Patent GB-585477-A (GB)
Title: Improvements in colour photography
IPC: G03C7/22
Filed: 1944-08-21
Abstract: 585,477. Colour photography. COOTE, J. H. Aug. 21, 1944, No. 15974. [Class 98 (ii)] Multi-colour photograpliic prints are obtained by exposing a silver halide emulsion to the first colour separation negative, developing and colouring to produce the required positive colour record, coating with a composition containing silver halide and the same colloid as in the first layer, the second emulsion layer having the normal coating weight of silver halide with a colloid/silver ratio of not more than 1.5 to 1, printing thereon from the record colour separation negative developing and colouring. A third emulsion layer with the same characteristics may be coated thereon and exposed to a third colour separation negative, developed and toned to the appropriate colour. The starting material may be a base, e.g. of pyroxylin or cellulose acetate with a primer and a layer of a normal silver halide emulsion or one as above of low colloid/silver ratio. It is printed from the blue record and the latent image developed in a solution comprising sodium sulphite, metol, hydroquinone, sodium carbonate, potassium bromide and water and toned in a bath containing potassium iodide and iodine to a yellow image of silver iodide. This image is dyed with a basic yellow dyestuff in acid solution, e.g. Photophor Basic Yellow, Acronal Yellow. The Provisional Specification suggests Auramine or Chrysoidin may also be used'. The dyed image is washed to free the high-lights of colour and then treated with a 2¢-5 per cent aqueous solution of phosphotungstic acid or silicotungstic acid to form an insoluble lake. The silver iodide is removed with sodium thiosulphate and'the film dried. A coating of emulsion of reduced gelatin/silver ratio is applied of a thickness which gives a normal coating weight of silver halide and allowed to dry. The green record is then printed thereon, developed as above and toned in a bath containing nickel nitrate or chloride and potassium ferrocyanide in addition to citric acid, potassium nitrate, formaldehyde and water. The image is further toned in an alkaline bath containing dimethylglyoxime, the silver ferrocyanide which remains being removed by sodium thiosulphate solution. A watermiscible alcohol, e.g. methanol, may be added to the dimethylglyoxime to reduce its swelling action on the emulsion. The film is dried and a further coating of the emulsion of reduced gelatin/silver ratio is added and dried. The red record is printed and the emulsion developed, toned in a bath containing potassium ferricyanide and potassium bromide and washed in a solution of ferric ammonium sulphate or citrate before washing in water and drying. A final supercoat of gelatin or polyvinyl alcohol may be applied to preserve the film from damage. The concentrated emulsions are propared as in the normal manner up to the first ripening stage, after which 40 per cent of tho normal weight of gelatin is added and the emulsion precipitated with sodium sulphate. The emulsion is redispersed and set, shredded and washed free from soluble salts, adjusted to the desired ratio and the whole after-ripened. Alcohol and phenol may be added. Alternatively, concentrated emulsion may be produced by freezing and removing the ice as described in Specification 313,63], [Class 98 (ii)], or by centrifuging the emulsion at 40‹ C. in a relatively slow-runnirig centrifuge, e.g. 1000-1200 revs. per minute. The bulk of the silver halide, together with some adherent gelatine on the wall of the centrifuge, is redispersed with the requisite amount of gelatine and after-ripened. The order of production of yellow and magenta images is interchangeable, but the blue-green image normally exercises a strong desensitizing action on silver halide and should be produced last. The colour images may be produced by colour development. The blue-green image is obtained by direct colour development with dimethyl or diethyl - paraphenylene - diamine hydrochloride and 2,3,4-tri-chloro alpha-naphthol. The colour developer solution which also contains sodium sulphite, sodium carbonate potassium bromide and methyl alcohol has a strong desensitizing action on silver halide emulsion. The magenta image is colour developed by a solution containing 2-amino-5-diethylaminotoluene and paranitrobenzyl . cyanide. The yellow image is colour developed by a solution containing 2,5 - dichloro - aceto - acetoacetanilide and dimethyl paraphenylene diamine. The fully developed image layer may be treated, prior to coating, with a potassium iodideiodine bath followed by a bath of an alkali cyanide. Both sides of a base material may be coated or up to four images may be produced on one side of a single film. Specifications 168,100, [Class 98 (ii)], 553,377 and 572,464 also are referred to.