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Fig. 4. Lap dissolve.
through the polygon and thus compensates for film shrinkage.
Optical film-shrinkage compensation of this type in turn introduces another complication which must not be overlooked, namely that varying the relative amounts of glass and air in the optical path between the projection lens and the film will in turn cause considerable defocusing of the image. Refocusing could of course, be accomplished by readjusting the projection lens, but this in turn would produce a slight change in magnification of the optical system. In order to avoid these difficulties, a second planeparallel plate of variable thickness is introduced into the optical system between the projection lens and the polygon. This again consists of a stationary and moving wedge operating as an entity. This is called the "refocus compensator." The sum of the thicknesses of the shrinkage compensator and refocus compensator is kept constant by means of push-pull operation of the shrinkage-compensating wedge and refocus-compensating wedge respectively. This manner of operation then results in a shrinkage-compensating device of constant focus and constant image size irrespective of setting. In practice, the two wedges are ganged together and operated from a common control.
Methods of Picture Transition
In film devices of all kinds which use continuously moving film in conjunction with optical compensation, some specific method of picture transition from one film frame to the next must be used in the place of the intermittent pull-down
which is employed in a conventional projector. Numerous methods of picture transition are possible. The method used in television film scanner described here is that called "lap dissolve" which can best be described as the alternate fading in and fading out of successive film frames. The light intensity from the odd and even-numbered film frames is illustrated in Fig. 4, as this varies with time. For instance, while an oddnumbered film frame is fading from maximum to minimum light intensity on the screen, the successive even-numbered film frame is fading in from zero to maximum intensity. The important thing from the point of view of truly continuous and flicker-free operation is that the sum of the two light intensities should always add up to unity. Thus it will be seen that in lap dissolve, two successive film frames are always superimposed on the screen, except at one infinitely small moment when one film frame alone appears on the screen because the preceding frame has faded down to zero intensity and the succeeding frame has not yet had a chance to rise to a finite level of intensity.
The dynamic manner of operation is shown in Fig. 5 which relates the position of rotation of the polygon to the lapdissolve diagram. The first view shows the polygon in the "normal" position, namely that which would be used if it were desired to transmit merely one stationary film frame (T0). The second view shows the polygon in a position of rotation one-half frame-time later (To + 1/48 sec). At this moment the light from the raster is seen to be evenly
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January 1954 Journal of the SMPTE Vol. 62