Patent GB-620580-A (GB)
Title: Improvements in or relating to continuous-feed cinematograph projectors
IPC: G03B41/08
Filed: 1945-12-12
Abstract: 620,580. Kinematograph projectors. KUDAR, J. Dec. 12, 1945, No. 33587. [Class 97 (i)] In a continuous feed projector with a polygonal compensating prism and a light source producing a beam substantially parallel to the axis of the projection lens, there is inserted in the path of said beam a rotary beam deflector, synchronized with the rotation of the compensating prism, which deflects the beam so that the ratio of the angle of incidence of the principal ray at the prism face to the angle between the optical axis and prism face normal, is less than unity and substantially constant. In the form shown in Fig. 5, a 12-sided compensating polygon 4 is rotated between the projection lens 3 and film 1. A linear light source 2, with reflector 6 and condenser lens 7, is mounted within a transparent ring 8, round the outside of which the film is guided and which rotates at one-third of the speed of the polygon 4. The lens 7 is axially adjustable to allow for film shrinkage. The outside of the ring 8 is formed with 36 concave lens surfaces. The light source is preferably a linear incandescent or discharge lamp 2 with cylindrical reflector 6 and may be water cooled, the body of water acting as a condenser lens. The film may, alternatively, be guided in a straight path for projection. In a modification, a second condenser lens is provided, the rear surface being plane and the front surface conforming to the inside of the ring 8. The outer faces of the ring 8 can then be made plane. In a further modification, the light source is outside the ring and projects a beam along the axis of the ring whence a prism integral with the condenser lens reflects it into the plane of the ring. The compensating polygon may be hollow as shown at 20 in Fig. 10 and contain two plano-convex lenses 22, the convex faces 24 of which conform to the inner surface 25 of the polygon. To avoid chromatic errors each plano-convex lens may be built of a thin plano-convex crown lens and a plane parallel plate of flint glass of the same mean refractive index. The air gap 21 may be made variable, to vary the properties of the polygon to take account of film shrinkage. For this purpose the plane faces 23 are parallel, but slightly inclined to the polygon axis and to adjust the air gap one of them is moved axially. In a modification, reflecting prisms are secured to both lenses so that the light path in the air gap is in an axial direction. The adjustment of the lenses 22 may be effected automatically by the film itself passing over rollers on pivoted arms so that the arms are moved by any film shrinkage-this movement being coupled to the lens adjustment. The movement of the plane of the film as a result of the roller movement is utilized to assist the correction. Another correcting device shown in Fig. 16 consists of a drum 40 round which the film 1 passes with one row of perforations projecting into a light beam passing diametrically from a lamp 41 with condenser 42. Light passes through the perforations and is focused by the spherical lens 43 on to the perforations at the opposite side of the drum. According to the phasing of the passage of perforations at the two sides of the drum so more or less light falls on a photo-cell 44 which operates a servo system controlling the lenses in the compensator.