Image-storage panels based on field-effect control of conductivity

Using the principle of field-effect conductivity control, a new method of image storage has been demonstrated. Based on this a new type of solid-state image panel has been developed capable of producing a stored luminescent image. This employs an electroluminescent powder layer for generation of the output image and a ZnO powder layer for control and storage purposes. In operation the exposed ZnO surface is first uniformly corona charged to a negative potential to reduce its conductivity and "erase" old information. Following this the panel is exposed to an optical image which discharges local areas, producing a stored charge pattern on the ZnO surface. In accordance with this charge pattern a conductivity pattern is created in the ZnO layer which in turn controls the luminescent output of corresponding areas of the adjacent phosphor layer. For producing a stored image, approximately one microjoule/cm2of input radiation in the wavelength range of 3500-4000 A is required. Reciprocity exists between the exposure time and the radiation level. Stored images have a brightness as high as 20 foot T lamberts and a maximum contrast ratio of about 100:1 and exhibit good halftones. Although a hafftone output image can be retained for periods of the order of an hour, it can be rapidly erased when desired by recharging the ZnO surface to a uniform negative potential. Present panels are 12 by 12 inches in size and have a limiting resolution between 400 and 800 TV lines.