Patent GB-604280-A (GB)
Title: Improvements in or relating to iodine images and methods of producing same
IPC: G03C9/04
Filed: 1944-12-06
Abstract: 604,280. Photographic development of polarizing images. INTERNATIONAL POLAROID CORPORATION. Dec. 6, 1944, No. 24447. Convention date, Jan. 12. [Classes 97 (i) and 98 (ii)] A method of producing an iodine image, e.g. for polarizing screens, comprises forming a silver image in a permeable relatively water insoluble carrier, converting said silver image into an image formed of a relatively water insoluble oxidizing agent and reacting the last-mentioned image with an iodide and releasing the iodine to form a corresponding image. The silver image may be formed in a sheet or layer of an oriented plastic, in an emulsion layer thereon, or in a suspension comprising a mixture of gelatin and polyvinyl alcohol. In a further modification, a positive silver image of a paper print is transformed into iodine and imprinted by a suitable transfer process on to a sheet of oriented plastic having a greater affinity to iodine than the paper print. The molecularly oriented carrier in or on which the dichroic image of iodine is formed may comprise either polyvinyl alcohol, partially hydrolysed polyvinyl acetals, polyvinyl alcohol esters or regenerated cellulose. The carrier may itself be supported on a sheet of transparent plastic, e.g. cellulose acetate or nitrate, vinyl acetal, vinyl acetate, vinyl chloride, a vinyl acetate-vinyl chloride copolymer or a condensation type superpolymer such as polyamide or nylon. In one embodiment, a silver halide emulsion dispersed in polyvinyl alcohol is exposed and developed 'and fixed, and the silver image is transformed into a silver ferrocyanide image by immersion in a bath containing ammonium oxalate, cupric sulphate, potassium ferricyanide and oxalic acid. It is washed in 5 per cent sodium thiosulphate solution to remove silver salts and in water, then in a 1.5 per cent solution of sodium hydroxide and again in water before being placed in an acid solution of potassium iodide, sodium iodide or ammonium iodide. The image in iodine is fixed in a boric acid bath which may also contain sodium sulphate and potassium iodide to prevent softening of the sheet and control the colour. Cupric chloride, ferric chloride, ferric sulphate and ferric ammonium sulphate are alternatives to the cupric sulphate. A single bath containing the metallic salt and the ferrocyanide may be used to convert the silver direct to the metallic ferrocyanide, direct reaction of the salt and the ferrocyanide being prevented by adding ammonium oxalate, potassium oxalate, potassium citrate, potassium sodium tartrate, ammonium tartrate or sodium succinate, and as an accelerator, either oxalic acid, hydrochloric acid or a weak alkali, e.g. sodium, potassium or ammonium carbonate. If the iodine image is transferred from a paper print, e.g. to sheet comprising at least a surface layer of molecularly oriented polyvinyl alcohol, the iodine is not released until the paper print and the plastic sheet are in intimate contact. To do this, the silver image is converted to a hydroxide of a polyvalent metal in its higher valence form and immersed in an iodide solution not containing an acid, an acid, e.g. maleic acid being contained in the oriented surface. In an alternative method, a silver image is reacted with potassium permanganate to form an image of manganese dioxide which will release iodine from an acid solution of an iodide. Still another method is to react the silver with a solution containing a bichromate and a metallic salt, e.g. lead nitrate. A modification of this method is to omit the metal salt, the silver chromate releasing the iodine. In view of the presence of the silver salt image, this method is used in the transfer process. The processes are applicable to the formation of dichroic images on material having both faces molecularly oriented each carrying silver images, e.g. for making stereoscopic pairs.