Patent GB-544034-A (GB)
Title: Improvements in or relating to photomechanical printing processes employing half-tone screens
IPC: G03F5/18
Filed: 1940-06-17
Abstract: 544,034. Photographic and photomechanical printing processes. KODAK, Ltd. (Eastman Kodak Co.). June 17, 1940, Nos. 10467/40 and 7179/41. [Classes 97 (i) and 98 (ii)] A patterned photographic reproduction of predetermined contrast, for example a half-tone negative for photomechanical printing, is made of a continuous tone record or object by forming an image thereof on a sensitive layer through a patterned screen in contact with the layer, the light absorption properties of the screen elements being of a different nature from those of the record or object and the composition of the printing light being so selected in relation to its absorption by the record and screen and to the sensitivity of the sensitive layer as to give the desired contrast to the reproduction. Thus, the record and the screen elements may be of different colours, or have different light-polarizing properties, the colour or the plane of polarization respectively of the printing light being variable to adjust the apparent contrast of either, or differentially of both, when illuminated by such light in order to obtain the desired contrast in the reproduction. The screen elements may each be of smoothly-graded density from a minimum to a maximum, and may be of the kind described in Specification 544,033. The absorption curves of the record and screen should preferably be widely different so that by changing the printing light a wide degree of change can be obtained in the contrast of the reproduction. Thus, the minimum value of the ratio of the density of the colouring material of the screen to that of the continuous-tone record should not be more than 90 per cent, and preferably not more than 60 per cent over the range of wavelengths to which the sensitive layer is usefully sensitive. To obtain satisfactory dot sharpness in the half-tone reproduction the variation in density of the screen with wavelength should be not more than 60 per cent of its highest value, and preferably not more than 40 per cent. The contrast given by most silver halide emulsions varies with the colour of the light used in exposing, being generally higher the greater the wave-length, which fact must be taken into account in choosing the printing light. The desired composition of the light may be obtained in any known manner, it being preferred to employ white light with coloured filters. A light, or lights in combination, having a natural colour, such as those from mercury vapour and gas discharge lamps may be used. The screen should have a very low degree of light diffusion, as described in Specification 544,035. A modification of the method therein described of processing a screen to produce a low-diffusion magenta dye image is referred to, in which the developer contains p-aminodiethyl aniline hydrochloride, sodium hexametaphospate, sodium sulphite, potassium bromide, sodium hydrate, the substance known under the Registered Trade Mark "Kodalk," 1-p-nitrophenyl-3- methyl-5-pyrazolone, and n-butanol, development being stopped in a 10 per cent solution of benzatriazole; after fixing, the screen is treated in the reducing solution P described in Specification 544,035. Figs. 2 and 3 show diagrammatically the manner in which the contrast of a half-tone reproduction is varied. In Fig. 2 white light passes through a continuous tone record, in the form of a neutral grey step wedge, and a blue filter 26, from which it emerges as blue light of the intensities indicated at 43. It then passes through a yellow contact screen 16, the apparent density of the elements of which the blue light is indicated at 45. The intensity of the light passing the screen and falling on the sensitive surface is represented by the area 49 and 51 represents the effect on the sensitive layer, which is unaffected by intensities below the threshold value indicated by the horizontal line. The resulting half-tone dots are indicated at 57. Fig. 3 shows the effect of substituting a yellow filter 28 for the blue filter 26, the apparent density variations of the yellow screen, indicated at 46, being less than when blue light is used as in Fig. 2. The areas 50 and 52 correspond respectively to the areas 49 and 51 in Fig. 2, and 58 shows the resulting half-tone dots, which vary in size to a greater extent than those in Fig. 2 and therefore produce greater contrast. The continuous-tone record, instead of being neutral, may be of a colour different from that of the contact screen, so that by varying the colour of the printing light the ratio between their apparent contrasts may be varied. The method may be used for obtaining half-tone reproductions all of the same contrast from continuous-tone records which differ in contrast from each other, by selecting the appropriate printing light in each case; the apparent contrast of either the records or of the screen, or of both, varying with the light. Halftone colour separation records may be made from a coloured original which will have the same contrast when processed identically. The continuous-tone records or the screens employed in the process, or both, may have images partly of silver and partly of dye, of pigmented gelatine, or of toned silver, but preferably of pure dye. The invention may be applied to a combined continuous-tone record and half-tone screen as described in Specification 474,911 by rehalogenizing the silver images and then re-developing them, for example by colour development, to form images of different colours, or of which one only is coloured, the silver being subsequently removed. Screens having light-polarizing elements, which may be made as described in Specification 528,796, may be used to control the contrast of the reproduction by varying the polarization of the printing light, for example by a filter. When the continuous tone record is coloured, the printing light may be coloured and polarized, and both properties may be varied independently. Multi-coloured continuous-tone records may be combined with a coloured filter to give a separation image and the contrast of the copy be varied by using a polarizing contrast screen and a rotatable polarizing filter. If no coloured filter is used a copy in monochrome of all the colours is obtained.