# Computer-Generated Holograms Application in Security Printing
## 4 Smart Verification  
Another application of CGH is a hidden security feature combined with a classic rainbow hologram. In this case, a relief-phase hologram is supplemented with low-frequency relief, which a in zero-diffraction order forms a coded feature of a certain configuration observed and detected with a smartphone under specific conditions and while using a point source of visible light moved conventionally to an infinite distance. Such relief may be conventionally called pseudo-kinoform, though it deviates from the classic relief of this type due to a specific method of generation, while calculation is performed by discretization of the object light wave phase in the recording plane. As for a coded feature consisting of a set of points, kinoform was calculated according to Equation (4) or Equation (5). The kinoform recording was carried out using a frame-matrix holographer or electron beam lithography or optical lithography, as described in detail in the article [25], where a hidden image was realized by means of kinoform. Moreover, the image restored by laser radiation on the visualizer-screen did not contain an encoded data page, in contrast to the solution proposed by the authors. A perfectly generated kinoform (Figure 4) restores only one image, either virtual or real, depending on what is used as a kinoform--a negative or a positive. For this purpose, it is required that the delay between the light falling on the area with 0 radian phase and the light falling on the area with 2\(\pi\) radian phase amounted to one wavelength exactly. In case of a reflecting kinoform, it means that the height of relief must be equal to half the wavelength or 270 nm for the green (\(\lambda\) = 540 nm). The calculation of kinoform structures by the Gechberg-Sexton method was carried out in three cycles and 256 gradations of relief height were specified. When kinoform was placed on the diffraction grating, such deep relief of the kinoform worsened the quality of the main rainbow hologram. In order to avoid it, the depth of the kinoform relief must be several times less than the depth of the relief of the main hologram (Figure 5), while the kinoform itself will become similar to axial hologram where the real and virtual images are partially overlapping each other, and part of the light diffracts to the zero order, creating a bright spot in the center of the image. However, it is still possi  
Figure 4: Synthesized kinoform fringe pattern. Figure 3: B2C interaction model with the use of Holocode. out the DataMatrix image not overlapping with the main image and zero order. Thus, for the main hologram, the height of the relief was 300 nm with a spatial period of 1000 nm. For kinoforms, the height of the relief was about 100 nm with a spatial period of 5000 to 7000 nm. After making a hologram combined with kinoform relief by electron beam lithography or with the use of a frame-matrix holographer and having set the optimal exposure to restore both images (Figures 6 and 7), one can obtain an invisible hidden optical mark to be subsequently registered by a smartphone camera with LED flash. To recognize the coded mark formed by the CGH, a common smartphone with a camera and LED backlight was used. Smartphone computing capacity was operated by a specialized Android-based SW designed for recognition and registration of marks. To ensure the focusing on the mark, a smooth change in the focal length in the range of 10-25 cm with increments of 0.5 cm was used based on an open smartphone camera interface. For optical coded mark with CGH restored using an integrated LED of a smartphone, see Figure 8.  
If CGH is used as a coded mark, being an extra smartphone-readable feature in optically variable elements of security printing, it can be stated that protective functions are improved and another public feature that can be recognized by a user with a smartphone is added.