Journal of the Society of Motion Picture Engineers (1930-1949)

12 LOUIS DUNOYER [J. S. M. P. E. gation also applies to the case when apparatus without slit is used, provided that the illumination of the exploring zone is uniform. We also suppose that the exploring zone and the film do not coincide, and we shall examine the effect of the degraded illumination of the explored zone. In both cases it is necessary to form a hypothesis of the manner in which the transparency of the film varies along its length or, which is the same, the manner in which the total luminous flux would vary, which would go through the film if it were explored by means of a zone infinitely narrow in regard to the length occupied by a period of the transparency. In the case of a variable density film this transparency is the same as that of the film. For a constant minimum film \trans parency .. Y overage transparency FIG. 8. Curve representing sinusoidal variation of the film transparency along its length. density film it is defined by the ratio between the width of the part which is entirely clear and that of the opaque part. The most natural hypothesis which we can choose for the law of variation of the transparency of the film is that of a sinusoidal variation which would correspond to the perfect recording of a musical sound. In Fig. 8 the curve representing the variations of the transparency of the film along its length is plotted on the right. The abscissa XQ of the beginning of a period in regard to the axis of the lighting apparatus defines a given position of the film. Its transparency y at a point M, with the abscissa x, of the explored zone then will be expressed by: x) y = a + b sin (5)