Holograms

This article presents methods of synthesis and objective control of special flat optical elements (computer-synthesized holograms) as elements of protection. Due to the limited number of pages, we cannot go into great detail to explain the technology of how computer-synthesized holograms are created. The goal of this publication is to discuss following subjects ... 

FAGCI has the equipment for mass production of computer-synthesized holograms The technology of computer-synthesized holograms is widely used in Russia for protection of precious, authentic, and important documents, such as passports, securities, licenses, certificates, identification cards, etc. 

Acheson process Achlorhydria Achondrites Achromatic hologram Achromobacter sp. Achromycin Achromycin [60-54-8]... 

Refining centers Refinishing Reflectance spectrum Reflection hologram Reflective tapes Reflectivities Reformate... 

Troubleshooting Trouton constant Troxerutin [7085-55-4] Troysan 174 Troysan 192 True color holograms TrueType Truex process Truncatella sp. Truscottite [12425-42-2] Truxal Trycol... 

Fig. 1. Reconstructions from a hologram, (a) Normal viewing angle (b) viewing from left side (c) viewing from right side. Courtesy of Mr. Tae Jin Kim.

Reconstmction of the object wave is achieved by illumination of the developed hologram with the reference wave as shown in Figure 3a. The diffracted wave amphtude from the hologram is given by equation 3, where the first term represents the attenuated reference wave after passage through the hologram. 

The second term represents a virtual image of the original object signal which can be viewed by an observer looking at the hologram along the original... 

Phase Materials. Phase holograms can be recorded in a large variety of materials, the most popular of which are dichromated gelatin, photopolymers, thermoplastic materials, and photorefractive crystals. Dichromated gelatin and some photopolymers require wet processing, and thermoplastic materials require heat processing. Photorefractive crystals are unique in that they are considered to be real-time materials and require no after-exposure processing. 

Values of Q < 1 imply a thin hologram and Q > 1 imply a thick hologram (23). 

A uniform plane wave at wavelength A, incident at an angle 0 as shown in Figure 5b, has a complex amphtude given by equation 7 at the hologram plane. 

The diffracted field from the hologram is then given by the amphtude function as shown in equation 8. 

Fig. 6. Volume hologram recording where (a) is the transmission, and (b) the reflection.

Because the losses tend to be excessive in volume absorption gratings, volume holograms are realized in most cases using index (phase) gratings. 

For both geometries the diffraction efficiency approaches unity in value for Atransmission hologram exhibiting a periodic behavior (24,25) efficiency as a function of the grating strength ( ), whereas the reflection efficiency exponentially approaches unity. 

Kinoforms are phase-only masks which are prepared by setting the amphtude values to unity and changing only the phase across the hologram (30). The kinoform has the advantage that proper preparation can lead to holograms which can diffract almost ah. of the light into the usefiil first order. 

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