Lens alignment and positioning method and apparatus

Abstract: A method and apparatus for aligning and positioning a lens such that the optical and/or cylinder axis of the lens is precisely aligned with a fixture for the purpose of assembly or further mechanical operations such as machining and polishing. According to another aspect of the invention, the lens apex is precisely positioned a constant distance from a reference point. In order to implement the above, a method and apparatus is disclosed for optically aligning and positioning lenses using digitized video imaging, an X-Y-Z micron stage and a microprocessor (or computer) capable of performing image analysis.

Description: 1. Field of the Invention This invention relates to a method and apparatus for aligning and positioning a lens, so that an axis of the lens is precisely aligned with a fixture and a surface of the lens is precisely positioned a constant distance from a reference point on the fixture. The invention has particular application in the field of manufacturing contact lenses. 2. Description of Prior Art In the manufacture of lenses, the degree of accuracy of alignment of a partially finished lens to its fixture, e.g., a mandrel, determines the limit of concentricity possible through subsequent machining operations. A high degree of concentricity is advantageous in the manufacture of lenses since greater concentricity results in less prism. The accuracy of positioning the apex of the lens to a reference point or shoulder of the fixture determines the accuracy of the manufactured lens thickness. A high degree of consistency of lens thickness is advantageous for the manufacture of lenses. An example of the use of this invention is in the manufacture of contact lenses. However, the invention is not so limited and the principles of the invention may be applied to the manufacture of other types of lenses. For purposes of example only, the process of manufacturing contact lenses will be described. Contact lenses are typically manufactured by way of a complex multi-step operation as described in US-A-4,460,275 whereby the fabricated lens goes through many precision operations. As shown in Fig. 1, generally the first stage of manufacture forms a polished lens surface on a "blank" of plastic. The second stage of manufacture transfers this optical lens surface to a block whereby the polished surface is precisely fixed to a block with a suitable material such as wax or cement so that a second surface of the lens can be machined. This stage is commonly referred to as blocking a lens. The third stage forms a polished optical surface of a fixed diameter on the second surface of the lens thereby forming a contact lens. The fourth stage involves removing the finished lens and polishing the edges of the lens in a known manner. In order to minimize prism and maximize concentricity of the manufactured lens, and to precisely control the thickness of the manufactured lens, it is important to precisely align an axis of the lens, such as the optical axis, and to ensure that a portion of the lens, e.g., the apex, is precisely positioned a constant distance from a reference point. However, such precise positioning can be time consuming and difficult to obtain if done manually. While some attempts have been made to automate this process to some extent, the prior art falls short of achieving the results of the present invention. Patent Abstracts of Japan, Vol. 10, Number 134 (M-479), & JP-A-60-259365 discloses a lens centering device to perform centering of the lens prior to grinding. A laser light source transmits a laser beam through the lens which is held by a lens chuck shaft and which has polished lens faces. The laser light point is received by a light point position detector which generates x and y coordinate information for the light point to an operational display which generates correction commands. A first motor rotates the lens according to the command and a second motor moves a rod to radially displace the lens such that it will coincide with the axis of the chuck shaft. In view of the foregoing, it is an object of the present invention to overcome the drawbacks of the prior art. More specifically, it is an object of the present invention to provide an automatic manufacture process. It is another object of the invention to provide automatic alignment and positioning of a workpiece. It is another object of the invention to minimize prism in the manufacture of a lens. It is another object of the invention to maximize concentricity in the manufacture of a lens. It is another object of the invention to maximize control of the thickness of a manufactured lens. It is a further object to provide greater precision in a manufacture process than is currently typical in the art. SUMMARY OF THE INVENTION This invention relates to a method as claimed in claim 1 and to an apparatus as claimed in claim 4. The present invention offers several distinct advantages over the current methods used for blocking contact lenses. It provides greater accuracy than is typically achieved by present methods. Specifically, it provides greater accuracy of concentricity and position of the apex of the lens. Greater accuracy of concentricity means less prism in the manufactured lens. Greater accuracy of positioning of the apex means better control of the manufactured lens thickness. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic representation of a blocking operation for a lens. Fig. 2 is a diagrammatic representation of an apparatus capable of carrying out the present invention. Fig. 3 is a flow chart illustrating the steps for carrying out a preferred embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Fig. 2, there is shown an apparatus for implementing the steps of the present invention. Specifically, there is disclosed an alignment and positioning apparatus for aligning and positioning a workpiece in order to enable precise alignment to be performed automatically under computer control without the need for manual alignment. In Fig. 2 there is shown a controller 1 such as a computer or microprocessor which has digital analysis capability and which can function to control movement of an X-Y-Z micron stage. The controller is calibrated to determine the X and Y distances corresponding to the width of one pixel. Z-axis movement is in small, arbitrary, but known, units from some reference point designated Z=0. There is also provided a CCD TV camera 2 or other camera-type apparatus for generating a visual image as will be discussed below. A Z-axis element of an X-Y-Z micron stage is indicated as element 3. The Z-axis further includes a microscope objective (preferably 2-20X) or other suitable lens structure 4, a chuck or other suitable holding structure 5 for holding a block 6, and a lens 7, attached to a fixture assembly 8. Fixture assembly 8 is operably connected to the X-Y axis of an X-Y-Z micron stage 9. The chuck can be mounted in the optical axis or line of sight (LOS) of the workpiece whereby camera 2 views the workpiece through a hollow chuck, or the chuck can be mounted to the side a predetermined distance from the optical axis. This latter alternative is shown in dotted lines in Fig. 2. According to a preferred embodiment of the present invention, camera 2 generates an image of a focusing reticle (not shown) located in the optical path between objective 4 and camera 2. This image is digitized, and digital analysis is performed to determine the position of best focus. According to the amount of speed and accuracy desired, which will vary from application to application, various digital analysis techniques may be used. As is well known, a digital image is comprised of a plurality of pixels where each pixel has a unique X and Y coordinate. For pixels all lying in an X-Y plane, the Z coordinate will be the same for each pixel. Each pixel has an intensity level between 0 (black) and some value N (white) which may be for example 256. These values can also be referred to as gray levels. Often, a threshold value for the gray or intensity level may be predetermined such that any pixel having an intensity below the predetermined value will be assigned a binary value of 0 (black) and any pixel having an intensity level above the predetermined value is accorded a binary value of 1 (white), or vice versa. In operation, a video image of a focusing reticle located in the optical path between the microscope objective and the video camera can be generated by camera 2 and digitized according to the above. Then, under control of computer 1, the components of the X-Y-Z micron stage are moved to align the image with the optical axis of the video camera and focus the digitized reticle image of the lens focal point. This may be done by using a centering subroutine of a type commonly known. Briefly, information corresponding to the configuration of the reticle (e.g., size, shape, pattern, etc.) is pre-stored in the computer. When a reticle image is generated by reflection from the lens, back to the imaging device, the reticle image falls within the field of view of the imaging device. (If it does not, it can be brought into the field of view under computer control.) Under computer control, the X-Y stage is moved to locate the reticle image in the center of the field of view based on the previously stored reticle information. During initial setup, the center of the optical axis is made to correspond to the center of the field of view for the imaging device. Therefore, centering the reticle image causes the optical axis of the lens to be aligned with the optical axis of the imaging device. The (X,Y) position corresponding to this condition is stored in the computer. Preferably, during initial setup, the optical axis of the holder 5 is aligned so that alignment of the lens axis with the imaging device also aligns it with the axis of holder 5. Once the reticle image (lens focal point) is positioned in the center of the optical axis, the Z-axis position is adjusted under computer control to focus the reticle on the lens surface at its apex. According to a preferred embodiment, to determine the degree of focus, a focusing algorithm is used which looks for transitions from light to dark either above or below the predetermined threshold value. Alternatively, it is desirable to use a focusing algorithm which looks for a change in intensity over a given range of pixels. Two techniques currently used to decrease analysis time are averaging values of groups of pixels with a position value consistent with their location, or, analyzing every nth pixel (where n is some number greater than or equal to 1). The best focus for one light to dark transition is the Z-position which gives the steepest slope in gray level difference moving a predetermined direction in the X-Y plane. The best focus overall is the Z-position with the highest average slope for one or more of the light to dark transitions scanned. Using one or more of the above processes, the computer determines the position on the Z-axis which produces the sharpest reticle image (the lens focal point). The Z-axis position corresponding thereto is then stored in memory associated with the computer 1. According to the above, the centering determines the exact optical center line of the lens. The surface focus determines the exact position of the apex of the lens. After these positions are determined and information corresponding to the respective positions is stored in the computer memory, the block may be inserted into chuck 5 which is attached to the microscope of objective 4 and hot wax is deposited onto the base curve surface. The computer then moves the Z-axis down to a point above the lens which allows the apex to be fixed at the desired distance from the reference position, based on the information stored in the computer. Once any desired operations have been performed, the computer moves the Z-axis away so that the base curve which is now fixed to the block can be removed and the process repeated for the next lens. With reference to Fig. 3 there is shown a flow chart illustrating the operation of the present invention. After the lens is inserted into a holder connected to the X-Y part of the X-Y-Z micron stage, the reflected image from the lens is found (301). This is done by movement of the Z-axis to bring the reflected image of a reticle or light source into focus somewhere in the field of view. X and Y axis movements are then made, if necessary, to bring the reflected image into the field of view. Next, the reflected image (or a portion of it) is centered in the current field of view by computer controlled manipulation of the X-Y stages of the X-Y-Z micron stage (302). When the reflected image is centered, the position (X,Y) corresponding to this position is stored in a memory associated with a computer (303). Next, the centered image is focused upon by computer controlled manipulation of the Z-axis stage of the X-Y-Z micron stage (304). Once the position of best focus of the reflected image is determined, information corresponding to this position is stored in a memory associated with the computer (305). Then a determination is made as to whether to repeat the centering operation or not (306). This determination can be made based on a degree of quality and accuracy desired and considering the speed with which it is desired to perform the manufacture process. If the entire reflected image is not in the field of view or if increased accuracy is desired, multiple centering and focusing operations may be desirable. If it is desired to repeat the center focus process, a control signal is generated by the computer which causes step 302 to be repeated. If it is not desired to repeat the center focus, control passes to step 307 where the image of the lens surface is found. Then, a focusing operation is performed on the surface of the lens (308). Information corresponding to the position of best focus of the surface of the lens is recorded in a memory associated with the computer (309). Then a block is loaded into a precision holder (310), wax is placed on the back surface of the lens so that the lens can be mounted to the block (311) and based on the stored information, computer controlled movement causes alignment of the center of the block with the optical axis of the lens (312). Then the block is moved in the Z-axis based on the stored Z information to achieve the exact offset (spacing or distance) with respect to a reference point (313). At this point, the lens is in proper position to perform additional machining operations or other desired operations (314). After any desired operations are performed, the block (fixture) and lens can be removed (315). This process can then be repeated for the next lens.

Claims: A method for accurately positioning a lens (7) having at least one axis and an apex, with respect to a holder (6) and a reference point on said holder, in which an axial position of said lens is determined (301), first information corresponding to said determined axial position is stored (303), and an optical axis of said lens is automatically aligned with an axis of an imaging device (312), characterized by: determining (304) the position of the apex of said lens (7); storing (305) second information corresponding to the apex position; automatically positioning (313) the apex of said lens (7) a predetermined distance from said reference point; and using the first and second stored information to automatically operatively position said lens with respect to said holder such that the axis of said lens is in optical alignment with said holder (312) and said apex is located a predetermined distance from a reference point (313); wherein the step of determining (301) an axial position of said lens (7) comprises the steps of: locating a lens (7) in a fixture (8) attached to a portion of a movable support (9); controlling said movable support (9) to move said lens (7) within a field of view of an imaging device (4); generating a first video image of a focusing reticle (2); digitizing (1) said first video image and analyzing the digitized first video image to determine the position of said movable support (9) which causes the first video image (2) to be located in the center of the field of view of the imaging device (4) to thereby align the optical axis of said lens with an optical axis of said imaging device; and wherein the step of determining (304) the position of the apex of said lens (7) comprises the steps of: generating a second video image of a focusing reticle (2); moving the focusing reticle through a plurality of positions within a field of view of an imaging device (4); digitizing (1) said second video image and analyzing the digitized second video image of said focusing reticle at said plurality of positions to determine the position corresponding to where the focusing reticle is focused on the lens (7) apex. The method of claim 1, wherein said controlling step comprises controlling said movable support (9) to move said lens (7) within a field of view of a video camera (4). The method of claim 2, wherein said digitizing and analyzing steps each comprise separating the video images using a computer into a plurality of pixels where each pixel has a unique X, Y and Z coordinate, with an intensity level between 0 (black) and some value N (white), and assigning a binary value of 0 (black) or 1 (white) to each pixel depending upon whether the intensity level thereof is above or below a predetermined threshold. Positioning apparatus for positioning a lens (7) and having: a holder (6) capable of holding said lens (7); a first determining means (1, 9) for determining an axial position of said lens (7); a first storage means (1) for storing information corresponding to said determined axial position; and a means (9) for automatically aligning an axis of said lens (7) with an axis of said holder (6); characterized by: second determining means (1, 3) for determining the position of the apex of said lens (7); second storage means (1) for storing information corresponding to the apex position; means (3) for automatically positioning the apex of said lens (7) a predetermined distance from a reference point; and control means (9, 3) responsive to said stored information for automatically controlling the relative position of said lens (7) with respect to a holder such that the axis of said lens (7) is in optical alignment with said holder and the apex of said lens (7) is located a predetermined distance from a reference point; said first determining means comprising: image means (2), having a field of view, for generating a first video image representing the position of the axis of said lens; means (1) for digitizing and analyzing said first video image; and control means (9) responsive to said digitized video image for causing movement of said lens (7) within the field of view of said image means to center the video image within the field of view of said image means; and said second determining means comprising: means (2) for generating a second video image representing the quality of focus of a focusing means (4) with respect to the apex of said lens (7); control means (3) for relatively moving the position of said focusing means (4) with respect to said lens (7); and means (1) for digitizing said second video image and analyzing the quality of focus represented by said digitized second video image during said relative movement at a plurality of positions and for determining the relative position of said focusing means (4) and lens (7) corresponding to the best focus of said second video image. The apparatus of claim 4, wherein said means for digitizing and analyzing said first and second video images comprises a computer. The apparatus of claim 5, wherein said control means responsive to said stored information and said control means responsive to said digitized video image comprises an X-Y-Z micron stage.

Inventors: ADAMS RICHARD W, BENZ PATRICK H, BUMPUS PETER B, ECKLES ANDREW J IV, GRANT STEPHEN R, ADAMS, RICHARD W., BENZ, PATRICK H., BUMPUS, PETER B., ECKLES, ANDREW J. IV, GRANT, STEPHEN R.
Applicants: BENZ RES & DEV, BENZ RESEARCH AND DEVELOPMENT CORP.
Application year: 1991
Main classification: B23Q16
IPC codes: G02C13, B23Q17, B23Q15, G02C7, G01M11, B24B13