Abstract:
An ophthalmic apparatus for measuring a characteristic of an eye to be examined, the apparatus comprising a moving device for relatively moving a measurement unit with respect to the eye, wherein the measurement unit includes a measurement system for measuring the characteristic of the eye, a position detecting device for detecting a relative position of the measurement unit with respect to the eye at the time of measurement, a storing device for storing detected position data, and a processing device for performing a predetermined processing based on a plurality of position data stored in each measurement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ophthalmic apparatus for examining or measuring an eye to be examined. 
     2. Description of Related Art 
     With an ophthalmic apparatus for obtaining data about characteristics of an eye to be examined, such as eye refractive power or corneal shape, an examinee&#39;s head is stably fixed on a head rest or the like. Thereafter, a measurement part (a measurement optical system) is aligned with the eye and then measurement is performed. 
     Recently, in order to save trouble of an examiner to make fine alignment, there has been suggested an apparatus having an auto-alignment mechanism which detects an alignment condition of the measurement part with respect to the eye and automatically moves the measurement part based on the thus detected result. 
     With this kind of apparatus, however, it is often the case where an examinee&#39;s head is inclined with respect to a horizontal reference although the examinee&#39;s head is fixed by using a head rest or the like. There is also possibility that the examinee&#39;s head may move at the time of shifting the measurement part from one eye to the other after completion of measurement on the first eye. Measurement performed under such a condition is likely to end up in inaccurate measurement results, namely astigmatic axial (cylindrical axial) angles, refractive power in the principal meridian direction, or the like. Especially in the case where refractive power data or corneal shape data is used in keratorefractive surgery for correcting refractive error by ablating the cornea using an excimer laser beam, the astigmatic axial angle data is desired to be highly accurate. 
     Further, with this kind of apparatus having an auto-alignment mechanism, the measurement part automatically tracks movement of the eye and the measurement is performed automatically upon completion of the alignment. Therefore, it is difficult to grasp the movement of the eye. If the eye is not stably fixed, measurement results obtained thereby will be of poor reliability. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide an ophthalmic apparatus with which highly reliable measurement results are obtained. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, an ophthalmic apparatus for measuring a characteristic of an eye to be examined, the apparatus comprises moving means for relatively moving a measurement unit with respect to the eye, wherein the measurement unit includes a measurement system for measuring the characteristic of the eye, position detecting means for detecting a relative position of the measurement unit with respect to the eye at the time of measurement, storing means for storing detected position data, and processing means for performing a predetermined processing based on a plurality of position data stored in each measurement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the objects, advantages and principles of the invention. In the drawings, 
     FIGS. 1A and 1B are external views showing construction of an apparatus in accordance with one preferred embodiment of the present invention; 
     FIG. 2 is a schematic view showing construction of optical systems arranged inside a measurement part of the apparatus; 
     FIG. 3 is a schematic block diagram showing a control system of the apparatus; 
     FIG. 4 is a view showing an arrangement of photodetectors included in a photo receiving part of an eye refractive power measurement optical system; 
     FIG. 5 is a view illustrating a way of detecting the relative position of the measurement part in up, down, right and left directions with respect to a fixed base; 
     FIG. 6 is a view showing a monitor screen displaying movement history of a measurement point; 
     FIG. 7 is a view showing a Y direction movement device of the apparatus; and 
     FIG. 8 is a view showing an X direction movement device and a Z direction movement device of the apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A detailed description of one preferred embodiment of an ophthalmic apparatus embodying the present invention will now be given referring to the accompanying drawings. FIGS. 1A and 1B are external views showing construction of the apparatus in accordance with the preferred embodiment of the present invention: FIG. 1A is a front view, looking from an examinee&#39;s side and FIG. 1B is a side view. FIG. 2 is a schematic view showing construction of optical systems arranged inside a measurement part  5 . FIG. 3 is a schematic block diagram showing a control system of the apparatus. 
     Reference numeral  1  denotes a fixed base. The base  1  has a head support part  2  fixedly attached thereto for fixedly supporting an examinee&#39;s head.  5  is the measurement part containing a measurement optical system, an alignment optical system and the like which will be described later. The measurement part  5  is provided with a measurement window  5   a  at an approximate center thereof in the right and left direction (X direction) on a side facing to the examinee. A main body  3  to which the measurement part  5  is mounted slides along the base  1  in the right and left direction (X direction) and the back and forth direction (Z direction) by a known slide mechanism which works in response to operation of a joystick  4 . In addition, the measurement part  5  moves in the up and down direction (Y direction) relative to the main body  3  by a Y direction movement device  130   b  which is driven via a control part  50  in response to rotation operation of a rotation knob  4   a  mounted at the joystick  4 . Further, to be ready for auto-alignment, the measurement part  5  is movable in the X, Y and Z directions relative to the main body  3  by the control part  50  as well as an X direction movement device  130   a,  the Y direction movement device  130   b  and a Z direction movement device  130   c.    
     The device  130   b  (see FIG. 7) rotates a feed screw  132   b  by a pulse motor  131   b  fixedly provided to the main body  3 , thereby causing a female screw portion  133   b  fixedly provided to a Y table  136   b  to descend and ascend. As the result, the Y table  136   b  moves vertically.  134   b  is a guide shaft fixed to the Y table  136   b.  The device  130   a  (see FIG. 8) rotates a feed screw  132   a  by a pulse motor  131   a  fixedly provided to the Y table  136   b,  thereby causing a female screw portion  133   a  fixedly provided to an X table  136   a  to move right and left. As the result, the X table  136   a  moves laterally.  135   a  is a guide groove fixedly provided to the Y table  136   b,  and  134   a  is a guide plate fixedly provided to the X table  136   a.  The device  130   c  (see FIG. 8) has a configuration similar to the device  130   a,  and through rotation of a feed screw  132   c  by a pulse motor  131   c  fixedly provided to the X table  136   a,  a female screw portion  133   c  fixedly provided to the Z table  136   c  is moved back and forth. As the result, the Z table  136   c  moves back and forth.  135   c  is a guide groove fixedly provided to the X table  136   a,  and  134   c  is a guide plate fixedly provided to the Z table  136   c.  Each of these devices  130   a,    130   b  and  130   c  allows the measurement part  5  mounted on the Z table  136   c  to move in each of X, Y and Z directions. 
     Reference numeral  39  is a color monitor (display) for displaying various information to the examiner, such as an image of an eye to be examined, alignment information, measurement results and the like. 
     Reference numerals  10 A and  10 B denote light sources emitting infrared light such as LEDs or the like. The light sources  10 A and  10 B are arranged on the base  1  at the examinee&#39;s side with a predetermined distance therebetween and with its center at the center of the base  1  in the X direction.  11  is a photo receiving part comprised of a slit  12  and a one-dimensional photodetector (a position detector)  13 . The slit  12  is arranged generally at an approximate center in the X direction and has one slit aperture  12   a  extending in a Z direction. The photodetector  13  has a detecting plane along the X direction. 
     Light emitted from each of the two light sources  10 A and  10 B illuminates the slit  12 . Thereafter each of the light is limited by passing through the slit aperture  12   a  and reaches the photodetector  13 . Based on incident positions of each slit light on the photodetector  13 , the control part  50  detects the relative position of the measurement part  5  in the X and Y directions with respect to the base  1 . 
     Description is now given to a way of position detection of the measurement part  5  with reference to FIG. 5 (for the sake of simplicity in the description, it is supposed that the measurement part  5  is at a constant position in the Z direction and the two-dimensional position in X and Y directions needs to be detected). 
     First, it is supposed that a reference position P in the X direction is on a perpendicular bisector of the light sources  10 A and  10 B arranged to have a distance d therebetween. Then, the slit aperture  12   a  (i.e. the measurement part  5 ) moves to a position P′ that is a distance x away from the position P in the X direction. In the Y direction, on the other hand, it is supposed that the slit aperture  12   a  (i.e. the measurement part  5 ) is at a position that is a distance L away from the axial line of the light sources  10 A and  10 B in a vertical direction, and that the distance between the slit aperture  12   a  and the detecting plane of the photodetector  13  is a distance Lo (predetermined distance). Here, let D′ denote the distance between the incident positions of the slit light emitted from each of the light sources  10 A and  10 B on the photodetector  13 , then the distance L is obtained by the following expression. 
     
       
           L =( d/D ′)× Lo   Expression 1 
       
     
     Once the distance L is obtained, the travel distance x of the position P′ from the reference position P is then obtained through detecting deviation of at least one of the incident positions on the photodetector  13  from a certain reference point on the photodetector  13 . For example, a point O on the photodetector  13  located right above the slit aperture  12   a  is determined as the reference point (in the figure, the direction to the right relative to the reference point O is referred to as “+”, and the direction to the left is referred to as “−”). When considering the slit light by the light source  10 A is at the position P′, let D′A denote the deviation amount of the incident position of the slit light to the reference point O, then judging from the figure the travel distance x is obtained by the following expression. 
     
       
           x =( L/Lo )× D′A −( d/ 2) 
       
     
     By substituting the above Expression 1 into this expression, the following expression is obtained. 
     
       
           x =( D′A/D′ )× d −( d/ 2) 
       
     
     On the other hand, when considering the slit light by the light source  10 B at the position P′, let D′B denote the deviation amount of the incident position of the slit light to the reference point O. Then in a like manner as the case of the light source  10 A, the travel distance x is obtained by the following expression. 
     
       
           x =( D′B/D ′)× d +( d /2) 
       
     
     Accordingly, through detecting and calculating the distance D′ between incident positions of each slit light and the deviation amount D′A of the incident position of the slit light by the light source  10 A (or alternatively the deviation amount D′B of the slit light by the light source  10 B), the travel distance x of the position P′ from the reference position P are obtained (preferably, an average of the deviation amounts D′A and D′B of the respective incident positions of the slit light is obtained). For the details of this position detection, see U.S. Pat. No. 5,764,341 (Japanese unexamined publication No. HEI 9 (1997)-149885). 
     It should be noted that in this preferred embodiment, the light sources  10 A and  10 B are disposed to the base  1  and the photo receiving part  11  is disposed to the measurement part  5 . However, it is equally applicable to dispose the light sources  10 A and  10 B to the measurement part  5  and the photo receiving part  11  to the base  1 . Further, it is also applicable to provide one light source and a photo receiving part having two slit apertures (See U.S. Pat. No. 5,764,341 corresponding to Japanese unexamined publication No. HEI 9 (1997)-149885). 
     In FIG. 2, reference numeral  101  denotes a corneal shape measurement optical system.  102  is a placido plate generally of a dome-shape having an opening at the center thereof, in which a ring pattern is formed with a number of concentric circles of light transmitting portions and light shielding portions with an optical axis L 1  as the center.  103  are illumination light sources emitting light in a range between red light and infrared light such as LEDs or the like. Light emitted from the light sources  103  is reflected by a reflecting plate  104  and illuminates the placido plate  102  approximately uniformly from behind so that an image of the ring pattern (an image of the placido ring) is formed on a cornea Ec of a patient&#39;s eye E. On an outer circumference of the placido plate  102 , anterior eye segment illumination lights  105  which emit near infrared light are disposed. 
     Disposed behind the reflecting plate  104  are a working distance target projecting optical system  110  and a working distance target detecting optical system  115 : the former comprises a light source  111  emitting near infrared light such as an LED and a lens  112 , and the latter comprises a lens  116  and a one-dimensional photodetector (a position detector)  117 . Light emitted from the light source  111  is made generally parallel light by the lens  112  and enters into the cornea Ec obliquely through openings provided in the reflecting plate  104  and the placido plate  102 , thereby forming a target image by the light source  111  on the cornea Ec. The light of the target image that has been formed on the cornea Ec passes through openings provided in the placido plate  102  and the reflecting plate  104 , and enters into the photodetector  117  via the lens  116 . Based on the incident position of the light of the target image on the photodetector  117 , an alignment condition of the apparatus with the eye E in a working distance direction (Z direction) is detected. 
     An eye refractive power measurement optical system  20  is provided in a rear direction along the optical axis L 1 .  120  is a photo receiving part having on its photo receiving surface eight photodetectors  115   a - 115   h  located at conjugate positions with the cornea Ec relative to a photo receiving lens  121  (see FIG.  4 ). Among the eight photodetectors, the photodetectors  115   a - 115   f  are located along the line passing though the center of the photo receiving surface (the optical axis L 1 ) such that the photodetectors  115   a  and  115   b,    115   c  and  115   d,  and  115   e  and  115   f  are symmetrical in relation to the center of the photo receiving plane. These three pairs of photodetectors are determined their configuration distances so as to be able to detect the refractive power at each corresponding position in the meridian direction on the cornea Ec (FIG. 4 is shown in the equivalent size on the cornea Ec). On the other hand, the photodetectors  115   g  and  115   h  are disposed on a line orthogonal to the photodetectors  115   a - 115   f  to be symmetrical in relation to the optical axis L 1 . The eye refractive power measurement optical system  20  can be of any conventional type. In this embodiment, one disclosed in U.S. Pat. No. 5,907,388 (Japanese Unexamined Patent Publication No. HEI 10 (1998)-108837) is used. Thus, based on output signals from the photodetectors  115   a - 115   f  indicating the phase difference, the refractive power, which varies along a meridian direction is obtained at a plurality of corneal portions and distribution of the eye refractive power is obtained. 
     In addition, a beam splitter  25  is provided on the optical axis L 1 . Disposed on an optical axis L 2 , which is made coaxial with the optical axis L 1  by the beam splitter  25  are half mirrors  26  and  27 , a lens  28 , a fixation target plate  29  and an illumination light source  30  which emits visible light. The fixation target plate  29  has a fixation point in the middle and an area surrounding the fixation point transmits visible light. The lens  28  is movable along the optical axis L 2  so as to change visibility of the fixation point on which the eye E is fixated. 
     Disposed on an optical axis L 3 , which is made coaxial with the optical axis L 2  by the half mirror  27  are a lens  33 , a light source  34  which emits near infrared light. Light emitted from the light source  34  is made generally parallel light by the lens  33  and enters into the cornea Ec from the front via the half mirrors  26  and  27  and the beam splitter  25 , thereby forming a target image on the cornea Ec by the light source  34 . The light of target image formed on the cornea Ec enters into a CCD camera  38  via the beam slitter  25 , the half mirror  26  and a lens  35 . 
     Disposed on an optical axis L 4 , which is made coaxial with the optical axis L 2  by the half mirror  26  are the lens  35  and the CCD camera  38  serving as a photographic element. Output from the camera  38  is inputted into the monitor  39  directly or via the control part  50 . An anterior eye segment of the eye E photographed by the camera  38  is displayed on the monitor  39 . Also, an image of the placido ring and a target image by the light source  34  are conducted image processing by the control part  50  to obtain the corneal shape and the alignment condition in the up and down and the right and left directions (X and Y directions). 
     Next, description is given to operations of the apparatus. First, description is given to eye refractive power measurement. An eye refractive measurement mode is selected by using a mode change-over switch  40 . 
     The measurement is started, for example, from the right eye. While observing an anterior eye segment of the right eye displayed on the monitor  39 , the examiner operates the joystick  4  and the rotation knob  4   a  to slide the main body  3  in the X and Y directions and to move the measurement part  5  in the Y direction, thereby making rough alignment. Then, when the camera  38  photographs a target image formed on the cornea Ec by the light source  34 , the control part  50  obtains the coordinates of the target image to detect the alignment condition in the X and Y direction. Thereafter, the control part  50  further moves the measurement part  5  relative to the eye E based on the detection result. In other words, the control part  50  drives the X direction movement device  130   a  and the Y direction movement device  130   b  to move the measurement part  5  automatically in the X and Y directions relative to the main body  3  (to perform auto-alignment). The control part  50  also detects the alignment condition in the Z direction from signals outputted from the photodetector  117 , and further moves the measurement part  5  based on the detection result. In other words, the control part  50  drives the Z direction movement device  130   c  to move the measurement part  5  automatically in the Z direction relative to the main body  3  (to perform auto-alignment). 
     Once the alignment has been completed, the control part  50  stops the drive of each of the movement devices  130   a,    130   b  and  130   c  and then automatically issues trigger signals to start eye refractive measurement. An eye refractive power calculation part  52  obtains the distribution of the eye refractive power from the phase difference of the output signals from each photodetector disposed in the photo receiving part  120 . To meet this end, first preliminary measurement is performed as in the case of conventional phase difference method to obtain refractive power. Based on the thus obtained result, the lens  28  is moved to fog the eye E. Thereafter, based on the output signals from the photodetectors  115   g  and  115   h  that vary in response to the movement of the slit image on the photo receiving part  120 , the eye refractive power calculation part  52  obtains the corneal center in the meridian direction in which the photodetectors  115   a - 115   f  are disposed. Next, based on the phase difference of the output signals from each of the photodetectors  115   a - 115   f  with respect to the thus obtained center, refractive power at a plurality of corneal portions corresponding to each photodetector is obtained. While rotating a slit light projecting optical system  21  and the photo receiving part  120  180 degrees at a predetermined angle step (1 degree) about the optical axis, the calculation to obtain refractive power is performed for each meridian at every angle step, whereby distribution of the refractive power which varies in the meridian direction is obtained (for the details, see U.S. Pat. No. 5,907,388 (Japanese unexamined publication No. HEI 10 (1998)-108837). Here, it should be noted that values of eye refractive power are obtained with the corneal vertex as the reference. The obtained data about the distribution of the eye refractive power is stored in a HDD  55   a  or a memory  55   b.    
     When measurement result of the first measurement is obtained, the control part  50  again performs auto-alignment (moves the measurement part  5 ) based on position information of the target images obtained by the camera  38  and the photodetector  117 . After completion of the auto-alignment, eye refractive measurement is performed in the same manner as the first measurement. 
     Eye refractive power measurement is programmed to be repeated automatically until a predetermined number (for example, five) of measurement results are obtained on one eye. When the measurement is repeated for the predetermined number of times on the right eye, the measurement is performed on the left eye for the predetermined number of times similarly to the right eye. 
     Every time the measurement as above is performed, a position detection calculation part  51  included in the control part  50  obtains the relative position (hereinafter referred to as a measurement point) of the measurement part  5  in the X and Y directions based on information obtained by the photodetector  13  in a manner described above. The thus obtained measurement point is stored in the memory  55   b  as movement history in correspondence with the order that the measurement point is obtained. When the predetermined number of measurement results on one eye or the both eyes are obtained, the movement history of the measurement points are displayed graphically on the monitor  39 . 
     FIG. 6 shows a monitor screen displaying the movement history of the measurement points. FIG. 6 shows an example of the case where measurement is performed five times on the right eye and another five times on the left eye. The numbers indicating the order of measurement are displayed near each measurement point  61 , and theses measurement points  61  are connected by arrows  62  in the order of measurement (in the figure the numbers and arrows appear as {circle around ( 1 )}→{circle around ( 2 )}→{circle around ( 3 )}→{circle around ( 4 )}→{circle around ( 5 )}). In addition,  63  shows an allowable dispersion range of the measurement points. In the example shown in FIG. 6, the allowable dispersion range is set to have φ2 mm. Here, the allowable dispersion range  63  may be changed as the examiner intends. The allowable dispersion range  63  is set to have its center at the center of gravity of the measurement points  61 . Similarly, the origin of the up and down as well as the right and left (XY coordinates) is set to be at the center of gravity of the measurement points  61 . As the number of measurement points  61  increases to two points, three points and so on, the center of gravity shifts in response. 
     Here, when performing the measurement repeatedly for plurality of times on one eye, the control part  50  determines whether the measurement point  61  is within the allowable dispersion range  63 . That is to say, the measurement point  61  is out of the allowable dispersion range  63 , that situation is displayed on the monitor  39  to inform the examiner. Or the situation may be informed by means of a voice generator  49 . In this way, the examiner can see if the examinee&#39;s head is not properly fixed by the head support part  2  so that the eye moves or if the eye moves so that the gaze is not stable. 
     When such a situation is informed, to avoid decreasing the reliability of measurement results, the examiner takes necessary steps such as instructing the examinee to firmly fix his head, and then redo the measurement. This eliminates the possibility of using measurement results that are obtained without properly fixing the eye so that accurate measurement results are obtained. Here, it is the control part  50  that determines whether or not the apparatus goes out of the allowable dispersion range. However, it is also applicable that the examiner make that determination based on the movement history of the measurement points that is graphically displayed on the monitor  39 . 
     Here, in the case of auto-alignment, the measurement part  5  moves in response to the movement of the eye without requiring the examiner to perform fine alignment. Therefore, the examiner may not recognize the movement of the eye. Accordingly, it is desirable to display the movement history of the measurement points or to inform the determination regarding the stability of the eye fixation. It goes without saying that the similar effect is achieved also in the case of alignment performed manually. 
     In FIG. 6, the predetermined number of measurement points are measured within the allowable dispersion range on the right eye and the left eye respectively, a deviation angle θ from a horizontal reference line in the condition of measurement is obtained based on where the respective centers of the gravity of the measurement points  61  on right eye and left eye are located. Based on this deviation angle θ, the astigmatic axial angle as obtained in the measurement is corrected. In FIG. 6, the deviation angle θ is obtained from a distance in X and Y directions to the respective centers of the gravity of the measurement points of the right eye and the left eye. That is to say, if the examinee&#39;s head is inclined on the head support part  2 , the astigmatic axial angle is based on the condition where the head is inclined. Yet, by correcting the angle with the deviation angle θ, the astigmatic axial angle will be of accurate one. In the case where the astigmatic axial angle is used in keratorefractive surgery for correcting the eye to be emmetropia, the measurement results are required to be highly accurate. Hence, the correction of the astigmatic axial angle is highly useful. In addition, it is also possible to obtain an interpupillary distance using the respective centers of gravity of the measurement points  61  on the right eye and the left eye. 
     When performing corneal shape measurement, a corneal shape measurement mode is selected using the switch  40 . Similarly to the case of eye refractive power measurement, when alignment is completed using the auto-alignment mechanism, the light source  103  is illuminated for a predetermined period of time and an image of the ring pattern is photographed. A corneal shape calculation part  53  performs predetermined image processing on the photographed ring pattern image to detect the edge of the image. By obtaining each edge with respect to the corneal center at a predetermined angle step (1 degree), the distribution of the corneal curvature is obtained. The data about corneal curvature distribution is stored in the HDD  55   a  or the memory  55   b.    
     Also in the corneal shape measurement, the predetermined number of measurement points are obtained and stored in correspondence with the order that the measurement point is measured (photographed) as movement history. The movement history is graphically displayed on the monitor  39 . Further, the stability of the eye is determined. Thereafter, when measurement is completed on the both eyes, the deviation angle θ from the horizontal reference line is obtained from the center of gravity of the right eye measurement points and the center of gravity of the left eye measurement points, whereby the angle of inclination with respect to the data about distribution of the corneal curvature (the angle of inclination in meridian direction) is corrected. 
     Further, a continuous mode of eye refractive measurement and corneal shape measurement may be selected using the mode change-over switch  40 . In this mode, the eye refractive measurement and the corneal shape measurement are continually performed on one eye. In this case too, the measurement points obtained in each measurement are stored as movement history. Based on the display of the movement history or the determination of whether the measurement point is out of the allowable dispersion range, reliability of the measurement results to be used is determined. In addition, the axial angle (the angle of inclination) of the both measurement is corrected so that even though the examinee&#39;s head is inclined, the axial data can be accurate. 
     By operating a mouse  57  or a key board  58 , the thus obtained measurement results may be transmitted to a corneal surgery apparatus  90  via a FDD  59   a  (and FD  59   c ) and a communication port  59   b.  The corneal surgery apparatus  90  calculates ablation data (such as ablation amount and an ablation range) from the data transmitted thereto. 
     In the measurement described above, the examiner may select the measurement results based on the movement history of the measurement points obtained in respective measurement on the right eye and on the left eye. For example, the data obtained when a measurement point has relatively large deviation may be excluded. Alternatively, data obtained when the measurement points are relatively stable is designated to by used in calculation of the corrective value for the astigmatic axial angle (the inclination angle). Certainly, this exclusion and designation may be done by the control part  50 . 
     As has been described above, according to the present invention, highly reliable measurement results are obtained. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in the light of the above teachings or may be acquired from practice of the invention. The embodiments chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.