Patent GB-498420-A (GB)
Title: Improvements in or relating to optical projection apparatus
IPC: G02B21/00
Filed: 1937-09-24
Abstract: 498,420. Optical-projection apparattus. KAPELLA, Ltd., WARMISHAM, A., and REASON, R. E. Sept. 24, 1937, No. 25955. [Classes 97 (i) and 98 (ii)] Apparatus for projecting an image of a plane profile comprises an optical system arranged to produce a rectified image free from distortion due to oblique projection, means for rotating the system about an axis inclined to the direction in which the profile is viewed, and means for maintaining the correct angular relationship between the optical axis of the system and the image and object planes, whereby partial images of the profile obtained in different rotational positions of the optical system will be properly correlated with one another in the image plane. In Fig. 1, the section plane A is illuminated by focusing thereon an image of a knife-edge A<1> by means of an objective A<2> having a diaphragm with a small aperture A<3> which may be in the form of a narrow slit. The plane A is obliquely projected without distortion on to a photographic plate B by a system of the kind described in Specification 498,419, comprising a projection lens B<1> and lenses B<2>, B3 having their principal foci in the nodal planes of the lens B<1>, the system preferably being symmetrical so as to give unit magnification, and the planes A, B being equally inclined to the axis. The system B<1>, B<2>, B<3>, together with the plate B, is mounted for limited rotation about an axis C, which may be at right-angles to the optical axis. To maintain the image stationary and sharply focused during such rotation, the plate B is at the same time rotated relatively to the optical axis about an axis C<1> parallel to the axis C at the same rate as that of the system about the axis C, so as to maintain the planes A, B equally inclined to the optical axis. To enable a stationary photographic plate to be used, reflecting devices may be included in the rotating optical system. In Fig. 2, the plate B is fixed at right-angles to the plane A and the axis of rotation C is inclined at 45‹ to both planes. The optical system comprises, in addition to the lenses B<1>, B<2>, B<3> of Fig. 1, two reflectors D, D<1> each of which deflects the rays through a right-angle. Other arrangements using the same lens system with reflectors are described; in one the planes A, B are parallel and the axis C is inclined. at 45‹ to both ; in another the surfaces A, B are in the same plane which also contains the axis C ; while in a further form the planes A, B are parallel and both are at right-angles to the axis C. In Figs. 10 and 11 the projection is effected by a wide-angle lens H, the planes A, B being parallel to each other and to the nodal plane of the lens. The lens H and plate B are mounted in a frame J rotatable about the axis C at right-angles to the plane A, and the plate B is rotatable in the frame and is kept in a constant orientation, to obtain correlation of the partial images, by a parallel linkage consisting of an arm J<1> pivoted at J3 to the rotatable carrier J<2> of the plate B and at J<4> to the fixed base. The form shown in Fig. 13 consists of a lens K<1> with reflectors K<3>, K<4> for forming on a diffusing screen K a focused but distorted image of the plane surface A, and another lens K<2> with reflectors K<5>, K<6> for reprojecting on to the fixed plate B the image on the screen K at such an angle as to correct the distortion. The surfaces A, K, B are parallel to each other, and the projection system is rotatable about the axis C at right-angles to these surfaces. Specifications 498,377, 498,378, 498,382, and 498,458 also are referred to.