**Synthetic Image Holograms**

Jakub Svoboda, Marek Škere ˇn and Pavel Fiala *Czech Technical University in Prague Czech Republic*

### **1. Introduction**

This chapter is dedicated to synthetic image holograms - the elements which can create a reconstruction of a 3D object for observation with the human eye. Holography as a technique of image recording and reconstruction has been extensively developed from sixties of the twentieth century. During this time there have been various attempts to synthesize holograms artificially without the presence of the real object in the classical recording setup. Different approaches have been used, several trying to synthesize the three-dimensional object from two-dimensional views using the classical recording setup, the others trying to calculate the microstructure of the hologram completely in a computer. Today, we can divide synthetic holography into two major streams, the first containing the methods for creating the image for observation by human eye and the second consisting of approaches for designing the synthetic diffractive structures for general wavefront generation. The former techniques can exploit various imperfections of human vision and omit several parameters of the optical wave. The latter techniques are usually based on the direct calculation of the microstructure and they try to create the reconstruction in its full complexity. Only the first group of synthetic image holograms will be analyzed in this chapter.

The synthetic approach to hologram creation can have several advantages, but also noticeable disadvantages. The most important advantages are connected with flexibility in modifying the recorded object. First, the object need not to exist in reality in a form of a physical model. For most synthetic approaches, it is fully sufficient to have a 3D computer model for preparing the recording data. Also for real physically existing objects it could be tricky to perform the recording process in a classical setup. For example, various outdoor scenes such as buildings and others could not be included in the laboratory setups. Generally, the scaling possibility is very limited in classical holography, so the recorded object (or its model) must be of final size. On the other hand, it is easy to scale the computer model of an object. The next problem is in various corrections of color properties, surface textures, and general fine tuning of the recorded object. While such operations are very simple in the case of computer models, they could bring insoluble problems for real physical models. The stability of the object is also very important. It is crucial to highly stabilize the object for recording in classical holographic recording setup (when exposing with a continuous-wave laser), whereas in a computer stability is not a problem. This can apply also for holograms of living objects or dynamic scenes, where it is easy to take snaphots using photographic techniques, but holographic exposure is almost impossible. Finally, according to the recording technology chosen, other parameters of the synthetic hologram can be highly superior to those of classical holograms (e.g. fidelity of color mixing, contrast of the image, etc.).

The most important cues in 3D imaging are the following physiological cues - stereopsis and movement parallax. *Stereopsis* is based on the difference between the image viewed by the left eye and the image viewed by the right eye. Each eye has slightly different angle of viewing due to the different spatial position of the eyes (common distance between the eyes is about 65 mm). The effect is called binocular disparity or binocular parallax. The image information from the two different images is then processed in the brain's visual center and the relative spatial position of the viewed objects is conceived. Stereopsis is probably the most important and strongest cue, from close up to medium viewing distances. The principle of stereopsis is

Synthetic Image Holograms 211

Fig. 1. Stereopsis: When viewing a 3D object, the images that are incident on each eye's retina slightly differ in angle. Brain interprets this change in angle as the depth of the object.

*Movement parallax* is also based on processing an image from retina, but it is a monocular cue. The difference in the position of the image is caused by the observer moving his head. This technique can be found in animals whose eyes are placed on the opposite sides of their head - e.g. some types of birds that swing their heads to achieve motion parallax. There are also surveys showing that people with monocular vision use the movement parallax to obtain the

There is a variety of methods of providing 3D perception for the observer. Until the "invention" of binocular vision, 3D space was represented mainly by the monocular means of psychological cues as the relative size, occlusion, and shading. By using perspective, the images started to be more real. The first 3D perception as we know it from real life came with Sir Charles Wheatstone's invention in 1838 [Wheatstone, 1838; Wheatstone, 1852]. He described the binocular vision and proposed the device called the *stereoscope*. This device used stereopsis for imaging depth - it created two separated viewing zones with two different images, one for each eye. Since the stereoscope was invented before photography, the first 3D

From that time on, many stereoscopic methods have been developed. Modern 3D display methods can be classified according to the number of viewing zones and the way they are separated. The methods with two viewing zones only are denoted *binocular methods* or *methods of selective observation*. The observer has to wear some kind of glasses to split the particular views into the left and right eye. On the other hand, there are methods of spatial display, which create more viewing zones. In such a case, these zones need to be spatially displaced

Besides movement parallax, this is a crucial cue, that contributes to 3D vision.

depth perception [Faubert, 2002; Ferris, 1972].

**1.2 Stereoscopic methods and holography**

images were paintings.

illustrated in Fig. 1.

Unfortunately, the synthetic approach brings also a whole range of specific problems and disadvantages. Generally, the recording setup is much more complicated in synthetic holography than in classical holography (although it can be much easier to operate some synthetic writers than to tune the classical laboratory recording setup). Furthermore, most synthetic techniques are limited in final size of the hologram (e.g. for directly written synthetic elements the typical area is in cm2, while for classical holograms it is usually in dm2). The recording process itself takes usually much longer in synthetic holography (although the preparation for exposure could be longer when using the classical recording). It is also important to realize, that in most synthetic approaches, the reconstructed optical wavefront does not exactly correspond to the one from a similar classical hologram. Various parameters are often omitted due to the limited performance of human eye, which can not evaluate the optical signal in full complexity.

The most important features of the synthetic hologram can be summarized as follows: the object data could be obtained without necessity to work with a physical model, various modifications of the object could be possible; the object could be potentially dynamic, spacious, etc.; the hologram itself could be as large as possible, it could enable general color mixing, the reconstruction could be possible in white light.

In the following sections the key steps of the design and recording processes are described together with the common technology used for recording the synthetic holograms. Section 1 contains analysis of the human vision with focus on 3D perception by human eyes. Section 2 describes the approaches to the hologram synthesis using two most common techniques - synthesis at the hologram plane and synthesis at the eye-pupil plane. Various details concerning the color mixing, special 3D properties, and kinetic behavior of the holograms are also discussed. Finally, in Section 3 the most common devices and recording materials are briefly mentioned.

### **1.1 Human vision**

The visual sense of a 3D scene is caused by the brain interpreting the image information from the eye nerves from both eyes. A significant advantage is that the eyes are quadratic detectors (they do not feel the phase); therefore, 3D vision can be "faked" by only transmitting proper intensity images into the eyes as it will be shown.

Due to the relative positions of our eyes, the image from the left eye slightly differs from the image from the right eye. This is the crucial aspect causing the 3D sense and that is why it is used in hologram synthesis.

The image of the scene in front of our eyes is displayed by the eye's optical system on the photosensitive layer of the eye - the retina. This image information is processed by the photoreceptors (rods and cones) and transmitted to the brain through the optic nerve for further processing. What happens in the eye is not the only relevant factor for vision. There are also factors supporting depth perception, which relate to the shape of the object. Vision cues can be divided into physiological and psychological ones [Najdek, 2008].

The physiological cues relate to the eye physiology and the principles of vision - the necessary information from the viewed scene is either obtained by tracking the position of the image on the eye's retina (*stereopsis*, *movement parallax*) or by sensing the tension in the eye's muscles (*accommodation*, *convergence*). The physiological cues express the depth of the viewed scene by the means of 2D images. These cues are for example *perspective*, *relative size*, *occlusion*, *shading*, *depth of field*, etc.

2 Will-be-set-by-IN-TECH

Unfortunately, the synthetic approach brings also a whole range of specific problems and disadvantages. Generally, the recording setup is much more complicated in synthetic holography than in classical holography (although it can be much easier to operate some synthetic writers than to tune the classical laboratory recording setup). Furthermore, most synthetic techniques are limited in final size of the hologram (e.g. for directly written synthetic elements the typical area is in cm2, while for classical holograms it is usually in dm2). The recording process itself takes usually much longer in synthetic holography (although the preparation for exposure could be longer when using the classical recording). It is also important to realize, that in most synthetic approaches, the reconstructed optical wavefront does not exactly correspond to the one from a similar classical hologram. Various parameters are often omitted due to the limited performance of human eye, which can not evaluate the

The most important features of the synthetic hologram can be summarized as follows: the object data could be obtained without necessity to work with a physical model, various modifications of the object could be possible; the object could be potentially dynamic, spacious, etc.; the hologram itself could be as large as possible, it could enable general color

In the following sections the key steps of the design and recording processes are described together with the common technology used for recording the synthetic holograms. Section 1 contains analysis of the human vision with focus on 3D perception by human eyes. Section 2 describes the approaches to the hologram synthesis using two most common techniques - synthesis at the hologram plane and synthesis at the eye-pupil plane. Various details concerning the color mixing, special 3D properties, and kinetic behavior of the holograms are also discussed. Finally, in Section 3 the most common devices and recording materials are

The visual sense of a 3D scene is caused by the brain interpreting the image information from the eye nerves from both eyes. A significant advantage is that the eyes are quadratic detectors (they do not feel the phase); therefore, 3D vision can be "faked" by only transmitting proper

Due to the relative positions of our eyes, the image from the left eye slightly differs from the image from the right eye. This is the crucial aspect causing the 3D sense and that is why it is

The image of the scene in front of our eyes is displayed by the eye's optical system on the photosensitive layer of the eye - the retina. This image information is processed by the photoreceptors (rods and cones) and transmitted to the brain through the optic nerve for further processing. What happens in the eye is not the only relevant factor for vision. There are also factors supporting depth perception, which relate to the shape of the object. Vision

The physiological cues relate to the eye physiology and the principles of vision - the necessary information from the viewed scene is either obtained by tracking the position of the image on the eye's retina (*stereopsis*, *movement parallax*) or by sensing the tension in the eye's muscles (*accommodation*, *convergence*). The physiological cues express the depth of the viewed scene by the means of 2D images. These cues are for example *perspective*, *relative size*, *occlusion*, *shading*,

cues can be divided into physiological and psychological ones [Najdek, 2008].

optical signal in full complexity.

briefly mentioned.

**1.1 Human vision**

*depth of field*, etc.

used in hologram synthesis.

mixing, the reconstruction could be possible in white light.

intensity images into the eyes as it will be shown.

The most important cues in 3D imaging are the following physiological cues - stereopsis and movement parallax. *Stereopsis* is based on the difference between the image viewed by the left eye and the image viewed by the right eye. Each eye has slightly different angle of viewing due to the different spatial position of the eyes (common distance between the eyes is about 65 mm). The effect is called binocular disparity or binocular parallax. The image information from the two different images is then processed in the brain's visual center and the relative spatial position of the viewed objects is conceived. Stereopsis is probably the most important and strongest cue, from close up to medium viewing distances. The principle of stereopsis is illustrated in Fig. 1.

Fig. 1. Stereopsis: When viewing a 3D object, the images that are incident on each eye's retina slightly differ in angle. Brain interprets this change in angle as the depth of the object. Besides movement parallax, this is a crucial cue, that contributes to 3D vision.

*Movement parallax* is also based on processing an image from retina, but it is a monocular cue. The difference in the position of the image is caused by the observer moving his head. This technique can be found in animals whose eyes are placed on the opposite sides of their head - e.g. some types of birds that swing their heads to achieve motion parallax. There are also surveys showing that people with monocular vision use the movement parallax to obtain the depth perception [Faubert, 2002; Ferris, 1972].
