CA1274017A

CA1274017A - Method and apparatus for checking the authenticity of documents and documents used therefor

Info

Publication number: CA1274017A
Application number: CA000518396A
Authority: CA
Inventors: Johan Samyn
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1985-09-19
Filing date: 1986-09-17
Publication date: 1990-09-11
Also published as: JPS63501250A; DK169225B1; ES2001689A6; ATE85442T1; EP0236365A1; US4820912A; DE3687707T2; DE3687707D1; DK238987D0; JP2659014B2; NL8502567A; EP0236365B1; DK238987A; WO1987001845A1

Abstract

ABSTRACT :

A method and an apparatus for chocking the authenticity of documents, such as banknotes or credit-cards. The documents comprise a number of randomly distributed conductive fibres of which the distribution is scanned by microwaves and the response is transformed into a digital coded signal. A
digital mark on the document, which is representative for the distribution of an individual document, is read off, transcoded, and compared with said coded signal for producing an approval signal. The use of a microwave scanning system in conjunction with documents having a random distribution of suitable fibres ensures a great repetitivity and security of the system.

Description

~. ~ d ~ f;) H~TilOD A~D APP~RATUS FOR Cfl~C~YINC 'I'~l~ Auq~ NrrcI~ry OF~
I~U NTS US61~ 'rfl~R~F'OR

The lnventlon relates to a method of checklng the authentlclt~ of documents made of a non-conductln~ materlal, such as paper or plastlc, and havlng a physical characterlstlc of whlch the value random~y changes over a number of sub-areas of a checklng area, each of sald ~ocuments further ~omprlstng a dlgltal mark whtch ls c~aracterlsttc ~or the dlstrlbutton of s~ld Yalue over sald sub-a~eas. Such documents are e.g. ~anknotes, passports, credl~ cards, bonds or other securl~y papers. In 50me cases, the packlng ~otl In whlc~ a product ls enYeloped, is also to conslder as a document of whlch the authentlclty must be chec~ed.

It ls kno~n to check ~he authe~tlctty o~ suc~ documents by scannlng the checking-area o~ su~h doc~ments ~or detectlng the dlstrtbutlon of sald v~lue over sald sub-areas, produclng a digltal scannlny slgnal whlch tS characterlstlc for such dlstrlbutlon as scanned, readlng o~f satd dlgttal m~r~ on satd document and produclng a dtgttal marX-s~gnal whlch ls characterls~lc ~or the dtgital ~ark as read o~f, and comparlng sald scannlng-s~gnal wlth sald mark-sl~nal. In such methods the scann~n~ is conducted ~y photo-elect~lc means for detectinq the dlstrlbut~on of the darkness of the sur~ace of the docum~nt. It ls also known per se to scan by mea~s of a magnetlc head ~or detectlng the dls~r~bu~lo~ of ma~netlc partlcles ln the document.

An adapted sca~nlnq method, ln order to ensure a maxlmum of securlty, has to comblne an accurate repet~t1vlty and a hlgh resolutlon, l.e. capabllity to ~ollow sharp varlatlons over short dlstances. ~hen a scanning system ~. " ~
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cannot follow such sharp varlatlons, lt wlll only be capable to dlstlngulsh two coarsel~ dlfferent pat~erns from each other, and wlll not be capable to dlstlngulsh two sllyhtly dlfferent documents from each other. As a result, the docu~ents are less lndlvldualtzed and falsl~tcatton ts more eas~.

~estdes a hlgh resolutlon, an accurate re~et1tlvt~y of the scannlng sl~nal ~s also lmportant. ~ecause of magnettc prehlstory or dlsturblng of ma~net1c ~lelds or defo~matlons of the document, the repet~tlvtty of a magnetlc scanntng system ls not ensured, whereas loss of brlghtness or colour or other damage of the document wlll disturb the repetltlvlty of a photo-electrlc scannlng system. flowever, lf the scannlng s~s~em ts not suff~clentl~ repe~l~tve, the rlsk tha~ an authenttc document would produce a sl~ghtly di~ferent scannlng-slgnal and be ldentlfled as a ~alslf~catton ls not negllgtble. In these clrcumstances lt ls not posslble to submlt the scannlng-slgnal to a severe crlterlon of ldentlty ~lth a gl~en st~nal, and tt has no sense to lmprove the resolutlon as long as the repetlttvlty ls not also lmpro~ed, a~d lnversely.

It i~ an ob~ect o~ the present lnventlon to provlde ~or thls chec~lng method a sca~nlng ~ethod ln whl~h a good resolutlon ls comblned wlth a very hlqh repet~tlvtt~ ln s~ch a way that the scannlng-slgnal can be submltted to ver~ se~ere crtterla for the approval o~ the document, desplte of any wear or llmlted damage o~ the document.

rhe method accordlng ~o the lnventlon ls characterlzed by the ~act that doc~ments are used comprtslng a number of partlcles wlth elec~romagnettc propertles whlch are senslbl~

dlfferent from those of sald non-conductlng mate~lal of the dvcu~ent and whtch are randomllJ dlstr1buted ln at least the chec~lng area of the documænt, and that ~he scannlng ls conducted by means of a mtcrowave bundle that 1mplnges on sa~d checklng-area and by measurlng the response mlcrowave bundle.

Although a mlcrowave bundle leavlng a wavegulde has a thtckness dtmensl~ of ~t least half the waveleng~h, l.e. tn the mtlltmeter to centlmeter range, lt has sur~rlslngly be ~ound that the scannlng allows sufflclent resolutlon to dtvtde the checklng area tn sub-areas o~ the order of 0.25 mm, and that ~he measured values for each sub area are repetttive 1n the order o~ 2 X of the maxlmum ob~alnable ~alue. As a conseguence, a chec*1nq area ln the form o~ e.q.
a rect1llnear strlp of 8 c~ length allows the subdtvlslo~
lnto 320 sub-areas, where the measurable ~alue can he trans*or~ed lnto l out of 64 posslble repetltl~e values.
Thls means 64 d~fferen~ poss.tble comblnatlons for ~he dlgltal mar~ on the docu~ent. It ls consequently very dl~flcult for a falslflcator to ma~e a document b~tb its randomly dlstrlbuted partlcles and to afftx dlfferent dl~tal n~a~ks the one after the o~her and to try ~7em out tr~
a check~n7s apparatus unt~l he f.~nds the ~tght rnar~ 7 the otl~er hand, lf a fa?slflcat~ t~tes to tmltaee al7 e~sttnS~
doc~ment, he mæy easlly read ou~ the mark fro~ ~he authentlc document al7d a~flx tt o/7 t~e fal-~lfled document, but he wlll not flnd a document wtth preclsely the same dlst~lbutlon of partlcles on whlc17 he could af~lx the mar* and whlch the s~anner would not be able to dtsttnqulsh as ha~n~ 2 dlfferent dls trl~utton ~

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The partlcles used ln the tnaterlal o~ the document shall have dlfferent electrvmaqnetlc propertles than the mater_al of the documer~t. Thls means a dlfferent dlelectr~c constant ~, or magnetlc permeablllty ~ or reslstlvlty p or other Yalue ~n~luenclnq the mlcrowave so that the response mlcro~ave bundle, l.e. the ref7ected bundle or the bundle whtch has traversed the docu~ent, has unde~gone a change~ent, e.g. ln amplltude or pola~lzatlon, wl h respect to the bundle that lmplnqes on the document, 50 that thls changemen~ can be measured. rhæ parelcles wtll preferably be ~n the ~or~ of electrlcal conductlng flbres, and mo~e sp~clf~cally ln the form o~ me~alllc f~bres, d~strlbuted o~er sald checklnq area ln a proportton o~ less than I
g~m . The length o~ the f~bres wlll preferabl~ be ln the range of 0.5 to lS mm and thetr dla~eter betw~en 2 and 25 ~m. Flbres shorter than 0.5 mm are less e~lclent ~nd lo~ger than 15 ~m are more dlf~cult to mtX ~nto the paper or plast1c or other non-conductlng materlal durtn~ lts manu~actu~lnq. Plbres of less than 2 ~m th~c~ness would be well usable, bu~ are not easy o manufac~ure, and i~ m~re than 2S ~m thtck, ~hey would be vfslhle and aFfect the aspec~ o~ the document. For no~-round cross-sec~lons, the ~dla~eter~ means the avera~e d_ameter o~ thlckness. ~he f~bres are pre~erabl~ made of s alnless steel, but can also be made o~ carbon. Polymer flbres covered w~th a ~etal coatlng are also usable as conductlng flbres.

~he lnvent~on also relates to an apparatus for conducting the above process, ln whtch the apparatus co~prtses a checklng statlon adapted for recelvlnq such document, means for scannln~ ~he checklng area of such document for detectlng the dlstrlbutlon of such pa~ttcles over a nun~e~

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of sub-areas of s~ld checklnq area and for producln~ a dlqt ~al scannlng-slgnal whlch ls characterlstlc for such dlstrlbutlon as scanned, means for produclng a second dlgttal slgnal and ~or comparinq the latter wlth sald scannln~ slgnal, whlc~ apparatus ls charactertzed by the fact that sald scannln~ means comprlse an emltter of a mlcrowave bundle dlrected towards the checklng area of the document when tn sald checklng statlon for produclng a response mlcrowave bundle, and a recelver o~ he ~esponse bundle, and that sald apparatus further co~prlses means for readlng-off sald d1gl~al mar* ~rom sald document ~hen ln sald checklng-statl~n, the output of the latter ~eans belng connected to the input of sald means for produclng sald second dlqltal slgnal.

Pre~erably, the response mlcrowave bundle ~hlch tS to measure, 15 the bundle transml~ted through the document. In thts case, the checklng s~atlon ~or place where the pvstt~on of the document ts adapted to be chec~edJ ca ~e ln ~he fo~r of a transverse passage for the document through a wavegu~de between sa~d e~ltter and recelver, and the scannlng means comprlse means for productng a recttllnear relatl~e movement of the document thraugh the passa~e. The scanntng 13 then co~ducted o~er a rectll1near strtp of t~e dv~ument, and the mlcro~2~e bundle sweeps over subsequent sub-areas of th~
strlp, whlch forms the checklnq area.

The dlg~tal mark ts preferably ln the form o~ a magnetlc flux pattern ln a magnetlc strlp of sa1d document, whlch 15 read off ~y a mugnetlc reading-head. As the readlng ls ln the form of a seguence of polarlza~lons ln one or ~n the other sense, and not ln the form of guantltles o~
magnetlzation, the danger of non-repetltlvlty vf the readlng .. .. . . ..
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of thls strlp .Ls much less c~ltlcal than an~ ma~netlc scannlng of ~he docuwent ln order to reproduce the dlstrlb~ltlon o~ ~he flbres.

The lnventton also relates to the documents, as descr~bed above, wh~ch are speclf~cally adapted for the present method, amon~ whlch specl~lcally the documents havlng the appea~ance of usual credlt-cards, l.e. ln the ~orm of a stiff rectangular card ~f a lenq~h rangln~ fro~ 8 to 12 c~
and a wtdth ranglng ~rom 4 to 8 cm ~nd havtng a lengthw~se runnlng n~gnettc strl2. Such card can ~e made ~f two or more subsequent layers o~ plasttc and/or paper materlal, where e.qO only one lnslde layer comprtses the flbres.

~he lnventlon wlll now further ~e expla~ned wlth reference to the drawings, ~ven ~y way o~ example onl~ and ln whlch :
F~gure I is a perspect~e s~de-vlew of ~7 apparatus : accordlng ~o t~e lnventlon.
F~g~re 2 shows a detalled transverse sectlon of the chec~lng-statton and the mtcrowave scannlnq means assoclated ~o thls chec*lng-statlon, as used ln the apparatus of Flgure 1.
F~gure 3 and 4 show a number of dlagrams of s~g)7als, produced by the m~cro~ave recel ver when ¢hecking tl2e check~ng-area of a docwnent pass~ng throug~ the chectclng-statlon ~ F~gure 1, for d:lfferent comblna~lorls of flbre length, flhre ~hlc*ness and flbre concentra~lon ln ~he documents passlng throuqh ~he checklng-statlon.

For the docwnents to check, a paper of the slze of a ban*note was used, hartng a welght of 80 g/m, a mun~er o~

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stalnless steel flbres beln~ embedded and randoml~
d~str~buted over the whole paper. Dlfferent comblnatlons of flbre thlckness f~ ~m Ol 12 ~mJ, flbre lenqth ~3 mm or 5 mmJ, and flbre concentratlon 10.05 ~ to 1 XJ were used. The document 1 ls lntroduced ln a slot 2 between two flat guldlng plates 3 and 4, and leaves the apparatus at the left end of the guldln~ pldtes, as shGwn ln flgures 1 and 2. On either 5tde of each of the s~atlonary guldl~g plates 3 and . 4, there are two toothed ~lexlble endless belts ~5' and 6' resp~cttvely 5 and 6J runnlng tn parallel wlth the guidlng plates, the belts 5, 5 ' and 6, 6' ~elng pressed respect~el~
agatnst e~ch other ~or ensurtng the transpor o~ ~he document througb slot ~. The flextble belts are each led aro~nd four tovthed wheels 7, 8, ~, lO ; 7', 8', 9', lO' ;
12, 14 ; 11', 12', 13', 14'. The four toothed wh~els 7', 8', 9', lO' for belt 5' are ~txed ln palrs on the same ro~at~on-axle w~th the ~our toothed wheels, respec~tvely 11', 1~', 13' and 14', for belt 6', ln order to ensure s~n~h~ontsm between belts 5' and 6', and the same ls done w~th the toothed wheels for belts 5 and 6. A toothed ~heel 17, ft~ed on the axle of one of the palrs of toothed wheels for bel s ~ and 6, connected ~t:th ~oothed wheel 18, flx~d o~
the axle of o~e of the patrs of toothed wh~els for belts S' and 6' for drlvlng the latter belts by neans of an endless belt 20 wh~ch ls toothed on etther slde, ensures the synchronlsm between belts S, fi on one hand and ~elts 5' and 6' on ~he other hand (~heel 19 ~s freely rota~able around ttS axlsJ. The whole ls drlven b~ an endless belt 16, engagl~g w~th toothed wheel 15 ~htch ls flxed on the axle o~
wheels ~ and 12, and thls endless belt 16 tS drlven by motor fno~ shown).

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l'he belts 5, 6, 5', 6' transport the document wlth a constant speed along a detectlon apparatus 21. It ls clear that many other sorts of transport ~eans for paper documents can be desi~ned for produclng the movement along the de~ectlon apparatus. For stl*~ documents of small s~ze ln card ~orm, such as credlt cards, the transport means can e.q. largely be slmpllf~ed b~ passlng the card between palrs of rollers. It ls also posslble ~o have a stàtlonary checklng-stat~on, where the mlcrowave-beam wlll sweep along the document. It ls su~flctent to create a relatlve ~o~ement between the detectlon apparatus and ~he document, allowtng the scannlny o* subseque~t sub-areas of the checking area of the document.

T~e detectlon appara~s 21 ~FIgure 2J c~mprlses a mlcrowave osclllator 22, e.g. a Gunn-dlode, a wavegulde 23 connected to satd oscllla~or and a mlcrowave rece~ver ~4, e.g.
comprtslng e.g. a SchottXy dlode. As shokn on Ftgure 2, the wavegulde 23 whlc~ connects the osctllator ~th the recelver ls dlrected perpendlcularly throuqh the gu~d~ng plates 3 and 4, and at the cross-poln~, the pla~es are provlded with an openlng or w~ndow of the same transverse dlmenston of the wav~qulde. ~n thls way, a transverse passage for the document through the wavegulde ~ provlded and the recelver ls able to ~easure the amplttude o~ the ~crowave after traverslng the docu~ent. In order to change the m~crowave pattern l~ the wavegulde and ln satd w~dow, so as ~o ob~ln an opt~mal detect~o~ ~ tbe ~lbres, the wavegulde 23 is prov~ded wl th a number of tun~ng-screws 25, 25' and 25~ . rhe necesslty of such tunfng can howe~er, lf deslred, also be 3~J avolded when the recelver ~s provlded wlth an lsolator ~or preventlng the waves whlch have entered the ~ecelve~ to reflect back lnto the wavegulde a~aln, as well known ln the art of micro~aves.

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_ 9 _ ~he amplltude of the m~crvwave after traversfng the document ls representatlve for the presence of one or more flbres. It ls however also posslble to put the recelver on the same 5Ide of the yuld1ng plates as the emlt~er, tn order to measure the radlatton re~lected by ~he ~lbres. ~he emltted mlcrowave ra~latlon can be polarlzed ln one dlrectlon, e.q.
ln the preference dlrectlon of the flbres ln the paper, lf any, but can also be clrcularly polarlzed so as to be equall~ senslble to all flbres, ln~ependently from thelr dt rect t on in the documen~.

~lgure 3 shows 6 ~ypes of response patterns fa to f~, obtatned by the rece1ver 24 of the apparatus accordlnq to Flgure 1, worklng at a ~requency of 24.15 GWz ~nd ~ransmtttlng a wave ~n a r~l 0-mode through the wavequlde 23 o~ t~e type ~42 ~rectangular 10.668 m~ by 4.31~ mm) fo~
papers o~ dlfferent co~blnatlons of concentrat~on, thlc~ness and length of the flbres as menttoned abo~e.

In Flgure 3, the length and thlckness of the flbres ls kept cons~ant, and the concentrat~on ls var~ed from 0.05 æ to 1 X
(percent~qes by we~qhtJ. In Ftgure 3a the concentratlon ls so lo~, that the response pattern compr~ses a larqe horlzontal part and thls brlnys dowD ~he posslbll~tles to dlstlnquish a great number of patterns from one ano~her. The same occurs for the hlgh concentratlon of Flgure 3f, where the response pattern comprlses a large horizontal maximum par~. ~ehween both concentrattons an opttmum can be sought.
rt ls clear that ln each case the optlmum concentratlon w~ll depend on the used wavelen~th and the slze of tAe ~lbres and that thls wlll ln general lle below 1 g/m .

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rhe recelver ls less senslble lndeed to flbres of less than a quar~er w2velength and the senslblllty shows a small peak at a quarter w~velength, and further ra~ses to a hlgher peak at a half waveleng~h. q~hls ma~es that, the more the flbre length ls ln the range of hlgher senslbillty, the lower the optlmum concentratlon w~ll be. ~hls ls shown ln Flgure 4 a to d, where tt ls shoN7l that, for a flbre length of 3 mm, a hlgh concentratlon above O.5 X wlll be deslrable, whereas for a flbre length of 5 mm, the op~lmum wlll l1e between 0.05 and O.5 %. ~hls makes that, for the suttable waveleRgths, the flbre length wlll preferably range from 5 mm to lS mm, althougb shorter flbres ln ~he range o~ 0.5 ~o 5 ~m, and less eff~clen~, can also be mlxed ln the materlal ~or ~he documents.

The lnfluence of the flbre d~ameter ls shown ln Plqure 4, e to h. For a same welght and leng~h o~ ~he ftbres, there are less flbres. I~ ls consequently clear that the senslbllity o~ the recelver for a glven wel~ht of flbres, wlll be lower ~or thtcker dlameters, and tha~ the opttmum percentage wlll ra~se accord~ng as the dlameter ls hlgher.

~he ~esponse slqna7 o~ the re~elYer 24 ls further, ln a way well known to those skllled ln the ar~, transformed lnto a dl~ltal scanntng slgnal. To thls end, the obtalned slgnal is e.g. dlvlded in 12B absclssa-posltlons. ~or each posltlon, the ordlnate ls trans~vrmed ln a dl~ttal Yalue whlch ranges be~ween O and 63 and tAls needs 6 blnary code blts. For the 12~ absclssa posltlons ln total, a blnary word of 6 x 128 blts wlll then be represen~at~ve for ~he obta1ned respo~se sl~nal or for the dlstrlbut1On of the ftbres over 128 su~-areas of the ~trlp-~orm part of the docu~ent whlch has , .. ~

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passed the sald wlndow between the emltter and the recelver.
Th~s converter of the analog slgnal of Flgure 3 to a dlgltal scannlng slgnal forms part o~ ~he scannlng s~stem and can be reallzed accordlng to various prlnclples ln varlous posslble s~stems, and lts detalls are not o~ tmporta~ce for thls invent~on. rhe only lmpor~ant feature ls, that the recetv~r 24 produces such ~epetltlve response slgnal, that a sa~e su~dlvlslon of the ordlnate lnto 64 values and e~en more, appears to be posslble, and that there ls sufflclent resolutlon ~or checklng the absclss~ ln 128 posltlons.

rhe document 1 to chec~, ls fur~her provtded wtth a dlgltal n~rk ~htCh hæs a unlvocal relattonshlp accordlng to a secret transcodIng Por~ula, wlth t~e dlgltal scannlng slgnal, Gbtatned when scannlnq the doc~men~ w1 h the scannlnq syste~. ~hls d~qltal ~ar~ can be put on ~he documen~ ln the ~orm of e.g. readable printed flgures, a bar code, perforatlons, a programmed lntegrated clrcult or a magnettc strlp whlch can be read off wlth a magnettc readlng-head. In order to brlng the d~glta1 mark on the docu~ent to check, the document ls lntroduced ~n a scannln~
sys em for productng sald dlgltal word and the output o~
t~e scar~lng system ts cor~3ected to ~ transcoder and further to e.g. ~ maq~etlc wrltin~ head ~or w~ltln~ the transcode~
word ln~o a mayne~lc strlp on ~he document. Other parts of ~he magnet~c str~p can then be reserved for other data. In th1s way, the dlgltal mark~ as lntroduced on the document~
ls also charactertstlc, o~er a certaln transcodinq formula, o~ the dlstrlbutlon of the f~bres o~er the 12~ sub-areas of the checklng area of the document. If not necessary, the transcod~ng formula can be reduced to l~s slmplest form, l.e. ldentlty betw~en the dlgltal scannlnq slgnal and the dlgltal ~ord whlch has been put on the document.

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rhe appara~us accordlng to cZalm l fur~her comprlses the necessary me~ns ~not shown~ for readln~ the dl~l~al mark on the document. In dependence o~ the nature of ~Je mark, thls may be an optlcal flqure reader, an optlcal bar code reader, 5 or a maqnetlc readlng-head. The outpu~ of th.s reader 15 the~ connected to a comparator tnot shown), where the mark-sl~nal, as read o~f, ls flrs~ly transcoded and compæred wlth the dlgltal scanntng-slgnal obtalned ~rom the scannlng sys~em. The comparator wlth lts transcoder ca~ be desiqned 0 ln ~arlous forms accordlnq to rarious pr~nclples, whlch are well ~nown by those sktlled ln the ~rt and are not of lmpor~ance Por thls lnventlon. Only when the scannlnq slgnal corresponds to the transcoded mar~ slgnal, the compar~tor wlll produce an output-slqnal, whtch can be ~sed as an lndlcatlon that the checked document 13 authentfc.

~t tS clear tha~ other scannlng means, usfn~ a mlcrowave bundle whlch lmplnges on ~he document can be deslgned, _n so far as they al7Ow to measure a response bundle whlch keeps apart the measurements fvr ~he dlfferent sub ~reas, wl~hout departlng from the scope o~ thls lnre~tlon. It ls also clear thae tAe dl~ltal sl~nals and marks can ~e coded in any sor~
of code, elther purely blnary, or blnary c~ded declnal or ot~er. ~he noD-conductlng ~rlaterlal ~or the document can be paper or plast~c or any other na~erlal, ln so far as lt doesnot re~lect the mlc~ow2ve e~ergy to such ~xtent that the de~ectlon of the f~bres would no lon~er be posslble.

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Claims

Claims:

1. A method of checking the authenticity of documents, made of a non-conducting material and having a physical characteristic of which the value randomly changes over a number of sub-areas of a checking-area, and further comprising a digital mark which is characteristic for the distribution of said value over said sub-areas, the method comprising the scanning of said checking-area of such document for detecting the distribution of said value over said sub-areas, producing a digital scanning signal which is characteristic for such distribution as scanned, reading off said digital mark on said document and producing a digital mark-signal which is characteristic for the digital mark as read off, and comparing said scanning signal with said mark signal, characterized by the use of documents comprising a number of particles with electromagnetic properties which are sensibly different from those of said non-conducting material and which are randomly distributed in at least said checking area, and that said scanning is conducted by means of a microwave bundle that impinges on said checking-area and produces a response microwave bundle and by measuring this response bundle.

2. A method according to claim 1, characterized by the use of documents with particles in the form of electrical conducting fibres.

3. A method according to claim 2, characterized by the use of documents with particles in the form of metallic fibres of a length ranging from 0.5 to 15 mm and a diameter ranging from 2 to 25 µm.

4. A method according to claim 3, characterized by the use of documents with particles in the form of stainless steel fibres which are randomly distributed over said checking are in a density of 1 g/m2.

5. A method according to any one of claims 1 to 3, in which the response microwave bundle is the bundle after traversing said document.

6. A method according to claim 1, in which the scanning of the document is conducted over a rectilinear strip part of said document, by producing a rectilinear relative movement between the microwave bundle and the document, so that the bundle sweeps over subsequent sub-areas of said document.

7. A method according to claim 6, in which a digital mark is used in the form of a magnetic flux pattern in a magnetic strip of said document.

8. A method according to any one of claims 1 to 3, in which a document is used in the form of a rectangular card of a length ranging from 8 to 12 cm and a width ranging from 4 to 8 cm, having a lengthwise running magnetic strip.

9. An apparatus for checking the authenticity of documents made of a non-conducting material comprising a number of particles with electromagnetic properties which are sensibly different from those of said non-conducting material and which are randomly distributed in at least a checking area of such document, the documents being provided with a digital mark, the apparatus comprising a checking station means for receiving such document, means for scanning the checking area of such document for detecting the distribution of such particles over a number of sub-areas of said checking area said scanning means comprising an emitter of a microwave bundle directed towards the checking area of the document when in said checking station for producing a response microwave bundle, and one receiver means of the response bundle for providing a repetitive response signal and obtaining a sufficient subdivision of ordinate and abscissa values for resolution and checking the positions thereof.

10. An apparatus according to claim 9, in which said checking station is in the form of a transverse passage for the document through a waveguide between said emitter and receiver, and that said scanning means comprise means for producing a rectilinear relative movement of the document through said passage.

11. An apparatus according to claim 9 or 10, in which said reading-off means comprise a reading-head of a magnetic flux pattern in a magnetic strip.