fig1 shows the surface of security document 1 on which security layer 2 of inventive material m was produced by a printing operation . this may be for example unstructured security layer 2 completely covering the surface of security document 1 . material m contains here in binder 6 a plurality of microcapsules 7 in which individual particles 8 are freely movable floating in a swelling agent . “ pigment particles ” 8 are electrically polar and have a black side and a white side . in the initial state , without a defined electric field previously acting on the particles , particles 8 are aligned randomly . fig2 shows the effect that occurs when this security document is brought into electric field e . it is assumed here that the black half of particles 8 is negatively charged and the white half positively charged . electrophoretic particles 8 floating freely movably in individual microcapsules 7 will then align with applied electric field e in accordance with their polarity . in fig2 the field is so applied that particles 8 all point upward with their black half so that material m , i . e . security layer 2 , appears altogether black when regarded from above . it is to be heeded that for clarity &# 39 ; s sake the figures show microcapsules 7 with particles rather isolated in security layer 2 . in reality , particles 8 or microcapsules 7 are packed relatively densely within binder 6 . likewise , for the sake of better representability the individual layers are not shown in the correct ratios of size . on white background paper , security layer 2 would be clearly recognizable as an all - over black color of the paper . when the electric field is reversed , the white halves of particles 8 turn upward and security layer 2 becomes virtually invisible . if a structured security layer or information is printed on with inventive material m instead of all - over security layer 2 , the electric field can cause the structure or information to switch back and forth between a visible and an invisible state . fig3 and 4 show a further variant . the essential difference over the examples according to fig1 and 2 is that individual particles 9 have two halves of transparent material that are separated by an opaque middle layer . when aligned according to fig3 , security layer 2 is thus opaque . print 10 with certain information applied to document 1 under security layer 2 is then invisible . fig4 shows the position of particles 8 in case of a field rotated 90 °. here the opaque separation layers within particles 8 are perpendicular to the surface of document 1 so that one can see through security layer 2 from above and the information in printed layer 10 becomes recognizable . the example according to fig5 involves a variation of somewhat more complicated construction in which certain information was applied to document 1 with inventive optically changeable material m . fig5 shows a section through a printed letter in a greatly enlarged representation . specifically , background layer 18 , for example a black or dark - gray background , is printed on the basic material of document 1 here . thereabove is layer 4 that is produced of optically changeable material m in the area of the letters to represent information 3 , and of further material 5 , for example conventional printing ink 5 , in edge areas . material m again has particles 8 embedded in microcapsules 7 in binder 6 as shown in fig1 and 2 . particles 8 consist here of a white half and a dark colored half . the dark color of particles 8 corresponds precisely to the color of colored material 5 surrounding information 3 . background layer 18 also has this color . in the state shown in fig5 a viewer looking at document 1 from above will therefore no longer be able to recognize information 3 applied in layer 4 in surrounding material 5 by material m . by applying a suitably electrically oriented field , particles 8 can be rotated so that information 3 appears white on a dark background and in dark surroundings . to prevent information 3 on security document 1 from being made visible at will by applying an external electric field , security document 1 is coated on each side with layer 11 , 12 of transparent conductive material , for example a conductive polymer or the like . layers 11 , 12 are shorted via lines 14 , 15 and microelectric circuit 13 , which is provided for example in the paper of document 1 . on the back of document 1 , i . e . on lower electric layer 11 , there are contact areas 17 likewise connected via line 16 with microcircuit 13 . by inputting an electronic code via contacts 17 to microcircuit 13 the short between electrically conductive layers 11 , 12 can be eliminated . it is then possible to apply a voltage to layers 11 , 12 and thus produce a suitable electric field , thereby rotating particles 8 within material m and making information 3 visible . by polarity reversal of the electric field , i . e . by reversing the voltage on layers 11 , 12 , particles 8 are switched back to the state shown in fig5 . by inputting a further code it can be ensured that microcircuit 13 shorts the two electrically conductive layers again . this again prevents the security document from being made readable in unauthorized fashion by applying voltage to conductive layers 11 , 12 , i . e . document 1 is protected again . fig6 shows an example in which security document 1 is coated with large - surface security layer 2 . in the test device an external structured field is applied in which structured metallic electrode 20 is used on the upper side and all - over counterelectrode 19 on the underside . in unstructured layer 2 on the security document , precisely structure s of structured electrode 20 is then imaged . the examples described in the figures are fundamentally also possible for particles with magnetic polarity that can be aligned in a magnetic field .