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44
BRITISH KINKMATOGRAPHY
Vol. 21, No. 2
of the position of the shutter edge. The first shutter employed produced flicker at the top and bottom of the frame for this reason.
To avoid this flicker it was decided to use the movement of the spot on the screen to obscure the image. This can be done by making a shutter which has a helical slot cut in it for each image. As the shutter rotates, the projection of the slot moves on the film in a vertical direction so that the spot of light from the television picture which is recording the image is in its centre. When the spot of light moves from bottom to top of the cathode ray tube during the television frame suppression period the image falls outside the slot and is so cut off. Unfortunately it was not mechanically possible to cut a double slot in the shutter and so a single slot of variable width was used. This is shown in Fig. 3.
Although the phosphor used on the cathode-ray tube (zinc sulphide) has a short decay, it is found that the integrated effect of
Fig. 4.
16 Dim. T. V. Recording Camera.
afterglow over the full exposure time cannot be neglected. Indeed there is evidence to show that it contributes up to 25 per cent of the exposure. It is therefore important to give an equal exposure time to all particles of the emulsion, and this is the reason for the shape of slot shown in
LP Fjg3
]p At the end of the cycle
* i the full exposure time for
afterglow cannot be allowed
> K*^t and the exposure is there
^r fore reduced for the last few
/lines in the picture unless the pull down time is shortened. It would seem desirable to use a cathode-ray tube phosphor of shorter decay to make this effect completely negligible.
(iv) Film Transporting Mechanism
In any double gated camera great accuracy of film registration is required since errors do not repeat on successive frames. For this reason it was decided to use a shuttle gate which has fixed register pins, and is known to provide great accuracy of registration. A further advantage of the shuttle gate is the lack of friction on the film during pull down, since it is then held loosely in the film guide. This low gate friction prevents kk emulsion pile up," which has been found to be serious on recording cameras as they are expected to run for long periods without cleaning.
Two register pins are used. For silent film both are between the two frames in the gate, the one on the sprocket hole side being full fitting and the other fitting top and bottom only. For sound film the full fitting pin is between the frames, and a side fitting pin is put in the upper frame. Interchangeable