with a screen having 65 lines to the inch are considered coarse. Those having 150 lines to the inch are considered fine. History The first photoengraving process was developed in the 1820s by Nicéphore Niépce, which used photoresist to make a one-off camera photograph rather than a printing plate. His usual test subjects were paper prints of conventional engravings, and exposure was by contact under direct sunlight rather than by the use of a camera. Several metals were tried for the printing plate, as well as glass and lithographic stone. His first success came in 1822. The earliest known surviving example of a paper print made from one of his photoengraved plates dates to 1825 and reproduces a 17th-century engraving. Niépce used Bitumen of Judea as the photoresist. Initially soluble in various spirits and oils, a thin coating of bitumen hardens (polymerizes) where it is exposed to light. The unexposed parts can then be rinsed away with a solvent, baring the underlying material, which can then be etched to the desired depth. Niépce's process lay dormant for many years, but it was revived in the 1850s and bitumen was widely used as a photoresist far into the 20th century. Very long exposures in bright light were required, but bitumen had the advantage that it was superbly resistant to strong acids. The use of photoengraving for a halftone process that could be used to print grayscale photographic images dates all the way back to the 1839 introduction of the daguerreotype, the first practical photographic process. The daguerreotype image consisted of a microscopically fine granular structure on the surface of a silver-plated copper sheet that had been polished to a mirror finish. Methods were soon devised for differentially etching the image grains and the ground so that the daguerreotype could be used as a printing plate. In some instances, very pleasing results were obtained, but exceptional skill and care were required and the very fine structure of the image limited the useful life of each plate to a few hundred prints at best. Henry Fox Talbot is usually credited with the first workable process for converting a grayscale image into a varying structure of stark black and white that resulted in a reasonably durable printing plate. As with other early halftone processes, the plate could not be combined with ordinary type, so for inclusion in a book or periodical each image had