Patent GB-896140-A (GB)
Title: Photoconductive devices
IPC: H04N9/04
Filed: 1958-07-21
Abstract: 896,140. Photo-conductive devices. RADIO CORPORATION OF AMERICA. July 21, 1958 [July 23, 1957], No. 23400/58. Class 37. [Also in Groups XL (a) and XL (b)] Compensation for the effect of " lag in photo-conductive devices, i.e. the time delay between a change in the light stimulus and the resulting change in resistance, is effected by combining the reponses of two photo-conductors which have different responses or distinct portions of one photoconductor arranged to have different responses. The invention is described in connection with photo-conductive pick-up tubes for colour and black-white television and photo - conductive photo-electric cells and electroluminescent panels. Fig. 4 shows an embodiment for black-white television in which image light is directed on to two photoconductive type pick-up tubes 30, 301 which may be of the kind employing a signal plate of tin chloride or tin oxide as described in connection with Fig. 3 (not shown), the tube 30<1> being provided with a filter 64 to reduce the amount of light incident on this tube. The response characteristic of tube 30<1>, after inversion in 70, is then similar to curve 26 (Fig. 2), which shows the response, i.e. the change in resistance of the photo-conductive material in the target electrode between the points A (light-on), D (light-off) and F (initial dark-resistance). When this (inverted) response is combined in adder 72 with the response of tube 30 (curve 24, Fig. 2), the resultant overall response (curve 28, Fig. 2), has faster rise and decay components than the uncompensated response of tube 30. Instead of employing an inverter 70 signals of the required polarity may be derived (1) from the screen electrode adjacent the target of tube 30<1> (Fig. 7, not shown); (2) from the normal target electrode by operating this at a lower potential than the screen electrode (Fig. 8, not shown); or (3) by employing electron-multiplier type tubes and deriving the output from the last but one dynode (Fig. 9, not shown). For colour operation two tri-colour tubes or two groups of three monochrome tubes with appropriate filters may be employed (Figs. 5 and 6, not shown). In an arrangement in which both sets of signals are derived from a single tube the signal electrode associated with the photo-conductive target 58 (Fig. 10), of the latter consists of a plurality of conducting strips alternate ones 76 of which are transparent and are connected together to provide the main signal output of the tube whilst the intervening strips 79 are partially light-absorbing and are connected together to provide the compensating output signals. The latter signals are then inverted in 70 and combined with the main signals in adder 72 to produce the required " lag " corrected signal response. In a modification of the latter arrangement the two groups of strips are biased by potentials of opposite polarity so that combining the two outputs without polarity inversion produces the required compensated signal. Since the scanning electron beam will not impinge on the target area adjacent the strips which are negatively biased a leakage path is provided by a semi-conductive layer which may consist of evaporated germanium slightly oxidized (Fig. 12, not showh). Fig. 11 shows an alternative single tube arrangement in which the main signals are derived from a transparent signal electrode 38 which co-operates with the photo-conductive electrode 58 whilst the compensating signals are derived from a grid electrode 82 provided on the side adjacent the electrode 58 with photo-conductive material 84 which receives light of reduced intensity due to the filtering action of electrode 58 and is scanned by electrons reflected by the latter electrode. Fig. 13 shows a single pick-up tube arrangement for tri-colour signals in which the target comprises a plurality of groups of colour sensitive elements each group including colour filters 90R, 90B and 90G, each of the latter having superimposed thereon in order a transparent signal strip 76; a first photo-conductive strip 92; a semi-conductive strip 94; a second photo-conductive strip 96; and a signal strip 98, the assemblies being mounted on the faceplate 100 of the tube. In operation the main signals are derived from the signal strips 76 and the compensating signals from the signal strips 98 since the light incident on the photoconductor 92 associated with strips 98 is reduced in intensity by passage through the photo-conductor 92. By supplying a negative bias to the signal strips 98 the compensating signals are derived in inverse polarity and may therefore be directly combined with the main signals to produce the " lag compensated signal. Fig. 14 shows an application of the invention to an electroluminescent panel the latter comprising a glass support plate 100, a transparent conductive coating 102, an electroluminescent phosphor 104, a semi-conductive layer 106 and a grooved photo-conductive layer 108, the protruding portions of the latter being provided with conducting strips 112, e.g. of air-drying silver paste. Alternate-protruding parts of the photo-conductor 108 are covered with partially light-absorbing strips 110, e.g. of a resin containing lamp-black and alternate strips 112 are connected together and to one side of an A.C. source 109 the intervening strips 112 being similarly connected to the other side of the source 109. It is stated that by suitably adjusting the tapping point on the source 109 which is connected to the transparent conductive coating 102 the " lag " of the photo-conductive material 108 which is exposed directly to incident light is compensated by the " lag " of the photo-conductive material which is under the light-absorbing strips 110. Fig. 15 shows an application of the invention to a photo-electric cell comprising a sheet of photo-conductive material 118 provided with three sets of conductive strips 120, 124, 126 of which 124 is partially shielded by a light-absorbing mask 122. With this arrangement and correct adjustment of the potentiometer 129 the signals produced, by incident light, between the strips 120 and 124 provided " lag " compensation for the signals produced between strips 120 and 126. In all of the embodiments described above the " lag " compensation is effected by subtraction of signals produced by a partially light shielded photo-conductor from signals produced by a photo-conductor which receives the full light stimulus available. If, however, the photo-conductive material which provides the compensating signals has a response of the kind shown at 134, Fig. 17 (e.g. such as produced by a thick evaporated layer of amorphous selenium) the required compensation may be effected by the addition of this response to that of the unshielded photo-conductive material, shown by curve 132, the compensated response being shown by curve 136. Fig. 18 shows a practical embodiment of the latter method of producing " lag " compensation and comprises a transparent face-plate 40 supporting a transparent conductive electrode 56 and a photo-conductor electrode consisting of parallel strips alternate ones 142 of which produce a response of the kind shown by 134 (Fig. 17), whilst the other alternate ones 140 produce a response of the kind shown by curve 132 (Fig. 17) so that when the photo-conductor electrode is scanned by an electron beam the two responses are added to produce the " lag " compensated signal. An arrangement similar to that of Fig. 18, but having the signal plate in the form of a plurality of parallel strips each of which is associated with a strip of photoconductive material is described in connection with Fig. 19 (not shown). It is stated that some of the arrangements employed for the subtraction type of compensation may be employed for compensation by addition of the response such as e.g. by applying a positive potential to the signal strips 96 (Fig. 13), and connecting them in parallel with the signal strips (Fig. 13) for a particular colour,