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CH. XI] FLICKER WITH MOVING PICTURES 423
With the outside shutter, beyond the objective, the wing of the shutter as it advances removes light from the whole of the picture, a phenomenon which tends to reduce flicker.
The diameter of the inside shutter is limited by the size of the mechanism, while the outside may be made as large as is desired.
As will be seen below, the diameter of the shutter has an effect on the light.
The picture should be entirely covered by the shutter before it commences to move, and it should not be uncovered until it has ceased to move. This requires that the wings of the shutter need to be about 3 cm. (i^ in.) wider than the theoretical J^6th of the circumference of the circle.
The larger the circle the nearer to Yd of the circle is the width of the shutter wing.
With a shutter of large diameter, the actual velocity is greater and the interruption of the light is more sudden, therefore a shutter of large diameter is to be preferred.
§ 591. Flicker. — The standard speed of the film is given as 18 meters (60 ft.) per minute, 30 cm. (i ft.) per second. There being 16 films per 30 cm. (foot), this gives 16 pictures per second.
It is the general intention to run films at this speed though they are often run either faster or slower to get the best effects. The time during which one picture is shown (Vie second) can be divided into 6 equal periods, during five of these periods the picture is stationary and during the 6th the film is moved and the next picture substituted.
One complete change will be called a cycle.
The films could be run through the machine with no shutter at all, the film being in place an instant and then moved and the next picture substituted by a quick movement. This will cause a spreading out of white patches into a vertical streak called travel ghost, and will also give a general gray appearance and lack of contrast to the screen image.
To avoid this appearance some kind of a shutter is used to obliterate the pictures while the film is in motion. The shutter can be either translucent or opaque.