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The opaque shutters were originally made to cover the picture 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 three- 

 wing 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 an d 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). 



