**1.2 Stereoscopic methods and holography**

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 images were paintings.

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

image information into proper directions. The diffractive methods are discussed in detail in

Synthetic Image Holograms 213

The most perfect autostereoscopic method is "true" holography. The hologram is capable of reproducing a wavefront of an object in its full complexity and therefore it works in fact as an autostereoscopic display with infinite number of viewing zones. An illustration of

(a) (b) (c) (d)

Fig. 2. 3D displaying methods: **(a)** In binocular methods, only two channels are employed

photography is an example of non-diffractive autostereoscopic methods, in which multiple viewing zones are spatially separated. **(c)** Diffractive autostereoscopic methods create the spatially separated viewing zones as in case (b). **(d)** Hologram is capable of reconstructing the image wavefront in its full complexity. It acts as if there is infinite number of viewing

In this section, the basics of synthetic image holography are discussed. According to the above described principles of autostereoscopy, it is desirable to exploit the directionality of the diffraction of light on the hologram (or more generally on the diffractive structure) for creating the spatially separated image channels in space. The problem consists of two main tasks, first the acquisition of image data for recording of particular channels and the second dealing with the recording process itself. Thus in Section 2.1 the general ideas of decomposition of the three-dimensional signal to the set of two-dimensional views are described. In Section 2.2 two particular approaches to the multiplexing of the 2D information within a single holographic element are presented. In the last part (Section 2.3), more detailed aspects of the recording and reconstruction processes are analyzed such as the true-color mixing based on holographic elements, various spatial properties of the reconstructed image, and possible synthesis of the

The analysis of human vision (Section 1.1) showed that three-dimensional image perception can be artificially generated by creating the spatially separated discrete image channels in space. It has been also mentioned, that due to the inability of the human eye to register the phase properties of the incident optical wave (except for the directionality of the wave), the real image signal can be substituted by two-dimensional intensity views. The question is, how the discretization of the spatial channels should be done to create a satisfactory

which overlap each other in space. In this case, an anaglyph is shown. **(b)** Integral

Section 2. An illustration of a synthetic diffractive structure is shown in Fig. 2c.

reconstruction from a hologram is shown in Fig. 2d.

zones.

**2. Synthetic holography**

holograms with kinetic behavior.

**2.1 3D image as a set of 2D views**

by the displaying device itself. These methods are therefore denoted *the methods of selective display* or *autostereoscopic methods*.

As shown in Fig. 2a, in the binocular methods, there are two 2D images (binocular pair) overlapping each other, which are transmitted in the direction of the observer [Takanori, 1976]. To view the correct image by the corresponding eye, the observer must wear glasses with some sort of filters that will block the image for the other eye and let only the correct one to pass. The first device ever, that was used for this kind of display was the already mentioned *stereoscope*. Typical representatives of these display methods nowadays are e.g. anaglyphs or the 3D cinema. In *anaglyphs* [Zone, 2003], the two observation channels are superimposed, but each one of them is encoded by a different color filter. The observer wears glasses with colored filters that separate properly the two images into the eyes. Using a pair of pure colors from R-G-B for filtering enables only monochromatic observation. Therefore, to achieve a color result, mixed color filters are used for the glasses. The most common is red-cyan (50 % blue and 50 % green). Other color filters have been invented to improve color rendering. An example is Infitec - narrow band interference filters.

The modern 3D *cinemas* use either polarization filters or shutter systems [Turner, 1986]. The polarization principle is the same as in anaglyphs - the two viewing channels are superimposed, each encoded into a different polarization state. The glasses are equipped with proper polarization analyzers to split the two channels. The shutter systems split the binocular pair in the time domain. The movie frames are projected with about twice the frequency of a 2D movie. The shutter glasses are synchronized with the projector and they block periodically the left and the right eye, so that the observer sees only one frame with the correct eye at a time.

The main disadvantages of all the binocular methods are the necessity of wearing the filtering glasses, the lack of motion parallax, and the distortion of the image. The image can be viewed undistorted only from the same position from which it was taken. That means, for example, that in the cinema, most spectators see the scene in a somewhat distorted way. Nevertheless, these methods are satisfying for 3D imaging and are widely used in the entertainment industry.

Unlike separating the channels at the observer by using the selective observation methods, autostereoscopic methods create the separated viewing zones using the displaying device itself [Halle, 1997]. These methods are capable of creating more than two spatially separated viewing zones and motion parallax can be obtained without the necessity of wearing any glasses. Another advantage over binocular imaging is the fact, that when multiple viewing zones are involved, there is no constraint on the observer's position. Each zone contains the image information from the particular direction and wherever the observer is located relative to the displaying device, he sees the 3D image in the correct perspective.

A typical representative of this imaging method is integral photography. The object is captured through an array of tiny convex lenses. Each of the lenses creates an image of the object with a slightly different perspective. These images are recorded on a photographic plate. After being developed, the plate is illuminated from behind and the light travels again through the array of lenses. Since the setup is exactly the same as it was during the recording, the light rays from various lenses meet again in the original position, where the surface of the object was located (Fig. 2b). For viewing such an image, the observer does not need to wear any glasses.

Another improvement in autostereoscopic methods comes with using diffractive structures. The tiny lenses are replaced with a diffractive structure, that controls the distribution of the 4 Will-be-set-by-IN-TECH

by the displaying device itself. These methods are therefore denoted *the methods of selective*

As shown in Fig. 2a, in the binocular methods, there are two 2D images (binocular pair) overlapping each other, which are transmitted in the direction of the observer [Takanori, 1976]. To view the correct image by the corresponding eye, the observer must wear glasses with some sort of filters that will block the image for the other eye and let only the correct one to pass. The first device ever, that was used for this kind of display was the already mentioned *stereoscope*. Typical representatives of these display methods nowadays are e.g. anaglyphs or the 3D cinema. In *anaglyphs* [Zone, 2003], the two observation channels are superimposed, but each one of them is encoded by a different color filter. The observer wears glasses with colored filters that separate properly the two images into the eyes. Using a pair of pure colors from R-G-B for filtering enables only monochromatic observation. Therefore, to achieve a color result, mixed color filters are used for the glasses. The most common is red-cyan (50 % blue and 50 % green). Other color filters have been invented to improve color rendering. An

The modern 3D *cinemas* use either polarization filters or shutter systems [Turner, 1986]. The polarization principle is the same as in anaglyphs - the two viewing channels are superimposed, each encoded into a different polarization state. The glasses are equipped with proper polarization analyzers to split the two channels. The shutter systems split the binocular pair in the time domain. The movie frames are projected with about twice the frequency of a 2D movie. The shutter glasses are synchronized with the projector and they block periodically the left and the right eye, so that the observer sees only one frame with the correct eye at a

The main disadvantages of all the binocular methods are the necessity of wearing the filtering glasses, the lack of motion parallax, and the distortion of the image. The image can be viewed undistorted only from the same position from which it was taken. That means, for example, that in the cinema, most spectators see the scene in a somewhat distorted way. Nevertheless, these methods are satisfying for 3D imaging and are widely used in the

Unlike separating the channels at the observer by using the selective observation methods, autostereoscopic methods create the separated viewing zones using the displaying device itself [Halle, 1997]. These methods are capable of creating more than two spatially separated viewing zones and motion parallax can be obtained without the necessity of wearing any glasses. Another advantage over binocular imaging is the fact, that when multiple viewing zones are involved, there is no constraint on the observer's position. Each zone contains the image information from the particular direction and wherever the observer is located relative

A typical representative of this imaging method is integral photography. The object is captured through an array of tiny convex lenses. Each of the lenses creates an image of the object with a slightly different perspective. These images are recorded on a photographic plate. After being developed, the plate is illuminated from behind and the light travels again through the array of lenses. Since the setup is exactly the same as it was during the recording, the light rays from various lenses meet again in the original position, where the surface of the object was located (Fig. 2b). For viewing such an image, the observer does not need to wear

Another improvement in autostereoscopic methods comes with using diffractive structures. The tiny lenses are replaced with a diffractive structure, that controls the distribution of the

to the displaying device, he sees the 3D image in the correct perspective.

*display* or *autostereoscopic methods*.

time.

entertainment industry.

any glasses.

example is Infitec - narrow band interference filters.

image information into proper directions. The diffractive methods are discussed in detail in Section 2. An illustration of a synthetic diffractive structure is shown in Fig. 2c.

The most perfect autostereoscopic method is "true" holography. The hologram is capable of reproducing a wavefront of an object in its full complexity and therefore it works in fact as an autostereoscopic display with infinite number of viewing zones. An illustration of reconstruction from a hologram is shown in Fig. 2d.

Fig. 2. 3D displaying methods: **(a)** In binocular methods, only two channels are employed which overlap each other in space. In this case, an anaglyph is shown. **(b)** Integral photography is an example of non-diffractive autostereoscopic methods, in which multiple viewing zones are spatially separated. **(c)** Diffractive autostereoscopic methods create the spatially separated viewing zones as in case (b). **(d)** Hologram is capable of reconstructing the image wavefront in its full complexity. It acts as if there is infinite number of viewing zones.
