# Computer-Generated Holograms Application in Security Printing
## 1 Introduction  
Computer-generated holograms [1] are widely used in different fields of science and technology. They are most popular in holographic memory systems [2], augmented reality projectors [3], wavefront sensors [4], and images spatial filtering and pattern recognition systems [5], etc. One of the application areas of computer-generated holograms (CGH) is security printing. Security printing usually means the print area characterized by special print methods used individually or in combination with optically variable features such as holograms aimed at protection against duplication or falsification of goods or documents. Identification documents (passports, different documents certifying the holder's identity such as a driver's license), banknotes, bank cards, excise stamps, and other valuable documents or controlled forms are often secured with such methods and features. Recently, along with physical security methods, digital methods such as QR codes, Data-Matrix codes, and 1D and 2D codes, etc. have become widespread. In such cases, part of the appropriate data is decoded, and most often a connection to a database takes place on the basis of the decoded identifier to obtain the required information on the secured item.  
CGH has been successfully used for a long time in applied optical technology, including but not limited to security holograms, also known as diffractive optically variable image devices (DOVIDs), e.g., to generate a so-called hidden image. In such cases, a point or coherent source is needed to restore the image from the hologram surface. The image can be restored and become visible either to the naked eye or with the use of simple portable devices. On the other hand, CGHs have been efficiently implemented in optical memory systems, where image is 2D amplitude and/or phase-modulated data page specially designed for automatic read-out [2]. It was shown that with the use of a simple SLM-based projection system, a holographic element with the square size of about 1 mm\({}^{2}\) with data capacity up to 1 MB can be obtained [6].  
The article writers offer several unconventional ways of using CGH-based holographic memory in the area of security printing through modern analogue-digital methods of their generation, and information read-out (restoration) with portable data-processing computer modules such as smartphones or other devices where automatic control or data acquisition is required. This article considered the following use of CGH: as a printed hologram applied to the surface of paper, cardboard, metallized PET film, and other substrates by thermal-transfer printing or laser ablation; as a smart identifier on the surface of a rainbow security hologram restored as an optical image of DataMatrix code in the point source moved to an infinite distance; and as a personalized micro-CGH (non-recurrent, unique for each hologram) located on a photopolymer security hologram. Different algorithms of calculation, coding, and generation of CGH were selected for each area of application, and a smartphone with preinstalled specialized SW was taken as a direct or coded data read-out device.