fig1 a - 1 i disclose a metal substrate during a number of consecutive method steps of a method in accordance with an embodiment of the present invention . eventually , in fig1 i , there is disclosed an optical element as created using a manufacturing method of the present invention . in fig1 a , the method starts with metal substrate 1 , schematically illustrated as a block . the surface 2 of the metal substrate 1 will be the subject of a plurality of method steps for creating nanostructures forming a negative master structure for manufacturing an optical element therewith . the metal substrate 1 is preferably made of aluminium having a degree of purity of at least 99 . 99 %. alternatively , the metal substrate may be made of aluminium having a different degree of purity , or even a different metal that is suitable for processing by means of anodization , such as titanium , zinc , magnesium , niobium , and tantalum , and any alloys comprising at least one of these elements . the use of aluminium having a degree of purity of at least 99 . 99 % is preferred in order to provide a surface 2 which is ( to a large degree ) void of impurities that may give rise to undesired deformations during the anodization step . optionally , prior to the actual start of the manufacturing process , the surface 2 of the metal substrate may be heat treated , polished and / or cleaned such as to obtain a smooth surface that fits the requirement for desired optical surface roughness . with or without the optional steps of heat treating , polishing and cleaning of the surface , the manufacturing method according to the present invention primarily exists of a preparation phase and a forming phase . during the preparation phase , the substrate is prepared in such a manner that it will be straightforward during the forming phase to create nanostructures of different sizes . as will be appreciated , this is required in order to achieve a hybrid moth eye structure type surface rather than a moth eye structure type surface having nanostructures that on averagely have all the same size . the preparation step starts with a step of anodizing at a first voltage . the results of the step of anodizing are illustrated schematically in fig1 b . fig1 b again shows the metal substrate 1 . on a surface 2 of the metal substrate , as a result of the anodization step , a first metal oxide layer is present . assuming that the method has started with a metal substrate made of aluminium having a degree of purity of at least 99 . 99 %, the first metal oxide layer 3 is a layer of alumina ( al 2 o 3 ). the anodization step has created a typically closed aluminium oxide ( alumina ) layer , which is amorphous and consists of interconnected islands 5 of alumina with nanopores 6 in between . typically , the size of the interconnected alumina islands 5 is proportional with the anodization potential : the first voltage . the packing and localization of the alumina islands is caused by coulomb interactions between the charged domains . as a result , more or less regularly distributed across the surface 2 of the metal substrate 1 the nanopores 6 are formed in between the alumina islands 5 . the depth of the nanopores is slightly less than the thickness of the alumina layer 3 . as a result , the bottom of each nanopore 6 is not in contact with the aluminium of the metal substrate 1 underneath the alumina layer 3 . the next step in the process is the step of selective etching performed mainly in the nanopores 6 . the selective etching is performed using an etchant such that etching of the aluminium is performed more intensively than etching of the metal oxide . an example of such an etchant is known as pes 77 - 19 - 04 ( the chemical composition at 30 ° c . being : h 3 po 4 : ch 3 cooh : hno 3 = 77 : 19 : 04 ). this etchant provides good results , although this is certainly not the only suitable etchant available and the invention is not limited to this specific etchant . the etching of the nanopores 6 results in deepening of the bottom of each pore until the aluminium is reached . then , since the etchants more intensively etches away the aluminium than the aluminium oxide , the etching step mainly results in the nanopores 6 extending a certain distance 8 into the aluminium of the metal substrate 1 underneath the alumina layer 3 . this is illustrated schematically in fig1 c . in fig1 c , the metal substrate 1 with the alumina layer 3 is illustrated , again indicating the alumina islands 5 and the nanopores 6 . each of the nanopores 6 extends over a certain distance 8 into the metal underneath the alumina layer 3 as a result of the selective etching step . after the step of selective etching , the alumina layer 3 is removed by means of any suitable method therefore . as will be appreciated , the alumina layer 3 may be removed by another selective etching step using chromic acid . this leaves the aluminium underneath the alumina layer 3 in tact , and primarily removes only the alumina layer . the result of this step is illustrated in fig1 d . fig1 d again illustrates the metal substrate 1 with this time only the ends 8 of the nanopores present on the ( new ) top surface 2 ′ of the metal substrate 1 . optionally , and in accordance with the preferred embodiment , a further step of anodization is performed at a second voltage level , the second voltage being lower than the first voltage . since the second voltage is lower than the first voltage , the size of alumina islands 11 will be smaller than the size of the alumina islands 5 that resulted after the first anodization step . as a result of the present anodization step at the second voltage level , a large number of alumina islands 11 with nanopores 10 in between will be present on the surface 2 ′, as illustrated in fig1 e . further to this , the ends 8 of the original pores 6 that extend to a more deeper level into the aluminium of the metal substrate 1 are also still present in the surface 2 ′, as illustrated in fig1 e . in the next method step , the alumina layer 9 created by the last anodization step is removed , and the result is illustrated in fig1 f . the new surface 2 ″ is a rather rough surface consisting of small indentations alternated by the pores 8 . this is the end of the preparation phase of the manufacturing method . in the subsequent forming phase of the manufacturing method , a multistep anodization and etching procedure is carried for forming the 5 desired nanostructures on the surface 2 ″. the step of anodization may be carried out at the second voltage level creating small islands of alumina with nanopores in between . the etching step will widen all of the pores slightly , after which a further deepening is carried out by ( optionally ) performing an additional anodization and etching step . the steps of anodization and etching may be carried out a number of times for creating nanostructures of desired size ( having pores of a desired width ). in fig1 g , the end result of this step is visible , again illustrating the metal substrate 1 of aluminium , with an aluminium oxide layer 14 on top . the aluminium oxide layer 14 consists of a large number of pores 15 which are every so often alternated by the pores 8 . fig1 g resembles the result after the manufacturing method of the present invention providing a negative master structure or mould that enables to create a positive master structure or optical element comprising nanostructures as desired . the mould is formed by the metal substrate 1 . the mould 1 as such created is covered by a suitable polymer or metal in a liquid ( e . g . molten ) state for manufacturing the positive master structure or optical element . the result of covering the surface 2 ″ of the mould 1 with the liquid polymer is illustrated in fig1 h . here , the metal substrate 1 or mould 1 is illustrated having the alumina layer 14 comprising the large pores 8 and the smaller pores 15 in between . these are covered with a layer of liquid or molten material 18 , e . g . a polymer of a suitable kind , or a metal or the like . the liquid layer 18 is then cured using any suitable method , after which the positive master structure 18 is to be separated from the negative master structure or mould 1 . separation may be performed by removing the alumina layer 14 using any suitable method , e . g . selective etching . in fig1 i , the optical element 18 which is created using this method is illustrated , having smaller nanostructures 20 and larger nanostructures 22 on the surface thereof . the surface consisting of small nanostructures 20 and large nanostructures 22 resembles a hybrid moth eye structure type surface . as will be appreciated , the larger nanostructures 22 have been created as a result of the preparation step or preparation phase performed prior to the forming step . the preparation step included a step of anodizing of the surface 2 of the metal substrate 1 at a first voltage level . it is recalled that the size of the alumina islands 5 in fig1 b , and thereby the locations of the nanopores 6 and the distance in between , is proportional to the magnitude of the first voltage used during anodization . the nanopores 6 were then deepened by means of selective etching . as will be appreciated , these steps may optionally be repeated at again a different voltage level e . g . for creating eventually nanostructures of three or more different sizes . fig2 schematically illustrates the method of the present invention . the method 30 starts with providing the metal substrate 31 , and subjecting the surface of the metal substrate consecutively to the steps of heating 34 , polishing 35 and cleaning 36 . as already explained above , the steps of heating , polishing and cleaning ( 34 , 35 , 36 ) are optional steps and may be dispensed with . however , in the preferred embodiment , these steps are performed prior to the preparation phase of the manufacturing method 30 . after cleaning in step 36 , the preparation phase 40 starts with the step of anodizing 41 which is performed at a first ( high ) voltage . this voltage is preferably taken to be within a range of 80v through 150v , more preferably 80v through 120v , for example 100v . this creates a first oxidized metal layer containing a number of first pores . these pores in step 42 are deepened by means of a selective etching process , as explained above . then in step 45 , the first metal oxide layer created during the anodization step is removed , for example again by means of a selective etching step using a different etchant ( e . g . chromic acid ). optionally , and in accordance with the preferred embodiment , in step 46 a further anodization is performed at a second voltage , wherein the second voltage is smaller than the first voltage . comparatively , the second voltage may be a voltage selected within a range of 20v through 60v ( preferably 30v through 50v , for example 40v ). this anodization may be performed for any suitable duration . it has been experienced that , in accordance with the preferred embodiment , the duration of this anodization step at low second voltage is performed for a duration of at least 12 hours . comparatively , the anodization at the first high voltage during the preparation phase 40 may also be performed for a long duration , preferably 12 - 15 hours . the anodization step at the lower second voltage in step 46 creates a third metal oxide layer on the metal substrate 1 . in step 48 , after the anodization of step 46 , the third metal oxide layer is removed by means of for example , an etching step using chromic acid . the preparation phase 40 is then complete , and the method continues with the forming phase 50 . during the forming phase 50 of the method 30 , first the metal substrate 1 is subject to anodization at a low voltage level , e . g . this may be the second voltage level that is also used during step 46 . this is followed by etching step 54 wherein the second pores formed in the second oxidized metal layer during anodization 52 are widened . then after etching step 54 , at the choice of the skilled person ( step 55 ), the steps 52 and 54 may be repeated for as long as necessary such as to create nanostructures having the desired shape and size . preferably , the steps 52 and 54 in this stage of the method of the present invention are performed multiple times the alternating manner as indicated . it has been found that good results are achieved by performing these steps five times , although the skilled person will appreciate that the number of times wherein these steps are performed may depart from this ( e . g . once , 2 times , 3 times , 4 times , 6 times , 7 times , 8 times , etc . ), dependent on the depth end width of the pores to be formed . the forming step 50 of the method then ends and a negative master structure or mould has been created from the metal substrate . then in step 60 , for creating an optical element or positive master structure , the mould is filled with a metal or polymer in a liquid state . this is cured or solidified , after which in step 63 the positive master structure is separated from the negative master structure , e . g . by removing the oxidized metal layer in between the positive master structure and the negative master structure as illustrated in fig1 h . the above described procedure is a wet chemical procedure which is based on anodization and etching . as a result , this process can be performed on large surfaces at once , for creating large hybrid moth eye structure type surfaces with a relatively small degree of defects . the density , shape , and size control of the nanostructures created are all based on physical and chemical parameters that may directly be controlled throughout the manufacturing process such as voltage , electrolyth , chrystal grain size , reaction time , etc . the method of the present invention is free of any mechanical , optical or other techniques for creating the desired surfaces . a skilled person however appreciates that , if this is for any reason considered beneficial , the method of the invention may be enhanced with any such steps at the choice of the skilled person . in addition to the above , although the figures and embodiments described illustrate the method by applying it to a metal substrate in the form of a block ( or plate ), the method is not limited to such applications , and as a result of the fact that the method of the present invention is a wet chemical procedure , it may be applied to any object of arbitrary size and shape . the present method can be used in the production of coatings for any optical system that is required to work over a broad spectral range . for example , the method may be applied for creating optical elements for ( semiconductor ) lithographic systems , lenses , or any other anti reflective surfaces . with the method of the present invention , the anti reflective properties of a surface may be extended to wave length ranges as large as between 200 nm through 1100 nm . the method of the present invention may also beneficially be applied for creating optical elements for use in astronomic appliances or in space . the coatings or elements created herewith will help to reduce stray light and may be applied to curved surfaces for which there is presently no suitable technique available . the present invention has been described in terms of some specific embodiments thereof . it will be appreciated that the embodiments shown in the drawings and described here and above are intended for illustrative purposes only , and are not by any manner or means intended to be restrictive on the invention . the context of the invention discussed here is merely restricted by the scope of the appended claims .