Abstract
Mottle is one of major print defects found in all printing process. It is related to print uniformity which is normally evaluated by observation. As the tendency to specify relatively very intense light for many visual tasks is prominent, the purpose of this work is to present a dependency of print mottle acceptance on ambient illumination and to scrutinize some of the data from which the recommendations are derived. The experiment was designed to analyze print uniformity based on measured mottle index (MI) and human perception under varying illumination. Printed samples were obtained by offset printing technique on different paper grades giving a mottle index between 0.4 and 1.0. They were viewed under illumination that varied from 40 to 2,500 lux. Results showed that the intensity of illumination can be prescribed with coordinating it with MI and Acceptance response parameters. With the exception of the illumination below 450 lux, more than 80% of all observer tended to lose visual power to detect non-uniformity of printed samples at MI less than 0.6. Regarding the standard working environment under the Illuminating Engineering Society's recommendation and based on this finding, it is possible to specify the mottle index of any print at maximum value of 0.6. in order to avoid non-uniformity appearance.
