Holographic multi-stereogram constructed from computer images : Applied 3-D printer
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The aim of this thesis is to develop a method to convert computer images to a holographic film plate. In the recording process of holographic multi-stereograms, one uses a liquid crystal display (LCD) to show 70 different pictures of a constructed object. These pictures show the object from 70 different positions, and in a sequence the object appears as though turned over the LCD. Each of the pictures is exposed in a different area of the film and the film position is controlled by the holographic printer. The result of this recording process is a 3-D image of the object with horizontal parallax, constructed from 70 different 2-D computer pictures. At the start of thesis search for relevant literature was given high priority, where the purpose was to find what kind of optical equipment and quality was needed for these experiments. Most papers recommended the He-Ne laser, silver halide film emulsion and developers like Agfa GP 62 and Kodak D-19. As for the quality of the optical equipment, mirrors need a flatness of the order of 1/10⋅λ, and lenses must have minimal aberrations. Before the making of a holographic multi-stereogram, the optical equipment and set-up for recording holograms together with developing process were examined. In order to find a practical favourable method, preliminary holography experiments where carried out. In these experiments, there were practical problems like instability, reflections, depolarisation and exposure. The developing and bleaching processes were also tried out, and gave good results. In order to produce a holographic multi-stereogram, it was necessary to procure some more optical equipment. Some of the equipment was made in the work-shop in the Institute of Physics: - Micrometer screws for the spatial filter. It is possible to change the pinhole from 25μm to 10μm. The result is a cleaner laser beam. - New slit on the holographic printer. The angle is larger and the possibility of shadows is reduced. - Micrometer screws on the holographic printers slit. It is possible to adjust the width of the slit easily and precisely to the right size. - Table extension for the optical breadboard, used in the production of reflection and rainbow holograms. - Table holder for breadboard and the table extension. - Mirror holder to transfer laser light from the optical table (lower level) to the breadboard (upper level). - Glass cage mounted on the optical table to protect the equipment from air vibrations. Other optical equipment bought from different companies : 4 34 35 - High quality lenses - Mirrors with flatness 1/10⋅λ. - Liquid crystal display ( LCD ). - Optical breadboard. The optical breadboard made it possible to arrange the optical equipment at two levels on the table. This is very practical in the production of holographic multi-stereograms. - High power red He-Ne laser with 24 mW output power. The production of holographic transmission multi-stereogram is time-consuming; the recording time can be 30 minutes and for reflection holograms several hours. Therefore, the laser output power stability must be constant in the exposure of holographic multi-stereogram. The power stability is measured for 2 different periods at different sampling rate, which shows the laser power stability is very good. It is important the laser is sufficiently heated before recording holograms. The heating time must be at least 1 hour. The laser’s coherence length is measured with help of Michelson’s set-up, and is measured to be 30 cm. From the visibility plot the contrast of the hologram will fall from 1 to 0.8 for 5 cm laser beam path difference. It is very important that the reference and object beam have the same path length. To produce high quality holograms the stability of the laser beam and the optical equipment must be good. Vibrations of the interference fringes on the film plate for more than 1/10⋅λ reduce the brightness of the hologram. Therefore, the optical equipment’s stability and quality have to be very good to meet this requirement. Several different vibrational measurements have been made on the optical table. The first is made with the glass cage and the second without. The plot shows distinctly that the measurement without the glass cage has higher vibrational amplitude than the measurement with the class cage. A vibrational measurement of the recording process of the holographic multi-stereogram recording is also made. This measurement shows the printer makes a high vibration amplitude when the motor is running, but the optical table is stable 1-2 seconds after the motor has stopped. There are no vibrations measured when the shutter moves (open/close). With the use of the printer and the shutter, which is the equipment that moves under the recording process, there are no vibrational problems for the production of holographic multi-stereogram. The thermal stability requirement for the film plate is very high, a change of temperature for more than 0.1°K makes reduction of the hologram quality. Several holographic recordings were made: transmission, reflection and rainbow holograms made with 1-step and 2-step methods. For all of the recordings made with the 1-step method, it is not possible to get rid of the vertical lines from the slit of the printer. With the use of the 2-step method, where a master hologram is copied onto a new hologram the problem of lines from the slit was solved. From a master transmission holographic multi-stereogram, high quality 2-step reflection and rainbow holograms were produced. There were also produced 13 reflection holograms of good quality for EPM Consultants, who have co-operated in this thesis work.