Abstract
A method for determining geometrical parameters of a soft contact lens comprises the steps of providing an OCT imaging device comprising an OCT light source; providing a soft contact lens arranging the soft contact lens relative to the OCT imaging device so light coming from the OCT light source impinges on the back surface of the soft contact lens; generating a three-dimensional OCT image of the soft contact lens; from the three-dimensional OCT image determining a plurality of edge points located on the edge of the soft contact lens, connecting adjacent ones of the edge points by individual straight lines; summing up the lengths of all individual straight lines to a length U of the approximated circumference of the soft contact lens; from the length U determining a diameter D of the lens according to D=U/π.
Claims
1. Method for determining geometrical parameters of a soft contact lens (CL), the method comprising the steps of providing an OCT imaging device comprising an OCT light source; providing a soft contact lens (CL) having a back surface (BS) and a front surface (FS); arranging the soft contact lens (CL) relative to the OCT imaging device such that light coming from the OCT light source impinges on the back surface (BS) of the soft contact lens (CL); irradiating the soft contact lens (CL) with light from the OCT light source and generating a three-dimensional OCT image of the soft contact lens (CL) using the OCT imaging device; from the three-dimensional OCT image determining a plurality of edge points (E.sub.i) located on the edge (E) of the soft contact lens (CL); connecting adjacent ones (E.sub.i,E.sub.2; E.sub.2,E.sub.3) of the edge points in a circumferential direction by individual straight lines, each of the individual straight lines having a length (U.sub.i), to form an approximated circumference of the soft contact lens (CL); summing up the lengths (U.sub.i) of all individual straight lines, the sum of the lengths of all individual straight lines representing a length U of the approximated circumference of the soft contact lens (CL); from the so determined length U of the approximated circumference of the soft contact lens (CL) determining a diameter D of the soft contact lens (CL) according to the equation D=U/Tr.
2. Method according to claim 1, the method further comprising the step of determining a center (C) of the back surface (BS) of the soft contact lens by extracting the back surface (BS) of the soft contact lens (CL) from the three-dimensional OCT image, and by determining that point on the back surface (BS) to be the center (C) of the back surface (BS) from which a sum of all shortest distances from that point along the back surface (BS) to all edge points (E.sub.i) is minimal.
3. Method according to claim 2, further comprising the step of determining a length L along the back surface (BS) from a first edge point (E.sub.1) of the plurality of edge points (E.sub.i) through the center (C) to a second edge point (E.sub.10) located opposite to the first edge point (E.sub.1) relative to the center (C), and further determining a base curve equivalent BCE of the soft contact lens from the length L and the diameter D of the soft contact lens using the equation: D=2 BCE sin (L/[2.Math.BCE]).
4. Method according to claim 3, wherein in case a central reflex occurs in the three-dimensional OCT image in a central area of the back surface (BS) of the soft contact lens (CL) caused by irradiating the soft contact lens with light from the OCT light source, a further three-dimensional OCT image is generated by irradiating the soft contact lens (CL) with light of lower intensity from the OCT light source, with the central reflex being attenuated in the further three-dimensional OCT image to allow for determining the length L along the back surface (BS) or the center thickness (CT) of the soft contact lens, and wherein in the central area of the back surface of the soft contact lens (CL) the further three-dimensional image is used for determining the length L along the back surface (BS) or the center thickness (CT) of the soft contact lens (CL), or for determining both the length L along the back surface (BS) and the center thickness (CT).
5. Method according to claim 2, wherein further a center thickness (CT) of the soft contact lens (CL) is determined from the three-dimensional OCT image by calculating, at the center (C) of the back surface (BS) of the soft contact lens (CL), a plane (TP) tangential to the back surface (BS) of the soft contact lens (CL), determining an axis (CA) running perpendicular to this tangential plane (TP) through the center (C) of the back surface (BS) of the soft contact lens (CL) and through the front surface (FS) of the soft contact lens (CL), and determining the center thickness (CT) of the soft contact lens (CL) to be the distance between the center (C) of the back surface (BS) of the soft contact lens (CL) and the intersection of the axis (CA) with the front surface (FS) of the soft contact lens (CL).
6. Method according to claim 1, further comprising the steps of: from the three-dimensional OCT image generating a plurality of two-dimensional OCT light sections of the soft contact lens (CL), each individual two-dimensional OCT light section of the plurality of two-dimensional OCT light sections of the soft contact lens (CL) comprising two edge points (E.sub.1, E.sub.10) of the plurality of edge points (E.sub.i); defining a boundary curve bounding each individual two-dimensional OCT light section; determining the second derivative of the boundary curve of the two-dimensional OCT light section; and determining the two edge points (E.sub.1, E.sub.10) to be located at those locations at opposite ends of the two-dimensional OCT light section where the second derivative of the boundary curve of the two-dimensional OCT light section has a maximum.
7. Method according to claim 1, wherein the soft contact lens (CL) is arranged in a container (10) filled with an aqueous liquid (11).
8. Method according to claim 7, wherein the step of providing an OCT imaging system comprises providing an OCT imaging system having a probe head (12) comprising a planar liquid dip window (120) through which the light from the OCT light source irradiates the soft contact lens (CL) immersed in the aqueous liquid (11) in a predetermined irradiation direction, with a normal on the planar liquid dip window (120) being arranged inclined relative to the predetermined irradiation direction.
9. Method according to claim 7, wherein the aqueous liquid is water, buffered solution or a mixture of the water and the buffered solution.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Further advantageous aspects of the invention will become apparent from the following description of embodiments of the invention with the aid of schematic drawings in which:
(2) FIG. 1 shows a measurement assembly that can be used for performing the method according to the invention;
(3) FIG. 2 shows a contact lens having a circular edge;
(4) FIG. 3 shows how the circumference of the contact lens of FIG. 2 is approximated by straight lines connecting adjacent edge points located on the edge of the contact lens;
(5) FIG. 4 shows a two-dimensional OCT light section through a soft contact lens that has substantially deformed with the boundary curve bounding the two-dimensional OCT light section;
(6) FIG. 5 shows the soft contact lens of FIG. 4, with the boundary curve bounding the two-dimensional OCT light section being approximated through straight lines;
(7) FIG. 6 shows a flow chart for determining the diameter D and the base curve equivalent of the soft contact lens; and
(8) FIG. 7 shows a flow chart for determining the center thickness of the soft contact lens.
DESCRIPTION OF EMBODIMENTS
(9) In FIG. 1 a measurement assembly is shown that can be used to perform the method according to the invention. The measurement assembly 1 comprises a container 10 filled with an aqueous liquid 11, for example with buffered saline. The container 10 comprises a support surface 100 on which a soft contact lens CL to be inspected rests, the lens being completely immersed in the aqueous liquid 11. A probe head 12 of an Optical Coherence Tomography (OCT) imaging device comprises a scanning mirror (not shown) for directing light from an OCT light source (not shown) of the OCT imaging device across the soft contact lens CL to be inspected. The probe head 12 further comprises a planar liquid dip window 120 which is arranged in the aqueous liquid 11 and through which light coming from the OCT light source impinges on the soft contact lens CL to be inspected. A normal to the planar liquid dip window 120 is arranged slightly inclined relative to the direction of the light coming from the OCT light source in order to avoid back reflections of the light coming from the OCT light source at the planar liquid dip window 120. The double-headed arrow in FIG. 1 indicates OCT light impinging on and back scattered from the soft contact lens CL (for generating the OCT image).
(10) As can be seen in FIG. 1, the concave back surface BS of the soft contact lens CL is arranged such that light coming from the OCT light source impinges on the back surface BS of the soft contact lens, whereas in prior art measurement assemblies the light has impinged on the front surface FS of the lens. As can also be seen in FIG. 1, the soft contact lens CL rests on the support surface 100 in a manner slightly tilted relative to a normal on the support surface 100 (although in FIG. 1 the soft contact lens CL is not deformed).
(11) FIG. 2 shows the edge of a soft contact lens CL which is neither deformed nor tilted and therefore has a perfectly circular shaped lens edge E. FIG. 3 illustrates the manner of approximation of the circumference of the soft contact lens CL shown in FIG. 2. The tilted arrangement of the soft contact lens CL shown in FIG. 1 is not shown in FIG. 2 and FIG. 3, since FIG. 2 and FIG. 3 are only used for the purpose of explaining the general principle how the length U of the approximated circumference of the soft contact lens CL is determined (FIG. 3). In general, once the length U of the approximated circumference is determined, the diameter D of the soft contact lens is determined according to the equation D=U/π.
(12) This determination of the diameter is also represented by the first three steps 600, 601, and 602 of FIG. 6.
(13) The length U of the approximated circumference is determined with the aid of a plurality of edge points E.sub.i(E.sub.1, E.sub.2, E.sub.3, . . . , E.sub.18) located on the lens edge E of the soft contact lens (see FIG. 3). Adjacently arranged edge points E.sub.i (e.g. E.sub.1, E.sub.2; E.sub.2, E.sub.3; etc.) are connected by (individual) straight lines as shown in FIG. 3, and the sum of the lengths U.sub.i (U.sub.1, U.sub.2, . . . ) of these (individual) straight lines represents the length U of the approximated circumference. While this principle is shown in FIG. 3 for a number of eighteen edge points for the sake of better illustration, the number of edge points E.sub.i is typically much higher than eighteen (and may amount to one hundred or more). Obviously, the higher the number of edge points the shorter is the distance between adjacently arranged edge points E.sub.i, and the smaller is the difference between the actual circumference and the length U of the approximated circumference represented by the sum of the lengths U.sub.i of the individual straight lines.
(14) Generally, the locations of the edge points E.sub.i (see FIG. 3) on the edge of the soft contact lens can be determined from the three-dimensional OCT image using any method suitable for this purpose. According to a particular aspect, however, they can be determined with the aid of a plurality of two-dimensional OCT light sections which can be generated from the three-dimensional OCT image of the soft contact lens, with each such individual two-dimensional OCT light section comprising two edge points E.sub.i. This is possible even without knowing the center of the soft contact lens and regardless of whether or not the center of the soft contact lens is contained in such individual two-dimensional OCT-light sections, since each of the two-dimensional OCT-light sections comprises two edge points located at opposite ends of the two-dimensional OCT-light section. For the sake of simplicity, let us assume that such two-dimensional OCT-light section comprises the edge point E.sub.1 and the edge point E.sub.10 (see FIG. 3) arranged oppositely relative to the center of the soft contact lens (although that center may not yet be known). A boundary curve bounding this two-dimensional OCT-light section is then defined. The curvature of this boundary curve (which is represented by the second derivative of the boundary curve) has two maxima which are located at opposite ends of the boundary curve. The locations (coordinates) of these two maxima of the second derivative of the boundary curve are then determined to be the edge points E.sub.1 and E.sub.10. This also applies when the soft contact lens is deformed.
(15) Once the edge points E.sub.i and the diameter D (D=U/π) are determined (in whichever manner this is done), the center C of the back surface of the soft contact lens may be determined by extracting the back surface from the three-dimensional OCT image of the soft contact lens. As has been explained further above, the center C of the back surface BS is that point on the back surface BS of the soft contact lens from which the sum of all shortest distances along the back surface BS to the individual edge points E.sub.i (see FIG. 3) is minimal. As also mentioned, determination of such point on a surface is a known problem for surfaces having an arbitrary geometry and can be solved with the aid of a suitable software like MATLAB available from the company MathWorks, Natick, Mass., U.S.A.
(16) In the case of an edge E of a contact lens that has a round (circular) edge, the distance from the center C of the back surface to all edge points along the back surface of the contact lens has the same length (see FIG. 3). To find the said center of the back surface, a virtual center (i.e. a possible center) of the back surface of the soft contact lens is selected. For each of the edge points E.sub.i located on the edge E of the soft contact lens the shortest length from that virtual center (which may not be the true center of the back surface of the contact lens) along the back surface to the respective edge point is determined. Thereafter, it is determined whether all individual shortest lengths from the virtual center along the back surface BS of the soft contact lens to the edge points E.sub.i have the same length. If they all have the same length, then this virtual center is determined to be the center C of the back surface BS of the soft contact lens. Otherwise, a new virtual center is selected and the shortest lengths from the new virtual center to the edge points E.sub.i are determined again, until they all have the same length. This process is typically (iteratively) carried out by the software running on a computer. In a practical embodiment, the term ‘same length’ in this regard means that the difference between the longest one and the shortest one of the shortest lengths from a virtual center along the back surface to the edge points of the soft contact lens is smaller than a predetermined (small) threshold. The first virtual center for which this threshold condition is fulfilled is determined to be the center C of the back surface BS of the soft contact lens (in order to minimize the computer time for calculation). Otherwise, the computer may continue running the (iterative) process endlessly in an attempt to find a virtual center for which the condition regarding ‘the same length’ is met is even better.
(17) The same principle is equally applicable for tilted and/or deformed soft contact lenses from which the three-dimensional OCT image is generated. In such case, the individual straight lines connecting adjacent edge points E.sub.i are not arranged in a single plane (as shown in FIG. 3 for illustrative purposes) but extend in the three-dimensional space. The individual lengths U.sub.i of the straight lines connecting the adjacent edge points E.sub.i can be calculated from the coordinates of the edge points in the three-dimensional space. From the individual lengths U.sub.i the length U of the approximated circumference can be calculated by summing up the lengths U.sub.i of the individual straight lines, and the diameter D can be determined as described above according to D=U/π. This is possible due to the assumption that the overall volume and the overall surface of a soft contact lens do not change in case the soft contact lens deforms.
(18) FIG. 4 shows a two-dimensional OCT light section (generated from the three-dimensional OCT image) of a (tilted) and substantially deformed soft contact lens, with a boundary curve bounding the two-dimensional OCT light section. This two-dimensional OCT light section comprises the center C of the soft contact lens as well as the first edge point E.sub.1 and the second edge point E.sub.10 located opposite to the first edge point relative to the center C of the back surface BS of the soft contact lens. The first edge point E.sub.1 and the second edge point E.sub.10 have been chosen for reasons of simplicity although other edge points E.sub.i located opposite to the center C of the back surface BS may be chosen.
(19) In the following, determination of the base curve equivalent BCE of the soft contact lens is described. As has been explained above, the center C of the back surface BS of the soft contact lens is that point from which the sum of all shortest distances to the to the individual edge points E.sub.i (see FIG. 3) on the edge E of the soft contact lens is minimal and from which all these shortest distances are equal.
(20) For the determination of the base curve equivalent BCE the length L from the first edge point E.sub.1 along the back surface BS of the soft contact lens to the second edge point E.sub.10 is to determined. This length L from E.sub.1 to E.sub.10 along the back surface may be determined through approximation by the sum of a plurality of straight lines connecting adjacent points arranged on the back surface of the soft contact lens, as this is shown in FIG. 5. The lengths L.sub.i (L.sub.1, L.sub.2, L.sub.3, . . . ) of the individual straight lines along the back surface are summed up, and this sum represents the length L along the back surface. Again, the higher the number of individual straight lines along the back surface, the better is the approximation. The base curve equivalent BCE is then numerically determined from the equation D=BCE.Math.sin (L/[2.Math.BCE]), with D representing the diameter of the soft contact lens and L representing the length along the back surface of the respective two-dimensional OCT light section.
(21) This manner of determining the base curve equivalent BCE (with the foregoing determination of the lens diameter D) is shown in the flow chart of FIG. 6. In particular, the determination of the center C of the back surface of the soft contact lens, the determination of the length L along the back surface and the subsequent numerical determination of the base curve equivalent BCE is shown in steps 603, 604, and 605 of FIG. 6.
(22) In case a two-dimensional OCT light section extends through the central reflex (which may in particular occur in a central area of the back surface when the soft contact lens is not tilted at all or is tilted only very slightly, so that the light from the OCT light source impinges vertically on the back surface in a central area of the back surface) it may not be possible to properly determined the length L due to strength of the central reflex (brightness). In such case, a further three-dimensional OCT image may be generated by irradiating the soft contact lens with light of lower intensity from the OCT light source, so that the central reflex is attenuated in the further three-dimensional OCT image. In the central area (which may have a radius of 1 mm about the center of the back surface) the further three-dimensional image may then be used (instead of the three-dimensional OCT image at normal intensity) for determining the length L.
(23) For determining the center thickness CT of the soft contact lens, it must be taken into account that the soft contact lens may be deformed and/or tilted so that it is necessary to determine the proper direction in which the center thickness CT of the soft contact lens must be measured, see FIG. 4. For that reason, a plane TP tangential to the back surface at the center C (already determined) of the back surface is calculated, and a central axis CA is determined that runs through the center C of the back surface of the soft contact lens and perpendicular to (i.e. normal to) the tangential plane TP, and which in its further course intersects the front surface of the soft contact lens. Since the light from the OCT light source (indicated by the unlabeled arrows above the lens) may not impinge in the same direction in which the center thickness is to be determined (i.e. the lens may be tilted), a dewarping calculation may become necessary for the single point at which the central axis CA intersects the front surface of the soft contact lens to be able to properly determine the center thickness CT (i.e. the refractive index of the lens material at the wavelength of the OCT light must be taken into account). These steps are shown in FIG. 7, with step 700 representing the generation of the three-dimensional OCT image, step 701 representing the determination of the center C of the back surface, step 702 representing the calculation of the tangential plane TP, step 703 representing the dewarping calculation, and step 704 representing the calculation of the distance between the center C of the back surface and the point of intersection of the axis CA and the front surface as representing the center thickness CT.
(24) While the invention has been described with reference to particular embodiments shown in the drawings, the invention is not intended to be limited to these embodiments, but rather various modifications may be possible without departing from the teaching underlying the invention. Therefore, the scope of protection is defined by the appended claims.
