Patent GB-721946-A (GB)
Title: Method and apparatus for reproduction of pictorial records
IPC: G03G15/22
Filed: 1951-06-27
Abstract: 721,946. Producing photographs electrically. JACOB, C. W. June 27, 1951 [July 15, 1950; Nov. 2, 1950], No. 15251/51. Class 98 (2). [Also in Groups XVI, XXXVI amd XL (a)] A method of making a pictorial record of an original comprises exposing a photo-sensitive matrix-a photo-conductive or photo-emissive layer with or without a support and a recordreceiving sheet-to a light pattern or image to produce an electrical image of the original, different areas of which have different potentials in accordance with differences in tonal values of corresponding areas of the original, electrically isolating different areas of the matrix from one another by using a grid structure close to the matrix to produce a particle-controlling electric field adjacent the matrix, and then directing electrically-charged marking particles into the electric field of said image while maintaining the electrical isolation of said different areas so as to cause different amounts of particles to be deposited on said areas in accordance with the varying potentials of said areas. As shown, a photoconductive insulating layer 22, as of sulphur or anthracene, is exposed through a transparent conducting plastic supporting plate 21, e.g. of cellulose acetate butyrate, to an original 31 by means of a projector 30, 33 so as to discharge imagewise a uniform negative electrostatic charge formed on the layer by illuminating it evenly whilst the plate 21 is held at a negative potential and removing the light prior to placing the plate at a small positive potential. Simultaneously or thereafter, the electrostatic image is developed with ink droplets passed through an electrostatic separator 50, which removes other than negativelycharged droplets of a selected charge to mass ratio, and introduced through ports 27 into a developing chamber 20 having a control grid 23 and a coarser space-charge shielding grid 24 parallel to but at different distances from the layer 22. By earthing the chamber and grids the drops falling under gravity are directed towards the layer from which they are repelled to the grid 23 where the plate is negatively charged, so that the ink print is a negative of the original. The grid may be formed of spaced pairs of almost touching wires which serve to retain the repelled drops by capillary attraction. Alternatively it may be formed of spaced parallel plates which are either porous or grooved to retain the repelled drops. In the construction shown in Fig. 4, plates 73 having a U-shaped flange and four apertures 87 are sandwiched between plates 72 having registering apertures 85 surrounded by spacing collars 75 so as to produce entry channels 72a arid channels 78 leading to exit ducts 88 formed by the apertures 85, 87. The plate 73 between the channels is made thin to avoid coarseness in the developed image, a discontinuous image being avoided by shifting the developing chamber along guides on the support for the plate 100 and sulphur layer 101 engaging grooves which are parallel to the ducts 88 and disposed in the bottom of the chamber walls. The ink image may either be deposited directly on the sulphur layer and printed on to a paper sheet or deposited directly on a cellulose acetate sheet or a paper sheet soaked in insulating oil placed on the sulphur layer. The sulphur layer may be replaced by a glass plate 132 supported on the conducting plastic plate by spacers 131 and coated with globules 133 of caesium or other electron-emitting photo-electric layer. On exposing the layer, the emitted electrons are attracted to the transparent conducting plate 130 so that the globules become positively charged and so control the ink deposit on the paper sheet 135 on the glass plate. The charge on the globules can be removed by circulating ionized gas through the spaces 136 or by means of radioactive material. The cycle of operations is repeated a number of times to form the image and between each cycle the glass plate structure is displaced a short distance relative to the paper to avoid a shadow effect of the spacers 131. Instead of the plate 130 there may be used a glass block having a very thinly silvered layer preferably covered by a thin layer of caesium located adjacent the layer 132 so that by applying a negative potential to the halfmetallized coating, electrons will flow to the globules 133 when the block is illuminated. Alternatively the photo-sensitive plate comprises a plate of transparent plastic insulating material in which are embedded fine insulated wires, the light-receiving ends of which are coated with caesium, or the insulated wire may be sheathed and placed touching with the sheathings at a fixed potential. By successively exposing the photo-sensitive layer to red, green, and blue colour separation records and depositing after each exposure cyan, magenta, or yellow inks in separate developing chambers movable into register with the exposing station, colour prints may be made. By employing two or more projectors at the exposing station several prints may be made simultaneously. Colour correction may be effected by initially exposing the photo-sensitive layer to a separation record different from that to be printed and ink-developed so as to form an electrostatic masking image. In another embodiment, Fig. 11, a layer 24<SP>1</SP> of sulphur or anthracene on an earthed metal drum 21<SP>1</SP> is first discharged by means of a lamp 106<SP>1</SP> and an earthed transparent conducting plate 105<SP>1</SP> and is then exposed through a transparent electrode 23<SP>1</SP> raised to a high potential to an image of a line display on a cathode-ray tube 20<SP>1</SP> formed by a lens 22<SP>1</SP> so as to produce an electrostatic image on the layer 24<SP>1</SP>. A paper or other non-conducting strip 30<SP>1</SP>, which is freed of static charge by means of a radioactive block before it is led around the drum, is fed from a spool 311 to a spool 32<SP>1</SP>. An endless grid 65<SP>1</SP> comprising a wire mesh having its wires extending diagonally of its length and secured to rubber strips fixed to marginal metal strips having feed perforations is passed over the paper between it and the developing chambers 40<SP>1</SP> which are similar to those shown in Fig. 4. The paper strip is fed anticlockwise by contact with the drum and the grid is fed in the same direction at a different speed by sprockets coaxial with the drum to which they are geared. The developing ink drops are fed into the chambers 40<SP>1</SP> through ports 41<SP>1</SP> and extracted through ports 59<SP>1</SP>. To ensure that the drops are stationary relatively to the paper during actual deposition, a current of air imparting a speed to the drops tangentially of the drum approximately equal to that of the grid and drum is passed over the grid from an inlet 102<SP>1</SP> to a vent 104<SP>1</SP>. The ink drops deposited on the grid both directly and by repulsion from the image are removed by passing the grid through an ink solvent 97<SP>1</SP> and thence upwardly through a downward current of hot air. If the plates 45<SP>1</SP>, 46<SP>1</SP> are disposed normally to the rotating drum, the plates should be shaped at their extremities to direct the drops tangentially to the drum. To permit variations in the tone reproduction, the plates 45<SP>1</SP>, 46<SP>1</SP> may be raised successively to different potentials so as to ensure deposition of ink only on areas of the image having a potential not lower than those of these plates, the plates in each chamber being divided into two or more sets at different potentials, as by an insulating plate 60<SP>1</SP>, and if desired ink drops of different mass and charge may be fed into the different compartments of these chambers. If desired a complete picture may be recorded on the layer 24<SP>1</SP>, the drum being fed intermittently and stopped for each exposure by a Geneva mechanism. The grid 65<SP>1</SP> may be dispensed with and the cathode-ray tube may be replaced by a film which is fed either continuously and covered by a narrow slit if slit images are to be used or intermittently and a complete frame produced if complete pictures are to be recorded. If the drum is fed intermittently, the developing ink drops are preferably introduced intermittently into the supply channels between the plates 45<SP>1</SP>, 46<SP>1</SP> so that deposition will occur only with the paper stationary and to ensure this a field deflecting the drops from the paper may be applied between the drum and the plates during drum movement. In a further arrangement for continuous production of three-colour prints, a paper strip is fed around a transparent conductive plastic drum and at each of three stations the layer 24<SP>1</SP> is exposed to a projected image produced on a cathode-ray tube, developed, and then recharged uniformly, the electrode employed for this purpose being also radioactive to disperse static from the paper or ionized air may be fed past the strip at each station for this purpose. A single cathode-ray tube disposed within the drum is provided with three separate electron guns to produce red, green, and blue separation images which are projected, two by means of mirrors, to the three stations disposed around the drum, and at corresponding stations cyan, magenta, and yellow ink drops respectively are deposited to develop the images so formed. A time delay unit, e.g. employing a magnetic recording tape, may be connected in each of two of the electron gun circuits to compensate for the interval elapsing between the recording of the components of a single view so that the components may be produced in registry on the paper strip. To reduce charge migration due to the conductivity of the deposited liquid marking particles, the drum may be cooled by circulating a refrigerant through pipes within the drum to increase the viscosity of, and so reduce the ionic migration in, the liquid. Specification 672,767 is refe