Patent GB-1368819-A (GB)
Title: Automatic focusing device
IPC: G02B7/28
Filed: 1971-11-25
Abstract: 1368819 Automatic focusing apparatus NIPPON KOGAKU KK 25 Nov 1971 [4 Dec 1970] 54809/71 Heading G1A An automatic focusing provision in lens testing apparatus includes an arrangement of two spaced, rotating discs provided with grid patterns. The system shown in Fig. 2 includes a lamp 17, a target member 18 having a slit (18b), an optical system 22 forming an image of the slit somewhere between the rotary discs 12, 11, and a lens 27 plus photo-cell 28 detecting light passing through discs 12, 11. The latter are of the form shown in Fig. 4, disc 12 comprising one half which is completely transparent and the other bearing a grid pattern of opaque and transparent portions 12a, 12b, and disc 11 comprising one completely transparent half and the other bearing a similar grid pattern to that of disc 12. The grid of one disc is aligned with the transparent half of the other, and the discs rotate in synchronism. The grid patterns are arranged to rotate through and thus alternately act to chop the main optical path. In this way, if the positioning of the optical path elements is such that the infocus image of slit 18b is nearer to a particular disc, this one allows more light to pass through its grid pattern portion (when this is effective). This will cause an output from the photo-cell 28, Fig. 6a, b or c, (not shown) modulated at the common frequency of the grids but whose envelope defines a square waveform whose phase indicates the proximity of the focused image to one of discs 12, 11, except when the image is equally spaced from both when the square wave disappears. An auxiliary photo-electric system comprising lamp 25 and detector 26 monitors the edges of discs 12, 11 the latter of which includes an opaque section 11C, the purpose being to derive a  reference signal indicating which disc grid pattern is effective at a particular time. This is compared with the output of amplifier 29, which is the photo-cell 28 output free of grid modulations, in phase difference meter 31. When the signals are either in phase or in anti-phase, the member in the optical path e.g. the target 18 is moved respectively in one direction or the other along the optical axis to tend to centre the image of the slit between the two discs, in which centralized condition the square waveform envelope on the photo-cell 28 output disappears and a zero output from 31 is obtained. In a lens testing procedure, a reference lens is located at position 21; automatic focusing takes place by e.g. movement of slit 18b; the reference lens is removed and replaced by the lens to be tested; compensating movement of e.g. the slit again takes automatically to regain focus; and the movement along the axis of the controlled member is a function of the strength of the lens under test as compared with the reference. Alternative arrangements. Instead of operating on the slit position, the servo-drive may be connected to the optical system 22 or to the support arrangement of the discs to move these bodily along the optical axis. The rotary discs may alternatively comprise a plurality of sectors consisting of alternate transparent and grid pattern portions. They may comprise mirror portions instead of transparent and thus light is reflected to the photo-cell 28.