Patent GB-1076166-A (GB)
Title: Improvements relating to image pick-up devices
Inventor: VOLES ROGER
IPC: H04N3/30
Filed: 1963-03-21
Abstract: 1,076,166. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. March 23. 1964 [March 21, 1963], No. 11208/63. Heading H4F. The invention relates to apparatus for obtaining images under low lighting conditions of a scene which is in motion with respect to the apparatus, e.g. the apparatus is mounted in an aircraft, the images being compensated for variation in the attitude, speed and altitude of the aircraft. Fig. 3 shows an embodiment of the invention which is arranged to scan successive strips such as CD, Fig. 1, with a small rectangular frame E, the aircraft being at the point P flying along the line PA at a height h and speed v having roll, pitch and yaw angles at #, P and γ respectively. In Fig. 3, electrical signal paths are denoted by solid lines and mechanical couplings by dotted lines, the rectangles which are marked with mathematical symbols or expressions being potentiometers which multiply their input signals by the magnitude and sign of the symbol or expression. Light received from frame E after being applied to the rotating mirror 2 in the direction of arrow 1 is focused by lens 3 on to the photo-cathode of image intensifier 4 and the intensified image produced on the fluorescent screen of the intensifier 4 is directed by lenses 5 and 7 and Dove prism 6 on to the photocathode of camera tube 8. Mirror 2 is driven by motor 9 which also rotates the Dove prism 6 at half the speed of the mirror 2. The circuit arrangement of Fig. 3 is described in detail in the Specification but in general the video output signal from camera tube 8 is applied via gate 14 to the control electrode of cathoderay tube (C.R.T.) 15, the scanning waveforms for C.R.T. 15 being derived from the waveforms for camera tube 8. Gate 14 is controlled by the output of discriminator 28 and inhibits the passage of signals through the gate 14 when the output of block 17 exceeds a threshold value of K (see later). A signal representing is applied to terminal 29, f being the focal length of lens 3 and d the diameter of the photo-cathode of intensifier 4, and after being multiplied by #/h is used to determine the slope of the saw-tooth generated to be fv/h. A square wave of half the frequency of the saw-tooth on conductor 36 is produced on conductor 41 by generator 35 and applied to image intensifier 4 as a blanking signal (see later) and via differentiating network 42 to the raster generator 16 as a synchronizing signal. The image on the screen of C.R.T. 15 is focused for recording by lens 55 on to a strip of photographic film 56 moving at a speed representing v cos γ. The distance along the track of the aircraft is represented by the co-ordinate along the film strip and the angle of depression Á is represented across the film strip. So that the scanned strips such as CD are displayed side-by-side across the screen of the C.R.T. 15 a step waveform is applied to the C.R.T. from scanning circuit 23. In operation the image of the rectangular frame E appears on the photo-cathode of intensifier 4 inclined at the angle of the mirror 2. A landmark on the ground within frame E will move across the frame as the mirror rotates and the image of the landmark on the photo-cathode will move down the image of the frame parallel to the short sides of the frame. So that an intensified image of a fixed patch of ground appears on the fluorescent screen of intensifier 4, although rotating with mirror 2, a steadily increasing translation in a direction which rotates with mirror 2 is applied to the intensified image. This translation is produced by means of a saw-tooth deflecting waveform multiplied by the sine and cosine of the angle of mirror 2. Thus during the steady rise of the saw-tooth a fixed image is applied to the photocathode of the camera tube 8, which changes rapidly to a new image during the flyback of the sawtooth, the sawtooth being of such an amplitude as to cause a shift of half the height of the image of the frame E, so that an image of a completely new section of strip CD appears every two cycles of the sawtooth waveform. Dove prism 6 removes the rotation displacement of the image. The phosphur and photocathode of intensifier 4 have short lags while the photo-cathode of tube 8 has a long lag to allow for integration. The blanking signal on conductor 41 is applied to the final stage of intensifier 4 for the period of reading the image on the photo-cathode of the camera tube 8 in order to produce a video signal which is not contaminated, the reading time being equal to the integration time. To obtain a constant scale for distances along the track of the aircraft, i.e. transverse to strip CD in the film record, it is necessary to control the amplitude of the scan along the track of the aircraft on the C.R.T. 15 in response to the angle of depression Á. To achieve this the video signal for the portion of the lines for which Z sin Á<1/K only is passed by gate 14 where Z is the instantaneous value of line deflection. To avoid striations the video signals allowed through gate 14 may be arranged to overscan with a linear tapering off in brightness at the edges. To obtain exact registration of successive mirror scans the two-mirror scanner 2 must rotate at a velocity A quadrature sawtooth component is applied via blocks 37 and 40 to intensifier 4 to improve the resolution and reduce smearing caused by the velocity induced across the frames. Some smearing is produced due to the yaw γ reducing the effective forward velocity to v cos γ and inducing a lateral velocity of v sin γ, this is reduced by means of blocks 37, 40, 44, 46 and 47. Roll # is taken account of by applying p to the shaft 54 in the correct sense. Due to pitch P the effective height of the aircraft is changed and this is taken account of by blocks 32 and 33 and also a velocity is induced normal to the scanner axis and this is taken account of by blocks 37, 38, 40, 43, 49, 50 and 51. Because yaw and pitch induce changes in the effective rate of scanning the frame frequency is adjusted to correspond, block 52. Due to drift, i.e. yaw γ the detail in each strip such as CD has a sawtooth distortion and thus if the aircraft flies over a straight road the recorded image of the road appears as a series of disconnected short sections inclined at the angle of yaw γ to the length of the film strip 56 (Fig. 2, not shown). To remove this the picture is sheared and this is done by means of blocks 21 and 22. By using two cameras alternately to integrate the intensifier output via Kerr cells a continuous output at half the rate used in the above may be obtained, thus doubling the integration time. A storage tube may be used instead of a film strip and instead of using a Dove prism the camera may be rotated at w. Mirror 2 may have any number of facets and they may be concave.