Separation of Granularity into Uniformity Deterioration Factors for Electrophotographic Images

It is important to not only evaluate granularity but also clarify the factors affecting it in order to improve the graininess of an image. A method for separating graininess into elements on the basis of the fluctuation of the background, lightness, and toner adherent area is proposed. By using this method following estimations will be enabled; 1. By obtaining the relation between the ordinary graininess and elements, the effect of each elements to the image quality can be estimated. This makes it possible to categorize printer. 2. Measuring by means of a polarizing plate, an effect of gloss ununiformity to the image quality can be detected. 3. By defining the standard contrast for a monochrome image, even though the dot density differs, the area fluctuation factor of graininess can be compared. Introduction Electrophotographic halftone images consist of a screen of 20 ~ 30 micron-sized dots made of toner. Therefore, the pattern shapes and image lightness of the screen may fluctuate even when there is a minimal amount of unevenness in toner adhesion, leading to a deterioration in granularity. Generally, the graininess in electrophotography imaging makes the images inferior to those of other imaging systems like offset and inkjet printing. Therefore, improving granularity is an important technological goal. To do so by optimizing the electrophotographic system, it is necessary to quantify graininess. Dooley and Shaw of Xerox Corp. reported a method for measuring graininess on b/w images [1]. Imakawa et al. of Ricoh Co. Ltd. reported a method for color images based on the work by Dooley et al. [2]. The method uses frequency domain analysis for the fluctuations in colorimetric and lightness values, including corrections made by using the human visual transfer function (VTF) and the average lightness. The correlation between the subjective evaluated values and the estimated values shows good agreement at 0.95. This method enables us to evaluate graininess objectively. Even though methods for evaluating graininess evaluation have been developed and made it possible to compare the image quality between print outputs quantitatively, graininess deterioration cause has not been specified yet. The object of this study is to develop a method for revealing the cause of graininess deterioration in order to optimize the electrophotographic system.