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25 holography lessons

Glossary FAQ


6. Object

It is mentioned in Lesson 1, that we should select rigid objects made of metal, glass, stone, etc. to be recorded with the continuous-wave laser. If the object made of some soft material, e.g. paper, it will be shifted by warm and air flows during recording.
An interference pattern of the reference and object beams also shifts and it will not be recorded on the plate. The object on the hologram will look black.The most important point for the same reason is the fastening of a photoplate and object on the base within the scheme of hologram recording. Even when the object is rigid but it is not fixed properly on the base, it's possible that hologram will not be recorded.

Let's examine a simple but reliable and approved method - recording of a metal object (a small statue), which is placed on the metal plate. In such a scheme the photoplate fastening is considerably simplified. It can be placed on three metal supports, see the photo. It's well known the most stable position - when the object is placed on three supports. Therefore all the tripods have three supporting points.
Look at the photo. The object is placed against the white background. The white background is more preferable than the black one because the statue's shadow will be visible and this factor underlines three-dimensional effect of a holographic image. The background should be hard. White cardboard is not suitable for this purpose. It can be shifted during hologram recording. So, the background is made of white paper sticked on the glass plate.
The object has to be checked for possible shift. Try to shake the object lying on the table with your finger. If you feel that the object shifts easily, put it onto three small nuts (three supporting points again). The nuts should not be visible on the photoplate side. It is possible to put one nut under the statue head and two other nuts will be under the statue base. Try to shake the object with finger again. If it shifts, vary position of the nuts. It is easy to record coins, medals and other flat hard things on three supporting points. There is another more reliable way to fix the object on the base. That is sticking with hard epoxy glue. The epoxy glue includes two components, which are mixed according to a special recipe directly before gluing. After mixing, the glue is polymerized and it solidifies and becomes as rigid as a stone.
Three screws with large heads are used to set a photoplate. Their length is such, that the photoplate is 2-3 mm higher than the object. To protect the object against screw shadows, screws are placed in the following way: two screws are along the edges of the photoplate front side and one screw is in the middle of the photoplate rear side. It will be useful to fix the screws in the metal base or to stick them with epoxy glue. We haven't to do this because stability of the photoplate position is quite enough for the present conditions of recording.
This simplest proposed variant of the photoplate setting is possible, just to record a hologram of small size that is not more than 9x12 cm and exposing time should not be more than 10 sec. Photoplates of this size have their own hardness which is enough and they do not bend on three supporting points. Photoplates of big size are like membranes and weak external sources of vibration can easily cause their resonant motion. Photoplates of big size are like membranes and weak external sources of vibration can easily cause their resonant motion. Special metal frames of complicated design are demanded in order to fasten them.
The metal plate itself where the object and the photoplate are fixed must be also isolated against exterior vibrations. If its weight is about 5-10 kg, the plate can be put on the tyre tube, which is weakly pumped, or on the thick sheet of a foam-rubber. Unfortunately, there is no universal way to protect the plate against exterior vibrations if the hologram is recorded under home conditions. The level of vibration is very high on the upper floors of high-rise building and near freeways or railroad. Conditions are more favourable in the basement rooms or in the silent city districts. However, the old refrigerator or air-conditioner can spoil your holograms too. Holograms, which are recorded at night, are better than ones, which are recorded during day time. It is necessary to record test-holograms in order to achieve good results. If the object's image on the hologram is dark or there are dark lines, it means that the object is shifted during recording. If there are dark lines on the photoplate, it means that the photoplate shakes during recording.
The metal plate in our scheme is put onto the assembly table. The basement room and short exposure (5 sec) allow to record holograms without any special methods to protect them against exterior vibrations. -->