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AXIS OF POLARIZATION
Fig. 30. Diagram of the "rotating polarizer" method of alternate frame stereoscopic projection.
There are two ways to project and view eclipse stereograms. One is by using rotating or vibrating shutter devices held up in front of the eyes. These are synchronized electrically with the projector. The other method is to employ a rotating polarizer in front of the projector lens and polarizing spectacles for the viewer (Fig. 30). In one position the polarizer delivers light through the left spectacle filter, in the other through the right filter.
Alternate-frame, or eclipse, projection should have at least twice the number of frames required for conventional films. That means doubling the length and providing for faster projection speed. If the alternate frames are photographed alternately, there is a very objectionable fringing in pictures of moving objects. This is a cause of eyestrain, especially in a picture where the action seen by one eye is in quite a different stage of progress than the action seen by the other. Difficulty in fusion invariably results. This combination of disturbing effects, caused by flickers out of phase between the eyes and by fusion trouble, limits the appreciation of the eclipse method.
Complete visual comfort can be attained in stereo movies only if the two images are projected simultaneously, if they are rock-steady, if they are of equal brightness, if they are of equal contrast,
if they are properly aligned vertically and horizontally, if far distant points are not separated too far in one image from that of the other, and if they are of exactly the same size.
Improved Single-Film Methods
Several inventions by the author eliminate the drawback of the singlefilm dual-image arrangement, namely the unequal distribution of light. These methods employ novel optical systems which are accessory attachments to standard projector heads.
If one member of a stereogram has even a slightly different brightness, some eyestrain • will result. If the difference is large, the resulting eyestrain will be great. This is because the action of the eyes' iris diaphragms is entirely automatic and not by voluntary control. If strong light falls on one eye, both pupils will contract and the eye seeing the darker image will have its pupil closed down more than it should be properly to see the image. In accommodation too, both eyes act together and it is impossible for one independently to accommodate to a different extent from the other.
The improved single-film methods satisfy one of the basic requirements for good stereograms, easy to look at, in that both members of the pair are of the same brightness. The illustration
J, A, Norling: The Stereoscopic Art — A Reprint
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