Patent GB-620032-A (GB)
Title: Improvements in means for producing light of variable colour for use for example in colour television receiving apparatus
IPC: H04N1/50
Filed: 1945-08-09
Abstract: 620,032. Colour television. SCOPHONY, Ltd. Aug. 9, 1945, No. 20504. Convention date, Aug. 26, 1944. [Class 40 (iii)] In a colour television system the diffraction action of a supersonic frequency light-modulator produces a colour band spectrum, the position of the bands of which, relative to an apertured diaphragm, may be varied by varying the supersonic frequency. An image of a slit 12, Fig. 1, normally focused on the opaque portion 20 of diaphragm 19, is diffracted from this portion towards the apertures 21 when the cell 15 is actuated. This is achieved by applying electrical oscillations of suitable frequency to the crystal 16, thus creating compressional waves in the liquid of the cell, the wavefronts of which are parallel to the light issuing from slit 12. The position of the diffracted light (i.e. band spectrum), and consequently the colour, relative to the slits 21, is a function of the frequency N applied to the crystal, and is related to the dimensions &c. of Fig. 2, by the equation N=s/#.v/# v being the velocity of the compressional waves in the liquid and A the optical wavelength of that portion of the band spectrum which is passed by the slit s. One practical form of a three-colour television receiver utilizing the partial colours blue, green, red, is realized by employing an electronic switching device controlled by the frame synchronizing signals to connect the crystal 16 alternately to one of three oscillators operating at the appropriate frequencies N for the derived colours A, the light issuing from slit 21 being collected by lens 22 for the formation of the coloured picture after scanning by suitable means. The crystal used in this case must have a frequency response, the width of which depends on the required variation in N. Alternatively three separate crystals tuned to different frequencies may be employed, each being switched electronically as before. One oscillator, the frequency of which can be varied by a variable-reactance valve, may be used instead of three separate oscillators. Fig. 3 shows part of an arrangement alternative to that of Fig. 2, in which the diaphragm 19 is replaced by a rotating diaphragm disc 30 divided into three sections of 120 degrees, and each containing an annular opening of suitable radius and width to permit passage of light of one of the desired colours, the rotation of the disc being controlled by the frame synchronizing signals. Compensation for any difference in the intensities of the three spectral bands is provided by giving each of the openings a suitable radial width. When the arrangement employing a fixed slit instead of the rotating disc is used, this compensation may be provided by arranging filters of suitable absorption characteristics in the optical paths, or by covering the slit with a transparent sheet, the transparency of which has a maximum in the centre and decreases towards the edges. Otherwise the centre of the slit may be arranged to pass the required optical wavelengths with maximum efficiency and neighbouring wavelengths with intensities which diminish towards the edges. An arrangement in which the three partial colours are simultaneously active instead of successively, is illustrated in Fig. 5, in which three crystals 16a, 16b, and 16c each excite zones of the cell 15 at the desired frequencies, suitable diaphragm members 53, 54 serving to permit light to pass only through these zones. A lens system 55 in conjunction with scanning members 56, 57 is used to form an image on screen 58 in known manner. Instead of a single cell 15, Fig. 5, three separate cells, each with its individual crystal, may be used, Fig. 6 (not shown). An arrangement permitting a wide separation of the cells utilizes individual lenses for each cell and also includes deflecting prisms associated with the outermost cells, Fig. 3 (not shown). Separation of the functions of light intensity and of colour modification may be realized as described in connection with Figs. 8, 9 (not shown), by causing the light beam to traverse a first supersonic cell, identical with the type employed in a black-and-white system, and then causing the intensity-modulated white light thus produced to traverse a second supersonic cell which imparts the desired colour modulation as before described. Specifications 439,236 and 513,776 are referred to.