Patent Description:
Optical imaging systems are used in a variety of different fields, such as ophthalmic imaging. Ophthalmic imaging relates to the imaging of parts of the eye, such as the retina, which can allow early signs of disease or pathology to be detected.

It is often desirable to control an optical property of the imaging system during image acquisition. For example, it is preferable to provide an imaging system with the capability to vary its focal length while the surface is being imaged, so that an in-focus image of a curved surface can be obtained, or so that an in-focus image of a flat or curved surface can be obtained with an imaging system using an optical component with a curved surface. As another example, it may be desirable to vary the scan angle of a raster scan pattern using an oscillating mirror such that some play in the mirror's movement mechanism can be compensated for. As yet another example, it may be desirable to vary the intensity of light used to back-light a sample of spatially varying thickness in transmission microscopy in order to obtain an image with uniform exposure, in which features across the sample can be easily viewed and compared.

Further background is provided in the following documents.

<CIT> discloses a system and method of driving an electrowetting display device including a plurality of sub-pixels. A target reflectance value for a sub-pixel in the plurality of sub-pixels is determined. A reflectance value of the sub-pixel is set to the target reflectance value by setting the reflectance value of the sub-pixel to a first reflectance value greater than a threshold value, and setting the reflectance value of the sub-pixel to the target reflectance value.

<CIT> discloses a method and apparatus for recalibrating a liquid lens. In one example, a lens holder is provided to adjust the focal length of the lens as a function of temperature. In another example, a recalibration circuit including a second lens of similar characteristics to the imaging lens is used to determine an appropriate focus. In other examples, an open loop calibration process is used.

<CIT> discloses a scanning device having a drive system to angularly reciprocate a rotatable mirror about an axis in a desired waveform in accordance with an excitation signal supplied by a programmable waveform generator. A position sensor generated as signal representing the actual waveform of reciprocation of the mirror, and a microprocessor determines a signal representing the error between the actual waveform and the desired waveform. The microprocessor includes a waveform optimizer that receives this error signal and delivers an excitation correction signal to the waveform generator to adjust the excitation signal from the waveform generator to cause the mirror to have an actual waveform substantially in accordance with the desired waveform.

<CIT> discloses a scanner/reader for reading target objects, such as a barcode, that incorporates a variable focus liquid lens whose focal distance is constantly and continuously varied through the lens' predefined focal range provides an efficient way to scan or read a barcode. A driver circuit for the liquid lens continuously generates a lens driver signal that provides the liquid lens with a continuously varying voltage input to the liquid lens and thus varying the focal distance of the liquid lens cyclically through a predefined focal range.

<CIT> discloses an imaging assembly in an imaging reader for electro-optically reading indicia, which includes a solid-state imager having an array of image sensors, and a focus control element, such as a liquid imaging lens, having a variable transfer function characteristics for capturing, optically modifying and directing return light from the indicia over a field of view onto the imager for processing into an electrical signal indicative of the indicia during a reading mode of operation; and a controller for controlling the imaging assembly to optically modify the return light in accordance with a known transfer function stored in a memory accessible to the controller during a calibration mode of operation prior to the reading mode.

Optical systems having a variable optical property often exhibit a hysteresis in the variation of the optical property with a control signal applied to an optical element of the optical system that is used to control the optical property. This hysteretic behaviour can degrade image quality of the imaged surface.

The present invention provides an optical system according to Claim <NUM>, and a method of controlling an optical property of an optical element according to Claim <NUM>.

To assist understanding of the invention, there is described in the following an optical system according to a background example not forming an embodiment, which comprises an optical element responsive to an applied signal to vary an optical property of the optical element, the variation of the optical property with the applied signal exhibiting hysteresis in a first range of values of the signal, and no hysteresis in at least one second range of values of the signal. The optical system further comprises a memory storing data representative of a variation of the optical property with either increasing values of the signal or decreasing values of the signal, and a controller configured to control the optical property of the optical element by generating a control signal based on the stored data and applying the generated control signal to the optical element. In a case where the memory stores data representative of the variation of the optical property with increasing values of the signal, the controller is configured to use the stored data to change the value of the optical property from an initial value corresponding to a first value of the signal, to a subsequent value corresponding to a second value of the signal that is smaller than the first value of the signal, by setting the value of the control signal to a third value that is within the second range of values, and subsequently increasing the value of the control signal from the third value to the second value. Alternatively, in a case where the memory stores data representative of the variation of the optical property with decreasing values of the signal, the controller is configured to use the stored data to change the value of the optical property from an initial value corresponding to a first value of the signal, to a subsequent value corresponding to a second value of the signal that is greater than the first value of the signal, by setting the value of the control signal to a third value that is within the second range of values, and subsequently decreasing the value of the control signal from the third value to the second value.

There is also described in the following, in accordance with a background example not forming an embodiment, a method of controlling an optical property of an optical element, wherein the optical element is responsive to an applied signal to vary the optical property, and the variation of the optical property with the applied signal exhibits hysteresis in a first range of values of the electrical signal, and no hysteresis in a second range of values of the signal. The method comprises: storing data representative of the variation of the optical property with either increasing values of the signal or decreasing values of the signal; and generating a control signal based on the stored data, and applying the generated control signal to the optical element. In a case where the memory stores data representative of the variation of the optical property with increasing values of the signal, the value of the optical property is controlled to change from an initial value, corresponding to a first value of the signal, to a subsequent value, corresponding to a second value of the signal that is smaller than the first value of the signal, by setting the value of the control signal to a third value that is within the second range of values, and subsequently increasing the value of the control signal from the third value to the second value. Alternatively, in a case where the memory stores data representative of the variation of the optical property with decreasing values of the signal, the value of the optical property is controlled to change from an initial value, corresponding to a first value of the signal, to a subsequent value, corresponding to a second value of the signal that is greater than the first value of the signal, by setting the value of the control signal to a third value that is within the second range of values, and subsequently decreasing the value of the control signal from the third value to the second value.

In the system and method of the above background examples, in the case where the memory stores data representative of the variation of the optical property with increasing values of the signal, where the controller uses the stored data to change the value of the optical property from an initial value corresponding to a first value of the signal, to a subsequent value corresponding to a second value of the signal that is larger than the first value of the signal, the control signal is not set to a third value that is within the second range of values, and subsequently increased from the third value to the second value. Instead, the value of the signal is increased from the first value to the second value, since no inaccuracies caused by hysteresis would arise in this case.

Similarly, in the system and method of the above background examples, in the case where the memory stores data representative of the variation of the optical property with decreasing values of the signal, where the controller uses the stored data to change the value of the optical property from an initial value corresponding to a first value of the signal, to a subsequent value corresponding to a second value of the signal that is smaller than the first value of the signal, the control signal is not set to the third value that is within the second range of values and subsequently decreased from the third value to the second value. Instead, the controller is configured to change the value of the signal from the first value to the second, smaller value, since no inaccuracies caused by hysteresis would arise in this scenario.

There is also described in the following an optical system comprising an optical element responsive to an applied signal to vary an optical property of the optical element, the variation of the optical property with the applied signal exhibiting hysteresis in a first range of values of the signal, and no hysteresis in a second range of values of the signal. The optical system further comprises a memory storing data representative of the variation of the optical property with either increasing values of the signal or decreasing values of the signal, and a controller configured to control the optical property of the optical element by: generating, based on the stored data, a cyclic signal having one or more discontinuities in each cycle of the cyclic signal, and setting the size of at least one of the one or more discontinuities in each cycle based on the stored data or live system response such that a part of the variation of the optical property with the cyclic signal coincides with a part of the variation represented by the stored data; and applying the cyclic signal to the optical element.

There is also described in the following a method of controlling an optical property of an optical element, wherein the optical element is responsive to an applied signal to vary the optical property, and the variation of the optical property with the applied signal exhibits hysteresis in a first range of values of the electrical signal, and no hysteresis in a second range of values of the signal. The method comprises: storing data representative of the variation of the optical property with either increasing values of the signal or decreasing values of the signal; generating, based on the stored data, a cyclic signal having one or more discontinuities in each cycle of the cyclic signal, the size of at least one of the one or more discontinuities in each cycle being based on the stored data or live system response such that a part of the variation of the optical property with the cyclic signal coincides with a part of the variation represented by the stored data; and applying the cyclic signal to the optical element.

Embodiments of the invention will now be explained in detail, by way of example only, with reference to the accompanying figures, in which:.

An optical system <NUM> according to a background example which is helpful for understanding the present invention is shown schematically in <FIG>. The optical system <NUM> comprises an optical element <NUM>, and a controller <NUM> having a memory <NUM>. These components of the optical system <NUM> are described in more detail below. Other well-known components of the optical system <NUM> that are not necessary for understanding the present invention (such as a beam delivery pre-focussing optical system and a post-processing optics, control systems therefor, etc.) are not illustrated or described herein, for sake of clarity. The optical system <NUM> implements a dynamic focusing mechanism which is used to compensate for systematic and target-related focus changes to ensure that an optical imaging system remains in focus with respect to a target, and may be used for correcting optical power change of an ellipsoidal mirror of an ophthalmoscope, for example. The control techniques described herein may be applicable for an imaging system where light is delivered and collected through the same beam delivery mechanism, or where light is only collected or delivered to a target.

The optical element <NUM> may, as in the present background example, be provided in the exemplary form of a liquid lens. The optical element <NUM> is responsive to a control signal SC applied thereto by the controller <NUM> to vary, as an optical property of the optical element <NUM> that can be varied by the applied control signal Sc, the focal length of the optical element <NUM>. As will be described in more detail below, the variation of the optical property with an applied signal shows hysteresis in a first range of values of the signal, and no hysteresis in at least one second range of values of the signal. It should be noted, however, that hysteretic behaviour is not specific to liquid lenses, and may also be observed in a variety of other optical components.

For example, some types of thin-film membranes show hysteresis in transmitted light intensity as incident light intensity is increased and decreased (and vice versa) over a common range of values.

Furthermore, an optical scanning element, which comprises a member having reflective surface for reflecting light that is configured to rotate about an axis (such as oscillating mirror that can be used to vary a scan angle of an optical scanner, for example a polygonal mirror or galvo mirror used in a scanning laser ophthalmoscope, SLO) under the control of a drive mechanism may exhibit hysteresis that is dependent upon the movement direction of the reflective surface. For example, an oscillating mirror may yield a certain scan angle when moving to a particular position in one direction, but yield a different scan angle when moving to the nominally same position from the opposite direction. The "mismatch" in the scan angle that is dependent upon the movement direction of the scanning element may be caused by play in the mechanism that is used to rotate the scanning element.

An optical property of a liquid lens or other type of optical element, such as its focal length, can be varied in one of a number of different ways. For example, a signal in the form of pressure changes applied to the liquid lens may be used to deform a fluid-filled membrane of the liquid lens. The application of different pressures can produce different curvatures of the fluid inside the membrane and thus vary the focal length of the lens. However, in the present background example, the variation of the focal length of the lens is based on the electrowetting principle, as described in more detail below.

The controller <NUM> is configured to control the optical property of the optical element <NUM> by generating the control signal SC using data stored in the memory <NUM> that is described in more detail below, and applying the generated control signal Sc to the optical element <NUM>. The controller <NUM> may be implemented in a number of different ways.

<FIG> shows an exemplary implementation of the controller <NUM> in programmable signal processing hardware, which may take the exemplary form of a personal computer (PC) or the like. The signal processing apparatus <NUM> shown in <FIG> comprises an interface (I/F) section <NUM> for outputting the control signal Sc, and optically for receiving instructions from an external computer or the like (not shown) for setting different values of the focal length of the optical element <NUM>. The signal processing apparatus <NUM> further comprises a processor <NUM>, a working memory <NUM> and an instruction store <NUM> storing computer-readable instructions which, when executed by the processor <NUM>, cause the processor <NUM> to perform the processing operations hereinafter described to control the focal length of the optical element <NUM>. The instruction store <NUM> may comprise a ROM which is preloaded with the computer-readable instructions. Alternatively, the instruction store <NUM> may comprise a RAM or similar type of memory, and the computer-readable instructions can be input thereto from a computer program product, such as a computer-readable storage medium <NUM> such as a CD-ROM, etc. or a computer-readable signal <NUM> carrying the computer-readable instructions.

In the present background example, the combination <NUM> of the hardware components shown in <FIG>, comprising the processor <NUM>, the working memory <NUM> and the instruction store <NUM>, is configured to implement the functionality of the controller <NUM>. The instruction store <NUM> may serve as the memory <NUM> shown in <FIG>. It should be noted, however, that the memory <NUM> need not form part of the controller <NUM>, and may alternatively be provided as an external components that is communicatively coupled to the controller <NUM> in any appropriate way known to those skilled in the art, for example via a network such as a local area network (LAN) or the Internet.

It should be noted that the controller <NUM> need not be implemented in programmable signal processing hardware of the kind described above, and may alternatively be implemented in dedicated hardware such an appropriately configured field-programmable gate array (FPGA), for example.

The memory <NUM> may be any kind of data storage device well-known to those skilled in the art, and stores data (also referred it herein as a "calibration curve") representative of a variation of the optical property of the optical element <NUM> with either increasing values of an applied signal or decreasing values of the applied signal. More particularly, the memory <NUM> may, as in the present background example, store values indicative of a measured focal length of the optical element <NUM> for corresponding values of the electrical signal applied by the controller <NUM>, wherein the focal length is measured for each increment in the value of the applied signal. However, the memory <NUM> may alternatively store values indicative of a measured focal length of the optical element <NUM> for corresponding values of the applied signal, wherein the focal length is measured as a function of decreasing values of the signal. The data representative of the variation of the optical property with increasing or decreasing values of the applied signal need not, however, be provided in the form of correlated measured values of the optical property and of the applied signal, and may alternatively be represented by a function representing the variation, which may be derived from experimental results or by modelling of the behaviour of the optical element <NUM>, for example.

The operation of the liquid lens as an example of the optical element <NUM> in the present background example will now be described with reference to <FIG>, which figures illustrate the liquid lens in different states of operation.

As illustrated in <FIG>, the liquid lens may comprise two immiscible liquids, namely a first liquid <NUM> and a second liquid <NUM>, that are disposed between two electrodes, <NUM> and <NUM>, and are both substantially transparent. By way of example, the first liquid <NUM> may be water, and the second liquid <NUM> may be oil. The oil-water interface <NUM>, the oil-electrode interface <NUM> and the water-electrode interface <NUM> all have an associated Gibbs free energy. The Gibbs free energies associated with the oil-electrode interface <NUM> and the water-electrode interface <NUM> influence the angle of curvature of the interface <NUM> between the two immiscible liquids <NUM> and <NUM>. Other types of electrowetting liquid lenses operate on a similar principle but may vary in their structure. For example, other types of electrowetting liquid lenses may only have a single transparent or translucent liquid, and may vary the curvature of this single liquid.

The curvature of the interface <NUM> between the two immiscible liquids <NUM> and <NUM> can be varied by applying the control signal SC to the electrodes <NUM> and <NUM>, which signal changes the Gibbs free energy of the solid-liquid interfaces. The applied signal may, as in the present background example, correspond to an applied voltage, or may alternatively be a current signal.

As illustrated in <FIG>, an applied signal corresponding to a voltage V<NUM> produces a corresponding water contact angle at the water-electrode interface <NUM>. This water contact angle influences the curvature of the oil-water interface <NUM>. In <FIG>, the curvature of the oil-water interface <NUM> is convex, and therefore any light incident on the liquid lens will be made to diverge by the liquid lens, as represented by the arrows in this figure.

As shown in <FIG>, under a different applied voltage V<NUM>, the water contact angle at the water-electrode interface <NUM> is changed, thereby changing the curvature of the oil-water interface <NUM>. With an applied signal corresponding to a voltage V<NUM>, the oil-water interface <NUM> becomes flat (neither concave nor convex), such that light incident on the liquid lens is unaffected (neither diverged nor converged) by the liquid lens, as represented by the arrows in this figure.

If the signal applied by the controller <NUM> is changed further (for example, see <FIG>, in which the applied voltage is changed to V<NUM>), the water contact angle between the water <NUM> and the surface of the electrode <NUM> is changed still further, thereby further changing the curvature of the oil-water interface <NUM>. In <FIG>, the curvature of this oil-water interface <NUM> is concave, such that light incident on the liquid lens is converged by the liquid lens, as illustrated by the arrows in this figure.

Liquid lenses of this kind are suitable for use in ophthalmic applications, since their optical properties may be rapidly and accurately controlled using an applied voltage. This type of focussing mechanism can be driven as a function of another systemic parameter. However, these types of liquid lenses exhibit hysteresis effects, which can cause errors in the set focal lengths and the like of the liquid lens. As will be described in the following, the calibration and the drive mechanism employed in embodiments of the invention compensate for these hysteresis effects.

The hysteresis effects arise due to the behaviour of the water contact angle at the water-electrode interface <NUM> of the liquid lens. Specifically, this water contact angle will be different for a particular value of applied voltage from the controller <NUM> depending on whether the applied voltage across the liquid lens has been increased from a smaller voltage to that value, or decreased from a larger voltage value to that value.

As noted above, the water contact angle at the water-electrode interface might be different for a particular voltage value if the applied voltage was increased to reach that particular voltage value than if the applied voltage was decreased to reach that particular voltage value. An increase in voltage would cause the water contact angle at the water-electrode interface <NUM> to assume an advancing water contact angle, whereas a decrease in voltage would lead the water contact angle of the water-electrode interface <NUM> to assume a receding water contact angle. In general, advancing water contact angles are larger than receding water contact angles. Since the curvature of the oil-water interface <NUM> depends on the water contact angle at the water-electrode interface <NUM>, any differences in the water contact angle (such as the difference between an advancing water contact angle and a receding water contact angle) would produce a difference in the curvature of the oil-water interface <NUM>, and this difference in curvature would affect the converging/diverging action of the liquid lens, thereby changing its focal length.

<FIG> is a schematic illustration of how the focal length, F, of the liquid lens changes as a function of both increasing and decreasing values of a drive voltage signal S that is applied to the electrodes of the liquid lens, wherein the direction of change of the drive signal S (towards higher or lower values) is shown by the arrows in the two parts of the hysteresis curve. As can be seen in <FIG>, in a first range of values of the drive signal S (shown at <NUM> in <FIG>) that lies between SMin and SMax, the variation of the focal length F of the liquid lens with the applied drive signal S exhibits hysteresis, while no hysteresis is present in the ranges <NUM> and <NUM> of the applied drive signal S that are on either side of the range <NUM>, i.e. for S > SMax and S < SMin. In ranges <NUM> and <NUM>, where no hysteresis occurs, there is a one-to-one correspondence between the value of S and the resulting focal length F. On the other hand, in hysteresis range <NUM>, the values of focal length F depend on whether the drive signal S is increasing or decreasing.

The aforementioned data that is stored in memory <NUM> may, as in the present background example, represent the variation of the focal length F of the liquid lens from the minimum value, S<NUM>, of the drive signal S shown in <FIG> to the maximum value, S<NUM>', of the drive signal. Although the stored variation thus covers regions <NUM> to <NUM>, it need not cover both of regions <NUM> and <NUM>, and may alternatively cover regions <NUM> and <NUM> (or parts thereof) only, for example. Alternatively, the stored data may represent the variation of the focal length F of the liquid lens from the maximum value S<NUM>' of the drive signal S shown in <FIG> to the minimum value S<NUM> of the drive signal S shown in <FIG>, covering regions <NUM>-<NUM>, or regions <NUM> and <NUM> (or parts thereof) only, for example.

The controller <NUM> is configured to operate in a 'step-and-lock' mode to generate control signals Sc such that changes in the focal length F of the liquid lens that occur in response to changes in the control signal SC follow the stored variation not only for increases in the value of Sc but also for decreases in the value of Sc. In this mode of operation, the rate of change of focus drive is greater than or equal to the settling time of the focus mechanism. There is therefore a one-to-one correspondence between each applied value of the control signal Sc and the resulting value of the focal length F, even in the range of values <NUM> shown in <FIG>. As illustrated in this figure, the controller <NUM> is configured to use the data stored in the memory <NUM> to change the value of the focal length F from an initial value Fi, corresponding to a first value, S<NUM>, of the control signal Sc, to a subsequent value, Fn, corresponding to a second value, S<NUM>, of the control signal Sc that is smaller than the first value S<NUM>, by setting the value of the control signal SC to a third value, S<NUM>, that is within range <NUM>, and subsequently increasing the value of the control signal Sc from the third value S<NUM> to the second value S<NUM>. The third value S<NUM> may, as in the present background, be the lowest signal value in the stored variation. Since the changes in the settings of the focal length F of the liquid lens occur on timescales that are greater than the response time (latency) of the liquid lens, changes to F will follow the stored variation (i.e. a portion of the lower part of the hysteresis curve in <FIG>).

<FIG> shows an experimentally determined variation of the focal length of the liquid lens as the drive current (as an example of the control signal Sc) is changed from an initial value corresponding to a starting focus value, to a lower value at the 'reset point', and then increased to a value corresponding to the target focus value. As shown in <FIG>, the measured focal length follows the upper solid curve while the drive current is being decreased towards the rest point, and then follows the stored variation (i.e. the 'calibrated curve' that is stored in memory <NUM>) while the drive current is being increased from the reset point towards the value that corresponds to the target focus.

In an alternative background example in which the memory <NUM> stores data representative of the variation of the focal length F with decreasing values of the drive signal S, the controller <NUM> is configured to use the stored data to change the value of F from an initial value, Fi', corresponding to a first value of the signal, S<NUM>', to a subsequent value, Fn' corresponding to a second value, S<NUM>', of the control signal Sc that is greater than the first value of the control signal, by setting the value of the control signal Sc to a third value, S<NUM>', that is within the range of values <NUM>, and subsequently decreasing the value of the control signal Sc from the third value S<NUM>' to the second value S<NUM>' (as also illustrated in <FIG>). In this alternative, the third value S<NUM>' may be the highest signal value in the stored variation.

By controlling the focal length of the liquid lens in this way, the controller <NUM> can compensate for the hysteresis and ensure that both increments and decrements in the control signal SC cause the resulting focal length to substantially follow the variation stored in the memory <NUM>. Focus variations due to optical system and/or the geometry of the imaged target can thus be effectively corrected. Similarly, for background examples where the scan angle of (for example) a mirror is controlled in this way, the controller <NUM> can compensate for hysteresis effects associated with any mechanical accuracies of a mirror movement mechanism.

The process by which the controller <NUM> of the present background controls the optical property of the optical element <NUM> is illustrated in <FIG>.

In process S10-<NUM>, the controller <NUM> stores data representative of the variation of the optical property with increasing values of the signal in the memory <NUM>. This data may represent measurements of the optical property obtained by driving the optical element <NUM> with gradually increasing control signal values and measuring the optical property value obtained at each control signal value, the data corresponding to the measured values themselves or a curve fitted to such data. As noted above, the stored data may alternatively be obtained by modelling the hysteretic behaviour of the optical element <NUM>.

In process S20-<NUM>, the controller <NUM> controls the optical property of the optical element <NUM> by generating a control signal Sc based on the stored data, and applying Sc to the optical element <NUM>, wherein the value of the optical property is changed based on the stored data from an initial value Fi corresponding to a first value S<NUM> of the signal, to a subsequent value Fn corresponding to a second value S<NUM> of the signal that is smaller than S<NUM>, by setting the value of Sc to a third value S<NUM> that is within the second range of values <NUM> (where no hysteresis is observed), and subsequently increasing the value of Sc from S<NUM> to S<NUM>. The third value S<NUM> (which may be regarded as a 'reset' point) may, as in the present background example, be the lowest signal value in the stored variation.

As detailed above, the controller <NUM> stores data representative of the variation of the optical property with increasing values of the control signal Sc. As such, where the value of the optical property is changed from an initial value corresponding to a first value of the signal to a subsequent value corresponding to a second value of the signal that is larger than the first value of the signal, the controller <NUM> is not configured to set the value of the signal to a third value that is within the second range of values <NUM>, and then subsequently increase the signal value from the third value to the second (subsequent) value; instead, the controller <NUM> is in this case configured to change the value of the signal from the first value directly to the subsequent, larger value, since no inaccuracies caused by hysteresis would be introduced by such a change.

<FIG> illustrates the process by which the controller <NUM> controls the optical property of the optical element <NUM> in an alternative background example, in which the data stored in memory <NUM> is representative of a variation of the optical property with decreasing values of the signal.

In process S10-<NUM> of <FIG>, the controller <NUM> stores data representative of the variation of the optical property with decreasing values of the signal in the memory <NUM>.

In process S20-<NUM>, the controller <NUM> controls the optical property of the optical element <NUM> by generating a control signal Sc based on the stored data, and applying Sc to the optical element <NUM>, wherein the value of the optical property is changed based on the stored data from an initial value Fi' corresponding to a first value S<NUM>' of the signal, to a subsequent value Fn' corresponding to a second value S<NUM>' of the signal that is larger than Si', by setting the value of SC to a third value S<NUM>' that is within the second range of values <NUM> (where no hysteresis is observed), and subsequently decreasing the value of Sc from S<NUM>' to S<NUM>'. The third value S<NUM>' (which may be regarded as a 'reset' point) may, as in the present example embodiment, be the highest signal value in the stored variation.

As detailed above, the controller <NUM> stores data representative of the variation of the optical property with decreasing values of the control signal Sc. As such, where the value of the optical property is changed from an initial value corresponding to a first value of the signal to a subsequent value corresponding to a second value of the signal that is smaller than the first value of the signal, the controller <NUM> is not configured to set the value of the signal to a third value that is within the second range of values <NUM>, and then subsequently decrease the signal value from the third value to the second (subsequent) value; instead, the controller <NUM> is in this case configured to change the value of the signal from the first value directly to the subsequent, smaller value, since no inaccuracies caused by hysteresis would be introduced by such a change.

The above methods allow for compensation of hysteresis behaviour in optical systems where the rate at which the focal length of the optical system is changed is greater than or equal to the settling time of the focussing mechanism. The accuracy with which a target value of the optical property of the optical element <NUM> can be set may be increased in an optical system as described above, since the inaccuracies introduced by hysteresis effects are suppressed.

The above-described background example compensate for hysteresis behaviour when there is time to reset the signal value to the third value and then to the desired signal value before the subsequent focal length of the focusing mechanism is needed.

However, for some applications, the drive signal must be changed at a high frequency, such that, with the latency of the optical element, it is not possible to "reset" and then change the signal value fast enough. This may occur when the rate of change of focus drive is less than or equal to the settling time of the focusing mechanism. In such situations, the above system and method might not adequately compensate for hysteresis effects.

The controller <NUM> is operable in a 'continuous' mode of operation in embodiments hereinafter described, in which the focussing mechanism comprising the optical element <NUM> is driven continuously by the controller <NUM>. In the following description of these embodiments, features of the optical system that are common to those of the preceding background examples will not be described again, and the discussion will focus instead on the differences between these embodiments and the background examples.

The controller <NUM> in these embodiments is operable in the 'continuous' mode of operation to generate a cyclic signal having one or more discontinuities in each cycle of the cyclic signal, and to set the size of at least one of the one or more discontinuities in each cycle based on the above-described data that is stored in memory <NUM> such that a part of the variation of the optical property with the cyclic signal coincides with a part of the variation represented by the stored data. The controller <NUM> is further configured to apply the cyclic signal to the optical element <NUM>.

As used herein, the term "cyclic" means a signal that varies in a cyclical manner, in which the period of the cycles may or may not be constant. The frequency of the cyclic signal may thus be fixed or varying.

By way of example, <FIG> illustrates a variation of focal length F of the liquid lens with an applied drive signal S when the drive signal is a cyclic signal (specifically, in the form of a sinusoid with a fixed frequency) that has discontinuities at each of its turning points (i.e. the maxima and minima of the sinusoid), as illustrated in <FIG>. Cyclic drive signals other than sinusoids are, however, also possible. With a sinusoidal drive signal S having such discontinuities, the variation of F with S during a cycle with a fixed period of S can be approximated as a parallelogram, as illustrated in <FIG>. Specifically, the variation of F with S follows the calibration (hysteresis) curve representing the stored data while the sign of the change in S (e.g. positive in the case of increasing values of S) is the same as the sign of the change in S used to record or otherwise determine the calibration curve stored in memory <NUM>. Thus, as the value of S increases from one discontinuity shown in <FIG> (labelled "drive advance") to the next, the resulting values of focal length F follow the dashed curve shown in <FIG>. The discontinuity provided at each maximum of the drive signal S shown in <FIG> has no significant effect on the focal length F, which stays substantially constant when the discontinuity occurs (owing to the change in S at the discontinuity occurring on a smaller time scale than the response time of the liquid lens), as illustrated by the top horizontal side of the parallelogram in <FIG>. The discontinuity in the drive signal S, however, allows the variation of focal length F with decreasing values of S from one discontinuity in S to the next discontinuity in S (see <FIG>) to follow a curve in the F-S plane that has a similar slope to the variation of F with S while S increases between one discontinuity and the next, as also shown in <FIG>. The next discontinuity in S similarly has little or no effect on the value of F (see bottom side of the parallelogram in <FIG>) so that the loop followed by the variation of F with S becomes closed.

Thus, as S increases after having been set at a minimum value of S to the next maximum value of S, then abruptly decreases when the maximum value of S is reached, then decreases to the next minimum value of S, and then abruptly increases when the minimum value of S is reached, as shown in <FIG>, the variation of F with S follows one loop of the parallelogram shown in <FIG>, with the focal length F smoothly increasing to a maximum value before smoothly decreasing to a minimum value, as would be the case with an applied sinusoidal drive signal and no hysteresis being exhibited by the optical element <NUM>. The discontinuities in the cyclic drive signal S thus compensate for the hysteretic behaviour of the optical element <NUM>.

Although the controller <NUM> may thus be operable in the 'continuous' mode to generate a cyclic drive signal S having two discontinuities in each cycle of the drive signal, and to set the size of both of the discontinuities based on the stored calibration data such that a part of the variation of the focal length F with the S coincides with a part of the variation represented by the stored data, the controller <NUM> may alternatively generate a cyclic drive signal S having only one discontinuity in each cycle of the drive signal, and to set the size of this single discontinuity based on the stored calibration data such that a part of the variation of the focal length F with the S coincides with a part of the variation represented by the stored data; this alternative is applicable in cases where the range of focal lengths F covered during a cycle of the drive signal S is not contained entirely within a range of focal lengths F corresponding to values of S in hysteresis region <NUM> shown in <FIG>, but extends into one of the ranges of F corresponding to values of S in non-hysteresis region <NUM> or <NUM> also shown in <FIG>. For example, the drive range may include a transition from hysteresis region <NUM> to non-hysteresis region <NUM>, as illustrated in <FIG>.

In these cases, the controller <NUM> is configured to generate an asymmetric drive signal as shown in <FIG>, with a discontinuity at each maximum of the sinusoidal drive signal S but no discontinuity at any of the minima. In this example, the drive advance is thus applied only for one turning point of the drive signal which is within the hysteresis region <NUM>. No drive advance is required for the turning point within the no-hysteresis region <NUM>. It can be seen from <FIG> that the slope of the drive curve is different for low-to-high and the high-to-low directions of change of the drive signal S. Applying an asymmetric drive advance as in <FIG> would result in this different slope for the drive.

The process by which the controller <NUM> of the present embodiment controls the optical property of the optical element <NUM> is summarised in <FIG>.

In process S30, the controller <NUM> stores data representative of the variation of the optical property with either increasing values of the signal or decreasing values of the signal in the memory <NUM>.

In process S40, the controller <NUM> controls the optical property of the optical element <NUM> by generating a cyclic signal having one or more discontinuities in each cycle of the cyclic signal, the size of at least one of the one or more discontinuities in each cycle being based on the stored data such that a part of the variation of the optical property with the cyclic signal coincides with a part of the variation represented by the stored data, and applying the cyclic signal to the optical element <NUM>.

Claim 1:
An optical system arranged to maintain focus of an optical imaging system for ophthalmic imaging with respect to a target, the optical system comprising:
an optical element (<NUM>) responsive to an applied signal to vary a focal length of the optical element (<NUM>), the variation of the focal length with the applied signal exhibiting hysteresis in a first range of values of the signal, and no hysteresis in a second range of values of the signal;
a memory (<NUM>) storing data representative of a hysteresis curve which indicates the variation of the focal length with increasing values of the signal and decreasing values of the signal; and
a controller (<NUM>) configured to continuously control the focal length of the optical element by:
generating, based on the stored data, a cyclic signal having one or more discontinuities in each cycle of the cyclic signal, and setting the size of at least one of the one or more discontinuities in each cycle based on the stored data such that a part of the variation of the focal length with the cyclic signal coincides with a part of the variation represented by the stored data; and
applying the cyclic signal to the optical element (<NUM>).