HIGH SPEED MOTION PICTURE PHOTOGRAPHY 395 



the same velocity. Cameras operating on both principles have been 

 developed and are used in scientific and engineering research. A large 

 part of the work now being done by other workers in this field is with 

 taking speeds extending from a few hundred pictures per second to 

 2,000 or 2,500 pictures per second. 



Cameras of the first type, that is those in which the film is driven 

 continuously and in which the object is brightly illuminated for a 

 short length of time (usually of the order of 2 to 10 microseconds) are 

 relatively simple in construction. Provision is made for the reel of 

 unexposed film which may be 25, 50 or 100 feet in length and have 

 either 16 millimeter or 35 millimeter width. The film may be guided 

 past a fixed exposure aperture or around a rotating drum or toothed 

 sprocket from which it passes to the take-up reel. Power may be 

 supplied through an electric motor either to the take-up reel or to 

 the toothed sprocket. If a stationary exposure aperture is used, two 

 drive sprockets, placed one above and one below the gate, may be 

 employed. Periodic flashing of the illuminant may be secured by the 

 use of a commutator actuated by the camera mechanism. 



The second type of camera above referred to employs an optical 

 intermittent to produce the image movement required to avoid 

 blurring when the film is continuously driven past the exposure 

 aperture. A variety of optical intermittents has been developed 

 employing either lenses, mirrors, prisms, or a plane parallel glass plate 

 or block. Regardless of choice of means, the optical intermittent 

 serves to produce a series of rapidly moving images which move with 

 the film velocity. Of these available methods perhaps the simplest 

 is the plane parallel glass plate or block which, by reason of its thick- 

 ness, deviates or displaces an inclined ray in a manner nearly pro- 

 portional to rotation about a chosen axis. 



At Bell Telephone Laboratories a high speed camera has been 

 developed which normally operates at a taking speed of 4,000 pictures 

 per second. This camera employs optical compensation of the type 

 in which a cube of glass is rotated at a high rate of speed (60,000 r.p.m. 

 for 4,000 pictures per second) between the camera lens and the sprocket 

 as shown in Fig. 1. The compensator cube has four polished faces, 

 each parallel to its axis of rotation and parallel to the axis of the 

 film sprocket. One picture is taken for each quarter revolution of 

 the compensator. The index of refraction of the glass and the dimen- 

 sions of the cube are chosen to cause correct movement of the image as 

 the film is continuously advanced past the exposure area of the 

 sprocket. The cube rotates in the direction of the arrow which is 

 opposite to that of the sprocket. Downward movement of the image 



