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22. Scheme of transmission hologram copying

So we came to the last step of making the pulse hologram – copying a three-dimensional image from the transmission master-hologram to the reflection hologram. Copying solves two important problems: First of all the reflection hologram restores the adequate three-dimensional image visible in normal white light. Secondly, position of the image can be changed with respect to photoplate. As a rule a part of the image is moved forward before the photoplate what leads to increase of the effect of image reality. You can “touch” the outstanding part of the object for example a nose of a dog.

For usual observation of the image restored by the transmission hologram it’s sufficient to illuminate it by a laser beam from the side from which the recording reference beam had been falling (see the second fig. in the Lesson 13). At that the transmission hologram restores a virtual, pseudoscopic image. So a full illusion of presence of the object behind the hologram is created. But if the transmission hologram is illuminated by a laser beam from the opposite side the real, so-called orthoscopic image located before the hologram is restored. This image is called a real image because it's really formed in the space before the hologram and it can be seen on the white screen set in the image plane. Properties of the orthoscopic image are quite unusual - it is as if turned inside out (look at the Denisyuk hologram from the side of emulsion layer and You’ll see the real, orthoscopic image). But namely such way of illumination of the transmission hologram allows copying the restored three-dimensional image with its transfer to the plane of the photoplate and even before the photoplate.

The scheme of copying of the reflection hologram is shown on the fig. below. In order to better understand the peculiarities of copying process the scheme is shown in such a way that positions of the transmission hologram and the restored image correspond to positions of the photoplate and the object by recording the transmission hologram (see the first fig. of Lesson 13). Position of the observer’s eyes is also shown in the figure and this position is the same both for observation of the virtual image restored by the transmission hologram (by the opposite direction of the reconstruction beam!) and for the final image restored by the reflection hologram got through exposure and chemical treatment of the photoplate. The laser beam 1 is divided in two beams by the semitransparent mirror 2. The reconstruction beam 3 is directed by the mirror 4 to the spatial filter 5. The enlarged and cleared beam falls on the transmission hologram 6 from the side of the glass substrate and restores the real, orthoscopic image 7, located in the same place as the object during recording the transmission hologram. In the region of the reconstruction of real image the photoplate 8 is set. Adjusting the distance between the transmission hologram and the photoplate it's possible to change position of the reconstruction image by placing it on the plane of the reflection hologram and even before it! The reference beam 9 is directed to the spatial filter 5 by the mirror 4 and falls on the photoplate under the Brewster’s angle. Since the object and reference beams are fall on the photoplate from different sides the reflection hologram is registered on it and this hologram can restore a three-dimensional image in the usual white light. Direction of incidence of the reference beam is due to be specified more exactly. As the image restored by the reflection hologram is orthoscopic the reference beam shall have the direction opposite to the direction of the beam which will illuminate the obtained reflection hologram. Direction of illumination from the reconstructing light source 10 is shown on the figure by dotted lines. For this reason the photoplate is turned to the reference beam by its emulsion side. The examined scheme of copying the reflection hologram demonstrates unusual possibilities of manipulating three-dimensional images in the coherent optical systems with diffraction elements. -->