High dynamic range capture and sensor calibration to improve microscale halftone ink and paper surface quantification.

Abstract

Microscale measurements of halftone dots allow us to investigate the ink transmittance as a function of the position on the halftone dots and to characterize the edges of the halftone dots. In this work, an optical microscope mounted with a commercial camera and a High Dynamic Range (HDR) capture method were combined to measure accurately the printed dots at the microscale. Each stage of the method development has been quantified and compared to simple imaging. On the equipment, the parameters of acquisition have been optimized: size of the observed area under the microscope, exposure time, and fusion of different images resulting from the HDR method. The development of this method allowed comparing three types of printing: offset, inkjet, and electrophotography. The interest of microscopic analysis is to retrieve information that is not accessible at the macroscopic level: descriptors of the ink dot shape and a more precise model of the light diffusion in paper. Furthermore, variables classically measured at the macroscopic level can be assessed: optical densities and colorimetric values. Compared to the single method, the HDR method makes it possible to measure accurately the print characteristics. The coefficient of variation is reduced from 30% to 5%. The perspective of this work is to provide additional information for authenticating printed documents.