Patent GB-1096741-A (GB)
Title: Colour projection system
IPC: H04N5/74
Filed: 1965-01-06
Abstract: 1,096,741. Colour television; light modulation. GENERAL ELECTRIC CO. Jan. 6, 1965 [Feb. 11, 1964], No. 604/65. Heading H4F. A colour television picture image is projected on to a screen 9 with the aid of a cathoderay tube 4, the electron beam 5 in which is modulated by electrical signals corresponding to the point by point variation of the red, blue and green video signal of the image to be projected so that phase diffraction gratings are formed in a light modulating medium 2 e.g. an oil. The red and blue video signals are arranged as shown to modulate the horizontal velocity of the electron beam to form two vertical phase diffraction grating of different spacings, the spacings corresponding to the frequency of the carrier waves on which the colour signals are modulated before being added to the horizontal scanning signal. The green video signal is modulated on a carrier wave of higher frequency than the carrier waves of the red and blue signals and applied to the vertical deflection plates of the electron beam device so as to form a horizontal diffraction grating having a spacing equal to the spacing between alternate lines of a frame, the electron charge deposited on the modulating medium being spread out or smeared transverse to the scanning direction of the beam, the amplitude of the smear varying proportionally with the amplitude of the high-frequency carrier signal, which carrier signal amplitude varies inversely with the amplitude of the colour signal. The size of the raster on the light modulating medium is approximately ¥ of an inch in height and one inch in width so that the spacing between alternate lines of a frame is approximately 3 mils. White light from a source 6 is transmitted through the light modulating medium via an elliptical reflector 11, a circular filter member 12 having a vertically oriented central portion passing only the red and blue or magenta components and having sectors on each side of the central portion passing only the green component, a first lens plate member 13 comprising an array of lenticules, a second lens plate and input mask member 14 comprising an array of lenticules on one face and an input mask, as shown, on the other, and a mask imaging lens system 15, and the deformations in the light modulating medium modifies the light transmitted in point by point correspondence with the coloured image to be projected on to screen 9. On the output side of the light modulating medium there is an apertured light mask 7 similar to mask 14 and a projection lens system 8. Since the red and blue information diffraction gratings have parallel lines i.e. they are both vertical gratings a beat diffraction grating is formed in which the spacing between successive lines corresponds to a frequency which is the difference of the frequency of the other two gratings. The line to line spacing of such a grating is greater than the spacing of either of the other gratings and accordingly the primary components of light incident thereon i.e. red and blue or magenta components, are deviated less than they are by the other two gratings and whereas in prior Specifications these deviated first order beat grating components were blocked by the light mask the object of the present Specification is to use these first order beat grating components to improve the efficiency of the system and colour rendition. Thus the grating for each of two primary colours having parallel lines i.e. red and blue, is selected such that the component having longer wavelength (red) has the shorter spacing, and the carrier frequencies corresponding to the spacings are substantially in direct relationship to the ratios of the dominant wavelengths of the two primary colours. Two pairs of apertures 20 in the output mask 7, one pair on each side of the line of undeviated light parallel with respect to the lines of the red and blue gratings, are arranged to be spaced and of such a width that the apertures adjacent the path 25 of undeviated light pass substantially all the red and blue components of light in the first order spectra produced by the beat frequency grating and first order light of the blue component produced by its grating and the other two apertures pass first order light of the red component and second order light of the blue component produced by their corresponding gratings. The green light components are deviated by the green diffraction grating which is orthogonal to the red, blue and beat diffraction gratings. In the embodiment described, Fig. 4 (not shown), the red grating frequency is three times the beat frequency and the blue grating frequency is twice the beat frequency i.e. the carrier frequencies of the red and blue signals are in direct relationship to the ratio of the wavelengths of red and blue colours (3: 2). Fig. 1 shows how red, blue, green and magenta dots are formed in the projected image by following through the system the bundles of rays 90, 91, 93 and 92 respectively from the light source to the screen, areas 30, 31, 32 and 33 of the light modulating medium representing red, blue, green and magenta diffraction gratings. The light modulating medium instead of controlling the transmission of light may control the reflection of light.