LeRoy A. McGrew Ball State University Muncie, Indiana 47306
Stereoscopic Projection in the Chemistry C ~ S S ~ O O ~
A
recent paper' described the use of a planar 35 mm camera for the preparation of stereo slides of molecular models. The method resulted X 4in. in a set of color stereo pairs mounted in mounts and intended for individual study using battery powered 3-D viewers. The success of this project in our department prompted an investigation of stereo projection in order that the stereo method be extended to classroom discussion of structural principles. Although an excellent method of projecting in three dimensions has been known for over thirty years, it has never enjoyed the popularity it deserves. Stereo movies have achieved very limited distribution and acceptance. Dual-lens slide projectors once aimed a t the amateur market are no longer manufactured, and economical used models in good condition are hard to find. Stereo attachments made to fit on the lens of a planar camera during the talung process and on the lens of the projector during the showing have appeared from time to time. These attachments change the format of the camera and usually give inferior results. The problem of stereo projection, then, is one of equipment availability, and it is the intent of this report to describe modern, planar equipment which will allow classroom 3-D projection of still pictures, and will yield a significant improvement in performance over that offered by the older-model stereo systems. For stereoscopic viewing there must be presentation to the two eyes separately of a stereo pair: disparate left-eye and right-eye views of the same scene. Individual viewing of the pair, as in the battely-powered viewer, is possible with the two pictures mounted side-by-side, but mass viewing on large screens requires that the two pictures be superimposed, but still separated by some means so that each image will be seen only by the appropriate eye. Stereoscopic projection thus requires two optical systems, and the integrity of the two channels must be preserved at all points from projector lens to eye. In spite of the existence of laser holography, and reported advances made by Russian workers in the design of lenticular screens, the only practical method of preserving image separation is that which cross-polarizes the projection light of the two channels, and which requires the audience to wear polarizing glasses. The alleged disadvantages of the glasses may have been partially responsible r M c G ~ ~ LEROY, w, A,, J. CAEM.EDUC.,48, 531 (1971). =KAISER, JULIUS B., "Make Your Own Stereo Pictures," The Macmillsn Company, New York, 1955, p. 283. 'Nikon, Tnc., Garden City, New York, 11530, Subsidiary of Ehrenreich Photo-OpticalIndustries, Inc.
for the failure of the stereo cinema, but they are relatively unimportant in the shorter viewing sessions attempted in the more formal atmosphere of a classmom. The polarization method yields fine image quality, excellent separation, and the full color of the transparencies is retained in the screen images. The only major requirement of a stereo projection system, then, is that it be capable of throwing two images on the same screen area. In place of a duallens stereo projector, we may closely couple two planar projectors, each one carrying one frame of a stereo pair, at the same distance from the ~ c r e e n . ~We may place polarizing sheet material in front of the lenses so that the light forming the two images will be polarized in perpendicular fashion. Image separation will then be preserved, and the stereo impression will be experienced, if the audience wears polarizing glasses in which the orientation matches that of the left-eye and right-eye projectors. This technique allows us not only to take advantage of recent improvements in planar projector design, but also to project full-frame, 35 mm stereo pairs which have needed no special processing or mounting. Equipment
Two planar, 2 X 2-in. projectors are required, and there are several models available which would appear to be suitable. The choice of a particular make is not as important as matching the pair's optical and mechanical performance. The lenses of the two projectors should be closely matched in focal length to insure that they throw the same size image when they are equidistant from the screen. They must have an accurate registration system in order that the stereo pairs remain superimposed on the screen as the slides are advanced. For a smooth transition from one slide to the next, the automatic advance system of one must not lag appreciably behind the other. An efficient illumination system is also necessary, since image brightness is lost as the light travels through two polarizing devices before reaching the eye. The 500-W quartz-halogen bulb used by most modern projectors is adequate, but anything less powerful may pEoduce marginal brightness. Nikkormat Autofocus projectorsS were used in our work, the choice being made largely on the basis of local availability of accessories and service. The rather shallow vertical dimension of these projectors allows them to be coupled one above the other. Locating them in the same horizontal plane would require an undesirably large separation of their optical axes. The projectors are equipped with automatic focusing and remote-control slide advance. These two features Volume 49, Number 3, March 1972
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Figure 2.
The complete projection fromewark,
Figure 3.
Framework with projectors in place.
Figure I . Exploded drawing of the proiection framework: A, circular bracket holding polarizing sheet; B, circular platform, 14-in. diameter, suppork the top pmje~tor; C, brocket with nylon screw for locking position of top projector platform; D, rock-and-pinion unit, no. 60,572 and 40,891, Edmund Scientific Co., 1 5 0 Edrcorp Building, Borrington, N. J. 08007, E, ball-bearing turntable, No. Edmund SsientiRc Co.
40,602,
are not necessary for stereo projection, but they are highly recommended, since they result in a convenience of operation which the original stereo projectors could never accomplish. Although the chosen projectors were not close in lens or body serial number, they matched well in all aspects necessary for the attainment of good stereo images. Our projectors are supported in a framework constructed especially for this project from '/a-in. aluminum plate (Figs. 1 and 2). One projector rests on the lower level of the framework and is elevated and leveled by means of its own adjustments. The other rests on a ball-bearing turntable on the top plate. A rack-and-pinion unit also supports the top plate at the rear of the framework so that the top projector is rapidly adjustable, both vertically and horizontally, for quick and accurate superimposition of the images. Circular brackets at the front hold the polarizing sheet,4 and are adjustable vertically so as to be concentric with the optical axes of the projectors when the images are superimposed. A slot near the outer edge of the brackets allows adjustment of the polarizing axis for attainment of a perfect match with the orientation of the stereo glasses. When properly adjusted, the distance between the lens axes is about 7 in., and keystone distortion of the images, arising from vertical toe-in of the projectors, is negligible. The entire projection apparatus is mounted on a rolling instrument table for portability (Fig. 3). The projection room must be equipped with a screen that has a metallized surface. The silverlenticular type of screen, currently popular for both home and educational use, works very well. Conventional beaded or white matte screens do not work because they cause depolarization of the projection light. Attempts to make an economical stereo screen met with only limited success. The best effort resulted from brush application of flat aluminum paint to a piece oi clear acetate measuring 4 X 6 ft. The 196
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acetate was bonded to a composition panel of equal size for support. The cost of materials was low, but the screen proved inferior to the silver-lenticular models,, primarily because of excessive brightness falloff a t only moderate viewing angles. Preservation of the polarization coding was, however, excellent, and such a screen would be suitable for viewing by small groups. With any type of screen, every effort must be made to provide a flat, reflecting surface. The buckles and bulges which often plague projection screens cause shadow areas which are particularly annoying in stereo. For perfect image separation, the axes of the polarizing chips before the eyes of the spectators must be exactly perpendicular. Sunglasses will not work, but inexpensive stereo glasses are SuitabIe glasses for our project were made locally and quite inexpensively. A number of lS/s X 4 in. cardboard stereo slide mountse were obtained and each was inletted to provide a bridge for the nose. These frames were then reinforced by gluing a wood splint across the top, and were provided with a wooden handle. The wood and cardboard parts were given a coat of spray shellac. Polarizing sheet material, the same as was used in front of the projector lenses, was cut into 1-in. squares, and two of the squares were cemented to the frame openings of each mount with the polarizing axes of the two squares at a right angle. The absolute 4 Cut from 12-in. square, E. H. Sargent and Co., Catalog No. S-70948. 6 Bernell Corporation, 316 So. Eddy Street, South Bend, Ind. 46617. 6 "Stereo Easymounts," made by Photographic Enterprises Guild Co., Sherman Oaks, Calif.
orientation of the polarizing axes was not considered important, but all glasses were, of course, constructed with the same relative orientation. The finished glasses (Fig. 4) are used in the fashion of a lorgnette. They may be used by students who already wear vrescri~tion glasses. and they allow easy removal ior note taking.
Figure 4.
Homemade stereo glasses.
Photographing Models for Projection
The equipment and techniques used to take projectable stereo pairs are essentially the same as are used to provide pairs for the battery-powered viewers, and they have been described in detail in an earlier article.' Projection of fullframe pairs simplifies the previous procedure in that the exposed film may be processed and mounted in the normal fashion. The parallactic disparity necessary for three-dimensional perception is introduced by taking two pictures of the model from laterally-displaced camera positions. The magnitude of the lens separation between the two positions controls the amount of parallax introduced, and therefore also controls the apparent depth range of the final image. For stereo pairs which are to be seen in a viewer, a lens separation which is onefortieth of the distance from the camera lens to the center of the model works well if the taking lens is in the 50-55 mm focal length range. I n stereo projection, the parallax is also subject to the degree of magnification of the image, and considerable latitude in the choice of lens separation values may be allowed, depending on screen size and the depth effect which is desired. Lens separation values which will produce the desired effectscan only be chosen from a knowledge of the mathematical relationships which govern stereoscopic transmission. A rigorous treatment of the theory has been given by Spottiswoode and Spottiswoode.' Using their notation, the equation most suitable for our purposes may be stated as follows
to that point in the stereoscopically-viewed scene; V, the actual distance from the observer to that point on the projection screen; iM,the linear magnification of the projection system, i.e., screen image width divided by film image width; t, the interocular separation (taken as 62.5 mm); j,, the focal length of the taking lens; t,, the lens separation used in taking the stereo pair. Equation (1) is actually a simplified form of the general equation and applies only to a projection system in which infinity points actually appear at infinity to the observer. Such a system is said to be "ortho-infinite."8 For the system under discussion here, the ortho-infinite condition may be assured by proper adjustment of the projection apparatus, and we may use eqn. (1) to derive acceptable lens separation values for our models. It will be seen that, for a given observer distance, the apparent position of points in the scene, and thus the depth range, is controlled by the product MjJ,. The terms j, and t, are camera variables and they combine to introduce the range of parallaxes which locates the image points in space. Their effect on the exposed film could only be changed by special processing methods not normally available to the amateur photographer. The image magnification, M, amplifies these parallaxes by an amount which depends upon the size of the projection ~ c r e e n . ~For a given screen and a given taking lens, the variation of t, is the only means by which the stereographer can control the apparent depth of his picture. At this point it is useful to introduce a quantity, N, called the "nearness ratio."'0
This quantity is helpful in that it allows us to relate the depth of the image to the plane of the projection screen, and will allow us to discuss certain properties of the stereo image which do not depend on the spectator-screen distance. A point which appears at the plane of the screen has an N-value of one. The separate images of the point in the stereo pair are projected without parallax; they are superimposed on the screen. The value of N is greater than unity for points which appear in front of the screen plane, and less than unity for points which appear behind the screen plane. Since we are interested in t, values, we may write, from eqns. (1) and (2), the following expression
For our project we wished to make 35-mm stereo pairs using a camera equipped with a lens of 55-mm focal length. Projection of the slides, with their 1.5-in. film width, on our 70-in. screen represents an image magnification, iM, of 47. Assuming a value
7
where p is the distance from the camera lens to a particular point in the scene during the taking of the stereo pair; P, the apparent distance from the observer
SPOTTISWOODIE, RAYMOND, AND SPOTTISWOODE:, NIOEL,"The California
Theorv of S k r e o s ~ o ~ Transmission." io University of ~ & , " ~ e r k e l and e ~ i o s Angeles, 1953;pp. 23-93. Zbid., pp. 32, 62,63. Ibid., p. 75. lo Ibid., p. 25
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of 62.5 mnt for the interocular distance, t, eqn. (3) reduces to
Now we may choose a point in the molecular model which we wish to appear at the screen plane. We assume that the model is oriented appropriately and that the camera is positioned so as to nearly fill the photographic frame with the model image. If the model has a number of planes of depth, it is usually best to choose the approximate center point (center of mass) of the model. The distance p is then the distance from camera lens to center point, the N-value is one, and eqn. (4) yields the appropriate value of t,. On projection, the usable depth range will extend in both directions from the screen plane; points in the model closer than 41 t, will appear in front of the screen, and points further away than 41 t , will appear behind the screen. The conclusion is seen, for our camera equipment, to approximate the 40:l rule mentioned earlier. Other workers may calculate an appropriate p:t, relationship for their equipment from eqn. (3). If an N-value of one is assigned to a point, say, twice the distance from the camera lens to the center point of the model, and t , is calculated accordingly, then the center point will appear at N = 2, and the entire image of the model will appear before the screen plane. Indeed, slides containing points with N-values of 3, 4, or 5 may be fused by the audience when the projection system is in perfect adjustment. Such slides occasionally may be used to produce a striking effect; spectators at the rear of the audience see the model suspended in space above the heads of those at the front, while the closer viewers experience the image in startling proximity. No part of the model should overlap the photographic frame, or the screen border, or psychological factors in the perceptual mechanism will prevent this effect. In spite of its attractive noveky, this technique should be used sparingly. The spectator's eyes,. which are at all times focused on the screen, must converge at a distance somewhat in front of the screen i n order to fuse a point with an N-value greater than unity. A little reflection will reveal that the visual system is not often called upon to interpret situations in which there is a large discrepancy in the point of focus and the point of convergence. Continued exposure to such a condition may result in serious eyestrain. Use of the Projection System
Successful use of the projection system requires that it he set up and adjusted prior to the beginning of the class period. With their optical axes in the median plane of the screen, the projectors are located so as to fill the screen, and they are focused at that distance. The two images are superimposed by means of the framework adjustments. The top projector throws the left-eye image, and the orientation of the polarizing material before the lenses is adjusted to match the orientation of the stereo glasses. The ortho-infinite condition of the projection system must be assured. Even though the close-up views of models contain no infinity points, a deviation from 198
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Journal of Chemical Education
this condition will alter the depth range and introduce distortion. The taking procedure we have described is essentially that which uses a zero-convergence camera; the camera is not toed in on a central point in the model from either position used in taking the pair. A point a t infinity in the median plane of the camera lens positions will be imaged on the center lines of the film, since rays from that point are parallel. If the pair is then projected with the images exactly superimposed, the infinity point will have no screen parallax and will appear to lie at the screen plane." Other points will appear with their parallaxes unduly magnified, and will lie in front of the screen. The problem is easily corrected by slightly toeing in the projectors to insure a screen parallax for infinity points. The left frame border should lie 2.5-in. to the left of the right frame border at the screen. Infinity points will then appear with a parallax of 2.5-in., a situation approximating the direct viewing of such points. Nearer points will be imaged with the true parallax range, assuming that the proper value of t, was used in taking the pair. The ortho-infinite condition could also be accomplished by introduction of the offset during a special reprinting of the stereo pair, but this is much more difficult than the projection adjustment described above. The desired sequence of slides is chosen and the stereo pairs are separated and loaded into 36-slide straight trays labeled "left eye" and "right eye." The trays are placed in the corresponding projectors and the first slide of each is inserted into the gate. At the beginning of the period students are instructed in the proper use of the stereo glasses, which requires that they be held level to prevent leakage of images into the wrong eye. Turning on the projector lamps then completes the preparatory sequence. Some of the problems which may beset a planar slide presentation become more serious in a stereo session. A pointer cannot be used accurately, since it always appears at its true distance. It is impossible, for instance, to point in unambiguous fashion at an image area which appears in front of the screen. A flashlight pointer, used by the instructor from his position beside the projection apparatus, works better, although it is always imaged at the screen plane. Perhaps the best procedure would be to minimize the need for pointing by labeling the original model at those positions which require special emphasis. Anot,her concern arises in regard to the audience seating arrangement. The stereo impression varies from place to place in the seating area. The depth range increases past congruency with the original scene as one moves back, and is compressed as one moves forward. Angular distortion occurs as one moves to higher viewing angles a t a constant screen distance, although this distortion does not seem more serious than that which occurs with planar images, and the depth impression persists at very high viewing angles. The best arrangement is not always possible in conventional classrooms, but it may be suggested that the audience he seated in the range of 2-5 image widths, and in as narrow an area as possible. The
" Reference in footnote 7, p. 101.
system, as presently described, is not suitable for large auditorium use. Projected stereo has been used at our institution in the beginning course in organic chemistry to illustrate concepts of stereochemistry and conformational analysis: conformations of ethane and other simple hydrocarbons, dissymmetry and optical isomerism of noncyclic molecules, geometric isomerism of alkenes, conformations of cyclohexane and substituted cyclohexanes, and stereoisomerism of cyclic molecules. Most of our slides show hall-and-stick models, and we have a~talogedabout 100 stereo pairs. We hope to extend the technique to discussions of orhital hybridization, orhital symmetry, the geometry of inorganic complexes, and stereospecific and stereoselective reactions. Careful attention to projection details has resulted in very satisfactory viewing sessions. The projectors maintain their registration so that no readjustment for superimposition is necessary during presentation of a 36slide tray, and the automatic focusing feature eliminates the only other projector adjustment. Although the principles of stereo-
scopic transmission may be unfamiliar, the instructor who is acquainted with planar slide photography and projection should have no trouble when he decides to add the third dimension. Students have reacted favorably to the method and have experienced no eyestrain. It is of course helpful if they have read the pertinent textual material prior to the slide presentation, and if they have had an introduction to the uses of molecular models. The need for adequate discussion usually limits the presentation to somewhat less than 30 slides in one 50-min class period. Even for one such period, the purchme, storage, and presentation of the same number of actual models of equivalent size would clearly he impossible. Acknowledgment
A University Science Improvement Project grant, funded by the National Science Foundation and administered by Ball State University, provided financial support and is gratefully acknowledged. Mr. Eric Langdon, machinist, made the aluminum dual projection framework. The exploded drawing of the framework was prepared by Susan Nelson.
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