Patent GB-1251226-A (GB)
IPC: H01J29/45
Filed: 1968-11-06
Abstract: 1,251,226. Electroluminescence. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. 6 Nov., 1968 [20 Nov. 1967 (2)], No. 52598/69. Heading C4S. [Also in Division H1] A memory cell for storing information from a radiation input has a photoconductive element which when cooled shows persistent conductivity after the cessation of causative radiation, and has means for detecting the conductivity of the element. The photoconductive material is doped and is chosen from cadmium sulphide, cadmium selenide and cadmium sulphoselenide; the dopants are copper with chlorine or bromine. With the material in its dark resistance state, a conductivity increase may be produced by illumination with visible or U.V. light, or with X-rays or other high energy radiation (e.g. radioactive rays). A saturation level of conductivity is reached if the exposure time-intensity product is large; this saturation level is also reached if an intermediate intensity-time illumination is used and the material subsequently left subjected to a potential of more than a certain value. Dark level conductivity may be restored by infra-red quenching, or by raising the material from its cooled state (- 30 C.) to above this temperature, e.g. to 0 C. Any particular conductivity is preserved (or may be changed by appropriate illumination) irrespective of the presence or absence of current through the elements (except under the high potential condition mentioned above). Readout of stored information may take place by measuring the current through a cell. Embodiments are described which are respectively a single element and a series of elements defined in the body by orthogonal sets of parallel electrodes on opposite sides of the body. Instead, a photoconductive layer provided with a transparent electrode may be scanned by an electron beam in an evacuated envelope and the signal displayed on a conventional television unit. Other embodiments use electroluminescent layers (e.g. ZnS in a plastic binder) to give a direct image corresponding to a positive or negative (e.g. with infra-red radiation) of the original radiation image applied to the photoconductive layer. The simplest of these uses A.C. applied across a series combination of E/L layer and photoconductor, there being an opaque layer (505) (Fig. 5, not shown) e.g. black paint on carbon black, between the two and reflecting and insulating layer (504) of powdery barium titanate in a plastic binder. The electrode on the photoconductor may be a series of parallel lines, a mesh, or a transparent film. A thicker p.c. layer (601) (Fig. 6, not shown) may be provided for X-radiation input. In the more complex embodiment of Fig. 7 the photoconductor layer 506 is between an electroluminescent layer 503 and an insulating polyester resin layer 701; by suitable adjustment of phase, frequency, and voltage of the two sources the dark current may be backed off, contrast varied etc. In all embodiments the conductivity image may be stored as positive or negative-visible or higher frequency energy applied as a pattern to a dark state element gives a positive image; I.R. applied as a pattern to an element showing radiation saturated conductivity gives a negative conductivity image.