Patent GB-856006-A (GB)
Title: Improvements in measuring apparatus having optical and photoelectric means
Inventor: SCHUCH KURT
IPC: G01J1/42
Filed: 1956-12-31
Abstract: 856,006. Photo-electric distance-measuring apparatus. ZEISS JENA, C., VEB. Dec. 31, 1956, No. 39716/56. Class 40 (3). Photo-electric apparatus for measuring the displacement of an object along the optical axis of an optical system from a predetermined point on said axis comprises a positive lens forming a real image of the object and, in one embodiment Figs. 1-5, means for comparing the relative intensity of different portions of the light rays forming said image such that when the object is at said predetermined point said intensity ratio has a predetermined value, e.g. unity, any displacement of the object from said predetermined point causing a change in said ratio proportional to said displacement. In a second embodiment, Fig. 6, the image is oscillated along the optical axis so that the intensity of the light received by a photo-cell varies cyclically, the ratio of the light intensities corresponding to the two extreme positions of the image being dependent on the displacement of the object from its predetermined point. Preferably the apparatus includes optical compensating means which is adjusted to maintain said predetermined intensity ratio, the magnitude of said adjustment being proportional to the displacement of the object. First embodiment. In Figs. 1, 3, 4 and 5 the two portions of the image forming light rays comprise the axial and non-axial portions thereof and in Fig. 2 the two portions comprise different non- axial portions; in Figs. 1 - 4 the two portions are directed onto separate photo-cells providing a differential output and in Fig. 5 the two portions are directed sequentially to a single photo-cell. As shown in Fig. 1 a first image of a spot B 1  on the object 1 is produced at an image plane B 1  by a lens 2 and a second image is produced at an image plane B 1 <SP>11</SP> by means of a semi-transparent mirror 3, the light passing the image planes B 1 <SP>1</SP>, B 1 <SP>11</SP> being received by photo-cells 4 and 5 respectively. An opaque axial stop 6 in front of the image plane B 1 <SP>1</SP> cuts out the axial rays and an apertured screen 7 in front of the image plane B 1 <SP>11</SP> passes the axial rays, the relative dimensions and positioning of the stop 6 and screen 7 being such that when the object 1 is at a predetermined distance a from the lens 2 the cells 4 and 5 receive equal light intensities. In the absence of the optical compensator comprising a parallel plate formed of two glass wedges 8 and 9 any displacement of the object 1 from its predetermined position would alter the intensity ratio of the light received by the two cells 4 and 5 by an amount proportional to said displacement but as shown said intensity ratio is maintained constant by adjusting the thickness of the compensating plate by moving the wedge 8 transversely to the optical axis, the magnitude of said adjustment being proportional to the displacement of the object. In a modification, Fig. 2, the apertured screen 7 is replaced by an opaque stop 7<SP>1</SP> located behind the image plane B 1 <SP>11</SP>, the object 1 is illuminated by light projected thereon from a source 11 via the lens 2 and a semi-transparent mirror 10 interposed between the lens 2 and mirror 3 and the compensating means comprises an axially adjustable negative lens 12. In a further modification, Fig. 4 (not shown), the predetermined position of the object 1 is at the focus of the lens 2 and the resultant parallel beam of light is focused on to the image planes B 1 <SP>1</SP> and B 1 <SP>11</SP> by means of two additional lenses. In Fig. 3 the semi-transparent mirror 3 and screens 6 and 7 are replaced by a small fully silvered mirror 13 which reflects the axial rays to the photo-cell 5 and cuts out the axial rays from the light reaching the photo-cell 4. A modification, Fig. 5, uses a single photo-cell 20, the light passing around the mirror 13 being reflected onto the cell by a fully silvered annular mirror 21. A rotating shutter 22 having radial slits is mounted in front of the photo-cell 20 so that it receives the light from the mirrors 13 and 21 alternatively. The output from the photocell 20 thus comprises two interleaved pulse trains and the wedge 8 is adjusted to equalise the amplitudes of the two trains. Second embodiment. As shown in Fig. 6 an image of the spot B1 is produced at a single image plane B 1<SP>1</SP> and the image light is received by a single photo-cell 27, the axial rays being excluded therefrom by an axial stop 23. The image plane B i <SP>1</SP> is oscillated longitudinally along the optical axis by means of a variable thickness parallel-plate system comprising two wedges 24, 25, the wedge 25 being oscillated tranversely to the optical axis by means 26 so that the intensity of the light received by the photo-cell 27 varies cyclically, said intensity being a minimum when the image plane Bi<SP>1</SP> coincides with the stop 23 and the ratio of the light intensities corresponding to the two extreme positions of the image being dependent on the displacement of the mean position of the image plane B i 1 from the stop 25 and hence on the displacement of the object 1 from its predetermined point. Alternatively the stop 23 may be replaced by an apertured screen in which case the intensity of the light received by the photo-cell 20 is a maximum when the image plane Bi<SP>1</SP> coincides with the screen and the image plane may be oscillated by oscillating a negative lens along the optical axis. In any of the embodiments the compensating means may be adjusted automatically by the output from the photo-cell or cells.