Patent GB-1249302-A (GB)
Title: Improvements in or relating to optical beam splitter devices and apparatus
Inventor: GARDNER PETER
IPC: G02B27/10
Filed: 1968-01-16
Abstract: 1,249,302. Detecting movement optically. MINISTER OF TECHNOLOGY. 5 April, 1969 [16 Jan., 1968], No. 2269/68. Heading H4D. [Also in Divisions G1 and G2] An optical beam-splitter for detecting movements of a real image comprises a periodic array including two sets of plane facets 12a, 12b and means for focusing the illumination incident on the sets of facets on to separate areas 14, 15 so that such separate area is associated with a separate one of the said facets and receives illumination from all the facets of the set. Preferably (Fig. 2) two such beam-splitters 24, 23 at right angles to each other are used. The beam-splitters 24, 23 have first surfaces comprising arrays 25, 26 of parallel bi-prisms and second surfaces comprising condenser lenses 23A, 24A. The periodic array is symmetrically constructed with all facets equally inclined to the axis of symmetry of the device. One beam-splitter 24 has one set of facets associated with a detector 29 and the other facets with a detector 30; and beam-splitter 23 has associated therewith photo-detectors 27, 28. Each pair of detectors has a common output terminal 32, 31 feeding frequency measuring circuits 37, 36 and summing and differencing devices 33, 34, which latter produces an analogue voltage fed with the sum signal to a circuit 35. If desired, a single beam-splitter may be used with the four sets of facets comprising the faces of similar pyramids. The device may be used in airborne photographic cameras to compensate image movements due to the forward motion of the aircraft. Thus in the Fig. 2 device the sum voltage is proportional to the image velocity in the direction of D, which bisects the angle between the arrays 25, 26. This bisector D is aligned with the direction of flight. The frequency measuring circuits 36, 37 each comprise a Schmitt trigger circuit feeding a monostable circuit an output of which is connected to an output terminal and to one plate of a capacitor also connected to earth. In operation the periodically varying voltage signal produced at 31, 32 is supplied to its respective Schmitt trigger circuit in 36, or 37. The trigger circuit changes its state each time the input signal exceeds a threshold level in either the positive or negative directions. This gives a periodic output pulse waveform which is fed to the monostable. The monostable produces a positive output signal of predetermined duration each time the output of the trigger circuit swings positively. The ratio of the predetermined duration of a positive pulse to the time interval between successive positive pulses in the monostable output is proportional to the frequency of its respective signal at 31, 32 within set limits. The output voltage is used to modify the action of the camera film control mechanism. Alternatively it can be used to move the camera system when a stationary image is produced on the plate; or where the scene is viewed through a television camera to produce a corresponding shift in one of the camera time base deflection circuits.