Photoengraving also known as photo-chemical milling is a process of engraving using photographic processing techniques. The full form of photoengraving is photo mechanical process in the graphic arts, used principally for reproducing illustrations. The subject is photographed, and the image is recorded on a sensitized metal plate, which is then etched in an acid bath. In the case of line cuts (drawings in solid blacks and whites without gradations of color), the photoengraving is done on zinc, and the result is called a zinc etching. In the case of halftone cuts, the work is done on copper. The halftone effect is accomplished by photographing the subject through a wire or glass screen, which breaks the light rays so that the metal plate is sensitized in a dotted pattern; the larger dots create the darker areas, the smaller dots the highlights. The finer the screen, the greater the precision of detail in the printed product. Halftones made with a screen having 65 lines to the inch are considered coarse. Those having 150 lines to the inch are considered fine. The most common type of photoengraving involves using a material that is photosensitive and resistant to acids or other etching compounds. This material, called a photoresist, is applied to a metal to be engraved. It is then exposed to light (usually strong ultraviolet) through a photographic negative causing it to harden where the negative allows light to pass. The photoresist is then developed by washing in a solvent that removes the unhardened parts. Finally, the metal to be engraved is exposed to an acid or other etching compound which dissolves the exposed parts of the metal.