# Computer-Generated Holograms Application in Security Printing
## 5 Holomemory
To experimentally verify the recording method, text string KRYPTEN AB12345678 was taken as useful data to be recorded on the photopolymer material. Taking into account the redundancy introduced by means of the Reed-Solomon correction code, the input data were converted to 78 bytes. In our case, a region of 32 \(\times\) 32 SLM elements corresponds to 1 byte in the data page after encoding. The generated data page had a dimension of 79,872 elements and corresponded to half the area of the SLM. To obtain a synthesized hologram that is uniform in illumination, a pseudo-random phase mask was added to the generated data page. Using a projection optical system, sequential recording was performed. The result of consecutive recording of combined holograms will be an optical security element to be used to secure e.g., personal identification documents and containing level 1 visual features and micro-CGH which may be classified as expert level 3 feature requiring special devices for identification and reading. For combined exposure and enlarged image of recorded micro-CGH, see Figure 12.  
The obtained micro-CGH may be read with an optoelectronic device, including a microscope type optical projection system with autofocus, LED light source and photodetector, and a protocol of data transfer to PC. The coded image of data page is restored on the read-out device by the use of fast inverse Fourier transform algorithm. An alternative read-out method is physical data recovery by illuminating the micro-hologram with coherent laser radiation using a Fourier transform objective and a matrix photodetector that registers the reconstructed encoded image [26]. In our case, preference was given to the optical projection reading method using incoherent radiation and digital restoration. This method was chosen to avoid the influence of a significant level of speckle noise from the hologram and the substrate to the reading result. Figure 13 shows the reconstructed encoded image, and its discrete fragments (pixels) are highlighted in green or red. Some data may be lost or damaged in the course of recording due to defocusing of the recording system or local defects of the carrier. Figure 11: Stages of digital data transformation into CGH. Figure 12: Examples of optical security element with micro-CGH. Correctly reconstructed pixels of the encoded image are highlighted in green color, and incorrectly reconstructed pixels in red. Therefore, inverse Reed-Solomon transform was used in the data decoding algorithms to recover data. In cases where at least 70% of the area was identified as correct, then the encoded image was corrected, and all data could be restored without loss. A screenshot of the program window with the result of reading the initial data, presented as the text string \"KRYPTEN AB12345678\", is shown in Figure 14.  
The developed method of obtaining and recording unique individualizing micro-CGH, used together with optical photopolymer security elements, may be applied to secure plastic documents in the form of embedded element, which guarantees impossibility to replace or falsify it without damaging the document integrity, as shown in the Figure 15.  
The recorded individualized micro-holograms fulfil the functions required for document registering and commodity turnover, while the individual data cannot be read without a special device. As a result, the application of micro-CGH to obtain a secured material medium for individual data with modern data storage and exchange tools allows us to assume CGH application as a document security feature integrated into their creation, distribution, and control. For model of interaction between B2G and G2C with the use of Holo  
Figure 14: Result of successful decoding. Figure 13: Reconstructed encoded image. Figure 9:0 Example of an individualized micro-CGH.