Patent GB-570498-A (GB)
Title: Improvements in or relating to photographic tone correcting masks
IPC: G03C7/18
Filed: 1943-08-16
Abstract: 570,498. Photographic tone-correcting masks. KODAK, Ltd. Aug. 16, 1943, Nos. 13263, 13264 and 13265. Convention dates, Aug. 15, 1942. [Class 98 (ii)] A photographic tone - correcting mask for use in the reproduction of a transparency and made from the transparency, has a masking image which is a record of the transparency and which 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 'densities 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 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 masks are particularly useful in printing from transparencies where the prints are to be processed by reversal.- In addition, the masks may also give colour correction, and have complementary neutral grey or coloured images. As shown, the curve M is the characteristic curve of an image obtained by low intensity exposure through a transparency having the characteristic curve T. The curve M<1> is the characteristic curve of an image obtained by either (a) exposure through the transparency and the image having the characteristic curve M on to another lightsensitive layer, or (b) exposure through the transparency on to the unexposed area of the light-sensitive layer on which the image having the characteristic curve M was made (after or before removal of that image). The curve T<1> is the corrected reproduction curve of the image obtained when a light-sensitive layer is exposed through the transparency and the image having the curve M<1> in register. In example 1, a high contrast ortho film is so exposed to a colour transparency of characteristics shown in curve T that substantially only those densities less than 0.6 in the transparency are recorded, and the film is developed in a high - contrast developer, fixed, washed and dried to give a mask of the characteristics shown in curve M. The transparency and mask in register are then printed with red light on to a low contrast panchromatic emulsion, e.g. using a tri-colour red filter, giving sufficient exposure to record all the detail in the transparency, and the film is developed in a metol-hydroquinone-borax developer, fixed, washed and dried to give a mask having the characteristics shown by curve M<1>. A stripping film may be used. In example 2, a film carrying a positive kinematograph film emulsion highly sensitized to a region of the spectrum such as red, green, or yellow, is exposed through the transparency with blue light so that substantially only those densities below 0.6 are recorded, and the film is hardened in a formaldehyde bath, developed in a high-contrast metol-hydroquinone-sodium hydroxide developer, washed, dried, and after removal of the developed silver image in a potassium dichromate - sulphuric acid bath followed by treatment in an ammonium hydroxide bath is re-sensitized (e.g. to red light) in a bath comprising pinacyanol, alcohol, pyridine, benztriazole, and water, washed, dried and given a second exposure to red light through the transparency so as to record all the detail thereof. The film is developed in a metol - hydroquinone - borax developer, fixed, washed and dried. An ordinary blue-sensitive film may be used initially and sensitized at the point indicated. In example 3, a film having a variable contrast emulsion giving high contrast on exposure to blue light and low contrast on exposure to red light is exposed through the transparency and a blue filter so that substantially only those densities below 0.6 are recorded, and the film is then developed, washed and dried. The film is then registered with the transparency (to which alternatively it may have been joined before exposure) and exposed through it using a red filter, the exposure being sufficient to record all the detail thereof, and is developed in a coupling developer comprising diethyl -pphenylenediamine, pbenzylphenol, and NaOH to give a silver plus grey dye image. The film is then bleached in ferricyanide solution, fixed and washed. Other coupling developers containing one or more couplers may be used. Couplers giving a coloured image may be used to provide a coloured masking image. A variable contrast emulsion giving high contrast to green and low contrast to red or yellow may be used. A variable contrast emulsion giving high contrast to a region beyond blue and low contrast to blue is not normally useful. Alternatively, a positive kinematograph emulsion highly sensitive to red, green or yellow light may be given a low intensity exposure through the transparency with blue light, development effected in a high contrast developer, and the film re-exposed with the light chosen for colour correction and redeveloped in a low contrast developer. The finished mask is registered with the transparency and prints made on to a colour film, e.g. by the process of Specification 507,841. Alternatively, colour component records -may be printed from the combination. The mask may also be used in the projection or'viewing of colour transparencies, in printing from black- and-white transparencies, and in making prints from a coloured negative transparency, e.g. one made on material containing colour couplers in the emulsion. The masks may be designed to correct not only for tone reproduction in the original transparency, but also for errors introduced in printing. Specifications 551,614, 553,196, 553,229 and 553,230 also are referred to.