , are indicated by arrows in circles, respectively. 
     The optical system  20  may have other elements (optical components, etc.). 
     The polarizer  251  is, for example, a Nicol prism or the like, and causes linearly polarized light from incident light, which has a predetermined vibrating surface, to pass therethrough. 
     The compensator  291  is, for example, a magneto-optical element such as a Faraday element using a garnet or the like, and rotates the vibrating surface of the linearly polarized light by a magnetic field. 
     The analyzer  292  is the same member as the polarizer  251 , and allows the linearly polarized light having a predetermined vibrating surface to pass therethrough. 
     The light receiving element  293  is a silicon diode or the like, and outputs an output signal corresponding to intensity of light. 
     The light source  21  emits light having a random vibrating surface. Further, the polarizer  251  allows the linearly polarized light having a predetermined vibrating surface to pass therethrough. In  FIG. 4 , for example, the polarizer  251  allows linearly polarized light having a vibrating surface parallel to the drawing sheet to pass therethrough. 
     The vibrating surface of the linearly polarized light passing through the polarizer  251  is rotated by the optically active substance included in the aqueous humor in the anterior chamber  13 . In  FIG. 4 , the vibrating surface is rotated by an angle α M  (optical rotation α M ). 
     Next, the vibrating surface, which is rotated by the optically active substance included in the aqueous humor in the anterior chamber  13 , is returned back to the original state by the compensator  291 . In a case in which the compensator  291  is the magneto-optical element such as the Faraday element, the vibrating surface of the light passing through the compensator  291  is rotated by applying a magnetic field to the compensator  291 . 
     The linearly polarized light passing through the analyzer  292  is received by the light receiving element  293 , and converted into an output signal corresponding to intensity of light. 
     Here, an example of a method of measuring the optical rotation α M  by the optical system  20  will be described. 
     First, in a state in which the light emitted from the light source  21  does not pass through the anterior chamber  13 , the compensator  291  and the analyzer  292  are set by using the optical system  20  including the light source  21 , the polarizer  251 , the compensator  291 , the analyzer  292 , and the light receiving element  293  such that an output signal from the light receiving element  293  is minimized. In an example illustrated in  FIG. 4 , in a state in which light does not passes through the anterior chamber  13 , the vibrating surface of the linearly polarized light passing through the polarizer  251  is orthogonal to the vibrating surface of the light passing through the analyzer  292 . 
     Next, the light passes through the anterior chamber  13 . Then, the vibrating surface is rotated by the optically active substance included in the aqueous humor in the anterior chamber  13 . For this reason, an output signal from the light receiving element  293  exceeds the minimum value. Therefore, the vibrating surface is rotated by applying a magnetic field to the compensator  291  so that an output signal from the light receiving element  293  is minimized. That is, the vibrating surface of the light projected from the compensator  291  becomes orthogonal to the vibrating surface of the light passing through the analyzer  292 . 
     An angle of the vibrating surface rotated by the compensator  291  corresponds to the optical rotation α M  generated by the optically active substance included in the aqueous humor. Here, a relationship between the magnitude of the magnetic field applied to the compensator  291  and the angle of the rotated vibrating surface is known in advance. Therefore, the optical rotation α M  may be found from the magnitude of the magnetic field applied to the compensator  291 . 
     Specifically, lights having plural wavelengths λ (wavelengths λ 1 , λ 2 , λ 3 , . . . ) are emitted to the aqueous humor in the anterior chamber  13  from the light source  21 , and thus the optical rotations α M  (optical rotations α M1 , α M2 , α M3 , . . . ) are obtained for each wavelength. The sets of the wavelengths λ and the optical rotations α M  are received by the signal processor  30 , and thus the concentration of the optically active substance, which is desired to be obtained, is calculated. 
     Plural optically active substances are included in the aqueous humor, as described above. Therefore, the measured optical rotation α M  is a sum of the optical rotations α M  provided by the plural optically active substances. Therefore, it is necessary to calculate the concentration of the optically active substance (here, glucose), which is desired to be obtained, from the measured optical rotation α M . Because a publicly known method may be used to calculate the concentration of the optically active substance which is desired to be obtained, a description thereof will be omitted. 
     In  FIG. 4 , the vibrating surface of the polarizer  251  is parallel to the page surface, and the vibrating surface before passing through the analyzer  292  is perpendicular to the drawing sheet. However, in a case in which the vibrating surface is rotated by the compensator  291  in a state in which the light emitted from the light source  21  does not pass through the anterior chamber  13 , the vibrating surface before passing through the analyzer  292  may be inclined with respect to a surface parallel to the drawing sheet. That is, the compensator  291  and the analyzer  292  may be set such that an output signal from the light receiving element  293  is minimized in a state in which light does not pass through the aqueous humor in the anterior chamber  13 . 
     An example in which the compensator  291  is used as a method of obtaining the optical rotation α M  is described herein, but the optical rotation α M  may be obtained by using a component other than the compensator  291 . Further, an orthogonal polarizer method (provided that the compensator  291  is used), which is the most basic measuring method of measuring the rotation angle (optical rotation α M ) of the vibrating surface, is described herein, but other measuring methods such as a rotating analyzer method, a Faraday modulation method, and an optical delayed modulation method may be applied. 
     Fourth Exemplary Embodiment 
     In the optical measurement apparatus  1  to which the third exemplary embodiment is applied, the angle of light entering the mirror  27  is fixed by using the telecentric fθ lens for the telecentric optical system  24 . In the optical measurement apparatus  1  to which the fourth exemplary embodiment is applied, the optical path is switched by moving the mirror  231  of the deflector  23  instead of using the telecentric optical system  24 . 
     In the fourth exemplary embodiment, the concentration of the optically active substance such as glucose is measured by providing the polarization controller  25  and using an optical rotation. Further, the concentration of the optically active substance such as glucose may be measured by concentration without using the polarization controller  25 . 
       FIGS. 5A and 5B  are views illustrating an example of a configuration of the optical measurement apparatus  1  for an eyeball to which the fourth exemplary embodiment is applied.  FIG. 5A  is a view illustrating the eyeball  10  when viewed from the top side (a cross-sectional view in the upward and downward directions), and  FIG. 5B  is a view illustrating the eyeball  10  when viewed from the front side. The parts similar to those of the optical measurement apparatus  1  to which the third exemplary embodiment is applied (the optical measurement apparatus  1  to which the first exemplary embodiment is applied, except for some parts) are designated by the same reference numerals, and descriptions thereof will be omitted. 
     The optical measurement apparatus  1  to which the fourth exemplary embodiment is applied is provided with a condensing lens  26  instead of the telecentric optical system  24 . Further, the deflector  23  includes a mirror  231  and a linear motion stage  233  on which the mirror  231  is mounted so that the mirror  231  is moved in one direction by the linear motion stage  233 . The linear motion stage  233  is an example of a moving unit. 
     That is, the reflecting surface of the mirror  231  is moved by the linear motion stage  233  in a direction of the optical path (the front-back direction in which light travels) Therefore, an incident position of light to the mirror  27  is switched. Further, an optical path is set to the state of the optical path α suitable to measure the concentration of the optically active substance included in the aqueous humor in the anterior chamber  13 . That is, the optical path is set to pass across the anterior chamber  13 . 
     Here, the deflector  23  and the condensing lens  26  are also an example of a switching unit. 
     In the fourth exemplary embodiment, an incident position of light to the mirror  27  is restricted by a movement direction of the linear motion stage  233 . That is, an incident position of light to the mirror  27  is switched in a one-dimensional direction. For example, in  FIG. 5A , the optical path is restricted by a movement of the face in the front-back direction. 
     Therefore, in a case in which the optical path is moved in the up-down direction of the face as illustrated in  FIG. 5B , the light source  21  and the collimator lens  22  are disposed in a direction perpendicular to the drawing sheet, the movement direction of the linear motion stage  233  is also set to a direction perpendicular to the drawing sheet, and a direction of the mirror  231  on the linear motion stage  233  is set such that the light emitted from the light source  21  through the collimator lens  22  is reflected to the mirror  27 , as illustrated in  FIG. 5A . 
     A piezo element may be attached to the back surface of the mirror  231 , and the front surface of the mirror  231  may be moved instead of using the linear motion stage  233 . In this case, the linear motion stage  233  may be a driving device for operating the piezo element. 
     Fifth Exemplary Embodiment 
     In the optical measurement apparatus  1  for an eyeball to which the fifth exemplary embodiment is applied, a circumference of the anterior chamber  13  of the eyeball  10  is immersed in a liquid. This state is called liquid immersion in some cases. 
       FIGS. 6A and 6B  are views illustrating an exemplary configuration of the optical measurement apparatus  1  for an eyeball to which the fifth exemplary embodiment is applied.  FIG. 6A  is a view obtained when the eyeball  10  is viewed from the top side (a cross-sectional view in the upward and downward directions), and  FIG. 6B  is a view obtained when the eyeball  10  is viewed from the front side. Further, the configuration of the optical measurement apparatus  1 , except for a liquid immersion part  50  to be described below, is identical to that in the third exemplary embodiment illustrated in  FIGS. 3A and 3B . Therefore, the same parts are designated by the same reference numerals, descriptions thereof will be omitted, and different parts will be described. 
     The liquid immersion part  50  includes a container  51 , and a liquid  52  that fills the container  51 . The container  51  of the liquid immersion part  50  is moved close to a surface of the face at the periphery of the eyeball  10 , so that the circumference of the anterior chamber  13  of the eyeball  10  is immersed in the liquid  52 . The liquid  52  may have a refractive index that is less different from the refractive index of the aqueous humor. For example, water, saline solution, or the like may be used. 
     The liquid immersion part  50  includes an incident window  53  and an emission window  54 , through which light passes, at portions corresponding to the optical path of the container  51  so that the light passes through the anterior chamber  13  while traversing the anterior chamber  13 . The incident window  53  is configured such that the light reflected by the mirror  27  enters in a direction perpendicular to the incident window  53 , and the emission window  54  is configured such that the light passing through the liquid  52  and the anterior chamber  13  is projected in a direction perpendicular to the emission window  54 . Further, a size or a shape of the container  51  is not particularly limited as long as an incident position of light at the circumference (e.g., the cornea  14 ) of the anterior chamber  13  of the eyeball  10  is immersed in the liquid  52 . 
     As described above, the liquid immersion part  50  inhibits the light reflected by the mirror  27  from being refracted by the surface of the cornea  14 , thereby inhibiting the direction of the light from being changed. That is, a shape of the cornea  14  or the like hardly affects the light, so that the optical path traversing the anterior chamber  13  is easily set. Further, the optical path β runs without being reflected by the surface of the cornea  14 , but the distance of the optical path β, which passes through the anterior chamber  13 , is short. 
     The liquid immersion part  50  may be applied to the optical measurement apparatus  1  for an eyeball to which the other exemplary embodiments are applied. 
     Sixth Exemplary Embodiment 
     In the optical measurement apparatus  1  for an eyeball to which the second to fourth exemplary embodiments are applied, the mirror  27  is set to have a predetermined incident angle. Further, the mirror  27  is disposed to be spaced apart from the eyeball  10 . 
     In the sixth exemplary embodiment, the mirror  27  is provided on a contact member  60  having a mirror used in a state of being in contact with the surface of the eyeball  10 . The contact member  60  having the mirror is an example of a mounting member. 
       FIGS. 7A and 7B  are views illustrating an exemplary configuration of the optical measurement apparatus  1  for an eyeball to which the sixth exemplary embodiment is applied.  FIG. 7A  is a view obtained when the eyeball  10  is viewed from the top side (a cross-sectional view in the upward and downward directions), and  FIG. 7B  is a view obtained when the eyeball  10  is viewed from the front side. Further, the configuration of the optical measurement apparatus  1 , except for the contact member  60  having the mirror, which will be described below, is identical to that in the third exemplary embodiment illustrated in  FIGS. 3A and 3B . Therefore, the same parts are designated by the same reference numerals, descriptions thereof will be omitted, and different parts will be described. 
     As illustrated in  FIG. 7A , the contact member  60  having the mirror is a member for an eyeball such as so-called contact lens, and the contact member  60  is mounted on the surface (eyeball surface) of the cornea  14  of the eyeball  10 . Further, the configuration in which the contact member  60  is mounted on the surface (eyeball surface) of the cornea  14  of the eyeball  10  is expressed herein as the configuration in which the contact member  60  is mounted on the eyeball  10 . 
     The contact member  60  having the mirror has a mirror  27  that is provided in a base body  61 . 
     The base body  61  is made of resin such as, for example, polyhydroxyethylmethacrylate, polymethylmethacrylate, silicone copolymers, or fluorine-containing compounds. In a case in which a refractive index of the base body  61  is close to refractive indexes of the aqueous humor in the anterior chamber  13  and the cornea  14  of the eyeball  10 , refraction is inhibited at an interface between the contact member  60  having the mirror and the eyeball  10 . Therefore, it is easy to set the optical path traversing the anterior chamber  13  of the eyeball  10 . Further, the optical path β runs without being reflected by the surface of the cornea  14 , but the distance of the optical path β, which passes through the anterior chamber  13 , is short. 
     A portion of the base body  61 , through which light enters toward the mirror  27 , is configured as a flat surface  62  perpendicular to the light. In addition, a portion of the base body  61 , through which light is projected toward the detector  29 , is configured as a flat surface  63  perpendicular to the light. Therefore, when the light enters the contact member  60  having the mirror and the light is projected from the contact member  60  having the mirror, the optical path is inhibited from being bent due to the refraction of the base body  61 . 
     As illustrated in  FIG. 7B , the mirror  27  has a quadrangular external shape. Further, the external shape of the mirror  27  may be other shapes such as an arc shape. 
     The base body  61  needs not have a circular shape, and may have other shapes such as a quadrangular shape as long as the base body  61  may be mounted on the cornea  14 . 
     The contact member  60  having the mirror, which is described in the sixth exemplary embodiment, may be applied to the second to fourth exemplary embodiments. 
     While various exemplary embodiments have been described above, these exemplary embodiments may be combined with each other. 
     The present disclosure is not limited to the aforementioned exemplary embodiments, but may be implemented in various forms without departing from the subject matter of the present disclosure. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.