CA1110476A - Latent photo system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A latent image of an object which appears to be invisible to the unaided eye is formed on a substrate by photographing the object through a half tone screen to produce at least one halftone image of the object, thereafter placing the halftone image in reg-istry with a strip of light sensitive film and a dark screen having therein a plurality of extremely small apertures arranged in parallel rows oriented in predetermined angular offset relation, for instance 30° to the rows of halftone dots forming the halftone image, exposing the strip of film to the halftone image through the small openings in the screen, developing the exposed film to produce thereon the latent image of the object, and then imprinting the latent image on said substrate. The printed image can be viewed through another dark screen having apertures smaller than those in the first-named screen, and will appear automatically on the face of any print made of the substrate by means of a xerographic process or the like.

Description

'L'he pre~serlt lnvention relates to the art of printing or litho~rclphy and more partlcularly to a methocl of producing an lnvisible photograph, laterl1; in f`orm, by the use of a dark screen, and a method of' viewing the photograph either by utilizing the same dark screen, or by making a print of the photograph by an offset printing process, and ~hen reproducing the print by a Y~erGc,raphLc process or the like.
The techni.qtle of' half'tone photography is well known in the printing art. It consists of' employing a.n offset camera, a halftone screen, and suittable llght senslt:ive :fi.lm for accepting the Lmage. The off'set camera is equi.pped with a copyboard, and suitable li.ghting and lens to ref'lect the tonal values of' the photograph through the hal.rtont:3 scxeerl and onto the l:l.ght, st3n-s:l.ti.ve rl.lm 'L'he h.ll.f'torle s~ri?t.~rllrl.ly h..IVCI ~'e`Ir\~.Lar'ly ar'r'aIl!,ed :ltJt;~;, t;(~nt~.~, ()r :I.I.r~t3~;; f!;lth ~:lt:)t;, t,(:~ I' L Lr)i.! ~l-lVl~f~; $1 CIl~r'li.
ctrlti.~K~ and a l:l.ghtt3K~ out(3r~ sha~)i? tO clllOW the ~ ;ht; or dar~ tone of the orlginal photograph to re~lect a given dot, positLve l~o to 59/0 and negative 60~a/o to gc~0 on the light; sensitlve f-ilm. The halftone screen is normally arranged at an angle relative t-o the upper margin of' the photograph, the angle of 45 normally is used when photosraph-Lng black and whit;e p:'Lctures clut-~ to the ang:le of' inc:ldence of the dots in rel~tlon tc) t;ht.? upper mc~ bln of.' t,he r)hoto~.r,raph. 'L'he dot,3J 1;one~;, or .L1neE~ oL' t;h~? ha:l.ftorle ~cret?n ma~ be arraZlgt;~d rcln(lorrlly to prOd~C`t? dLf'.rert-rlt e.f'f'ects .L
the fLnal printing, theref'o:re no spec:lfic angle is need~?d f'or the printlng process. Angles of 75, 105 or c~o can be used depending on whether the halftone is bein~ overprinted wi-t~.
other screensJ or ln the case of a color photograph al::L foU-r angles rna~ be used to produce four separate ha].ftone negatives.
The use of 30 angles between halftones produces a pleasing effect called a rosette. ~lowever, minor errors in angulatior of 1 or 2 can produce unwant!:.d patterns caLled moir~C;. The term rosette then can be applLed to the cl:i.splacernent of 30 $~

between angles of two or more half'to-ne screens, whereas when t'ne angles are more or less than the 30 di.splacement 'he effect would produce a mo-lr~. Reprocluctions o~ photoe;raphs may be produced using a process called continuous tone printing.
The original -photograph is reproduced on a spec-lal ~ntinuo-us tone f'llm conta-ln~ng no dots or lines, th.erefore no angle is possible or needed for reproduction. A s~ecial grained printing plate converts -the continouS tone lmage to 2. random ha]ftone for printing. A reproductlon of a photograph then can be ac-complished by either a random, halftone, or cont-lnuous tone process. A latent or lnvis:ible photograph can be produced by usi.ng a reproduction made from any of the three above -processes.
The reproduction process described in the present i.nvention will be geometric or halftone :l.n nat;ure.
In Wl.cker U.S. Paterlt~ No. 3,675,9)1~3 and 3~'7~34,2~)9 a mettlod `I.S df.!C~Cr;bed WtliCh per~ s an Imag.~ to be h:~d(lerl l.n a prln~ (3 half'tone f':Leld, usln~ a <;creer1 which can have openings of about 40~ in value to hide a negative of an imaga by exposing the negat-Lve through a screen onto a new film, a second exposure ls made usl.ng the positive of~ the negative image and -the c1ark screen whlch is angled 30 from the angle of the nega.tl.ve screen in respect to the upper margin of the lma~e. The sc:reen :Ls placed l.n contact and ref~:Læter w:Lth the -pos:1.. t:l.ve l.mage and arl exposure is rnade onto 2, n~w f':l'lm. A.fter devc?:Loprr1t?rlt ~h~ nefr,ltlve c3ot; o~' about L~o~r~ opaci~y take on the exact shape of' the or.l~inal image and are disposed at an angle of 45, and in the case of the positive the 40~ opaque dots take on the reverse shape of the original image covering the background and meeting or ~oining the 4~0 dots of the negative image which are disposed at an angle of 75. The image i3 hidden by khe use of small halftone dots spaced at lO0 l-1r1es or more per inch and the exact reglstration of the positive and negative i.mage~.
To view the hldden image a clark screen of' 6P~ or more opacity is employed in a device or devlces which is dic3posed above the i.mage plane of the printed hidden image, a sui-table light is trans-mitted or reflected through or onto the dark screen and the hidden image appears to blinlc when the screen is moved or vibrated over the surface of the imaae. This is accomplished by the use of a small vibrator motor in ~he devices. The move-ment of the screen was necessary because the alignment of the screen's minute openings and the ~0% hal~Etone dots in the hidden image had to coincide exactly with respect to each other. As the screen moved over the surface of the hidden image it caused a light interference effect or a misalignment between the ~0%
opaque dots and the minute openings of the screen. Normal e~pansion or contraction of printed surface could cause size changes of .002 to .005 of an inch, causing the misaligrlment problem to exis-t.
'rhe us~ of a solicl imacJer nec~ative arld pos:i-t:ive, pllot:o~
graphed through a screen at -two separate angles produces an edging effect at the point where the two angles of the negative and positive images intersect, which can cause the shape of the hidden image to be partially visible. The movement of the screen over the hidden image can have the effect of seeing the image as partially hlack or partially white. It is the intent of the present invention to show a method of hidlng reproductions o~ oxiginal photographs which will eliminate the edgirlg effect caused by the solid images of the negative and positive, and a viewing screen which eliminates the need for moving or vibrating the screen over the surface of the hidden photograph.
Accordingly, the present invention provides a method of producing on a substrate a latent image of an ob~ect which appears to be invisible to the unaided eye~ comprising photo-graphing the object through a halftone screen to produce at least one halftone image of the object, placing said halftone image in registry with a s-trip of li~ht sensitive film and a dark screen having therein a pluralit~ of micro-dot sized apertures arranged in parallel rows oriented in predetermined angular offset relation to the rows of halftone dots forming said halftone image, exposing said strip of film to said half-tone image through the apertures in said dark screen, developing the exposed film to produce thereon the latent halftone image of said object, the imprinting said latent image on said substrate.
The present invention describes a new method of half-tone photography utilizing an ex-tremely dark screen to produce a halftone with micro-miniaturized positive dots which contain a negative and positive halftone irnage separated by the difference of 30, renderincJ the visihlc halfto~les :inv:isiblc or :laten-t.
To v:iew the h.iclclorl :i.rna~Je, a dark scroen of similar p~r.iod:ic:ity, hclv.i.ncJ apOXtU.I^O; IllUCh smaller thClll tlle sma-l.~.c`C;t microdot, is placed over the ilm and turned -to the positive angle, the . ~a-emerging picture retains the reflective aspects of the or-iginal positive halftone wh-lle the dark viewing screen iso:Lates the negakive halftone, due to the 30 angle d-lfference.
Still another way of v-lewing the hidderl image is to make an offset print thereof, and then reproduce the print by way of a xerographic reproductlon process, or the lLke. Although the image is hidden in the offset print, the process Or re--producing the print through a lens cau~3es the latent image to become visible on khe "reproduction" of the print.
It will be understood to those familiar with the art that a photograph which is invisible untll combined with a dark screen can be an effective and posit-lve system of personal -ldentiflcation.
The fahrication o~ the dark scr-eerl c:an be ol unl(lue pattern wlth llnes ~)er Inch varl.ahle ovcr- the Im.l(re area, pr~verltlrlfg ;-lrly knowrl methc)d o~ orJ~r,kla:L dupliccltl.orl. Thl3 colnbLrled wlth wlde tonal differerlces between lndLviduals would renc3er any counter-feitlng by photography or altering useless.
The above and other novel features o:EI the Lnvent-lon will become mvre readily apparent w-lth the followLng description of the drawings.

Flg. 1 ls an overhead vlew of the or-lginal photograph and copy mounted for exposure in an offset camera;
~ -ig. 2 illustrates the negative halftone produced by the camera after exposure through a contac-t halftone screen;
Fig 2A is an enlarged section of the halftone negative In Fig. 2 illustrating the halftone pattern and -the angular direc-tion of the rows of dots for-mirlg the halftone;
Fig. 3 is an overhead v:iew of a c:ontact frame contalnirlg the negative in Fig. 2 and a new tLlm in reglster ready for exposure;

Fi~. 3~ illustrat;e3 the posltlve halE`torle procl-lced after ex-posure and devel~,pment of ~he new fllm i~ Fig. 3;
Fig. 3B ls an enlarged section of the hal~tone in Fig. 3A, indicating the angle of its row6 Or halftone dot.s, which are opposite in contrast to those shown in the negative in Fig. 2A;
Fig. 4 is a print of the dark screen which may be used to create the latent halftcne, with a section of the screen en larged to show the eff'o,ctive angle of the rows of' lts mic~o~
openings when the dark screen is incne operating position;
Fig. 5 is an overhead view of a cont~Q~, frame containing the - 10 positive halftone shown ln Fig, 3A in contact with the dark screen and a new f'ilm ready f'or the first exposure of the latent halftone;
Flg. 6 ls a view of the unde~eloped latenk negative pro duced from the poFsLtLve half't;orle .~nd t;he dark scroerl ln l~'ig. ~;
~ i'lt,~ ;A l'l'L~It,tr~at;es arl erl'klr~JJe(,l versl,orl o~ he lln(,l~ve~'ioped m~c~c)-c30l, ,lrn~t~;o in ,L~ lndic:al,lrlti~; the l;~JO atl~ In Whl.(,~l I;he rows of dotc: representlrltr the screerl and the positlve halftone extend and the resulting rosette pattern;
Fig. 6B is the same dark screen as shown in Fig. 4 turned ZO ko an operating posit-lon in whlch ;t~- rows of micro-open-Lngs are positioned at 75 for the second expolsure of khe latent half'torle;
FLg. 7 Ls an overheacl vt,ew of' the contact f'rarrle, the nega kl,ve hal~'torle ôf I~'l.g~. 2 ar-ld the clark screen Ln re~,!;lster ,,wit;h th~ ~ilm us~d ln ~'Lg. 5;
F~g. 7A is a vLew of the undeveloped latent poslt:Lve pro-duced from the negatlve and dark screen in F-lg. 7;
Fig. 7B illustrates an enlarged view of the undeveloped image in Fig. 7A -Lndicating the remaining two angles in which the rows of dots representirlg the new positlon of the screen and the negative halftone extend and the resulting rosette pattern;
Fig. ~ is a combination of the negative in F-Lg. 6A and the positive in Fig. 7B illustrating the f'our angular directlons of the rows of micro-dots and the ~lnal rosette pattern of' the latent halftone;
F-Lg. 8A is a maJor enlargement of the rosekte patt,ern in Fig. ~;
Fig. 8B :is a print of the laten-t photograph at normal size;
Fig. 9 illustrates the visible latent halftone or a pro-Jected image of the visible latent hal~tone.

_ ReferrLng to Fig. l an original photograph continllous in form ls mounted in the copyboard 5 of an o:~fset comera. The photograph or cop~ contains dark shadows 6 and ~ight hlghlights 7 which will be shown and followed throughout the drawings and descriptions. At the lower center o~ the copy l, two purlch-ed hole,s ~0 are regl~te~red arld ~ecured by t,wo regL~ter p~.ns 13 whLch proJect rrom cop,yboard 5. ~ow wlth the oopy ln pc)~Lt,lorl the camera lamp l~ l.s turnet3 on ref'lectLng I.l~ht 9 oL`f' the copy 1 through a suLtable lens, direct-lng the dark and llght tones of the copy through a vignetted halftone screen and onto a new film. The new fllm is now developed producLng the halftone negatlve as shown in Flg. 2, whereLn the photo portion o~ the negative halftone is indicated at la whlle the hLghlight dots o~ the negatlve are indLcated at 6a and the .shadow dot,s Or the ne~at:ive are indlcaked at '7a.

' ' ' ~ ~ ~ '. ' Th-is is better illustrated in Fig. 2A which shows an en-largement of the light and dark ha:Lftone dOtsg and the direc-tion of the rows of dots in relation to the upper edge of the - negative. The negative is now prepared ~or contact and is mounted as shown in Fig. 3 in the frame 5 which contains a new piece of light sensitive film 12, which has kwo punched holes for receiving the two regLster pins 13. A vacuum frame is now turned on securing the negative to the new film in known manner, and the exposure is now made using the lamp 5 to trans-fer the image of the negative halftone la to the new film 12.
The film 12 is now developed and processed to produce the posL-tive hal~kone 12a as shown in ~:ig. 3A, the positive halftone be:1ng the direct opposlte, as far as dark shadows ~nd h-lgh-ll~r,hts are concelnec1) as the neg,.ltlve. Tt~lr; hcl:lf'torle Ls shown er~]~r~ r-l r~l.g. 3B~ r~ r~ ; ~ cl 7 ~l~r~ rl ~;o ~ ;tl~tll-r-~(~(t op~o~)lte of' a~ea~3 6a arld '7a stlown ln 1~1tg. 2A. 'L'tle negatlve halftone la ~dthe pos-Ltlve halftone 12a can now ~e used to produce the latent photograph as shown in Fig. 8B.
In Fig. 4, nurneral 1~ illustrates a dark screen having smal]. openings which have been aligned at the angle of 105 as shown by arrow 15, Ln relatlonship to the axls Or the~
screen, The ~creen 14 also has two punched holes 10 whLch rc:E)resent the axl~3 of the Clcre~er~ ncl whlctl a-re used to reçg~
Lster the ~3creen to the new ~`llrn 16 beneattl the posltlve ha:Lf-tone 12a as shown Ln Flg. 5. ~'ig. 5 1s an overhead vlew o~ the contact framr-~ containing the dark sc:reen angled at 105 and the posikive halfkone 12a, bokh o~ which are held in regisker ko the new film 1~ using the punched hole! 10 and the register pins 13, The vacuum pump is activated and the la~p l~ is -turnecl on transmltting the image o~ the half'tone dots throllgh: the small openings o~ the dark screen to the new rilm. The fl]m 16 is left in position for a second exposure o~ the latent photograph. However to clearly understancl the process, the drawlng ~Lgs. 6 and 6A illustrate what -the undeveloped latenk negati.ve lrnage produced f-rom the first exposure would look like i.~ film 16 were to ~e developed at this sta~e. Fig. 6A
is an enlarged version of l~i.g. 6 i.llustrat-lng the dot patkerns as s'nown ln areas 6 and 7 which were produced f'rom the posi-tive and negative dots of 12a interfering with the dark screen 14. Arrow 8a indicates the angle from the positive in Fig. 3B and arrow 15 indicates the angle of the rows of open-ings in darlc screen 14. N~neral 19 (Fig. 6A) illustrates the resulting rosekte pattern from the 30 angular difference between ~a and 15.
The dark screen 14 Ls now prepared f'or the second exposure on the new filrn 16. Fi.g. 6B i.s a plcture of the modlfiecl screerl 1ll' cut with l.ts ax:Ls extend,l.ng a~ the arlrs:Le O~ '7cj to :I.t~s row:.~ o.E' nperll.nr~;s, as .Lncl.l.ccll;ed b~ r.~r~ow :L-3. Mu~rlera:l.:lllB

) t3~rlOt~ nLar~ ,';111(3r1t (~' t;~lf~ C.St't'~'rl ''I~l(Jw;tlr~'; t;~l( small apertures of' ~he screerl. Numerclls 10 and :l3 I.ndl.cate -the means ~or registering the new fllrn 16 as shown in Fi.g. 7. Fi~. 7 is an overhead vlew of the contact frame 5 containing the new fi.lm 16 (once exposed) and dark screen 14' J and the negat-Lve halftone la re~istered by holes 10 and secured by reg-Lster p-Lns 13, The darlc screen 1l~' I.s displacecl at the angle o' 75 an(l L~ b~tweerl the new fllrn 16 ~Ind the negatlve hcl'lftone :la.
'rhe vacuwrl~pumE) L~ now actlvtlked arld the lamp 1~ 1,c3 t~.l:r'lleCI , on transmitting the negat-Lve halftone dots Or the negative la through the dark screen 14' to the new f-ilm 16. The second exposure is now completed and the f'ilm 16 is now ready for development.
~'igs. 7A and 7B are views O:r what the undeveloped image produced from the second exposure would look like if devcloped~
and -lf the f-lrst exposure had not already been made~ Fig. 7A
ls -the positive latent halftone which was produced on a separate piece of fllm ~not film 16) to show the image produced from _9..

9L'7~

the interference of` the halftone negative la and the dark screen 14' angles at 75 as shown by arrow 1~ in F-ig. 7~ Thls is better illust-rated by the enlargement of Flg. 7A as shown in Fig. 7B. The angle indicated by arrow 8) 45, was produced from the halftone negative la as shown in Fig. 2A. The angle denot-ed by the arrow 1~, 75, was procluced by the dark screen 14' which was angled to place its rows of openings at 75 as sho~n in Fig. 7. Fllm 16 is now developed and processed producing the final latent halftone, an'enlarged portlon of which is shown '10 in Fig. 8.
In Flg. ~, the areas 6-6a, 7-7a3 l9a, 19b are excellent illustrations of the v-Lsible circular pattern known as rosettes wh-Lch was produced by combining the latent posltive halftone whlch contaln~ mlcro~(lot~ angles a~ l~5 and '75 ~ arld the~ laterlt negatlve half'torle~ whlch cont.lln~ rnlcro-(lots ang'le~E, at; l~,j ancl :Ins . :~t should be noted t~lat t;tle vl~Lble rose~e pakt;~rn 1 ~eometric in ~ize and shape, ancl symmetrLcal, producing a nega--tive and positive halftone that ls invisib'le or latent.
F-lg. 8A is an enlarged view of any area of Flg. 8 wllich shows the uniformity of the rosette pattern.
F-lgr. 8B ls the latent halftone photograph at normal sLzeJ
The size of the latent photograph can be either enl~rFr,ed or re-duced.
It should be noted that thes~ hal~'torle rn1cro-('lots anf~,le~
at the two ang~les produce a ~eometrlc effect that -ls completely symmetrical The negative latent ha]ftone being the ~posite of the positive halftone cancels the tonal values that would normally be visible if only the negatlve or the posit-Lve halftone was prlnted. The enlarged vers-lon shown in Fig. 8 does have some vis-ible efI'ect. This can normally be expected from a minor error in density between the negatlve and positive halftones.

The drawings and prlnts were purposely producecl at a higher con-trast to enable the invent-Lon to be thorou~hly descrlbed In 7~

norma.l practice however, the kon3.1 patterrl is completely in~
v-isible, Fig. t'3B is the latent halftone photograph at normal size. The latent, halftone photograph can be reduced f~ther to a very small slze to enable imprinting on ereclit cards that are universal and are accepted by both creditors and the buying publie. This reduetion of' the iraage can be aceomplished due to the size of the small miero-dots in relationship to a normal half'tone, whieh has dots mueh ~rger in siz.e. The shadow or negcative dots do not fill in or elose up as in the ease with a normal halftone.
Fig, 9 is either the v-Lsible lakent halftone , or a projeet-ed image of the latent halftone, produced by combining a dark sereen having openings smaller than those in ~creen lL~ whlch the latent half'torle photor~r,rcl~h o~ f~ E3, ar~ )o~ll;lorlr!(l re~lcltlve~
to t;ht-~ lattnt po,3Lt;lve half't()rle at; the arlr~;'Le or 7,jC~. 'I'~liL~ rlc scretrl f'or ~)roducLrl~ t;he vlsl~)le Lrnarrt-~ ~s sim~Lar t,o l;ht! dcl-rlc sereen 14 or 14' exee~pt that Lt;s ope~nLngs are reduced by at least 50~0 in diameter. This reduction of the diameter of' the openings of' the clark sereen will overeome any dimensional dlfferenee be-tween the material of the latellt ha'lftone and the rnaterial Orthe dark viewing sereen.
Durlng the pcast deeade the use~ of crec'l:it cards as replace-ment f'or rnoney ha.c~ bt-eome wk~e~pread; wlth orlly a L Lm:lt(?d nurrlber of' the card~3 havlnt~ e:le(t,rorllc or pt~-intlrltg code~l f`or ust;~
cas personal identi~icatlon.' The reason for this being the ease in which the eore ean be eopied or changed by a method ealled electronic skimming. The use of a v:isible photograph on the card is the most vulnerable to colmterfeiting or tampering, since the photograph ean easily be altered or even changed to the pleture of the eounterfeiter or iorger.
It ls one o~ the -lntents of this invent-ion to provide a sys-tem of identifLeation that is virtually foolproof as far as alter-lng or ehanging the photograph that is used for lclentlfi~

7~

Cr~ti&n. I~he dark screen 14 (and the viewing sGreen~can be fabricated f`rom origi.nal ~t to prevent original duplication of a lat2nt photograph~ The small micro-dots of the latent hal.f-tone, which contains the total tonal range Or the ind-lvidual, are completely tamper proof since the visi~le pic-ture can not be seen until placed on a viewer contai.ning the dark screen.
Further embodiments of this invention will be named below but should not be limited to the securi.ty or credit card f'ield.
In Fig. 7B the positive latent halftone combined with the dark v-lewing screen will produce a visible positive half`tone as seen in Fig. 9 but without the cancellati.on ef.~ect of' the negative latent halftone. Since the tonal range is still slightly vlsible, the halftone cauld be rtc~uced to ~1 very ~mall ~sl.ze makl.nr~ the~:~mar~e l.mpoE~.:Lble t;o :re~c()gr~ e . ~r~le u~se of a pr~c),Jector could ~t.3 ut,L:1:1ze~d to vl.ew t,he l~.ll;erll, h~l.L'tone ..lt norlrl;ll s:l.zt;~ arl;al.n us.Lrlr, the d.~rk: screerl ~`or~ v:Lew1ng.
A fu~th~r embodiment o~ the invention can be seen in Fi~. 8 where the negative latent halfto.ne is adJacent to or 30 from the posit-lve latent halftone, uslng only two angles of' the dark screen 14, 75 and lp5 . The rernal.rllng two angle~ 45 and 90, could be utilized to produce a secorld latent ha:lftone in the ~.
sam~ square -~nch arca. In thls same vei.n the~ four angl.es, 1~5 , 7~ , ~() and 105 coll1cl be~ se('l to ~.)hoto~rcl~.)h ~ u-r dl:['.f'~3rent posi.tlve or negatlve hal:~torles throllgh the dark screen 1l~.
These latent halftones can be vlewed by turnin~ the dark view-ing~; screen to the angle of' the half'tone, producing only one halftone in the visible form while isolating the remainin g three f'rom view because of the 15 or 30 angle dif.ference between them~
therefore four latent irnages can be stacked one on top of the other on the same frame of film.
Still another way was d:Lscovered to produce a v-Lsibl.e image of the latent i~age ernbodied in the developed f`ilm of Flg. 8~, whlch lncludes the above-noted roset-te pattern~ This was done by us-lng a positive of the hidden or latenk credi.t card image which included the roset~ pattern ~e.g. F-lg ~i), photographing it 'GO produce a negative~ and then exposirlg this negative to an o~fset printing plat~ From th:Ls plate printed samp~.es of` the credit card were made on an of`fset printing press. The image of the credit card on such printed sarnp].e was not visibler However7 when a copy of the printed sample was made by way of a black and whi.te xerographic process, the copy or "reproduction"
reproduced the latent credit card :image in a manner in which the rosette pattern was much darker than the orig-l.nal, to the exten~
that the hidden e~fec~ o~ the picture (credit card) was destroy-ed, ~nd the picture became visible and darker, although not as per.~ectly vls-lble as woulcl be the case oE us.Lng the screerl and f`.l.lm as descr:Lbecl :~n connect.l.orl w:~th F~ c~

'.I.'he above-rlo'ce(l pr.l.rlte(l salrlL)l.e, wherl r~eprn(1llced h~ way uL`
a colored xerographl.c proce<, re~ lted t.n a rep,r~luct:Lorl l.n which the now v:lslble credit card :lrrlage was three times darker than in the case of the black and white copy.
The above-noted printed sarnple (with hidden credit card image) also defled reproduction by phc~graphy and printing~
For example, the sample print was ph~tographed~ an of`fset; prlrltirlg p1ate was made rrom the re~u:l-tclrlt negative, and oEIset pr-i.n-ts were rnade The result3 were r,uch that the ner!;a~..lve bect.lnle l~ght;-er, the prlntln~ plate c~arker, artd the .r Lnal pr.~nl;ed sarnple~
(:Erom orlg:Lnal sample) became much darker, there~ore making the :formerly latent -lmage visible.
It is bel-leved that this "darkening" e~fect which occurs when efforts are rnade to reproduce the latent image in the manners noted above, results wherever a lens is employed to perform one of the reproduction steps It is believed that the lens or lenses cause the dots Or the rosette patterns to "join" or group together to destroy the original photographic range of light to dark, or small dots to :Large dots, -that pro-~-13-duced the hldden effect of the picture orl the credit card.
Thus another way of producin.; the hidt~en image disclosed herein ls by attempt-in~ to reproduce i.t by conventional photo~raphic and printing processes.
It will become apparent to those familiar with the art that further improvements of -thi.s invention woul~i be too numerous to describe fully in this patent, and while I have illustrated and disclosed only some of my preferred emb~di:ments of this invention, it is to be understood that since the various details departing from the basic prlnciples and teachlngs of thiC; invèn-tion, I do not limit myself to the precise process herei.n ciis-closed and the right is speciflcall~J reserved t) encompasC; all changes and mod-Lfications comin~ within the scope of the lnve~-ti.Qn c19 cie.~:Lned in the apperlcled c~.rns.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of producing on a substrate a latent image of an object which appears to be invisible to the unaided eye, comprising photographing the object through a half tone screen to produce at least one halftone image of the object, placing said halftone image in registry with a strip of light sensitive film and a dark screen having therein a plurality of micro-dot sized apertures arranged in parallel rows oriented in pre-determined angular offset relation to the rows of halftone dots forming said halftone image, exposing said strip of film to said halftone image through the apertures in said dark screen, developing the exposed film to produce thereon the latent halftone image of said object, and imprinting said latent image on said substrate.

2. A method as defined in claim 1, including pro-ducing both positive and negative halftone images of said object, and superposing said positive and negative halftone images onto said strip of light sensitive film by successively exposing the film to said halftone images through said dark screen with the rows of apertures in said screen being oriented in a first pre-determined direction during exposure of said film to one of said halftone images and oriented in a second direction different from said first direction during exposure of said film to the other of said halftone images.

3. A method as defined in claim 2, wherein during each exposure of said strip of film said screen is positioned so that its rows of apertures are offset up to 30° from the rows of halftone dots defining the halftone image then being exposed to said strip, and said first and second directions in which said .....

rows of apertures are oriented are angularly spaced up to 30° from each other,

4. A method in accordance with claim 2, including using two or more positive or negative halftones images and the dark viewing screen to produce two or more latent positive or negative halftone images.

5. A method in accordance with claim 2, including reducing the halftone images to smaller sizes.

6. A method in accordance with claim 5, using a projector to enlarge said reduced images onto a dark viewing screen at the same periodicity as that as the latent halftone images.

7. A method in accordance with claim 2, introducing using a continuous tone negative and positive reproduction with said first and second directions being angled at 30°
from each other to produce the latent halftone negative and the latent halftone positive images.

8. A method in accordance with claim 2, using a random negative and positive reproduction with said first and second directions being angled 30° from each other, to produce a latent positive and negative halftone image.

9. A method in accordance with claim 1, including using a negative continuous tone reproduction and the dark screen to produce a negative latent halftone image.

10. A method in accordance with claim 1, including using a positive continuous tone reproduction and the dark screen to produce a negative latent halftone image.

11. A method in accordance with claim 1, including using a positive continuous tone reproduction and the dark screen and positive negative film or paper to produce a positive halftone having percentages of from 95% to 99% reflecting the total range of the positive continuous reproduction.

12. A method according to claim 1 suitable for use in connection with an identification card, including the steps of preparing both positive and negative halftone reproductions of a person's image in sequence and in register by photographing said positive and negative reproductions through said dark screen, and separating said halftone images by an angle of up to 30° relative to each other to render the visible halftones invisible when viewed without the aid of a dark screen having smaller apertures than said first-mentioned dark screen placed thereover.

13. An article having printed on the face thereof a latent image of an object which appears to be invisible to the unaided eye, said image comprising a microdot reproduction of a photograph of a halftone image of said object taken through a dark screen having therein a plurality of parallel rows of micro-dot sized apertures arranged at an acute angle to the rows of halftone dots defining said halftone image.