WO2020157469A2

WO2020157469A2 - Methods and apparatus for determining prism in a lens

Info

Publication number: WO2020157469A2
Application number: PCT/GB2020/050163
Authority: WO
Inventors: Michal KRAWCZYNSKI; Joseph Edward DAVIES; Xiaoxi Chowshe ZHANG
Original assignee: Eyoto Group Limited
Priority date: 2019-01-31
Filing date: 2020-01-24
Publication date: 2020-08-06
Also published as: TW202030464A; WO2020157469A3; GB2580946A; GB201901353D0

Abstract

A method of determining prism in a lens comprises displaying a test pattern (25) comprising a plurality of discrete elements (26) arranged in a repeating pattern. A subject lens (22) is positioned between the display (12) and a digital camera (14) and an image of the lens affected test pattern as seen through the subject lens as captured. The lens affected test pattern is compared with the original test pattern to determine changes in position of the elements at discrete points of the lens and the magnitude of prism at said discrete points calculated from this data. In order that the discrete elements in the lens affected test image can be correctly identified, each of the elements in the test pattern has a uniquely identifiable visual characteristic or there may be at least one reference feature in the test pattern which has a uniquely identifiable visual characteristic. The uniquely identifiable visual characteristic may be colour and/or intensity.

Description

Methods and Apparatus for Determining Prism in a Lens

Technical Field of the Invention

The present invention relates to methods and apparatus for determining prism in a lens, especially an ophthalmic lens. The present invention relates particularly, but not exclusively, to methods and apparatus for mapping prism across an ophthalmic lens.

Background to the Invention

Automatic lensmeters are known which are able to automatically determine characteristics of a glasses lens such as the power of the lens. In one known arrangement, a test pattern is displayed on a digital screen, an ophthalmic lens is positioned between the screen and a digital camera and an image of the test pattern as seen through the lens is captured by the camera in a“lens image”. The test pattern will usually be distorted by the lens, unless the lens is plain, and by comparing the lens affected test pattern captured in the lens image with the original test pattern it is possible to determine the magnitude of magnification produced by the lens. Having determined the magnification (M), the power (P) of the lens is calculated using a function f(M)=P, which function will include the distance of the lens to the display screen (the object distance). The image processing and analysis is usually carried out by a computing means running appropriate software and the system is calibrated to take the effects of the camera and other parts of the apparatus into account.

In WO 2018/073577A2, we described a method of determining the power of a lens using a test pattern comprising a set of dots arranged so that they can be joined by a first ellipse of best fit. When viewed though a lens, the size and spacing between the dots will change depending on the magnification and by analysing these changes the magnitude of magnification and hence the power of the lens can be determined. Conveniently, the change in spacing between the dots in the set is analysed by producing a second ellipse of best fit for the set of dots in the lens image and comparing the major and minor axes of the second ellipse of best fit with those of the first. This test pattern can also be used to determine whether the lens includes astigmatic correction (cylindrical power) and, if so, the axis angle of the astigmatic correction. In the original test pattern, the dots are arranged on a circle so that in the first ellipse of best fit the major and the minor axes are the same. If the lens is cylindrical, the relative positions of the dots will change when viewed through the lens so that the major and minor axes of the second ellipse of best fit will not be the same. By analysing differences in the major and minor axes of the first and second ellipses of best fit, the cylindrical power and the axis angle of any astigmatic correction can be determined as well as the magnitude of magnification. In one embodiment, the test pattern comprises an array of dots arranged to define multiple overlapping sets of dots, each of which sets can be joined by an ellipse of best fit as described above. Using this test pattern, a lens can be analysed to determine its power and any astigmatic correction at a large number of positions across the lens from a single lens image and the results presented in the form of a contour map of power and/or astigmatic correction across the lens.

Another optical characteristic which may be present in an ophthalmic lens is prism. Prism is prescribed to patients with binocular vision problems such strabismus (lazy eye), heterophoria or diplopia (double vision) which can cause eye strain and headaches. The prism serves to move the image seen by one or both eyes, improving fusion, and relieving symptoms of binocular vision problems.

Prismatic power/correction is commonly specified in prism dioptres D. A prism dioptre being a unit of measurement used to express the angle of deviation of a ray of light by a prism or lens. Prism power, in these units, is measured as the displacement of the ray, in centimeters, perpendicular to its line of incidence at a distance of one meter, where 1 D is equivalent to 1 cm displacement per meter distance.

Prism can be applied to ophthalmic lenses by manufacturing (e.g. grinding) a physical prism structure or‘wedge’ on the back surface of the lens or by decentring the Optical Centre (OC) of a lens from the centre of a user’s pupil so that prism is induced as described by Prentice’s rule without manufacturing a prism structure into the lens.

Induced prism can be determined by the following equation derived from Prentice’s rule:

P = cF Equation 1 where:

P is the amount of prism correction (in prism dioptres) c is decentration (the distance between the pupil center and the OC, in centimeters)

F is lens power (in diopters)

When evaluating the prescription of ophthalmic lenses, measurements of the different optical characteristics are taken at particular reference points on the lens dependant on the type of lens. Prism is measured at the Prism Reference Point (PRP), a point on the lens where the lens manufacturer has indicated prism should be measured. For spherical power and sphero-cylinder single vision lenses and multifocal lenses, the PRP is assumed to be coincident with the Distance Reference Point (DRP), a point on the lens where the manufacturer indicates to measure the distance sphere power, cylinder power and axis.

In spherical power and sphero-cylindrical single vision lenses the DRP is coincident with the OC of the lens, which by definition has no induced prismatic component. Where a prism is manufactured into the lens, the manufactured component of prism will be a constant prism over the whole lens. However, on one half the lens it adds to the induced prism and on the other side of the OC it subtracts from the induced prism. Accordingly, at the OC the manufactured prism is present but either side of it the prism changes due to the combined effects of the spherical and manufactured prism components. Thus for a spherical power and sphero-cylindrical single vision lens with manufactured prism, the OC is the point in the lens where the prism does not change and the prism at the OC is indicative of the manufactured prism since there is no spherical component.

Where prism is induced by offsetting the OC relative to the patient’s pupillary centre, lens manufacturers use a Layout Reference Point (LRP) to indicate the decentring of the OC from the wearer’s pupil; however, current automatic lensmeters cannot detect this point without knowledge of the magnitude of the decentration.

For progressive, aspheric and finished multifocal lenses it is expected that the manufacturer should indicate a PRP point (rather than it being coincident with other reference points as in spherical power single vision lenses). For example, in progressive lenses, the PRP is located at the midpoint between two semi-visible engraved markings produced on the lens and which are typically separated by 34 mm. This is in turn is referenced back to the LRP which is coincident with the Fitting Cross (FC) in progressive lenses. The LRP itself is then referenced back to the OC or Geometric Centre (GC) of the lens.

The need to measure optical characteristics of a lens at specific reference points presents difficulties as known automatic lensmeters are not able to detect all reference points automatically. Accordingly, in some instances it is necessary to mark the lens to identify the relevant reference points. This adds to the complexity of the process and may result in error if incorrectly marked. In spherical power and sphero-cylindrical single vision lenses the power, cylinder and axes measurements are taken at the OC, which as noted above is a point in the lens having no prism or no prism change. Accordingly, the OC in such lenses can be detected by identifying the point of no prism or no prism change. However, current methods for measuring prism in an ophthalmic lens using an automatic lensmeter are only able to determine prism at a single location at a time. Detecting the OC using the known methods requires a series of iterative prism measurements and manipulation of the position of the lens between each measurement in order to ultimately align the measurement axis with the OC. There is currently no method and apparatus for detecting the OC of the lens, for the purpose of evaluation, which does not involve manipulation of the lens position.

The amount of prism present in a lens is generally determined by measuring the apparent shift in an object seen through the lens. This requires knowledge of the position of the object before and after the lens is introduced. Determining prism at multiple locations across a lens is complex as it involves uniquely identifying multiple points of reference in a test pattern before and after a lens under test (the subject lens) is introduced. The test patterns currently used in known automatic lensmeters for determining power and other optical characteristics across a lens typically comprise a plurality of identical dots arranged in a matrix of rows and columns. However, since a lens with prism will magnify and distort the test pattern, known automatic lensmeters are unable to determine with certainty how the dots in the lens affected pattern captured in the lens image correlate with the dots in the original test pattern. This in turn means that known lensmeters are unable to reliably determine the change in position of the dots caused by the lens, which data is required in order to calculate the magnitude and direction of prism.

Aspects of the invention attempt to overcome, or at least mitigate the drawbacks of the prior art.

It is an object of the invention to provide an alternative method for determining prism in a lens which overcomes, or at least mitigates, some or all or the drawbacks of the known methods.

It is an object of the invention to provide an alternative method for determining prism in a lens which can adopted in an automatic lensmeter.

It is a further object to provide an alternative method for determining prism in a lens which is capable of producing a map of prism across a lens without having to manipulate the lens during the prism measurement process.

It is a still further object to provide an alternative method for determining prism in a lens which is capable of producing a map of prism across a lens and which, in the case of spherical and sphero-cylinder single vision lenses, can be used to determine the OC of the lens.

It is also an object of the invention to provide an alternative apparatus, especially an automatic lensmeter, capable of determining prism in a lens.

Summary of the Invention

In accordance with a first aspect of the invention, there is provided a method of determining prism in a lens, the method comprising:

a) displaying a test pattern on a planar display surface, the test pattern comprising a plurality of discrete elements arranged in a repeating pattern, in which either:

(i) all of the elements in the pattern have a characteristic which is uniquely identifiable visually; or

(ii) the elements can each be referenced to at least one feature of the pattern which has a characteristic that is uniquely identifiable visually; b) positioning a subject lens between the display surface and a digital camera, wherein the distance of the lens from the display surface is either known or can be determined, and using a digital camera to capture an image of the test pattern as seen through the subject lens in a lens image; c) comparing the“lens affected” test pattern captured in the lens image with the original test pattern to determine changes in position of the elements at discreet points of the lens and calculating the magnitude of prism at said discreet points from this data.

The method may comprise calculating the magnitude of prism at said discreet points from said data and the distance of the lens form the display surface when the lens image is captured.

The method may comprise interpolating between the values for magnitude of prism at said discrete points and generating a map of prism across at least an area of interest of the lens. The map may be a 2-D map. The map may be a contour map and may be a coloured contour map. The map may be a 3-D map.

In an embodiment, the elements in the pattern are each rendered uniquely identifiable visually by each having a unique colour and/or intensity.

In embodiments where the elements can each be referenced to at least one feature of the pattern which has a characteristic which is uniquely identifiable visually, said at least one feature may be rendered uniquely identifiable visually by being a different colour and/or intensity from the other elements in the pattern. Said at least one feature may comprise at least one element of the repeating pattern.

In an embodiment, the test pattern comprises a plurality of discreet elements arranged in an array of rows and columns and said at least one feature comprises a row and/or column of said elements, wherein each of the elements in said row and/or column are of a colour and/or intensity different from the other elements in the pattern.

In an alternative embodiment, the test pattern comprises alternating light and dark stripes, and wherein said at least one feature comprises one of said stripes being of a different colour and/or intensity from all the other stripes. Said one of said stripes may be located substantially centrally of the lens. The dark stripes may be black and said one of said stripes may be grey. In this embodiment, steps b and c may comprise: d) displaying a first test pattern in which the stripes are aligned in a first orientation and capturing an image of the first test pattern as seen through the subject lens in a first lens image;

e) displaying a second test pattern in which the stripes are aligned in a second orientation different from the first test pattern and capturing an image of the second test pattern as seen through the subject lens in a second lens image; f) comparing the lens affected first test pattern captured in the first lens image with the original first test pattern to determine changes in position of the stripes along a first axis orthogonal to the orientation of the stripes in the original first test pattern at discrete points across the lens;

g) comparing the lens affected second test pattern captured in the second lens image with the original second test pattern to determine changes in position of the stripes along a second axis orthogonal to the orientation of the stripes in the original second test pattern at discrete points across the lens;

h) converting the changes in position of the stripes determined in steps f and g into components of prism along said first and second axes and calculating the magnitude of prism from said components at said discrete points.

The stripes in the second test pattern may be aligned orthogonal to the stripes in the first test pattern. Changes in position of the stripes may be used to determine the direction of prism in addition to magnitude at said discreet points.

The method may comprise determining a direction of change of position of the elements in the pattern relative to a defined co-ordinate system. In which case, the method may comprise taking the lens image with the lens positioned at a known orientation relative to the defined co-ordinate system. The defined co-ordinate system may be defined relative to the orientation of the digital camera.

In embodiments, comparing the lens affected test pattern captured in the lens image with the original test pattern to determine changes in position of the elements comprises comparing the lens affected test pattern in the lens image with a reference image of the original test pattern displayed on the display surface, which reference image is captured by the camera without a subject lens between the camera and the display surface. The method may comprise processing the lens image and the reference image to extract the relative positions of the test pattern elements in the lens image and the reference image.

The method may comprise correlating individual elements in the lens affected test pattern with those in the original test pattern using the uniquely identifiable visible characteristic of each element.

In embodiments where the elements can each be referenced to at least one feature of the pattern which has a characteristic which is uniquely identifiable visually, the method comprises identifying said at least one feature in the lens image and the original test pattern and correlating individual elements in the lens affected test pattern with those in the original test pattern by reference to said one feature.

In embodiments where the test pattern comprises alternating light and dark stripes, the method may comprise: i) comparing the lens affected first test pattern in the first lens image with a first reference image of the first test pattern displayed on the display surface, the first reference image being captured by the camera without a subject lens between the camera and the display surface; and

j) comparing the lens affected second test pattern in the second lens image with a second reference image of the second test pattern displayed on the display surface, the second reference image being captured by the camera without a subject lens between the camera and the display surface.

Where the method comprises generating a map of prism across at least an area of interest of the lens, the method may comprise interrogating the map to determine the point of minimum prism shift or point of no prism change of the lens.

The display surface may be an electronic display screen and the method may be carried out using apparatus comprising the electronic display screen, the digital camera with its optical axis aligned perpendicular to the display screen, a mount for holding a subject lens between the display screen and the camera lens, and a computing device operatively connected with the display screen and the digital camera, the computing device configured and programmed to control the electronic display screen and the digital camera and to carry out the processing steps on images captured by the digital camera in order to carry out the method as set out above.

In accordance with a second aspect of the invention, there is provided apparatus for carrying out the method according to the first aspect of the invention, the apparatus comprising a digital display screen, a digital camera having its optical axis aligned perpendicular to the display screen, a mount for holding a subject lens between the display screen and the camera, and a computing device operatively connected with the display screen and the digital camera, the computing device configured and programmed to control the digital display screen and the digital camera and to carry out processing steps on the images captured by the digital camera in order to carry out the method according to the first aspect of the invention. The apparatus may be an automatic lensmeter.

A third aspect of the invention comprises use of a lensmeter to carry out the method according to the first aspect of the invention.

Detailed Description of the Invention

In order that the invention in its various aspects may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic view from the side of a first embodiment of apparatus for carrying out a method of determining prism in a lens in accordance with an aspect of the invention.

Figure 2 shows a first embodiment of a test pattern for use in the method of determining prism in a lens in accordance with an aspect of the invention.

Figure 3 shows a second embodiment of a test pattern for use in the method of determining prism in a lens in accordance with an aspect of the invention.

Figure 4 shows a third embodiment of a test pattern for use in the method of determining prism in a lens in accordance with an aspect of the invention.

Figure 5 shows the test pattern of Figure 4 in a different orientation. Figures 6 and 7 are examples of lens images captured as part of the method of determining prism in a lens in accordance with an aspect of the invention illustrating how the test pattern in Figure 4 is affected by two different lenses.

Figure 8 is an example of a display used to present the results of the method of determining prism in a lens in accordance with an aspect of the invention to a user, the display including a contour map of prism across an area of interest of a lens.

Figure 9 is a view from the front of a second embodiment of apparatus for carrying out a method of determining prism in a lens in accordance with an aspect of the invention, with outer casing elements of the apparatus removed so show the internal detail.

Figure 10 is a cross-sectional view through the apparatus of Figure 9 taken on line A-A.

Embodiments of methods and apparatus for determining prism in an ophthalmic lens in accordance with aspects of the invention will now be described. The methods and apparatus can be used to determine prism of individual ophthalmic lenses which may or may not be mounted in a glasses frame. The methods and apparatus can be used to determine prism of a lens blank or partially finished lens. For the sake of clarity, the method and apparatus will be described and claimed with reference to use in determining prism in a lens but it should be understood that the term“lens” used in the description and claims encompasses a lens blank or a partially finished lens, unless the context requires otherwise.

Figure 1 illustrates schematically apparatus 10 in the form of an automatic lensmeter which can be configured to determine prism in an individual ophthalmic lens in accordance with the present invention. The apparatus 10 comprises a planer electronic digital display screen 12 and a digital camera 14. The display screen 12 and digital camera 14 are mounted to a supporting frame 16 so that the optical axis W of the camera lens 18 extends perpendicular to the plane of the display screen. The apparatus 10 has a mount 20 for holding a subject lens 22 between the display screen and the camera lens 18. The apparatus 10 has a computing device 24, including memory and processing means, which is conveniently located within a housing forming part of the supporting frame 16 but could be located elsewhere and could be remote from the screen, camera and supporting frame 16. The computing device 24 is operatively connected with the display screen 12 and the digital camera 14 and is programmed and configured to generate and display test patterns on the display screen 12 and to capture images of the displayed test patterns using the digital camera. The computing device is also configured to process the captured images according to the methodology described below.

The display screen 12 in this embodiment is a high-definition (4k plus) LCD panel whilst the digital camera 14 has a CMOS image sensor and a telecentric lens 18. However, other types of electronic display screen and digital imaging technology can be adopted.

A method of determining prism in a lens using the apparatus 10 will now be described.

With a subject lens 22 located at a test position between the display screen 12 and the camera 14, a test pattern comprising a plurality of elements arranged in a repeating pattern is displayed on the display screen and an image of the test pattern as seen through the lens is captured using the camera 14 in a“lens image”. The lens will usually distort the test pattern, unless it is a plain lens, and so the test pattern captured in the lens image will be referred to as a lens affected test pattern. It should though be appreciated that in the event the subject lens 22 is plain, the lens affected test pattern may be substantially the same as the original test pattern.

By comparing the positions of the elements in the lens affected test pattern with their positions in the original test pattern, it is possible to determine the change in position of the elements caused by the lens. Data relating to the change in position of the elements caused by the lens can then be used to determine the degree of prism, provided the distance of the lens 22 from the display screen 12 (the object distance) is known, from the following equation:

P = lOOtancr Equation 2 Where:

P is prism power in prism dioptres

a is the angle constructed by the shift in pattern and the object distance

The object distance may be pre-set so that this is known or it may be detected using a sensor arrangement. Alternatively, the object distance can be calculated if the power of the lens and the magnitude of magnification provided by the lens at the test position are known from the equation:

Equation 3

Where:

M is the magnification

f is the focal length of the lens

do is the object distance

Conveniently, the apparatus 10 is configured to determine the power of the lens and the magnitude of magnification at the test position using known methods, for example as disclosed in WO 2018/073577A2, and to calculate the object distance at the test position.

In order to be able to compare the positions of the pattern elements in the lens affected test pattern with their positions in the original test pattern, an image of the original test pattern as displayed on the screen is captured by the camera without a subject lens between the camera and the display screen in a“reference image”. A reference image may be captured every time the apparatus is used or periodically as part of an initialisation procedure carried out say once a day, once a week, once a month or whenever required. Alternatively, an image of the original test pattern or data relating thereto may be saved either in the computing device itself or at some remote location which the computing device is able to communicate. Since the subject lens 22 will magnify and distort the test pattern, a difficulty in comparing the positions of the test pattern elements in the lens affected test pattern with their positions in the original test pattern is being able to correctly correlate individual elements in the lens affected and original test patterns. That is to say, it is difficult to determining accurately which of the test elements in the original test pattern are which in the distorted test pattern in the lens image. To overcome this problem, the test pattern may comprise elements which are all uniquely identifiable visually so that each element in the original test pattern can be correctly identified in the lens affected test pattern. Each element of the test pattern then will have a uniquely identifiable visual characteristic. This could be the size and shape of the element. However, in a particularly advantageous embodiment, each element of the test pattern has a unique colour and/or intensity. Using colour and/or intensity to uniquely identify each element is advantageous as these characteristics are not significantly distorted by the types of ophthalmic lens which are expected to be examined using the method and apparatus of the invention. In contrast, the shape and size of the elements may be distorted by the lens making correct identification of each element more problematic. Figure 2 illustrates part of a test pattern 25 comprising a plurality of elements in the form of dots 26 arranged in an array of rows and columns against a black background. Each of the dots is a different colour from all the other dots so that each dot can be uniquely identified visually when analysing the test pattern in the lens image and reference image. In figure 2, the different colours of the dots are schematically illustrated by the use of different shading patterns. The elements 26 in the pattern could be provided against any suitable background so long as they are visually identifiable.

In an alternative embodiment, the test pattern has at least one feature which has a uniquely identifiable visual characteristic and which can be used as a reference from which all the elements in the pattern can be identified. In one embodiment the at least one feature is provided by one or more of the elements in the test pattern which have a visually identifiable characteristic different from the other elements in the test pattern. Again, any visually distinguishing characteristic can be used such as shape and or size. However, use of colour and/or intensity have been found to be particularly beneficial for the reasons discussed above. Figure 3 illustrates part of a further test pattern 28 comprising a plurality of elements in the form of dots 30, 30’ arranged in an array of rows and columns against a black background. In this test pattern, the dots 30’ in one a row and one column of the array are grey whilst the remaining dots 30 are white. The apparatus 10 is able to identify the grey dots 30’ in the lens affected test pattern captured in the lens image and from this correctly identify each of the individual dots 30, 30’ and correlate them with the dots in the original test pattern. In use, the row and column of grey dots are typically aligned substantially centrally of the subject lens 22 when the lens image is captured. As with the test pattern of figure 2, the elements in the pattern of figure 3 can be set against any suitable colour background and the colours of the elements can be selected as desired, provided the elements 30’ in the central column and row are visually distinct from the other elements 30 in the pattern.

Figure 4 illustrates part of a further test pattern 32 comprising a plurality of alternating dark and light stripes 34, 36. The majority of the dark stripes 34 are black but one 34’ is grey so that it is uniquely identifiable visually and can serve as a reference from which the remaining stripes can be individually identified. The light stripes 36 will typically be white but can be any suitable colour so long as they are distinguishable from the dark stripes 34 and the reference stripe 34’. The reference stripe 34’ can be any suitable colour so long as it is visually distinguishable from the light and dark stripes and is conveniently located substantially at the centre of the pattern 32 so as to be located substantially centrally of the subject lens when a lens image is taken

Using a test pattern 32 comprising stripes, it is not possible to extract from a single lens image data relating to changes in position of the stipes in a direction parallel to the orientation of stripes. To address this issue, two lens images are taken of the test pattern as seen through the lens with the stripes arranged in different orientations in each image. Accordingly, in this embodiment a first test pattern 32 is displayed in which the stripes 34, 34’, 36 are aligned in a first orientation and an image of the first test pattern as seen through the subject lens 22 is captured by the camera in a first lens image. After the first lens image 36 is captured, a second test pattern 32’ is displayed in which the stripes are aligned in a different orientation than in the first test pattern, as illustrated in Figure 5, and an image of the second test pattern 32’ as seen through the subject lens 22 is then captured in a second lens image. Other than the orientation of the stripes, the first and second test patterns 32, 32’ are identical and the apparatus remains otherwise unchanged and the subject lens is not moved between capturing the first and second lens images.

Figures 6 and 7 are examples of lens images 38, 40 of the first test pattern 32 of Figure 4 as seen through a positive 6 dioptre lens and a progressive 7 to 5 dioptre lens respectively, showing how the stripes are distorted by the lens to produce a lens affected test pattern 32a, 32b.

The first lens image 38, 40 and a reference image of the first test pattern 32 captured by the camera with no lens present (a first reference image) are processed to extract from the images data relating to changes in position of the stripes along a first axis orthogonal to the orientation of the stripes in the first test pattern 32. Similarly, the second lens image and a reference image of the second test pattern captured by the camera with no lens present (a second reference image) are processed to extract from the images data relating to changes in position of the stripes along a second axis orthogonal to the orientation of the stripes in the second test pattern 32’.

Data relating to changes in position of the stripes along the first and second axes is converted into components of prism along said first and second axes and from this the magnitude and direction of the prism in the lens is calculated, knowing the object distance of the lens to the display screen. A simple thresholding and edge detection algorithm can be used to extract the pixel positions of the edges of the stripes along the fore mentioned axes for prism calculation purposes. The stripes 34, 34’, 36 in the second test pattern 32’ can be configured orthogonally relative to the stripes in the first test pattern 32 as shown in Figures 4 and 5. In this case, the components of prism can be regarded as x and y components of prism or horizontal and vertical components. However, the stripes need not be oriented orthogonal to one another in the first and second test patterns as long as the change in orientation is sufficient to be able to reliably derive components of prism along two different axes (directions).

When using the test patterns in Figures 2 and 3 where the elements are arranged in an array of rows and columns, data regarding changes in position of the spots along two different axes (e.g. x and y axes) can be derived from a single lens image and a single reference image.

Typically, the lens and reference images are processed to determine the change of position of the elements at a number of discrete points across the lens to reduce processing requirements. This might involve determining the change of position of the elements at a number of discrete points spaced apart by a given number of pixels in the display screen 12. For example, data relating to the change in position of the elements might be obtained at points say every nine pixels along each row and column of the screen. Furthermore, analysis will usually be limited to those parts of the test pattern in the lens image covered by the lens. This might require processing the lens image to detect the edge of the lens or using a mask to limit the analysis to a desired area of interest in the lens image. This also reduces the amount of processing required.

The prism data derived at the various discreet points is interpolated to provide prism data across the whole of the lens, or at least over an area of interest of the lens, and to generate a map of prism across the lens or area of interest. The map will typically be a 2-D map but a 3-D map could be used. The data relating to prism across the lens or area of interest can be provided to a user in any suitable form and may be displayed on a screen, which could be same screen 12 as is used to display the test patterns or another screen. Figure 8 illustrates a display which can be generated using apparatus in accordance with the invention in which a contour map 42 for part of an area 44 of a subject lens 22 is displayed in the right-hand side. Conveniently, the magnitude of prism is indicated by colour against a scale 46. This is particularly advantageous for progressive lenses which have varying degrees of prism across the lens. In figure 8, the different colours in the contour map are illustrated schematically in greyscale but any suitable colours can be used to provide a map which is easy to read.

The prism map 42 generated according to the method can be used to identify the point of minimum prism and/or no prismatic change 48 of the lens. For spherical or sphero-cylinder single vision lenses, the point of minimum prism and/or no prismatic change of the lens 48 is coincident with the OC and the DRP, which is the position in the lens at which the power, cylinder and axis are determined. Accordingly, use of the method in an automatic lensmeter capable of determining power enables the lensmeter to detect the OC/DRP 48 in a spherical or sphero-cylinder single vison lens from the prism map and to determine the power, cylinder and axis from that point (DRP) using the known power measurement methods. This avoids the need to carry out an iterative series of prism measurements and/or to physically move the lens to align the measurement axis with the OC.

In the method according to an aspect of the invention, the direction of change in position of the elements can be related to a defined co-ordinate system, which may be aligned with the orientation of the digital camera and/or the display screen. For use in determining prism in a lens 22 in a glasses frame, the lens may be positioned in a known orientation relative to the defined co-ordinate system so that the values of magnitude and direction for the lens prism can be related to specific positions on the lens and to the orientation of the lens. However, for use in determining prism in a lens blank or unfinished lens which is not marked to indicate its intended orientation when fitted in a glasses frame, it may not be possible to relate the direction of prism to the final lens. Nevertheless, the prism data and prism map obtained using the method and apparatus of the invention can provide useful information. For example, the prism map can be interrogated to identify the point of minim prism or no prism change 48 as illustrated in Figure 8 which, as discussed, in a spherical or sphero-cylinder single vison lens blank will be coincident with the PRP, the DRP and the OC. Where the lens blank has a constant amount of prism ground into it, the prism map can be interrogated to ensure that the amount of prism at the point of minimal prism, or no prismatic change, is correct, within acceptable limits.

Figures 9 and 10 illustrate an alternative embodiment of apparatus 110 which can be used to carry out the method. The apparatus 110 is similar to that of the previous embodiment. Features of the apparatus 110 in accordance with the second embodiment which are the same as, or which perform the same function as, features of the first embodiment are given the same reference numeral but increased by 100.

The apparatus 110 in this embodiment comprises a supporting structure 150. A digital camera 114 is mounted in a lower region of the supporting structure. The camera 114 has a lens 152, whose optical axis W is aligned vertically upwards. A high definition display screen 112 for displaying test patterns is mounted to the supporting structure in an upper region above the camera lens 152. The display surface of the screen 112 faces the camera lens and is aligned horizontally, perpendicular to the optical axis W of the camera lens. The camera and the display screen are configured so that the optical axis W of the camera lens is aligned substantially at the centre of the display screen 112.

The display screen 112 in this embodiment is a high-definition (4k plus) LCD panel whilst the digital camera 114 has a CMOS image sensor and a telecentric lens 152. However, other types of electronic display screen and digital imaging technology can be adopted.

A subject lens carriage 154 is located between the camera lens 152 and the display screen for holding a subject lens 122 in an appropriate orientation for measuring prism using the method of the invention. The lens carriage 154 includes a female cartridge 156 mounted to a stage 158 and a male cartridge 160 removably engageable in the female cartridge. The male cartridge 160 includes a mounting arrangement for a subject lens 122. In use, the male cartridge can be fully or partially removed from the female cartridge to allow lenses 122 to be mounted and removed, the male cartridge being re-inserted in the female cartridge when a subject lens is mounted ready for use. The lens carriagel54 is configured to hold a subject lens 122 between the camera lens and the display screen 112 with the centre of the lens generally aligned with the optical axis W of the camera lens. The male and female cartridges 160, 156 have apertures arranged so that a test pattern displayed on the screen 112 can be seen through the subject lens 122 by the camera.

The stage 158 is mounted to the supporting structure via a drive arrangement 164 which is operative to move the lens carriage 154 vertically relative to the supporting structure so that the object distance between a subject lens 122 mounted in the carriage and the display screen 112 can be adjusted. The drive arrangement 164 includes a vertically aligned threaded shaft 166 driven by a stepper motor 168, both of which are supported on the supporting structure. The stage 158 is mounted to the shaft 166 by means of a drive nut 170 such that rotation of the shaft 166 by the motor 168 causes the nut 170 and the stage 158 to move linearly in a vertical direction along the shaft. The apparatus 110 includes an electronic control system (not shown) including a computing device having memory and processing means. The computing device is operatively connected with the display screen 112 and the digital camera 114 and is programmed and configured to generate and display test patterns on the display screen 112 and to capture images of the displayed test patterns using the digital camera 114. The computing device is also configured to process the captured images according to the methodology described above and to control operation of the stepper motor 168 in order to vary the object distance between the subject lens 122 and the test pattern display screen 112 as required.

Whilst not shown in the drawings, the apparatus 110 has an outer casing mounted to the supporting structure to enclose the internal components. The outer casing includes an access panel or door which is openable to allow access to the male cartridge 160 to enable a lens to be mounted in the device for testing and subsequently removed. The apparatus also has a second display screen which is visible externally for displaying information to a user and a user interface. The second display screen is operatively connected with the computing device and used to display information which may include instructions and/or results of prism measurement. The second display screen can also be used to enable a use to provide inputs to the apparatus and could be a touch screen. The user interface could include a key pad or other user input device.

The apparatus 110 in accordance with the second embodiment is otherwise arranged and configured to carry out the method of determining prism in a subject lens 122 as described above in relation to the first embodiment.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims and statements of invention.

Claims

1. A method of determining prism in a lens, the method comprising:

(ii) the elements can each be referenced to at least one feature of the pattern which has a characteristic that is uniquely identifiable visually;

b) positioning a subject lens between the display surface and a digital camera, wherein the distance of the lens from the display surface is either known or can be determined, and using a digital camera to capture an image of the test pattern as seen through the subject lens in a lens image;

c) comparing the lens affected test pattern captured in the lens image with the original test pattern to determine changes in position of the elements at discrete points of the lens and calculating the magnitude of prism at said discrete points from this data.

2. A method as claimed in claim 1, the method further comprising interpolating between the values for magnitude of prism at said discrete points and generating a map of prism across at least an area of interest of the lens.

3. A method as claimed in claim 1 or claim 2, wherein each of the elements in the pattern are rendered uniquely identifiable visually by having a unique colour and/or intensity.

4. A method as claimed in claim 1 or claim 2, wherein the elements can each be referenced to at least one feature of the pattern which has a characteristic which is uniquely identifiable visually, said at least one feature being rendered uniquely identifiable visually by being a different colour and/or intensity from the other elements in the pattern.

5. A method as claimed in claim 4, wherein said at least one feature comprises at least one element of the repeating pattern.

6 A method as claimed in claim 4 or claim 5, wherein the test pattern comprises a plurality of discrete elements arranged in an array of rows and columns and said at least one feature comprises a row and/or column of said elements, wherein each of the elements in said row and/or column are of a colour and/or intensity different from the other elements in the pattern.

7. A method as claimed in claim 4 or claim 5, wherein the test pattern comprises alternating light and dark stripes, and wherein said at least one feature comprises one of said stripes being of a different colour and/or intensity from all the other stripes.

8 A method as claimed in claim 7, wherein said one of said stripes is located substantially centrally of the lens.

9. A method as claimed in claim 7 or claim 8, wherein the dark stripes are black and wherein said one of said stripes is grey.

10 A method as claimed in any one of claims 7 to 9, wherein steps b and c comprise: d) displaying a first test pattern in which the stripes are aligned in a first orientation and capturing an image of the first test pattern as seen through the subject lens in a first lens image;

e) displaying a second test pattern in which the stripes are aligned in a second orientation different from the first and capturing an image of the second test pattern as seen through the subject lens in a second lens image;

f) comparing the lens affected first test pattern captured in the first lens image with the original first test pattern to determine changes in position of the stripes along a first axis orthogonal to the orientation of the stripes in the original first test pattern at discrete points across the lens;

11 A method as claimed in claim 10, wherein the stripes in the second test pattern are aligned orthogonal to the stripes in the first test pattern.

12. A method as claimed in claim 10 or claim 11, wherein the changes in position of the stripes are used to determine the direction of prism in addition to the magnitude at said discreet points.

13. A method as claimed in any one of the preceding claims, wherein the method comprises determining a direction of change of position of the elements in the pattern relative to a defined co-ordinate system.

14. A method as claimed in claim 13, wherein the method comprises taking the, or each, lens image with the lens positioned at a known orientation relative to the defined co-ordinate system.

15. A method as claimed in claim 13 or claim 14, wherein the co-ordinate system is defined relative to the orientation of the digital camera.

16. A method as claimed in any one of the previous claims, wherein comparing the lens affected test pattern captured in the lens image with the original test pattern to determine changes in position of the elements comprises comparing the lens affected test pattern in the lens image with a reference image of the original test pattern displayed on the display surface, which reference image is captured by the camera without a subject lens between the camera and the display surface.

17. A method as claimed in claim 16, wherein the method comprises processing the lens image and the reference image to extract the relative positions of the test pattern elements in the lens image and the reference image.

18. A method as claimed in claim 17 when dependent on claim 3, the method comprising correlating individual elements in the lens affected test pattern with those in the original test pattern using the uniquely identifiable visible characteristic of each element.

19. A method as claimed in claim 17 when dependent on claim 4 or claim 5, the method comprising identifying said at least one feature in the lens image and the original test pattern and correlating individual elements in the lens affected test pattern with those in the original test pattern by reference to said one feature.

20 A method as claimed in any one of claims 16 to 18 when dependent on claim 10, the method comprising:

i) comparing the lens affected first test pattern in the first lens image with a first reference image of the first test pattern displayed on the display surface, the first reference image being captured by the camera without a subject lens between the camera and the display surface; and j) comparing the lens affected second test pattern in the second lens image with a second reference image of the second test pattern displayed on the display surface, the second reference image being captured by the camera without a subject lens between the camera and the display surface.

21 A method as claimed in claim 2, or any one of claims 3 to 20 when dependent on claim 2, the method comprising interrogating the map to determine the point of minimum prism and/or no prismatic change of the lens.

22. A method as claimed in any one of the previous claims, wherein the display surface comprises an electronic display screen and the method is carried out using apparatus comprising the electronic display screen, the digital camera with its optical axis aligned perpendicular to the display screen, a mount for holding a subject lens between the display screen and the camera lens, and a computing device operatively connected with the display screen and the digital camera, the computing device configured and programmed to control the electronic display screen and the digital camera and to carry out the processing steps on images captured by the digital camera in order to carry out the method according any one of the preceding claims.

23. Apparatus for carrying out the method according to any one of claims 1 to 22, the apparatus comprising a digital display screen, a digital camera having its optical axis aligned perpendicular to the display screen, a mount for holding a subject lens between the display screen and the camera, and a computing device operatively connected with the display screen and the digital camera, the computing device configured and programmed to control the digital display screen and the digital camera and to carry out processing steps on the images captured by the digital camera in order to carry out the method according to the first aspect of the invention.

24. Apparatus as claimed in claim 23, wherein the apparatus is a lensmeter.