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25 holography lessons

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25. Reflection copy recording

So everything is ready for recording a copy of the hologram. We lower the copying arrangement from metallic supports on the inflated balls, turn off the light and finally check position of the recording beams. Then we remove the screen out of the frame for the photoplate, turn on the green lamp and block the laser beam. We take a photoplate out of the box, look through opening at the green lamp and check if there are particles of dust or pieces of glass on the plate surface. If there are some we brush them off with the help of a soft brush without damaging the emulsion layer. Then we carefully place the photoplate into the frame with the emulsion side turned to the reference beam. A big time of recording the hologram (30-120 seconds) and large sizes of photographic plates imposes certain requirements relating to fastening of photoplates and to subsequent endurance of the whole scheme (stabilization). It’s possible to say that during the whole period of existence of the pictorial holography using CW lasers there continued a permanent struggle against displacements and vibrations of the object and photoplate. But why only displacements and vibrations of the object and photoplate? – You may ask. Because other elements of the holographic copying scheme (and also elements of the Denisyuk scheme): the mirrors, lenses, spatial filters and so on are sufficiently rigid metallic structures which are far less subjects to influence of external disturbances. And what is a photoplate? Figuratively speaking it is a membrane, which catches the smallest external mechanical vibrations and air flows. And if the frequency of these vibrations coincides with its proper frequency a resonance will occur by which the amplitude of vibrations of the photoplate will increase considerably. But of course if sizes of the photographic plate are small, for example 6x6 cm, requirements to its vibroprotection reduce considerably. But if we record a hologram on the photographic plate with the sizes 28x40 cm it’s necessary to take all precautionary measures in order not to throw out the expensive photoplate and not to loose several hours of working time. There exist five principal moments, which facilitate solving the problem of stabilization of the photographic plate position during exposure.

1. Protection of the holographic scheme from vibrations of the building foundation through mounting the table on the antivibration stands (for example on the inflated footballs or car tyres) and also from air flows in the room with the help of the protective cover (depending on situation these cover can be of foam plastic, plywood or simply of thick fabric).

2. Fastening the master-hologram and the photographic plate in the vertical position. In this case influence of vibrations coming from the building foundation will be minimum.

3. Pressing the photographic plate to the rigid steel frame, see the photo. If the plate is pressed to the steel frame probability of its vibrations decreases sharply. But if the frame has a skew or uneven surface this can bring to nothing all your efforts. Internal stresses will appear in the photoplate, which won’t allow for the plate to reach a “calm down” during a long time. Therefore the frame surface to which the photoplate is pressed should be ideally plane and thoroughly polished. Then after pressing the internal stresses won’t appear in the photoplate and the prolonged stabilization of the scheme won’t become necessary.

4. Pressing the photoplate to the frame applying a minimum force. It might seem that it’s sufficient to press the photoplate to the steel frame using a good steel clamp along the whole diameter and stability of the photoplate will be secured. But on the contrary such a stiff pressing can again cause appearance of internal stresses in the photoplate and result in negative consequences. Therefore the plate should be pressed to the frame with the help of weak spring clamps in several places along its perimeter, see the photo. Such an option of the fastening on the one hand allows to dampen external influences acting on the photoplate due to its pressing to the massive metallic frame and on the other hand allows to avoid appearance of internal stresses.

5. Stabilization of the whole holographic scheme immediately before recording a hologram. Don’t forget that you have held the plate in your hands, have touched the spring clamps and that you simply breathed upon all these fixtures, mirrors and frames. So you have introduced a certain heat disbalance into the holographic scheme. Because of these invisible and weak thermal flows mechanical stresses will again occur in the scheme which will disappear gradually only during stabilization of the scheme in the process of equalization of temperature. The time of stabilization depends on many factors and it has its proper value for every holographic scheme, for every object and every size of the hologram. It depends even on location of the room and building in which the holographic unit is situated. If for example the building is situated near the underground line or big highway you can expect nothing good from such proximity. The time of stabilization should be chosen experimentally. For the copying apparatus described in our lessons the time of stabilization for the hologram with the sizes 28x40 cm doesn’t exceed 7 min. It’s a rather small time. After stabilization of the scheme the photoplate is exposed during the estimated time and then its photochemical treatment is carried out which is fully analogous to the treatment described in the Lesson 10. Photos below show the holograms recorded with the help of the pulse laser and copied on described unit.