1.  Field of the Invention
The present invention relates to a photographic film cassette.  More particularly, the present invention relates to a photographic film cassette an entire roll including a leading end of photographic film is contained in a cassette shell, and rotation of a spool causes the leading end to advance to the outside. 
2.  Description Related to the Prior Art
In a conventional photographic film cassette containing 135 photographic film, a leading end of photographic film is protruded from inside a cassette shell even before use of the photographic film.  To load a camera with the cassette, a user is required to handle it with while paying attention not only to the cassette shell but to the protruded leading end.  To overcome this difficulty of the conventional photographic film cassette at the time of inserting it into a camera, it has been proposed to utilize a cassette in which a leading end of photographic film is initially contained in a cassette shell, and rotation of a spool causes the leading end to advance to the outside, as disclosed in U.S. Pat.  Nos.  4,832,275 and 4,834,306 and a commonly assigned U.S. Pat.  No.  5,213,277. 
FIG.  12 illustrates the device disclosed in in U.S. Pat.  No.  4,832,275.  A cassette shell 52 (partially shown) contains a spool 50 rotatably.  Photo film 51 is Wound on the spool 50 and forms a photo film roll.  There are a pair of roll-regulating rings 53 and 54 disposed around the outermost turn of the roll and slanted relative to the roll, so as to prevent the roll from loosening.  The rings 53 and 54 spread in a passage port 55 having a port mouth (not shown), so that a leading end 51a of the photo film 51 is passed between the rings 53 and 54.  The rings 53 and 54 are rotatably received in annular grooves formed in an inner cylindrical face of the cassette shell 52 in slant orientations.  A pair of separating claws 56 and 57 are formed integrally with the passage port 55 to be in contact with the outermost turn of the roll. 
Upon a start of rotating the spool 50 in the unwinding direction, the roll, if there is a small gap between the roll and the rings 53 and 54, may come to loosen to a small extent, but is at once stopped from loosening by the rings 53 and 54 in course of the rotation of the spool 50.  The roll and the rings 53 and 54 are rotated together with the spool 50.  The leading end 51a comes near to the passage port 55, is separated by the separating claws 56 and 57 from the roll, and exited from out of the cassette shell 52 through the passage port 55. 
The roll-regulating rings 53 and 54 are replaced, according to U.S. Pat.  No.  5,213,277, with a pair of annular ridges formed on an inner cylindrical face of the cassette shell.  Any constructions structions of such leader-advancing photo film cassettes, such as disclosed in relevant prior documents, commonly require a structure for separating the leading end from the roll of the photo film, while coming in contact with the outermost turn of the roll
However the known separating claws have a low reliability of success in separation of a leading end from the roll.  This arises from difficulties in manufacturing the photo film while keeping the thickness of the photo film uniform with great precision. 
It is frequent that there are minute changes in amounts of emulsion for application to the photo film, and changes in thickness of a support material being manufactured.  There thus rises a substantial difference in photo film between the estimated entire thickness and an actual entire thickness.  Such a difference can amount to over 5% of the estimated entire thickness.  Should a separating claw be formed in a somewhat large size with an estimation that the photo film may have a small thickness, there would be a problem in that the separating claw would scratch a roll of photo film having a larger thickness unexpectedly. 
It can be also pointed out that, if a separating claw is formed in a somewhat small size with an estimation that the photo film may have a large thickness, there is a problem in that a roll of photo film having a smaller thickness unexpectedly does not come in contact with the separating claw. 
Severe environment also influences-the performance of the claw in separating the leading end.  When the roll of the photo film contained in the cassette shell is kept at high temperature or high humidity, the turns of the roll are tightened to have a small diameter irrecoverably.  There are occasions on which the small roll of the photo film does not come in contact with the separating claw, for example in a hot condition of 80.degree.  C. inside an automobile subjected to daylight in midsummer, or in a dry condition of 15% RH or below
Long preservation of the roll of the photo film contained in the cassette shell further influences the roll by tightening the turns to have a small diameter.  The roll is given a tendency to curl irrecoverably in the entire length o#the film.  It is difficult or impossible for the small roll of the photo film to come in contact with the separating claw.  Should the leading end be exited successfully, it would be highly difficult to keep the film flat within a camera loaded With the photo film. 
There is an occasion on which, when a camera is loaded with the cassette, the photo film fails to move for initial transportation within the camera.  The camera has an exposure aperture for effecting exposure, and a back lid having a pressure plate behind the exposure aperture.  The pressure plate supports the photo film on the rear of an exposure aperture.  A thin space, defined between the exposure aperture and the pressure plate for passage of the photo film, is as thin as 0.2 mm for the purpose of flattening the photo film within the camera.  In use of the leader-advancing cassette, the photo film having been exited from the cassette may fail to be inserted into the passage space.  There is a fear that the leading end might come in contact with a wall, be flex back by the wall, and fail to advance further. 
Under conditions of high relative humidity of 80% RH, stiffness, namely characteristic resistant to deformation, of photo film is lowered remarkably.  There are occasions in which turns of the photo film roll, lying one over another while wound on a spool, are stuck together nearly inseparably, in particular under an increase in friction between turns of the roll, and after long preservation under high humidity.  When the turns of the photo film absorb a great deal of moisture, they may be thickened to increase the diameter of the roll inside a cassette shell, until it becomes difficult or impossible to rotate the roll of the film relative to the cassette shell.  This causes difficulties in rotating a spool of the leader-advancing cassette.  Should a rotational torque be applied to the spool, the roll while kept nearly immovable within the cassette shell would concentrate application of rotational load upon a trailer of the photo film secured to the spool.  Increase of such load on the trailer would flex the trailer in the direction reverse to the turns of the roll, and prevents the trailer from transmitting the rotation of the spool to the roll.  It follows that there are drawbacks in known leader-advancing cassettes in reliability of outward advancement of the leading end.