Abstract:
A rotary gate for digitally scanning frames of a motion picture film has a motor drive wheel and a coaxially spaced free wheel which support and engage sprocket holes in the film. The free wheel has no connection to the driven wheel other than the film which rotates the free wheel as it is driven by the drive wheel for advancing the film through the gate. A light source and sensor are provided on opposite sides of the film for illuminating the film and digitally scanning motion picture frames. Sprockets on the drive wheel are profiled to stabilize the film laterally while it is advanced for scanning of each frame. A fixed disc with a smooth track my be substituted for the free wheel for supporting the film.

Description:
This Appln claims benefit of Prov. No. 60/157,373 filed Oct. 1, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to transport systems for digital scanning of the frames of a motion picture film. More specifically, the invention is directed to a method of, and the construction of apparatus for transporting film without slack or interference. 
     Prior art film transports have apparatus between the sprocket wheels which can interfere with even illumination of the film from behind, and with unobstructed sensing of the light transmitted through the film by a CCD. 
     Slack in the film is also caused when axially displaced sprocket wheels are used due to differences in rotational speed between the sprocket wheels. Pull-down claws, as are used in the prior art, also cause unevenness of movement and bunching of the film. 
     U.S. Pat. Nos. 5,088,813 and 5,548,328 to Wakefield disclose a scanner in which film is transported over two axially aligned spaced wheels and in which the film is illuminated from behind and the light transmitted through the film is focused by a lens onto a CCD. However, Wakefield&#39;s wheels are not able to have the film transmit torque from a driven wheel to a free wheel. Instead, Wakefield uses two respective friction belts which engage respective surfaces of the two wheels, both belts being driven by a common shaft. The belts and shaft can be obstacles to optimum placement of the film illumination source and scanning sensor. Also, slippage between the belts and shaft can result in the application of a net torque which can deform the film and prevent attainment of proper positioning for distortion-free scans. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems of the prior art are overcome by the present invention which provides a transport system for high speed digital scanning of the frames of a motion picture film, referred to as a “rotary gate”. In a rotary gate for film scanning, according to the invention, two axially aligned film sprocket wheels are spaced apart in parallel planes with no connection between them. The bottom sprocket wheel is rotatably driven by a motor and engages the bottom row of sprocket holes of the film for transporting the film from a supply reel to a take-up reel. The top sprocket, which is free-wheeling, engages the top row of sprocket holes in the film and is rotatably driven by the film while keeping the film taut for scanning of the individual frames. 
     Because there is no apparatus between the sprocket wheels, it is possible to illuminate the film from behind and sense the light transmitted through the film with a CCD, with none of the slack caused by axially displaced sprocket wheels, and with no interference from intermediate transport mechanisms such as a pull-down claw. Since the film is always maintained taut on the sprockets, registration of the film can be assured. 
     In an alternate embodiment of the invention, the film is suspended between a driven sprocket wheel and a stationary disc having a smooth track over which the upper margin of the film can glide as it is advanced by the driven wheel. 
     In order to stabilize the film in the direction of advancement, as well as laterally, the sprockets are provided with a cross sectional profile which facilitates their entry into the film perforations and enables them to tension the film to prevent unwanted movement of the film during each scan of a frame. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of a portion of the preferred embodiment of the invention; 
     FIG. 2 is a front elevation view of the preferred embodiment of the invention of the invention in its intended environment; 
     FIG. 3 is a side elevation view of the preferred embodiment of the invention of the invention in its intended environment; 
     FIG. 4 is a side elevation partial view of an alternate preferred embodiment of the invention; 
     FIG. 5 is an enlarged elevation view of one of the film perforations shown in FIGS. 1-3; 
     FIG. 6 is an enlarged end view of one of the sprockets shown in FIGS. 1-4; and 
     FIG. 7 is an elevation view of the interaction of the film perforation of FIG. 6 with the sprocket of FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 of the drawings, a free wheeling sprocket rotary gate  1  for motion picture film has a drive wheel  3  of the type used for transporting film  5  mounted on a shaft  7  rotatably driven by a motor  9 . Sprockets  11  on the wheel  3  engage corresponding sprocket holes or perforations  12  in a margin running along a bottom edge of a segment of a continuous web of film  5 . The film  5  is wound on a supply reel, and transported to a take-up reel. The supply reel, or the spindle (not shown) on which it is mounted, is preferably fitted with a friction mechanism, or other drag inducing device, to prevent coasting of the supply reel so that the film  5  is maintained under tension as it is advanced from the supply reel to the take-up reel. 
     Suspended from above, and in coaxial spaced relationship to the sprocket wheel  3  is a free wheel  13 . Sprockets  15  on the free wheel  13  engage corresponding sprocket holes or perforations  14  in a margin running along a top edge of a segment of the continuous web of film  5 . The wheel  13  is mounted on a shaft  17  which is journalled in a bearing for rotation in response to the force exerted by the film  5  against the sprockets  15  as the drive wheel  3  advances the film from the supply reel to the take-up reel. 
     The sprocket wheels  3  and  13  are spaced a distance apart so that the distance between the sprockets  11  and sprockets  15  is equal to the distance between the sprocket holes  12  in the lower margin of the film  5  and sprocket holes  14  in the upper margin of the film  5 . The film is tensioned by the sprockets  11  and  15  against bowing thereby ensuring that each frame  19  of film is substantially planar and parallel to the common axis of the sprocket wheels  3  and  13  as the frame passes a viewing position whereat a maximum number of the sprocket holes  12  and  14  are respectively engaged by the sprockets  11  and  15 . 
     Referring now to FIG. 2 of the drawings, a light source  21 , e.g., a projection lamp is mounted for directing a beam of light at a sensor in the form of a digital camera  24  having a charged coupled display (CCD) chip  25 . The lamp  21  and camera  24  are mounted so that the center of the beam of light passes through the common axes of the sprocket wheels  3  and  13  at an elevation equal to that of each film frame  19  when in the viewing position. That is, the light source  21  is mounted for projecting light along an axis in a plane intermediate the parallel spaced planes in which the drive wheel  3  and free wheel  13  are rotatable for illuminating the film  5 . The CCD  25  is mounted so that its center receives the central ray of light from the lamp  21 , the sensing surface of the CCD  25  being parallel to the frame  19  when the latter is in the scanning position. 
     As each frame enters the viewing position, the CCD  25  is scanned or read for digitizing the image on the frame as will be known to those skilled in the art. 
     The sprockets  11  and  15  must have a cross sectional profile which is smaller than, and which can be contained within, the profile of the film perforations  12 ,  14  so as to facilitate entry of each sprocket into a corresponding film perforation as the film is advanced. 
     A smaller profile for the sprocket relative to the perforation leaves a clearance with can enable free play between the film and sprocket resulting in shifting of the film in its plane and misalignment of a film frame with the light source  21  and camera  24 . The sprockets  11 , 15  of the invention have been designed to have a profile which prevents such shifting and resulting frame instability and misalignment during scanning. 
     A standard 35 mm film frame has the profile of a circle, 0.11 inches in diameter, truncated by diametrically opposed parallel straight edges spaced 0.073 inches apart. The length of each parallel side is 0.082 inches. For use with 35 mm motion picture film, each sprocket  11 ,  15  has a cross section with a frontal flat side  41  having a length equal to the length of the flat side  41  of the film perforation  12 , 14 , i.e., 0.082 inches. Opposite side curved surfaces  42 , 43  of each sprocket extend from the flat side  41 , tapering outwardly in conformity with the corresponding edges of the film perforation until maximum separation between the surfaces  42  and  43  is reached at a distance slightly less than one half the distance between the straight sides of the perforation. The side surfaces  42 , 43  have a radius of curvature equal to the radius of curvature of the perforation proximate the front straight surface  41  and bow inwardly where they intersect with a rear side  44  of the sprocket. 
     The rear, optionally curved, surface  44  of each sprocket  11 , 15  extends from the regions of maximum separation of the side curved surfaces  42 , 43 . In the preferred embodiment of the invention, as applied to 35 mm film, the maximum width of the sprocket cross section, i.e., the distance between the side curved surfaces  42 , 43  is 0.1050 inches as shown in FIG.  6 . 
     As can best be seen in FIG. 7, while penetrating a film perforation  12 , 14 , each sprocket  11 , 15  is constantly urged against the front and side edges of the perforation which is has penetrated. Each penetrated perforation therefore is held centered with respect to its sprocket. The rear side of the sprocket does not engage with the film, hence, the distance from the front flat surface of the sprocket to the rear surface is not critical. The front of each sprocket should, however, be the same size as the front of the perforation. The diameter of curvature of the side walls of each sprocket, adjacent the front wall  41  preferably should match the diameter of the perforation. 
     In the event of film shrinkage, even if the width of the front side  41  of the sprocket prevents it from meeting the front of the perforation, the sprocket will still be centered in the perforation by the symmetric curved sides  42 , 43 . To this end, ample space is provided between the rear surface  44  of the sprocket and the rear edge of the perforation to allow for shrinkage of the film base. Upon shrinkage the front wall  41  of the sprocket may not engage the corresponding front edge of the perforation. However, due to symmetry, the side walls  42 ,  43  of the sprocket will snugly engage the corresponding side edges of the sprocket where they taper outwardly, with the front edge  41  of the sprocket in parallel spaced relationship to the front edge of the sprocket. The net force exerted by the sprocket against the film will be in the forward direction with no freedom for lateral shifting of the film, i.e., in a direction transverse to the forward direction. 
     Referring now to FIG. 4 of the drawings, there is shown an alternate arrangement for a rotary gate in which a wheel  3 ′ is driven by a motor in similar arrangement to that shown in FIGS. 1-3. However, unlike the rotary gate of FIGS. 1-3, the rotary gate of FIG. 4 employs a fixed disc  13 ′ which has a stepped diameter forming an inner cylinder  31 ′ and an axially displaced outer cylinder  33 ′. Although the disc  13 ′ can be rotatably mounted similarly to the sprocket wheel  13  of FIGS. 1-3 and optionally rotated upon engagement of its track by frictional contact with the film, it is preferably fixed relative to the housing of the motor  9  for simplicity, reduced cost, and critical accuracy in positioning the film. 
     The circumference of the inner and smaller diameter cylinder  31 ′ is smooth and serves as a track over which the upper margin of the film  5 ′, with sprocket holes  14 ′, glides as the film  5 ′ is advanced by rotary action of the motor-driven sprocket wheel  3 ′ with sprockets  11 ′ engaging sprocket holes  12 ′ in the film&#39;s lower margin. 
     The diameter of the outer and larger diameter cylinder  33 ′ is greater than the diameter of the inner and smaller diameter cylinder  31 ′ so that it forms a circular shoulder or lip which extends, radially, beyond the circumference of the track  31 ′ and has a bottom surface  35 ′ which acts as a stop for preventing movement of the film  5  in a lateral direction, i.e., transverse to the direction along which the film is intended to be advanced. 
     It is to be appreciated that the foregoing is a description of a preferred embodiment of the invention to which variations and modifications may be made without departing from the spirit and scope of the invention.