Patent GB-1142338-A (GB)
Title: Improvements in or relating to devices comprising a laser light-source, and a digitally-controlled deflection device
IPC: G02F1/31
Filed: 1966-05-19
Abstract: 1,142,338. Scanning display system. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 19 May, 1966 [22 May, 1965], No. 22254/66. Heading H4F. The invention relates to producing a television picture utilizing a laser light-source, an electrically controllable polarizer for controlling the brightness of the individual image points and a digitally-controlled deflection device as described in Specification 994,955, which comprises a plurality of birefringent prism (P 1 ) and polarization Kerr cell switch (K 1 ) combinations, Fig. 1 (not shown). In order that the ordinary and extraordinary rays are deflected symmetrically with respect to the optical axis of the system each birefringent prism is combined with a non-birefringent, i.e. isotropic, prism or a birefringent prism suitably oriented. An image projection arrangement according to the invention comprises a laser light (visible or ultra-violet) source 1, Fig. 2, an amplitude modulator 2 based on the Kerr or Pockel effect and a digital light-deflector 3, the modulator and deflector being controlled by signal receiver and generator 6. Lens 0 1  focuses the collimated deflected laser light-ray in its focal plane F and since the resulting time-variable image cannot be viewed directly because it is very small, a second lens 0 2  projects it on to a projection screen P. An optical noise filter 4 comprising a polarizing switch and an analyser controlled by the generator 6 may be interposed in the arrangement as shown to eliminate any undesirable second light ray produced if the polarization switches in the deflector do not effectively rotate the plane of polarization of the laser ray through exactly 90 degrees. The time-variable images may be projected on to a moving photographic film instead of on to a projection screen. The images thus obtained are monochromatic and to obtain a multicolour projection three images, i.e. red, green and blue, are produced and projected so that they overlap on the projection screen, Fig. 3 (not shown). To reduce the number of projection lenses (0 1  and 0 2 ) used and also the occurrence of parallax error, dichroic elements (7), (8) and (9), Fig. 4 (not shown), may be utilized to combine the three coloured images. One laser source which emits three radiations of different frequencies may be used, Fig. 5 (not shown). A reduction in the number of the digital light-deflectors encounters difficulties since, firstly, the refractive indices of the birefringent prisms exhibits dispersion and secondly, the Kerr (or Pockel) constant of the polarizing switches exhibits dependence upon wavelength. The dispersion originating from the prisms may be compensated by suitable choice of the glasses used in the lenses 0 1  and 0 2  and the construction of the lenses. It is thus possible to work with only one deflection device 3 if the three light-frequencies are passed through the system 2, 3, 4 in time sequence, the dispersion of the Kerr constant being effectively compensated for by varying the voltage applied to the Kerr cells in corresponding time sequence between in general six different voltages or working points. The number of different voltages may be reduced by suitable choice of the three light frequencies, and for three working points each Kerr-cell is divided into two cells Z 1  and Z 2 , Fig. 6, separated by a dielectric D of a low dielectric constant, the two partial cells being activated independently of each other with voltages U 1  or U 2 . For more working points the Kerr-cell may be subdivided again. As a special case of the three-colour image a black-and-white image may be obtained, alternatively a black- and-white image may be obtained by projecting a monochromatic image on to a phosphor screen. Lens 0 2  may be combined with a Zoom lens. If the frame frequency of a signal is so low that the image flickers then an intermediate image produced on a long persistence phosphor display screen is scanned by a digital picture scanning device as described in Specification 1,137,267 or by a television camera with the required frame frequency and the resulting picture signal is handled for the purpose of a digital image projection according to the present invention, Fig. 7 (not shown). A monochromatic or multi-coloured stereoscopic image may be produced when viewed through a pair of polarizing spectacles by interposing a further polarization-optical switch (11, Fig. 8, not shown), e.g. a Kerr or Pockel cell, after the deflector (3) or noise filter (4), operated so that alternate images are polarized at right angles to one another. Alternatively, stereoscopic projection may be obtained by producing two images of different colours either alternately or simultaneously and since these would be monochromatic by the nature of laser radiation the filter glasses in the spectacles are simplified. Further, the two colours need not differ with regard to their wavelength so much that they appear to the eye as different colours and thus three-dimensional multi-colour projection is possible, alternate images being produced using wavelengths # 1 , # 2  . . ., and #<SP>1</SP> 1 , #<SP>1</SP> 2  . . ., the wavelengths # 1  and #<SP>1</SP> 1  &c. not being distinguishable for the eye. A linear digital light-deflector in combination with a rotary mirror which effects the frame scan, may be used to project the image on to a screen, Fig. 9 (not shown).