2098376 9217821 PCTABS00016 Negative-working lithographic printing plates comprise a radiation-sensitive composition having incorporated therein a photocrosslinkable polymer, containing the photosensitive group (I) as an integral part of the polymer backbone, and a print-out composition which produces an optical density difference upon exposure to activating radiation and thereby enables the exposed image on the printing plate to be seen before the plate is processed. The print-out composition comprises a leuco form of a dye having one or more removable hydrogen atoms, the removal of which forms a compound colored differently from the leuco form; a photo-oxidant which has a photoscissionable nitrogen-oxygen bond, the photo-oxidant serving to convert the leuco dye to the differently colored form when the composition is exposed to activating radiation; and a heteroaromatic amine N-oxide which functions to improve the photo-efficiency of the print-out composition and thereby increase the print-out density.
LITHOGRAPHIC PRINTING PLATES