Patent GB-498378-A (GB)
Title: Improvements in or relating to optical testing or measuring apparatus
IPC: G02B21/00
Filed: 1937-06-02
Abstract: 498,378. Optical testing-apparatus. KAPELLA, Ltd., WARMISHAM, A., and REASON, R. E. June 2, 1937, Nos. 19538/38 and 19539/38 (both divided out of 498,382), and 19540/38. [Class 97 (i)] An optical testing-apparatus suitable for testing screw threads and the like, comprises means, such as a slit or knife edge for defining the profile of a plane or curved section of the object, and a lens for obliquely projecting an image of the profile from a surface the line of intersection of each elementary portion of which with one nodal plane of the lens is parallel to and at the same distance from the optical axis of the lens as the line of intersection of the corresponding elementary portion of the image surface with the other nodal plane of the lens. Light from a source A passes through a condensing lens A<1>, a small aperture A<2>, and an objective A<3>, emerging as a parallel beam which casts a sharply defined shadow of a knife edge A<5> on a screw thread A<4>, thereby defining a section plane on the thread, which may be whitened to increase the brightness. The nodal planes of a projecting lens A<8> intersect the section plane A<6> and the plane of the screen A<7> in two parallel lines equidistant from the optical axis of the projecting lens. In the figure, the nodal planes are shown as coincident. The image on the screen A<7> is distorted due to the oblique projection, and measurements thereof are subject to calculated corrections. The screen A<7> may be replaced by a photographic plate. In Fig. 2, the distortion is corrected optically. A slit B replaces the knife edge A<5>, and the light is reflected by a mirror B' on to the object, the section plane on which is indicated by A<6>. A focused but distorted image is projected by a lens B<2> on a diffusing surface B<4> and this is reprojected by a lens B<3> on to a screen B<5> to correct the distortion. The magnifications of the two projecting lenses may be different provided that each satisfies the condition regarding its nodal planes referred to above. In Fig. 3, not shown, a knife edge is focused on the object by an objective provided with a narrow slit. The image formed on a photographic plate is reprojected, Fig. 4 (not shown), on a flat screen in a position relative to the plate corresponding to that occupied by the section plane. An enlarged photograph may be taken of the rectified image. This method corrects lack of flatness in the plate. The arrangement of Fig. 1 may be used for direct viewing of the profile, but is not suitable for measurement by means of a graticule in the focal plane of the eyepiece since the image is subject to keystone distortion.. A collimating field lens may be placed between the section plane and the projecting lens, whereby the image is subject only to rectangular distortion. In Fig. 5, a distorted image of the graticule is viewed simultaneously with the distorted image of the profile. A partly transparent reflector D is arranged between the section profile D<1> and the tilted projecting lens D<2>. The graticule D<3> is placed in a plane which is the image of the section plane in the reflector D, and is illuminated by a source D<4>. Collimating lenses D<6>, D<7> may be provided. Superposed and equally distorted images of the section profile and the graticule are formed in the image plane D<8> for viewing by the eyepiece D<9>. In another method of superposing the graticule and profile images, Fig. 6, a rectified image of the section profile D<1> is obtained in an inclined image plane E by the oblique projecting system described in Specification 498,419, comprising a projecting lens E<1> between two similar collimating lenses E<2>, E<3> whose principal foci coincide with the nodal points of the projecting lens. A graticule is arranged in the image plane E and a distorted image of the two is formed by the projecting lens D<2> in the plane D<8> for viewing by the eyepiece D<9>. Another system, Fig. 7 (not shown), is described for producing a rectified image for direct viewing. A rectangularly distorted image is first obtained and is then corrected by cylindrical lenses. For testing the pitch of a screw thread, the image of one tooth may be projected, and the thread then moved through the pitch distance to obtain the image of the next tooth. These may be compared by photographing in superposed position, or the second image may be reversed by rotating the knife edge used for illumination, through 180‹. Alternatively, two light sources, G, G<1>, Fig. 8, each with a condenser G<2>, G<3>, and knife edge G', G<5>, are employed with a transparent reflector G' and a single objective G<6>. Either knife edge may be used by rotating a shutter G<8>, one side or the other of the section plane being thereby illuminated. In another form, Fig. 9 (not shown), two knife edges forming the two edges of a slit are used with a single light source, the edge desired being selected by interposing in the beam a sliding traitsparent double wedge, or a tilting plate. The apparatus is applicable for determining whether two co-operating objects, e.g. a worm and worm wheel, will mesh properly. Two oppositely directed illuminating devices are used, for the worm and worm wheel respectively, each being tested with the other removed. The two images are projected, e.g. by the device shown in Fig. 2, on to the same screen. To avoid interference due to unwanted light from other parts of the object, the illuminating beam may be polarized, and a crossed analyzer used in the projecting beam so that the polarized reflected light is cut out and the depolarized diffused light is transmitted. A suitable polarizing device is described in Specification 412,179. The apparatus may be used for the examination of section profilles of hollow or deeply reentrant objects by the use of reflectors as described in Specification 498,378. Specifications 498,377, 498,379, 498,382, and 498,420 also are referred to.