Black & white continuous tone printing using multiple negative working plates, so that each plate prints an equal segment of a determined density range

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Title: Black & white continuous tone printing using multiple negative working plates, so that each plate prints an equal segment of a determined density range
Author: Forti, Frank C.
Abstract: Continuous tone printing, either collotype or screenless lithography, are processes that are in limited use. The halftone method of reproduction is still the most economical way to reproduce a continuous tone original. This study dealt with the mechanism of screenless lithography, more specifically on how to make the use of negative working plates a feasible alternative to positive working plates. It is known that the positive working plate can print a longer range of tones than a negative working plate. So the main point of the experiment dealt with how to extend the range of a negative working plate. A negative plate is of high contrast, and so multiple negative working plates were used to print one image. Each plate in itself can print three to four steps of a continuous tone step wedge. In this experiment, three plates were used in the hopes of printing twelve steps in a continuous gradation. A continuous tone step wedge along with a black and white photograph were divided among the three plates. Twelve steps on the step wedge were identified, and so each film had four of the possible thirty steps which existed on the step wedge. These films were exposed to plates, and mounted on the press. Each image was run in registration with one another, and the solid ink density was varied to produce the continuous tone effect. From the press sheets examined, in which there are examples in this report, a high contrast image was produced. One of the main problems in the experiment was differences in emulsion coating on each individual plate. Which resulted in inconsistent exposure times from one plate to another. From repeated plate testing using a .15 density increment continuous tone step wedge as a control guide, it was found that for the same exposure a hardening of the plate coating (and consequently of ink receptivity) corresponding to two steps equaling .30 density difference was to be expected. This made it virtually impossible to get a handle on the correct exposure time needed for the plate. Results from this experiment show, that it is not possible to effectively print a continuous tone image with a negative working plate and or multiple negative plates. This is due to the high contrast of the negative working plate, thus making it suitable for mainly halftone lithographic purposes.
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Date: 1986-05

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