The Relationship between the Dot Area Monitored and Printing Quality in Offset Lithography

The most popular numerical quality control method used in the press field is to use control strips and measure solid patches by a hand-held densitometer. Scanning densitometers have been developed and being used to save measuring time and to control ink keys recently. However, this method has two problems: 1. Solid Ink Density is not enough to assure the quality of images, which cannot recognize dot gain, doubling, and slur. 2. Control strips do not always represent the image quality, which are easily changed by the 'gap' of the plate cylinder and the amount of dot area on the plate. To solve the first problem, halftone patches seem to be better than solid patches, because the density of halftone patches are affected by dot gain, doubling, and slur. Some papers recommend about 3/4 tone to be monitored (Sigg, 1970), and furthermore this 3/4 tone is important to keep print contrast and watch dot gain changing. The dot area which should be monitored to keep the tone reproduction consistent is called the Special Dot Area (SDA) in this paper and was estimated by the simulation based on tone reproduction curves and the experiments using DDCP. The result of this study shows that the SDA which is suitable for the control strip for offset lithography is 65-85%, around 3/4 tone.