Holographic Stereograms and their Potential in Engineering Education in a Disadvantaged Environment. B. I. Reed, J Gryzagoridis, Department of Mechanical Engineering, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa Published as: Reed, B. I., & Gryzagoridis, J. (1998). Holographic Stereograms and their Potential in Engineering Education in a Disadvantaged Environment. The International Journal of Mechanical Engineering Education, 26(4), 303-308. The presentation of subject matter to students from disadvantaged backgrounds relies on using techniques that help them visualise various concepts and processes. Here an alternative demonstration technique is described, the holographic stereogram, where one can show some object or process behaving in the manner that it would in a real life environment. The method used to generate these stereograms is briefly described and an example is given as to how they can be integrated into an existing academic program. 1. Introduction Modern Engineering curricula strive to present courses and develop fresh techniques that have been designed with the aim of introducing students to traditional subject matter in new and innovative ways. This is particularly true in an environment where the students have predominantly a disadvantaged background. The bridging from this disadvantaged background is accomplished in part by developing contextual projects that develop what is known as Systems Thinking and the Engineering Design Process as well as developing teaching aids that augment the work done in traditional lectures.
These teaching aids should preferably take the form of low cost demonstration tools that will enhance this new approach to lecturing and aid in giving students the ability to visualise various concepts and processes. It is not foreseen that this should replace laboratory or demonstration equipment, but rather give these a new dimension in situations where necessary. To that end, one of the demonstration techniques available is the use of a holographic stereogram, where one can show some object or process behaving in the manner that it would in a real life situation. With this technique, complex concepts or procedures can be illustrated in a manner not possible before, with relatively inexpensive equipment. 2. General Introduction to the Holographic Stereogram Multiplexed holographic stereograms have evolved significantly since they were first proposed and developed during the late sixties and early seventies. 2.1. Multiplexed Holographic Stereograms Dominic DeBitetto s [1] ground breaking technique employed a method analogous to that used by the communications industry, i.e. sending many signals simultaneously down a coaxial cable and distinguishing them by using different transmission frequencies. He extended this principle to holography by capturing successive images on one piece of holographic film. The images were differentiated from each other by the way they are encoded in the film s emulsion. Different images are displayed by light being reconstructed in different directions. A problem with his approach was that the incoherently recorded two-dimensional photographs had to be taken from a series of equally spaced positions along a horizontal line. This does not meet the requirements of a fully 3D object visualisation tool. Further development has been done to reduce this limitation and Kasahara, et al [2] have shown how this technique can be extended to a cylindrical format.
Cross [3] took a different approach to DeBitetto. His technique involved having all the successive images information onto consecutive narrow strips. By moving ones plane of view, different images are reconstructed, i.e. the viewer looks through a narrow strip hologram at a virtual image of the slide at the centre of the hologram [4]. Therefore, where the former set-up has a camera moving past the subject while following a horizontal path, here the camera remains stationary while the object rotates. 2.2. Modern White-Light Transmission Holographic Stereograms By the mid 1980 s, Stephen Benton [5] had taken the work pioneered by DeBitetto further as he was dissatisfied with the number of optical and other shortcomings related to Cross s multiplexing technique. A major concern of his was the fact that many of the modern day techniques used in Cross s Stereograms were of a proprietary nature and thus protected by various patents. By reworking DeBitetto s original ideas he developed the present-day standard for flat stereograms. With this development came a flurry of advances in techniques resulting in the ability to create achromatic and multicolour stereograms as well as introducing animation that was optically defined and dimensionally accurate. 3. Computer Generated 3D Animated Images In generating the individual frames that are subsequently used to create a stereogram, traditionally a 16mm ciné camera has been used. However, this has resulted in stereograms generally depicting scenes that can only successfully be filmed in a model/camera arrangement. The introduction of computer animation was something that was initially restricted to very specialised environments. In the past half a dozen years, there has been a real reduction in the cost of computer processing power and at the same time, the level of sophistication of computer animation programmes has increased significantly. There is tremendous scope now, in our acquired ability to generate a stereogram, that moves it beyond the simple tool of artistic
expression, to one where the technique can be developed into a useful commercial tool for utilisation in a variety of technical and educational fields. One of the problems faced by researchers in the early days of computer generated animation was how to simply transfer their animated frames to celluloid for projection onto the holographic film. The most common method employed was to simply send each digital frame in turn to be transferred to slide by a specialised graphics bureau. An alternative to this has been used which is rather simpler than that described above but has proved to be surprisingly effective. Here one simply photographs the output displayed on the computer s monitor using a macro lens on a single-lens reflex camera. Ordinary colour reversal film has successfully been used. Under normal circumstances, unless the camera has a fairly wide aperture (> f/1.5), one will not be able to see the object stereoscopically as there will be insufficient parallax. However, this is not relevant here as the photographs are taken from a flat screen of a series of animated frames showing no parallax. 4. Optical Layout for the Master (or H1) hologram The basic layout used in the creation of the Master holograms is shown diagramatically in Figure 1. A conventional transmission holography layout is employed with a metalised variable beamsplitter (BS) placed in the position shown to split the HeNe laser into the object and reference beams. The object beam path is determined by reflecting the unexpended laser beam off the adjusting mirror (M2) to fall onto the centre of the opaque focusing screen (OS) where the slide image is to be projected. A spatial filter (SF1) is introduced to clean up the beam and
L M3 M CL2 SF2 BS F 30 O OS PL SF CL1 SF1 M1 M2 Figure 1 - Optical Layout for Master Holograms to expand it to the collimating lens (CL1). Built into the housing of this collimating lens is the slide film (SF) positioning mechanism which in turn projects a collimated image of the object onto a diffuse screen. The collimated image is now passed through the projector lens (PL) of a 35mm slide projector. This lens configuration is well corrected and results in a crisp, well-defined real image at its focal plane (adjusted in this case to be the opaque screen). The reference beam is reflected off the adjusting mirror (M3) to fall onto the centre of the space that the holographic film (F) will occupy. A spatial filter (SF2) is introduced to clean up the beam and expand it to the collimating lens (CL2). The collimated reference beam is now directed at the slit that is formed by the mask (M). This setup allows a series of sequential images to be recorded onto a strip of holographic film for use in the generation of the Transfer hologram which will become the holographic stereogram.
5. Optical Layout for the Transfer (or H2) Hologram The optical layout of the procedure used in creating the stereogram is similar in process but completely different in execution to that for the Master. Generation of these Transfers still uses the transmission holographic approach but here one adds not only the white light rainbow effect but also an overhead reference beam for display purposes. The basic layout used in the creation of the Transfer holograms is shown diagramatically in Figure 2. Reference beam strikes the film from above M7 Laser M1 M2 CM M5 CL2 H2 BS M M3 SF1 M4 CM SF2 Reference beam continued above M6 H2 H1 CL1 Figure 2 - Optical Layout for Transfer Holograms From the beamsplitter (BS), the laser beam is positioned by mirror M2, M3 and M4 to be incident on the Master hologram (H1). This is done without the spatial filter (SF1) and the collimating lens (CL1) present, in order to align the object beam to be incident on the H1 from the same angle as the reference beam used to create it. The Master is held in position by sandwiching it between two perspex sheets, anchored in position by two support brackets. The reference beam used to create the Master must be collimated as, in order to reconstruct a geometrically correct (though pseudoscopic) real image from the Master, one must illuminate the flipped Master with the conjugate of the reference beam.
A spatial filter (SF1) and collimating lens (CL1) are introduced to fulfil the illumination requirements described above. A secondary mask (M) is introduced to restrict the collimated light to the width of each incremental image. The theory of white light rainbow transmission holography requires that only a narrow slit of the master be exposed. The image slit width is set to 10mm to allow the restricted real image to be as bright as possible without sacrificing clarity in the final stereogram. With the collimated beam now incident on the Master, the real image is projected to the plane of the holographic film. The layout of the reference beam differs markedly to that used for the generation of the Masters, in that the reference beam has to be positioned to be incident on the film from above. This requirement was determined by the manner chosen to reconstruct the stereogram. In order to maintain constant polarisation vectors, the beam can only have its elevation off the table changed in one plane. If the beam is skewed away from the vertical when reflected from mirror M6, then there will be a discrepancy in the directions of the vectors where the reference beam intercepts the object beam and the quality of the final hologram will be degraded. The reference beam is directed by mirrors M3 and M4 onto a collimating mirror (CM1). This collimating mirror reflects the beam via M5 down to the film. Note that from M4 to the film, only the elevation of the beam is altered and not its angular position. A spatial filter is positioned just after M4 in order to clean and expand the laser beam. The focal length of the collimating mirror is effectively shortened by employing a secondary simple lens (CL2). The mirror M5 directs the collimated reference beam down onto the film at an angle of incidence of 38 O.
The resultant hologram is a stereogram that displays both parallax as well as any animation that is required. 6. Integration into Existing Academic Programs With the ability of producing low cost, visually effective demonstration media, the challenge is now to integrate this into an existing curriculum as effectively as possible. Various techniques exist for displaying these stereograms. In a lecture type environment where it is necessary to have a large viewing area, it is preferable to use a planer stereogram, as it is then possible to use a simple overhead projector as the light source and then a mirror to reflect the image to the students eyes. In a laboratory or demonstration environment it is often necessary to use a curved stereogram (up to 360 O ) in order to successfully demonstrate or show a full 360 O of parallax. As a typical example of a specific application for this technique, one needs look no further than the Engineering Drawing course taken in most first year curricula. It has been found in the course as it is presented here, that there are many students that struggle to visualise threedimensional lines and planes in space. This impacts negatively on a student when they come to draw this plane on a drawing board and creates a sense of disillusionment and frustration. The use of a holographic stereogram in this situation will allow a student to view the drawing exercise on a flat hologram in its three-dimensional orientation before they transfer this view to a drawing board. This technique will also help with the explanation of the concept of true length as an extension to the concept of planes in space.
7. Conclusion By using the holographic stereogram as an aid to both enhance and improve current teaching methods, one can not only stimulate interest in a topic by the introduction of visually stimulating media, but also present traditional subject matter in a fresh and innovative way. This is of particular importance when one takes into account the diverse educational backgrounds of our students. 8. References [1] DeBitetto (1969) Holographic Panoramic Stereograms Synthesised from White-Light Recordings. Applied Optics, vol. 8, no. 8, p1740 [2] Kasahara T, Kimura Y, Kawai M, (1971) 3-D Construction of Imaginary Objects by the Method of Holographic Stereogram, in E S Barrekette, et al, eds., Applications of Holography (New York: Plenum Press), pp19-34 [3] Cross L (1977) Multiplex Holography. Paper presented at SPIE but not offered for publication [4] Saxby G, Practical Holography (New York: Prentice Hall,1988), p56 [5] Benton S A, (1983) Photographic Holograph. Proceedings of SPIE, vol. 391, pp2-9