Patent GB-544033-A (GB)
Title: Improvements in or relating to half-tone screens for photomechanical printing processes
IPC: G03F5/16
Filed: 1940-06-17
Abstract: 544,033. Half-tone screens for photomechanical printing processes. KODAK, Ltd. (Eastman Kodak Co.). June 17, 1940, No. 10466. [Class 98 (ii)] A contact half-tone screen, especially for use in photolithographic and other processes in which etching of the plates is not employed, comprises a pattern of screen elements of smoothly graded density which increases continuously from the corner to the centre of each element, the density gradient being substantially zero at the corner and centre, the average gradient being between 0.9 and 2.0, and the relative gradients at points 0.1 and 0.3 of the way from the corner to the centre being respectively at least 0.08 and between 0.3 and 0.7. The density readings are taken on the line joining the corner to the centre of the element, the length of which is taken as unity, so that the average density gradient is the difference between the densities at the centre and corner of the element. The relative gradient is the actual gradient at the point in question divided by the relative gradient. Preferably the density characteristics are within narrower limits than those given above, the average gradient being within 1.2 and 1.6, or better between 1.3 and 1.4, and the relative gradient not more than 0.17 at the 0.1 point. Preferably the relative gradient reaches unity at a point between 0.38 and 0.55, or better between 0.4 and 0.5, of the way from the corner to the centre. The relative gradient is preferably between 1.2 and 2.5 at the 0.75 point and decreases to unity at a point between the 0.85 point and the centre, and preferably between the 0.92 and 0.98 points. The maximum value of the relative gradient may be from 1.9 to 2.0. On Fig. 11B the curves marked I and XII represent the relative density gradients of the elements of two screens according to the invention, and any screen having elements of which the average gradient is between 0.9 and 2.0 and the relative gradients between the zero and 0.45 points lie wholly between those of curves I and XII also falls within the invention. Fig. 6 shows in full lines the general form of the desired density distribution curve in relation to that, shown in broken lines, ordinarily obtained in making a contact screen by exposing a sensitive layer 18, Fig. 1, to the shadow image thrown by a spaced half-tone screen 17 on which the light from an illuminated aperture 16 falls. In the latter curve the density gradients are too high and too low respectively in the regions of the points of minimum and maximum density, resulting in insufficient contrast in the highlights and too high contrast in the shadows of prints made by the use of such a screen. The full-line curve has higher density gradients in the region of maximum density 63 and lower gradients in the region of minimum density 64 thus correcting these defects. Several methods of producing screens having the desired density gradation across the elements are described. In one method, in addition to the main exposure through a spaced screen made as shown in Fig. 1, the sensitive layer 18 is given an overall "flash" exposure, which has the effect of increasing the densities in the regions 64 least exposed in the main exposure. The reduction of contrast caused by the flash exposure may be compensated by using a contrasting emulsion in order to obtain the necessary average gradient. The main exposure may be underexposed to reduce the overall density and may be slightly less than is necessary to produce a latent image in the regions 64. The flash has the same effect as if the characteristic curve of the emulsion had a long "toe," and its effect may be controlled by variation of its total amount, and by the use of filters to vary its colour in relation to the colour-sensitivity of the emulsion. Additional exposure may be given to the centres of the screen elements by the use of a diaphragm 13 with an aperture 14 of less size than the aperture 16, or by the use of a neutral grey filter with a similar opening. The spaced screen 17 made of lines and openings, preferably of equal width, is placed at a distance K in front of the layer 18 where K=aS<SP>2</SP>/w, S being the distance from centre to centre of the screen elements, w the mean wavelength in mÁ, of the actinic exposing light, and a a constant depending on the form of the image desired, a suitable value being 3.32X10<SP>6</SP>. The diameter D of the illuminated aperture is given by the equation D=SL/bK where L is the distance between the aperture and the layer 18 and b is between 1 and 4, preferably between 2 and 3. In another and preferred method, the exposure is made by ultra-violet light instead of visible light, which gives the same effect as increasing the " toe " of the characteristic curve of the emulsion. Mercury vapour lamps may be used with a filter to cut off visible light. The film, particularly when exposures are made by ultra-violet light, preferably should have a very fine grain, a slow speed, a high or moderately high gamma such as 1.7 and a high resolution, the film known under the Registered Trade Mark "Microfile" being suitable. It is preferably held in a vacuum holder and tests employing different separations between the screen 17 and the film may be made by packing-strips of cardboard beneath the film. The resulting density distribution across the elements can be measured by a micro-densitometer to determine whether a given separation gives elements of the desired kind. Fig. 13 shows various forms of the aperture D which are preferred to a circular aperture. Apertures a and b are suitable for use with visible light, the inner shaded area of the latter being a neutral grey filter; aperture a produces screens giving greater shadow contrast than those made with aperture b. Apertures c and d are suitable for use with ultra-violet light, and have respectively apaque circular and annular areas within them. A diaphragm having a number of apertures may also be used, Fig. 14 showing an arrangement of five apertures which may be of any of the types shown in Fig. 13. The spaced screen is shown at 25, and the rays from the apertures passing through corresponding points in it meet at a point 26 in the plane of the emulsion. The screens may also be made by photographing a large drawing of the screen made with the desired distribution of densities. The screen may have the "non-diffusing" characteristics described in Specification 544,035, and may be used in the manner described in Specification 544,034. The invention may be applied to the screens embodied in the photographic sensitive elements described in Specification 474,911.