Altering Grayscale Images to Compensate for Press Fingerprints

Grayscale images are generally made from blackand-white or color continuous-tone photographs. Original photographs are digitized using one of a number of scanning techniques—ranging from desktop scanners to Kodak PhotoCD equipment— and saved as computer files. Saved digital files can then be opened directly by page layout programs, such as Adobe PageMaker or QuarkXPress, placed in the desired position on a page, and then output to laser printers, imagesetters, platesetters, or digital ondemand printing equipment. Unfortunately, grayscale images placed directly into page layout programs will probably look terrible when printed on a printing press because such images have not been altered to compensate for the effects caused by the printing process. In particular, each printing process—indeed each printing machine—causes two types of changes to the grayscale image: tone range compression and halftone dot deformation.To produce the best possible reproduction of grayscale images, it is necessary to determine the actual effect of a particular printing press on tone range compression and halftone dot deformation through careful testing. Upon completion of these tests, the unique characteristics of a press, known as its fingerprint, are recorded.The fingerprint can then be used, in conjunction with image editing software such as Adobe Photoshop, to match a given photograph to the particular press. Matching a photograph to a particular press’ fingerprint will result in the best possible reproduction of a grayscale image. This paper will describe the variables affecting tone range compression and halftone dot deformation, describe a method for fingerprinting an individual press, then explain how Adobe Photoshop can be used to match a grayscale image to a particular fingerprint.