Indian Journal of Pure & Applied PhysicsVol. 44, December 2006, pp. 896-902

Holographic optical elements encoded security holograms with enhanced features

Sushil K Kaura*, S P S Virdi # & A K Aggarwal

Coherent Optics Division, Central Scientific Instruments Organisation, Chandiigarh 160030

*E-mail: skkaura_22@rediffmail.com

"Physics Department, Punjabi University, Patiala 147002

Received 3 August 2006; accepted 10 October 2006

A simple and cost-effective two-step method for forming encoded security holograms with enhanced features isdescribed in this paper. These security holograms contain enhanced encoded/concealed anti-counterfeit security Features,which can only be decoded using a key hologram in the final reading process. The encoded key hologram and the securityhologram are in the form of special encoded complex holographic optical elements. When the security hologram isilluminated with the decoding beam, specific moire-like fringe pattern's are formed on the security hologram and in additionseveral spatially separated bright focused spots are also generated from the security hologram. A careful spatial filtering ofthese bright spots results in specific moire patterns at different locations in the observation plane and moreover thesepatterns contain variable interferometric features. Further, these moire patterns disappear when the security hologram isperfectly repositioned and only the variable interferometric features are formed. Since these security holograms containvariable interferometric features in addition to the specific moire patterns and bright focused spots, thus making theseholograms suitable for both visual and as well as machine inspection.

Keywords: Security holograms, Holographic optical elements encoded holograms,' Concealed coded holograms, Opticalsecurity

IPC Code: G03H

1 IntroductionSince the earliest days of market trade, counterfeit

goods have existed. However, in recent years, theproblem of counterfeit goods/documents has attaineda serious dimension. Optical techniques areincreasingly finding their usefulness in the fields ofsecurity and product authenticity verification. Opticalsecurity features can be inspected by either visualchecking without using any special equipment or withthe help of technical facilities for automaticinspection. In order to deter the counterfeiting,various optical validation and security verificationtechniques based on double random phase encodingand joint transform correlations have been widelyinvestigated!". These techniques though excellent intheir own right, are inherently complex and needspecific and costly equipment to visualize or verifytheir security features. Embossed holograms are alsoused extensively as security seal on various productsand documents to guard them against duplication andforgery but face a serious threat from counterfeiters,as the holographic pattern/image can be acquired froma security hologram (photographed or captured with a

"

CCD camera) and a new look alike hologramsynthesized using .cornmercially available hologramproducing equipment. [t is difficult, for a normal eye,to determine whether- such a' hologram is genuine orcounterfeit. In ordel~to. -enhance the anti-counterfeitability of security holograms, various methods basedon phase encoding have been discussed'i". Encodingh h . / 9-11 lib I . dt roug moire patterns tas a so een exp oite to

enhance the anti-counterfeit ability of securityholograms for visual inspection. Recently, a methodhas been proposed in which both machine-readableand visual verifiable features are incorporated toincrease the anti-counterfeit ability of the securityholograms'<. Though this method increases the levelof difficulty for the counterfeiter but it still offerslimited security.features and there is a possibility thatby knowing about the shape and number of fringes,these holograms could be regenerated by hit and trialmethod by an expert holographer. In order to furtherenhance the anti-counterfeit ability of securityholograms, a simple and cost-effective method formaking holographic optical elements encoded securityholograms with enhanced features is described in this

paper.securitysecurityobjectholograkey holform ofelementand mfeatureskey hole

2 PriiThe I

forrnatirsecurityencodecseparateencodecseparatethe samconvergwith a I

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KAURA et al.: HOLOGRAPHIC OPTICAL ELEMENTS ENCODED SECURITY HOLOGRAMS 897

ures

paper. In this method, the enhanced/additionalsecurity features- have been incorporated in thesecurity holograms by using multiple convergentobject beams in the formation of encoded keyhologram and the security hologram. The encodedkey hologram and the security hologram are in theform of special encoded complex holographic opticalelements. These security holograms contain enhancedand multifold encoded anti-counterfeit securityfeatures, which can only be decoded by using encodedkeyhologram in the final reading process.

2 Principle ofthe MethodThe method reported in this paper is based on the

formation of an,' encoded key hologram and thesecurity hologramIwhich are in the form of a specialencoded complex holographic optical elements)separately in two recording steps. The formation ofencoded key hologram (Fig. 1) in turn involves twoseparate and independent holographic recordings 'onthesame recording plate. In the first recording case, aconvergent object beam 01 is used in conjunctionwith a collimated reference beam R. Before makingthe second recording on the same plate, theconverging lens, used for the generation of objectbeam 01, is given a minute movement in thetransverse direction and a slightly different objectbeam O2 is generated. In the second recording, theobject beam O2 is used in conjunction with the samereference beam R. This encoded key hologram (KH),when illuminated with a collimated beam, provides anencoded reference beam for the second recordingstep. The so generated encoded reference wave isused, in .two separate holographic exposures, inconjunction with two spatially separated convergentobject waves S I and S2, respectively for the formationofthe security hologram (Fig. 2). For further incorpo-ration of enhanced security verification features inthese security. holograms, each of these holographicrecordings in turn involves two separate holographicexposures independently and on the same recordingplate. In the first recording case, the convergent objectwave SI is used in the first exposure. Before makingthesecond exposure on the same plate, the converginglens, used for the generation of beam S I, is given aminute movement along the longitudinal direction andtheso generated object beam S'I is used in the secondexposure. Similarly, in the second recording case, theconvergent object wave Sz is used in the firstexposure. Prior to making the second exposure on thesame plate, the converging lens, used for the

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.thod forJ securityed.in this

L BS

M3

...................... ,.

"""'-·,0 ..

KH

Fig. I-Schematic of experimental layout for recording encodedkey holograms

l'v\( )~1z

L ~~L ~ Xt Y 71

Fig. 2-Schematic of experimental layout for recording securityholograms

generation of beam S2, is given a minute movementalong the transverse direction and the so generatedobject beam S'2 is used in the second exposure. Whenthese security holograms are read through the encodedkey hologram, specific moire-like pattern is formed.These moire patterns are formed due to thesuperposition of complex holographic sinusoidalphase diffraction grating patterns 13 of high spatialfrequencies generated from key hologram and thoserecorded in security hologram. Further, severalspatially separated bright focused spots also getgenerated as it is read through the key hologram. Acareful spatial filtering of these bright focused spotsresults in spatially separated high contrast moirepatterns at different locations in the observation plane.However these moire patterns in addition also containvariable interferometric features due to longitudinal

898 INDIAN J PURE & APPL PHYS, VOL 44, DECEMBER 2006

and transverse motion of the converging beams,respectively. By making careful adjustments, themoire-like patterns get disappeared (when securityhologram is perfectly repositioned) as the complexdiffraction patterns generated from key hologramcompletely overlap on those already recorded on thesecurity hologram and only the variable inter-ferometric features (circular and linear interferencefringe patterns) are formed. For the sake of simplicityin mathematical formulations 0], O2, S], S\ S2 and S'2have been taken as plane wavefronts. In the firstrecording case for forming KH, we take 0]propagating at an angle ao to the axis, O2 propagatingat an angle ao + 8ao to the axis and R propagatingalong the axis. The complex amplitude distribution of0], O2 and R can be considered as:

01 = Ao exp [-iax]; O2= Ao exp [-i (a + E) x] andR = A, exp [ikx]

where a = k sin ao; a + E = k sin (ao + 8ao) and E = k(8ao) cos ao

The amplitude transmittance of the processed KHis:

For forming the security hologram (SH) in thesecond recording step, KH is again illuminated withthe same collimated reference beam R. The complexamplitude of the transmitted field from KH is:

UI-R tl =R\Od2+R\R\2 +OdR\2 +01*R2 +R\02\2+R\R\2 +02\R\2 +02* R2 (2)

We can consider \R\2to be constant across KH, as aplane reference wave R is used for illumination ofKH. Thus only 3rd and 7th terms on the right-hand sideof Eq. (2) are of interest to us as they represent twodiffracted-orders of beams 01 and O2, i.e.

... (3)

These two generated beams (serving as encodedreference beam) are used, in two separate holographicexposures, in conjunction with two spatially separatedconvergent object beams SI and S2, respectively forthe formation of the security hologram. Each of theseholographic recordings in turn employs two separateholographic exposures {in the first case, using S I inthe first exposure and S'] in the second exposure; andin the second case, using S2 in the first exposure andS'2 in the second exposure} independently and on the

same recording plate. The complex amplitude distri-bution of S], S'I, S2 and S'2 can be considered as:

SI = A] exp [-ibx]; S'] = A] exp [-i (b + 11) x];S2= A2 exp [-icx] and S'2=A2 exp [-i (c + ~) x]

After processing, the SH is repositioned at thesame location at which it was recorded. Theamplitude transmittance of the processed SH is:

2 2t2-\0]+02+Sd +\0]+02+S']\ +\0]+02+S2\2+ \0]+ O2+ S'2\2 ... (4)

As all the four terms on the right hand side ofEq. (4) are almost similar, so for the sake ofsimplicity in further mathematical formulations onlythe first term is considered. Thus, the amplitudetransmittance'< of the processed SH is:

t2 -11 + cos Ex + cos (a - b)x + cos (a + E - b)x]

In this configuration, when KH is illuminated witha collimated beam, it provides two illuminating beams01 and O2 for SH [Eq. (3)]. The irradiance at SH canbe written as:

(1) \ 'P (x) \2- {l + cos 2n ).lox}

where ).lo= lid = (8ao) cos aol ADuring the final reading process" when SH is

slightly misaligned by an angle 8 with respect to y-axis, then the amplitude transmittance'? of the SH is

t'2 - [1 + cos 2n (ux -vy) + cos 2n (a - b)().lX -vy)+ cos 2n (a + E - b)().lX -vy)] , .. .. (7)

., ,

where ).l = cos 8 Id and v = sin 81d .',' .Thus, the complex amplitude distributionatSll is:

t (x,y) = \'P (x) \2t'2 - 1 + cos 2n ().lX "vy)'+cos 2n (a - b)().lX -vy) .+cos 2n (a + E - b)().lX -V y) + cos 2n /loX+cos 2n ).lo X cos 2n ().lX -vy)+cos 2n /lo X cos 2n (a - b)().lX-vy)+cos 2n ).lo x cos 2n (a + E - b)().lX -vy) ... (8)

It may now be seen that the 6th' term denotes the

presence of a complex moire pattern 15 on the securityhologram. Likewise, the resultant intensity distri-bution I(x,y) in the observation plane, due to mis-aligned SH, could be written as:

I(x,y)= \'P (x) t'2\2 - 2cos 2n ).loXcos 2n().lX -vy)+[cos 2n ).loXcos 2n ().lX -vy)][cos 2n ().lX -vY)

(5)

3 Exp(In OUI

(Coheren

(6)

where.moiremodulemergipresencarefulgeneraobservhas br(contaiSimilarand fcgeneralpatternIt mayinterferintensitcreatedside ofresult irintensirpatternsvariableand trailS2, resp(the mollperfectl-interfereferencethat in Ibeams ikey holetageousability a:several sspots fnprocess.

ude distri-d as:

x]ned at therded. TheHis:

)2+... (4)

md side ofIe sake ofations only

amplitude

- b)x]

rinated withating beams.e at SH can

... (6)

vhen SH isrespect to y-f the SH is

ux -vy)

on at SH is:

;·2n).l0 x

y)x -vy) ... (8)

n denotes the.n the securitytensity distri-, due to mis-

ux -vy), 2n (ux -vy)

'"

(5)

(7)

KAURA et al.: HOLOGRAPHIC OPTICAL ELEMENTS ENCODED SECURITY HOLOGRAMS 899

+ 2cos 2n (a - b)(J..lx-vy)+ 2cos 2n (a + e - b)(/lX -vy)]+ cos 2n).loX cos' 2n (a - b)().1X -vy)+ cos 2n).lox cos2 2n (a + E - b)().1X -vy)+ 2cos 2n).loX cos 2n (a - b)().1X -vy)+ 2 cos 2n).loX cos 2n(a + £ - b)().1X -vy)+ 2 cos 2n).loX cos 2n (a - b)().1X -vy)X cos 2n (a + E - b)().1X -vy) ... (9)

where, the I" term denotes the presence of a complexmoire pattern; the 2nd to 4th terms indicate themodulation of the moire patterns -in the other beamsemerging from SH; and the 5th to 9th terms depict thepresence of extra noise terms. It is further seen that acareful spatial filtering of 'the 1st term results in thegeneration of high contrast moire pattern in theobservation plane. It is to be pointed that the Eq. (9)has been obtained by using only _ the first term(containing beam SI) on the right hand side ofEq. (4).Similarly by using the third term (containing beam S2)and following the same procedure results in thegeneration of a spatially separated high contrast moirepattern at a different location in the observation plane.It may be noted that, due to the standard holographicinterferometry, interference fringe patterns withintensity distribution _4A1

2 cos2 (11/2) are additionallycreated due to the first and second terms on right handside of Eq. (4). Similarly, the third and fourth termsresult in additional different interference patterns withintensity distribution -4A/ cos" (~/2). Thus, the moirepatterns observed in the observation plane hasvariable interferometric features due to longitudinaland transverse motion of the converging beams SI andS2, respectively. By making appropriate adjustments,the moire patterns would get disappeared when SH isperfectly repositioned (i.e., 8=0) and only the variableinterferometric features (circular and linear inter-ference fringe patterns) are formed. It is to be notedthat in the proposed method, the use of convergingbeams instead of plane wavefronts in recording thekey hologram and the security holograms are advan-tageous in terms of enhancing their anti-counterfeitability as it additionally facilitates in the generation ofseveral spatially separated bright focused verificationspots from the security hologram in the readingprocess.

3 Experimental DetailsIn our experimental' arrangement, a He-Ne laser

(Coherent model 31-2140, 35 mW output power,

632.8 nm wavelength) was used in the recording ofencoded key hologram, security hologram and in thefinal reading process of security holograms. Theexperimental layout for the first recording step offorming the encoded key hologram is schematicallyshown in Fig. 1. A variable beam splitter (BS) splits alaser (L) beam into two components. The reflectedcomponent from BS is used for the generation of aconvergent object wave 01 through a beam expander(BEl) in conjunction with a combination of twocollimating lenses CI (f/3.5; 100 mm-diameter) andC2 (f/5; 100 mm-diameter). The transmittedcomponent from BS, used for the generation of acollimated reference beam (R), is expanded andcollimated by using a beam expander (BE2) and acollimating lens C3 (f/4; 50 mm-diameter). These twobeams (R and 01) are used to make the first recordingon the hologram recoding plate (KH). Before makingthe second recording on the same recording plate(KH), the converging lens C2 is given a minutemovement (-300 p,m) in the transverse direction. Theso generated convergent object beam O2 is used inconjunction with R for making the second recordingon the same holographic plate (KH). The experi-mental layout for second recording step of makingsecurity holograms is schematically shown in Fig. 2.Light beam from the laser (L) is split through avariable beam splitter (BS I) into a reflected and atransmitted component. The transmitted component isfurther split into two parts by another variable beamsplitter (BS2). The reflected component from BSI isused for the generation of a convergent object waveS I on the hologram recording plate SH through abeam expender (BEl) in conjunction with acombination of two collimating lenses CI (f/3.5; 100mm-diameter) and C2 (f/5; 100 mm-diameter). Thetransmitted component from BS2 is used for thegeneration of another convergent object wave S2 onthe hologram recording plate SH through a beamexpender (B~) in conjunction with a combination oftwo collimating lenses C3 (f/5; 100 mm-diameter) andC4 (f/3.5; 100 mm-diameter). The reflected compo-nent from BS2 is used for the generation of aconjugate reference beam for the KH through a beamexpender (BE3) in conjunction with a collimating lensC5 (f/4; 50 mm-diameter) and the real image derivedfrom KH serves as the encoded reference wave inmaking the security hologram. The so generatedencoded reference wave is made to interfere, in twoseparate holographic exposures, with the two spatially

900 INDIAN J PURE & APPL PHYS, VOL 44, DECEMBER 2006

separated convergent object waves S I and S2 on thehologram recording plate to form the securityhologram (SH). However, each of these holographicexposures in the second recording step in turninvolved two separate holographic exposures indepen-dently on the same recording plate. In the first case,the converging lens C2 is given a minute movement(-500 run) along the longitudinal direction (i.e. alongthe optical axis) between the two holographicexposures and whereas in the second case, theconverging lens C4 is given a minute movement(- 40 urn) in the transverse direction (i.e. perpen-dicular to the optical axis) between the twoholographic exposures. Standard Kodak D-19developer and R-9 bleach bath solutions are used withSlavich PFG-01 plates to give high efficiency and lownoise encoded key holograms and security holograms.The experimental layout for the final reading processof these security holograms is schematically shown inFig. 3. Here, a collimated beam [generated through abeam expander BE in conjunction with a collimatinglens C (f/4; 50 mm-diameter)] is used as a conjugatereference beam to illuminate the KH, where the KH isplaced at a predetermined fixed position. The realimage derived from the KH serves as a decodingreconstructing beam for reading the SR. It is observedthat when SH is slightly misaligned in it's reposi-tioning, specific moire pattern gets formed on thesecurity hologram (Fig. 4). In addition, severalspatially separated bright focused spots are generatedfrom the security hologram (Fig. 5). A careful spatialfiltering of these bright focused spots results in thegeneration of spatially separated high contrast moirepatterns at two different locations in the observationplane OP and these moire patterns in addition alsocontain variable interferometric features, i.e. typical

SF

" Fig. 4- Typical moire pattern on security hologram

Fig. 5-Photograph of spatiallyseparated bright focused spots

circular and linear interference fringe patterns due tolongitudinal and transverse motion of the convergingbeams S I and S2 (used for making the securityhologram), respectively (Fig. 6). These specific moirepatterns are obtained by giving a typical tilt of -3degree in the vertical direction and linear movementof -1.2 mm along the horizontal direction to thesecurity hologram in the reading process. Further,these specific moire patterns disappear as the securityhologram is perfectly repositioned (where in this case,the complex diffraction patterns generated from keyhologram completely overlap on those alreadyrecorded on the security hologram) and only thevariable interferometric features, i.e. typical circularand linear interference fringe patterns are formed

(Fig .. 7position

4 Cor.. A sit

holograjgrams \paper. ~encodedwhich c:holograrencoded

op

Fig. 3-Schematic of experimental layout for reading securityholograms

.ologram

focused spots

tterns due toe converging'the securitypecific moireal tilt of -3ir movementction to theess. Further,; the security~in this case,ed from keyose alreadynd only theiical circular~re formed

KAURA et al.: HOLOGRAPHIC OPTICAL ELEMENTS ENCODED SECURITY HOLOGRAMS 901

Fig. 6- Typical moire patterns along with variable interferometric features due to spatially filtered bright focused spots

Fig. 7- Typical variable interferometric features with perfectly repositioned SH

(Fig. 7). It may be noted that the sensitivity 111

positioning of either KH or SH is not very critical.

4 ConclusionsA simple and cost-effective method for making

holographic optical elements encoded security holo-grams with enhanced features is discussed in thispaper. These security holograms contain enhancedencoded/concealed anti-counterfeit security features,which can only be decoded by using an encoded keyhologram in the final reading process. In this case, theencoded key hologram and the security hologram are

in the form of special encoded complex holographicoptical elements rather than binary patterns. In thefinal reading process, a specific moire pattern isformed on the security hologram only when thesecurity hologram is illuminated by the decoding,reconstructing beam, generated from the encoded keyhologram. These moire patterns are formed only inthe case of an authentic security hologram and arevisually verifiable. In addition, several spatiallyseparated bright focused spots are generated from thesecurity hologram. These bright focused spots,formed at a predetermined fixed location (angular and

902 INorAN J PURE & APPL PHYS, VOL 44, DECEMBER 2006

azimuth position), may be used advantageously formachine inspection by using a relatively simplemachine-readable device. A careful spatial filtering ofthese bright focused spots results in spatiallyseparated high contrast moire patterns at differentlocations in the observation plane and these moirepatterns in addition also contain variable interfero-metric features, i.e. circular and linear interferencepatterns, respectively in them, and are used for visualinspection. When the security hologram is perfectlyrepositioned, the specific moire patterns disappear andonly the variable interferometric features, i.e. circularand linear interference fringe patterns me formed andme available only in the case of an authentic securityhologram. These variable interferometric featuresfurther facilitate in the visual inspection of theenhanced/additional security verification featurescontained in these security holograms. It is furtherobserved that different specific moire verificationpattern~ can be obtained by giving different tilt in thevertical direction and linear movement in thehorizontal direction to the security hologram in thefinal reading process. It may be noted that thesesecurity holograms contain complex sinusoidal phasediffraction grating patterns, which makes themextremely difficult to counterfeit. Since the verifi-cation/identification patterns in these securityholograms are variable interferometric features (i.e.circular and linear interference fringe patterns) inaddition to the specific moire patterns and spatiallyseparated bright focused spots, this type of securityholograms are suitable for both visual and as well asmachine inspection. It may further be seen that theanti-counterfeit ability of these security hologram isenhanced manifold by using multiple convergent

object beams while making the encoded key hologramand the security hologram. This type of hologramscan also be used as a security code for betterprotection against counterfeiting in embossed holo-grams.

AcknowledgementThe authors are grateful to Dr Paw an Kapur,

Director, CSIO, Chandigarh, for his constant encou-ragement, support and permission to publish thiswork. They wish to thank the Department of Scienceand Technology, Govt of India, New Delhi, for thefinancial support for carrying out this work.

References1 Refregier P & Javidi B, Optics LeI!, 20 (1995) 767.2 Wang R K, Watson I A & Chatwin C, Optical Engg, 3S

(f996) 2464.3 Neto L G & Sheng Y, Optical Engg ; 35 (1996) 2459..~ Weber D & Trolinger J, Optical Engg, 38 (1999) 62.5 Javidi B & Horner J L, Optical Eugg ; 33 (1994) 1752.6 Lai S, Optical Engg, 35 (1996) 2470.7 Kaura S K, Chhachhia D P, Sharma A K & Aggarwal A K,

Indian J Pure & Appl Phys, 41 (2003) 696.8 Aggarwal A K, Kaura S K, Chhachhia D P & Sharma A K, J

Optics A: Pure & Appl Optics, 6 (2004) 278.9 Liu S, Zhang X & Lai H, Appl Optics, 34 (1995) 4700.

10 Aggarwal A K, Kaura S K, Chhachhia D P & Sharma A K,Optics & Laser Tech, 38 (2006) 117. .

11 Zhang X, Dalsgaard E, Liu S, Lai H & Chen J, Appl Optics,36, 19978096.

12 Kaura S K, Chhachhia D P & Aggarwal A K, J Optics A:Pure & Appl Optics, 8 (2006) 67.

13 Santos PAM D, Nunes LCD S & Correa I, Appl Optics, 39(2000) 4524.

14 Sirohi R S & Chau F S (Ed), Optical methods ofmeasurement (Marcel Dekker Inc, New York, USA) 1999,pp 227.

15 Sciarnmarella C A, Optical Engg, 21 (1982) 447.

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