Abstract:
The invention relates to a tool for polishing optical surfaces, comprising a rigid body ( 10 ) which is equipped with a polishing face ( 12 ) and an elastic sheet ( 20 ) which is disposed between the polishing face ( 12 ) and the optical surface to be polished. According to the invention, the polishing face ( 12 ) of the rigid body ( 20 ) takes the form of a portion of a geometric surface which is defined by at Least three parameters. In a preferred embodiment of the invention, the aforementioned surface is atoric. The inventive method consists in selecting the above-mentioned tool from a set of available tools, applying an elastic sheet ( 20 ) thereto, polishing at least one optical surface and changing the elastic sheet ( 20 ) before polishing other optical surfaces. The original shape is maintained with greater precision in relation to the entire surface to be polished, but particularly close to the edges and/or at the centre thereof.

Description:
[0001]     The present invention relates to a tool for polishing optical surfaces comprising a rigid body having a polishing face at least as large as an optical surface to be polished, and a resilient pad arranged between said polishing face and the optical surface to be polished.  
         [0002]     The invention also relates to a method that uses such a polishing tool.  
       BACKGROUND OF THE INVENTION  
       [0003]     In the field of the manufacture of optical lenses it is known to polish a previously machined tens surface by means of a polishing tool.  
         [0004]     A conventional type of polishing tool comprises a spherical support surface having a diameter similar to that of the lens to be polished, on which a polishing material, such as felt, is applied. The tool is brought into contact with the tens, and the lens surface is polished by the combined effect of a pressure between the tool and the lens and an orbital rotation between them, and with the aid of an abrasive slurry.  
         [0005]     By virtue of the developments in the machining techniques in recent years, the machining operation can now provide a lens surface matching very closely the final desired shape; as a result, the polishing operation of such lenses is intended to reduce the average roughness Ra of the surface to values of between 0.002 and 0.005 μm, but without altering or deforming the machined shape. The effect of the polishing operation must therefore be accurately controlled.  
         [0006]     Some polishing tools have been developed in which the polishing surface can adapt to a certain extent to the shape of the tens to be polished: for example, U.S. Pat. No. 3,583,111 describes a tool for a smoothing operation, having a curved base surface on which a resilient cushion is applied; and EP1249397 describes a toot for polishing atoric optical surfaces, in which a rigid base surface underlying a layer of resilient material is toric in shape; this allows matching more closely the tool with an atoric lens surface to be polished, and therefore in general it allows a higher degree of accuracy.  
         [0007]     However, in order to avoid any changes in the machined shape there should be a perfect contact between the polishing surface and the Lens surface, and this is possible only with tenses having rotational symmetry: spherical and toric surfaces.  
         [0008]     Even the polishing tool known from EP1249397, which in general maintains quite well the original shape of an atoric lens as machined, tends to rub too much or too little in some areas, especially near the edges of the tens and in the portion that is closer to the centre, and therefore the tens tends to result slightly deformed, or remain too rough, in such areas.  
       DESCRIPTION OF THE INVENTION  
       [0009]     It is an object of the present invention to provide a tens polishing tool and a method for polishing complex optical surfaces, such as atoric surfaces and free forms, i.e. progressive-power surfaces and such surfaces on which additionally a toric or atoric surface is overlapped, by which the mentioned drawbacks are at least reduced.  
         [0010]     In particular, it would be desirable to maintain as much as possible the original shape of the surface to be polished across the whole lens surface, including the edges and/or the central portion thereof.  
         [0011]     It would also be desirable to provide a tool and method by which the polishing operation can be carried out in a reliable and fast way on a large variety of optical surfaces.  
         [0012]     According to a first aspect, the present invention relates to a toot for polishing optical surfaces as defined in the introduction, wherein the polishing face of the rigid body is configured as a portion of a geometric surface, said geometric surface being defined by at least three parameters.  
         [0013]     By virtue of this features, the toot can match closely the shape of an optical surface to be polished, namely in the case of surfaces with free forms, and can be more accurate than existing tools in polishing evenly all the lens surface, including the portions near the edge and in the centre of the optical surface.  
         [0014]     In embodiments of the invention, said geometric surface is an atoric surface.  
         [0015]     Preferably said atoric surface is a surface of revolution; in one advantageous embodiment, the generatrix of said atoric surface is an ellipse.  
         [0016]     The resilient pad may comprise a first Layer of flexible material of a thickness of at least 3 mm and a second layer of polishing material; the first layer of flexible material has preferably a thickness between 3 and 15 mm, for example between 3 and 8 mm.  
         [0017]     In preferred embodiments of the invention, the resilient pad is provided separate from said rigid body and comprises an adhesive film covered with a peelable protection paper.  
         [0018]     This allows storing the resilient pads before use, and to apply them when necessary to a polishing too( in a fast and simple way.  
         [0019]     In order to facilitate the change of the resilient pad applied to a tool, said resilient pad may comprise means for its removal from the polishing face after use.  
         [0020]     According to a second aspect, the invention relates to a method for polishing optical surfaces, characterised in that it comprises the following steps:  
         [0021]     (A) providing a polishing tool comprising a rigid body, said body having a polishing face at least as large as an optical surface to be polished and configured as a portion of a geometric surface defined by at least three parameters;  
         [0022]     (B) applying a resilient pad to said polishing face;  
         [0023]     (C) polishing at least one optical surface with said tool, by contacting an optical surface with the toot and causing relative motion between them while applying a predetermined pressure, during a predetermined time; and  
         [0024]     (D) changing said resilient pad before polishing further optical surfaces.  
         [0025]     Said step (A) of providing a polishing toot preferably comprises determining by the least square method the parameters of the atoric surface matching more closely the optical optical surface to be polished, and selecting from a set of available tools the toot having a polishing surface having parameters closer to those determined by the least square method.  
         [0026]     In some embodiments of the invention, said step (B) of applying a resilient pad to the polishing face is carried out by means of a tool comprising a support for the rigid body and means to press the resilient pad against the body with a predetermined pressure.  
         [0027]     The use of such a tool facilitates the mounting operation and avoids the risk of uneven adherence between the resilient pad and the body due to insufficient pressure.  
         [0028]     Preferably said means for means to press the resilient pad against the rigid body comprise a cylindrical silicone block of medium hardness; with this system, a single silicone block is enough for exerting pressure against a whole set of rigid bodies, without the need of providing a counterbody for each body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     For a better understanding of what has been set forth, the invention will be described in the following with reference to the appended drawings, which schematically and by way of non-limiting example show embodiments of the invention.  
         [0030]     In said drawings:  
         [0031]      FIGS. 1   a  and  1   b  show a polishing toot according to an embodiment of the invention, with the two components separate and assembled, respectively;  
         [0032]      FIG. 2  shows a detail of the resilient pad of the tool of  FIGS. 1   a  and  1   b ; and  
         [0033]      FIG. 3  is a graphical representation of an atoric surface with an elliptic generatrix, showing the parameters needed for its definition. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]     As shown in  FIGS. 1   a  and  1   b , a polishing toot according to the invention comprises an essentially cylindrical rigid body  10  having a mounting side  11 , intended to mount the tool by conventional means in a polishing device, and a polishing face  12 , opposite the mounting side.  
         [0035]     The diameter of the rigid body  10  is similar to that of the optical surface to be polished, or preferably larger.  
         [0036]     The polishing face  12  of the rigid body is a curved surface, which is configured as a portion of a complex geometric surface defined by at Least three parameters.  
         [0037]     In the embodiment shown in  FIGS. 1   a  and  1   b  the polishing face  12  is convex and it is a portion of an atoric surface. The parameters defining said atoric surface depend on the shape of the optical surface to be polished; their determination will be described later on.  
         [0038]     The skilled man will understand that the polishing face may also be concave, depending on the curvature of the optical surface to be polished.  
         [0039]     A resilient pad  20  is applied to the rigid body over the polishing face  12 ; the resilient pad  20  comprises a first layer of flexible material  21 , which is relatively thick and is applied on the polishing face  12 , and a second layer of polishing material  22 , which is thinner than said first layer  21  and constitutes the outer surface of the pad  20 , intended to come into contact with the optical surface to be polished.  
         [0040]     A suitable material for the first layer  21  is a physically cross-linked polyolefin foam, for example a foam having a density of 40 kg/m 3  and a resistance to a 50% compression of 95 kPa. Another suitable material for the first layer  21  is a closed cell polyurethane foam with a Shore 00 hardness lower than 50, chemically foamed.  
         [0041]     With these materials, the first layer  21  is ideally at least 3 mm thick; a thickness under 8 mm should be enough for the vast majority of applications, but depending on the hardness of the material, thicknesses of up to 15 mm could be foreseen, since this would allow using a smaller number of different tools.  
         [0042]     The second layer  22  is made of felt or another similar material, which may be natural or synthetic. These materials are well known in the polishing of optical tenses, and have the function of retaining the abrasive particles contained in suspension in the polishing liquid.  
         [0043]     The second Layer  22  is firmly attached to the first flexible layer  21 , for example by means of an adhesive  23 .  
         [0044]     As shown in the perspective view of  FIG. 2 , where the different layers of the resilient pad  20  have been partly separated for a better understanding of its structure, in this embodiment the resilient pad  20  also comprises an adhesive film  24  applied on the surface which is intended to be in contact with the polishing face  12  of the rigid body  10 , and a protection layer  25  that is peeled off when the resilient pad  20  has to be applied to the polishing face  12  of the rigid body  10 .  
         [0045]     As can be seen in  FIG. 2 , the contour of the resilient pad  20  is essentially cylindrical, much like that of the rigid body  10 , but the pad has a projecting tab  26  on one side, for allowing easy removal of the pad  20  from the rigid body  10  after use, as will be described below.  
         [0046]     Essentially, a method for polishing an optical surface with this toot comprises selecting a rigid body  10  having an atoric polishing face which is appropriate for the optical surface to be polished, from a set of available rigid bodies; applying a resilient pad  20  to the polishing face of the selected rigid body  10  and placing it in a polishing machine; and applying a predetermined pressure and relative motion between the toot and the optical surface, for a predetermined period of time. After this, the resilient pad  20  may be stripped off from the rigid body  10  and changed with another pad. This change can be made after every optical lens is polished, or a small number of tenses can be polished with the same pad  20  before it is changed, for example between two and five.  
         [0047]     In the following, each step of the method will be explained with the aid of examples.  
         [0048]     A set of rigid bodies is provided for use in the polishing method, each having a polishing face configured as a portion of a geometric surface defined by at least three parameters; for the sake of simplicity, in general only atoric surfaces of revolution, and more particularly those generated by an ellipse travelling along a circumference, will be used.  
         [0049]     Such a surface needs to be defined by three parameters: the two semi-axes (a,b) of the ellipse that generates the atoric surface, and the radius (R) of the circumference. A representation of the parameters of such a surface is shown in  FIG. 3 .  
         [0050]     It has been verified that in order to polish progressive lenses with prescriptions between −6 and +6 dioptres sphere, between 0 and −4 dioptres cylinder, and additions between 0.75 and 3.50 dioptres, a set of 108 rigid bodies is enough for obtaining a good result in the polishing operation, i.e. a very high proportion of tenses that are polished correctly.  
         [0051]     Starting from a user&#39;s tens prescription, the atoric surface which more closely matches the tens shape (“best atoric surface”) is determined by the least square method.  
         [0052]     The following table summarizes the results of this determination, for three different lens prescriptions Pr 1 , Pr 2  and Pr 3 .  
         [0053]     To each prescription is further overlapped the particular progressive lens design; the resulting lens surface is defined by a number of points each representing the distance from a given plane. In the present examples the determination was made on the basis of surfaces defined by 90000 points.  
                                                                                                                                   Lens prescription   Parameters of best atoric                    sphere   cylinder   axis   addition   a   b   R   χ 2             (Dp)   (Dp)   (°)   (Dp)   (mm)   (mm)   (mm)   (mm)                        Example   0   −4   0   3.5   35.8263   48.7000   108.6473   3395.42       Pr1       Example   4   −4   0   1   54.6479   63.2862   155.5229   346.61       Pr2       Example   −2   −2   45   2   62.0229   72.1739   62.4988   1071.2388       Pr3                  
 
         [0054]     The last column of the table shows the chi-square value for each example, that is, the sum of the square errors between the points of the surface to be polished and the corresponding points of the best atoric surface.  
         [0055]     On the basis of the parameter values obtained, one tool is selected among a number of available tools having atoric polishing faces; a tool having parameters a, b, R as close as possible to those determined by the least square method will be selected in each case.  
         [0056]     It has to be taken into account that the resilient pad provides a certain degree of adaptation between the polishing face of the rigid body and the optical surface to be polished, and thus allows very good results in the polishing operation without the need to provide an extremely large number of tools having different atoric surfaces.  
         [0057]     A comparative example was carried out, to show the advantage of using an atoric polishing surface.  
         [0058]     In this comparative example the best toric surface, i.e. the toric surface more closely matching the shape of the tens to be polished, has also been determined for the same tens prescription and the same progressive tens design indicated above, also by the least square method; the following table summarizes the results, namely the parameters a′ and b′ (radius of the two circumferences) of the best toric surface for each example, and the corresponding chi-square value.  
                                                                                                                                             Lens prescription   Parameters of                    addi-   best toric                    sphere   cylinder   axis   tion   a′   b′   χ 2             (Dp)   (Dp)   (°)   (Dp)   (mm)   (mm)   (mm)                        Example   0   −4   0   3.5   79.5383   79.5383   13914.3       Pr1       Example   4   −4   0   1   98.1671   98.1671   15127.0       Pr2       Example   −2   −2   45   2   72.4372   69.5343   5058.9       Pr3                  
 
         [0059]     As can be seen in the last column of the table, in this case the chi-square values for each example Pr 1 , Pr 2 , Pr 3  are remarkably higher than in the case of atoric surfaces; this clearly show that the best atoric surface matches more closely the desired Lens surface than the best toric surface.  
         [0060]     Consequently, the polishing operation with a toot having such an atoric surface will give a better result in terms of maintaining the shape of the machined lens surface than if the polishing operation was carried out using a toric toot.  
         [0061]     Once the appropriate rigid body with an atoric polishing surface has been selected as described in the example above, a resilient pad  20  is applied to it. This operation can be carried out manually, but for a better result it is preferable to apply the pad  20  by means of a small press: the rigid body is arranged in the press and the pad  20 , previously stripped of the protection layer  25 , is placed in position on the polishing face  12 ; the press is then lowered such that a curved rubber cylinder, loosely matching the curvature of the rigid body and having a diameter larger than that of the rigid body, or preferably a medium hardness cylindrical silicone block, presses the resilient pad  20  against the rigid body  10  causing it to adhere firmly. The diameter of the silicone block is preferably larger than that of the rigid body  10 , and the end of the block that presses the pad is concave in shape, in case the surface  12  is convex; thus, a single silicone block is enough for a whole set of convex rigid bodies. As mentioned, this silicone block is of medium hardness and preferably about 50 mm high.  
         [0062]     The operation of stripping the resilient pad from the rigid body after use can be carried out manually, by pulling the projecting tab  26 , or by means of a simple tool.  
         [0063]     The rigid body  10  with the applied resilient pad  20  is placed in a known polishing machine, together with the tens to be polished, and the polishing operation is started. The polishing time may be less than one minute for most operations, and the polishing pressure is relatively small.  
         [0064]     For example, the parameters of the polishing process for polishing a tens of organic material having a diameter of 70 mm can be as follows: 
        inclination between the rotation axis of the lens and the axis of the tool: 0° to 5°, preferably 3°    offset distance between axes: 14.0 mm     displacement between axes: 7 mm     force between lens and polisher: 4.5 kg     rotation speed: between 960 and 1000 rpm, preferably 960 rpm     polishing time: 45 to 60 seconds, preferably 45 seconds     polishing slurry: suspension of Al 2 O 3  in water at 7° C., with a concentration of 18° (Beaumé) at 20° C.        
 
         [0072]     Although only particular embodiments of the invention have been shown and described in the present specification, the skilled man will be able to introduce modifications and substitute any technical features thereof with other technically equivalent, depending on the particular requirements of each case, without departing from the scope of protection defined by the appended claims.  
         [0073]     For example, the resilient pad  20  could be applied and maintained on the rigid body by other appropriate means, different from an adhesive film.