GB2628990A

GB2628990A - Lens examination equipment and method

Info

Publication number: GB2628990A
Application number: GB2305343.2A
Authority: GB
Inventors: Crouch Bradley; Poolman Rhys
Original assignee: Eyoto Group Ltd
Current assignee: Eyoto Group Ltd
Priority date: 2023-04-12
Filing date: 2023-04-12
Publication date: 2024-10-16
Also published as: GB202305343D0

Abstract

A method of examining a sample lens (4, 5, figure 1) comprises steps of positioning a sample lens between a display surface (13) and a camera (28) and displaying on the display surface a test pattern (e.g. figure 8) comprising at least one fiduciary marker 400 presented in an initial orientation. The camera is used to capture an image of the usually-distorted test pattern as seen through the lens so as to determine the position of a reference point within the marker. The previous steps are repeated so as to capture a series of lens images wherein the marker is rotated to a different orientation in each successive lens image. An eclipse 408 of best fit is derived, the eclipse joining the positions of the reference point of the marker in successive images, and characteristics of the eclipse are analysed to determine the degree and nature of the distortion of the test pattern to determine at least one parameter of the sample lens.

Description

LENS EXAMINATION EQUIPMENT AND METHOD

Technical Field of the Invention

The invention relates to methods and equipment for examining lenses and especially, but not exclusively, ophthalmic lenses in glasses.

Background to the Invention

It is often necessary to he able to determine the optical parameters of a lens such as those used in glasses. This may he required as part of the manufacturing process to ensure a lens conforms to the prescription and may be carried out either before the lens has been assembled into a glasses frame or after. It is also sometimes necessary to determine the optical parameters of a lens in a pair of glasses which have been used, say as part of an ophthalmic examination where the person does not have their prescription to hand or to check that the lens is in conformity with their prescription.

Ideally, this determination can he carried out readily without reliance on highly skilled labour and preferably is at least somewhat portable and at least somewhat automated.

An exemplary method for determining optical parameters of a lens, as disclosed in W02018/073577, involves displaying a test pattern comprising a set of dots which define a first ellipse of best lit in which the major and minor axes RI, R2 are equal (i.e. a circle) on a plane surface. A digital image of the (usually distorted) test pattern seen through a lens is captured. A second ellipse of best fit joining the dots in the set is derived from the distorted test pattern in the image. Characteristics of the first and second ellipses are compared to determine the degree and nature of distortion to the test pattern, from which the power of the lens is calculated. Typically, the test pattern include a number of said sets of dots distributed over an area of the surface with each set being analysed to determine the optical parameters of the lens at multiple locations.

One of the challenges with this approach is that the dots are circular, with the centre of the dot used as a reference point. Accurately identifying the centre of a dot is relatively straightforward for dots close to the centre of the lens or where the lens has a low power. For higher powered lenses or close to the edges of lenses this is less straightforward as the image of the dots become increasingly distorted. Especially towards the edge of lenses the distortion tends to result in teardrop shapes rather than simple ellipses which makes accurate identification of the dot centre even more challenging. A further issue is that the spatial distortion can make it difficult to accurately associate dots that are visible in measurement images with specific dots visible before the lens is inserted. Whilst this inaccuracy is relatively small with low powered lenses and can be partially alleviated by using smaller dots, it is still suboptimal.

It is an object of the present invention to provide an alternative method and equipment for examining a lens so as to determine an optical parameter of a lens at multiple locations simultaneously which at least partially alleviates or overcomes the above issues.

Summary of the Invention

According to a first aspect of the invention, there is provided a method of examining a sample lens, the method comprising; a. positioning a sample lens between a display surface and a camera b. displaying on the display surface a test pattern comprising at least one fiduciary marker, presented in an initial orientation; c. using the camera to capture an image of the (usually distorted) test pattern as seen through the sample lens ("the lens image") and to determine the position of a refence point within the fiduciary marker; d. repealing steps b and c so as to capture a series of lens images wherein the fiduciary marker is rotated to a different orientation in each successive lens image e. deriving an ellipse of best fit joining the positions of the reference point of the fiduciary marker in successive lens images; f. analysing characteristics of the ellipse to determine the degree and nature of distortion to the test pattern produced by the sample lens and from this determining at least one parameter of the sample lens.

A reference point, such as a particular vertex, within a fiduciary marker is relatively simple to identify in successive images. Furthermore, the position of the reference point can he more accurately identified than the centre of a dot, especially under distortion. Beneficially, individual fiduciary markers may be smaller than the set of dots required to generate an ellipse of best fit in the prior art discussed above. This can enable more accurate determination of lens properties at a particular location.

hi the present application, the term "ellipse" should be understood as encompassing a circle, which is a special case of an ellipse in which the major and minor axes are equal.

The fiduciary marker may comprise a two-dimensional pattern. The pattern may be black on white or white on black. In many embodiments, the two dimensional pattern may comprise a series of square or rectangular blocks. The reference point may comprise a vertex within the two dimensional pattern. In particular embodiments, the reference point may be an external vertex.

The fiduciary marker may correspond to a particular standard or protocol. In such cases the fiduciary marker may be selected from a predefined set of fiduciary markers. Any suitable set of fiduciary markers may be used. In one embodiment, the fiduciary marker set is an AprilTags set. In particular, the fiduciary markers may he a TagCircle valiant of AprilTags. The skilled person will however appreciate that alternative fiduciary markers or fiduciary marker sets could be used to implement the invention.

The fiduciary marker may be rotated about a central point between successive lens images. The fiduciary marker may be rotated by a constant angle between successive lens images. The angle may he less than 90°. In particular embodiments, the angle may be 60°, 45°, 30° or 22.5°.

The method may comprise determining the major axis and the minor axis of the ellipse of best fit. The method may include the step of determining the orientation of the major axis and the minor axis of the ellipse of best fit. The method may include the step of determining any deviations from regularity of the ellipse of best fit.

The method may comprise the step of correcting for system distortion from sources other than the sample lens. Such system distortion may be due to the camera lens arrangement or other features. Correcting for system distortion may comprise applying a transform algorithm to the test pattern in the captured lens image to remove system distortion. Alternatively, the method may comprise comparing the characteristics of the ellipse of best fit with reference values. The reference values may vary by location.

The method may comprise the step of determining the power of the sample lens from determined major axis and minor axis of the ellipse of best fit. When the major and minor axes of the second ellipse are not equal, the method may comprise determining a degree of astigmatic correction (cylindrical power) of the sample lens and the axis angle of the astigmatic correction.

In one embodiment, the method is used to determine the at least one optical parameter at a single point in the sample lens. In such an embodiment, the test pattern may consist of a single fiduciary marker only and the method will determine the optical properties of the sample lens at the point aligned with the fiduciary marker.

hi an alternative embodiment, the method is used to determine said at least one optical parameter of the sample lens at multiple locations within an area of interest of the lens, the method comprising: g. in step b above, displaying a test pattern comprising a plurality of fiduciary markers distributed over an area of the surface; h. in step a above, positioning the sample lens so that at least the area of interest of the lens is positioned between the displayed test pattern and the camera before capturing the lens image; and i. performing the analysis in steps e and f above in respect of the reference point of each of the fiduciary markers in the test pattern recorded in the lens image within the area of interest to determine said at least one optical parameter at various locations with the area of interest of the sample lens.

The area of interest may comprise substantially the whole of the sample lens.

In some such embodiments, the test pattern may comprise a two dimensional array of fiduciary markers. In some embodiments, the array may he a regular array. Suitable examples of a regular array include but are not limited to a square array, a rectangular array, a hexagonal array or the like. In square or rectangular arrays, the fiduciary markers may be arranged in an array of rows and columns, wherein the fiduciary markers in each row are equally spaced apart by a distance which is equal to the spacing between adjacent rows, and wherein the fiduciary markers in each column are equally spaced apart by a distance which is equal to the spacing between adjacent columns. The column spacing is equal to the row spacing for a square array and is not equal to the row spacing for a rectangular array. In other such embodiments, alternate rows are off-set so that the fiduciary markers in any given row lie midway between the fiduciary markers in an adjacent row or rows, such that each fiduciary marker (other than those at the edges of the array) is surround by six other fiduciary markers located at the apexes of a notional regular hexagon, wherein each set of six other fiduciary markers comprises one of said sets of fiduciary markers.

Where the test pattern comprises multiple fiduciary markers, each fiduciary marker displayed may be a different fiduciary marker. In such embodiments, each different fiduciary marker may be selected from the same fiduciary marker set. Use of different fiduciary markers within the test pattern makes identification of a particular fiduciary marker in successive images very convenient. In other embodiments, each fiduciary marker may he the same fiduciary marker or each fiduciary marker may he selected from a limited subset within a fiduciary marker set. This may beneficially allow the technique to be used where the set of fiduciary markers is insufficient to provide different fiduciary markers across the full extent of the lens.

In some embodiments, where the test pattern is a square (or rectangular) array, each fiduciary marker in a single row (or column) may be the same and fiduciary markers in neighbouring rows (or columns) may differ. In such embodiments, each successive row (or column) may comprise a different fiduciary marker. Alternatively, successive rows (o columns) may cycle through a number of different fiduciary markers before a fiduciary marker is repeated on a subsequent row (or column). This can help distinguish between fiduciary markers in the lens image, where the set of fiduciary markers is not large enough for each fiduciary marker in the test pattern to be different.

For instance, where only two different fiduciary markers are selected, alternating rows (pr columns) could be comprise each of the markers.

In some such embodiments, the method may involve the steps of carrying out the above analysis for a full rotation of the fiduciary markers in a first configuration comprising like fiduciary markers in each row (or each column) with markers differing in adjacent rows (or columns) and then repealing the analysis for a second configuration comprising like fiduciary markers in each column (or row) with markers differing in adjacent columns (or rows). Where there are sufficient fiduciary markers in the step for each column and row to have a different fiduciary marker in each configuration, this can enable each location within the array to he identified by reference to a unique marker combination.

The method may comprise displaying the results of the examination graphically in the form of a map of the lens in which the at least one optical parameter is represented as a colour or contour.

The method may include calibration of the system used to carry out the method by: j. using the camera to capture an image of the test pattern displayed on the surface without a sample lens between the camera and the surface ("the system image"); k. comparing the test pattern in the system image with the original test pattern to determine the degree of distortion to the original test pattern produced by the system; 1. deriving a transform algorithm which applied to the test pattern in the system image will substantially restore it back to the original test pattern and saving the transform algorithm.

Alternatively, the method may include calibration of the system used to carry out the method by: m. using the camera to capture a series of images of the test pattern displayed on the display surface without a sample lens between the camera and the surface ("the system images"), each successive system image with the or each fiduciary marker in the test pattern rotated to a different orientation; n. deriving an ellipse of best fit joining the positions of the reference point of the or each fiduciary marker in successive system images; o. determining and storing the characteristics of the or each ellipse of best.

fit as reference values; or p. deriving and storing a transform algorithm which applied to the or each ellipse of best fit in the system image will substantially restore it back to expected ellipse characteristics calculated from the original test pattern.

The expected ellipse characteristics may comprise circles centred on the or each fiduciary marker in the test pattern. The or each circle may have a diameter equal to 10 the maximum transverse measurement of the or each fiduciary marker.

In such embodiments, when subsequently examining a sample lens, the transform algorithm may be applied to remove system distortion before carrying out the analysis in steps e and f above. Similarly, in appropriate embodiments, the reference values may be used to carrying out the analysis in steps e and f above.

The display surface may be a display screen. The display screen may be an LED display screen, LCD display screen or any other suitable form of display screen. The display screen may be a dedicated display screen or may he the display screen of another device such as a tablet computer or the like.

The display surface may be substantially planar. The display surface may be substantially perpendicular to the optical axis of the camera. The optical axis of the camera may be aligned with the centre of the display surface. The sample lens may be positioned such that the optical axis of the sample lens is substantially perpendicular to the display surface. The sample lens may be positioned such that the optical axis of the sample lens is aligned with the centre of the display surface. Alternatively, the sample lens may be positioned such that the optical axis of the camera and the sample lens are aligned but offset from the centre of the display surface. In such embodiments, the method may comprise displaying the test pattern on one section of the display surface and displaying results of the examination on another section of the display surface.

The method may include the step of aligning the test pattern with particular 30 measurement points of the sample lens. In such embodiments, this may comprise aligning the centre of a fiduciary marker in the test pattern with the particular measurement point. Suitable particular measurement points of the sample lens include but are not limited to optical centre, frame centre, near reference point, far reference point, distance reference point, prism reference point or the like.

The alignment may be achieved by identifying the particular measurement point of the sample lens as currently positioned and offsetting the displayed test pattern. Alternatively, the alignment may be achieved by identifying the particular measurement point of the sample lens as currently positioned and moving the sample lens position relative to the display surface. To achieve such displacement the sample lens may be movably positioned relative the display surface.

The particular measurement points may be identified by carrying out the method of steps a to f above wherein the analysis is directed to identifying a selected particular measurement point or each selected particular measurement point. Alternatively, the particular measurement points may he identified by other techniques prior to or after mounting the sample lens.

In some embodiments, the method of the present invention may he performed with the sample lens at a first lens distance from the display surface and repeated with the sample lens at a second lens distance from the display surface, different from the first lens distance. In such embodiments, the method may involve determining the power of the sample lens by analysing the lens images captured at the different lens distances or by analysing the or each ellipse of bets fit derived from the lens images captured at the different lens distances.

In such embodiments, the method may comprise determining the magnitude of magnification MI of the test pattern or ellipse of best fit at the first lens distance and the magnitude of magnification M2 of the test pattern or ellipse of best fit at the second lens distance M2 and calculating the power P of the test lens from the magnification values MI, M2 at the first and second lens distances and the change in lens distance Adl between the first and second lens distances.

The term "lens distance" as used herein, including in the claims, refers to the 30 distance between any given reference point in or on the sample lens and the display surface when measured in a direction perpendicular to the plane of the display surface. It can be helpful to think in terms of the lens distance being the distance between the display surface and a reference plane extending parallel to the display surface and which passes through the refence point. In a simple lens at least, a suitable reference plane would extend orthogonal to an optical or principal axis of the sample lens. However, in practice it is not essential to actually identify such a reference plane or to measure the actual distance between the display screen and the reference plane as the change in lens distance Adl can be determined from the movement of the sample lens between the first and second lens distances. For example, the change in lens distance Adl can be determined with relative accuracy by incorporating into apparatus for carrying out the method a mechanism for accurately moving the sample lens by a set distance perpendicular to the plane of the display surface between the first and second positions and/or by incorporating a means to determine, measure, detect or sense, either directly or indirectly, the distance moved by the sample lens between the first and second lens distances. Provided the sample lens is held in the same orientation relative to the display surface as it is moved between the first and second lens distances, the change in lens distance Adl will be same for all points in the sample lens regardless of the shape of the sample lens.

The power P of the sample lens can be determined using the following equation or an equivalent: P = f = A dl Where MI and M2 are the values of the magnification of the test pattern measured with the sample lens at the first and second lens distances respectively and Adl is the change in lens distance between the first and second lens distances. In a preferred embodiment Adl is perpendicular or substantially perpendicular to the display screen.

The method may comprise moving the sample lens a predetermined distance Adl. Alternatively, the method may comprise determining the degree of magnification of the test pattern caused by the sample lens at the first lens distance and then moving 1 1 1 111, Ag, the sample lens until a second lens distance is reached at which the change in magnification of the test pattern caused by the sample lens is at or above a predetermined amount suitable to enable the power of the sample lens to be calculated and to determine the distance moved by the sample lens. The method may comprise monitoring the position of a reference point on the sample lens, either directly or indirectly, in order to determine the change in lens distance Adl. The method may comprise mounting the sample lens in a lens carriage for holding the sample lens between the display surface and the camera wherein the lens carriage is movable relative to the display surface in said linear direction perpendicular to the display surface. In one embodiment, the method comprises moving the lens carriage in said linear direction to place the sample lens at the second position after the first lens image test pattern has been captured. The method may also comprise determining the change in lens distance Adl from the movement of the lens carriage. The lens carriage may he part of a lens movement system comprising an electronic actuator operating under the control of an electronic control system for controlling movement of the lens carriage.

The actuator may comprise a stepper motor and the method may comprise determining the change in lens distance Adl by monitoring the number of steps taken by the motor to move the lens carriage from the first position to the second position. The stepper motor may drive a threaded shaft of known pitch to which the lens carriage is mounted by way of a drive nut.

The method may comprise determining the magnitude of magnification MI, Mz of the test pattern at the first and second lens distances by comparing each of the first and second lens image test patterns with the original test pattern. The method may he used to measure properties of a sample lens or sample lenses in a pair of glasses. Where the above method is carried out on a sample lens in a pair of glasses, once one or more of the steps set out in the previous paragraphs have been completed, the same steps would normally be carried out for the other sample lens of the pair of glasses.

In accordance with a second aspect of the invention, there is provided a system for carrying out the method according to the first aspect of the invention, the system 30 comprising a computing device having a planar display screen, a camera mounted above the screen with the axis of the camera perpendicular to the plane of the screen, 1I the camera being operatively connected to the computing device for storing and processing image data captured by the camera; and a glasses mount for holding a pair of glasses with a sample lens located between the camera and the test pattern on the display screen in use.

The computing device may be programmed to carry out the image data processing and analysis steps of the method in accordance with the first aspect of the invention.

The computing device may be a portable computing device such as a tablet computer. Using a tablet style computer, it is possible for the visual display and the 10 computing facility to be combined in one unit.

The camera axis may be offset to one side of the display screen.

The system may comprise a frame mounted to the computing device, the camera being mounted to the frame. The frame may comprise a casing partially enclosing the computing device. The casing may be attached to a base to which the camera is 15 mounted.

The system may comprise a casing partially enclosing the computing device, the casing being attached to a base, the camera being mounted to the base.

The computing device may be configured in use to display the test pattern in a first section of the display screen below the camera and to display the results of the examination in another section of the display screen. The computing device may he configured to process image data capture by the camera as part of the lens examination.

The glasses mount may comprise a plinth slidably mountable on the display screen. The computing device may he arranged in use to display graphic symbols on the display screen to guide a user in correctly positioning the plinth. The means for securing a pair of glasses to the plinth may he a glasses clamp mounted to the plinth.

The plinth may define an aperture through which the display screen/display is visible from above when the plinth is positioned on the display screen/display in use and the glasses clamp may be mounted to the plinth such that it can be moved to selectively to position either one of the lenses in a pair of glasses mounted in the clamp above the aperture in use. The glasses clamp may be pivotally mounted to the plinth. The plinth may hold the lenses in a pair of glasses mounted to it at a distance from the display screen/display. The distance may be fixed or the plinth may have at least one separable spacer section which can be selectively removed to vary the distance. The distance may be set substantially at the focal length of a +20D lens. In some embodiments, the plinth could he provided with a powered spacing arrangement. This could be controlled by a user or other system such as a computer to vary the height of the plinth and hence the spacing between the sample lens and the display screen. The powered spacing arrangement may comprise a motor, which may, in some embodiments, he a stepper motor.

By using the system and/or methods set out in this invention, it is possible readily and easily to identify the characteristics of glasses being used by a person and whether these are in conformity with what would he prescribed, or whether replacement is necessary. It is also possible to use the system and methods as quality control in a manufacturing facility for glasses and ophthalmic lenses. In both cases the system and methods greatly reduce the need to use skilled optometrists in what is a routine task, allowing the specialist's skills to be employed in identifying persons' needs.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to 20 the accompanying drawings, of which: Figure 1 is a perspective view of a first embodiment of a system for examining sample lenses in a pair of glasses in accordance with an aspect of the invention, showing the system in an open deployed position; Figure 2 shows a part of the system of Figure 1 from above, with parameters calculated for the sample lens concerned displayed on a display screen of the system; Figure 3 is a view similar to that of Figure 2 but illustrating how data relating to the type of sample lens under examination can be displayed in the form of a map of the sample lens; Figure 4 is a perspective view of the system of Figure 1 in a folded configuration; Figure 5 is a perspective view of a second embodiment of a system for examining lenses in a pair of glasses in accordance with an aspect of the invention; Figure 6 is a view from one side of the system of Figure 5; Figure 7 is a schematic representation of an alternative a system for examining a sample lens which can he used to carry out the various methods of examining a lens described herein; Figure 8a is an annotated lens image captured using the method of the prior art using dots as a test pattern; Figure 8b schematically illustrates the nature of distortion to dots within the captured lens image of figure 8a; Figure 9a is an example of a fiduciary marker suitable for use in the method of the present invention; Figure 9b is an annotated lens image of the fiduciary marker of figure 9a; Figure 9c is an annotated overlay of successive lens images of the fiduciary marker of figures 9a and 9b wherein the fiduciary marker is rotated between images; Figure 9d is a schematic flow chart of a method of determining lens properties according to the present invention; Figure 10a shows a lens image of a first array of fiduciary markers used in the method of the present invention; Figure 10c shows a lens image of a second array of fiduciary markers used in the method of the present invention; Figure 10c shows an overlay of the first and second lens images of figures and 10h illustrating how particular positions within the array can be uniquely located; Figure 10d is a schematic flow chart of a method of determining lens properties according to the present invention; and Figure 11 provides a schematic illustration of recentring a test pattern from an initial position ion figure 1 la to a recentred position shown in the expanded extract of figure 1 lb. Figures 1 to 4 illustrate a first embodiment of a system 1 for examining a sample lens in a pair of glasses 2 having a pair of lenses 4 and 5. The system comprises a square frame 10 mounted to a tablet style computer 12 having a built-in visual display unit with a display screen 13. The tablet style computer 12 can he any computing device having a planar digital display screen 13 of a suitable size, such as an i-Pad (RTM). The frame is attached to the computer 12 by a number of mounts 14 extending rearwards from the frame 10 to engage with a rear surface of the computer and which clamp the frame to the computer. A glasses clamp 16 is mounted within the interior of the frame above the display screen 13.

The glasses clamp 16 has a pair of jaws 18, 20, the jaws having grooves 22 in opposing surfaces into which the top and bottom of the pair of glasses 2 are located and held in use. The jaws are mounted on a runner 24 which extends over the display screen from front to rear (as considered with the screen in landscape mode, the longer edges at the front and rear). The jaws 18, 20 are slidably mounted for movement along the runner 24. A spring 25 (seen in figure 2) urges jaw 20 towards jaw 18 to grip the top and bottom of the pair of glasses. The jaws can be separated using a knob 20a on the movable jaw 20, which can be gripped by hand to pull jaws 18 and 20 apart. Runner 24 is slidably mounted on a pair of rails 26 which are attached on the inside of opposite sides of the frame 10 at the front and rear of the display screen. It can he seen that the clamp 16 is movable in a plane parallel to the surface of the visual display unit 13 by movement of the jaws along the runner 24 and movement of the runner along the rails 26.

A camera 28 is mounted on a camera mount 30 which locates the camera above part of the display screen with the axis X of the camera perpendicular to the display screen 13. The camera 28 may he a digital camera and is operatively connected to the computer 12 by a suitable cable or other means so that image data captured by the camera can he saved in the computer for processing and analysis and to allow control of the camera by the computer. The camera mount has a hinge 32 enabling the camera to be folded inside the frame 10 when the system is not in use (see figure 4).

In the first embodiment as shown in Figures 1 to 4, the frame 10 surrounds the left hand half 34 (figure 2) of the display screen 13, when considered in landscape mode. In this particular example, the computer 12 is 28 cm wide across, thus the inside dimensions of the frame 10 are 14 x 14 cm. The right hand half 36 of the display screen 13 (see figure 2) is clear for direct viewing.

For use in examining a sample lens 4, 5 in a pair of glasses, a test pattern is displayed in the left hand half 34 of the display screen below the camera. The pair of glasses 2 is mounted in the glasses clamp 16, the position of the jaws 18, 20 being adjusted to ensure a firm but not overtight grip of the glasses 2 in the grooves 18. The position of the clamp 16 is adjusted by moving the jaws along the runner and/or the runner 24 on the rails 26 to position one of the lenses between the test pattern and the camera. The test pattern is now seen by the camera 28 through the lens 4 and a digital image of the test. pattern as viewed through the lens can be captured by the camera and image data saved in the computer memory for analysis. This image will he referred to as the "lens image". The test pattern as seen through the lens and captured in the lens image is usually distorted by the lens and by comparing the degree and nature of the distortion of the test pattern from the lens image data with the original image it is possible to derive one or more optical parameters or characteristics of the lens. Several methods which can be used for this analysis in the system 1 will be described below. The results of the analysis are displayed on the right hand side 36 of the display screen 13 as illustrated in figures 2 and 3. The format and nature of the results of the analysis displayed can be varied to suit examination requirements and thus the results visible in these figures should be understood as indicative only. Once the examination of one sample lens 4 is complete the process can be repeated for the other sample lens 5. The camera mount and camera are omitted from figures 2 and 3 for clarity.

The frame 10 and camera 28 could be mounted centrally of the display screen 13. In this case, the test pattern would be displayed in a central region of the display screen below the camera and the results displayed on display screen to one or both sides of the frame.

In the arrangements set out in figures 1 to 4, the glasses clamp 16 may be calibrated to measure the separation of the centres of the two sample lenses.

A second embodiment of a system 101 for examining a lens in a pair of glasses will now be described with reference to figures 5 and 6. In the following description, features of the system 101 that are the same as those described above in relation to the first embodiment 1 or which fulfil the same function will be identified using the same reference numbers but increased by 100.

In the system 101 in accordance with the second embodiment, the frame 110 is in the form of a casing 150 in which a display unit 112 (or optionally a tablet style computer) is partially encased and a base 152 to which the casing 150 is mounted. The casing 150 has an aperture 154 in an upper wall so that substantially the whole of the display screen 113 of the computer is exposed. The casing 150 may have a number of parts that are releasably or permanently attached when assembled to the computer. The camera mount 130 is provided on the base 152 and has an upright section 130a extending upwardly to the rear of the display unit 112 and a horizontal section 130b extending forwardly from the upper end of the upright. section over the display screen 113. The camera 128 is mounted proximal the forward end of the horizontal section 130b so that the axis X of the camera is aligned perpendicular to the display screen 113.

The camera mount 130 is off-set to the left side of the computer and is configured so that the axis X of the camera is located generally at the centre of the left-hand half of the display screen, with the camera a suitable distance above screen. The camera 128 is operatively connected to a computer by a suitable cable or other means so that image data captured by the camera can be saved in the computer for image processing and analysis and to allow control of the camera by the computer.

A principal difference of the second embodiment of the system 101 over the first is that the glasses clamp 116 is not mounted to the frame but mounted to a separate plinth 156 which sits directly on the display screen 113. The plinth is placed directly on the screen and can he slid across the screen 113 for correct positioning of a lens relative to the camera and/or the test pattern.

The plinth 156 comprises a main body having a central aperture 158 through which the display screen 113 is visible from above when the plinth is located on the display screen. The glasses clamp 116 is mounted to the plinth body to one side of the aperture 158 and can be pivoted to selectively position either of the lenses in a pair of glasses mounted in the clamp 116 above the aperture.

The glasses clamp 116 in this embodiment has a pair of arms 160a, 1601) pivotally coupled together at their centres in a scissor-like manner. The arms 160a, 160b are mounted to a pivot pin 162 so that the arms can pivot relative to each other and so that both arms can be pivoted together about the pin to selectively position one or other lens in a pair of glasses above the aperture 158. Each arm 160a, 160b is generally S shaped and carries an abutment 164 at either end. Each abutment 164 extends upwardly from the arm and is generally cylindrical with a groove 118 about its side for engaging with a pair of glasses. The aims 160a, 160b are configured so that one of the abutments 118 on each arm engages a top edge of the glasses and the other engages the bottom edge. The arms 160a, 160b are biased by a torsion spring to bring the opposed abutments at either end of the clamp together. Lugs 166 are provided on each arm 160a, 160b on one side of the clamp which can be used to move the arms to separate the abutments when mounting a pair of glasses. Once the glasses are in place, the lugs 166 are gently released and the bias force firmly clamps the top and bottom edges of the glasses between the abutments. The abutments 164 may engage with the glasses frame and/or edges of the lenses depending on the style.

The plinth 156 and glasses clamp 116 hold the top and bottom edges of the glasses substantially in a plane parallel to the display screen 113 with the lens being examined spaced from the display screen by a distance, which is usually less than the focal length of the lens. The plinth 156 as illustrated holds a sample lens at a fixed spacing from the display screen, which in this embodiment is set to the focal length of +20D lenses, around 50mm. It has been found that this distance enables accurate results to be obtained in lenses ranging from -20D to +15D. This range covers the majority of lens used by the general public, which tend to fall in the range of -SD to +SD. However, it will be appreciated that the plinth could be configured to provide a different spacing between the sample lens and screen to cope with sample lenses outside the normal range and the system could he provided with a set of two or more plinths which each provide a different spacing between the sample lens and the screen. In a further alternative, the plinth 156 could be provided with one or more removable spacer sections which can be selectively used to change the height of the plinth and hence the spacing between the sample lens and the display screen. For example, to extend the range above +15D the distance between the sample lens and the screen would have to be decreased from that discussed above. This could be achieved by removing a spacer section to reduce the height of the plinth above the screen. In a further alternative, the plinth could he provided with a powered spacing arrangement for controlling g the lens distance between the sample lens and the display screen. The powered spacing arrangement could be controlled by a user or other system such as a computer to vary the height of the plinth and hence the spacing between the sample lens and the display screen. In order to achieve such control, the powered spacing arrangement may comprise a motor or stepper motor. In steeper motor embodiments, any change in lens distance Adl can be measured by monitoring the number of steps taken by the stepper motor. In such embodiments, the stepper motor may drive a threaded shaft of known pitch to which the sample lens carriage is mounted by way of a drive nut.

The system 101 in accordance with the second embodiment is used to examine sample lenses in a pair of glasses in a manner similar to that of the first embodiment. A pair of glasses is mounted in the clamp 116 on the plinth 156 and the clamp pivoted to position one of the sample lenses over the aperture 158. The plinth 156 is placed on the display screen to locate the sample lens below the camera. The system 101 is configured to display information on the screen 113 to guide the user in correctly positioning plinth and the sample lens. This could comprise graphic symbols displayed on the screen 113 which show the user where to position the plinth 156, for example, which may be generated by the computer in response to data from the camera 128. The plinth can be slid over the screen to correctly position the sample lens and to move the sample lens position during examination if required. With the sample lens in position, a test pattern is displayed on the display screen 113 below the camera and the sample lens and the camera used to capture an image of the test pattern as seen through the sample lens the lens image. The (usually distorted) test pattern in the lens image data is analysed the computer which determines the degree and nature of the distortion of the test pattern caused by the lens and from this is able to derive one or more optical parameters of the sample lens. The results of the analysis are displayed on the right hand side of screen which is not obscured by the plinth.

The camera 128 could he mounted centrally of the display screen 113. In this case, the test pattern would be displayed in a central region of the display screen below the camera and the results displayed to one or both sides of the display screen.

The system 1, 101 in accordance with either of the first and second embodiments is compact, highly portable and self-contained. In optional versions of these embodiments, a tablet computer can be used to generate both the test image and display the results analysis and carry out the image processing and analysis eliminates the need to have multiple display units and a separate computing unit. The use of a tablet computer also saves on costs as it eliminates the need to design and manufacture dedicated computing and display hardware.

The system 1, 101 as described above can be adapted to carry out examination of a lens using a number of different test patterns and methods of analysis, several embodiments of which will now he described in relation to the examination of a sample lens in a pair of glasses using the system 101 according to the second embodiment described above. However, the methods described can also be used in the system 1 according to first embodiment. Indeed, the underlying principles of these methods can he adapted for use in any other suitable lens examining system. Figure 7 for example illustrates an alternative lens analysis system 201 that can be used to carry out the methods described below. The same reference numerals but increased by 100 will be used to identify features that are the same, or which fulfil the same function, as those of the previous system 101.

The system 201 comprises a camera 228 looking perpendicularly at a display 213 on which a test pattern is displayed as described below. Between camera 228 and display 213 is a lens frame 267a. The camera is connected to a computer 212 which in turn is connected to a screen 268 for the user, and optionally to the display 213, to enable dynamic adjustment of the test pattern. A sample lens 203 to be examined is inserted in the lens frame 267a so that the sample lens is orthogonal to the camera 228 and the midpoint of the lens is approximately on the line between the midpoint of the camera lens and the midpoint of the display. The camera is used to capture an image of the distorted test pattern seen through the sample lens (the lens image) and the distorted test pattern in the lens image is analysed and compared to the original test pattern and the distortions caused by the lens 203 are calculated in computer and used to determine the optical parameters of the lens which are displayed.

Where the distance between the lens 203 and the display 213 is fixed, to allow for examination of sample lenses having a greater focal length than the spacing, a supplementary compensating lens 269 is inserted in a second frame 267b.

Turning now to figure 8, a prior art method of examination of sample lenses uses a test pattern comprising a hexagonal array of dots. In such an array, dots are arranged in an array of rows and columns, wherein the dots in each row are equally spaced apart by a distance which is equal to the spacing between adjacent rows, and wherein alternate rows are off-set so that the dots in any given row lie midway between the dots in an adjacent row or rows. Accordingly, each dot (other than those at the edges of the array) is surrounded by six other dots located at the apexes of a notional regular hexagon. Similarly, each set of six other dots comprises one of said sets of dots.

A sample lens 4,5, 203 such as a lens from a pair of glasses is mounted over display screen 13, 113, 213 and below camera 28, 128 228. With the sample lens in position, a test pattern is displayed on the screen 13, 113, 213 below the camera 28, 128 228 and the lens 4,5, 203 and the camera 28, 128 228 are used to capture an image of the distorted test pattern as seen through the sample lens 4, 5, 203, the "lens image".

The lens image data is saved in the computer for image processing and analysis.

In the test pattern the dots are regularly spaced and regularly sized. In the lens image 300 illustrated at figure 8a, the position of each of the dots is distorted. An ellipse of best fit joining the dots in each set is derived from the distorted test pattern for each set of dots in the image. Each such ellipse can then be analysed to determine the proprieties of the lens at different locations across the sample lens corresponding to each set of dots. In particular, analysis of the relative distortion of each ellipse both in terms of relative size and orientation of the axes compared to the test image can enable calculation of the spherical power, cylinder and axis of the sample lens. Examples of such calculations are known to the skilled person and are discussed in detail in documents such as W02018/073577.

Two such ellipses 310, 320 are identified in figure 8a. Ellipse 310 is relatively close to the centre of the lens image and ellipse 320 is relatively close to the edge of the lens. Ellipse 320 is larger and deviates further form a simple circular form.

In order to derive the ellipse of best fit, the centre point of each dot in the lens image is identified. The dots in the test pattern are circular and dots close to centre of the lens are minimally distorted in shape. At the edge of the lens, similarly to the ellipses 310, 320 above, the dots in the lens image are subject to greater distortion. This is illustrated by the exemplary dots 311, 321 in figure 8b.

Dot 311 is equivalent to one of the dots from ellipse 310. Dot 311 is therefore substantially circular and thereby identifying the centre 312 of dot 311 can be carried out relatively simply and accurately, by identifying the point at the centre of the lateral and upright axes of the dot 311.

Dot 321 is equivalent to one of the dots from ellipse 320. Dot 321 has a teardrop shape rather than a simple ellipse. As such the true centre 322 of dot 321 is displaced from the centre of the lateral axis 323. Accurately identifying the degree of distortion and correctly locating the centre 322 of the dot 321 is therefore difficult. Indeed, identifying the true centre may he impossible beyond a certain limit of screen/camera resolution. This leads to inaccuracy in the determination of best fit ellipses and thus a limitation in the accuracy of the prior art method.

Turing now to figures 9a-9c, a method of examination of lenses according to the present invention uses a test pattern comprising one or more fiduciary markers. In figure 9a, an exemplary fiduciary marker 400 is illustrated. The fiduciary marker 400 comprises a two dimensional black and white pattern formed from a series of rectangular blocks. The particular fiduciary marker illustrated is selected from the TagCircle21h7 set of AprilTags. The skilled person will nevertheless appreciate that other suitable fiduciary markers may he used instead. The TagCircle2lh7 set of AprilTags comprises a series of such two dimensional patterns each fitting within the dotted outline 409.

The vertices of the dotted outline 409 each lie on a circle with an origin at the centre of the fiduciary marker 400. Accordingly, in a lens image of a test pattern comprising the fiduciary marker 400, it would be possible to draw an ellipse around the marker 400. Deviations in the size and shape of the ellipse from a circle in the test pattern can therefore be used to determine lens properties, similarly to the prior art.

Turning to figure 9b, an individual fiduciary marker 400 is shown. The fiduciary marker is the same as the fiduciary marker of figure 9a, but is rotated by 22.5° from the base image of figure 9a. As illustrated, there arc insufficient vertices of the marker 400 corresponding to the dotted outline 409 to accurately identify multiple points and thereby draw an accurate ellipse 408 from a single image of the marker 400.

The present invention, as set out in figure 9d therefore involves capturing a series of lens images, wherein the fiduciary marker 400 is rotated between successive lens images. At step 1001, the sample lens 4, 5, 203 is positioned between display surface 13, 113, 213 and camera 28, 128, 228. At step 1002, a test pattern comprising the fiduciary marker 400, presented in an initial orientation is displayed on display surface 13, 113, 213. At step 1003, the camera 28, 128, 228 captures a lens image of the test pattern and the position of a rcfence point within the fiduciary marker 400, typically one of the exterior vertices, is determined. Steps 1002 and 1003 can then be repeated with the fiduciary marker 400 rotated to a different orientation in each successive lens image. At step 1004, an ellipse of best fit is derived, the ellipse of best fit joining the positions of the reference point of the fiduciary marker 400 in successive lens images. At step 1005, the characteristics of the ellipse arc analysed to determine at least one parameter of the sample lens 4, 5, 203. In particular, analysis of the ellipse both in terms of relative size and orientation of the axes can enable calculation of the spherical power, cylinder and axis of the lens at a location aligned with the fiduciary marker 400. Examples of such calculations are known to the skilled person and are discussed in detail in documents such as W02018/073577.

An example of a succession of lens images (45° rotation between images) overlaid on one another is shown in figure 9c. By selecting one particular vertex of the fiduciary marker 400 (in this example an external vertex coincident. with the dotted line) as a reference point, the position of the reference point can be readily and accurately identified in successive lens images. The location of the reference points in the successive images may then be used to derive an accurate ellipse of best fit 408.

Even under distortion, the location of an external vertex within a fiduciary marker 400 can he less ambiguously and more accurately determined than the centre of a dot. Accordingly, the present invention provides greater accuracy than the use of dots.

The method described in relation to figure 9 can use a single fiduciary marker 400. If so, the fiduciary marker can be aligned with a with a particular measurement. point of the lens such as the optical centre, frame centre, near reference point, far reference point, distance reference point, prism reference point or the like.

The position of a particular measurement point is illustrated schematically by loop 407 in figures 9h and 9c. It is possible to align a marker 400 with the particular measurement point for more accurate measurement of the lens properties at that location. Recentring is discussed further in relation to figure 11 below.

The skilled person will appreciate that rather than applying to a test pattern comprising a single fiduciary marker 400, the method of figure 9 may be applied to a test pattern comprising multiple fiduciary markers 400. As above, at step 1001, the sample lens 4, 5, 203 is positioned between display surface 13, 113, 213 and camera 28, 128, 228. In such embodiments, at step 1002, a test pattern comprising multiple fiduciary markers 400 is displayed, on display surface 13, 113, 213 each fiduciary marker 400 presented in an initial orientation. Then, at step 1003, the position of a reference point (typically one of the exterior vertices) within each fiduciary marker 400 in the captured lens image is determined. Steps 1002 and 1003 can then be repeated with each fiduciary marker 400 rotated to a different orientation in each successive lens image. At step 1004, an ellipse of best fit is derived for each fiduciary marker 400, each ellipse of best fit joining the positions of the reference point of the fiduciary marker 400 in successive lens images. At step 1005. the characteristics of the ellipses are analysed to determine at least one parameter of the sample lens 4, 5, 203. In particular, analysis of each ellipse both in terms of relative size and orientation of the axes can enable calculation of the spherical power, cylinder and axis of the sample lens 4, 5, 203 at locations aligned with each fiduciary marker 400.

In such instances, if the test pattern comprises a regular array of fiduciary markers, sample lens properties may be measured at locations across the whole sample lens 4, 5, 203 by capturing a series of lens images with each fiduciary marker 400 rotated between successive images Whilst the method can be applied in cases where each fiduciary marker 400 in the test pattern is the same, this is not optimal. This is because it can be difficult to associate the location of particular fiduciary markers 400 in the test pattern with the corresponding fiduciary marker 400 in the lens image. This is a more significant issue towards the edge of the sample lens 4, 5, 203 or where the sample lens 4, 5, 203 is of a higher power.

The above issue can he addressed by using a different fiduciary marker 400 for each position in the test pattern. Given that a complete set of fiduciary markers 400 has a finite size, this limits the number of fiduciary markers 400 available and hence the amount of lens locations where lens properties can be measured.

Turing now to figure 10, a further embodiment of the method of the present invention utilises a square array of fiduciary markers 400. In the example shown the fiduciary markers 400 in each row are each spaced apart by a distance which is equal to the spacing between adjacent rows, and the fiduciary markers 400 in each column are each spaced apart by a distance which is equal to the spacing between adjacent columns and the row spacing is equal to the column spacing. The skilled person will appreciate that alternative array formations may be utilised if desired.

Figure 10a shows a lens image 401 of an array 410 of fiduciary markers 400. In the array 410, each fiduciary marker 400 in a single row 411a-m is the same and fiduciary markers 400 in neighbouring rows 411a-m differ. Each column 412a-m thereby comprises a like sequence of differing fiduciary markers 400.

Figure 1 Oh shows a lens image 402 of an array 420 of fiduciary markers 400. In the array 420, In the like array 420 of figure 10b, each fiduciary marker 400 in a single column 422a-m is the same and fiduciary markers 400 in neighbouring columns 422a-m differ. Each row 421a-m thereby comprises a like sequence of differing fiduciary markers 400.

Figure 10c shows a lens image 403 overlaying lens images 401 and 402. As is illustrated in figure 10c, if lens images 401, 402 are overlaid, the location of each fiduciary marker 400 within the images can be uniquely referenced by the specific combination of markers in terms of row 411, column 422. For instance, highlighted location 431 can be identified by the fiduciary marker of row 41 If and the fiduciary marker of column 422g. In this manner, 13 separate fiduciary markers in the arrays 410, 420 can uniquely identify 169 positions in a test pattern.

As illustrated in figure 10d, to measure the properties of sample lens 4, 5, 203 at locations corresponding to the fiduciary markers 400, the sample lens 4, 5, 203 to be examined is positioned between display surface 13, 113, 213 and camera 28, 128, 228 at step 1011. At step 1012, a test pattern comprising the array 410 with each fiduciary marker 400 in an initial orientation is displayed on display surface 13, 113, 213. At step 1013, the camera 28, 128, 228 captures a lens image of the array 410 and the position of a refence point within each fiduciary marker 400 is determined. Steps 1012 and 1013 can then be repeated with each fiduciary marker 400 in array 410 rotated to a different orientation in each successive lens image.

At step 1022, a test pattern comprising the array 420 with each fiduciary marker in an initial orientation is displayed on display surface 13, 113, 213. At step 1023, the camera 28, 128, 228 captures a lens image of the array 420 and the position of a refence point within each fiduciary marker 400 is determined. Steps 1022 and 1023 can then be repeated with each fiduciary marker 400 in array 420 rotated to a different orientation in each successive lens image.

At step 1014, an ellipse of best fit 438 is derived for each fiduciary marker position 431, the ellipse of best fit joining the positions of the reference point of each fiduciary marker 400 in successive lens images of both array 410 and array 420. At step 1015, the characteristics of the ellipses are analysed to determine at least one parameter of the sample lens 4, 5, 203. In particular, analysis of each ellipse both in terms of relative size and orientation of the axes can enable calculation of the spherical power, cylinder and axis of the sample lens 4, 5, 203 at locations aligned with each fiduciary marker 400 in the arrays 410, 420.

The skilled person will appreciate that it is also possible to alternate test patterns made up of array 410 and array 420 rather than displaying a series of test patterns based on array 410 with each fiduciary marker 400 in different orientations followed by a series of test patterns based on array 420 with each fiduciary marker 400 in different.

orientations.

In some implementations, separate ellipses of best fit 418, 428 can he calculated for each of the fiduciary markers in each array 410, 420 rather than a combined ellipse of best fit 438.

Centring/Recentring In some examples, the method of the present invention may be carried out on a first series of captured lens images so as to calculate the position of a particular measurement points of the sample lens. The particular measurement points of the sample lens include but are not limited to optical centre, frame centre, near reference point, far reference point, distance reference point, prism reference point or the like.

For instance, in the example of figure 1 la, the optical centre of the sample lens has been determined to he at position 407.

In such examples, if it is determined that the particular measurement point 407 is not aligned with a fiduciary marker 400, the displacement between the particular measurement point 407 and the centre of a particular fiduciary marker (typically the closest fiduciary marker 400n), can be determined. The test pattern can then be displaced by the determined value so as to align the particular measurement point 407 with the centre of the particular fiduciary marker 400n, as is illustrated by the enlarged extract shown in figure 1 lb. The skilled person will of course appreciate that recentring may be carried out in response to the determination of a particular measurement point 440 by any other

suitable technique.

Calibration Regardless of which of the methods of analysis described above are used, it may he necessary to calibrate the system used to carry out the method in order to remove 30 distortions to the test pattern in the lens image captured by the camera produced by the system itself rather than by the sample lens. Such system distortions may arise, for example, in displaying the test pattern on the screen 13, 113 and in capturing an image of the test pattern using the camera 128.

Accordingly, the methods described above may include a calibration step in which an image of the original test pattern displayed on the screen 13, 113, 213 is captured by the camera 28, 128, 228 without a sample lens in place. This can he called a "system image". The test pattern in the system image is compared to the original test pattern by the computer to determine what distortions have been introduced by the system. The computer 12, 112, 212 using appropriate algorithms, calculates error coefficients and generates a transform algorithm which can he applied to the distorted test pattern in the system image data to transform it hack to the original, perfect test pattern. The transform algorithm is stored in the computer.

When a sample lens 4, 5, 208 is subsequently tested, the test pattern in the lens image captured by the camera 28, 128, 228 includes distortions introduced by the system and distortions introduced by the sample lens 4, 5, 208. The transform algorithm is applied to the test pattern in the lens image data to remove the system distortions. The resulting test pattern can he called a "transformed test pattern". Any remaining distortions in the transformed test pattern are the result of the lens only and the transformed test pattern can be compared with the original test pattern to determine the optical parameters of the lens in accordance with the methods described above.

Calibration may not be required where the system is of sufficient quality such that any system distortions are negligible and is less important for the spot. mode in the first method. Where calibration is required, it may only be necessary to derive a suitable transform algorithm once when the system is first commissioned and then to use the stored transform algorithm each time a lens is examined. However, for other systems it may be necessary to check the calibration and derive a transform algorithm periodically to ensure that further system errors have not been introduced over time. For portable systems such as the systems 1, 101 described above, it may he necessary to check the calibration and derive a transform algorithm each time the system is set up and/or transported. In some cases, it may be necessary to check the calibration and derive a transform algorithm each time a lens is tested or at least each time a new pair of glasses is to be tested.

Alternatively, calibration may be carried out by capturing using camera 28, 128, 228 a series of system images of the test pattern without a sample lens 4, 5, 208 in place, with each fiduciary marker rotated to a different orientation between successive system images. Subsequently, the method may involve deriving an ellipse of best fit joining the positions of the reference point of the or each fiduciary marker in the successive system images. For each ellipse of best fit, the major axis and the minor axis can be determined. The determined values may be stored as reference values and/or used to derive a transform algorithm. Typically, the above method would also include determining the orientation of the axes of each ellipse and/or any deviations from a regular ellipse shape.

The method according to the first embodiment can also he carried out using the first embodiment of the system 1 or indeed any other suitable lens examination system having a means to display the test pattern, a digital camera or other means for capturing an image of the test pattern through the lens, computing means to carry out the required image processing and computational analysis on the image data, and means such as a display screen to display the results. Further, it is not essential that the test pattern be displayed on a digital screen and it could be displayed in other ways such as on printed media. However, the use of a digital display screen, such as the screen 13, 113 of the systems 1, 101 described above, is advantageous as the test pattern can be rapidly and accurately changed or realigned. This is advantageous, for example, where the lens is held at a fixed distance above the screen as the size and/or spacing of the dots 374 in the test pattern 370 displayed can be varied to accommodate lenses have differing focal length. For example, in the system 101 where the plinth 156 is configured to hold the lens at a distance from the display screen 113 which is set to the focal length of a +20D lens, the system can handle lenses designed for myopia by adjusting the size and/or spacing of the dots in the test pattern without the need to adopt any particular physical measure or the use of additional lenses.

In embodiments where the lens distance between the sample lens 4, 5, 208 can be varied, the method described above may be performed with the sample lens 4, 5, 208 at a first lens distance from the display surface13, 113, 213 and repeated with the sample lens 4, 5, 208 at a second lens distance from the display surface, 13, 113, 213 different from the first lens distance. Performing the method at two different lens distances, differing by a distance Adl can determination of eth power of the sample lens 4, 5, 208 by either analysing the lens images captured at the different lens distances or by analysing the or each ellipse of best fit derived from the lens images captured at the different lens distances. Typically, this could be achieved by determining the magnitude of magnification MI of the test pattern or ellipse of best fit at the first lens distance and the magnitude of magnification M2 of the test pattern or ellipse of best fit at the second lens distance M2 and calculating the power P of the test lens from the magnification values MI, M, at the first and second lens distances and the change in lens distance Adl between the first and second lens distances. One way the sample lens power can be detemiined would be by use of the following equation or an equivalent: 1 M M P 1 f A dl Where MI and M, are the values of the magnification of the test pattern measured with the sample lens at the first and second lens distances respectively and Adl is the change in lens distance between the first and second lens distances. Examples of such methodology and calculations are known to the skilled person and are discussed in detail in documents such as W02021/170984.

The one or more embodiments are described above by way of example only.

Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

CLAIMS1. A method of examining a sample lens, the method comprising; a. positioning a sample lens between a display surface and a camera b. displaying on the display surface a test pattern comprising at least one fiduciary marker, presented in an initial orientation; c. using the camera to capture an image of the (usually distorted) test pattern as seen through the sample lens ("the lens image") and to determine the position of a refence point within the fiduciary marker; d. repeating steps b and c so as to capture a series of lens images wherein the fiduciary marker is rotated to a different orientation in each successive lens image e. deriving an ellipse of best fit joining the positions of the reference point of the fiduciary marker in successive lens images; f. analysing characteristics of the ellipse to determine the degree and nature of distortion to the test pattern produced by the sample lens and from this determining at least one parameter of the sample lens.
2. A method of examining a sample lens as claimed claim 1 wherein the fiduciary marker comprises a two-dimensional pattern and the reference point comprises a vertex within the two dimensional pattern.
3. A method of examining a sample lens as claimed in claim 1 or claim 2 wherein the fiduciary marker is rotated about a central point by a constant angle between successive lens imaaes.
4. A method of examining a sample lens as claimed in any preceding claim wherein the method comprises determining the major axis and the minor axis of the ellipse of hest fit and, optionally, the orientation of the major axis and the minor axis of the ellipse of best fit.
5. A method of examining a sample lens as claimed in claim 4 wherein the method comprises the step of determining the power of the sample lens from the determined major axis and minor axis of the ellipse of best fit and where the major and minor axes of the second ellipse are not equal, determining a degree of astigmatic correction (cylindrical power) of the lens and the axis angle of the astigmatic correction 7. A method of examining a sample lens as claimed in any preceding claim wherein the method is used to determine said at least one optical parameter of the lens at multiple locations within an area of interest of the lens, the method comprising: g. in step b above, displaying a test pattern comprising a plurality of fiduciary markers distributed over an area of the surface; h. in step a above, positioning the sample lens so that at least the area of interest of the lens is positioned between the displayed test pattern and the camera before capturing the lens image; and i. performing the analysis in steps e and f above in respect of the reference point of each of the fiduciary markers in the test pattern recorded in the lens image within the area of interest to determine said at least one optical parameter at various locations with the area of interest of the lens.8. A method of examining a sample lens as claimed in claim 7 wherein the test pattern comprises a square array of fiduciary markers.9. A method of examining a sample lens as claimed in claim 7 or claim 8 wherein the test pattern comprises multiple fiduciary markers, and each fiduciary marker displayed is a different fiduciary marker selected from the same fiduciary marker set.10. A method of examining a sample lens as claimed in claim 9 wherein the test pattern comprises multiple fiduciary markers, and each fiduciary marker is selected from a limited subset within a fiduciary marker set.11. A method of examining a sample lens as claimed in any one of claims 7 to 10 wherein each fiduciary marker in a single row (or column) is the same and fiduciary markers in neighbouring rows (or columns) differ.12. A method of examining a sample lens as claimed in claim 11 wherein the method involves the steps of carrying out the analysis of steps e and f above for a full rotation of the fiduciary markers in a first configuration comprising like fiduciary markers in each row (or each column) with markers differing in adjacent rows (or columns) and then repeating the analysis for a second configuration comprising like fiduciary markers in each colunm (or row) with markers differing in adjacent columns (or rows).13. A method of examining a sample lens as claimed in any preceding claim wherein the method comprises displaying the results of the examination graphically in the form of a map of the sample lens in which the at least one optical parameter is represented as a colour or contour.14. A method of examining a sample lens as claimed in any preceding claim wherein calibration of the system used to carry out the method includes the steps of: j. using the camera to capture an image of the test pattern displayed on the surface without a sample lens between the camera and the surface ("the system image"); k. comparing the test pattern in the system image with the original test pattern to determine the degree of distortion to the original test pattern produced by the system; 1. deriving a transform algorithm which applied to the test pattern in the system image will substantially restore it back to the original test pattern and saving the transform algorithm.15. A method of examining a sample lens as claimed in any preceding claim wherein calibration of the system used to carry out the method includes the steps of: m. using the camera to capture a series of images of the test pattern displayed on the display surface without a sample lens between the camera and the surface ("the system images"), each successive system image with the or each fiduciary marker in the test pattern rotated to a different orientation; n. deriving an ellipse of best fit joining the positions of the reference point of the or each fiduciary marker in successive system images; o. determining and storing the characteristics of the or each ellipse of best fit as reference values; or p. deriving and storing a transform algorithm which applied to the or each ellipse of best fit in the system image will substantially restore it back to expected ellipse characteristics calculated from the original test pattern.16. A method of examining a sample lens as claimed in claim 15 wherein the expected ellipse characteristics comprise circles centred on the or each fiduciary marker in the test pattern with a diameter equal to the maximum transverse extent of the or each fiduciary marker.17. A method of examining a sample lens as claimed in any one of claims 14 to 16 wherein the transform algorithm is applied to remove system distortion before carrying out the analysis in steps e and f above or the reference values are used in carrying out the analysis in steps c and f above.18. A method of examining a sample lens as claimed in any preceding claim wherein the method includes the step of aligning the test pattern with particular measurement points of the lens.19. A method of examining a sample lens as claimed in claim 18 wherein the alignment is achieved by identifying the particular measurement point of the lens as currently positioned and offsetting the displayed test pattern such that the centre of a fiduciary marker in the test pattern is aligned with the particular measurement point.20. A method of examining a sample lens as claimed in any preceding claim wherein the method is performed with the sample lens at a first lens distance from the display surface and repeated with the sample lens at a second lens distance from the display surface, different from the first lens distance, so as to determine the power of the sample lens by analysing the lens images captured at the different lens distances or by analysing the or each ellipse of best fit derived from the lens images captured at the different lens distances.21. A method of examining a sample lens as claimed in claim 20 wherein the method may comprise determining the magnitude of magnification MI of the test pattern or ellipse of best fit at the first lens distance and the magnitude of magnification M2 of the test pattern or ellipse of best fit at the second lens distance M2 and calculating the power P of the test lens from the magnification values MI, Isd2 at the first and second lens distances and the change in lens distance Adl between the first and second lens distances.22. A method of examining a sample lens as claimed in any preceding claim wherein the method is used to measure properties of a sample lens or sample lenses in a and wherein once the method is carried out on a sample lens in a pair of glasses, the method is carried out for the other sample lens of the pair of glasses.23. A system for carrying out the method according to any preceding claim, the system comprising a computing device having a planar display screen, a camera mounted above the screen with the axis of the camera perpendicular to the plane of the screen, the camera being operatively connected to the computing device for storing and processing image data captured by the camera; and a glasses mount for holding a pair of glasses with a lens located between the camera and the test pattern on the display screen in use.24. A system as claimed in claim 23 wherein the computing device is configured in use to display the test pattern in a first section of the display screen below the camera and to display the results of the examination in another section of the display screen.