An ophthalmic lens in its simplest form has two distinct surfaces, each on a respective side of the lens. These two surfaces cooperate with each other to either converge or diverge light and thereby correct vision problems. When a position or shape of one of these surfaces relative to the other is imprecise, optical errors are created. This is the type of optical errors basically that is of interest herein, and that is primarily addressed by the method of the present invention.
A blocking of an ophthalmic lens blank is required prior to working the lens blank in a lens generating apparatus. Modern ophthalmic lens generating apparatus especially are computer-controlled machines capable of generating a final optical surface on an ophthalmic lens in a single operation. These apparatus may also include profiling equipment capable of shaping the edge contour of the lens while the lens blank remains bonded to the original support block used during the surface generation process.
As can be appreciated, the precision of these machines depends greatly upon the accuracy with which the lens blank is affixed to the support block. Conventional lens blocking typically follows one of two approaches, the first and most common method is referred to as geometric centre blocking, and the second method is called optical centre blocking. In the first case, a support block is attached to the geometric centre of the lens blank, (Blank Geometric Center). In the second method, the block is attached to a point on the front surface of the lens through which the optical axis of the lens passes, or at a point where the prescribed prism is to be found, (Major Reference Point). The support block which is made of metal or plastic is bonded to the front surface of the lens in order to serve as a work-holder having known positional reference points and datum plane relative to the lens' front surface. All surfacing and shaping work is then done relative to these positional references.
The machines of the prior art for blocking a lens have evolved from a basic manual and visual alignment of markings on the lens blank with references on the support block, to semi-automatic equipment using a pick and place manipulator and LCD generated target images for positioning the lens blank. In this respect, a representative group of lens blockers of the prior art is presented hereinbelow.
In a first example of lens blockers of the prior art, the U.S. Pat. No. 3,663,983 issued on May 23, 1972 to Hamilton B. Bole describes an apparatus for forming a block on a lens. The apparatus has a block molding station relative to which a plurality of blocking molds are selectively successively indexed for receiving both a lens blank to be blocked and material for forming the block. The apparatus includes lens aligning indicia which become automatically illuminated with the positioning of each mold at the blocking station. The apparatus further includes spring clamping means with which lens blanks to be blocked are clamped to respective blocking molds and securely retained in desired aligned relationship with the molds during the formation of the blocks.
In a second example of lens blockers of the prior art, the U.S. Pat. No. 3,804,153 issued on Apr. 16, 1974 to Luc Andre Tagnon illustrates a device for positioning a mold for casting a metal block onto the surface of a lens. The mold is movable by means of a spherical swivel joint and pantograph linkage such that the reference axes of the mold are movable to coincide with the optical axes of the lens blank, and such that the mold always makes direct contact with the curved surface of the lens. The reference axes on the metal block are later used for controlling the movements of a lens trimming and bevelling machine.
Another example of lens blocking devices of the prior art is described in the U.S. Pat. No. 4,288,946 issued on Sep. 15, 981 to Bela J. Bicskei. The device comprises a tripod on which an ophthalmic lens is placed, and a projector and mirror assembly for projecting target image from under the tripod. The lens is manually positioned on the tripod with the optical markings on the lens corresponding to the target image of the projection. When the lens is properly positioned, an articulated blocking arm is used for precisely positioning and securing a support block to the lens blank.
In yet another example, the U.S. Pat. No. 4,319,846 issued on Mar. 16, 1982 to David W. Henry et al. describes a method and apparatus for aligning a lens blank upon a lens blocking station. A transparent indicia is movable back and forth over a lens supported on a blocking station. The indicia is firstly used to properly position the lens blank over the blocking station, with the optical axes of the lens blank being in a consistent orientation relative to the blocking station.
Another example of lens blocking devices of the prior art is described in U.S. Pat. No. 5,283,980 issued on Feb. 8, 1994 to Marold H. Lohrenz et al. With this device, a lens blank is placed over a sheet of non-slip transparent material covering a liquid crystal display connected to a computer. The LCD exhibits a target image generated by the computer. The lens is manually positioned over the sheet of non-slip material according to the markings of the target image. The computer calculates and compensates for the optical error in the refractive characteristics of the non-slip sheet and the viewing glass, and shifts the target image to account for the error. Once the lens is properly positioned, a lens support block is affixed to the lens blank using a blocking arm capable of applying a constant force to the lens independently of the height of the lens blank.
The U.S. Pat. No. 5,498,200 issued on Mar. 12, 1996 to Ralf Werner describes a device for attaching a holder to a lens blank before grinding the edge of a lens blank. The lens blank is placeable over a glass plate carrying a tripod stand. A LCD screen is used to project a scale, a template image or an eyeglass frame image. A prism is positioned between the lens blank and the eye of an operator of the device for superimposing the projection of the LCD screen onto the image of the lens blank. The tripod is raiseable to a predetermined height before the lens blank is adjusted on the tripod and a support block is affixed thereto.
A further example of a lens blocker of the prior art is described in U.S. Pat. No. 5,505,654 issued on Apr. 9, 1996 to Kenneth O. Wood et al. The apparatus comprises an alignment station for supporting and aligning the lens blank with a target image generated by a liquid crystal display. Both the image of the lens and the target image are projected on a viewing mirror in front of the operator of the apparatus. The apparatus also has a movable pick and place arm with a vacuum picking cup for moving the lens from the alignment station to a blocking station while maintaining the lens orientation. The blocking station includes a support for a lens block, a support for the lens blank and a system for injecting heated liquid bonding material between the lens and the block which solidifies on cooling to join the lens and block together.
Other lens blocking apparatus of the prior art are describe in the following U.S. Patents. All these machines are also characterized by the fact that the blocking process includes the placement of a lens blank on a support ring or on the support block itself during the bonding of the block to the lens.
U.S. Pat. No. 4,330,203 issued on May 18, 1982 to Oppenheim et al.; PA1 U.S. Pat. No. 4,677,729 issued on Jul. 7, 1987 to A. H. Morland et al.; PA1 U.S. Pat. No. 4,737,918 issued on Apr. 12, 1988 to J. P. Langlois et al.; PA1 U.S. Pat. No. 5,425,665 issued on Jun. 20, 1995 to B. H. Kennedy; PA1 defining a spatial coordinate system in a computer environment; PA1 analyzing an image of a lens blank and locating this image in the spatial coordinate system; PA1 measuring a back curvature of the lens blank and locating the back curvature in the spatial coordinate system; PA1 probing a front curvature of the lens blank and locating the front curvature in the spatial coordinate system; and PA1 integrating the image, the back curvature and the front curvature and generating a virtual entity of the lens blank in the computer environment. PA1 defining a spatial coordinate system in a computer environment; PA1 analysing an image of a lens blank and locating a contour of the lens blank and manufacturer's markings on that lens blank in the spatial coordinate system; PA1 measuring a back curvature of the lens blank and locating a position of the back curvature in the spatial coordinate system; PA1 probing a front curvature of the lens blank and locating a position of the front curvature in the spatial coordinate system; PA1 generating a virtual entity of the lens blank in the computer environment and locating the virtual entity in the spatial coordinate system; PA1 locating a support block and a molding ring around the support block within the spatial coordinate system; and PA1 while monitoring a position of the virtual entity relative to the spatial coordinate system, positioning the lens blank above the support block and the molding ring, at a distance from the support block and the molding ring, and injecting bonding material between the lens blank and the support block for bonding the lens blank to the support block. PA1 defining first and second spatial coordinate systems in a computer environment; PA1 analysing an image of a lens blank and locating a contour of the lens the blank and manufacturer's markings on that lens blank in the first spatial coordinate system; PA1 measuring a back curvature of the lens blank and locating a position of the back curvature in the first spatial coordinate system; PA1 probing a front curvature of the lens blank and locating a position of the front curvature in the first spatial coordinate system; PA1 generating a virtual entity of the lens blank in the computer environment and locating this virtual entity in the first spatial coordinate system; PA1 locating a support block and a molding ring around the support block within the first spatial coordinate system; PA1 while monitoring the virtual entity in the first spatial coordinate system, blocking the lens blank on the support block at a distance from the support block and from the molding ring; PA1 assigning reference features to the support block in the first spatial coordinate system, and assigning the virtual entity to these reference features; PA1 locating a lens generating apparatus in the second spatial coordinate system; PA1 installing the lens blank and support block assembly in the lens generating apparatus; PA1 locating the reference features of the support block in the second spatial coordinate system; PA1 locating the virtual entity in the second spatial coordinate system; PA1 simulating first and subsequent tool paths on the virtual entity; PA1 evaluating optical error in each of the first and subsequent tool paths; PA1 defining one of the first and subsequent tool paths having minimum optical error; PA1 assigning the one of the first and subsequent tool paths having minimum optical error as the ideal tool path; PA1 programming the ideal tool path in the lens generating apparatus; and PA1 while monitoring a position of the virtual entity in the second spatial coordinate system, generating a surface on the lens blank using the programmed ideal tool path.
Given that a large percentage of the lenses have aspherical front surfaces, segmented bifocals or spherical-aspherical front halves, the placing of a lens on a circular support ring or on a circular block is prone to positioning errors. In these cases, the contact surface between the lens and the support ring or block is only a partial circle or few contact points wherein the lens may tilt during the clamping motion and the axes thereof may be out of referential position relative to the block's datum plane.
The efficiency of computerized numerically controlled (CNC) lens generating apparatus depends greatly upon the precision of the lens blocking process and the available information about the lens blank, for programming the controller of the CNC machine. For example, when the locations and curvatures of both surfaces of a lens blank are known precisely, it is possible for the controller of the machine to calculate the amount of material to be removed during the surface generating process, the exact coordinates for an initial cut, the total number of cuts required, the depth of cut for a finishing pass and the total duration of the surface generating process. These values may thereafter be analysed and integrated for programming a lens generating apparatus and generating lenses of a highest quality, at an optimum rate of processing.
Generally, it has been difficult with the lens blockers of the prior art, to obtain consistent blocking and accurate information about the front and back surfaces and thickness of a lens blank for supporting a significant optimization of the efficiency of a lens generating apparatus. In the past, emphasis has been placed on the precision in the positioning of the optical references of a lens blank relative to the support block. The physical locations and curvatures of both surfaces have been of a secondary interest.
For this reason, and despite the use of ultra-precision computer-controlled apparatus, ophthalmic lens generated nowadays are not always errorless. Ophthalmic lens are often thicker than they could be for providing a good appearance when mounted in a thin frame for example.
As explained before, the devices and apparatus of the prior art have attached little importance to the measurement of the exact positions of both surfaces of a lens blank. As a result, a first surface is often generated in a lens blank without knowing precisely where the second surface really is.
Furthermore, some devices and apparatus of the prior art use lens supports and vacuum picking cups made of resilient material such as rubber for example. Although the positioning of the lens is precisely effected at the imaging station, the manipulation of the lens using flexible members prior to bonding the lens can cause slight misalignment of the lens during the bonding of the lens to the support block thus causing a prism error or other defects in the generated lens.