**3.3 Commonly used recording materials, light sources, and mass replication techniques**

A very important issue in the image holography is the replication process. For many applications it is desirable to produce the final hologram in large quantities (e.g. holograms

<sup>2</sup> Since the master-hologram plate is moved during the recording process, the relative position of the recording device and the already recorded images is changed. But in the final hologram, all the recorded views have to "emerge" from one particular place to form the 3D impression. That is the reason why all the possible recording setups either have to include tilting the objective or have to implement some kind of movement compensation. There are several possibilities for the mechanical movement that can be considered. However, the simplest and the most robust way from the mechanical point of view is one where the "writing" objective is static. In such a case, the signal spot is always in the same position and the reference beam also does not have to be moved. The only moving elements would be the holographic plate and the SLM (as the compensating element). The step of the holographic plate is equal to the size of the holopixel. The step of the SLM is in the opposite direction and it is smaller simply in the ratio of the objective magnification.

22 Will-be-set-by-IN-TECH

recorded, and so on. The process is more complicated, since the movement of the holographic plate needs to be compensated by moving also the SLM.2 The scheme of the particular setup can be seen in Fig. 16. All the views obtained by sampling the object are recorded in this manner and the plate is developed and bleached. Since the reference wave is collimated, the replay beam can be a wide collimated wave that reconstructs all the recorded holopixels at once. The reconstructed images from all the holopixels overlap at the place where the object was situated during capturing. The final hologram plate is placed into this place and the final hologram is recorded (similar to Fig. 7b). The device is driven by a PC - the images are sequentially uploaded to the SLM and the two *x* − *y* stages are operated as shown in the

Fig. 16. Scheme of the eye pupil synthesis device: **1** laser, **2** shutter, **3** mirror, **4** beam splitter, **5** microscope objective with spatial filter, **6** beam expander, **7** collimator, **8** holographic diffuser,

**9** square aperture, **10** signal shade, **11** SLM, **12** special objective, **13** holographic plate.

simply in the ratio of the objective magnification.

**3.3 Commonly used recording materials, light sources, and mass replication techniques** A very important issue in the image holography is the replication process. For many applications it is desirable to produce the final hologram in large quantities (e.g. holograms

<sup>2</sup> Since the master-hologram plate is moved during the recording process, the relative position of the recording device and the already recorded images is changed. But in the final hologram, all the recorded views have to "emerge" from one particular place to form the 3D impression. That is the reason why all the possible recording setups either have to include tilting the objective or have to implement some kind of movement compensation. There are several possibilities for the mechanical movement that can be considered. However, the simplest and the most robust way from the mechanical point of view is one where the "writing" objective is static. In such a case, the signal spot is always in the same position and the reference beam also does not have to be moved. The only moving elements would be the holographic plate and the SLM (as the compensating element). The step of the holographic plate is equal to the size of the holopixel. The step of the SLM is in the opposite direction and it is smaller

figure.

for document security applications are often made in series of a million). Thus it is extremely important to look for a proper technology which could enable cost effective production of large series. In classical image holography the holograms are usually copied optically in an optical setup, which is still a relatively expensive process. Volume gratings could not be copied in any other way. However, if the grating is of the relief type, it can be also copied using some of the imprint techniques (like mechanical embossing, etc.). To make this possible, the hologram must be exposed in proper relief recording material. The light source used usually depends on the requirements of the particular recording medium.

For exposure of relief gratings photoresists are often used (if exposing with laser beam). For e-beam exposure the electron beam sensitive resists are used. Unfortunately, the gratings in photoresists are usually thin, so this material can be used only when the volume properties are not needed (theoretically, the relief gratings in resist can be "thick" and possess volume effects, but they can not be copied using an embossing technology). Photoresist can be used for all exposure steps or the hologram can be finally transfered to the resist material from a different recording medium. The spectral sensitivity is maximum in UV part of the spectrum so photoresists are usually exposed using short wavelength visible sources (such as semiconductor lasers with wavelength ∼ 400nm or gas lasers with wavelengths within the range 400 − 460nm). Because of their overall low sensitivity the photoresists are not perfectly suitable for primary exposure of large areas.

After the hologram is recorded in the relief material such as photoresist it is usually metalized and copied to a hard metal (usually Nickel) relief copy using the electro-forming process. Such a matrix is then used for mechanical embossing.

For recording of the volume gratings whole range of holographic materials can be used according to particular needs. Silver halide gelatin, dichromated gelatin, and some photopolymers can satisfy the requirements. In contrast to the resist materials, they can be easily sensitized for laser sources within the whole visible range. Recently, diode pumped solid state lasers are widely used in holography because of their high output power, very good coherence properties, high efficiency, and relatively low operational costs. Very common are 532nm sources based on the second harmonic from Neodymium doped active material. For further details concerning the recording materials and other components necessary for the recording process see [Bjelkhagen, 1993; Collier et al., 1971].

The synthetic image holograms can be constructed in various ways according to the particular application and other demands on the hologram properties. Within this chapter only the basic ideas have been presented. Within the Optical Physics Group at the Faculty of Nuclear Sciences and Physical Engineering of the Czech Technical University in Prague, various approaches to image synthesis are researched. Besides research in the field of holographic techniques, also recording materials are developed and automated recording devices are constructed. All samples presented in this text have been fabricated using the technology available at the Optical Physics Group.
