As the movie projector casts intermittently a series of pictures, it is necessary to shut down the light by a rotating shutter inorder to keep screen clear of the unnecessary shadow accompanied by the change of action of successive pictures.
each change of successive pictures. But it is common practice at present to adopt picture cycles range between 16 and 28 cycles per second.
Thus if the projected light shut down once for each picture cycle, there will arise some flickering effects of intermitted illumination.
To prevent such flickering, the present practice is to shut the light once or twice in the same picture being projected on the screen, in addition to the above said necessary shutting.
On the other hand the light intensity of the A.C. arc or A.C. incandescent lamp fluctuates with double the frequency of the supply source. Thus the alternating current used for the light source is apt to interfere with the action of rotating shutter.
The result is that the illumination of the picture screen produces some breathing effect with such low frequencies observable to the eye.
This paper describes the method of measurement of screen interference by the use of photoelectric cell and oscillograph, and also shows the calculated results.

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