GB831201A - Improvements in or relating to optical measuring devices - Google Patents

Abstract

831,201. Optical testing apparatus. OPTICAL MEASURING TOOLS Ltd. Jan. 24, 1958 [Oct. 26, 1956], No. 32751/56. Class 97(1) Optical apparatus for determining distances (e.g. focal length of a lens or height of an object) comprises a lens holder movable relatively to an object holder for focusing and, on the side of the lens holder away from the object, two optical elements for bringing together two beams passing through different portions of the lens in the holder so that, when said lens is in focus, the images formed by these beams are superposed. For determining the dimensions of a work-piece 12, the apparatus may take the form shown in Figs. 1 and 2 with the work-piece mounted below the objective holder 19 of a microscope on a slide movable on a table 44 which is itself movable at right angles to the slide on a base 31, each movement being controlled by a screw against which the slide or table is spring pressed. The microscope is clamped to a carriage 37 movable vertically by fine and coarse adjusting knobs and having a projection 58 contacting a feeler 57 slidably operating a dial indicator 56. The objective holder 19 accommodates a meniscus lens 15 cemented to a concavo-convex objective 11 having a central reflecting portion 14 and peripheral reflecting portions 13 so that light from the work-piece 12 is doubly reflected within the objective and emerges therefrom as a double beam 16, 17. Behind the objective is a prism 21 having inclined faces 22, 23 in the path of each beam 16, 17 so that the latter pass to the eyepiece 28 along the same path through a lens 26 and prism 29. The two beams form superposed images when the objective 11 is correctly focused on the work-piece, the reading on dial 56 at this time indicating the dimension of the work-piece. In the form shown in Fig. 4, an illuminated scale 74 is carried by a slide 63 and a holder 60 for a lens 80 is carried by a slide 61, the slides 61, 63 being movable on parallel guides 65, 66 fixed to end castings 67, 68 which are movable on rollers 71 in a direction perpendicular to the length of the guides. Slide 61 carries a movable crossslide 83 which supports an iris diaphragm 88, a mirror 84 and a beam-combining cube 85. The mirror and cube serve to combine two parallel beams from lens 80, the combined beam passing to an optical goniometer 64 mounted on a divided circle 90. With the scale 74 in the focal plane of lens 80, cross-slide 83 is moved to bring the diaphragm 88 into the light path and the goniometer is used to measure the angle # between two of the lines of the scale which are symmetrically disposed with respect to the optical axis of the lens. The focal length F of lens 80 is then deduced from the relation F = S, where S 2 tan # is the distance between said two lines.