MEASUREMENT OF BACK RADIUS OF CURVATURE AND CENTER MATERIAL THICKNESS OF A CONTACT LENS IN SOLUTION

Abstract: The application relates to a method and device for simultaneously measuring the center material thickness and back radius of curvature of a lens, and in particular a toric disposable opthalmic lens. An ultrasound wave is radiated by a transducer towards a lens and reflected off the front and back surfaces of the lens. Based on the lapse in time between respective echoes the center material thickness and the sagittal height of the lens is determined. The back radius of curvature is calculated from known geometric relationships between the chord of a circle and the sagittal height.

Description: MEASUREMENT OF BACK RADIUS OF CURVATURE AND CENTER MATERIAL THICKNESS OF A CONTACT LENS IN SOLUTION BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates generally to field of contact lenses, and more particularly to the measurement of the front radius of curvature, back radius of curvature and center material thickness of a contact lens in solution 2 Discussion of the Prior Art Conventional techniques are widely used for measuring optical parameters of a contact lens Although methods are available for measuring the diameter (D) of a contact lens, no method exists for the simultaneous measurement of the front radius of curvature (FC), the back radius of curvature (BC), and the center material thickness (MT) of a lens U S Patent No 5,062,297 discloses a method of measuring a profile of an object, wherein an ultrasonic wave is radiated from an ultrasonic transducer toward an object supported by a supporting member in water, and a wave reflected at the surface of the object is detected by the transducer The method disclosed in the patent, however, requires that the transducer be moved to different measurement points to obtain multiple profiles in order to generate average BC, FC & BC SAG and is therefore inefficient This measurement technique is inefficient in that it requires repeated parameter measurements that result in increased costs Furthermore, the patented method uses a diffused ultrasound beam that is more susceptible to changes in the curvature of the lens and thus, is not suitable for use with a tone lens It is therefore desirable, to develop a measuring technique for measuring optical parameters of a contact lens that solves the aforementioned problems BRIEF DESCRIPTION OF THE DRAWING The present invention may be more readily understood by one skilled in the art with reference being had to the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views, and in which Figure 1 diagrammatically depicts an example measuring system in accordance with the present invention, and Figure 2 diagrammatically depicts a top view of the support member of Figure 1 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is directed to a method and device for simultaneously measuring the center material thickness and back radius of curvature of a lens, and in particular a tone lens An ultrasound wave is radiated by a transducer towards a lens and, in turn, reflected off of the front and back surfaces of the lens Voltage-versus-time waveforms are generated based on the output of the transducer in response to the received reflected ultrasound waves The center material thickness is determined by measuring the lapse in time between receipt at the transducer of the reflected waveform off of the first surface and the reflected waveform off of the second surface of the lens Based on the lapse in time the center material thickness may be determined using a look-up table or database that accounts for the speed of sound in the lens material at the current experimental temperature In addition, the sagittal height is measured by first detecting the time lapse between a reflected ultrasound wave, or echo, off of the first surface of the lens and the echo of a reference plane membrane below the lens The reference plane may be a physical membrane or alternatively a virtual time offset from mam bang or trigger signal simulating an echo from a virtual reference membrane This lapse in time is also transformed to a distance measurement representing the sagittal height of the lens using a look-up table (LUT) or calibration standard In this case, the LUT accounts for the diameter of the lens pedestal, and acoustic velocity of the immersion medium, which varies with temperature and type of immersion fluid Based on the measured sagittal height, the back radius of curvature is calculated using known geometric relationships between the chord of a circle and the sagittal height Referring to Figure 1 , an example of a preferred measuring system is shown A containing means 1 , such as a bowl, is supported by a base 18 Ring 17 preferably is interposed between the bowl and the base to prevent slippage The containing means 1 includes outer and inner fluid reservoirs, depicted in Figure 1 as outer reservoir 19 and an inner reservoir 20 The inner reservoir 20 is filled with a first fluid 23a, such as a buffered saline solution Outer reservoir 19 is covered by a lid 16 and filled with a thermal transfer medium 23b, for example distilled water, maintained at a substantially constant temperature to stabilize the temperature of an object under test 2 and a transducer 3 A lid or cover 22 is placed over the lid 16 and interior container 20 during measurement of the material under test 2 Preferably, lid 22 includes a knob 25 for facilitating its removal and placement on and off of the containing means 1 The transducer 3 is disposed in a casing 29 along with a delay media/filter 28 and a focusing or acoustic lens 30 The focusing lens narrows the beam to a small point, on the order of approximately 400 micrometers or less, ensuring that measurements are taken over a relatively small portion of the lens surface, and not averaged over the entire optic zone The outer reservoir 19 has a support member 21 , such as a conically tapered pedestal with a passage or channel 21 a defined longitudinally therethrough, projecting upwards into the inner reservoir 20 Holding means 27, such as a nosepiece, is placed over the support member 21 An annular groove or ridge 27b is defined in the top surface of the holder 27, as shown in Figure 2 One or more radial slots 27c are defined in the groove 27b to allow the free flow or passage of the first fluid 23a between the inner reservoir 20 and channel 21 a Instead of a groove, an annular ridge may be used with one or more radial slots defined therein Multiple holding means are provided, each having a different diameter annular groove 27b The operator selects a particular holding means 27 based on the diameter of the contact lens under test In particular, a holder 27 is selected having an annular groove 27b of sufficient dimension so that when placed therein the lens 2 has negligible, if any, tolerance to move This ensures that the ultrasound beam will strike substantially perpendicular to an incident surface of the lens Otherwise, if too much tolerance is provided between the lens 2 and annular groove 27b, then the ultrasound beam transmitted by the transducer 3 will not strike the lens substantially perpendicular to the surface of the lens and the ultrasound wave reflected off the surface of the lens will be relatively weak and fluctuating Furthermore, holding means 27 ensures that the thickness of the lens is measured substantially at the center of the lens under test Thus, the holding means 27 provides a self-centering adjustment of the lens 2 relative to the transducer 3 The object under test 2, for example, a tone contact lens, is submerged in the fluid 23a and positioned concave side down in the groove 27b of the support member 21 Orientation of the lens 2 with its concave side down provides a more convenient SAG height reference Prior to commencement of measurement of the optical parameters, the contact lens 2 is preferably allowed to settle into place for a predetermined period of time, preferably approximately 10 seconds, so as to come to rest in the groove 27b The transducer 3 is disposed below the support member 21 with its side walls surrounded by the outer reservoir 19 so that the thermal transfer medium 23b stabilizes the temperature of the transducer In a preferred embodiment, the lens 2 is placed approximately 67 mm away from the focal point of the transducer Securing means 24, such as O-πngs, are interposed between the transducer 3 and outer reservoir 19 The first fluid 23a from the inner reservoir 20 passes freely through the permeable contact lens 2 and slots 27c defined in the holder 27 and into the channel 21 a, but does not mix or exchange with the thermal transfer medium 23b in the outer reservoir 19 Means, such as circuitry, connected to the bowl assembly 1 is used to determine back curve sagittal depth (BC SAG) and material thickness (MT) of the contact lens 2 is used In particular, transducer 3 radiates ultrasound waves generated by an ultrasound voltage pulse generator 4, such as a pulser, towards the lens 2 which, in turn, are reflected off the surface of the lens and received by the transducer In a preferred embodiment, the pulser may pulses between approximately 1 to approximately 10,000 times per second, with each pulse signal having a frequency from approximately 5 MHz to approximately 100 MHz In particular, the lens 2 has a first surface (closest to the transducer 3) and a second surface (furthest away from the transducer 3) Each surface produces a reflected ultrasound wave A timing gate 5 is connected to the ultrasound voltage pulse generator 4 and synchronized with the pulse signal or main bang Gate 5 is also connected to processors 8, 10, limiting the processing of return signals from transducer 3 to include only ultrasound waves reflected off of one or both of the surfaces of the lens 2, and suppress those waves reflected off of the surface of the fluid 23a A voltage is used to drive the transducer crystal or polymer Specifically, the applied voltage has a piezoelectric effect causing the transducer to become deformed and transmit an ultrasound wave The wave passes through the solution or interface between the transducer 3 and material under test 2 A portion of the ultrasound wave is reflected off of the first surface of the lens while the remaining portion of the wave passes through the lens 2 and is reflected off of the second surface Each reflected ultrasound wave impacts and deforms the transducer 3 thereby changing its associated voltage over time, which may be represented as a voltage versus time waveform The voltage output from the transducer is reproduced by a splitter 26 and transmitted to a first receiver 6 for determining the material center thickness (MT) and a second receiver 7 for determining water path (WP), from which the BC SAG may be derived Receivers 6, 7 measure the voltage output of the transducer over time to generate MT and WP waveforms, respectively Each surface of the lens produces an associated reflected ultrasound wave that is represented by a voltage versus time waveform The MT waveforms generated for the reflected waves off of the first and second surface of the lens may be used to determine the material thickness of the lens In particular, a time lapse between the MT waveform of the reflected ultrasound wave off of the first surface of the lens and the MT waveform of the reflected ultrasound wave off of the second surface of the lens is proportional to the material thickness of the lens Voltage information detected by receiver 6 over time is transmitted to a processor 8 for computing the material thickness of the lens based on the time lapse between receipt by the transducer of the reflected ultrasound waves off of the first and second surfaces, respectively In particular, the transformation from time to the corresponding spatial distance may be performed using a look-up table (LUT) or database of objects of different thicknesses Absolute temporal measurements are not used due to variations cause by different materials and thermal effects Hence a LUT is preferably used in which the effects are calibrated against known standards Processor 10 receives the voltage output from the second receiver 7 and calculates the WP or SAG height Prior to taking measurements a reference plane must be established from which the SAG height is to be measured In a preferred embodiment, a physical reference plane structure, for example, a plastic plate, is submerged into the first fluid 23a The reference plate is preferably made of a material having substantially the same acoustic impedance as the lens polymer under test This is advantageous because the reflection amplitude depends on the difference between the immersion fluid and the reflective object at the interface between the two materials Since the amplification gain circuit 9 shifts the electrical waveform in time to a different degree for signals which are not of equivalent amplitude, a more accurate results will be realized if the reference plate and lens under test have substantially equivalent acoustic impedance For example, an aluminum plate is not very desirable, while a glass plate is preferred, and a hydrophobic plastic, such as an oriented polyolefin is more preferred The time lapse between the WP waveform of the reflected ultrasound wave off of the first surface of the lens reaching the plastic reference plate is measured for a variety of lenses of different SAG height to develop a calibration standard or database used to convert between time and distance measurements Thereafter, the physical plate structure is removed and the reference plane is represented electronically by an artificial or virtual reference plane at the same location as the plastic plate Processor 10 determines the time lapse for the WP waveform of the reflected ultrasound wave off of the first surface of the lens to reach the artificial reference plane The detected time lapse is then converted to a distance measurement representing the SAG height using the developed calibration standard In turn, the BC is determined from the equation X_ h_ R (1 ) 8Λ 2 where, C is the chord, R is the back radius of curvature, and h is the sagittal height of the object. The SAG height of the object has been ascertained as described above When measuring a contact lens, the chord C represents the fixed artificial reference plane at the ends of the contact lens, e g the diameter of the lens as long as the support 27 intersects with the edge of the lens If the lens under test is a Tone lens having two radii, then plugging in the known variables in equation (1 ) and solving for the radius R results in a generated value representing the effective or average BC SAG of the two radii In practice, a LUT or database is generally used to determine the effective or average BC SAG based on the SAG height A first automatic gain control (AGC) 9 is connected via a feedback loop between the first processor 8 and the first receiver 6 Similarly, a second automatic gain control (AGC) 1 1 is connected via a feedback loop between the second processor 10 and the second receiver 7 The AGC circuits 9, 1 1 are used to change the signal gam when the ultrasound beam radiated from the transducer does not strike substantially perpendicular to the incident surface of the lens or if the acoustic impedance of the lens yields a weaker or stronger echo The AGC compensates for the narrow dynamic range of the receiver/processor In a preferred embodiment shown in Figure 1 , the first processor 8 is connected to a first display 12 and first alarm or indicator 13, while the second system logic circuit 10 is connected to a second display 14 and second alarm or indicator 15 An operator is able to view the results on the display If the ultrasound does not strike substantially perpendicular to the incident surface of the lens and is below a predetermined minimum threshold level which may be compensated for by adjusting the gam, the alarm or indicator 13, 15 is activated instructing the operator to adjust the position of the lens on the support member Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation The spirit and scope of the present invention are to be limited only by the terms of the appended claims

Claims: CLAIMS We Claim: 1 A method of measuring characteristics of a lens having a first surface and a second surface, comprising the step of simultaneously determining center material thickness and back radius of curvature of the lens 2 The method of claim 1 , wherein the material thickness and back radius determination are carried out using reflected ultrasound waves 3 The method of claim 2, further comprising the steps of radiating an ultrasound wave toward a lens, and receiving the ultrasound wave after it is reflected off of the lens 4 The method in accordance with claim 1 or 3, wherein the determining step comprises deriving the center material thickness of the lens 5 The method in accordance with claim 3, wherein the receiving step comprises receiving a first ultrasound wave reflected off of the first surface and a second ultrasound wave reflected off of the second surface 6 The method in accordance with claim 5, wherein the determining step comprises detecting a first time lapse between receipt of the reflected first ultrasound wave and the reflected second ultrasound wave 7 The method in accordance with claim 6, wherein the determining step further comprises transforming the first time lapse to a distance measurement representative of the center material thickness of the lens 8. The method in accordance with claim 1 , wherein the determining step comprises deriving the back radius of curvature of the lens. 9 The method in accordance with claim 8, wherein the determining step comprises: detecting a time lapse between the ultrasound wave being reflected off of the first surface of the lens and passing a virtual reference plane from which to measure the sagittal height of the lens, converting the time lapse to a distance measurement representing the sagittal height of the lens; and calculating the back radius of curvature based on the sagittal height. 10. The method in accordance with claim 9, wherein the calculating step comprises determining the back radius of curvature using the equation where, C is the chord, R is the back radius of curvature; and h is the sagittal height of the lens. 1 1 The method in accordance with claim 10, wherein the lens is a tone lens having two radii and the back radius of curvature is an average back radius of curvature of the two radii. 12. The method in accordance with claim 2, further comprising adjusting a gain of the ultrasound wave reflected off of at least one of the first surface and the second surface of the lens to correct for non-perpendicular incidence. 13 The method in accordance with claim 2, further comprising the step of limiting the reflected ultrasound wave to include only reflections off of the first and second surfaces of the lens 14 A device for measuring optical characteristics of a lens having a first surface and a second surface, comprising a containing means including an inner reservoir and an outer reservoir, a support member projecting upwards into the inner reservoir, and an interchangeable holding means disposed on the support member, the holding means being selected based on a diameter of the lens and supporting the lens concave side down, a fixed transducer for transmitting an ultrasound wave and receiving the ultrasound wave reflected off of the lens, and means for determining center material thickness and back radius of curvature of the lens based on the reflected ultrasound wave 15 The device in accordance with claim 15, wherein the inner reservoir is filled with a first fluid and the outer reservoir is filled with a second fluid 16 The system in accordance with claim 15, wherein the first fluid is a buffered saline solution and the second fluid is a thermal transfer medium for temperature stabilizing the lens and transducer 17 The system in accordance with claim 16, wherein the holding means comprises a holder having an annular groove defined in its top surface, the groove having a diameter substantially equal to the diameter of the lens 18 The system in accordance with claim 17, wherein the holder has an annular ridge projecting from its top surface, the ridge having a diameter substantially equal to a diameter of the lens 19. The system in accordance with claim 14, wherein the determining means comprises circuitry comprising an ultrasound voltage pulse generator connected to the transducer for generating the ultrasound wave, a splitter for reproducing an output voltage signal received by the transducer as a first waveform and a second waveform, a first receiver for receiving the first waveform and detecting a first time lapse between a first ultrasound wave reflected off of the first surface of the lens and a second ultrasound wave reflected off of the second surface of the lens, a first processor for converting the first time lapse into a distance measurement representing the center material thickness of the lens, a second receiver for receiving the second waveform and detecting a second time lapse between the first ultrasound wave reflected off of the first surface and a virtual reference plane, and a second processor for converting the second time lapse into a distance measurement representing the sagittal height of the lens and determining the back radius of curvature of the lens based on the sagittal height 20 The system in accordance with claim 19, wherein the second processor determines the back radius of curvature by the equation where, C is the chord, R is the back curve sagittal height, and h is the sagittal height of the lens 21 The system in accordance with claim 20, wherein the circuitry further comprises a gate for limiting the reflected ultrasound wave to include only reflections off of the first and second surfaces of the lens 22 The system in accordance with claim 20, further comprising an automatic gain controller for adjusting a gam of the ultrasound wave reflected off of at least one of the first surface and the second surface of the lens to correct for non-perpendicular incidence

Inventors: EBEL JAMES A, LEWIS YULIN, LONGO JEFFREY, RICE BRIAN, ROSS DENWOOD F, EBEL, JAMES, A., LEWIS, YULIN, LONGO, JEFFREY, RICE, BRIAN, ROSS, DENWOOD, F.
Applicants: JOHNSON & JOHNSON VISION CARE, JOHNSON & JOHNSON VISION CARE, INC.
Application year: 2001
Main classification: G01B17
IPC codes: G01B17, G01B21, G01M11