Holographic optical elements encoded security holograms with enhanced features

Indian Journal of Pure & Applied Physics Vol. 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 is described 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 security hologram are in the form of special encoded complex holographic optical elements. When the security hologram is illuminated with the decoding beam, specific moire-like fringe pattern's are formed on the security hologram and in addition several spatially separated bright focused spots are also generated from the security hologram. A careful spatial filtering of these bright spots results in specific moire patterns at different locations in the observation plane and moreover these patterns contain variable interferometric features. Further, these moire patterns disappear when the security hologram is perfectly repositioned and only the variable interferometric features are formed. Since these security holograms contain variable interferometric features in addition to the specific moire patterns and bright focused spots, thus making these holograms suitable for both visual and as well as machine inspection. Keywords: Security holograms, Holographic optical elements encoded holograms,' Concealed coded holograms, Optical security IPC Code: G03H " 1 Introduction Since the earliest days of market trade, counterfeit goods have existed. However, in recent years, the problem of counterfeit goods/documents has attained a serious dimension. Optical techniques are increasingly finding their usefulness in the fields of security and product authenticity verification. Optical security features can be inspected by either visual checking without using any special equipment or with the help of technical facilities for automatic inspection. In order to deter the counterfeiting, various optical validation and security verification techniques based on double random phase encoding and joint transform correlations have been widely investigated!". These techniques though excellent in their own right, are inherently complex and need specific and costly equipment to visualize or verify their security features. Embossed holograms are also used extensively as security seal on various products and documents to guard them against duplication and forgery but face a serious threat from counterfeiters, as the holographic pattern/image can be acquired from a security hologram (photographed or captured with a CCD camera) and a new look alike hologram synthesized using.cornmercially available hologram producing equipment. [t is difficult, for a normal eye, to determine whether- such a' hologram is genuine or counterfeit. In ordel~to. -enhance the anti-counterfeit ability of security holograms, various methods based on phase encoding have been discussed'i". Encoding h h. / 9-11 t roug moire patterns lib tas a so een exp I oite. d to enhance the anti-counterfeit ability of security holograms for visual inspection. Recently, a method has been proposed in which both machine-readable and visual verifiable features are incorporated to increase the anti-counterfeit ability of the security holograms'<. Though this method increases the level of difficulty for the counterfeiter but it still offers limited security.features and there is a possibility that by knowing about the shape and number of fringes, these holograms could be regenerated by hit and trial method by an expert holographer. In order to further enhance the anti-counterfeit ability of security holograms, a simple and cost-effective method for making holographic optical elements encoded security holograms with enhanced features is described in this

KAURA et al.: HOLOGRAPHIC OPTICAL ELEMENTS ENCODED SECURITY HOLOGRAMS 897 paper. In this method, the enhanced/additional security features- have been incorporated in the security holograms by using multiple convergent object beams in the formation of encoded key hologram and the security hologram. The encoded key hologram and the security hologram are in the form of special encoded complex holographic optical elements. These security holograms contain enhanced and multifold encoded anti-counterfeit security features, which can only be decoded by using encoded keyhologram in the final reading process. 2 Principle ofthe Method The method reported in this paper is based on the formation of an,' encoded key hologram and the security hologramiwhich are in the form of a special encoded complex holographic optical elements) separately in two recording steps. The formation of encoded key hologram (Fig. 1) in turn involves two separate and independent holographic recordings 'on thesame recording plate. In the first recording case, a convergent object beam 0 1 is used in conjunction with a collimated reference beam R. Before making the second recording on the same plate, the converging lens, used for the generation of object beam 0 1, is given a minute movement in the transverse direction and a slightly different object beam O2 is generated. In the second recording, the object beam O2 is used in conjunction with the same reference beam R. This encoded key hologram (KH), when illuminated with a collimated beam, provides an encoded reference beam for the second recording step. The so generated encoded reference wave is used, in.two separate holographic exposures, in conjunction with two spatially separated convergent object waves S I and S2, respectively for the formation ofthe security hologram (Fig. 2). For further incorporation of enhanced security verification features in these security. holograms, each of these holographic recordings in turn involves two separate holographic exposures independently and on the same recording plate. In the first recording case, the convergent object wave SI is used in the first exposure. Before making thesecond exposure on the same plate, the converging lens, used for the generation of beam S I, is given a minute movement along the longitudinal direction and theso generated object beam S'I is used in the second exposure. Similarly, in the second recording case, the convergent object wave Sz is used in the first exposure. Prior to making the second exposure on the same plate, the converging lens, used for the L...,. """'-,0.. KH Fig. I-Schematic of experimental layout for recording encoded key holograms L 1 l'v\( BS )~1z M3 t Y 7L ~ X ~~ Fig. 2-Schematic of experimental layout for recording security holograms generation of beam S2, is given a minute movement along the transverse direction and the so generated object beam S'2 is used in the second exposure. When these security holograms are read through the encoded key hologram, specific moire-like pattern is formed. These moire patterns are formed due to the superposition of complex holographic sinusoidal phase diffraction grating patterns 13 of high spatial frequencies generated from key hologram and those recorded in security hologram. Further, several spatially separated bright focused spots also get generated as it is read through the key hologram. A careful spatial filtering of these bright focused spots results in spatially separated high contrast moire patterns at different locations in the observation plane. However these moire patterns in addition also contain variable 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, the moire-like patterns get disappeared (when security hologram is perfectly repositioned) as the complex diffraction patterns generated from key hologram completely overlap on those already recorded on the security hologram and only the variable interferometric features (circular and linear interference fringe patterns) are formed. For the sake of simplicity in mathematical formulations 0], O2, S], S\ S2 and S'2 have been taken as plane wavefronts. In the first recording case for forming KH, we take 0] propagating at an angle ao to the axis, O2 propagating at an angle ao + 8ao to the axis and R propagating along the axis. The complex amplitude distribution of 0], O2 and R can be considered as: 01 = Ao exp [-iax]; O2= Ao exp [-i (a + E) x] and R = 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 KH is: same recording plate. The complex amplitude distribution 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 the same location at which it was recorded. The amplitude transmittance of the processed SH is: 2 2 t2-\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 of Eq. (4) are almost similar, so for the sake of simplicity in further mathematical formulations only the first term is considered. Thus, the amplitude transmittance'< 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 with a collimated beam, it provides two illuminating beams 01 and O2 for SH [Eq. (3)]. The irradiance at SH can be written as: (5) (1) \'P (x) \2- {l + cos 2n ).lox} (6) For forming the security hologram (SH) in the second recording step, KH is again illuminated with the same collimated reference beam R. The complex amplitude of the transmitted field from KH is: UI-R tl =R\Od2+R\R\2 +OdR\2 +01*R 2 +R\02\2 +R\R\2 +02\R\2 +02* R2 (2) We can consider \R\2 to be constant across KH, as a plane reference wave R is used for illumination of KH. Thus only 3 rd and 7th terms on the right-hand side of Eq. (2) are of interest to us as they represent two diffracted-orders of beams 01 and O2, i.e.... (3) These two generated beams (serving as encoded reference beam) are used, in two separate holographic exposures, in conjunction with two spatially separated convergent object beams SI and S2, respectively for the formation of the security hologram. Each of these holographic recordings in turn employs two separate holographic exposures {in the first case, using S I in the first exposure and S'] in the second exposure; and in the second case, using S2 in the first exposure and S'2 in the second exposure} independently and on the where ).lo= lid = (8ao) cos aol A During 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 6 th ' term denotes the presence of a complex moire pattern 15 on the security hologram. Likewise, the resultant intensity distribution I(x,y) in the observation plane, due to misaligned 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)

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 cos 2 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 complex moire pattern; the 2 nd to 4th terms indicate the modulation of the moire patterns -in the other beams emerging from SH; and the 5 th to 9 th terms depict the presence of extra noise terms. It is further seen that a careful spatial filtering of 'the 1 st term results in the generation of high contrast moire pattern in the observation 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 the generation of a spatially separated high contrast moire pattern at a different location in the observation plane. It may be noted that, due to the standard holographic interferometry, interference fringe patterns with intensity distribution _4A 1 2 cos 2 (11/2) are additionally created due to the first and second terms on right hand side of Eq. (4). Similarly, the third and fourth terms result in additional different interference patterns with intensity distribution -4A/ cos" (~/2). Thus, the moire patterns observed in the observation plane has variable interferometric features due to longitudinal and transverse motion of the converging beams SI and S2, respectively. By making appropriate adjustments, the moire patterns would get disappeared when SH is perfectly repositioned (i.e., 8=0) and only the variable interferometric features (circular and linear interference fringe patterns) are formed. It is to be noted that in the proposed method, the use of converging beams instead of plane wavefronts in recording the key hologram and the security holograms are advantageous in terms of enhancing their anti-counterfeit ability as it additionally facilitates in the generation of several spatially separated bright focused verification spots from the security hologram in the reading process. 3 Experimental Details In our experimental' arrangement, a He-Ne laser (Coherent model 31-2140, 35 mw output power, 632.8 nm wavelength) was used in the recording of encoded key hologram, security hologram and in the final reading process of security holograms. The experimental layout for the first recording step of forming the encoded key hologram is schematically shown in Fig. 1. A variable beam splitter (BS) splits a laser (L) beam into two components. The reflected component from BS is used for the generation of a convergent object wave 0 1 through a beam expander (BEl) in conjunction with a combination of two collimating lenses CI (f/3.5; 100 mm-diameter) and C 2 (f/5; 100 mm-diameter). The transmitted component from BS, used for the generation of a collimated reference beam (R), is expanded and collimated by using a beam expander (BE 2 ) and a collimating lens C3 (f/4; 50 mm-diameter). These two beams (R and 0 1 ) are used to make the first recording on the hologram recoding plate (KH). Before making the second recording on the same recording plate (KH), the converging lens C2 is given a minute movement (-300 p,m) in the transverse direction. The so generated convergent object beam O 2 is used in conjunction with R for making the second recording on the same holographic plate (KH). The experimental layout for second recording step of making security holograms is schematically shown in Fig. 2. Light beam from the laser (L) is split through a variable beam splitter (BS I) into a reflected and a transmitted component. The transmitted component is further split into two parts by another variable beam splitter (BS2). The reflected component from BSI is used for the generation of a convergent object wave S I on the hologram recording plate SH through a beam expender (BEl) in conjunction with a combination of two collimating lenses CI (f/3.5; 100 mm-diameter) and C2 (f/5; 100 mm-diameter). The transmitted component from BS 2 is used for the generation of another convergent object wave S2 on the hologram recording plate SH through a beam expender (B~) in conjunction with a combination of two collimating lenses C3 (f/5; 100 mm-diameter) and C 4 (f/3.5; 100 mm-diameter). The reflected component from BS 2 is used for the generation of a conjugate reference beam for the KH through a beam expender (BE3) in conjunction with a collimating lens C 5 (f/4; 50 mm-diameter) and the real image derived from KH serves as the encoded reference wave in making the security hologram. The so generated encoded reference wave is made to interfere, in two separate 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 the hologram recording plate to form the security hologram (SH). However, each of these holographic exposures in the second recording step in turn involved two separate holographic exposures independently 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. along the optical axis) between the two holographic exposures and whereas in the second case, the converging lens C 4 is given a minute movement (- 40 urn) in the transverse direction (i.e. perpendicular to the optical axis) between the two holographic exposures. Standard Kodak D-19 developer and R-9 bleach bath solutions are used with Slavich PFG-01 plates to give high efficiency and low noise encoded key holograms and security holograms. The experimental layout for the final reading process of these security holograms is schematically shown in Fig. 3. Here, a collimated beam [generated through a beam expander BE in conjunction with a collimating lens C (f/4; 50 mm-diameter)] is used as a conjugate reference beam to illuminate the KH, where the KH is placed at a predetermined fixed position. The real image derived from the KH serves as a decoding reconstructing beam for reading the SR. It is observed that when SH is slightly misaligned in it's repositioning, specific moire pattern gets formed on the security hologram (Fig. 4). In addition, several spatially separated bright focused spots are generated from the security hologram (Fig. 5). A careful spatial filtering of these bright focused spots results in the generation of spatially separated high contrast moire patterns at two different locations in the observation plane OP and these moire patterns in addition also contain variable interferometric features, i.e. typical op SF Fig. 3-Schematic of experimental layout for reading security holograms " Fig. 4- Typical moire pattern on security hologram Fig. 5-Photograph of spatiallyseparated bright focused spots circular and linear interference fringe patterns due to longitudinal and transverse motion of the converging beams S I and S2 (used for making the security hologram), respectively (Fig. 6). These specific moire patterns are obtained by giving a typical tilt of -3 degree in the vertical direction and linear movement of -1.2 mm along the horizontal direction to the security hologram in the reading process. Further, these specific moire patterns disappear as the security hologram is perfectly repositioned (where in this case, the complex diffraction patterns generated from key hologram completely overlap on those already recorded on the security hologram) and only the variable interferometric features, i.e. typical circular and linear interference fringe patterns are 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 Conclusions A simple and cost-effective method for making holographic optical elements encoded security holograms with enhanced features is discussed in this paper. These security holograms contain enhanced encoded/concealed anti-counterfeit security features, which can only be decoded by using an encoded key hologram in the final reading process. In this case, the encoded key hologram and the security hologram are in the form of special encoded complex holographic optical elements rather than binary patterns. In the final reading process, a specific moire pattern is formed on the security hologram only when the security hologram is illuminated by the decoding, reconstructing beam, generated from the encoded key hologram. These moire patterns are formed only in the case of an authentic security hologram and are visually verifiable. In addition, several spatially separated bright focused spots are generated from the security 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 for machine inspection by using a relatively simple machine-readable device. A careful spatial filtering of these bright focused spots results in spatially separated high contrast moire patterns at different locations in the observation plane and these moire patterns in addition also contain variable interferometric features, i.e. circular and linear interference patterns, respectively in them, and are used for visual inspection. When the security hologram is perfectly repositioned, the specific moire patterns disappear and only the variable interferometric features, i.e. circular and linear interference fringe patterns me formed and me available only in the case of an authentic security hologram. These variable interferometric features further facilitate in the visual inspection of the enhanced/additional security verification features contained in these security holograms. It is further observed that different specific moire verification pattern~ can be obtained by giving different tilt in the vertical direction and linear movement in the horizontal direction to the security hologram in the final reading process. It may be noted that these security holograms contain complex sinusoidal phase diffraction grating patterns, which makes them extremely difficult to counterfeit. Since the verification/identification patterns in these security holograms are variable interferometric features (i.e. circular and linear interference fringe patterns) in addition to the specific moire patterns and spatially separated bright focused spots, this type of security holograms are suitable for both visual and as well as machine inspection. It may further be seen that the anti-counterfeit ability of these security hologram is enhanced manifold by using multiple convergent object beams while making the encoded key hologram and the security hologram. This type of holograms can also be used as a security code for better protection against counterfeiting in embossed holograms. Acknowledgement The authors are grateful to Dr Paw an Kapur, Director, CSIO, Chandigarh, for his constant encouragement, support and permission to publish this work. They wish to thank the Department of Science and Technology, Govt of India, New Delhi, for the financial support for carrying out this work. References 1 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 of measurement (Marcel Dekker Inc, New York, USA) 1999, pp 227. 15 Sciarnmarella C A, Optical Engg, 21 (1982) 447.