Patent GB-720554-A (GB)
Title: Improvements in and relating to colour television apparatus
IPC: H04N9/077
Filed: 1952-01-11
Abstract: 720,554. Colour television. PHILCO CORPORATION. Jan. 11, 1952 [Jan. 19, 1951], No. 909/52. Class 40 (3). In a colour television system an image of the object to be televised is resolved into a plurality of parallel line elements separated by blank spaces, the image being subsequently projected through an optically dispersive system on to a photo-sensitive surface-electrode of a television camera tube so that each line of the resolved image is dispersed into a colour spectrum upon the photo-sensitive electrode. An electron image corresponding to the optical image formed within the tube is subsequently scanned to derive signals at the output of the tube comprising information concerning the colour components of the image; as applied to a " dotsequential " type of system, the scanning is carried out in a direction perpendicular to the lines in the resolved image while as applied to a " line-sequential " type of system the scanning is carried out in a direction parallel to the lines. An image of the object 50 is focused by means of lenses 52, 61 on to a grating 54 comprising a large number of narrow parallel slits. Alternatively the grating may be formed as shown in Fig. 2, comprising a plurality of cylindrical elements each of which forms a line image from the light falling thereon. The grating 54 may have a lens 59 formed on its back surface. The image, resolved into lines by the grating 54, is projected on to the photocathode of an image-orthicon 10 passing through a multiple prism 60 whereby each line of the image is spread out into a spectrum on the cathode 20. A charge image is formed upon a mosaic electrode 22, and, as applied to a " dotsequential " system, this is scanned by an electron beam in a direction perpendicular to the lines into which the image is resolved. The output from the tube consequently has the form shown in Fig. 4 and comprises a series of pulses 65, each corresponding to successive components of a corresponding spectrum, and spaces 66 corresponding to gaps between the spectra formed on 'the cathode 20. This signal is applied to a distributive sampling device having three outputs which are gated in succession by pulses 92, 94 and 96. These pulses are derived from a delay line which is synchronized by the spaces 66 and are so timed that the signals in the three outputs correspond respectively to the red, green and blue components of the image. These pulse signals are smoothed by passing them through respective low-pass filter circuits and are then applied to separate inputs of a sampling device which effects time-division multiplexing of the signals applied thereto to derive an output signal of the convention " dot-sequential " type. " Dotinterlacing," i.e. variation of the positions of the dots corresponding to any one colour so that the dots in one frame lie midway between the corresponding dots in the next succeeding frame, may be effected as described in Specification 720,552. In order to cause the portions 66 of the signal shown in Fig. 4 to lie in the " blacker-than-black " region and thus distinguishable from the pulses 65 for all amplitudes of these pulses, an edge-illuminated lightly frosted or scratched screen 51 is provided between the lens 52 and 61. The screen 51 also assists in cases where the object has a fine periodic pattern by reducing the definition of the pattern and so preventing " beats " between the pattern and the grating 54.