Patent Publication Number: US-2022214566-A1

Title: Electrically-tunable vision aid for treatment of myopia

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/856,065, filed Jun. 2, 2019, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical devices, and particularly to electrically-tunable lenses and other vision aids. 
     BACKGROUND 
     Tunable lenses are optical elements whose optical characteristics, such as the focal length and/or the location of the optical axis, can be adjusted during use, typically under electronic control. Such lenses may be used in a wide variety of applications, including particularly serving as vision aids. For example, U.S. Pat. No. 7,475,985 describes the use of an electro-active lens for the purpose of vision correction. The term “vision aid,” as used in the context of the present description and in the claims, refers to transparent optical elements that are positioned in front of the eye of a subject and have optical properties, which may be fixed and/or tunable, that are chosen so as to enhance the subject&#39;s vision. 
     Electrically-tunable lenses typically contain a thin layer of a suitable electro-optical material, i.e., a material whose local effective index of refraction changes as a function of the voltage applied across the material. An electrode or array of electrodes is used to apply the desired voltages in order to locally adjust the refractive index to the desired value. Liquid crystals are the electro-optical material that is most commonly used for this purpose (wherein the applied voltage rotates the molecules, which changes the axis of birefringence and thus changes the effective refractive index), but other materials, such as polymer gels, with similar electro-optical properties can alternatively be used for this purpose. 
     Some tunable lens designs use an electrode array to define a grid of pixels in the liquid crystal, similar to the sort of pixel grid used in liquid-crystal displays. The refractive indexes of the individual pixels may be electrically controlled to give a desired phase modulation profile. (The term “phase modulation profile” is used in the present description and in the claims to mean the distribution of the local phase shifts that are applied to light passing through the layer as the result of the locally-variable effective refractive index over the area of the electro-optical layer of the tunable lens.) Lenses using grid arrays of this sort are described, for example, in the above-mentioned U.S. Pat. No. 7,475,985. 
     PCT International Publication WO 2014/049577, whose disclosure is incorporated herein by reference, describes an optical device comprising an electro-optical layer, having an effective local index of refraction at any given location within an active area of the electro-optical layer that is determined by a voltage waveform applied across the electro-optical layer at the location. An array of excitation electrodes, including parallel conductive stripes extending over the active area, is disposed over one or both sides of the electro-optical layer. Control circuitry applies respective control voltage waveforms to the excitation electrodes and is configured to concurrently modify the respective control voltage waveforms applied to excitation electrodes so as to generate a specified phase modulation profile in the electro-optical layer. 
     Myopia (near-sightedness) is a condition in which the human eye over-focuses light, creating an image in front of the retina instead of on the retina. Consequently, the image perceived on the retina is blurred. U.S. Patent Application Publication 2012/0133891 describes an electro-optical apparatus and method for correcting myopia that includes at least one adaptive lens, a power source, and an eye tracker. The eye tracker includes an image sensor and a processor operatively connected to the adaptive lens and the image sensor. The processor is configured to receive electrical signals from the image sensor and to control the correction power of the adaptive lens to correct myopia, with the correction power dependent on a user&#39;s gaze distance and myopia prescription strength. 
     Several treatments have been studied for slowing down the progression of myopia (i.e., retarding the increase of the optical power required to enable sharp view of far distances). These methods are generally referred to as “myopia control.” For example, U.S. Pat. No. 7,503,655 describes a method and apparatus for controlling optical aberrations to alter relative curvature of field by providing a predetermined corrective factor to produce at least one substantially corrective stimulus for repositioning peripheral, off-axis, focal points relative to the central, on-axis or axial focal point while maintaining the positioning of the central, on-axis or axial focal point on the retina. The invention is said to provide continuous, useful clear visual images while simultaneously retarding or abating the progression of myopia or hypermetropia. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved methods and devices for vision correction, and particularly for treatment of myopia. 
     There is therefore provided, in accordance with an embodiment of the invention, apparatus for vision correction, including an electrically-tunable lens, which is configured to be mounted in proximity to an eye of a subject having a refractive error. Control circuitry is configured to apply drive signals to the electrically-tunable lens so as to generate in the electrically-tunable lens a first phase modulation profile in a central zone that intercepts a line of sight of the eye and a second phase modulation profile in a peripheral zone extending peripherally around the central zone over at least 180° of arc. The first phase modulation profile applies to light that is incident on the electrically-tunable lens a first refractive power of a first magnitude that is selected to correct the refractive error, and the second phase modulation profile applies to the light that is incident on the electrically-tunable lens a second refractive power of a second magnitude that is less than the first phase modulation profile. 
     In a disclosed embodiment, the refractive error is due to a myopia of the eye, and the first refractive power is a negative refractive power, with the first magnitude selected so as to correct the myopia. 
     In some embodiments, the apparatus includes an eye tracker, which is configured to detect an angle of the line of sight of the eye, wherein the control circuitry is configured to modify the drive signals so as to shift the central and peripheral zones responsively to changes in the angle of the line of sight. Additionally or alternatively, the control circuitry is configured to assess a distance from the electrically-tunable lens to an object viewed by the eye and to adjust the first refractive power responsively to the distance. 
     In some embodiments, the peripheral zone extends peripherally around the central zone over at least 270° of arc. In one such embodiment, the peripheral zone includes an annulus extending around the central zone over 360° of arc. 
     In a disclosed embodiment, the electrically-tunable lens is a compound lens including a fixed lens component having a predefined refractive power and a tunable component having a variable refractive power, which is controlled by the drive signals. 
     In one embodiment, the control circuitry is configured to apply the drive signals so that the second phase modulation profile varies continuously across the peripheral zone in a radial direction relative to the line of sight. 
     In other embodiments, the control circuitry is configured to apply the drive signals so that the second phase modulation profile includes a pattern of peaks and troughs that alternate across the peripheral zone in a transverse direction relative to the line of sight. In one such embodiment, the pattern is selected so that the second phase modulation profile emulates a Fresnel lens of the second refractive power. Alternatively, the pattern is selected so that the second phase modulation profile emulates an array of microlenses having the second refractive power. 
     In a disclosed embodiment, the electrically-tunable lens includes an electro-optical layer, having an effective local index of refraction at any given location within an active area of the electro-optical layer that is determined by a voltage waveform applied across the electro-optical layer at the location, and conductive electrodes extending over opposing first and second sides of the electro-optical layer, wherein the control circuitry is configured to apply the drive signals to the conductive electrodes. 
     There is also provided, in accordance with an embodiment of the invention, apparatus for vision correction, including an electrically-tunable optical phase modulator, which is configured to be mounted in proximity to an eye of a subject. Control circuitry is configured to apply drive signals to the optical phase modulator so as to generate in the optical phase modulator a first phase modulation profile in a central zone that intercepts a line of sight of the eye and a second phase modulation profile, different from the first phase modulation profile, in a peripheral zone extending peripherally around the central zone over at least 180° of arc. The first phase modulation profile is selected so as to enable clear vision by the eye in the central zone, while the second phase modulation profile is selected so as to blur light that is incident on the eye through the peripheral zone. 
     In some embodiments, the first phase modulation profile has a parabolic cross-section, while the second phase modulation profile has a non-parabolic cross-section. In a disclosed embodiment, the first phase modulation profile is selected so as to apply to light that is incident on the central zone of the optical phase modulator a refractive power selected so as to correct a refractive error of the eye. 
     Additionally or alternatively, the second phase modulation profile includes a pattern of peaks and troughs that alternate across the peripheral zone in a transverse direction relative to the line of sight. 
     In some embodiments, the apparatus includes an eye tracker, which is configured to detect an angle of the line of sight of the eye, wherein the control circuitry is configured to modify the drive signals so as to shift the central and peripheral zones responsively to changes in the angle of the line of sight. 
     There is additionally provided, in accordance with an embodiment of the invention, a method for vision correction, which includes providing an electrically-tunable lens for mounting in proximity to an eye of a subject having a refractive error. The electrically-tunable lens is driven so as to generate in the electrically-tunable lens a first phase modulation profile in a central zone that intercepts a line of sight of the eye and a second phase modulation profile in a peripheral zone extending peripherally around the central zone over at least 180° of arc. The first phase modulation profile applies to light that is incident on the electrically-tunable lens a first refractive power of a first magnitude that is selected to correct the refractive error, and the second phase modulation profile applies to the light that is incident on the electrically-tunable lens a second refractive power of a second magnitude that is less than the first phase modulation profile. 
     There is further provided, in accordance with an embodiment of the invention, a method for vision correction, which includes providing an electrically-tunable optical phase modulator for mounting in proximity to an eye of a subject. The optical phase modulator is driven so as to generate in the optical phase modulator a first phase modulation profile in a central zone that intercepts a line of sight of the eye and a second phase modulation profile, different from the first phase modulation profile, in a peripheral zone extending peripherally around the central zone over at least 180° of arc, such that the first phase modulation profile enables clear vision by the eye in the central zone, while the second phase modulation profile blurs light that is incident on the eye through the peripheral zone. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of adaptive spectacles, in accordance with an embodiment of the invention; 
         FIG. 2  is a schematic side view of an electrically-tunable lens system, in accordance with an embodiment of the invention; 
         FIG. 3A  is a schematic plot of the refractive power of an electrically-tunable lens as a function of location across the lens, in accordance with an embodiment of the invention; 
         FIG. 3B  is a schematic plot of a phase modulation profile applied by the electrically-tunable lens of  FIG. 3A  as a function of location across the lens, in accordance with an embodiment of the invention; 
         FIG. 3C  is a schematic plot of a phase modulation profile applied by an electrically-tunable lens as a function of location across the lens, in accordance with another embodiment of the invention; 
         FIG. 4A  is a schematic plot of the refractive power of the electrically-tunable lens of  FIG. 3A  as a function of location across the lens, showing a lateral shift in the refractive power in response to movement of the eye of a user of the lens, in accordance with an embodiment of the invention; 
         FIG. 4B  is a schematic plot of a phase modulation profile applied by the electrically-tunable lens of  FIG. 4A  as a function of location across the lens, in accordance with an embodiment of the invention; 
         FIG. 5A  is a schematic plot of a phase modulation profile applied by an electrically-tunable lens as a function of location across the lens, in accordance with another embodiment of the invention; and 
         FIG. 5B  is a schematic plot of the phase modulation profile applied by the electrically-tunable lens of  FIG. 5A  as a function of location across the lens, showing a shift in the phase modulation profile in response to movement of the eye of a user of the lens, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Research has shown that applying myopic defocus for peripheral vision can slow the progression of myopia. It is hypothesized that hyperopic defocus in the peripheral vision (meaning that the image is focused behind the retina) triggers the growth of the eye, resulting in progression of myopia. This phenomenon can be mitigated by applying myopic defocus (image formed in front of the retina) in the peripheral part of the field of vision. For example, in myopia control spectacles, the center of the lens may have the user&#39;s nominal myopia prescription, while the periphery of the lens is designed to provide a myopic defocus. 
     One of the challenges of applying this sort of myopic defocus, however, is that the eye rotates, and therefore different areas of the spectacle lenses are used for peripheral vision at different points in time. When the eye rotates, the line of sight crosses the area of the defocus, and therefore the center vision is perturbed, while some of the peripheral vision is not. To avoid this phenomenon, the user is forced to turn his or her head in order to reduce eye rotation. 
     Embodiments of the present invention provide an electrically-controlled spectacle lens for myopia control, in which peripheral vision defocus is applied without degrading the sharpness of the center vision. These embodiments use an electrically-tunable optical phase modulator, which can be mounted in proximity to the subject&#39;s eye, for example as spectacle lenses. Control circuitry, applies drive signals to the optical phase modulator so as to generate a first phase modulation profile in a central zone that intercepts the line of sight of the eye, and a different, second phase modulation profile in a peripheral zone extending peripherally around the central zone. The first phase modulation profile is selected so as to enable clear vision by the eye in the central zone, while the second phase modulation profile is selected so as to blur light that is incident on the eye through the peripheral zone (i.e., to blur the image that is incident on the peripheral area of the retina of the eye). 
     In some of the embodiments that are described below, the optical phase modulator is configured as an electrically-tunable lens. This lens may be a compound lens, including a fixed lens, made from glass or plastic, for example, to provide a baseline refractive power, which is modified dynamically by one or more electrically-tunable lens elements. The control circuitry applies drive signals to the electrically-tunable lens so as to generate a phase modulation profile in the central zone of the lens, which intercepts the line of sight of the eye, with a refractive power selected to correct the subject&#39;s refractive error. At the same time, in a peripheral zone extending peripherally around the central zone, the drive signals in these embodiments cause the electrically-tunable lens to generate a phase modulation profile with a refractive power having a lower magnitude than that in the central zone. 
     The term “magnitude,” in the context of the present description and in the claims, refers to absolute magnitude, and the sign of the refractive power may be either negative or positive. For myopia control, the refractive power in the central zone is negative, while that in the peripheral zone is less negative or may even be slightly positive. The peripheral zone typically extends peripherally around the central zone over at least 180° of arc, but it may extend around at least 270° of arc, or even over an entire annulus of 360°. 
     Alternatively, other phase modulation profiles, magnitudes and signs of refractive power may be chosen in order to treat ophthalmic disorders of other types, and the optical phase modulator is not necessarily configured as a lens for correcting the refractive error of the eye. In some of these embodiments, the phase modulation profile in the central zone of the optical phase modulator has a parabolic cross-section, which may be a flat cross-section, i.e., a parabola with its focus at infinity. (Profiles of this sort are shown in  FIGS. 3B, 3C, 4B and 5B , for example.) The phase modulation profile in the peripheral zone may be non-parabolic. 
     The use of an electrically-tunable optical phase modulator is advantageous in its ability to generate a wide range of different phase modulation profiles, with different patterns of spatial variation across the peripheral zone relative to the line of sight. The phase modulation profile may vary continuously over the peripheral zone, or it may comprise a pattern of alternating peaks and troughs, which may be chosen so as to emulate a Fresnel lens or an array of microlenses, for example. Furthermore, when an eye tracker is used to detect the angle of the line of sight of the eye, the control circuitry can modify the drive signals so as to shift the central and peripheral zones in response to changes in the angle of the line of sight. Thus, when the eye moves, the central zone will remain roughly centered on the line of sight, and the blurring applied by the peripheral zone will affect only the peripheral vision. 
     Embodiments of the present invention thus enable ophthalmic treatments to be tailored to the particular needs of each patient, for enhanced patient comfort and therapeutic effect. Since the phase modulation profiles can be changed simply by reprogramming the control circuitry, the characteristics of the electrically-tunable lens or other optical phase modulator can be updated as treatment progresses. Researchers can also use these reprogramming capabilities to evaluate the therapeutic impact of different phase modulation profiles. 
     System Description 
       FIG. 1  is a schematic, pictorial illustration of adaptive spectacles  20 , in accordance with an embodiment of the invention. Spectacles  20  comprise electrically-tunable lenses  22  and  24 , mounted in a frame  25 . The optical properties of the lenses, including the refractive power (including both central and peripheral zones of the lenses), and optical center (or equivalently, the optical axis) are controlled by control circuitry  26 , which is powered by a battery  28  or other power source. Control circuitry  26  typically comprises an embedded microprocessor with hard-wired and/or programmable logic components and suitable interfaces for carrying out the functions that are described herein. These and other elements of spectacles  20  are typically mounted on or in frame  25 , or may alternatively be contained in a separate unit (not shown) connected by wire to frame  25 . 
     Spectacles  20  comprise one or more sensors, which sense the direction of the line of sight of the eyes of the person wearing the spectacles, and possible also the distance from the eyes to an object  34  viewed by the person. Control circuitry  26  tunes lenses  22  and  24  according to the sensor readings. In the pictured example, the sensors include a pair of eye trackers  30 , which detect respective lines of sight  32  (gaze directions) of the right and left eyes. Control circuitry  26  typically shifts the respective optical axes of lenses responsively to the sensed gaze directions. Furthermore, the control circuitry can use the distance between the pupils, as measured by eye trackers  30 , to estimate the user&#39;s focal distance (even without analyzing the actual gaze direction), and possibly to identify the distance from the eye to object  34 . 
     On this basis, control circuitry  26  can adjust the refractive power of lenses  22  and  24  so as to assist the user&#39;s eyes in distance accommodation, and thus reduce or eliminate the user&#39;s need to accommodate, for example, as described in PCT International Publication WO 2019/012385, whose disclosure is incorporated herein by reference. Control circuitry  26  assesses the distance from the lenses to object  34  and adjusts the refractive power in at least a central zone  37  of the lenses based on the distance. Specifically, for a myopic eye with a negative refractive correction of a certain magnitude for distant vision, control circuitry  26  can reduce the magnitude of the correction when the user is viewing a nearby object. 
     Additionally or alternatively, a camera  36  captures an image of object  34 , for use by control circuitry  26  in identifying the object and setting the focal distance. Either eye trackers  30  or camera  36  may be used in determining the focal distance, but both of these sensors can be used together to give a more reliable identification of the object. Alternatively or additionally, camera  36  may be replaced or supplemented by a rangefinder or other proximity sensor, which measures the distance to object  34 . 
     Control circuitry  26  applies drive signals to lenses  22  and  24  according to phase modulation profile parameters that are stored in a memory  38 . In the case of myopia control, for example, these parameters indicate the characteristics that are to be applied in both central zone  37  and a peripheral zone  39  of the lenses. (In the pictured example, zone  39  is annular, extending 360° around zone  37 , but alternatively zone  39  may extend around a smaller angle of arc, as explained above.) As noted earlier, the phase modulation profile of central zone  37  in each lens  22 ,  24  is typically selected so that the refractive power of the central zone has a magnitude that corrects the refractive error of the respective eye. The parameters for peripheral zones  39  are selected so that the magnitudes of the refractive power in these zones are less than in the corresponding central zones. The reduced magnitudes are chosen so that the peripheral zones have a therapeutic effect on the eyes, for example in retarding the progression of myopia. As noted above, control circuitry  26  shifts the locations of zones  37  and  39  in response to changes in the angles of lines of sight  32 . 
       FIG. 2  is a schematic side view of electrically-tunable lens  22 , in accordance with an embodiment of the invention. Lens  24  is typically of similar design. 
     In the pictured embodiment, lens  22  is a compound lens, which comprises multiple elements: Fixed lenses  40  and  41 , typically made from glass or plastic, provide a baseline refractive power, which is modified dynamically by two electrically-tunable phase modulators  42  and  44 . Such phase modulators can be used to implement various phase modulation profiles, such as a spherical lens, a cropped lens, lens arrays, an aspherical lens, or combinations of these profiles in different areas on the panel. Furthermore, the phase modulators can switch dynamically between different phase profile implementations. 
     Although fixed lenses  40  and  41  are shown as being physically separate from tunable phase modulators  42  and  44 , in practice these components are typically encapsulated in a single package, in the form of a spectacle lens. (For this reason, lens  22  itself can be considered an electrically-tunable lens.) Thus, the total refractive power of lens  22 , over any zone within the area of the lens  22 , will typically be a sum of the fixed refractive powers of lenses  40  and  41  with the variable refractive power (or other phase modulation profile) applied by phase modulators  42  and  44 . Alternatively, lens  22  may comprise only electrically-tunable elements, and fixed lenses  40  and  41  may not be needed, particularly when the magnitude of the refractive correction is small. In some embodiments, lens  22  also comprises a polarizing element  46 , such as a polarizer and/or polarization rotator, with functionality as described hereinbelow. 
     Electrically-tunable phase modulators  42  and  44  adjust the phase modulation profile of lens  22  depending on the angle of the user&#39;s line of sight and possibly the distance to the object being viewed by the user, with central and peripheral zones  37  and  39  of lens  22  defined as described above. On this basis, an optical axis  48  of phase modulators  42  and  44  is shifted in response to changes in gaze direction  32 . Phase modulators  42  and  44  may comprise one-dimensional phase modulators (phase modulators for which the phase modulation profile is a function of the position in one axis), positioned such that they operate on orthogonal axes, for example electrically-tunable cylindrical lenses, with orthogonal cylinder axes. Alternatively, phase modulators  42  and  44  may be configured to generate two-dimensional phase modulation profiles and thus emulate spherical or aspheric lenses (or their Fresnel equivalents). Both of these sorts of lens configurations, as well as waveforms for driving the lenses, are described in detail in the above-mentioned WO 2014/049577. 
     As shown in FIGS. 2A-C and 3A-D of WO 2014/049577 and explained with reference thereto, modulators  42  and  44  in the present embodiments comprise an electro-optical layer, such as a layer of liquid crystal, and conductive electrodes extending over opposing first and second sides of the electro-optical layer. The electro-optical layer has an effective local index of refraction at any given location within its active area that is determined by a voltage waveform applied across the electro-optical layer between the electrodes at that location. Control circuitry  26  applies drive signals with the appropriate voltage waveforms to the electrodes, in accordance with the refractive and therapeutic phase modulation profile parameters in memory  38 . 
     In some embodiments in which phase modulators  42  and  44  comprise respective polarization-dependent electro-optical layers, the electro-optical layers are oriented so as to refract mutually-orthogonal polarizations: One of these phase modulators, for example, modulator  42 , operates on light polarized in the X-direction (pointing into the page in the view shown in  FIG. 2 ), and does not influence light polarized in the Y-direction (pointing upward in this view). Phase modulator  44  operates on light polarized in the Y-direction and does not influence light polarized in the X-direction. Unpolarized light passing through phase modulators  42  and  44  will thus be focused at both distances, with roughly half the light focused by phase modulator  42 , while the other half is focused by phase modulator  44 . Alternatively, polarizing element  46  may comprise a polarization rotator, which intercepts the incoming light and rotates its polarization so as to ensure that the light incident on the electro-optical layers of phase modulators  42  and  44  has a component at each of the respective polarizations. 
     In an alternative embodiment (not shown in the figures), an electrically-tunable lens comprises four optical phase modulators, combining the features of the embodiments described above: two one-dimensional phase modulators modulating light as a function of X position (for example emulating cylindrical lenses with cylinder axes parallel to the X-axis), operating on X-polarized and Y-polarized light, respectively; and two one-dimensional phase modulators modulating light as a function of Y position (for example emulating cylindrical lenses with cylinder axes parallel to the Y-axis), operating on X-polarized and Y-polarized light, respectively. This electronically-tunable lens is thus capable of applying two-dimensional refractive profiles to light of all polarizations. Alternatively, other combinations of electrode orientations and electro-optical layer polarizations may be used, depending on application requirements. All such combinations are considered to be within the scope of the present invention. 
     Tunable Lens Profiles for Myopia Control 
     The figures that follow show optical profiles of refractive power and phase shift of electrically-tunable phase modulators  42  and  44  as a function of location along a line running across the lens, for example along the horizontal (X) axis, crossing central and peripheral zones  37  and  39 . These profiles may be applied by each of phase modulators  42  and  44  individually (assuming the lenses are capable of generating two-dimensional profiles) or by the two phase modulators  42  and  44  in combination, for example along orthogonal cylinder axes. Alternatively, similar sorts of profiles may be generated in electrically-tunable lenses of other sorts. 
     Central zone  37  is assumed in these examples to have a diameter of 4 mm, although larger or smaller diameters may be used. For example, control circuitry  26  may increase or decrease the size of central zone  37  as a function of pupil size, as measured by eye trackers  30 . 
       FIG. 3A  is a schematic plot  50  of the refractive power (in diopters) of electrically-tunable phase modulators  42  and  44  as a function of location across the lenses, in accordance with an embodiment of the invention. In this example, it is assumed that fixed lenses  40  and  41  provide a baseline refractive power equal to the refractive correction required by the user, for instance −2D (two diopters, with negative sign). Therefore, control circuitry  26  drives phase modulators  42  and  44  to apply no additional refractive power in central zone  37 . In peripheral zone  39 , phase modulators  42  and  44  apply an additional refractive power of +1D, so that the net refractive power of lens  22  is −1D. The reduced magnitude of the refractive power in the peripheral zone can be useful in myopia control. 
       FIG. 3B  is a schematic plot  52  of the phase modulation profile applied by phase modulators  42  and  44  (in arbitrary units of phase, AU) as a function of location across the lenses, in accordance with an embodiment of the invention. In this case, the phase modulation profile represented by plot  52  varies continuously across the peripheral zone in the radial direction relative to the line of sight, which is located at the origin X=0. 
       FIG. 3C  is a schematic plot  54  of the phase modulation profile applied by phase modulators  42  and  44  as a function of location across the lenses, in accordance with another embodiment of the invention. The phase modulation profile in this case comprises a pattern of peaks and troughs that alternate across peripheral zone  39  in a transverse direction relative to the line of site (corresponding to the radial direction when the profile is circularly symmetric). The pattern is selected so that the phase modulation profile emulates a Fresnel lens. The Fresnel lens achieves the same refractive power (+1D) as does the smooth profile of plot  52 , but with a lower maximum phase shift. The use of the Fresnel profile, as opposed to a smooth profile, thus makes it possible to use thinner electro-optical layers in lenses  42  and  44 , as well as lower voltages in driving the lenses. 
       FIG. 4A  is a schematic plot  56  of the refractive power of phase modulators  42  and  44  as a function of location across the lenses, showing a shift in the refractive power in response to movement of the eye of a user of lens  22 , in accordance with an embodiment of the invention. In this example, eye tracker  30  has detected that the point of intersection of line of sight  32  with phase modulators  42  and  44  has shifted by 1 mm. Control circuitry  26  accordingly modifies the drive signals applied to the lenses so that central zone  37  shifts to be centered on the new line of sight. 
       FIG. 4B  is a schematic plot  58  of the phase modulation profile applied by phase modulators  42  and  44  as a function of location across the lenses, in accordance with an embodiment of the invention. In this example, phase modulators  42  and  44  apply a Fresnel phase modulation profile (as in  FIG. 3C ), with the center of the profile shifted to provide the desired shift in the location of central zone  37 . 
       FIGS. 5A and 5B  are schematic plots  60  and  62  of phase modulation profiles applied by phase modulators  42  and  44  as a function of location across the lenses, in accordance with another embodiment of the invention. Here, too, the phase shift is plotted in arbitrary units. The phase modulation profiles of plots  60  and  62  comprise patterns of alternating peaks and troughs that emulate an array of microlenses in peripheral zone  39 . Plot  62  shows the shift in the phase modulation profile in response to movement of the eye of the user. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.