Abstract:
Method of manufacturing a lens assembly by means of a replication process, wherein the following steps are carried out i) introducing a first, liquid, UV curable composition ( 2 ) into a first mould ( 1 ) provided with regularly spaced-apart cavities ( 6 ), ii) curing said first composition by UV radiation so as to obtain a first lens element comprising lenses arranged beside each other, wherein the surface of the obtained lens element is the negative of that of the cavities, in) applying a second, liquid, UV curable composition ( 5 ) to the first composition cured in step ii), iv) placing a second mould ( 4 ) on the second composition ( 5 ) applied in step iii), which second mould is provided with regularly spaced-apart recesses ( 7 ), in such a manner that said recesses will fill with the second composition, v) curing the second composition by UV radiation so as to obtain a second lens element comprising lenses arranged beside each other, wherein the surface of the obtained lens element is the negative of that of the recesses, and vi) possibly removing the first and/or the second mould.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of manufacturing a lens assembly by means of a replication process. The invention further relates to a lens assembly, a lens stack as well as a camera provided with such a lens assembly. 
     From U.S. Pat. No. 3,532,038 there is known an optical system in which a transparent base member is provided with lenticular lens cavities, which cavities are filled with a refractive fluid, the surface of which fluid is covered with a cover member. The cover member is provided with an aperture plate, on which finally a second base member is present, which is also provided with lenticular lens cavities, which cavities are likewise filled with a refractive fluid. 
     From US 2004/0100700 there is known a method of manufacturing a microlens array, wherein the cavities in a mould are filled with a UV curable resin, whilst the resin outside the cavities is removed by placing a transparent quartz board on top of the mould. The fluid present in the cavities is then formed into a plurality of separate lenses, whereupon a second UV curable resin layer is applied to the transparent board, which resin layer is cured by making use of the already formed separate lenses. The excess amount of the cured second resin layer is removed by using an organic solvent. Only one layer of replicated lenses is mentioned in said document, which lenses are separately arranged and do not exhibit any interconnection. 
     The replication process referred to in the introduction is known per se from U.S. Pat. Nos. 4,756,972 and 4,890,905, which disclose the possibility of manufacturing a high-quality optical component by means of a replication process. 
     Such a replication process is considered to be a quick and inexpensive manner of manufacturing optical components in large numbers. In the replication process, a mould having a precisely defined surface, for example an aspherical surface, is used, and a small amount of a radiation curable resin, for example a UV curable resin, is applied to the mould surface. Subsequently, the resin is spread over the mould surface, so that the cavities in the mould are filled with the resin, after which the whole is irradiated so as to cure the resin and the thus cured product is removed from the mould. The cured product is a negative of the mould surface. An advantage of the replication process is that lenses having a complex refractive surface, such as an aspherical surface, can be manufactured in a simple manner without having to subject the lens body to intricate grinding and polishing processes. 
     From International application WO 03/069740 in the name of the present inventor there is also known a replication process by which an optical element is formed. 
     From the above state of the art there are thus known methods by which optical systems are obtained which are made up of separately manufactured optical elements, as a result of which the dimensions of such systems may be considered to be large. In addition, the positional accuracy, viz. in the X, Y and Z directions (between the lens surfaces) of such systems may be called critical. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method of manufacturing a lens assembly by means of a replication process in which the glass substrate that is usually used is no longer required. 
     Another object of the present invention is to provide a method of manufacturing a lens assembly by means of a replication process which minimises the occurrence of shrink phenomena during the curing of the resin. 
     Another object of the present invention is to provide a method of manufacturing a lens assembly by means of a replication process in which a high positional accuracy of the lenses relative to each other is achieved. 
     Another object of the present invention is to provide a method of manufacturing a lens assembly by means of a replication process in which a substantially monolithic structure of optical elements is achieved, wherein various optical functions can be combined. 
     Yet another object of the present invention is to provide a method of manufacturing a lens assembly by means of a replication process by which lenses having a minimum thickness can be produced. 
     The method as referred to in the introduction is characterised in that the following steps are carried out: 
     i) introducing a first, liquid, UV curable composition into a first mould provided with regularly spaced-apart cavities, 
     ii) curing said first composition by UV radiation so as to obtain a first lens element comprising lenses arranged beside each other, wherein the surface of the obtained lens element is the negative of that of the cavities, 
     iii) applying a second, liquid, UV curable composition to the first composition cured in step ii), 
     iv) placing a second mould on the second composition applied in step iii), which second mould is provided with regularly spaced-apart recesses, in such a manner that said recesses will fill with the second composition, 
     v) curing the second composition by UV radiation so as to obtain a second lens element comprising lenses arranged beside each other, wherein the surface of the obtained lens element is the negative of that of the recesses, and 
     vi) possibly removing the first and/or the second mould. 
     One or more of the above objects are accomplished by carrying out the aforesaid steps i)-vi). Both lens elements are thus obtained by using a replication process. In the embodiment in which the first composition is different from the second composition it is possible to impart specific optical properties to the thus manufactured lens assembly, which can also be realised by using a different shape for the cavities of the first mould than for the recesses of the second mould. 
     In another embodiment it is preferable to remove the second mould in step vi) and subsequently carry out a step vii), comprising the placement of a spacer plate on the second composition cured in step v) so as to obtain a first assembly comprising a spacer plate, a second lens element and a first lens element. In this way an assembly comprising a spacer plate, a second lens element and a first lens element is obtained in which the spacer plate can function as a spacer between the first assembly and a second lens assembly to be subsequently provided. The spacer plate comprises an opening which is positioned coaxially with a main optical axis of the lens element in question, whilst in a special embodiment the side of said opening is provided with an anti-reflective coating. 
     According to another possibility, a film is arranged over the uncured first composition prior to carrying out step ii), which film seals the first composition yet to be cured which is present in the cavities of the first mould. When such a film is used, specific optical properties can be obtained, in particular such as diaphragm, anti-reflection, infrared reflection and aperture, but also electric conductivity. Using an electrically conductive film, it has been found to be possible to influence the refractive index of the lenses replicated on the film. Another possibility would be to supply a current to the electrically conductive film, thus making it possible to adapt the curvature of the lenses replicated on the film. In addition, the film has the special property that the occurrence of shrink phenomena during the curing of the resin can be minimised, so that lense irregularities are prevented. 
     A particularly suitable film that is transparent to the wavelength used, in particular in the visible range, viz. 400-700 nm, but also in the infrared range, is a flexible film having a thickness of maximally 0.75 mm, in particular maximally 0.5 mm, more in particular maximally 0.2 mm, which film does not become detached from the contours formed by the cured polymeric material present in the mould during the curing process. A film which is suitable for that purpose is transparent to the wavelength that is used, generally in the visible range, viz. 400-700 nm, but also in the infrared range. In addition, no air inclusions may be present between the composition to be cured and the film covering said composition. In addition to glass, also optical polymers of the acrylate, epoxy and similar types can be mentioned as materials for the film. Examples of suitable film materials are polycarbonate film having a thickness of 0.2 mm, glass type D263T (marketed by Schott) having a thickness of 0.2 mm, Melinex (trademark) PET (marketed by DuPont). Other materials for the film are polyvinyl butyral, polyester, polyurethane or PVC. Specific components may be added to the aforesaid film for the purpose of influencing the optical and mechanical properties, in which connection pigments, fillers and anti-shrinkage agents may be considered. The film functions to influence the optical properties. No bearing properties can be attributed to the film. 
     Using the present method, it has also been found to be possible to place the first assembly on the product obtained after step vi), in which the second mould is removed, so as to obtain a second assembly consisting of, successively, a first lens element, a second lens element, a spacer plate, a second lens element and a first lens element. 
     The present invention thus relates to a lens assembly consisting of, successively, a spacer plate, a first lens element and a second lens element, whilst in a special embodiment a film may be present between the first lens element and the second lens element, to which film specific optical properties can be imparted. It is also possible to arrange a third or fourth lens element on top of the spacer, on the side remote from the first lens element, in which case the above-described replication process may be used. 
     The present invention further relates to a lens stack as defined in the appended claims, as well as to a camera or light source in which such a stack is used. 
     Suitable UV curable compositions include GAFGARD233 (marketed by DuPont, type vinylpyrrolidone), Norland Inc. NOA-61, NOA-63, NOA-65, Three bond AVR-100 and Sony Chemical UV-1003, possibly provided with the usual additives such as initiators, reactive or nonreactive dilutants, crosslinking agents, fillers, pigments and anti-shrinkage agents. 
     The present lens assembly is in particular used in cameras, in which small-size lenses are required. In addition, large-scale production of such lenses must be possible, whilst the positional accuracy of the lenses relative to each other, in combination with the spacer plate, is highly critical. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be explained in more detail with reference to a flow sheet, viz.  FIGS. 1-9 , and a number of applications, viz.  FIGS. 10-13 , in which connection it should be noted, however, that the present invention is by no means limited to such a special embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows a first mould  1 , which is provided with regularly spaced-apart cavities  6 , which cavities  6  have been filled with a first, liquid, UV curable composition  2 . After the resin or the first composition  2  has spread over substantially the entire surface of the first mould  1 , with the cavities  6  and the parts present between the cavities  6  on the first mould  1  being provided with the still liquid first composition  2 , a film  3  is applied, as is shown in  FIG. 2   a.  To effect a proper adhesion between the mould and the UV curable composition, it is preferable to use a silane-type adhesion promoter. 
       FIG. 2   b  schematically shows another embodiment of the film  3 , which film  3  is provided with transparent parts  51 , alternated with non-transparent parts  52 , and which is positioned on the first composition  2  in such a manner that the transparent parts  51  more or less coincide with the light path that will pass through the composition  2  formed as a lens. The use of non-transparent parts  52  prevents the incident light on one lens part moving to the adjacent lens part, which phenomenon is also known as “crosstalk”. Subsequently, UV irradiation takes place, with  FIG. 3  clearly showing that the flexible film  3  present on the first composition  2  will follow the shrinkage caused by the UV curing of said first composition  2 . It is also possible to subject the already cured composition to a post-curing process, followed by stabilisation at a high temperature, for example for 9-12 hours at a temperature of 110-150° C. After the first lens element, comprising the cured composition  2  and the film  3  present thereon, which first lens element comprises an array of cured lenses arranged beside each other, has thus been obtained, a second, liquid, UV curable composition  5  is applied to the film  3 , as shown in  FIG. 4 , whereupon a second mould  4  provided with recesses  7  is placed on the still liquid second composition  5 , in such a manner that the second, liquid composition  5  will spread over the surface of the film  3  and the recesses  7  and the parts present between the recesses  7  on the second mould  4  will be provided with the second composition  5 . In one embodiment the second mould  4 , which is provided with recesses  7 , is so positioned relative to the first mould  1  that the cavities  6  of the first mould  1  will be located opposite the recesses  7  of the second mould  4 . Subsequently, UV curing will take place again, possibly followed by post-curing, after which the second mould  4  will be removed, as is shown in  FIG. 5   a,  with a first lens element  2  comprising cured lenses arranged beside each other thus being present in the first mould  1 , which first lens element  2  is provided with a film  3 , on which a second lens element  5  obtained from the second mould  4  is present. The lenses both of the first lens element  2  and of the second lens element  4  are cured compositions. Because use is made of a first mould  1  and a second mould  4 , the surfaces both of the first lens element  2  and of the second lens element  4  will be the negative of the surfaces of the moulds  1 ,  2  in question, which can be called typical of the present replication process. 
       FIG. 5   b  shows an embodiment in which the second mould  4  has a convex surface  50 , so that the second, liquid composition  5  will exhibit a concave configuration, in contrast to the convex configuration shown in  FIG. 5   a,  so that the second mould  4  will be pressed against the first mould  1 .  FIG. 5   b  furthermore schematically shows a film  3 , from which it appears that the transparent parts  51  are located in the light path that passes through the first composition  2  and the second composition  5 . It is also possible to use a second mould  4  (not shown) which comprises a convex surface as well as a surface provided with recesses. The non-transparent parts  52  are so positioned in the two lens elements  2 ,  5  that the crosstalk phenomenon cannot occur. Suitable UV curable compositions are: polycarbonates, including diethylene glycolbis-(allyl)carbonate, polystyrenes, including polychlorine styrene, polyacrylates, such as poly(trifluoroethyl methacrylate), poly(isobutyl methacrylate), poly(methylacrylate), poly(methyl methacrylate), poly(alphamethyl bromium acrylate), poly(methacrylic acid)-2,3-dibromium propylpoly(phenyl methacrylate poly(pentachlorine phenyl-methacrylate polymer), polyester compounds such as diallylphthalate, poly(vinyl-benzoate), poly(vinylnaphthalene), poly(vinylcarbazole) and silicones in the form of various types of resin materials, as well as acrylic resin, urethane resin, epoxy resin, enthiol resin or thiourethane resin or photopolymer. Exposure preferably takes place with an intensity of between 100 en 2000 W/cm 2 , in particular 700 W/cm 2 , and a dose of 1-15 J/cm 2 , in particular 7 J/cm 2 , a wavelength in the 320-400 nm range and an exposure time of 1-60 seconds, in particular 10 seconds. 
     After removal of the second mould  4 , as shown in the embodiment of  FIG. 5   a,  a spacer plate  8  as shown in  FIG. 6  is placed on the second lens element, in particular on the cured composition  5  present between the lenses of the second lens element, which spacer plate has a thickness or height such that the convexity of the second lens element is less than the aforesaid height or thickness, after which the first mould  1  is removed, as shown in  FIG. 7 , thus obtaining an assembly consisting of, successively, the spacer plate, the second lens element, the film, and the first lens element. The spacer plate  8  is bonded to the second lens element via a UV curable or thermosetting adhesive (not shown). The spacer plate  8  is made of a rigid material, for example glass, silicon or a composite material such as FR4. The spacer plate  8  is so configured that it will not interfere with the light path through the two separate lens elements, and possibly the film  3  present therebetween. The spacer plate comprises an opening which is positioned coaxially with a main optical axis of the lens element in question, whilst in a special embodiment the side of said opening is provided with an anti-reflective coating. The spacer plate  8  is thus only in contact with the second lens element at the position where the composition  5  has cured on the parts of the mould  4  present between the recesses  7  (see  FIGS. 5   a / 5   b ). The aforesaid assembly is then placed on the assembly shown in  FIG. 5   a,  with the thus obtained combination being schematically shown in  FIG. 8 , after which, as shown in  FIG. 9 , the first mould  1  is removed. Also in the embodiment shown in  FIG. 8 , the spacer plate  8  is connected to the assembly shown in  FIG. 5   a  by means of the aforesaid adhesive. In this way an assembly has been obtained which consists of a spacer plate  8  that is provided with two separate lens elements on either side thereof, whilst each individual lens element, which is made up of two lens parts, is provided with a film present therebetween. Using such a method, as disclosed in International application WO 2004/027880, it is possible to obtain separate lens constructions which can be suitably used in cameras. Although the embodiments shown in  FIGS. 2-9  have been described in conjunction with the use of a film, it should be understood that the use of such a film is optional. Although the embodiments shown in  FIGS. 6-9  are based on the assembly shown in  FIG. 5   a,  it should be understood that it is also possible to use the construction shown in  FIG. 5   b.  The four lens constructions shown in  FIG. 9  only serve by way of explanation, and in practice a large number of such lens constructions are produced simultaneously on a wafer level and separated from each other via usual techniques such as the technique disclosed in WO 2004/027880, which document may be considered to be incorporated herein. 
       FIG. 10  schematically shows a lens stack  20  in which a lens assembly obtained by using the present method is used. An optically active element, for example a VCSL (light source), a CCD/CMOS sensor  21 , is provided with a spacer  22 , whilst a glass plate  23  extending along the length of the optical element  21  is positioned on the spacer  22 , which glass plate  23  is provided on either side thereof with lenses  28 ,  29  replicated thereon. Subsequently a spacer  24  is disposed, on which spacer  24  a lens element manufactured according to the present method is present, which comprises a film  25  that is provided with replicated lens elements  26 ,  27  on either side thereof. The spacers  22 ,  24 , the glass plate  23  and the lens elements  28 ,  29 ,  27 ,  26  are bonded together by means of adhesives, for which thicknesses in the order of 5-100 μm may be used. The aforesaid spacers are made of glass, and the replicated lens elements are polymer-based. Although it is indicated herein that the film  25  and the two lens elements  26 ,  27  replicated thereon are located furthest away from the optically active element  21 , it is also possible to use embodiments in which the aforesaid film  25  and the lens elements  26 ,  27  are located closest to the optically active element  21 . The spacer  22  may be made of the same material as the previously discussed spacer  8 . The spacer  22  comprises an opening which is positioned coaxially with a main optical axis of the lens element in question, whilst in a special embodiment the side of said opening is provided with an anti-reflective coating. 
       FIG. 11  schematically shows a lens stack  30 , in which an optically active element, such as a VCSL (light source), a megapixel CMOS sensor  31 , is provided with a spacer  32 , on which spacer  32  a glass plate  33  is positioned, which glass plate  33  is provided on either side thereof with lens elements  43 ,  42  replicated thereon. In the illustrated embodiment, the spacer  34  is integrated in the lens element  43 , which means that the lens element  43  and the spacer  34  form a uniform or inseparable whole. Furthermore an embodiment is possible in which the spacer  34  is provided as a separate component, with the lens elements  40 , the spacer  34  and the lens element  43  thus being durably interconnected by means of an adhesive. According to yet another embodiment, the spacer  34  is integrated in the lens element  40 , so that only one layer of adhesive is required for durably interconnecting the glass plate  33  and the film  41 . Using such integrated spacers, it has been found to be possible to obtain more advantageous tolerance values for the stack height, because the number of adhesive layers and elements to be used has been reduced. Arranged on said spacer  34  is a lens assembly manufactured by means of the present method, comprising a film  41  provided with a first and a second lens element  39 ,  40  replicated on respective sides thereof. In addition thereto, a spacer  35  is provided, on which spacer  35  another lens assembly manufactured according to the present method is disposed, which comprises a film  37  provided with lens elements  36 ,  38  replicated on respective sides thereof. The spacers  32  and  35 , the glass plate  33  and the lens elements  42 ,  43 ,  40 ,  39 ,  38 ,  36  are bonded together by means of adhesives. Although it is indicated herein that the glass plate  33  provided with lens elements  42 ,  43  is located closest to the optically active element  31 , it is also possible to use embodiments in which a film  41  provided with the lens elements  39 ,  40  is arranged top of on the spacer  32 , for example, on which the glass plate  33  and finally the film  37  provided with the lens elements  36 ,  38  are in turn arranged. 
       FIG. 12  schematically shows a film  40  used in the present method, which film  40  is provided with a top coating  44 , which is shown in top plan view in  FIG. 13 . The top coating  44  is provided with regularly spaced-apart openings  45 , which are used as diaphragms. The top coating  44  may for example be a non-transparent layer, for example with a base of chromium oxide. In another embodiment it is also possible to apply an infrared coating as the top coating  44 , which top coating may also have a filter function, for example using a polycarbonate film that will absorb UV light. The film  40  shown in  FIG. 12  may be regarded as the films  25 ,  37  and  41  shown in  FIGS. 10 and 11 .