noncontact tonometers in preferred embodiments according to the present invention will be described hereinafter with reference to the accompanying drawings . referring to fig4 a noncontact tonometer in a first embodiment according to the present invention comprises , as principal components , a placido &# 39 ; s ring pattern projecting system ( light pattern projecting system ) 10 , an observation optical system 20 , an air jetting system ( air blowing means ) 30 , an xy alignment light projecting optical system 40 , an xy alignment detecting optical system 50 , a working distance measuring system 60 , a control system 100 and a display 200 . referring to fig4 the placido &# 39 ; s ring pattern projecting system 10 projects a light pattern consisting of a plurality of concentric rings on the cornea c of the eye e . the placido &# 39 ; s ring pattern projecting system 10 comprises a plurality of light - emitting diodes 11 arranged on concentric circles as shown in fig5 and a diffusing plate 12 provided with a ring pattern consisting of a plurality of concentric rings as shown in fig6 . the placido &# 39 ; s ring pattern projecting system 10 may be similar in configuration to a placido &# 39 ; s pattern projecting system disclosed in jp - a no . 8 - 275920 . in fig6 indicated at a1 to a6 are optically transparent parts and at b1 to b6 are optically opaque parts . the observation optical system 20 is used for observing the anterior segment of the eye e , and observing and recording an image of a placido &# 39 ; s ring . the observation optical system 20 comprises an objective lens 21 , a telecentric diaphragm 22 , an image forming lens 23 and a ccd ( light receiving means ) 24 . the optical axis o of the observation optical system 20 passes the center o &# 39 ; of the pattern of the plurality of rings of the placido &# 39 ; s ring pattern projecting system 10 . the ccd 24 is disposed so as to be optically conjugate with the focal plane of the cornea c of the eye e disposed at a proper working distance w with respect to the objective lens 21 and the image forming lens 23 . illuminating light beams projected by the placido &# 39 ; s ring pattern projecting system 10 are reflected by the cornea c , and a virtual image of the placido &# 39 ; s ring is formed near the focal plane f of the cornea c . since the observation optical system 20 is telecentric with respect to the side of the object , the image of the placido &# 39 ; s ring is formed on the ccd 24 in the same size even if the actual working distance is slightly different from the proper working distance w . the air jetting system 30 jets an air pulse whose pressure to the eye e varies in proportion to time . the air jetting system 30 comprises a cylinder 31 , a piston 32 , a solenoid 33 for driving the piston 32 and an orifice pipe 34 . as shown in fig4 the orifice pipe 34 has one end part penetrating a central part of the objective lens 21 and the other end part connected to a closed end of the cylinder 31 . the piston 32 is driven to compress air in the cylinder 31 , and compressed air is jetted toward the cornea c of the eye e . the present invention measures intraocular tension on the basis of a change in the shape of the image of the placido &# 39 ; s ring before and after an air pulse is begun to be jetted against the cornea c . therefore the applanation of the eye e is unnecessary , and the intensity of the air pulse may be lower than that of an air pulse used in a conventional noncontact tonometer . for example , an air pulse of 5 mmhg in peak pressure is sufficient for measurement . a conventional noncontact tonometer needs an air pulse of about 15 mmhg in peak pressure . the xy alignment light projection optical system 40 projects a position indicating light beam on the eye e to examine the position of the eye e relative to the optical axis o with respect to vertical direction ( direction along the y - axis ), and horizontal direction ( direction along the x - axis ). the xy alignment light projecting optical system 40 comprises a light source 41 , a pinhole plate 42 , a collimator lens 43 , a semitransparent mirror 44 and the objective lens 21 . the pinhole plate 42 is disposed at the front focal point of the collimator lens 43 . a position indicating light beam , i . e ., a beam of parallel light beams , is projected through the objective lens 21 on the eye e . the xy alignment detecting optical system 50 detects the positional relation between the eye e and the optical axis o with respect to directions along the x - axis and the y - axis . the xy alignment detecting optical system 50 comprises the objective lens 21 , the image forming lens 23 , a semitransparent mirror 51 and a light receiving device 52 . the light receiving device 52 is a photodiode capable of measuring the intensity of incident light . the photodiode receives the alignment index light beam projected by the xy alignment light projecting optical system 40 and reflected by the cornea c , and determines the position of the eye e relative to the optical axis o with respect to directions along the x - axis and the y - axis on the basis of the intensity of the incident light . the light receiving device 52 may be a psd ( position sensitive device ) capable of determining the centroid of the intensity of the incident light . the working distance measuring system 60 comprises a light source 61 , a slit plate 62 , a collimator lens 63 , a condenser lens 64 and a light receiving device 65 . light emitted by the light source 61 travels through the slit of the slit plate 62 and is collimated by the collimator lens 63 in a collimated working distance measuring light beam . the working distance measuring light beam falls obliquely on the eye e . the working distance measuring light beam reflected by the cornea c of the eye e travels through the condenser lens 64 and falls on the light receiving device 65 . the light receiving device 65 is , for example , a line sensor capable of supplying a position signal corresponding to the position of the light beams thereon . the light receiving device 65 supplies a signal corresponding to the distance of the eye e from the objective lens 21 . a position on the light receiving device 65 at which the working distance measuring light beam reflected from the cornea c falls corresponds to the distance between the cornea c and the objective lens 21 . the control system 100 comprises an arithmetic and control circuit 101 , an a / d converter 102 and a memory 103 . a detection signal supplied by the light receiving device 52 is given to the arithmetic and control circuit 101 . the solenoid 33 is controlled for operation by the arithmetic and control circuit 101 . the arithmetic and control circuit 101 transfers image data obtained by the a / d conversion of information about an image of the placido &# 39 ; s ring provided by the ccd 24 by the a / d converter 102 to the memory 103 at predetermined time . for example , first image data transfer operation for transferring image data is executed at time t1 between time t0 when an alignment completion signal is supplied and time t2 when an air jetting operation is started . a second image data transfer operation is executed at time t3 before time t4 when the pressure of an air pulse reaches the maximum and t seconds after the time t2 as shown in fig7 . image data thus transferred thereto is stored in the memory 103 . the degree of deformation of the cornea c at time t seconds after the time t2 when the air jetting operation is started when the intraocular tension is 10 mmhg and that of the same at time t seconds after the time t2 when the air jetting operation is started when the intraocular tension is 15 mmhg are greatly different from each other . fig8 shows a conceptional illustration of the deformation of the cornea c . in fig8 a curve ( 1 ) represents a degree of deformation of the cornea c at time t seconds after the start of the air jetting operation for jetting an air pulse of 5 mmhg in pressure when the intraocular tension is 10 mmhg , and a curve ( 2 ) represents a degree of deformation of the cornea c at time t seconds after the start of the air jetting operation for jetting an air pulse of 5 mmhg in pressure when the intraocular tension is 15 mmhg . as is obvious from fig8 the higher the intraocular tension , the more difficult is the cornea c to be depressed . therefore , the higher the intraocular tension , the less is the deformation of the ring pattern formed on the cornea c . accordingly , the intraocular tension of the eye e can be determined through the analysis of the deformation of the ring pattern . the memory 103 stores reference image data on the images of the placido &# 39 ; s ring pattern formed when an air pulse of 5 mmhg in peak pressure ( hereinafter referred to as &# 34 ; 5 mmhg air pulse &# 34 ;) is jetted against eyes respectively having known intraocular tensions . fig9 ( a ) to 9 ( i ) show reference image data on images of the placido &# 39 ; s ring pattern when an 5 mmhg air pulse is jetted against eyes respectively having intraocular tensions of 10 + i mmhg ( i = 1 , 2 , 3 , . . . and 10 ) by way of example . the arithmetic and control circuit 101 compares measured image data with the reference image data , and selects a reference image data most analogous with the measured image data . an intraocular tension corresponding to the selected reference image data is displayed on the display 200 . the arithmetic and control circuit 101 , the ccd 24 and the air jetting system ( air blowing means ) 30 constitute a cornea deformation measuring means . a light source , not shown , for illuminating the anterior segment of the eye e is turned on to illuminate the anterior segment of the eye e . a reflected light beam reflected from the anterior segment of the eye e travels through the objective 21 , the semitransparent mirror 44 , the telecentric diaphragm 22 , the image forming lens 23 and the semitransparent mirror 51 and fall on the ccd 24 . thus , an image of the anterior segment of the eye e is formed on the ccd 24 and the ccd 24 supplies an image signal . the image signal is transferred through the a / d converter 102 and the arithmetic and control circuit 101 to the display 200 . the display 200 displays the image of the anterior segment of the eye e . the light source 41 for xy alignment and the light source 61 for working distance measurement are turned on . xy alignment light emitted by the light source 41 travels through the semitransparent mirror 44 and the objective lens 21 , falls on the eye e , and is reflected by the cornea c of the eye e . the reflected xy alignment light travels through the objective lens 21 , the semitransparent mirror 44 , the telecentric diaphragm 22 , the image forming lens 23 and the semitransparent 51 and falls on the light receiving device 52 . working distance measuring light emitted by the light source 61 travels through the slit of the slit plate 62 , and is collimated by the collimator lens 63 in a collimated slit beam . the collimated slit beam falls on the cornea c of the eye e , is reflected by the cornea c , and the reflected collimated slit beam travels through the condenser lens 64 and falls on the light receiving device 65 . image signals supplied by the light receiving devices 52 and 65 are given to the arithmetic and control circuit 101 . the arithmetic and control circuit 101 makes the display 200 superpose a bright spot , not shown , on the image of the anterior segment displayed on the display 200 at a position represented by the image signal provided by the light receiving device 52 . a joystick , not shown , is operated for motions in directions along the x - axis and the y - axis to make the bright spot displayed on the display 200 coincide with the optical axis o of the noncontact tonometer . the joystick is operated for fore - and - aft motion to measure working distance w , i . e ., the distance between the objective lens 21 and a top point of the cornea c , by the agency of the light receiving device 52 and the arithmetic and control circuit 101 to position the cornea c properly with respect to directions along the optical axis of the noncontact tonometer ( z - axis ). xy alignment using the bright spot and working distance adjustment using the output signal of the light receiving device 65 can be achieved by the generally known methods and hence further description thereof will be omitted . upon the detection of the alignment of the noncontact tonometer with the top part of the cornea c on the basis of the output signals of the light receiving devices 52 and 65 at time t0 , i . e ., when an alignment completion signal is supplied at time t0 , the arithmetic and control circuit 101 drives multiple light - emitting diodes 11 to be turned on . light beams emitted by the light - emitting diodes 11 travel through the optically transparent parts of the ring pattern and fall on the cornea c of the eye e . reflected light beams reflected from the cornea c travel through the objective lens 21 , the semitransparent mirror 44 , the telecentric diaphragm 22 , the image forming lens 23 and the semitransparent mirror 51 , and fall on the ccd 24 . the image of the anterior segment of the eye e and an image of the ring pattern are formed on the ccd 24 . an image signal supplied by the ccd 24 is transferred through the a / d converter 102 to the arithmetic and control circuit 101 . the arithmetic and control circuit 101 stores sample image data on the images of the anterior segment of the eye e and the ring pattern sampled in a period between time t1 and time t2 when an operation for blowing an air pulse against the cornea c is started as first image data in the memory 103 . the arithmetic and control circuit 101 starts compressing air contained in the cylinder 31 at the time t2 by actuating the solenoid 33 to drive the piston 32 . the air pressure ap in the cylinder 31 increases substantially in proportion to time as shown in fig7 and reaches a peak of 5 mmhg at time t4 . the arithmetic and control circuit 101 samples image data on the images of the anterior segment of the eye e and the ring pattern , which are input from the ccd 24 , at time t3 t seconds after the time t2 when the operation for blowing an air pulse against the cornea c is started , and slightly before the time t4 and transfers the sampled image data as second image data to store the data in the memory 103 . after the second image data on the images of the anterior segment of the eye e and the ring pattern are thus stored in the memory 103 , the arithmetic and control circuit 101 compares the data on the ring pattern with the reference data on ring patterns as shown in fig9 ( a ), 9 ( b ), 9 ( c ) and 9 ( i ) for pattern matching . the arithmetic and control circuit 101 selects one of the reference data representing a ring pattern that is the most analogous with the ring pattern and makes the display 200 display an intraocular tension corresponding to the selected reference data . for example , the arithmetic and control circuit 101 makes the display 200 display information indicating that the intraocular tension of the eye e is 10 mmhg , 15 mmhg or p mmhg if the image of the ring pattern represented by the second image data stored in the memory 103 coincides with the image of the ring pattern shown in fig9 ( a ), fig9 ( b ) or fig9 ( i ). the arithmetic and control circuit 101 calculates the radius of curvature of the cornea c on the basis of the image of the ring pattern represented by the first image data stored in the memory 103 , and uses the results of calculation for correcting the intraocular tension determined by pattern matching . a ring pattern of a single ring or a pattern similar to a bar code may be used instead of the ring pattern consisting of the plurality of concentric rings . the noncontact tonometer in the first embodiment measures the deformation of the cornea c of the eye e by the foregoing method comprising projecting the light pattern on the anterior segment of the eye e by the placido &# 39 ; s ring pattern projecting system on the eye e , blowing an air pulse against the cornea c , and measuring the deformed light pattern represented by reflected light . the deformation of the cornea c may be measured by any suitable method other than the foregoing method . for example , a change in the shape of a section of the cornea of the eye caused by blowing air against the cornea may be measured by an optical system based on scheinplug &# 39 ; s principle . a noncontact tonometer in the second embodiment is not provided with any components corresponding to the placido &# 39 ; s ring pattern projecting system 10 and the telecentric diaphragm 22 shown in fig4 . as shown in fig1 , the noncontact tonometer in the second embodiment comprises a slit illuminating system 70 and a anterior segment section photographing system 80 . a section of the cornea c of the eye e is photographed by the slit illuminating system 70 and the anterior segment section photographing system 80 . the noncontact tonometer measures a change in the shape of a section of the cornea c caused by air blown against the cornea c by photographing the section of the cornea c by the slit illuminating system 70 and the anterior segment section photographing system 80 before and after blowing air against the cornea c . the slit illuminating system 70 shapes illuminating light emitted by an infrared light source 71 by a slit formed in a slit plate 72 in a slit light beam , collimates the slit light beam by a collimator lens 73 , and projects the collimated light beam through a semitransparent mirror 74 and 44 and an objective lens 21 on the cornea c . the anterior segment section photographing system 80 receives through an image forming lens 81 the slit light beam projected by the slit illuminating system 70 and reflected from the cornea c by an infrared - sensitive area ccd 82 ( image sensing means ). the ccd 82 has a light receiving surface inclined to the optical axis os of the image forming lens 81 . in this embodiment , the inclination of the surface of the ccd 82 to the optical axis os is 45 °. the slit plate 72 , the image forming lens 81 and the ccd 82 are disposed so that an extension of an optical section of an image of the slit of the slit plate 72 , the principal plane s1 of the image forming lens 81 and an extension s2 of the image forming surface 82a of the ccd 82 meet each other on a single line of intersection . a memory 103 stores reference image data on the images of the placido &# 39 ; s ring pattern formed when an air pulse of 5 mmhg in peak pressure ( hereinafter referred to as &# 34 ; 5 mmhg air pulse &# 34 ;) is jetted against eyes respectively having known intraocular tensions . for example , sectional shape data , not shown , on the shapes of sections of corneas when an 5 mmhg air pulse is jetted against eyes respectively having intraocular tensions of 10 + i mmhg ( i = 1 , 2 , 3 , . . . and 10 ) is stored in the memory 103 . when photographing a section of the cornea c , the noncontact tonometer is aligned with the cornea c with respect to directions along the x - axis and the y - axis . the noncontact tonometer is positioned roughly relative to the cornea c with respect to directions along the z - axis with reference to images displayed on a display 200 . an arithmetic and control circuit 101 turns on an illuminating light source 71 to project the slit light beam on the cornea c . after the noncontact tonometer has been aligned with the cornea c , the arithmetic and control circuit 101 forms an image of a section of the cornea c on the area ccd 82 in a period between time t1 and time t2 ( fig7 ) when an air jetting operation is started . an image signal supplied by the area ccd 82 , i . e ., first image data , is transferred through the arithmetic and control circuit 101 and stored in the memory 103 . the arithmetic and control circuit 101 starts compressing air contained in a cylinder 31 at the time t2 ( fig7 ) by actuating a solenoid 33 to drive a piston 32 . the air pressure ap in the cylinder 31 increases substantially in proportion to time as shown in fig7 and reaches a peak of 5 mmhg at time t4 . the arithmetic and control circuit 101 makes the area ccd 82 form a section of the cornea c at time t3 t seconds after the time t2 when the operation for blowing an air pulse against the cornea c is started , and slightly before the time t4 when the pressure of an air pulse reaches the maximum . then , an image signal supplied by the area ccd 82 is transferred through the arithmetic and control circuit 101 and is stored as second image data in the memory 103 stores . subsequently , the arithmetic and control circuit 101 makes the display 200 display superposed images of the sectional shapes of the cornea c represented by the first image data and the second image data stored in the memory 103 . the arithmetic and control circuit 101 reads sequentially the reference data stored in the memory 103 , compares the images of the sectional shapes of the cornea c represented by the first and the second image data with the reference data . the arithmetic and control circuit 101 selects one of the reference data representing a reference sectional shape that is the most analogous with the sectional shape , and makes the display 200 display an intraocular tension corresponding to the selected reference data . although the invention has been described in its preferred form with a certain degree of particularity , obviously many changes and variations are possible therein . it is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof .
a noncontact tonometer comprises an air current blowing means for blowing an air current against an eye , a cornea deformation measuring means for optically measuring a change in the shape of the cornea caused by the air current blowing means , and an arithmetic means capable of calculating the intraocular tension of the eye on the basis of the change in the shape of the cornea . since the intraocular tension of the eye can be calculated on the basis of the change in the shape of the cornea by the arithmetic means , the pressure of the air current blown against the eye need not be high , which lightens load on the subject .