Patent GB-552005-A (GB)
Title: Improvements in or relating to television image producing systems
IPC: H04N5/66
Filed: 1942-02-27
Abstract: 552,005. Television modulating light. WESTERN ELECTRIC CO., Inc. Feb. 27, 1942, No. 2607. Convention date, March 11, 1941. [Class 40 (iii)] In a television system utilizing a supersonic light-modulating device, for example as described in Specification 439,236, the apparatus for scanning the viewing screen comprises a doublemirror drum 42, Fig. 3, which in conjunction with the other parts of the scanning apparatus is adapted to maintain a certain length of line of illumination scanning the viewing screen, for example a whole picture line made up at one time of one single line, Fig. 19, and at other times of parts of adjacent lines, 'Fig. 20. In the form shown, light from a high intensity source 29, such as a mercury vapour discharge lamp, passes through condenser lenses 30, two apertures 32, 33 in an opaque screen 31, an aperture 34, a supersonic cell 25, a double cylindrical lens 36, and a cylindrical lens 37 to a plate 39. The image signals are impressed on the cell by a piezo-electric driver 24 and travel from one end of the cell to the other during the time corresponding to one line. The light modulations produced by the cell pass through a slit 40 in the plate 39 to a high-speed mirror-drum 42, thence to a low-speed mirror-drum 43 and thence to the viewing screen 41. The high-speed mirror-drum 42 has two sets of mirrors 45, 46 arranged to project the light on alternate adjacent lines on the viewing screen, and each set is made up of 10 mirrors. Each mirror operates during the period of one line to stabilize the light modulations of one line on the viewing screen, so that one revolution of the high-speed mirror drum will produce twenty lines. The low-speed mirror-drum 43 has 22 mirrors and rotates 1/22 of a revolution for one revolution of the high-speed mirrordrum 42. Each mirror of the low-speed mirrordrum, for example 11L, Fig. 14, is hinged on one side at 54 on a frame 53 and is guided on the other side by a roller 61 in a cam groove 62 so arranged that the mirror remains parallel with itself while moving in the range of the light beam from the high-spesd mirror-drum 42. Each mirror of the low-speed mirror-drum therefore acts as if it were stationary while the highspeed mirror-drum makes one revolution and produces 20 lines on the viewing screen. The successive mirrors of the low-speed mirror-drum are set to gradually different angles to make the succession sets of 20 lines follow each other to complete a full-scan of 440 lines for each revolution of the low-speed mirror-drum. At any moment during the scan, the modulation projected on the screen has the effective length of one line made up, for example of a single whole line A1-A440, Fig. 19, or of a part A-123-A440, Fig. 20, of one line and a part A441-A562 of an adjacent line. In this way, substantially all the points of the viewing screen receive the light modulation for substantially the same period of time, namely the period of one line. The period may be made longer or shorter by increasing or shortening the effective length of cell ; a masking device may be used for this purpose. Specification 496,964 also is referred to.