MAKE YOUR OWN STEREO PICTURES

see the red image but could see only the green image—and vice versa.

Many attempts have been made to remove the necessity of the observer having to wear glasses for three-dimensional projection. Some of these "free vision" experiments have been very interesting. The stereo pair has been projected into concave mirrors, into blocks of plastics and glass, onto screens of all shapes and even by means of an array of projectors aiming from strategic points at a "target" screen! Results of all efforts so far have failed to compete in clarity of pictures projected with use of the polarizing system. Also, "free vision," when finally accomplished, was available to occupants of certain seats only in an auditorium, and a slight movement of their heads would dispel the three-dimensional picture and result in distortion.

Pairs of pictures shown upon a screen cannot be seen as two individual pictures, but they must be fusable in the brain of the beholder as a single image—in three dimensions. Any process which projects only a single image on the screen—irrespective of how wide or high that image may be, or how many projectors are employed in placing that impression on the screen—must be two-dimensional and not stereo. If a process hasn't stereo pairs, it can't be three dimensional; it can be effective in its simulation of depth, but it cannot approach the depth qualities of three-dimensional projection. It takes both eyes to see depth, and it takes fusion of a pair of images in the brain of each member of the audience in order for him to see in depth, or three dimensionally.

THE POLARIZING SYSTEM

It has long been known that light can be polarized and made to travel in a controlled beam, rather than in scattered fashion as it does when coming from the sun or from artificial sources. It was realized that polarized light would not penetrate material which was polarized in a perpendicular direction. Polarizers were, at that time, available as Nicol prisms which are both heavy and costly and which could hardly be worn before the eyes.

PRINCIPLES OF STEREO PROJECTION

Polaroid, the polarizing material produced by the Polaroid Corporation of Cambridge, Massachusetts, is the filmy material from which "3-D" spectacles are made. In this product a weightless, cheap polarizer is used to make light travel in a single plane only-very much as it does through Venetian blinds. Also, when the axis of the polarizer is turned 90 degrees, Polaroid can be used in the making of eyeglasses; these then act as a filter to restrict vision. The standard method, although not always followed by projector manufacturers, is to tilt the Polaroid's axis so that the left eye's picture is polarized by Polaroid sheets within the projector to reach the screen vibrating 45 degrees left of vertical, while the image to be seen by the right eye is polarized at right angles, oriented to make the light reach the screen vibrating at 45 degrees right of vertical. The glasses worn by the audience contain polarizers which are oriented precisely as are their respective images. See Fig. 66.

If the left image alone were to be shown to the audience, the right eyes of the audience would be barred from seeing it; the Polaroid's microscopic grid before the right eye, at right angles to the image's light, would blot out vision. If the right image alone were projected, the audience's left eyes couldn't see it for the same reason. But the eye for which the image is intended can see the image very clearly, since the axes of the polarizing material, in the projector and glasses, both run in the same direction. There is, however, a small amount of light loss because of these two polarizers which must intercept each image, and this loss must be taken into account when the pictures are taken. That is why previous discussion on exposure recommended a very slight overexposure.

Polarizing spectacles which employ a product other than Polaroid have appeared. The great demand on the part of the motion picture industry has led to the production of cheaper filters to be discarded after one wearing. Before using such spectacles be sure that they contain effective polarizers. The wearer of inferior polarizers receives an impression of halo around close subject matter and one of doubleness of more distant material, either of which spoils the stereo presentation.

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