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1953-54 Theatre Catalog, 11th Edition, Page 260 (224)

1953-54 Theatre Catalog, 11th Edition
1953-54 Theatre Catalog
1953-54 Theatre Catalog, 11th Edition, Page 260
Page 260

1953-54 Theatre Catalog, 11th Edition, Page 260

BELL & HOWELL 35mm camera interlocked ior 3-D photography. Focusing done by rack and gears.

employ novel optical systems which are accessory attachments to standard projection heads.

If one member of a stereogram has even a slightlydifferent 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 diaphrams 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 accom BELL 6 HOWELL camera provided with three color filter wheels for 3-D separations negatives.

modate 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 shows a pair of pictures having image attitudes in a conventional arrangement. The diagram on the right shows an improved arrangement. It affords the best possible distribution of the light from the projector arc.

The illuminating spot from the arc is considerably more intense in its center than in its outer regions. With images having the attitudes shown in the left side diagram, more light falls at the top of one picture than at the top of the other, resulting in a different level of illumination in an area of one than in the corresponding area of the other. The arrangement shown on the right provides equal illumination in corresponding areas, because all portions of the light spot fall upon the same corresponding areas in each member. This meets the requirement that there be equal bright ventional attitude in a pentaprism and right angle prism forward of the lens. The polarizers are placed in front of the lenses away from the intense heat.

Prints for this system of projection are not made directly from.the original negatives. Duplicate negatives produced in an optical duplicating process are used instead.

Another improved single-film method has the images turned on their sides. This arrangement provides for the use of the full standard aperture in case the pair of images is to occupy only one standard frame. A series of prism elements between the film and the lens turns these images 90 degrees. The inverted image is "flipped over" in the pentaprism and the right angle prism in front of the lens. Otherwise, the practical advantages of this variation are comparable to those of the method which is discussed in the preceding few paragraphs.

Another method, devised by the author, for attaining equal illumination for both members of the stereoscopic pair is also illustrated. Here, there are two mirrors

DIAGRAM OF THE "rotating polan'zer" method of alternate frame stereoscopic projection.

ness for each image in order to attain complete visual comfort. It may seem surprising that the intensity difference between center and edges of the light circle is significant. Actually, in practice, projection engineers are quite happy if there is only a 40 per cent loss,e60 per cent as much light at the edges as in the center. In many theatre installations, the fall-off is as much as 50 per cent. The projection end of this system, using standard projector heads, has two optical trains containing prisms as well as lenses. These components are arranged so that the light beam enters and exits normal to the prism surfaces and there is no displacement or distortion such as would take place with wedge prisms. Alignment of the images on the screen is effected by a micrometer control to shift the lenses. In the lower optical train, transmitting the right eye image (the one having the conventional attitude), the image passes through in the conventional manner, reversing to Uheads up" in the projection lens. In the upper optical train, transmitting the left eye image (the one having the ftflipped over" attitude), the image is brought to con


arranged between the light source and the film. The lower mirror is a transmission-refiection type, with a refiective coating that will refiect half the light to the upper mirror, one-half the light passing through to one member of the pair. The upper mirror refiects one-half the original light beam to the other member. Again, two projection lenses are used, as in the other systems with the polarizers in front of them.

In the system the images do not have f0 be inverth in relation to each other, since the illuminating circle falls upon the same corresponding area in each. The light-circle can be reduced in size to fit the area occupied by one frame, adding ofhciency. Added efficiency also results from the reduction in the number of glass-to-air surfaces. Additionally, there is less possibility for dirt or t'foggingi' on the optical elements. In com tradistinction to the other methods, which require no lamphouse changes, this method does require a few changes in the lamphouse to accommodate the two mirrors.

A dual-image single-film does not add significantly to print costs, to the costs

1953-54 Theatre Catalog, 11th Edition, Page 260