Patent GB-570524-A (GB)
Title: Masks for use in photographic reproduction
IPC: G03C7/18
Filed: 1943-07-29
Abstract: 570,524. Photographic tone-correction masks. KODAK, Ltd. (Eastman Kodak Co., Evans, R. M., and Hanson, W. T.). July 29, 1943, No. 12314. [Class 98 (ii)] A photographic transparency has in register therewith a tone - correcting mask comprising a colloid relief masking image made from the transparency and which is a record of the transparency, but has densities which are so related to the densities of the transparency that over the greater part of the range of density variations the densities of the masking image increase as the densities of the transparency decrease, but at the range, covering the lowest densities of the transparency do not substantially increase and may even decrease. The densities of the masking image in the range covering the highest density of the transparency may increase at a lower rate than over the greater part of the range of density variations. The densities of the mask may be such that when it is placed in register with the transparency the combination has densities which when plotted against the logarithms of the exposures which produced the corresponding densities in the transparency are such that the resulting curve is substantially a straight line for the whole of its length and slopes in the same direction as the straight line portion of the characteristic curve of the transparency itself. The mask may be a negative of the original subject of which the transparency is a positive, and may be dyed neutral grey. In order to effect colour correction, the mask may be a record of the absorption of light of a wavelength longer than blue by the transparency, e.g. of the red light absorption, or in the case where the transparency has superimposed component images coloured minus blue,'minus green and minus red, of the blue light absorption of the minus green and minus red component images. The masking image may be dyed so as to absorb most highly light of a colour or colours undesirably absorbed by the transparency. As shown, the curve M represents the density curve of an image obtained by low intensity exposure through a transparency having the density curve T and development to a gamma of at least 1 ; curve M<1> represents the density curve of an image obtained either (a) by exposure through the transparency and the image having the density curves T and M respectively on to another light-sensitive layer and development to a gamma less than one, or (b) by exposure through the transparency having the density curve T on to the unexposed area of the light-sensitive layer on which the image having the density curve M was made (before or after removal of the image having the curve M) and development to a gamma of less than 1 ; and the curve T <1> the corrected density curve of the image obtained by printing through the transparency and the image having the curve M<1> and development to a gamma of about 1. The masks are preferably such that the straight line portion of the characteristic curve has a gamma of 0.3-0.6. The mask may be made by giving a light-sensitive emulsion a small exposure to light through the transparency, forming in the exposed layer an image of opposite sign to the transparency, exposing a second light-sensitive emulsion layer through the transparency and the image in the first light-sensitive layer in register, and forming a silver image in hardened colloid in the second light-sensitive emulsion of lower gamma than the gamma' of the image formed in the first emulsion layer and removing unhardened colloid and silver image from the second emulsion layer. Alternatively, a silver halide emulsion layer may be given a sufficiently small exposure through the transparency to record only the tone densities thereof, a silver image of opposite sign to the transparency developed with a high gamma non-hardening developer, the unexposed silver halide exposed through the transparency to record detail thereof, a second silver image of gamma lower than the gamma of the first image is developed with a low gamma hardening developer, and unhardened colloid and silver are removed. After the first development, the unexposed silver halide may be colour-sensitized. The first developed silver may be removed before re-exposure. A variable contrast emulsion giving an image of high relative contrast with light of one colour and an image of low relative contrast with light of another colour may be used, light giving high contrast being used in the first exposure and light giving low contrast in the second. The emulsion may comprise a gelatino-silver bromiodide emulsion sensitized with 5-10 mg. per litre of 2 : 2<1>-dimethyl-8-ethylthiacarbocyanine iodide and dyed with a cyan dye, e.g. the dyes made by coupling tetrazotized dianisidine with two molecular proportions of 1-amino-8-naphthol-2 : 4-disulphonic acid or of 1-amino-8- naphthol-3: 6-disulphonic acid, and is exposed through the transparency and a blue filter to record only densities of the transparency below 0.6. After development in a non-hardening metol-hydroquinone developer to a gamma of 4, washing and drying, it is re-exposed through the transparency to record detail thereof and is developed in a hardening catechol developer used at low pH and containing little or no sulphite to a gamma of about 0.5. To avoid using a developer-resistant red sensitizer, the film may be red-sensitized after first development and washing in a bath comprising pinacyanol, alcohol, pyridine, benztriazole and water. The unhardened gelatin and associated silver is removed by bleaching in an acid permanganate solution and fixing, e.g. in ammonia solution. The colloid image may be dyed with a grey or coloured dye. The combined transparency and mask may be used in the printing of a multi-layer film or in the production of colour component records. The relief mask containing silver may be used for printing colour corrected prints and the silver subsequently removed, the colourless relief mask being dyed up subsequently if other colourcorrect prints are required. The variable contrast emulsion may give high contrast to green and low contrast to red or yellow. Emulsions giving a high contrast to a spectral region beyond blue and low contrast to blue are unsatisfactory. A positive kinematograph emulsion may be used and the first developed silver removed with acid dichromate solution. The dye may be removable from the relief image by treatment with dilute ammonia solution and later replaced. A photographic stripping film having a substantially waterimpermeable permanent support may be used to provide the masking layer, the emulsion being stripped and secured to the transparency with a solution comprising gelatin, glycerine, turkey red oil, glacial acetic acid and water. A dyed relief may be produced as before, the impermeable support of the stripping film preventing the processing solutions from attacking the dye images in the transparency. The masks may be retouched as described in Specification 568,196, and may be used in the projection or viewing of colour transparencies, in the production of prints from black-and-white transparencies, and in the production of prints from coloured negative transparencies. The masks may be designed to correct also for tone reproduction errors introduced in the copying process. Specifications 507,841, 551,614, 553,196, 553,229, 553,230, 555,383 and 570,498 also are referred to.