Patent GB-1227869-A (GB)
IPC: H04N9/097
Filed: 1968-10-24
Abstract: 1,227,869. Cathode-ray tube circuits. FERNSEH G.m.b.H. 24 Oct., 1968 [25 Oct., 1967 ; 27 Nov., 1967; 19 Dec., 1967], No. 50592/68. Heading H4T. Persistence effects at low light levels in television pick-up tubes of the kind having a photocathode comprising a laminated photo semiconductor (e.g. tubes of the "Plumbicon" (Registered Trade Mark) type) are removed by producing an additional constant current in the photo-cathode so that the operating point on the characteristic for "black" level signals is positioned on the linear portion of such characteristic and then blanking the signal componendue to such additional currents. By thus shifting the operating point for black from the position 1 (Fig. 1) to the position 2 the electrical resistance du/di at low light intensities iL (where u is the signal output voltage of the tube and i the signal current) is smaller than the resistance when the black level operating point is at position 1 so that the discharge of the storage capacitance to position 2<SP>1</SP> occurs in a shorter time (due to the smaller time constant) than the discharge to position 1. There is still an incomplete discharge at the point 2<SP>1</SP>, however, and the signal component due to the residual charge between points 2 and 2<SP>1</SP> is removed by blanking whilst the level D (black shoulder) is clamped to a black reference level. The additional photo-cathode current may be produced by irradiating the photo-cathode by a light source, an auxiliary electron beam or by means of radioactive material embedded in the photo-cathode. In one embodiment (Fig. 2) a ring 6 of transparent material, preferably provided with a fluorescent substance, is illuminated from the outside by small lamps 7 such that a uniformly diffused light falls on the photocathode 5. When a number of pick-up tubes are employed as, e.g. in a colour television camera employing four tunes, i.e. a luminance tube and three, R, G, B, colour tubes, the photo-cathodes of each may be illuminated by means of separate electric lamps each provided with a regulating resistance for adjusting the brightness (Fig. 3, not shown). The photo-cathode may also be illuminated from the side scanned by the electron beam by means of lamps inside the tube envelope and the current leads to which are brought out via a socket pin to enable adjustment of the illumination to be effected (Fig. 4, not shown). In another arrangement, an optical system 38 (Fig. 5) normally employed for projecting the image of a diapositive 40 via a small mirror 39 on to the photo-cathode of a three tube colour television camera is utilized by removing the diapositive and directing light on to mirror 39 from an "Ulbricht" sphere 41 containing a white light source 45 and R, G, and B light sources 42, 43, 44 in such a manner that a desired spectral distribution is obtained, Fig. 6, shows a further arrangement in which a filter 49<SP>1</SP> comprising a neutral white sector W and red, green and blue sector R, G, B, separated by opaque sectors together with adjustable masking means for the former sector is positioned in plane 49 and illuminated by a light source 50 such that an image of plane 49 is produced by means of lens 48 and mirror 47 in an aperture diaphragm plane of the objective lens system 40 or in a conjugate plane. Other means for proiucing a light source having required spectral characteristics comprise four light tubes provided with respective lamps 51 ... 54 (Fig. 7) at one end and colour filters W, B, G, R at the other end or, as shown in Fig. 8, bundles of optical fibres each bundle being provided with a filter ---- - 58 illuminated by respective lamps 51 ... 54 at one end whilst the other ends are formed into a common bundle with a regular or statistical distribution of the fibres. In a further arrangement light from a lamp 911 (Fig. 9) is directed into a known beam splitting prism arrangement including dichroic surfaces 98, 99 via a ground surface 910 at such an angle that total reflection occurs at the opposite surface and such reflected light is then directed along the axis of the image forming light by means of a semi-reflecting surface 912 so that, each camera tube 95, 96, 97 receives a portion of the light. By regulating the spectral distribution and intensity of the source the irradiation of the different tubes may be adjusted, e.g. compensation for the low sensitivity of such tubes in the red spectral range may be effected.