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CH. XI] FLICKER WITH MOVING PICTURES 425
If the shutter is translucent the appearance during the change of pictures is that of a screen lighted to a uniform gray. This kind of shutter is not much used in practice as it has the disadvantage of slightly illuminating the parts of the screen which should be absolutely black.
The opaque shutters were originally made to cover the picture during the time the picture was in motion, i. e., from % to Y> of the cycle, the rest of the cycle the screen was lighted, but this was found to give a very bad flicker.
Recently to get rid of the flicker the shutters have been made with 2 or 3 opaque wings.
With the one-wing shutter a cycle is made up with
1 . Picture on the screen — screen light — Y* to % cycle.
2. Picture changed — screen dark — Y* to Yt> cycle.
There are 16 cycles per second. The average transmission is YI to % of the incident light.
It has been found that with a one-wing shutter the flicker is nearly as troublesome when the opaque part is Yf> as when it is Yt of the shutter. To avoid this, extra dark wings are added to the shutter, the form with 3 wings being the best With a threewing shutter a cycle is made up of :
1. Picture on the screen — screen light — % cycle.
2. Same picture on the screen but — screen dark — Yt> cycle.
3. Same picture on the screen but — screen light — Yt> cycle.
4. Same picture on the screen but — screen dark — Yb cycle.
5. Same picture on the screen but — screen light — Yb cycle.
6. Picture changed — screen dark — Y> cycle.
The screen is dark Yz and light Y* °f the time : Transmission of incident light, 50%.
Each picture is thrown on the screen three times before it is changed for the next. Thus, while there are 16 cycles per second; there will be 48 flashes per second.
At this speed, 48 flashes, flicker will altogether disappear (See § 592).