Positives for Photolithography by Chemical Reversal With special reference to screen positives using the contact screen F. G. S.

Abstract

In offset lithography, the deep-etch process of platemaklng which is the most used, demands a positive. The conventional methods of making screen positives are discussed briefly. Line and screen positives can be made directly on the camera and by contact by chemical reversal processes from suitable copy. A dark room camera Is especially suitable as lateral reversal is not required, and if the copy is reasonable the positives so produced can be used for printing down without further contacting. Film is the best material for chemical reversal and as this Is necessary for much work that has to be patched up its use is particularly appropriate. While some colour work has been done the method is at present most suitable for single colour work with “lith” emulsions. A vignetted contact screen can also be used and then the screen holding gear can be utilized for masks if necessary. When panchromatic material of the “lith” type is available for use with neutral contact screens, colour work may be more practical. Details rendition is good and sufficient dot reduction possible. Some curves are given which show that tone rendering follows that of negative work done with the contact screen. Screen positives by chemical reversal together with specimens of printed work were shown. The use of smooth plates would better tonal reproduction but In all operations standardization is essential.