GB2040066A - Method and apparatus for the measurement of contact lenses - Google Patents

Abstract

The radius of curvature of the base curve of a soft contact lens is measured by supporting the lens immersed in liquid on a transparent plane horizontal supporting surface of a transparent support member, with the lens resting on the supporting surface with it concave side facing vertically upwards. A beam of light is projected vertically through the support member and the lens on to a screen by means of an optical projection system capable of corming a sharply-focussed real image of the edge of the lens on the screen. The position of the optical system relatively to the support member is adjusted to a level such that a sharply- focussed real image of the circumferential edge of the base curve of the lens is formed on the screen, and the vertical distance of that position from a datum plane is measured. The diameter of the circumferential edge of the base curve is measured by reference to the projected image, and these measurements are utilised to derive the value of the radius of curvature of the base curve.

Description

SPECIFICATION Method and apparatus for the measurement of contact lenses This invention reiates to the measurement of contact lenses, and particularly of the base curve radius, that is to say the radius of curvature of the concave surface which is presented to the wearer's eye.

Contact lenses for personal wear are made by machining from a hydrophilic plastics material in the hard dry state, and are then immersed in a liquid saline solution (0.9% by weight salt to water) for a period of days or weeks to soften.

After softening, the lenses will be kept permanently either in the saline solution or in proprietary contact lens solutions both before being supplied to the wearer, and afterwards when not being worn, so that they preserve their shape and softeness.

After the newly-machined lenses have been initially immersed in the saline solution they swell and change shape, and this deformation continues throughout an "expansion curve" which may last for days or even weeks before the softened lenses finally reach total stability. Moreover the "swell factor" differs from batch to batch of the lenses, and indeed within a batch. Thus it is necessary for purposes of quality control to measure the diameter and base radius of every softened lens before its leaves the factory, for the purpose of dimension grading, in addition to inspecting each lens to check the integrity of its circumferentail edge (from which smail chips or cracks are liable to spread inwards) and to look for surface defects and blemishes.A similar procedure of measurement and checking is often performed by the optician at the time of fitting the lenses to a wearer, to determine that the lens size and shape has not altered during the period between the factory checkn and the ultimate supply to the user, or that the lens is not misidentified.

The three key dimensions of a spherical soft contact lens which require measurement are the overall diameter of the lens, the diameter of its base curve (if different from the overall diameter) and the radius of curvature of the base curve. The base curve is the spherical surface of the concave inner face of the lens, and the peripheral edge of the lens lies in or approximately in this base curve. The edge diameter can be measured by an optical projection instrument which projects a beam of light through the lens on to a screen focussed onto the edge of the lens, which shows up as a circular image on the screen. The lens diameter can be read off by means of a scale or graticule on the screen itself.

At the same time the integrity of the edge of the lens can be checked by inspection of the image on the screen. This diametral measurement and inspection is performed with the lens immersed in saline solution in a transparent cell.

The measurement of the radius of the base curve is difficult to perform because the lens, having a significant water content, e.g. 40% water, does not produce good light reflection whilst immersed in the saline solution, and thus its spherical surface is not easy to observe.

In fact the image of a soft contact lens immersed in liquid is almost a complete blank with the exception of its outer edge.

Accordingly, the measurement of the base curve has hitherto been performed with a probe projecting from the upper end of a cylindrical support. The immersed lens is supported with its concave side downwards on the circular upper end face of a cylindrical column of smaller diameter than the lens, and is observed through a microscope whilst a vertical probe is advanced upwardly through the centre of the column beneath the lens until its touches the centre of the underside of the lens and starts to lift the lens. The initiation of this lift-off movement is observed through the microscope and indicates the position of the vertical probe at which contact occurs.The vertical distance through which the probe has moved from the support plane of the top of the column up to the contact point is measured, and from this and the diameter of the column the base curve radius, that is the radius of curvature of the spherical concave surface of the lens, can be calculated. The observation of the lift-off of the lens is not easy and can be unreliable.

Another method has been to project an image of the lens edgewise onto a screen and to compare the image of the base curve with speciment curves of known radii. This again is not easy or reliable.

Moreover either of these measurements of the base curve radius has been performed as an operation additional to the edge diameter measurement and the inspection, and has required an additional measuring instrument, and handling of the lens.

Thus, an object of the present invention is to minimise handling the soft lens to eliminate moving the lens from one instrument to another, and in particular to enable the measurement of the base curve radius to be performed by means of one and the same instrument as is used to inspect and measure the edge diameter of the lens and as an extension of that operation.

According to the present invention, from one aspect, a method of measuring the radius of curvature of the base curve of a soft contact lens, comprises supporting the lens immersed in liquid on a transparent plane horizontal supporting surface of a support member, with the lens resting on the supporting surface with its concave side facing vertically upwards, projecting a beam of light vertically through the support member and the lens on to a screen by means of an optical projection system capable of forming a sharply-focussed real image of the edge of the lens on the screen, adjusting the position of the "object plane" (as herein defined) of the optical system relatively to the support member to a level such that a sharply-focussed real image of the circumferential edge of the base curve of the lens is formed on the screen, measuring the vertical distance of that portion of the "object plane" from a datum plane, measuring the diameter of the circumferential edge of the base curve by reference to the projected image, and utilising these measurements to derive the value of the radius of curvature of the base curve.

The "object plane" of the projection system is a surface defined by points each so positioned that a sharply-focussed image of an object at that point would be projected on the screen. Although the surface is referred to as a plane of focus, it is not necessarily flat but may be slightly curved, depending upon the optical characteristics of the projection system.

In general the thickness of the lens is taken as known. The justification for this will be explained below in connection with a specific embodiment.

Where the base curve of the lens extends right to the circumferential edge of the lens, the focussed image used will be that of the edge itself. The overall diameter of the lens can be measured by referring the projected image to a suitable scale marked on the screen.

In some cases the base curve of the lens does not extend right to the ede of the lens but is surrounded by a narrow annular lip curve of spherical form but greater radius of curvature which extends to the edge of the lens. This lip curve is provided to ensure that tear liquid will not be trapped between the lens and the cornea of the eye, but can escape freely past the edge of the lens. With a lens having such a lip curve, the circular junction between the lip curve and the base curve within it is used as the object line on the lens on to which the optical system is focussed, producing a clear circular line image focussed on the screen. This junction line and its image are used instead of the extreme edge of the lens, for deriving the base curve radius of the lens.

The adjustment of the position of the "object plane" as referred to above, may be effected by moving the projection lens of the system bodily relatively to the fixed support for the contact lens along the optical axis of the system. Preferably, however, this adjustment is made by moving the support member itself bodily towards andlor away from the projection lens without moving the projections lens or altering its focus. This simplifies the measurement of the distance through which the "object plane" has been moved relatively to the support member.

The invention from another of its aspects comprises an instrument for measuring the radius of curvature of the base curve of a soft contact lens, using the method referred to, the instrument including a support member with means for supporting the lens immersed in liquid, the support member providing a transparent horizontal supporting surface on which the immersed lens can rest with its concave side facing vertically upwardly, an optical projection system arranged to project a beam of light vertically through the support member and lens on to a screen and to form a sharply-focussed real image of a part of the lens on the screen, preferably magnified, means for adjusting the position of the "object plane" (as hereinbefore defined) of the optical system relatively to the support member, means for measuring the vertical distance of that position from a datum plane e.g. the supporting surface, and means for measuring the diamter of the circumferential edge of the base curve by reference to that image.

The means for measuring the diameter of the edge of the base curve may comprise a scale or graticule for measuring the size of the projected image of that edge on the screen. Alternatively it may comprise a movable probe unit interposed in thepath of the light beam through the optical system to throw an image on to the screen and which comprises moving probes of the said unit until their images coincide respectively with those of diametrically opposite points of the edge of the base curve, and deriving from the final position of the probes the diameter of the edge of the base curve.

The invention may be carried into practice in various ways, but certain specific embodiments thereof will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is an isometric view of an instrument for measuring the base curve radius of a soft contact lens, Figure 2 is a front elevation of the instrument in section on the line ll-ll of Figure 3; Figure 3 is a side elevation of the instrument in section on the line Ill-Ill of Figure 2; Figure 4 is a plan of the instrument in section on the line IV-IV of Figure 2; Figure 5 is a diagram showing on a larger scale a lens in a dish on the lens table of the instrument, and illustrating the geometrical derivation of the base curve radius; Figure 6 is an enlarged fragmentary view of part of a lens with a lip curve;; Figure 7 is a view similar to Figure 1 of a modified construction of the instrument with digital displays; and Figure 8 is a diagrammatic elevation of a probe unit for measuring the diameter.

In the embodiment of the invention illustrated in Figures 1-4 the instrument 10 comprises a base housing 11, an upper housing 12 supported above the base housing 11 by means of an upstanding bracket 13, and a lens table 14 mounted on a pair of movable pillars 15 above the base housing. A transparent lens dish 16 filled with saline solution rests on the lens table 14, to support a soft contact lens 17 resting on the bottom of the dish with its concave side facing uppermost and fully submerged in the saline solution, as shown in FigureS.

The instrument incorporates an optical projection system, comprising a lamp 20 in front of a concave mirror 21, a first condenser lens 22, and a 45" mirror 23 all these being mounted along a horizontal axis in a compartment 24 in the base housing lithe mirror 23 reflecting the light beam vertically upwardly through a second condenser lens 25 mounted on a bracket 26 in the housing 11 below the lens table 14. The condenser lens system 22,25, projects an image of the source lamp 20 through an aperture 27 in the top of the housing 11 and an aperture 28 in the lens table 14 and through the transparent lens dish 16, into the entrance pupil of a projection lens 29 mounted in the bottom of the upper housing 13, in the usual manner.The projection lens 29 projects a real image of part of a lens 17 in the lens dish 16, via a 45" mirror 30 on to a vertical translucent screen 31 mounted in the front of the upper housing 12.

The lens table 14 is movable vertically for focussing the image on the screen. Thus the two pillars 15 which support it are slidably mounted at their lower ends in close-fitting bores in an upper slide 33, which is slidable horizontally on a lower slide 35 by means of tongue-and-groove slideways 34. The lower slide 35 is itself slidable horizontally on a base 36 by means of tongue-and-groove slideways 38, the sliding direction of the lower slide being at right angles to that of the upper slide. For raising and lowering the lens table 14, a worm gear mechanism is provided comprising a nut 40 which its threaded on to a fine-pitch helical screw thread 41 formed externally on one of the pillars 15.The nut 40 is formed with external teeth engaged in the thread of a worm screw 42a formed on the end of an operating spindle 42 which is journalled in a supporting bracket 43 and carries an operating knob 44 on its outer end. The nut 40 is caged against vertical displacement. Thus rotation of the knob 44 rotates the spindle 42 and raises or lowers the pillar 15 and hence the lens table 14, the gearing 41,40, 42a providing a substantial reduction ratio between the knob and the table. A dial gauge 45 mounted on the top of the base housing 11 engages the underside of the lens table 14 to measure its height and vertical displacement relative to the base housing.The two slides 33 and 35 are provided with respective leadscrews 46 and 47 projecting through openings in the base housing 11 and having operating knobs 48 and 49 for transversing the table 14 horizontally in two directions at right angles.

For measuring the lens diameter, the screen may carry a graticule indicated in Figures 1 and 2. Since the soft contact lens is moved up and down while the screen and the projection lens remain stationary, the magnification will be constant and hence the gradicule enables a measurement of the diameter to be made.

Such a measurement may be not altogether convenient, since it presupposes accurately centering the contact lens in both of two directions, and in a further arrangement a probe unit shown in Figure 8 is provided for measuring the lens diameter.

This consists of a pair of aligned probes and is carried by the lens table 14, which is extended for the purpose. A base 60 hinged to the table at 61 so as to be capable of being swung clear, has an upstanding pillar 62 in which a left-hand micrometer screw 63 passes freely through a hole. The screw 63 has knob at one end (the left-hand end in Figure 8) and a flange to prevent endwise movement relatively to the pillar, while its other end is screw-threaded in a nut 65 forming part of a yoke 66 mounted to slide in guides on the base 60. The nut 65 carries a probe 67 shaped to extend over the edge of the lens dish, and down into the liquid in which the lens is immersed.

The right hand end of the yoke is viewed in Figure 8 has an upstanding pillar 69, and a second micrometer screw 70 passes freely through a bore in the pillar, and is provided with flanges (for example a nut and lock nut) to prevent it from moving endwise relatively to the pillar 69. The micrometer screw 70 is provided with a knob and scale 71 and is screw-threaded in a nut 72 mounted on the yoke so as to slide longitudinally without turning, and carrying a second probe 73 shaped similarly to the first probe 67.

Figure 8 is a diagrammatic in showing the yoke 66 as passing beneath the lens dish. In fact the yoke is in the form of a plate having in it a large hole to accommodate the lens dish.

The principle upon which the instrument is based for measuring the radius of curvature of the base curve of a contact lens will now be described with reference to Figure 5. The base curve is the spherical concave inner surface 60 of the lens 17, whose centre is at '0' and whose radius is shown at 'R'. The overall diameter 2a of the contact lens 17-is measured across the chordal plane 62 through the circumferential edge 61 of the lens, and is a measurement which is required for classification of the lens.It will be seen that if this diameter 2a is known, and the depth 'd' corresponding to the distance 'BA' between the mid-point 'B' of the chordal plane 62 and the mid-point 'A' of the base cruve 60, then the radius 'R' can be derived from the expression: R2 = a2 + (R-d)2 The measurement of the depth 'd' can be effected optically by means of the projection system. The dial gauge 45 is zeroed with the "object plane" (as defined above) of the projection system situated at the level 'X' in Figure 5, i.e. passing through the point 'A' on the base curve. This means that visible points on that plane 'X' would appear as focussed point images on the screen 31. Since the lens is transparent, no clear image is likely to be seen on the screen.The zering is done by first focussing the projection system to throw a sharp image on the screen of a graticule on the upper face of the bottom of the dish 16, using the vertical transverse knob 44, then raising the relative level of the "object plane" by lowering the table through a distance measured on the dial gauge equal to the known thickness 't' of the lens 17, and setting the dial gauge to zero in that position.

In general the thickness of the lens is taken as known. The justification for this is first that regarding the lens as a bowl its thickness is much less than its depth. It will be appreciated that contact lenses are always in the form of a comparatively deep bowl, since the concave surface must fit the eye of the wearer.

For example, in a typical lens, the thickness might be .16 mm and it is a requirement that this should not vary by more than .02 mm. Such a lens might have a radius of curvature of 8.2 mm a diameter of 13 mm and a depth of 3.2 mm. Thus the maximum possible error involved in assuming a nominal thickness is a small percentage of the depth of the lens and also a small percentage of the radius of curvature.

With the dial gauge thus zeroed, the table 14 and dish 16 are then lowered by means of the knob 44 until the projection system throws a sharp image of the circumferential edge 61 of the contact lens on to the screen 31. This will correspond to a raising of the "object plane" relative to the liquid surface until it exactly coincides with the chordal plane 62. The distance dl as measured on the dial gauge through which the dish 16 is thus lowered will equal d where CL is the refractive index of the saline solution in which the lens is immersed.

Accordingly, if the diameter 2a is known the radius of curvature 'R' of the base curve 'R' can be derived from the expression: R2 = a2 + (R~jld1)2 = a2+R2-2d1P+(iid1)2

The measurement of the overall diameter 2a can be made on the instrument 10 either before or after the measurement of the depth d1. For this purpose the annular graticule 50' scribed on the screen 31 is used for approximately centering the lens 17 with respect to the optical axis 52 by means of the table traverse controls 48 and 49. With the lens roughly centered and the circular image of its outer edge 61 forcussed onto the screen, the diameter of the contact lens may be measured from the graticule 70.

Alternatively it may be measured by means of the probe unit shown in Figure 8. In this case the probe unit is swung down about its hinge 61 so as to lower the probes 67 and 73 into the liquid to the level of the edge of the lens. Means not shown are provided for raising and lowering the probes so as to be in the appropriate plane, but it will be appreciated that this adjustment is made once and for all since even if the contact lenses of a series differ from one another, each will be brought to a position in which its edge lies in the object plane of the projection system.

The left-hand knob 64 is then turned, so as to move the left-hand probe 67 until its image on the screen coincides with that of the adjacent edge of the contact lens. During this adjustment the right-hand probe will also incidentally move to the same extent. The right-hand knob 71 is then adjusted to bring the image of the right hand probe into alignment with the opposite edge of the contact lens, with the right-hand and left-hand probe lying in the same vertical plane as a diametrical cord.

It will be appreciated that once again the measurement is made without the necessity to handle the contact lens, or transfer it from the position in which it had in any event to be placed, for inspection and for measurement of the radius of curvature. Moreover with the arrangement shown in Figure 8 the reading of the micrometer knob 71 gives a direct measurement of the diameter in the manner of a standard micrometer, and does not require two measurements to be made and one to be subtracted from the other.

Thus, in practice, each one of a batch of lenses 17 to be classified is placed in turn in the saline solution in the lens dish 16, and its overall diameter 2a and the depth 'd' are measured as described above, enabling the base curve radius 'R' to be calculated. Thus, the single instrument 10 enables the values 2a and 'R' to be measured in sequence for each lens of the batch.

It is also possible to utilise the instrument 10 for the derivation of the radius of curvature 'R' of the base curves of contact lenses 17a which are provided with outer lip curves. Such a lip curve is an annular concave spherical surface 70 (Figure 6) centered at 01 and formed around the margin of the lens 17' on its concave inner side to surround the base curve 60' which occupies the whole of the central portion of the inner face of the lens. The lip curve 70 which is quite narrow, perhaps only 0.2 to 0.5 mm in width, has a greater radius of curvature R1 than that of the base curve which it surrounds, so that the intersection at 72 of the lip curve and the base curve is a circle.The purpose of the flatter lip curve 70 is to prevent tear liquid from becoming trapped betwen the inner face of the lens and the cornea of the eyeball against which it is applied. When the light beam is projected through a lens 17' having a lip curve, a sharp image of the circle of intersection 72 of the lip curve and the base curve which constitutes the outer edge of the base curve 60' can also be focussed on the screen 31 and will be clearly visible.

Accordingly, when lenses 17' with lip curves 70 are to be classified by means of the instrument 10 the overall diameter of each lens is measured on the instrument 10 by focussing on the overall edge 61' and using the probes, as already described. Then the instrument is refocussed by means of the table height traverse knob 44 to throw a focussed image of the junction circle 72 on the screen, the diameter 2a' of that circle is measured using the probes, and the radius R' of the base curve is derived from the measurement of the depth d' precisely as before, d' being in this case the distance from the point A' to the chordal plane 62' through the junction circle 72 rather than to the chordal plane through the outer edge 61' of the lens.

Figure 7 illustrates a modified form of instrument 10' which is similar to the instrument 10 except that it provides a digital displays instead of the micrometer and knob dial display of the latter.

Thus, the instrument 10' incorporates electronic circuitry which transforms the probe movements and positions into a digital display of the lens diameter 29 on a screen 80, and calculates the base curve radius R or R' from the measurements of lens diameter and depth d or d' displaying the calculated values of R on a second digital screen 81. Athird digital display screen 82 is provided, with associated circuitry which enables it to display the "sag height" of the lens as it rests on the bottom of the dish 16. Thus "sag height" is either the overall depth measurement d or it is the value of d plus the thickness t of the lens. In other respects the instrument 10' is simlarto the instrument 10 as regards its construction and operation.

It will be seen that both instruments 10 and 10' are also provided with an on/off switch 90 and a brightness control 91 for the lamp 20. A motor driven fan 92 (Figure 3) is provided for cooling the lamp compartment 24.

Claims (10)

1. A method of measuring the radius of curvatures of the base curve of a soft contact lens, which comprises supporting the lens immersed in liquid on a transparent plane horizontal supporting surface of a transparent support member, with the lens resting on the support surface with it concave side facing vertically upwards, projecting a beam of light vertically through the support member and the lens on to a screen by means of an optical projection system capable of forming a sharply-focussed real image of the edge of the lens on the screen, adjusting the position of the "object plane" (as herein defined) of the optical sytem relatively to the support member to a level such that a sharply-focussed real image of the circumferential edge of the base curve of the lens is formed on the screen, measuring the vertical distance of that portion of the "object plane" from a datum plane, measuring the diameter of the circumferential edge of the base curve by reference to the projected image, and utilising these measurements to derive the value of the radius of curvature of the base curve.

2. A method as claimed in Claim 1 in which the datum plane is the supporting surface and the derivation of the radius is based on a nominal or known thickness.

3. A method as claimed in Claim 1 in which the base curve extends right to the extreme edge of the lens.

4. A method as claimed in Claim 1 or Claim 2 in which a lip curve of greater radius of curvature than the base curve surrounds the base curve at the margin of the concave face of the lens, and in which the said sharply-focussed image is that of the circular junction between the base curve and the lip curve.

5. A method as claimed in any one of Claims 1 to 4 in which the diameter of the base curve edge is measured by means of a probe unit interposed in the path of the light-beam through the optical system to throw an image on to the screen and which comprises moving probes of the said unit until their images coincide respectively with those of diametrically opposite points of the edge of the base curve, and deriving from the final position of the probes the diameter of the edge of the base curve.

6. A method as claimed in any one of the preceding claims in which the adjustment of the position of the "object plane" is made by moving the supporting member bodily towards or away from the projection lens of the optical system.

7. A method of measuring the radius of curvature of the base curve of a soft contact lens as specifically described herein with reference to Figures 1 to 6 or Figure 7 with or without the modification of Figure 8.

8. An instrument for measuring the radius of curvature of the base curve of a soft contact lens by the method claimed any one of Claims 1-6 the instrument including a support member with means for supporting the lens immersed in liquid the support member providing a horizontal transparent supporting surface on which the immersed lens can rest with its concave side facing vertically upwardly, an optical projection system arranged to project a beam of light vertically through the support member and lens on to a screen and to form a sharply-focussed real image of a part of the lens on the screen, means for adjusting the position of the "object plane" (as hereinbefore defined) of the optical system relatively to the support member, means for measuring the vertical distance of that position from a datum plane, and means for measuring the diameter of the circumferential edge of the base curve by reference to that image.

9. An instrument as claimed in Claim 8 in which the means for measuring the diameter of the edge of the base curve comprises a probe unit interposed in the path of the projected light beam so that a sharp image of the probesis thrown on the screen, the probes being adjustable to cause their projected image to coincide with the projected image of the said edge across a diameter thereof,, and indicator means arranged to indicate the length of the diameter of the said edge of the contact lens when the probe is adjusted to the condition of image coincidence.

10. An instrument for measuring the radius of curvature of the base curve of a soft contact lens as specifically described herein with reference to Figures 1 to 6 or Figure 7, with or without the modification of Figure 8.