Patent Publication Number: US-2004059320-A1

Title: Corneal-ablation-data calculation apparatus and a corneal surgery apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a corneal-ablation-data calculation apparatus and a corneal surgery apparatus for use in corneal surgery for correcting a refractive error by ablating corneal tissue with a laser beam, more specifically relates to an apparatus suitable for presbyopic correction.  
       [0003] 2. Description of Related Art  
       [0004] As a refractive error of an eye, presbyopia that it becomes harder to see a near point with age is mentioned in addition to myopia, astigmatism and hyperopia. Conventionally, as a method for presbyopic correction, spectacles are generally used for the correction. Recently, an apparatus which ablates a cornea by using a laser beam and changes a corneal curvature to correct the refractive error such as myopia, astigmatism or hyperopia has been introduced. In addition, a study for correcting presbyopia using that corneal surgery apparatus is conducted. For example, there is proposed a method for forming an area for near vision partly in a position deviated from a corneal center so that the cornea may become bifocal.  
       [0005] However, in such method of presbyopic correction, as a jump of an image occurs between an area for far vision and the area for near vision, there is a case where the post-operative condition of visibility is not favorable.  
       SUMMARY OF THE INVENTION  
       [0006] An object of the invention is to overcome the problems described above and to provide a corneal-ablation-data calculation apparatus and a corneal surgery apparatus which enable presbyopic correction with which a post-operative condition of visibility becomes favorable.  
       [0007] To achieve the objects and in accordance with the purpose of the present invention, an apparatus for calculating data on corneal ablation for corneal surgery for correcting a refractive error by ablating corneal tissue with a laser beam has a first input device for inputting a pre-operative corneal shape, a second input device for inputting a correction amount for far vision and additional power for near vision, a setting device for setting a far vision area in a central part of a cornea and an annular near vision area outside the far vision area, and a calculation device for obtaining the data on the corneal ablation based on the inputted preoperative corneal shape, the inputted correction amount for far vision and the inputted additional power for near vision, and the set far vision area and the set near vision area.  
       [0008] In another aspect of the present invention, a corneal surgery apparatus for correcting a refractive error by ablating corneal tissue with a laser beam has a laser irradiation system having an irradiation optical system for irradiating the laser beam onto a cornea, a first input device for inputting a pre-operative corneal shape, a second input device for inputting a correction amount for far vision and additional power for near vision, a setting device for setting a far vision area in a central part of the cornea and an annular near vision area outside the far vision area, a calculation device for obtaining data on corneal ablation based on the inputted preoperative corneal shape, the inputted correction amount for far vision and the inputted additional power for near vision, and the set far vision area and the set near vision area, and a control device for controlling the laser irradiation system based on the obtained data on the corneal ablation.  
       [0009] Yet, in another aspect of the present invention, a corneal surgery apparatus for correcting a refractive error by ablating corneal tissue with a laser beam further has a laser irradiation system having a laser source and an irradiation optical system for irradiating the laser beam onto a cornea, and a mask plate having a light shielding area corresponding to a far vision area in a central part and a first opening corresponding to a near vision area outside the light shielding area, the first opening having a shape where a ratio of a circumferential length at a radius position of the mask plate is made smaller as nearing a center of the mask plate.  
       [0010] Additional objects and advantages of the invention are set forth in the description which follows, are obvious from the description, or may be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by the corneal-ablation-data calculation apparatus and the corneal surgery apparatus in the claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] 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,  
     [0012]FIG. 1 is a view showing a schematic configuration of a laser irradiation optical system and a control system in a corneal surgery apparatus consistent with the present invention;  
     [0013]FIGS. 2A and 2B are views illustrating ablation for presbyopic correction;  
     [0014]FIGS. 3A and 3B are views illustrating ablation at each stage in hyperopic presbyopic correction;  
     [0015]FIGS. 4A to  4 C are views illustrating ablation at each stage in myopic presbyopic correction;  
     [0016]FIG. 5 is a view illustrating a method for forming a transition zone by ablation for myopic correction;  
     [0017]FIG. 6 shows results of ablation provided on a hyperopic presbyopic eye;  
     [0018]FIG. 7 shows results of ablation provided on a myopic presbyopic eye;  
     [0019]FIG. 8 is a view showing a schematic configuration of the corneal surgery apparatus in a case where a mask plate is disposed on a cornea;  
     [0020]FIGS. 9A and 9B are views showing a schematic configuration of a mask unit;  
     [0021]FIG. 10 is a view illustrating a shape of the mask plate;  
     [0022]FIG. 11 is a view illustrating a ratio of data on ablation depth distribution in a certain myopic correction amount; and  
     [0023]FIG. 12 is a view illustrating different shapes of the mask plate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0024] A detailed description of one preferred embodiment of a corneal-ablation-amount calculation apparatus and a corneal surgery apparatus embodied by the present invention is provided below with reference to the accompanying drawings.  
     [0025]FIG. 1 is a view showing a schematic configuration of a laser irradiation optical system and a control system in a corneal surgery apparatus consistent with the present invention.  
     [0026] A laser beam emitted from a laser source  1  (an excimer laser beam having a wavelength of 193 nm is used in the embodiment) is a pulsed beam, and in its typical shape, intensity distribution of the beam is approximately uniform distribution in a horizontal direction, and is the Gaussian distribution in a vertical direction. In addition, a cross section thereof on a plane intersecting at right angles with respect to a central optical axis L of the irradiation optical system is in a narrow rectangular shape.  
     [0027] The laser beam emitted from the laser source  1  is reflected by a plane mirror  2 , and further reflected by a plane mirror  3 . The mirror  3  is translatable (movable) in the direction of the arrow shown in FIG. 1 by a driving unit  4 , and translates (scans) the laser beam in the Gaussian distribution direction so that an object may be uniformly ablated.  
     [0028] An image rotator  5  is rotated about the optical axis L as its center by a driving unit  6  to rotate the laser beam about the optical axis L. Reference numeral  7  is a circular aperture having a circular opening for limiting an ablation area to a circular shape, and its opening diameter is changed by a driving unit  8 . Reference numeral  9  is a slit aperture having a slit opening for limiting an ablation area to a slit shape, and its opening width is changed by a driving unit  10 . In addition, the slit aperture  9  is rotated about the optical axis L by a driving unit  11  and its opening direction is changed. A projecting lens  12  projects the openings of the circular aperture  7  and/or the slit aperture  9  onto a cornea  15  of a patient&#39;s eye.  
     [0029] Reference numeral  13  is a dichroic mirror having a property of reflecting the excimer laser beam and transmitting visible light. The laser beam passed through the projecting lens  12  is reflected by the dichroic mirror  13 , and directed to and irradiated onto the cornea  15 .  
     [0030] Placed above the dichroic mirror  13  is an observation optical system  14  having a surgical microscope of a binocular type (commercially available one may be used) Besides, the patient&#39;s eye previously goes through positioning in a predetermined position on the occasion of surgery, and the positioning condition is maintained by continuously looking at a fixation target (a fixation light source) not illustrated.  
     [0031] A data input unit  21  inputs data necessary for refractive correction of the patient&#39;s eye, and a calculation unit (apparatus)  22  obtains data on corneal ablation (data on an ablation position, an ablation amount and the like) based on the inputted data. A control unit  20  controls the laser source  1 , each of the driving units  4 ,  6 ,  8 ,  10  and  11  based on the data on the corneal ablation obtained by the calculation unit  22 . Incidentally, a commercially available personal computer may be used for the data input unit  21  and the calculation unit  22 .  
     [0032] Next, ablation for presbyopic correction will be described. In the presbyopic correction, as shown in FIGS. 2A and 2B, an area  102  for far vision (hereinafter referred to as a “far vision area”) is set in a central part of the cornea, and an area  106  for near vision (hereinafter referred to as a “near vision area”) in an annular shape is set around (outside) the far vision area  102 . Also, outside the near vision area  106 , a transition zone  108  which smoothly connects the ablation area and a non-ablation area is set. In addition, it is preferable that a blend zone  104  for middle vision is further set between the far vision area  102  and the near vision area  106 . The blend zone  104  smoothly connects the far vision area  102  and the near vision area  106 . The ablation to form the far vision area  102  and the near vision area  106  is implemented by multi-stage operation. The data necessary for presbyopic correction as above are a pre-operative corneal shape, sizes of the far and near vision areas, a correction amount for far vision, and additional power (diopter) for near vision, and they are inputted by the data input unit  21 . The calculation unit  22  obtains a post-operative corneal shape based on the inputted data to calculate the data on the corneal ablation, and the control unit  20  (or the calculation unit  22 ) obtains data for controlling a laser irradiation system (the laser irradiation optical system and the control system thereof).  
     [0033] In the apparatus in the present embodiment, ablation for myopic correction and ablation for hyperopic correction are combined to form the far vision area  102  and the near vision area  106 . Thus, firstly, a brief description will be given on ablations for myopic correction, hyperopic correction and astigmatic correction, respectively, performed by the laser irradiation system in the present apparatus.  
     [0034] (Ablation for Myopic Correction)  
     [0035] In the case of the myopic correction, the laser beam of which the beam cross section is in the rectangular shape is moved (scanned) within the opening of the circular aperture  7  in the Gaussian distribution direction by the movement of the mirror  3 . Then, every time the laser beam has been moved (scanned) in one direction (one scan), movement (scanning) direction of the laser beam is changed by the rotation of the image rotator  5  (for example, in three directions at intervals of 120 degrees, preferably, in different angle direction at each scan) to perform approximately uniform ablation of the cornea within the opening of the circular aperture  7 . These processes are performed every time the opening diameter of the circular aperture  7  is sequentially changed. Thereby, the ablation for myopic correction may be performed deeply at the central part of the cornea and shallowly at the peripheral part (for more detail, see Japanese Patent Application Unexamined Publication No. Hei6-114083 corresponding to U.S. Pat. No. 5,637,109).  
     [0036] (Ablation for Hyperopic Correction)  
     [0037] In the case of the hyperopic correction, the opening diameter of the circular aperture  7  is fixed in alignment with an optical zone (an optical area where the correction is secured). The mirror  3  is moved and fixed so that the laser beam is deviated with respect to the optical axis L, and the laser beam of which the beam cross section is in the rectangular shape is moved (scanned) in an annular shape by the rotation of the image rotator  5  to ablate the cornea in an annular shape. Then, the mirror  3  is sequentially moved to increase the number of irradiation pulses (irradiation time) as the deviation amount of the laser beam from the optical axis L increases. Thereby, the ablation for hyperopic correction may be performed shallowly at the central part of the cornea and deeply at the peripheral part. Power (diopter) is controlled by changing the total number of irradiation pulses without changing a ratio of the number of irradiation pulses at each position of the laser beam deviated from the optical axis L by the movement of the mirror  3  (for more detail, see Japanese Patent Application Unexamined Publication No. Hei8-66420 corresponding to U.S. Pat. No. 5,800,424).  
     [0038] (Ablation for Astigmatic Correction)  
     [0039] In the case of the astigmatic correction, the opening diameter of the circular aperture  7  is fixed, and the mirror  3  is moved stepwise to move (scan) the laser beam in the Gaussian distribution direction. At this time, the moving (scanning) direction of the laser beam of which the beam cross section is in the rectangular shape is adjusted by the rotation of the image rotator  5  so as to correspond to a flattest meridian direction of the astigmatism. Then, as the laser beam is moved (scanned) stepwise and the number of irradiation pulses at each position is increased at a proper ratio as nearing the peripheral part of the cornea, ablation for hyperopic astigmatic correction may be performed shallowly at the central part and deeply at the peripheral part in the flattest meridian direction. On the contrary, if the number of irradiation pulses at each position at the time of moving (scanning) the laser beam stepwise is increased as nearing the central part of the cornea (at this time, the moving (scanning) direction of the laser beam is adjusted so as to correspond to a steepest meridian direction of the astigmatism), ablation for myopic astigmatic correction may be performed deeply at the central part and shallowly at the peripheral part in the steepest meridian direction (for more detail, see Japanese Patent Application Unexamined Publication No. Hei8-66420 corresponding to U.S. Pat. No. 5,800,424).  
     [0040] Further, in the case of the myopic astigmatic correction, the ablation may also be performed by the following method. The opening diameter of the circular aperture  7  is fixed, and the opening direction of the slit aperture  9  is previously adjusted so that its opening width is changed in the steepest meridian direction. As for the laser beam irradiation, as in the case of the above-described myopic correction, the laser beam is moved (scanned) in the Gaussian distribution direction by the movement of the mirror  3 , and every time the laser beam has been provided one scan, the movement (scanning) direction of the laser beam is changed by the rotation of the image rotator  5  to perform approximately uniform ablation of the cornea within the opening of the slit aperture  9 . These are performed every time the opening width of the slit aperture  9  is changed. Thereby, the ablation for the myopic astigmatic correction may be performed deeply at the central part of the cornea and shallowly at the peripheral part in the steepest meridian direction.  
     [0041] Next, the ablation for presbyopic correction in combination of the ablation for myopic correction and the ablation for hyperopic correction will be described. Hereinafter, description will be separately given on the ablation for hyperopic presbyopic correction and that for myopic presbyopic correction.  
     [0042] In the case of the hyperopic presbyopic correction, firstly, as a first stage, an area within an outer diameter of the near vision area  106  is set as an optical zone  110  and the ablation for hyperopic correction is performed thereon, as shown in FIG. 3A. In addition, the transition zone  108  outside the optical zone  110  is formed also utilizing the ablation for hyperopic correction. In FIG. 3A, a diagonally shaded portion is a portion to be ablated through the ablation for hyperopic correction. Incidentally, the transition zone  108  is formed by sequentially enlarging the opening diameter of the circular aperture  7  enlarged to a size of the optical zone  110 , in accordance with the deviation of the laser beam with respect to the optical axis L.  
     [0043] Next, as a second stage, an area within the far vision area  102  in the central part of the area  110  after the ablation for hyperopic correction is set as an optical zone, and the ablation for myopic correction is performed thereon, as shown in FIG. 3B. Also, in a case where the blend zone  104  for the middle vision is provided between the far vision area  102  and the near vision area  106 , the blend zone  104  is set as a transition zone and is formed by utilizing the ablation for myopic correction. In FIG. 3B, a diagonally shaded portion is a portion to be ablated through the ablation for myopic correction.  
     [0044] The transition zone formed by the ablation for myopic correction is in a curved shape inscribed in each curved surface of the cornea before and after the ablation. Owing to this, the non-ablation area and the ablation area are smoothly connected. FIG. 5 is a view illustrating a method for forming the transition zone by the ablation for myopic correction. An arc  200  indicates a curved surface of the cornea before the ablation having a radius of curvature R1, and an arc  202  indicates a curved surface of the cornea after the ablation having intended radius of curvature R2. Further, a circle  204  with a radius of curvature RT which inscribes in the arcs  200  and  202  at a point P1 on the arc  202  and a point P2 on the arc  200 , respectively, determined by a forming position of the transition zone makes the curved shape of the transition zone. For detail of the method for forming the transition zone, please refer to Japanese Patent Application Unexamined Publication No. Hei6-189999 corresponding to U.S. Pat. No. 5,445,633.  
     [0045] Besides, it is preferable that a size of the ablation for hyperopic correction is OZ 1  (a diameter of the optical zone  110 )=about 6.5 mm, and TZ 1  (an outer diameter of the transition zone  108 )=about 10.0 mm. In LASIK surgery where the ablation is performed after a corneal flap is formed, there is a case where TZ 1  is set small in accordance with a size of the corneal flap. On the other hand, it is preferable that a size of the ablation for myopic correction is OZ 2  (a diameter of the far vision area  102 )=about 3.0 to 4.0 mm, and TZ 2  (an outer diameter of the blend zone  104 )=about 4.0 to 5.0 mm. In addition, each of the ablation sizes for hyperopic correction and myopic correction are preferably determined based on a pupil size of the patient, respectively.  
     [0046] A correction amount for each ablation is set as follows. When desired refractive power after the operation is emmetropia, for the ablation for hyperopic correction at the first stage, the correction amount is set by near vision refractive power or far vision refractive power, and additional power. For the ablation for myopic correction at the second stage, the correction amount is set which corresponds to the additional power or a difference between the far vision refractive power and the near vision refractive power. When the desired refractive power after the operation is deviated from the emmetropia, each correction amount is adjusted by the deviation amount.  
     [0047] For example, when the emmetropia is desired with respect to the patient&#39;s eye of which the pre-operative far vision refractive power S (spherical power)=+1.0D, and the additional power=+2.25D, S=+3.25D is set for the correction amount for the ablation for hyperopic correction, and S=−2.25D is set for the correction amount for the ablation for myopic correction.  
     [0048] In the case of the myopic presbyopic correction, as a first stage, an optical zone  120  in the central part of the cornea and a transition zone  122  outside thereof are set, and the ablation for myopic correction is performed thereon, as shown in FIG. 4A. It is preferable that a size of the ablation for myopic correction is OZ 3  (a diameter of the optical zone  120 )=about 5.0 to 6.0 mm, and TZ 3  (an outer diameter of the transition zone  122 )=about 7.0 to 8.0 mm. In FIG. 4A, a diagonally shaded portion is a portion to be ablated through the ablation for myopic correction.  
     [0049] Next, as a second stage, an optical zone  124  and a transition zone  126  outside thereof are set, and the ablation for hyperopic correction is performed thereon, as shown in FIG. 4B. It is preferable that a size of the ablation for hyperopic correction is OZ 4  (a diameter of the optical zone  124 )=about 6.5 mm, and TZ 4  (an outer diameter of the transition zone  126 )=about 10.0 mm. Here, OZ 4  is made smaller than TZ 3 . The transition zone  126  becomes an area corresponding to the transition zone  108  in FIGS. 2A and 2B, and an area  128  where the transition zone  122  and the optical zone  124  overlap each other becomes an area corresponding to the near vision area  106  in FIGS. 2A and 2B. In this area, the refractive power is gradually changed because of the formation of the transition zone  122 . In FIG. 4B, a diagonally shaded portion is a portion to be ablated through the ablation for hyperopic correction.  
     [0050] Further, as occasion arises, as a third stage, additional ablation for myopic correction is performed on the area  120  after the ablation for hyperopic correction at the second stage, as shown in FIG. 4C. An optical zone  130  and a transition zone  132  outside thereof are set within the area  120 , and the ablation for myopic correction is again performed thereon. It is preferable that a size of the ablation for myopic correction is OZ 5  (a diameter of the optical zone  130 )=about 3.0 to 4.0 mm, and TZ 5  (an outer diameter of the transition zone  132 )=about 5.0 to 6.0 mm. In this case, the optical zone  130  becomes an area corresponding to the far vision area  102  in FIGS. 2A and 2B, and the transition zone  132  becomes an area corresponding to the blend zone  104  in FIGS. 2A and 2B. In FIG. 4C, a diagonally shaded portion is a portion to be ablated through the ablation for myopic correction.  
     [0051] If the third stage is not performed, the optical zone  120  becomes an area corresponding to the far vision area  102  in FIGS. 2A and 2B.  
     [0052] Besides, the far vision area in the central part of the cornea may be secured by performing the ablation for hyperopic correction (ablation in an annular shape) on an area except the optical zone  130  as shown in FIG. 4C at the second stage.  
     [0053] For the ablation for myopic correction at the first stage, the correction amount is set based on the far vision refractive power. For the ablation for hyperopic correction at the second stage, the correction amount corresponding to the additional power is set. If the ablation for hyperopic correction at the second stage causes somewhat myopia, the correction amount is set at the third stage so as to correct the deviation amount. For example, S=−0.75D may be set. If the third stage is not set, there is a case where the correction amount at the first stage or the second stage is adjusted.  
     [0054] At the time of each of the presbyopic corrections, the data on the pre-operative corneal shape, the data on the correction amount for each stage, and the data on the ablation size for each stage (OZ, TZ) are inputted by the data input unit  21 . The calculation unit  22  calculates the post-ablative corneal shape for each stage based on the inputted data to obtain the data on the corneal ablation for each stage. Based on the data on the corneal ablation for each stage, in the laser irradiation for performing the ablation for myopic correction, data for controlling the opening diameter of the circular aperture  7  is determined. In the laser irradiation for performing the ablation for hyperopic correction, data for controlling the movement of the mirror  3  and the number of irradiation pulses are determined. The control unit  20  controls the laser irradiation system based on the determined control data. The laser irradiation for each stage is sequentially performed.  
     [0055] Besides, the laser irradiation system may be the one which uses a scanning mirror (which may be constituted by two galvano-mirrors) for scanning the laser beam formed into a small spot of about 0.1 to 1.0 mm two-dimensionally in the X and Y directions. In this case, the laser irradiation for the post-operative corneal shape shown in FIG. 2B may be implemented at once without combining each of the stages.  
     [0056]FIG. 6 shows results of the correction of a hyperopic presbyopic eye performed by the present apparatus. As to a patient No.1, the pre-operative far vision refractive powers of both eyes (OD, OS) were S=+1.75D, and the additional power=+2.25D. Thus, the ablation for hyperopic correction at the first stage was set as S=+4.0D, OZ 1 =6.5 mm, TZ 1 =10.0 mm. Further, the ablation for myopic correction at the second stage was set as S=−1.5D, OZ 2 =4.0 mm, TZ 2 =5.0 mm.  
     [0057] The post-operative far vision refractive power of the patient No.1 was somewhat weak as S=−0.75D three days after the operation, and S=−1.0D three weeks after the operation. However, for far vision eyesight (visual acuity) of a naked eye, 1.0 was obtained. Also for near vision eyesight (visual acuity) of the naked eye, vision capable of reading a character at P2 (which indicates visual acuity of the near vision, and the character becomes larger in the order of P2, P3, P4, P5) was obtained. Both of the obtained results were equivalent to those of corrected eyesight (visual acuity) by means of the spectacles before the operation.  
     [0058] As to a patient No.2, the pre-operative far vision refractive power of a right eye (OD) was S=+2.0D, C (cylindrical power)=−0.5D, A (a cylindrical axial angle)=95°, and the additional power=+2.75D. The pre-operative far vision refractive power of a left eye (OS) was S=+2.0D, C=−0.5D, A=80°, and the additional power=+2.75D. For the hyperopic presbyopic eye, as the cylindrical power is corrected through the ablation for hyperopic astigmatic correction, the cylindrical power is read by plus. In this case, the pre-operative far vision refractive power of the right eye is S=+1.5D, C=+0.5D, A=5°, and the pre-operative far vision refractive power of the left eye is S=+1.5 D, C=+0.5D, A=170°.  
     [0059] For the patient No.2, the ablation for hyperopic correction at the first stage was set as S=+4.0D, OZ 1 =6.5 mm, TZ 1 =10.0 mm, and the correction for near vision was set slightly weak (by +0.25D). Further, the ablation for myopic correction at the second stage was set as S=−1.5D, OZ 2 =4.0 mm, TZ 2 =5.0 mm, and the correction for far vision was set slightly strong (by −0.25D).  
     [0060] The post-operative far vision refractive power of the patient No.2 was 0D (emmetropia). The far vision eyesight of a naked eye three days after the operation was 0.9; however, 1.2 was obtained two months after the operation. Also, for near vision eyesight of the naked eye, vision capable of reading small characters up to P3 was obtained.  
     [0061]FIG. 7 shows results of the correction of the myopic presbyopic eye performed by the present apparatus. As to a patient No.3, the ablation for myopic correction at the first stage was set as S=−1.25D, OZ 3 =5.0 mm, TZ 3 =7.0 mm, that are the same as the pre-operative far vision refractive power. Further, the ablation for hyperopic correction at the second stage was set as S=+1.5D, OZ 4 =6.5 mm, TZ 4 =10.0 mm, that are slightly stronger than the pre-operative additional power. Furthermore, the ablation for myopic correction at the third stage was set as S=−0.75D, OZ 5 =4.0 mm, TZ 5 =5.0 mm. For the far vision eyesight of the naked eye and the near vision eyesight of the naked eye three days after the operation, favorable results were obtained.  
     [0062] As to a patient No.4, the ablation was set up to the second stage. In the ablation for myopic correction at the first stage, OZ 3  and TZ 3  of the right eye (OD) were made 1 mm larger than those of the patient No.3, respectively. Further, the ablation for hyperopic correction at the second stage was set as S=+2.0D, that was slightly weaker than the additive diopter. Other conditions were set in the same manner as those for the patient No.3. For the far vision eyesight of the naked eye, such results were obtained that were close to the vision obtained through the correction by means of the spectacles. Also for the near vision eyesight of the naked eye, favorable results were obtained.  
     [0063] Next, a description will be given on another embodiment in which a mask plate having a light shielding area and an opening is disposed on the cornea  15 , the far vision area  102  is formed (set) in the central part of the cornea, and the annular near vision area  106  is set outside thereof. FIG. 8 is a view showing a configuration in a case where the mask plate is disposed on the cornea. Reference numeral  300  is a mask unit having the mask plate. FIGS. 9A and 9B are views showing a schematic configuration of the mask unit  300 .  
     [0064] In a circular mask plate  310 , an area for cutting off the laser beam and an opening for passing the laser beam are formed (see FIG. 10 for detail). The mask plate  310  is rotatably held on a ring block  320  by two ring bearings  313  arranged above and below. The ring block  320  is used for placing the mask plate  310  on the cornea  15 . Between a side face of an outer rim of the mask plate  310  and the ring block  320 , a space  311  is formed. Further, four fins  312  are attached to the side face of the outer rim of the mask plate  310 , at regular intervals. The fins  312  are made of an elastic material such as rubber, and the tip portions thereof are in contact with the inner wall of the ring block  320 .  
     [0065] Provided to the ring block  320  is an aspiration vent  322  which is connected to an aspiration source  324  via a tube. A convex part  326  for narrowing an aspiration path and enabling the fins  312  to pass is formed on the inner wall of the ring block  320  which is positioned near the aspiration vent  322 . Further, an inflow vent  318  for taking in air into the space  311  is provided near the convex part  326 . Besides, the space  311  is almost sealed except for the inflow vent  318  and the aspiration vent  322 . If the aspiration source  324  inhales the air in the space  311 , a negative pressure is developed in the space  311 . Due to such aspiration pressure, the fins  312  near the aspiration vent  322  are pulled, and the mask plate  310  rotates in the direction of the arrow A shown in FIG. 9A. When the fins  312  collide against the convex part  326 , they are bended to pass the convex part  326 .  
     [0066] Besides, a rotation mechanism of the mask plate  310  is not limited to the above-mentioned constitution. For example, it may have a constitution where a plurality of blades are provided to the side of the outer rim of the mask plate  310 , and compressed air is sent to the blades to rotate the mask plate  310 .  
     [0067] The ring block  320  is fixed while its lower end portion is in contact with a screla of the patient&#39;s eye. On the lower end portion brought into contact with the screla, a groove  330  for adsorption is formed, and the groove  330  is connected to an aspiration vent  332 . The aspiration vent  332  is connected to an aspiration source  334  via a tube. The aspiration source  334  provides an aspiration pressure, so that the ring block  320  is fixed to the patient&#39;s eye.  
     [0068] It is preferable that a sensor  340  for detecting the number of rotation of the mask plate  310  is provided to the mask unit  300 . The sensor  340  is comprised of a light source for emitting detection light and a photodetector, and photo-receives the detection light reflected by a groove  310   a  provided on the mask plate  310  to detect the number of rotation of the mask plate  310 . A control unit  336  in the mask unit  300  controls driving of the aspiration sources  324  and  334 .  
     [0069] In the presbyopic correction using the mask unit  300 , when the laser beam is irradiated while rotating the mask plate  310 , the ablation is not performed on the central part of the cornea in order to secure the far vision area  102 , and the ablation for securing the annular near vision area  106  and the transition zone  108  is performed outside the central part of the cornea as shown in FIGS. 2A and 2B. Further, preferably, the ablation for forming the blend zone  104  for middle vision is performed between the far vision area  102  and the near vision area  106 .  
     [0070] As for the size of each area, for example, the far vision area  102  is 3 mm in diameter (a radius R1=1.5 mm), the blend zone  104  is 4 mm in outer diameter (a radius R2=2 mm), the near vision area is 6.5 mm in outer diameter (a radius R3=3.25 mm), and the transition zone  108  is 10 mm in outer diameter (a radius R4=5 mm). This is only an example, and the size suitable for the patient may be used.  
     [0071]FIG. 10 is a view illustrating a shape of the mask plate  310 . The mask plate  310  has a light shielding area  351  having the radius R1 corresponding to the far vision area  102  in the central part, and an area  355  outside the light shielding area in which three openings  353  in the same shape are formed at regular intervals. The area  355  is an area for securing the blend zone  104  (radius R1 to R2), the near vision area  106  (radius R2 to R3), and the transition zone  108  (radius R3 to R4). The near vision area  106  is an area on which the ablation for a hyperopic shift in the amount of the additional power necessary for the presbyopic correction with respect to the far vision area  102  is performed. The blend zone  104  is an area on which the ablation is performed so as to be progressively changed to the additional power.  
     [0072] As for the openings  353  formed in the area  355 , in the range of the blend zone  104  (radius R1 to R2) and the near vision area  106  (radius R2 to R3), a ratio of a circumferential length of the opening  353  at a radius position of the mask plate  310  is made smaller as nearing a center of the mask plate  310 . An example for determining it will be described hereinafter. The ablation by the amount of the additional power as the amount for the near vision correction (for example, 1D) is assumed to be performed on the near vision area  106 , and data on ablation depth distribution of the near vision area  106  of radius R2 to R3 and that of the blend zone  104  of radius R1 to R2 are previously obtained (see FIG. 11). Maximum ablation depth for the radius R3 being an outermost circumference of the near vision area  106  is set as 1, and a ratio Kn of the ablation depth at a certain radius position rn with respect to the maximum ablation depth is obtained. The circumferential length of the opening  353  with respect to the circumferential length Ln at the radius position rn is obtained by an expression Ln×Kn. This is obtained for the range R1 to R3, so that the shape of the opening  353  in the range is determined.  
     [0073] On the contrary, in the range of the transition zone  108  (radius R3 to R4) of the opening  353 , the ratio of the circumferential length of the opening  353  at the radius position of the mask plate  310  is made smaller as nearing an outer rim of the mask plate  310 . As for the transition zone  108 , in a similar manner, data on ablation depth distribution in radius R3 to R4 is previously obtained, and the circumferential length of the opening  353  at the certain radius position is obtained based on the ratio Kn of the ablation depth at the certain radius position to determine the shape of the opening  353 . Incidentally, since the mask plate  310  shown in FIG.  10  is provided with three openings  353 , the circumferential length of the opening  353  is preferably determined while considering the size of connecting portions.  
     [0074]FIG. 12 is an example of the mask plate  310  in which one opening is formed. As to a shape of the opening  353 ′, as mentioned above, its circumferential length at the radius position rn is obtained as Ln×Kn. In this example, in order to make the light shielding area divided in two to be rotatable, the light shielding area is formed on a transparent member which transmits a laser beam. (The opening  353 ′ is a part on which the light shielding area is not formed.)  
     [0075] Presbyopic corrective surgery using the mask unit  300  will be described hereinafter. In a case where the correction is performed on the near vision area  106  by the amount of the additional power for near vision without correcting the far vision area  102 , the patient&#39;s eye is observed with the observation optical system  14 , and the mask unit  300  is placed above it. After the ring block  320  is adsorbed and fixed to the patient&#39;s eye by driving the aspiration source  334 , the mask plate  310  is rotated by driving the aspiration source  334 . At this time, the sensor  340  monitors the number of rotation of the mask plate  310 , and the control unit  336  controls the aspiration pressure of the aspiration source  324  so that a rotation period of the mask plate  310  is synchronized with the number of pulses of the laser beam emitted from the laser source  1  (for example, 60 Hz).  
     [0076] An irradiation method which enables the ablation with substantially uniform depth by the scan of the previously-described laser beam of which the beam cross section is the rectangular shape is used for the laser irradiation. Hereinafter, the description will be given on the irradiation method. The laser beam is moved (scanned) in the Gaussian distribution direction by the movement of the mirror  3  and is superimposed. When the laser beam is irradiated while rotating the mask plate  310 , the cornea is ablated by the laser beam passed through the opening  353 . Every time one scan has been provided, the control unit  20  adjusts and differentiates timing of emission of the pulsed laser beam from the laser source  1 . Thereby, a rotation position of the opening  353  on the cornea is deviated, and the irradiation position of the laser beam having passed through the opening  353  is also deviated. The ablation with the laser beam is superimposed, and the ablation depth at each of the radius positions is changed while being related to the number of scans and the circumferential length of the opening  353  at the radius position. The additional power for near vision may be controlled by changing the number of scans (laser irradiation time). The data input unit  21  inputs the additional power for near vision, and the control unit  20  (or the calculation unit  22 ) obtains the control data on the laser irradiation system. As a consequence, the far vision area  102  on which the ablation is not performed is secured in the central part of the cornea, and the annular near vision area  106 , the transition zone  108  and the blend zone  104  are formed outside the far vision area  102 .  
     [0077] Besides, the control of the correction amount may be performed by preparing several openings  353  with different shapes according to the additional power for near vision, instead of changing the number of scans (laser irradiation time).  
     [0078] In addition, in a case where a laser irradiation system capable of irradiating the laser beam broader than the mask plate  310  is used, instead of the laser irradiation system which irradiates the laser beam of which the beam cross section is in the rectangular shape as mentioned above, the number of pulses of the laser beam is adjusted with respect to the number of rotation of the mask plate  310 , so that the rotation position of the opening  353  at the time of the laser irradiation may be deviated.  
     [0079] In the case of the hyperopic presbyopic correction, the ablation for hyperopic correction as mentioned above is performed before or after the ablation using the mask unit  300 . At the time of the surgery, the data on the pre-operative corneal shape, the data on the correction amount for far vision, the data on the additional power for near vision, and data on a size of the optical zone for the ablation for hyperopic correction are inputted. In this case, it is preferable that the size of the optical zone is set to include the near vision area  106  for the presbyopic correction by the mask plate  310 , and the ablation for the transition zone is set around it. The control data for the laser irradiation system is prepared based on the inputted data. Incidentally, the control data may be calculated by obtaining the corneal shape at each of the steps of the ablation for hyperopic correction and the ablation performed by the mask unit  300 .  
     [0080] Further, also in the case of the myopic presbyopic correction, the ablation for myopic correction as mentioned above is performed before or after the ablation using the mask unit  300 . It is also preferable that the size of the optical zone for the ablation for myopic correction is set to include the near vision area  106 , and the ablation for the transition zone is set around it.  
     [0081] As described above, the presbyopic correction for making the post-operative condition of visibility favorable may be easily allowed.  
     [0082] 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.