# Computer-Generated Holograms Application in Security Printing
## 5 Holomemory  
Another alternative application of CGH is the offered method for photopolymer security holograms individualization [26]. The method consists of recording an extra computer-generated micro-hologram containing a hidden coded image with individual data in a small area of the photopolymer material. The micro-CGH fringe pattern is synthesized and recorded as an extra sub-hologram in the defined area of the security hologram (SH). The micro-hologram is recorded by projecting a fringe pattern of computer-generated Fourier holograms that contain images with an individual digital code into the plane of photopolymer carrier. CGH is displayed in the spatial light modulator on the basis of liquid crystals. When replicating the individualized SH, first the micro-CGH is exposed in accordance with the topology of the main holograms in the photopolymer strip, then the main 3D security hologram is recorded by placing it on the micro-CGH using the method of optical copying from master matrix. The main three-dimensional hologram is recorded by the Denisyuk method in the second exposure after recording the individualizing micro-CGH. The process of security hologram individualization must be carried out while continuously  
Figure 8: Restored optical coded mark with CGH using smartphone with LED flash. Figure 9: Model of B2C and B2B interaction with the use of smart verification. moving the plate with the photopolymer carrier. At the same time, while the optical head transfers to the next SH, CGH with proper individual data for the _i_-th micro-hologram must be calculated. Therefore, every subsequent micro-hologram contains the required individualized information for every security hologram (Figure 10). Using mathematical algorithms, the input digital information (in the form of digital texts, images, etc.) is coded with compression in the online mode and transformed into a coded image in the form of multidigit, e.g., 256-binary code. To generate a coded image, a coding algorithm from ECMA-378 [27] international standard which is used to record holographic memory disks (that is why the optical security element is called so) is applied. Additionally, the Cross Interleaved Reed-Solomon Code is used, which adds data redundancy and allows correction of damaged pixels of the coded image and complete restoration of data during subsequent read-out and decoding. Using the methods of computer-generation of Fourier holograms, the obtained coded image is transformed into computer-generated hologram presented in the form of gradation image (Figure 11). Algorithms based on Equations (3a) and (6) ensure their quick generation for seconds, while the data capacity of input information can be up to 1 Mb that is sufficient to store information in the form of digital identifier (digital code) consisting of a sequence number and link to the goods or documents from appropriate database where the data concerning the goods or documents have been entered. Figure 10: Photopolymer security holograms individualization: (**1**) is a LED or laser source; (**2**) is a condenser lens; (**3**) is a collimation lens of the lighting system; (**4**) is a spatial light modulator based on liquid crystals, where CGH is displayed; (**5**) is a Fourier lens; (**6**) is a spatial filter (diaphragm); (**7**) is a lens; (**8**) is a micro-lens; (**9**) is a personalized _i_-th micro-CGH; (**10**) is a photopolymer material; (**11**) is a personal computer for CGH calculation. In the implemented scheme, the laser radiation with a wavelength of 532 nm illuminates the space light modulator with a converging light beam and transfers the image onto a photopolymer with a ten-fold reduction. The SLM dimension was 1024 \(\times\) 768 elements and a pixel pitch was 36 \(\upmu\)m. The resolution of the recorded micro-hologram was about 300 mm\({}^{-1}\). The size of the recorded individualizing micro-hologram was 2 mm \(\times\) 1.5 mm.