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25 holography lessons

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17. Object setting

Setting the object for recording the hologram in particular and work with the object on the pulse setup in general are considerably easier than by the work with a continuous laser. There are no restrictions on a material of which the object is made. It can a domestic animal or flower, splashes of water and so on. Hence by setting the object basic attention is paid to work at composition and to optimum light solution. As a rule this work is individual and is determined by nature of the object being recorded. Let us consider several examples of setting different objects.

Classical objects

Classical objects are the objects which donít require specific lighting and for illumination of which a standard scheme described in lesson 16 is used. These are portraits of people, domestic animals, classical still lives with flowers and so on. In the fig. above a stuffed falcon ready for shooting is shown. The falcon is fixed on the object table of the shooting cabin and a dark fabric covers the table. On the back wall of the cabin a photo of sky is hung. The falcon is illuminated by wide beams from two sides. Scattered light illuminates the photo of light hung in the background. This fact creates additional illusion of space on the hologram. By setting the object distance from it to the photographic plate is checked up (about 30-35 cm) and also absence of screening the reference beam by the upper part of the object is verified. The reference beam passes above the object from above downward and incidents upon the photographic plate under the angle of 60 degrees and if the object is situated very close to the photographic plate its upper part can screen the reference beam. In the same way every of two illuminating beams which should exactly incident upon the object is checked.

Luminous inscriptions

Luminous inscriptions or symbols floating behind or before the main holographic image can be important elements in advertising or representative holograms. In combination with the main three-dimensional image they create an interesting and attractive effect. In the fig. such an element is a luminous arrow. For recording the inscription on a hologram the illumination scheme changes significantly, see fig. below. The beam divider and one of the glass diffusers are removed and the widened but not diffused beam is sent on the object table on which the additional optical scheme is mounted. The illuminating beam is directed on the first glass diffuser 3 by two big mirrors 1. This diffuser serves for increase of homogeneity of the illuminating beam. At the distance of about 5 cm from it the second diffuser 3 with the photomask of the inscription 4 is located. The photomask is fixed to the side of the diffuser, which looks towards the photoplate 5. For creation of the holographic luminous inscription a high-quality negative photomask of this inscription is needed (transparent letters on the dark background). At first the inscription is printed on a high-quality printer and then it's copied on a contrast photographic plate. The VRP photoplates are very well fit for these purposes. The photomask is fastened to the dim glass and at that the diffusive surface of the glass should be pressed to the emulsion side of the photomask in order to get a maximum sharpness of the inscription. The photomask is fixed on the shooting table at that spatial position where it should be situated in the holographic image. After adjustment of the scheme the plane around the photomask is closed by screens in order to isolate outside beams from the photoplate for recording.
The transparent master hologram of the object with the luminous inscription is recorded by two steps. At first the master hologram of the three-dimensional object is recorded and after shooting its position in the shooting cabin is measured. The photomask is set on the object table taking into account these measurements. Then the second master hologram with the luminous inscription is recorded. After chemical treatment both holograms are connected together and position of the image of the three-dimensional object and position of the inscription are finally superposed. Then copying of the doubled master hologram is executed. Sometimes itís necessary to make several test records of the master hologram with the inscription in order to position the inscription in the resulting image more precisely.

Punctual highlighting of the object using an optical fiber bundle

Sometimes local point highlighting of the object details, e.g. a filament of the electric bulb, is needed, see fig. (Itís known that self-luminous objects Ė candles, electric bulbs, wicks and so on, canít be recorded on the hologram, since only the laser light reflected from the object is registered on the hologram). For these purposes itís most convenient to use an optical fiber bundle, which can be fixed in any place on the object table or on the wall of the shooting cabin and to direct the beam into the required place. Scheme of the combined illumination of the object with the help of a standard wide beam and an optical fiber bundle is shown in the fig. below. Immediately behind the light splitter 1 a standard beam is widened by the negative lens 2 and is directed by the mirror 3 to the transparent glass diffuser 4 and further to the object 6. The second beam from the light divider is input into the optical fiber bundle 5 which is fixed in the shooting cabin and which highlights the required part of the object 6. Sometimes in order to secure more precise highlighting the lens is fixed on the end of the cord that allows correcting illumination field more precisely. Quality of illumination of the object in this case as in the previous case is determined only by recording test holograms of small dimensions.

References
1. Sergey P. Vorobyov, Some techniques of art hologram recording by pulse laser, Materials of the First International Forum "Holography EXPO-2004", Moscow, Oct. 2004 p. 50.
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