reference is now made to fig1 which illustrates schematically a prior art method of vision testing using a trial frame . the subject 10 views the test card 12 , located at an optically effectively infinite distance from him , through a pair of trial frames 14 , into which the optometrist can insert combinations of lenses 16 until the best visual acuity is obtained for the subject . the lens combination is changed incrementally until the best combination is determined , as per the subjective perception of the person 10 whose sight is being tested . such a trial frame is generally used for correcting defocus , by the use of combinations of spherical lenses , and astigmatism , by the addition of cylindrical lenses , mutually rotated . though probably the most widely used method in many parts of the world , this method has three disadvantages ( i ) the changes in correction lens are incremental , which in some cases makes it more difficult for the subject to determine the optimum combination ; ( ii ) it is operated by the optometrist , which adds to the subject &# 39 ; s uncertainty of the optimum combination ; and ( iii ) it is not generally used to correct aberrations other than simple defocus , astigmatism and tilt . reference is now made to fig2 which is a schematic diagram of a system for vision testing according to a preferred embodiment of the present invention , using an adaptive optical element . as with the prior art trial frame method , the subject whose vision is under test views the test card 22 , located at an optically effective infinite distance from his eye 20 . instead of the trial frame , a transmissive adaptive optical element 24 is located in front of the subject &# 39 ; s eye , and the test card is viewed through it . the transmissive adaptive optical element could preferably be a liquid crystal element , which introduces a phase shift into the wavefront which can be spatially modulated according to the individual voltage signals applied to each of the pixels in the lc element . the electronic control inputs 25 to the adaptive optical element 24 enable the individual pixels of the element to change their phase delay according to the spatial pattern generated by the control unit 26 . the spatial geometry of the element is determined by the settings of the control knobs 28 , which can be adjusted by the subject &# 39 ; s hand 30 , to provide the best visual acuity of the test card . when the optimum settings have been attained , these settings are output 32 from the control unit 26 , preferably in a form that enables the correction lens to be manufactured according to those settings , or that enables the laser refractive surgical treatment to be executed according to those settings . in order to correct defocus , the adaptive optical element need only be capable of changing its spherical power . in order to correct astigmatism , the numerical test card shown is replaced with a test card which incorporates orthogonally aligned visual acuity and resolution patterns , so that it is easy for the subject to determine when the level of astigmatism present has been optimally compensated using the controls 28 on the control unit 26 . the adaptive optical element is adjusted by means of the control unit to add a graduated level of cylinder to the lens , with the axis of the cylinder aligned according to the subject &# 39 ; s own perception of his view of the test card . [ 0060 ] fig3 schematically shows a further preferred embodiment of the present invention , in which the vision correction element is made up of a conventional lens 40 in combination with an adaptive optical element 42 . this embodiment is preferably used in order to simplify the operational requirements demanded of the adaptive optical element . such elements typically have a limited range of adjustment . in the embodiment shown in fig3 the conventional lens 40 , which may be interchangeable , may be preferably used to correct the lower order aberrations such as defocus and astigmatism , while the higher order aberrations , which may be only a small perturbation on the complete lens performance , are corrected interactively by means of the input control signals 44 to the adaptive optical element 42 . according to vet further preferred embodiments of the present invention , the adaptive optical element may also take part of the fixed refractive work of the correction lens , such that minor corrections of power can be undertaken interactively , as well as the higher order aberration corrections . according to yet a further preferred embodiment of the present invention , the conventional element 40 may be determined by an initial objective test of the subject &# 39 ; s vision , such as by using a refractometer , or by a prior art subjective measurement , such as the trial frame method described in the background section of this application , and this element is assumed to represent a first order vision - correction step . the adaptive optical element 42 is then used in order to fine - tune the vision correction according to the subject &# 39 ; s subjective perception of his best vision . this fine - tuning can be limited to additional fine correction of the low order aberrations approximately corrected by the initially chosen conventional element , or it can include also fine corrections for higher order aberrations , which are not even addressed in the initial objective test . reference is now made to fig4 which is a schematic diagram of a system for vision testing according to yet another preferred embodiment of the present invention , using a reflective adaptive optical element 50 , which reflects the light from the test card into the subject &# 39 ; s eye 54 . as previously , the subject interactively controls the element by means of control signals 56 , adjustment being made according to the subject &# 39 ; s perception of optimum visual acuity of the test card . this embodiment has an advantage over the transmissive embodiment shown in fig2 in that there is a wider range of adaptive elements available for reflective use than for transmissive use . the reflective element 50 may preferably be a deformable mirror , or a pixellated digital mirror device . the actuators of each spatially separate part of the adaptive optical element are controlled by the subject - operated control system . reference is now made to fig5 which is a view of a deformable mirror , such as could be used in the system of fig4 showing how deformations are programmed according to the symmetry and order of the correction polynomial required . the top surface 60 of the deformable mirror 62 shown in fig5 is the reflective surface . in fig5 the actuators have been programmed to provide a concave profile to the reflector at the center 64 , and a convex profile at the extremities 66 . the arrows are schematic indications of the direction of deformation . though only three arrows are shown , it is to be understood that in a real deformable mirror , the number of actuated pixels is many times higher than this . in the case of a dmd device , the number can run into tens of thousands . the simple symmetry shown in the mirror of fig5 if circularly symmetric , is suitable for correcting aberrations of spherical symmetry , such as the spherical aberrations . in order to correct astigmatism and coma , a deformation pattern with linear symmetry across the width of the mirror is needed . in general , when deformable mirrors such as that shown in fig5 or other slm &# 39 ; s are used , the deformations are computer generated according to the zernike polynomials , and the amount of the deformation is controlled by the subject . the adjustment commences with the low order corrections and progresses to the higher orders . the process is preferably repeated twice in order to iteratively achieve the best solution , as previously mentioned . control of the adjustments themselves may preferably be achieved by any of the commonly used computer control input devices , such as a joystick which may preferably be used for adjusting the intensity of the terms of the correction polynomial , with a pushbutton for switching orders . reference is now made to fig6 a to 6 c , which are schematic illustrations of yet further preferred embodiments of the present invention , wherein the optical element is adaptive since it can be adjusted by the user , but wherein this adjustment is performed mechanically . in fig6 a and 6b is shown an aspheric element 70 , which is shown as a refractive element , but could equally be reflective , with a variable radius of curvature as a function of spatial position on the element . by moving the element , either by a translation 72 , or by a non - axial rotation , different radii of curvatures are inserted into the optical path between the subject &# 39 ; s eye and the test plate , and the subject &# 39 ; s vision correction is thereby changed on a continuous basis . this variable radius of curvature element is thus suitable for correcting defocus defects in the subject &# 39 ; s vision . in fig6 a , the light is seen traversing the center of the lens 74 , having a strong corrective power , while in fig6 b , the element has been traversed mechanically such that the light passes through the lens at a distance from the axis 76 , which is of lower power . the position of the element can be calibrated in terms of its focal correcting power , such that as the user moves the element to the position of optimum visual acuity , the value of the correcting power can be determined from the lateral position of the element . though in fig6 a and 6b , the subject &# 39 ; s eye is shown in different positions in order to illustrate the different powers obtained , in practice , the subject &# 39 ; s eye would be at a fixed position , and the different powers used to provide different levels of vision correction . the slight additional prism introduced by the use of the lens non - paraxially can generally be neglected , especially if the lens is used to fine tune the aberration correction , using an element obtained by means of an objective measurement to approximately correct the aberration , as described in the embodiment of fig3 . in fig6 c is shown another preferred embodiment similar to that shown in fig6 a and 6b , wherein the single element 77 moved by the subject to change the corrective power resembles a section cut from the corridor of a progressive spectacle lens , shown virtually in dotted outline 78 . at one end of the element the power in the example shown is + 0 . 375 , and at the other it is − 0 . 375 . lateral motion of the element thus covers that range of corrective spherical power . according to yet another preferred embodiment of the present invention , the element may be such that it has a uniform radius of curvature , which is adjustable by the provision of hydraulic or pneumatic pressure within the element , such that control of the power is effected by the internal pressure applied . such an element 80 is shown in a refractive form in fig7 a and 7b , though it is clear to one skilled in the art that a reflective form may be likewise constructed . in fig7 a , a low internal pressure 82 is applied , providing a weak spherical power correction , whereas in fig7 b , a high internal pressure 84 is applied , providing strong spherical power correction . reference is now made to fig5 a to 8 c , which are illustrations of a pair of alvarez - humphrey plates , as described in u . s . pat . no . 3 , 507 , 565 for a “ variable power lens and system ”, hereby incorporated in its entirety by reference . this combination of superimposed cubic aspheric plates generates a variable amount of optical power when the places are laterally moved relatively to each other , ranging from a neutral plate as shown in the neutral position in fig8 a , to a positive lens , as shown in fig8 b , and to a negative lens , as shown in fig8 c , depending on the direction of the lateral mutual shift of the plates . such prior art plates can be used , according to a further preferred embodiment , to subjectively correct for defocus using the methods of the present invention . in an article entitled “ lateral - shift variable aberration generators ” published by i . a . palusinsky et al ., in applied optics vol . 38 no . 1 , pp . 86 - 90 ( january 1999 ), hereby incorporated in its entirety by reference , there is described a method of extending the alvarez - humphrey plate design to include elements capable of generating wavefront aberrations of higher order than only the defocus considered by alvarez and humphrey . in that article , there are given analytical formulae for calculating the plate surface profiles required to introduce variable amounts of the common aberrations up to fourth order , including tilt , defocus , astigmatism , coma , spherical aberration and line coma . reference is especially made to table 1 of the palusinski et al article , and its accompanying text . several surface profiles may be superimposed in one pair of plates , thus simultaneously generating a combination of aberrations , but with a fixed ratio between them . [ 0070 ] fig9 is a schematic illustration of how such plate pairs 90 may be utilized , according to more preferred embodiments of the present invention , as the variable adaptive optical element in the apparatus and method of the present invention . plate pairs with different surface profile are chosen , preferably according to the formulae given in the palusinsky et al article , to compensate for the various different aberrations required to be taken into account . in order to compensate for each aberration independently , a separate pair of plates is generally required for each aberration type . reference is now made to fig1 , which is a schematic illustration of a further preferred embodiment of the present invention , operative to correct low order aberrations . the variable refractive correction element is composed of a pair of cylindrical lenses 100 , 102 , mounted such that the distance d between them may be controllably varied , and their angular alignments , θ 1 and θ 2 may also be controllably varied . variation of the distance d between them results in a change in the power of the doublet , thereby correcting for defocus in the subject &# 39 ; s eye , while mutual rotation of one with respect to the other results in a change in the net cylinder and its axis , such that astigmatism can thus be corrected . in fig1 , the cylindrical nature of the lenses has been purposely exaggerated , in order to illustrate the way in which this preferred embodiment is constructed and operates . a common feature of all of the above mechanically adaptive optical elements is that the subject is able to control their optical corrective power monotonically and in such a way as to enable him to subjectively determine the position of optimum correction . the position can then be converted , preferably by means of an electromechanical interface device consisting of linear and angular encoders , to provide prescription information as to the corrective optic required for each of that particular subject &# 39 ; s eyes . it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove . rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art .
a system for determining with essentially continuous variability , wave aberrations originating in an eye of a subject , according to the visual perception of said subject .