| 摘要 |
A normally opaque recording sheet that can be converted by spark erosion into an image of an original that has low background in areas corresponding to black areas of the original. It comprises an optically transmissive insulating carrier sheet, an opaque layer of aluminium, chromium or nickel deposited on the carrier sheet, and optionally a spark-erodible protective layer. Both the protective layer and any over-layer are eroded away by passing electrical current between the recording sheet and a moving electrical stylus. The resulting positive or negative imaged recording sheet is cleaned by means of a cleansing liquid to remove residual opaque protecting layer and residual material in image areas. The resultant cleaned imaged recording sheet can be employed in transmission either to produce film intermediate positives or negatives, or by contact with pre-sensitised offset lithoplates to produce long run lithographic plates. It has considerably increased light transmission in the imaged areas and enables more faithful reproductions of the tonal gradations of the original to be achieved. |
