GB576838A - Drop-out half-tone negatives - Google Patents

Abstract

576,838. Photo-mechanical printing- processes. LIEBOWITZ, B. Aug. 9, 1943, No. 12828. Convention date, Aug. 25, 1942. Drawings to Specification. [Class 98 (ii)] A half-tone negative having opaque portions corresponding to the high-light sections of an original drawing. and half-tone portions corresponding to the tone sections of the drawing, is produced on light-sensitive photographic material by giving the tone sections of the drawing such a colour that at least the lightest portions of the tone sections have a tint to which the light-sensitive material is little sensitive and by thereafter photographing the drawing on to the light-sensitive material successively through a half-tone screen and without the screen. The drawing sheet may be coated with a colourless coating solution and the tone sections painted in with a colouring solution containing in addition to the paint therefor an additional chemical substance reacting with the colourless coating solution to give at least the lightest portions of the tone sections the required colour, preferably a yellowish, brownish, or reddish tint, to which the light-sensitive material is little sensitive. If the original drawing contains line sections, these are reproduced as transparent portions in the negative. In the second exposure, a glass plate having the same refractive index as the glass base of the half-tone screen is interposed between the drawing and the lightsensitive material. The first exposure through the half-tone screen is longer than usual and the second exposure is relatively brief. A contrasting developer is used. The half-tone sections may first be painted in with the colouring solution and the coating solution then applied to at least the tone sections of the drawing. In an example, paper is coated with a weak sodium thiosulphate solution, the tone sections are painted thereon with grey or black paint containing uranium nitrate (to give the lighter parts of the tone sections a yellowish tint), and the drawing is successively photographed through a half-tone screen and then without the screen, which is replaced by a transparent glass plate of the same thickness in the second exposure. The sensitive plate is developed in. highly contrasty developer. Half-tone screening may be ensured in the darker parts of the half-tone areas of the negative by making a third exposure with a white sheet interposed between the original drawing and the half-tone screen, this white sheet being photographed through the half- - tone screen on to the sensitive material. After preparing the half-tone sections of the original drawing, line sections may be made in a colour to which the light-sensitive material is insensitive, e.g. in black paint. If the third (" flashing ") exposure is given, half-tone screening is prevented in the line portions by the interposition of a mask having opaque portions corresponding to the line sections. Specification 525,300 is referred to.